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Jarvis KJ, Bell KM, Loya AK, Swank DM, Walcott S. Force-velocity and tension transient measurements from Drosophila jump muscle reveal the necessity of both weakly-bound cross-bridges and series elasticity in models of muscle contraction. Arch Biochem Biophys 2021; 701:108809. [PMID: 33610561 PMCID: PMC7986577 DOI: 10.1016/j.abb.2021.108809] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2020] [Revised: 01/22/2021] [Accepted: 02/09/2021] [Indexed: 01/11/2023]
Abstract
Muscle contraction is a fundamental biological process where molecular interactions between the myosin molecular motor and actin filaments result in contraction of a whole muscle, a process spanning size scales differing in eight orders of magnitude. Since unique behavior is observed at every scale in between these two extremes, to fully understand muscle function it is vital to develop multi-scale models. Based on simulations of classic measurements of muscle heat generation as a function of work, and shortening rate as a function of applied force, we hypothesize that a model based on molecular measurements must be modified to include a weakly-bound interaction between myosin and actin in order to fit measurements at the muscle fiber or whole muscle scales. This hypothesis is further supported by the model's need for a weakly-bound state in order to qualitatively reproduce the force response that occurs when a muscle fiber is rapidly stretched a small distance. We tested this hypothesis by measuring steady-state force as a function of shortening velocity, and the force transient caused by a rapid length step in Drosophila jump muscle fibers. Then, by performing global parameter optimization, we quantitatively compared the predictions of two mathematical models, one lacking a weakly-bound state and one with a weakly-bound state, to these measurements. Both models could reproduce our force-velocity measurements, but only the model with a weakly-bound state could reproduce our force transient measurements. However, neither model could concurrently fit both measurements. We find that only a model that includes weakly-bound cross-bridges with force-dependent detachment and an elastic element in series with the cross-bridges is able to fit both of our measurements. This result suggests that the force response after stretch is not a reflection of distinct steps in the cross-bridge cycle, but rather arises from the interaction of cross-bridges with a series elastic element. Additionally, the model suggests that the curvature of the force-velocity relationship arises from a combination of the force-dependence of weakly- and strongly-bound cross-bridges. Overall, this work presents a minimal cross-bridge model that has predictive power at the fiber level.
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Affiliation(s)
- Katelyn J Jarvis
- Department of Mathematics, University of California, Davis, CA, USA
| | - Kaylyn M Bell
- Department of Biological Sciences, Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, NY, USA
| | - Amy K Loya
- Department of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA
| | - Douglas M Swank
- Department of Biological Sciences, Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, NY, USA; Department of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA
| | - Sam Walcott
- Department of Mathematical Sciences, Worcester Polytechnic Institute, Worcester, MA, USA.
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2
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Muir WW, Hamlin RL. Myocardial Contractility: Historical and Contemporary Considerations. Front Physiol 2020; 11:222. [PMID: 32296340 PMCID: PMC7137917 DOI: 10.3389/fphys.2020.00222] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2019] [Accepted: 02/26/2020] [Indexed: 12/17/2022] Open
Abstract
The term myocardial contractility is thought to have originated more than 125 years ago and has remained and enigma ever since. Although the term is frequently used in textbooks, editorials and contemporary manuscripts its definition remains illusive often being conflated with cardiac performance or inotropy. The absence of a universally accepted definition has led to confusion, disagreement and misconceptions among physiologists, cardiologists and safety pharmacologists regarding its definition particularly in light of new discoveries regarding the load dependent kinetics of cardiac contraction and their translation to cardiac force-velocity and ventricular pressure-volume measurements. Importantly, the Starling interpretation of force development is length-dependent while contractility is length independent. Most historical definitions employ an operational approach and define cardiac contractility in terms of the hearts mechanical properties independent of loading conditions. Literally defined the term contract infers that something has become smaller, shrunk or shortened. The addition of the suffix “ility” implies the quality of this process. The discovery and clinical investigation of small molecules that bind to sarcomeric proteins independently altering force or velocity requires that a modern definition of the term myocardial contractility be developed if the term is to persist. This review reconsiders the historical and contemporary interpretations of the terms cardiac performance and inotropy and recommends a modern definition of myocardial contractility as the preload, afterload and length-independent intrinsic kinetically controlled, chemo-mechanical processes responsible for the development of force and velocity.
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Affiliation(s)
- William W Muir
- College of Veterinary Medicine, Lincoln Memorial University, Harrogate, TN, United States
| | - Robert L Hamlin
- College of Veterinary Medicine, The Ohio State University, Columbus, OH, United States
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3
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Caruel M, Moireau P, Chapelle D. Stochastic modeling of chemical–mechanical coupling in striated muscles. Biomech Model Mechanobiol 2019; 18:563-587. [DOI: 10.1007/s10237-018-1102-z] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2018] [Accepted: 11/21/2018] [Indexed: 01/15/2023]
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4
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Mamidi R, Li J, Doh CY, Holmes JB, Stelzer JE. Lost in translation: Interpreting cardiac muscle mechanics data in clinical practice. Arch Biochem Biophys 2018; 662:213-218. [PMID: 30576628 DOI: 10.1016/j.abb.2018.12.021] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2018] [Revised: 10/25/2018] [Accepted: 12/17/2018] [Indexed: 01/10/2023]
Abstract
Current inotropic therapies improve systolic function in heart failure patients but also elicit undesirable side effects such as arrhythmias and increased intracellular Ca2+ transients. In order to maintain myocyte Ca2+ homeostasis, the increased cytosolic Ca2+ needs to be actively transported back to sarcoplasmic reticulum leading to depleted ATP reserves. Thus, an emerging approach is to design sarcomere-based treatments to correct impaired contractility via a direct and allosteric modulation of myosin's intrinsic force-generating behavior -a concept that potentially avoids the "off-target" effects. To achieve this goal, various biophysical approaches are utilized to investigate the mechanistic impact of sarcomeric modulators but information derived from diverse approaches is not fully integrated into therapeutic applications. This is in part due to the lack of information that provides a coherent connecting link between biophysical data to in vivo function. Hence, our ability to clearly discern the drug-mediated impact on whole-heart function is diminished. Reducing this translational barrier can significantly accelerate clinical progress related to sarcomere-based therapies by optimizing drug-dosing and treatment duration protocols based on information obtained from biophysical studies. Therefore, we attempt to link biophysical mechanical measurements obtained in isolated cardiac muscle and in vivo contractile function.
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Affiliation(s)
- Ranganath Mamidi
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106, USA
| | - Jiayang Li
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106, USA
| | - Chang Yoon Doh
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106, USA
| | - Joshua B Holmes
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106, USA
| | - Julian E Stelzer
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106, USA.
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5
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The basic mechanical structure of the skeletal muscle machinery: One model for linking microscopic and macroscopic scales. J Theor Biol 2018; 456:137-167. [DOI: 10.1016/j.jtbi.2018.07.023] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2017] [Revised: 06/18/2018] [Accepted: 07/19/2018] [Indexed: 11/19/2022]
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6
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Kinetic coupling of phosphate release, force generation and rate-limiting steps in the cross-bridge cycle. J Muscle Res Cell Motil 2017; 38:275-289. [PMID: 28918606 DOI: 10.1007/s10974-017-9482-8] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2017] [Accepted: 09/12/2017] [Indexed: 10/18/2022]
Abstract
A basic goal in muscle research is to understand how the cyclic ATPase activity of cross-bridges is converted into mechanical force. A direct approach to study the chemo-mechanical coupling between Pi release and the force-generating step is provided by the kinetics of force response induced by a rapid change in [Pi]. Classical studies on fibres using caged-Pi discovered that rapid increases in [Pi] induce fast force decays dependent on final [Pi] whose kinetics were interpreted to probe a fast force-generating step prior to Pi release. However, this hypothesis was called into question by studies on skeletal and cardiac myofibrils subjected to Pi jumps in both directions (increases and decreases in [Pi]) which revealed that rapid decreases in [Pi] trigger force rises with slow kinetics, similar to those of calcium-induced force development and mechanically-induced force redevelopment at the same [Pi]. A possible explanation for this discrepancy came from imaging of individual sarcomeres in cardiac myofibrils, showing that the fast force decay upon increase in [Pi] results from so-called sarcomere 'give'. The slow force rise upon decrease in [Pi] was found to better reflect overall sarcomeres cross-bridge kinetics and its [Pi] dependence, suggesting that the force generation coupled to Pi release cannot be separated from the rate-limiting transition. The reasons for the different conclusions achieved in fibre and myofibril studies are re-examined as the recent findings on cardiac myofibrils have fundamental consequences for the coupling between Pi release, rate-limiting steps and force generation. The implications from Pi-induced force kinetics of myofibrils are discussed in combination with historical and recent models of the cross-bridge cycle.
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7
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Offer G, Ranatunga KW. Reinterpretation of the Tension Response of Muscle to Stretches and Releases. Biophys J 2017; 111:2000-2010. [PMID: 27806281 DOI: 10.1016/j.bpj.2016.09.031] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2016] [Revised: 09/03/2016] [Accepted: 09/21/2016] [Indexed: 11/29/2022] Open
Abstract
We have reexamined the experimental time courses of tension in frog muscle after rapid length steps. The early tension recoveries are biexponential. After 3 nm/hs stretches and releases, the rates of the immediate rapid tension changes are similar but the subsequent tension fall after a stretch is much slower than the rise after a release. After 1.5 nm/hs length steps, the entire tension responses are more nearly mirror images. To identify the underlying processes, we used a model of the muscle cross-bridge cycle with two tension-generating (tensing) steps. Analysis of the time course of the tension, the rates of the steps in the cycle, and their contributions to tension provided insights into previously puzzling features of the experimental response. After a stretch, the initial rapid tension fall in the model is caused principally by the reversal of the first tensing step, but after a few milliseconds the tensing step resumes its forward direction. We conclude that the remaining response should not be included in phase 2, the period of early tension recovery. With this exclusion, T2, the tension at the end of this period, rises with an increase of stretch. The rate of early tension recovery also increases with stretch size, showing that the reversal of the first tensing step is strain sensitive. After small length steps, the fast and slow components of the early tension recovery are both caused mainly by the first tensing step. The fast component is triggered by the initial sliding of the filaments, and the slow component is due to further sliding that occurs as the tension recovers. With small length steps (<0.5 nm/hs), the time course of the response to a stretch is the reverse of that to a release.
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Affiliation(s)
- Gerald Offer
- Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, United Kingdom.
| | - K W Ranatunga
- Muscle Contraction Group, School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, United Kingdom.
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8
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Rassier DE. Sarcomere mechanics in striated muscles: from molecules to sarcomeres to cells. Am J Physiol Cell Physiol 2017; 313:C134-C145. [PMID: 28539306 DOI: 10.1152/ajpcell.00050.2017] [Citation(s) in RCA: 46] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2017] [Revised: 05/22/2017] [Accepted: 05/22/2017] [Indexed: 11/22/2022]
Abstract
Muscle contraction is commonly associated with the cross-bridge and sliding filament theories, which have received strong support from experiments conducted over the years in different laboratories. However, there are studies that cannot be readily explained by the theories, showing 1) a plateau of the force-length relation extended beyond optimal filament overlap, and forces produced at long sarcomere lengths that are higher than those predicted by the sliding filament theory; 2) passive forces at long sarcomere lengths that can be modulated by activation and Ca2+, which changes the force-length relation; and 3) an unexplained high force produced during and after stretch of activated muscle fibers. Some of these studies even propose "new theories of contraction." While some of these observations deserve evaluation, many of these studies present data that lack a rigorous control and experiments that cannot be repeated in other laboratories. This article reviews these issues, looking into studies that have used intact and permeabilized fibers, myofibrils, isolated sarcomeres, and half-sarcomeres. A common mechanism associated with sarcomere and half-sarcomere length nonuniformities and a Ca2+-induced increase in the stiffness of titin is proposed to explain observations that derive from these studies.
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Affiliation(s)
- Dilson E Rassier
- Department of Kinesiology and Physical Education, McGill University, Montreal, Quebec, Canada
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9
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Kochubei PV, Bershitsky SY. Possible Cause of Nonlinear Tension Rise in Activated Muscle Fiber during Stretching. Bull Exp Biol Med 2016; 162:11-13. [PMID: 27878491 DOI: 10.1007/s10517-016-3532-x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2016] [Indexed: 11/29/2022]
Abstract
Tension in contracting muscle fiber under conditions of ramp stretching rapidly increases, but after reaching a critical stretch Pc sharply decreases. To find out the cause of these changes in muscle fiber tension, we stopped stretching before and after reaching Pc and left the fiber stretched for 50 msec. After rapid tension drop, the transient tension rise not accompanied by fiber stiffness increase was observed only in fibers heated to 25°C and stretched to Pc. Under other experimental conditions, this growth was absent. We suppose that stretch of the fiber to Pc induces transition of stereo-specifically attached myosin heads to pre-power stroke state and when the stretching is stopped, they make their step on actin and generate force. When the tension reaches Pc, all stereospecifically attached myosin heads turn out to be non-stereospecifically, or weakly attached to actin, and are unable to make the force-generating step.
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Affiliation(s)
- P V Kochubei
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Ekaterinburg, Russia.
| | - S Yu Bershitsky
- Institute of Immunology and Physiology, Ural Division of the Russian Academy of Sciences, Ekaterinburg, Russia
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10
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Mamidi R, Gresham KS, Verma S, Stelzer JE. Cardiac Myosin Binding Protein-C Phosphorylation Modulates Myofilament Length-Dependent Activation. Front Physiol 2016; 7:38. [PMID: 26913007 PMCID: PMC4753332 DOI: 10.3389/fphys.2016.00038] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2015] [Accepted: 01/28/2016] [Indexed: 11/13/2022] Open
Abstract
Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is an important regulator of contractile function, however, its contributions to length-dependent changes in cross-bridge (XB) kinetics is unknown. Therefore, we performed mechanical experiments to quantify contractile function in detergent-skinned ventricular preparations isolated from wild-type (WT) hearts, and hearts expressing non-phosphorylatable cMyBP-C [Ser to Ala substitutions at residues Ser273, Ser282, and Ser302 (i.e., 3SA)], at sarcomere length (SL) 1.9 μm or 2.1μm, prior and following protein kinase A (PKA) treatment. Steady-state force generation measurements revealed a blunting in the length-dependent increase in myofilament Ca(2+)-sensitivity of force generation (pCa50) following an increase in SL in 3SA skinned myocardium compared to WT skinned myocardium. Dynamic XB behavior was assessed at submaximal Ca(2+)-activations by imposing an acute rapid stretch of 2% of initial muscle length, and measuring both the magnitudes and rates of resultant phases of force decay due to strain-induced XB detachment and delayed force rise due to recruitment of additional XBs with increased SL (i.e., stretch activation). The magnitude (P2) and rate of XB detachment (k rel) following stretch was significantly reduced in 3SA skinned myocardium compared to WT skinned myocardium at short and long SL, and prior to and following PKA treatment. Furthermore, the length-dependent acceleration of k rel due to decreased SL that was observed in WT skinned myocardium was abolished in 3SA skinned myocardium. PKA treatment accelerated the rate of XB recruitment (k df) following stretch at both SL's in WT but not in 3SA skinned myocardium. The amplitude of the enhancement in force generation above initial pre-stretch steady-state levels (P3) was not different between WT and 3SA skinned myocardium at any condition measured. However, the magnitude of the entire delayed force phase which can dip below initial pre-stretch steady-state levels (Pdf) was significantly lower in 3SA skinned myocardium under all conditions, in part due to a reduced magnitude of XB detachment (P2) in 3SA skinned myocardium compared to WT skinned myocardium. These findings demonstrate that cMyBP-C phospho-ablation regulates SL- and PKA-mediated effects on XB kinetics in the myocardium, which would be expected to contribute to the regulation of the Frank-Starling mechanism.
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Affiliation(s)
- Ranganath Mamidi
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University Cleveland, OH, USA
| | - Kenneth S Gresham
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University Cleveland, OH, USA
| | - Sujeet Verma
- Department of Horticultural Science, Institute of Food and Agricultural Sciences Gulf Coast Research and Education Center, University of Florida Wimauma, FL, USA
| | - Julian E Stelzer
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University Cleveland, OH, USA
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11
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Temperature effect on the chemomechanical regulation of substeps within the power stroke of a single Myosin II. Sci Rep 2016; 6:19506. [PMID: 26786569 PMCID: PMC4726395 DOI: 10.1038/srep19506] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2015] [Accepted: 12/14/2015] [Indexed: 11/08/2022] Open
Abstract
Myosin IIs in the skeletal muscle are highly efficient nanoscale machines evolved in nature. Understanding how they function can not only bring insights into various biological processes but also provide guidelines to engineer synthetic nanoscale motors working in the vicinity of thermal noise. Though it was clearly demonstrated that the behavior of a skeletal muscle fiber, or that of a single myosin was strongly affected by the temperature, how exactly the temperature affects the kinetics of a single myosin is not fully understood. By adapting the newly developed transitional state model, which successfully explained the intriguing motor force regulation during skeletal muscle contraction, here we systematically explain how exactly the power stroke of a single myosin proceeds, with the consideration of the chemomechanical regulation of sub-steps within the stroke. The adapted theory is then utilized to investigate the temperature effect on various aspects of the power stroke. Our analysis suggests that, though swing rates, the isometric force, and the maximal stroke size all strongly vary with the temperature, the temperature can have a very small effect on the releasable elastic energy within the power stroke.
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12
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Effects of pseudo-phosphorylated rat cardiac troponin T are differently modulated by α- and β-myosin heavy chain isoforms. Basic Res Cardiol 2014; 109:442. [PMID: 25301196 DOI: 10.1007/s00395-014-0442-9] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/13/2014] [Revised: 08/31/2014] [Accepted: 09/18/2014] [Indexed: 10/24/2022]
Abstract
Interplay between the protein kinase C (PKC)-mediated phosphorylation of troponin T (TnT)- and myosin heavy chain (MHC)-mediated effects on thin filaments takes on a new significance because: (1) there is significant interaction between the TnT- and MHC-mediated effects on cardiac thin filaments; (2) although the phosphorylation of TnT by PKC isoforms is common to both human and rodent hearts, human hearts predominantly express β-MHC while rodent hearts predominantly express α-MHC. Therefore, we tested how α- and β-MHC isoforms differently affected the functional effects of phosphorylated TnT. Contractile measurements were made on cardiac muscle fibers from normal rats (α-MHC) and propylthiouracil-treated rats (β-MHC), reconstituted with the recombinant phosphomimetic-TnT (T204E; threonine 204 replaced by glutamate). Ca2+ -activated maximal tension decreased differently in α-MHC + T204E (~68%) and β-MHC + T204E (~35%). However, myofilament Ca2+ sensitivity decreased similarly in α-MHC + T204E and β-MHC + T204E, demonstrating that a decrease in Ca2+ sensitivity alone cannot explain the greater attenuation of tension in α-MHC + T204E. Interestingly, dynamic contractile parameters (rates of tension redevelopment, crossbridge (XB) recruitment dynamics, XB distortion dynamics, and XB detachment kinetics) decreased only in α-MHC + T204E. Thus, the transition of thin filaments from the blocked- to closed-state was attenuated in α-MHC + T204E and β-MHC + T204E, but the closed- to open-state transition was attenuated only in α-MHC + T204E. Our study demonstrates that the effects of phosphorylated TnT and MHC isoforms interact to bring about different functional states of cardiac thin filaments.
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van Dijk SJ, Boontje NM, Heymans MW, Ten Cate FJ, Michels M, Dos Remedios C, Dooijes D, van Slegtenhorst MA, van der Velden J, Stienen GJM. Preserved cross-bridge kinetics in human hypertrophic cardiomyopathy patients with MYBPC3 mutations. Pflugers Arch 2013; 466:1619-33. [PMID: 24186209 DOI: 10.1007/s00424-013-1391-0] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2013] [Revised: 10/04/2013] [Accepted: 10/19/2013] [Indexed: 12/23/2022]
Abstract
Mutations in the MYBPC3 gene, encoding cardiac myosin binding protein C (cMyBP-C) are frequent causes of hypertrophic cardiomyopathy (HCM). Previously, we have presented evidence for reduced cMyBP-C expression (haploinsufficiency), in patients with a truncation mutation in MYBPC3. In mice, lacking cMyBP-C cross-bridge kinetics was accelerated. In this study, we investigated whether cross-bridge kinetics was altered in myectomy samples from HCM patients harboring heterozygous MYBPC3 mutations (MYBPC3mut). Isometric force and the rate of force redevelopment (k tr) at different activating Ca(2+) concentrations were measured in mechanically isolated Triton-permeabilized cardiomyocytes from MYBPC3mut (n = 18) and donor (n = 7) tissue. Furthermore, the stretch activation response of cardiomyocytes was measured in tissue from eight MYBPC3mut patients and five donors to assess the rate of initial force relaxation (k 1) and the rate and magnitude of the transient increase in force (k 2 and P 3, respectively) after a rapid stretch. Maximal force development of the cardiomyocytes was reduced in MYBPC3mut (24.5 ± 2.3 kN/m(2)) compared to donor (34.9 ± 1.6 kN/m(2)). The rates of force redevelopment in MYBPC3mut and donor over a range of Ca(2+) concentrations were similar (k tr at maximal activation: 0.63 ± 0.03 and 0.75 ± 0.09 s(-1), respectively). Moreover, the stretch activation parameters did not differ significantly between MYBPC3mut and donor (k 1: 8.5±0.5 and 8.8 ± 0.4 s(-1); k 2: 0.77 ± 0.06 and 0.74 ± 0.09 s(-1); P 3: 0.08 ± 0.01 and 0.09 ± 0.01, respectively). Incubation with protein kinase A accelerated k 1 in MYBPC3mut and donor to a similar extent. Our experiments indicate that, at the cMyBP-C expression levels in this patient group (63 ± 6 % relative to donors), cross-bridge kinetics are preserved and that the depressed maximal force development is not explained by perturbation of cross-bridge kinetics.
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Affiliation(s)
- Sabine J van Dijk
- Laboratory for Physiology, Institute for Cardiovascular Research, VU University Medical Center, van der Boechorststraat 7, 1081 BT, Amsterdam, The Netherlands
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Nocella M, Bagni MA, Cecchi G, Colombini B. Mechanism of force enhancement during stretching of skeletal muscle fibres investigated by high time-resolved stiffness measurements. J Muscle Res Cell Motil 2013; 34:71-81. [DOI: 10.1007/s10974-012-9335-4] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2012] [Accepted: 12/21/2012] [Indexed: 10/27/2022]
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15
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Mamidi R, Mallampalli SL, Wieczorek DF, Chandra M. Identification of two new regions in the N-terminus of cardiac troponin T that have divergent effects on cardiac contractile function. J Physiol 2012. [PMID: 23207592 DOI: 10.1113/jphysiol.2012.243394] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
Abstract Cardiac troponin T (cTnT) has a highly acidic extended N-terminus, the physiological role of which remains poorly understood. To decipher the physiological role of this unique region, we deleted specific regions within the N-terminus of mouse cTnT (McTnT) to create McTnT1-44 and McTnT45-74 proteins. Contractile function and dynamic force-length measurements were made after reconstituting the McTnT deletion proteins into detergent-skinned cardiac papillary fibres harvested from non-transgenic mice that expressed α-tropomyosin (Tm). To further understand how the functional effects of the N-terminus of cTnT are modulated by Tm isoforms, McTnT deletion proteins were reconstituted into detergent-skinned cardiac papillary fibres harvested from transgenic mice that expressed both α- and β-Tm. McTnT1-44, but not McTnT45-74, attenuated maximal activation of the thin filament. Myofilament Ca(2+) sensitivity, as measured by pCa50 (-log of [Ca(2+)]free required for half-maximal activation), decreased in McTnT1-44 (α-Tm) fibres. The desensitizing effect of McTnT1-44 on pCa50 was ablated in β-Tm fibres. McTnT45-74 enhanced pCa50 in both α- and β-Tm fibres, with β-Tm having a bigger effect. The Hill coefficient of tension development was significantly attenuated by McTnT45-74, suggesting an effect on thin-filament cooperativity. The rate of cross-bridge (XB) detachment and the strained XB-mediated impact on other XBs were augmented by McTnT1-44 in β-Tm fibres. The magnitude of the length-mediated recruitment of XBs was attenuated by McTnT1-44 in β-Tm fibres. Our data demonstrate that the 1-44 region of McTnT is essential for maximal activation, whereas the cardiac-specific 45-74 region of McTnT is essential for augmenting cooperativity. Moreover, our data show that α- and β-Tm isoforms have divergent effects on McTnT deletion mutant's ability to modulate cardiac thin-filament activation and Ca(2+) sensitivity. Our results not only provide the first explicit evidence for the existence of two distinct functional regions within the N-terminus of cTnT, but also offer mechanistic insights into the divergent physiological roles of these regions in mediating cardiac contractile activation.
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Affiliation(s)
- Ranganath Mamidi
- Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, WA-99164, USA.
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16
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Rassier DE. The mechanisms of the residual force enhancement after stretch of skeletal muscle: non-uniformity in half-sarcomeres and stiffness of titin. Proc Biol Sci 2012; 279:2705-13. [PMID: 22535786 DOI: 10.1098/rspb.2012.0467] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
When activated skeletal muscles are stretched, the force increases significantly. After the stretch, the force decreases and reaches a steady-state level that is higher than the force produced at the corresponding length during purely isometric contractions. This phenomenon, referred to as residual force enhancement, has been observed for more than 50 years, but the mechanism remains elusive, generating considerable debate in the literature. This paper reviews studies performed with single muscle fibres, myofibrils and sarcomeres to investigate the mechanisms of the stretch-induced force enhancement. First, the paper summarizes the characteristics of force enhancement and early hypotheses associated with non-uniformity of sarcomere length. Then, it reviews new evidence suggesting that force enhancement can also be associated with sarcomeric structures. Finally, this paper proposes that force enhancement is caused by: (i) half-sarcomere non-uniformities that will affect the levels of passive forces and overlap between myosin and actin filaments, and (ii) a Ca(2+)-induced stiffness of titin molecules. These mechanisms are compatible with most observations in the literature, and can be tested directly with emerging technologies in the near future.
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Affiliation(s)
- Dilson E Rassier
- Departments of Kinesiology and Physical Education, Physics and Physiology, McGill University, Montreal (PQ), Canada.
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Park-Holohan S, Linari M, Reconditi M, Fusi L, Brunello E, Irving M, Dolfi M, Lombardi V, West TG, Curtin NA, Woledge RC, Piazzesi G. Mechanics of myosin function in white muscle fibres of the dogfish, Scyliorhinus canicula. J Physiol 2012; 590:1973-88. [PMID: 22310308 PMCID: PMC3491701 DOI: 10.1113/jphysiol.2011.217133] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The contractile properties of muscle fibres have been extensively investigated by fast perturbation in sarcomere length to define the mechanical characteristics of myofilaments and myosin heads that underpin refined models of the acto-myosin cycle. Comparison of published data from intact fast-twitch fibres of frog muscle and demembranated fibres from fast muscle of rabbit shows that stiffness of the rabbit myosin head is only ∼62% of that in frog. To clarify if and how much the mechanical characteristics of the filaments and myosin heads vary in muscles of different animals we apply the same high resolution mechanical methods, in combination with X-ray diffraction, to fast-twitch fibres from the dogfish (Scyliorhinus canicula). The values of equivalent filament compliance (Cf) measured by X-ray diffraction and in mechanical experiments are not significantly different; the best estimate from combining these values is 17.1 ± 1.0 nm MPa−1. This value is larger than Cf in frog, 13.0 ± 0.4 nm MPa−1. The longer thin filaments in dogfish account for only part of this difference. The average isometric force exerted by each attached myosin head at 5°C, 4.5 pN, and the maximum sliding distance accounted for by the myosin working stroke, 11 nm, are similar to those in frog, while the average myosin head stiffness of dogfish (1.98 ± 0.31 pN nm−1) is smaller than that of frog (2.78 ± 0.30 pN nm−1). Taken together these results indicate that the working stroke responsible for the generation of isometric force is a larger fraction of the total myosin head working stroke in the dogfish than in the frog.
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Affiliation(s)
- S Park-Holohan
- Molecular Medicine, National Heart & Lung Institute, Imperial College London, London SW7 2AZ, UK
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18
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Thin filament-reconstituted skinned muscle fibers for the study of muscle physiology. J Biomed Biotechnol 2011; 2011:486021. [PMID: 22131807 PMCID: PMC3216491 DOI: 10.1155/2011/486021] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2011] [Accepted: 08/12/2011] [Indexed: 11/20/2022] Open
Abstract
We review the use of thin filament-reconstituted muscle fibers in the study of muscle physiology. Thin filament extraction and reconstitution protocol is a powerful technique to study the role of each component of the thin filament. It is also useful for studying the properties of genetically modified molecules such as actin and tropomyosin. We also review the combination of this protocol with sinusoidal analysis, which will provide a solid technique for determining the effect of regulatory proteins on actomyosin interaction and concomitant cross-bridge kinetics. We suggest that thin filament-reconstituted muscle fibers are an ideal system for studying muscle physiology especially when gene modifications of actin or tropomyosin are involved.
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19
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Günther M, Schmitt S. A macroscopic ansatz to deduce the Hill relation. J Theor Biol 2010; 263:407-18. [PMID: 20045704 DOI: 10.1016/j.jtbi.2009.12.027] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/01/2008] [Revised: 12/23/2009] [Accepted: 12/24/2009] [Indexed: 11/26/2022]
Abstract
In this study, we derive the hyperbolic force-velocity relation of concentric muscular contraction, first formulated empirically by A.V. Hill in 1938, from three essential model assumptions: (1) the structural assembly of three well-known elements - i.e. active, parallel damping, and serial - fulfilling a force equilibrium, (2) the parallel damping coefficient explicitly depending on muscle force output and three parameters, and (3) the kinematic gearing ratio between active and serial element being assigned to a parameter. The energy source within the muscle represented by the force of the active element is an additional fifth parameter. As a result we find the Hill "constants" A and B as functions of our five model parameters. Using A and B values from literature on experimental data, we predict heat power release of our model. By calculating enthalpy rate and mechanical efficiency, we compare the model heat power to predictions from another Hill-type model, to Hill's original findings, and to findings from modern muscle heat measurements. We reconsider why the biggest share of heat rate during isometric contractions (maintenance heat) and the velocity-dependent heat rate during concentric contractions in addition to maintenance heat rate (shortening heat rate) may be traced back to the same mechanism represented by the kinematic gearing ratio. Namely, we suggest that the serial element transfers attachment-detachment fluctuations of actin-myosin crossbridges within one sarcomere to others in the same sarcomere and to those in parallel and in series. Numerically, in case of negligible passive muscular damping, we find the ratio between A and isometric force (relative A) to depend exclusively on the kinematic gearing ratio, whereas the maintenance heat rate scales with the square of relative A. Moreover, this mechanical coupling internal to the muscle fibres may also be behind the macroscopic force dependency of the overall parallel damping coefficient.
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Affiliation(s)
- Michael Günther
- Eberhard-Karls-Universität, Institut für Sportwissenschaft, Arbeitsbereich III, Wilhelmstrasse 124, D-72074 Tübingen, Deutschland, Germany
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20
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Fusi L, Reconditi M, Linari M, Brunello E, Elangovan R, Lombardi V, Piazzesi G. The mechanism of the resistance to stretch of isometrically contracting single muscle fibres. J Physiol 2009; 588:495-510. [PMID: 19948653 DOI: 10.1113/jphysiol.2009.178137] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Rapid attachment to actin of the detached motor domain of myosin dimers with one motor domain already attached has been hypothesized to explain the stretch-induced changes in X-ray interference and stiffness of active muscle. Here, using half-sarcomere mechanics in single frog muscle fibres (2.15 microm sarcomere length and 4 degrees C), we show that: (1) an increase in stiffness of the half-sarcomere under stretch is specific to isometric contraction and does not occur in rigor, indicating that the mechanism of stiffness increase is an increase in the number of attached motors; (2) 2 ms after 100 micros stretches (amplitude 2-8 nm per half-sarcomere) imposed during an isometric tetanus, the stiffness of the array of myosin motors in each half-sarcomere (e(m)) increases above the isometric value (e(m0)); (3) e(m) has a sigmoidal dependence on the distortion of the motor domains (Delta z) attached in isometric contraction, with a maximum approximately 2 e(m0) for a distortion of approximately 6 nm; e(m) is influenced by detachment of motors at z > 6 nm; (4) at the end of the 100 micros stretch the relation between e(m)/e(m0) and Delta z lies slightly but not significantly above that at 2 ms. These results support the idea that stretch-induced sliding of the actin filament distorts the actin-attached motor domain of the myosin dimers away from the centre of the sarcomere, providing the steric conditions for rapid attachment of the second motor domain. The rate of new motor attachment must be as high as 7.5 x 10(4) s(1) and explains the rapid and efficient increase of the resistance of active muscle to stretch.
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Affiliation(s)
- Luca Fusi
- Laboratory of Physiology, Department of Evolutionary Biology, University of Florence, 50019 Sesto Fiorentino, Italy
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21
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Galler S. Molecular basis of the catch state in molluscan smooth muscles: a catchy challenge. J Muscle Res Cell Motil 2008; 29:73-99. [PMID: 19039672 DOI: 10.1007/s10974-008-9149-6] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2008] [Accepted: 10/18/2008] [Indexed: 12/15/2022]
Abstract
The catch state (or 'catch') of molluscan smooth muscles is a passive holding state that occurs after cessation of stimulation. During catch, force and, in particular, resistance to stretch are maintained for long time periods with low (or no) energy consumption at basal intracellular free [Ca2+]. The catch state is initiated by Ca2+-stimulated dephosphorylation of the titin-like protein twitchin and is inhibited by cAMP-dependent phosphorylation of twitchin. In addition, catch is pH sensitive, but the reason for this is unknown. According to a traditional model, catch is due to slower cross-bridge cycles where myosin heads remain longer attached to the actin filaments after force generation, possibly caused by a hindered release of ADP from the myosin heads. However, this model was disproved by recent findings which showed that (i) inhibitors of myosin function, such as vanadate, do not affect catch force; (ii) factors which terminate the catch state do not accelerate myosin head detachment kinetics and (iii) a catch-like high resistance to stretch is still inducible when force development is prevented. Thus, catch probably involves passive linkage structures interconnecting the myofilaments (catch linkages). For example twitchin could (i) tie myosin heads to the thin filaments, (ii) mechanically lock them in a stretch resistant state or (iii) interconnect thick and thin filaments directly. However, it is questionable if these mechanisms are sufficient since twitchin seems to be about 15-times less abundant than myosin. Therefore, in addition, interconnections between thick filaments could exist, which could involve e.g. paramyosin or twitchin. Catch could even involve changes in the compliance of thick filaments. The function of myorod, found specifically in catch muscles in equal abundance with myosin, is not known. The suggestion is made here that catch linkages are present already during active contraction either as ratchet-like elements resisting stretch and not opposing shortening or in some kind of 'standby' mode ready to transform suddenly into the working mode by stretches or after Ca2+ removal following cessation of stimulation.
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Affiliation(s)
- Stefan Galler
- Department of Cell Biology, University of Salzburg, Hellbrunnerstrasse 34, 5020, Salzburg, Austria.
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22
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Tong CW, Stelzer JE, Greaser ML, Powers PA, Moss RL. Acceleration of crossbridge kinetics by protein kinase A phosphorylation of cardiac myosin binding protein C modulates cardiac function. Circ Res 2008; 103:974-82. [PMID: 18802026 DOI: 10.1161/circresaha.108.177683] [Citation(s) in RCA: 153] [Impact Index Per Article: 9.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
Normal cardiac function requires dynamic modulation of contraction. beta1-adrenergic-induced protein kinase (PK)A phosphorylation of cardiac myosin binding protein (cMyBP)-C may regulate crossbridge kinetics to modulate contraction. We tested this idea with mechanical measurements and echocardiography in a mouse model lacking 3 PKA sites on cMyBP-C, ie, cMyBP-C(t3SA). We developed the model by transgenic expression of mutant cMyBP-C with Ser-to-Ala mutations on the cMyBP-C knockout background. Western blots, immunofluorescence, and in vitro phosphorylation combined to show that non-PKA-phosphorylatable cMyBP-C expressed at 74% compared to normal wild-type (WT) and was correctly positioned in the sarcomeres. Similar expression of WT cMyBP-C at 72% served as control, ie, cMyBP-C(tWT). Skinned myocardium responded to stretch with an immediate increase in force, followed by a transient relaxation of force and finally a delayed development of force, ie, stretch activation. The rate constants of relaxation, k(rel) (s-1), and delayed force development, k(df) (s-1), in the stretch activation response are indicators of crossbridge cycling kinetics. cMyBP-C(t3SA) myocardium had baseline k(rel) and k(df) similar to WT myocardium, but, unlike WT, k(rel) and k(df) were not accelerated by PKA treatment. Reduced dobutamine augmentation of systolic function in cMyBP-C(t3SA) hearts during echocardiography corroborated the stretch activation findings. Furthermore, cMyBP-C(t3SA) hearts exhibited basal echocardiographic findings of systolic dysfunction, diastolic dysfunction, and hypertrophy. Conversely, cMyBP-C(tWT) hearts performed similar to WT. Thus, PKA phosphorylation of cMyBP-C accelerates crossbridge kinetics and loss of this regulation leads to cardiac dysfunction.
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Affiliation(s)
- Carl W Tong
- Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53706, USA
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23
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Skeletal muscle resists stretch by rapid binding of the second motor domain of myosin to actin. Proc Natl Acad Sci U S A 2007; 104:20114-9. [PMID: 18077437 DOI: 10.1073/pnas.0707626104] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
A shortening muscle is a machine that converts metabolic energy into mechanical work, but, when a muscle is stretched, it acts as a brake, generating a high resistive force at low metabolic cost. The braking action of muscle can be activated with remarkable speed, as when the leg extensor muscles rapidly decelerate the body at the end of a jump. Here we used time-resolved x-ray and mechanical measurements on isolated muscle cells to elucidate the molecular basis of muscle braking and its rapid control. We show that a stretch of only 5 nm between each overlapping set of myosin and actin filaments in a muscle sarcomere is sufficient to double the number of myosin motors attached to actin within a few milliseconds. Each myosin molecule has two motor domains, only one of which is attached to actin during shortening or activation at constant length. A stretch strains the attached motor domain, and we propose that combined steric and mechanical coupling between the two domains promotes attachment of the second motor domain. This mechanism allows skeletal muscle to resist external stretch without increasing the force per motor and provides an answer to the longstanding question of the functional role of the dimeric structure of muscle myosin.
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24
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Stelzer JE, Patel JR, Walker JW, Moss RL. Differential roles of cardiac myosin-binding protein C and cardiac troponin I in the myofibrillar force responses to protein kinase A phosphorylation. Circ Res 2007; 101:503-11. [PMID: 17641226 DOI: 10.1161/circresaha.107.153650] [Citation(s) in RCA: 136] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The heart is remarkably adaptable in its ability to vary its function to meet the changing demands of the circulatory system. During times of physiological stress, cardiac output increases in response to increased sympathetic activity, which results in protein kinase A (PKA)-mediated phosphorylations of the myofilament proteins cardiac troponin (cTn)I and cardiac myosin-binding protein (cMyBP)-C. Despite the importance of this mechanism, little is known about the relative contributions of cTnI and cMyBP-C phosphorylation to increased cardiac contractility. Using engineered mouse lines either lacking cMyBP-C (cMyBP-C(-/-)) or expressing a non-PKA phosphorylatable cTnI (cTnI(ala2)), or both (cMyBP-C(-/-)/cTnI(ala2)), we investigated the roles of cTnI and cMyBP-C phosphorylation in the regulation of the stretch-activation response. PKA treatment of wild-type and cTnI(ala2) skinned ventricular myocardium accelerated stretch activation such that the response was indistinguishable from stretch activation of cMyBP-C(-/-) or cMyBP-C(-/-)/cTnI(ala2) myocardium; however, PKA had no effect on stretch activation in cMyBP-C(-/-) or cMyBP-C(-/-)/cTnI(ala2) myocardium. These results indicate that the acceleration of stretch activation in wild-type and cTnI(ala2) myocardium is caused by phosphorylation of cMyBP-C and not cTnI. We conclude that the primary effect of PKA phosphorylation of cTnI is reduced Ca(2+) sensitivity of force, whereas phosphorylation of cMyBP-C accelerates the kinetics of force development. These results predict that PKA phosphorylation of myofibrillar proteins in living myocardium contributes to accelerated relaxation in diastole and increased rates of force development in systole.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin School of Medicine, Madison, WI 53711, USA.
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25
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Colombini B, Bagni MA, Romano G, Cecchi G. Characterization of actomyosin bond properties in intact skeletal muscle by force spectroscopy. Proc Natl Acad Sci U S A 2007; 104:9284-9. [PMID: 17517641 PMCID: PMC1890486 DOI: 10.1073/pnas.0611070104] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Force generation and motion in skeletal muscle result from interaction between actin and myosin myofilaments through the cyclical formation and rupture of the actomyosin bonds, the cross-bridges, in the overlap region of the sarcomeres. Actomyosin bond properties were investigated here in single intact muscle fibers by using dynamic force spectroscopy. The force needed to forcibly detach the cross-bridge ensemble in the half-sarcomere (hs) was measured in a range of stretching velocity between 3.4 x 10(3) nm.hs(-1).s(-1) or 3.3 fiber length per second (l(0)s(-1)) and 6.1 x 10(4) nm.hs(-1).s(-1) or 50 l(0).s(-1) during tetanic force development. The rupture force of the actomyosin bond increased linearly with the logarithm of the loading rate, in agreement with previous experiments on noncovalent single bond and with Bell theory [Bell GI (1978) Science 200:618-627]. The analysis permitted calculation of the actomyosin interaction length, x(beta) and the dissociation rate constant for zero external load, k(0). Mean x(beta) was 1.25 nm, a value similar to that reported for single actomyosin bond under rigor condition. Mean k(0) was 20 s(-1), a value about twice as great as that reported in the literature for isometric force relaxation in the same type of muscle fibers. These experiments show, for the first time, that force spectroscopy can be used to reveal the properties of the individual cross-bridge in intact skeletal muscle fibers.
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Affiliation(s)
- Barbara Colombini
- Dipartimento di Scienze Fisiologiche and Istituto Interuniversitario di Miologia, Università degli Studi di Firenze, Viale G. B. Morgagni 63, I-50134 Firenze, Italy; and
| | - M. Angela Bagni
- Dipartimento di Scienze Fisiologiche and Istituto Interuniversitario di Miologia, Università degli Studi di Firenze, Viale G. B. Morgagni 63, I-50134 Firenze, Italy; and
| | - Giovanni Romano
- Dipartimento di Fisiopatologia Clinica, Università degli Studi di Firenze, Viale G. B. Morgagni 85, I-50134 Firenze, Italy
| | - Giovanni Cecchi
- Dipartimento di Scienze Fisiologiche and Istituto Interuniversitario di Miologia, Università degli Studi di Firenze, Viale G. B. Morgagni 63, I-50134 Firenze, Italy; and
- To whom correspondence should be addressed. E-mail:
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26
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Stelzer JE, Moss RL. Contributions of stretch activation to length-dependent contraction in murine myocardium. ACTA ACUST UNITED AC 2007; 128:461-71. [PMID: 17001086 PMCID: PMC2151573 DOI: 10.1085/jgp.200609634] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The steep relationship between systolic force production and end diastolic volume (Frank-Starling relationship) in myocardium is a potentially important mechanism by which the work capacity of the heart varies on a beat-to-beat basis, but the molecular basis for the effects of myocardial fiber length on cardiac work are still not well understood. Recent studies have suggested that an intrinsic property of myocardium, stretch activation, contributes to force generation during systolic ejection in myocardium. To examine the role of stretch activation in length dependence of activation we recorded the force responses of murine skinned myocardium to sudden stretches of 1% of muscle length at both short (1.90 μm) and long (2.25 μm) sarcomere lengths (SL). Maximal Ca2+-activated force and Ca2+ sensitivity of force were greater at longer SL, such that more force was produced at a given Ca2+ concentration. Sudden stretch of myocardium during an otherwise isometric contraction resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed development of force, i.e., stretch activation, to levels greater than prestretch force. At both maximal and submaximal activations, increased SL significantly reduced the initial rate of force decay following stretch; at submaximal activations (but not at maximal) the rate of delayed force development was accelerated. This combination of mechanical effects of increased SL would be expected to increase force generation during systolic ejection in vivo and prolong the period of ejection. These results suggest that sarcomere length dependence of stretch activation contributes to the steepness of the Frank-Starling relationship in living myocardium.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53706, USA.
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27
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Stelzer JE, Brickson SL, Locher MR, Moss RL. Role of myosin heavy chain composition in the stretch activation response of rat myocardium. J Physiol 2006; 579:161-73. [PMID: 17138609 PMCID: PMC2075383 DOI: 10.1113/jphysiol.2006.119719] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022] Open
Abstract
The speed and force of myocardial contraction during systolic ejection is largely dependent on the intrinsic contractile properties of cardiac myocytes. As the myosin heavy chain (MHC) isoform of cardiac muscle is an important determinant of the contractile properties of individual myocytes, we studied the effects of altered MHC isoform expression in rat myocardium on the mechanical properties of skinned ventricular preparations. Skinned myocardium from thyroidectomized rats expressing only the beta MHC isoform displayed rates of force redevelopment that were about 2.5-fold slower than in myocardium from hyperthyroid rats expressing only the alpha MHC isoform, but the amount of force generated at a given level of Ca2+ activation was not different. Because recent studies suggest that the stretch activation response in myocardium has an important role in systolic function, we also examined the effect of MHC isoform expression on the stretch activation response by applying a rapid stretch (1% of muscle length) to an otherwise isometrically contracting muscle fibre. Sudden stretch of myocardium resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force (i.e. stretch activation) to levels greater than prestretch force. beta MHC expression dramatically slowed the overall rate of the stretch activation response compared to expression of alpha MHC isoform; specifically, the rate of force decay was approximately 2-fold slower and the rate of delayed force development was approximately 2.5-fold slower. In contrast, MHC isoform had no effect on the amplitude of the stretch activation response. Collectively, these data show that expression of beta MHC in myocardium dramatically slows rates of cross-bridge recruitment and detachment which would be expected to decrease power output and contribute to depressed systolic function in end-stage heart failure.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin Medical School, 601 Science Drive, Madison, WI 53711, USA.
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28
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Reconditi M. RECENT IMPROVEMENTS IN SMALL ANGLE X-RAY DIFFRACTION FOR THE STUDY OF MUSCLE PHYSIOLOGY. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2006; 69:2709-2759. [PMID: 19946470 PMCID: PMC2783642 DOI: 10.1088/0034-4885/69/10/r01] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/09/2023]
Abstract
The molecular mechanism of muscle contraction is one of the most important unresolved problems in Biology and Biophysics. Notwithstanding the great advances of recent years, it is not yet known in detail how the molecular motor in muscle, the class II myosin, converts the free energy of ATP hydrolysis into work by interacting with its track, the actin filament, neither it is understood how the high efficiency in energy conversion depends on the cooperative action of myosin motors working in parallel along the actin filament. Researches in muscle contraction imply the combination of mechanical, biochemical and structural methods in studies that span from tissue to single molecule. Therefore, more than for any other research field, progresses in the comprehension of muscle contraction at molecular level are related to, and in turn contribute to, the advancement of methods in Biophysics.This review will focus on the progresses achieved by time resolved small angle X-ray scattering (SAXS) from muscle, an approach made possible by the highly ordered arrangement of both the contractile proteins myosin and actin in the ca 2 mum long structural unit the sarcomere that repeats along the whole length of the muscle cell. Among the time resolved structural techniques, SAXS has proved to be the most powerful method of investigation, as it allows the molecular motor to be studied in situ, in intact single muscle cells, where it is possible to combine the structural study with fast mechanical methods that synchronize the action of the molecular motors. The latest development of this technique allows Angstrom-scale measurements of the axial movement of the motors that pull the actin filament toward the centre of the sarcomere, by exploiting the X-ray interference between the two arrays of myosin motors in the two halves of the sarcomere.
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Affiliation(s)
- Massimo Reconditi
- Università di Firenze, Lab di Fisiologia - DBAG, c/o Dip. di Fisica, via Sansone 1, I-50019 Sesto Fiorentino, ITALY
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29
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Stelzer JE, Patel JR, Moss RL. Acceleration of stretch activation in murine myocardium due to phosphorylation of myosin regulatory light chain. ACTA ACUST UNITED AC 2006; 128:261-72. [PMID: 16908724 PMCID: PMC2151564 DOI: 10.1085/jgp.200609547] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The regulatory light chains (RLCs) of vertebrate muscle myosins bind to the neck region of the heavy chain domain and are thought to play important structural roles in force transmission between the cross-bridge head and thick filament backbone. In vertebrate striated muscles, the RLCs are reversibly phosphorylated by a specific myosin light chain kinase (MLCK), and while phosphorylation has been shown to accelerate the kinetics of force development in skeletal muscle, the effects of RLC phosphorylation in cardiac muscle are not well understood. Here, we assessed the effects of RLC phosphorylation on force, and the kinetics of force development in myocardium was isolated in the presence of 2,3-butanedione monoxime (BDM) to dephosphorylate RLC, subsequently skinned, and then treated with MLCK to phosphorylate RLC. Since RLC phosphorylation may be an important determinant of stretch activation in myocardium, we recorded the force responses of skinned myocardium to sudden stretches of 1% of muscle length both before and after treatment with MLCK. MLCK increased RLC phosphorylation, increased the Ca(2+) sensitivity of isometric force, reduced the steepness of the force-pCa relationship, and increased both Ca(2+)-activated and Ca(2+)-independent force. Sudden stretch of myocardium during an otherwise isometric contraction resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force, i.e., stretch activation, to levels greater than pre-stretch force. MLCK had profound effects on the stretch activation responses during maximal and submaximal activations: the amplitude and rate of force decay after stretch were significantly reduced, and the rate of delayed force recovery was accelerated and its amplitude reduced. These data show that RLC phosphorylation increases force and the rate of cross-bridge recruitment in murine myocardium, which would increase power generation in vivo and thereby enhance systolic function.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin School of Medicine and Public Health, Madison, WI 53706, USA.
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30
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Andruchov O, Andruchova O, Galler S. Fine-tuning of cross-bridge kinetics in cardiac muscle of rat and mouse by myosin light chain isoforms. Pflugers Arch 2006; 452:667-73. [PMID: 16614852 DOI: 10.1007/s00424-006-0080-7] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2006] [Accepted: 03/20/2006] [Indexed: 11/29/2022]
Abstract
Cross-bridge kinetics underlying stretch-induced force transients was studied in cardiac muscle strips with different myosin heavy chain (MHC) and myosin light chain (MLC) isoforms. The force transients were induced by stepwise stretches of maximally Ca(2+)-activated skinned muscle strips. The MHC and MLC isoforms were analyzed by electrophoreses after the mechanical experiments. Muscle strips of euthyroid rats and mice exclusively containing alpha-MHC were used. In addition, muscle strips of hyper- and hypothyroid rats containing different combinations of MHC and MLC isoforms were used. The thyroid hormone is known to alter the expression of MHC but not of MLC isoforms. In muscle strips containing exclusively alpha-MHC, atrial MLC isoforms (all atria of rats and mice) were associated with about 30% faster kinetics than ventricular MLC isoforms (ventricles of hyperthyroid rats and some muscle strips of ventricles of euthyroid rats and mice). On the other hand, in muscle strips containing exclusively ventricular MLC isoforms, alpha-MHC (ventricles of hyperthyroid rats) was associated with about 2.6 times faster kinetics than beta-MHC (ventricles of hypothyroid rats). We conclude that the MLC isoforms fine-tune cross-bridge kinetics, which underlies stretch-induced force transients, whereas the MHC isoforms mainly determine this kinetics. The effect of MLC isoforms on the cross-bridge kinetics may partially contribute to the faster twitch contraction in atria than in ventricles. Furthermore, it may play a role in various cardiomyopathies where atrial MLC isoforms are partially expressed in ventricles or ventricular MLC isoforms are partially expressed in atria.
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Affiliation(s)
- Oleg Andruchov
- Department of Cell Biology, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria
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Stelzer JE, Larsson L, Fitzsimons DP, Moss RL. Activation dependence of stretch activation in mouse skinned myocardium: implications for ventricular function. ACTA ACUST UNITED AC 2006; 127:95-107. [PMID: 16446502 PMCID: PMC2151492 DOI: 10.1085/jgp.200509432] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Recent evidence suggests that ventricular ejection is partly powered by a delayed development of force, i.e., stretch activation, in regions of the ventricular wall due to stretch resulting from torsional twist of the ventricle around the apex-to-base axis. Given the potential importance of stretch activation in cardiac function, we characterized the stretch activation response and its Ca2+ dependence in murine skinned myocardium at 22°C in solutions of varying Ca2+ concentrations. Stretch activation was induced by suddenly imposing a stretch of 0.5–2.5% of initial length to the isometrically contracting muscle and then holding the muscle at the new length. The force response to stretch was multiphasic: force initially increased in proportion to the amount of stretch, reached a peak, and then declined to a minimum before redeveloping to a new steady level. This last phase of the response is the delayed force characteristic of myocardial stretch activation and is presumably due to increased attachment of cross-bridges as a consequence of stretch. The amplitude and rate of stretch activation varied with Ca2+ concentration and more specifically with the level of isometric force prior to the stretch. Since myocardial force is regulated both by Ca2+ binding to troponin-C and cross-bridge binding to thin filaments, we explored the role of cross-bridge binding in the stretch activation response using NEM-S1, a strong-binding, non-force–generating derivative of myosin subfragment 1. NEM-S1 treatment at submaximal Ca2+-activated isometric forces significantly accelerated the rate of the stretch activation response and reduced its amplitude. These data show that the rate and amplitude of myocardial stretch activation vary with the level of activation and that stretch activation involves cooperative binding of cross-bridges to the thin filament. Such a mechanism would contribute to increased systolic ejection in response to increased delivery of activator Ca2+ during excitation–contraction coupling.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin Medical School, Madison, WI 53706, USA
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Stelzer JE, Dunning SB, Moss RL. Ablation of cardiac myosin-binding protein-C accelerates stretch activation in murine skinned myocardium. Circ Res 2006; 98:1212-8. [PMID: 16574907 DOI: 10.1161/01.res.0000219863.94390.ce] [Citation(s) in RCA: 95] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Cardiac myosin binding protein-C (cMyBP-C) is a thick filament accessory protein that binds tightly to myosin, but despite evidence that mutations in the cMyBP-C gene comprise a frequent cause of hypertrophic cardiomyopathy, relatively little is known about the role(s) of cMyBP-C in myocardium. Based on earlier studies demonstrating the potential importance of stretch activation in cardiac contraction, we examined the effects of cMyBP-C on the stretch activation responses of skinned ventricular preparations from wild-type (WT) and homozygous cMyBP-C knockout mice (cMyBP-C(-/-)) previously developed in our laboratory. Sudden stretch of skinned myocardium during maximal or submaximal Ca2+ activations resulted in an instantaneous increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force (ie, stretch activation) to levels greater than prestretch force. Ablation of cMyBP-C dramatically altered the stretch activation response, ie, the rates of force decay and delayed force transient were accelerated compared with WT myocardium. These results suggest that cMyBP-C normally constrains the spatial position of myosin cross-bridges, which, in turn, limits both the rate and extent of interaction of cross-bridges with actin. We propose that ablation of cMyBP-C removes this constraint, increases the likelihood of cross-bridge binding to actin, and speeds the rate of delayed force development following stretch. Regardless of the specific mechanism, acceleration of cross-bridge cycling in cMyBP-C(-/-) myocardium could account for the abbreviation of systolic ejection in this mouse as a direct consequence of premature stretch activation of ventricular myocardium.
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Affiliation(s)
- Julian E Stelzer
- Department of Physiology, University of Wisconsin School of Medicine, Madison, WI 53711, USA.
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33
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Burton K, Simmons RM, Sleep J, Smith DA. Kinetics of force recovery following length changes in active skinned single fibres from rabbit psoas muscle: analysis and modelling of the late recovery phase. J Physiol 2006; 573:305-28. [PMID: 16497718 PMCID: PMC1779726 DOI: 10.1113/jphysiol.2005.103382] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Redevelopment of isometric force following shortening of skeletal muscle is thought to result from a redistribution of cross-bridge states. We varied the initial force and cross-bridge distribution by applying various length-change protocols to active skinned single fibres from rabbit psoas muscle, and observed the effect on the slowest phase of recovery ('late recovery') that follows transient changes. In response to step releases that reduced force to near zero ( approximately 8 nm (half sarcomere)(-1)) or prolonged shortening at high velocity, late recovery was well described by two exponentials of approximately equal amplitude and rate constants of approximately 2 s(-1) and approximately 9 s(-1) at 5 degrees C. When a large restretch was applied at the end of rapid shortening, recovery was accelerated by (1) the introduction of a slow falling component that truncated the rise in force, and (2) a relative increase in the contribution of the fast exponential component. The rate of the slow fall was similar to that observed after a small isometric step stretch, with a rate of 0.4-0.8 s(-1), and its effects could be reversed by reducing force to near zero immediately after the stretch. Force at the start of late recovery was varied in a series of shortening steps or ramps in order to probe the effect of cross-bridge strain on force redevelopment. The rate constants of the two components fell by 40-50% as initial force was raised to 75-80% of steady isometric force. As initial force increased, the relative contribution of the fast component decreased, and this was associated with a length constant of about 2 nm. The results are consistent with a two-state strain-dependent cross-bridge model. In the model there is a continuous distribution of recovery rate constants, but two-exponential fits show that the fast component results from cross-bridges initially at moderate positive strain and the slow component from cross-bridges at high positive strain.
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Affiliation(s)
- Kevin Burton
- The Randall Division of Cell and Molecular Biophysics, Guy's Campus, King's College LondonLondon SE1 1UL, UK
| | - Robert M Simmons
- The Randall Division of Cell and Molecular Biophysics, Guy's Campus, King's College LondonLondon SE1 1UL, UK
| | - John Sleep
- The Randall Division of Cell and Molecular Biophysics, Guy's Campus, King's College LondonLondon SE1 1UL, UK
| | - David A Smith
- Department of Physiology, Monash UniversityVictoria 3800, Australia
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Andruchov O, Andruchova O, Wang Y, Galler S. Dependence of cross-bridge kinetics on myosin light chain isoforms in rabbit and rat skeletal muscle fibres. J Physiol 2005; 571:231-42. [PMID: 16357018 PMCID: PMC1805649 DOI: 10.1113/jphysiol.2005.099770] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Cross-bridge kinetics underlying stretch-induced force transients was studied in fibres with different myosin light chain (MLC) isoforms from skeletal muscles of rabbit and rat. The force transients were induced by stepwise stretches (< 0.3% of fibre length) applied on maximally Ca2+-activated skinned fibres. Fast fibre types IIB, IID (or IIX) and IIA and the slow fibre type I containing the myosin heavy chain isoforms MHC-IIb, MHC-IId (or MHC-IIx), MHC-IIa and MHC-I, respectively, were investigated. The MLC isoform content varied within fibre types. Fast fibre types contained the fast regulatory MLC isoform MLC2f and different proportions of the fast alkali MLC isoforms MLC1f and MLC3f. Type I fibres contained the slow regulatory MLC isoform MLC2s and the slow alkali MLC isoform MLC1s. Slow MLC isoforms were also present in several type IIA fibres. The kinetics of force transients differed by a factor of about 30 between fibre types (order from fastest to slowest kinetics: IIB > IID > IIA >> I). The kinetics of the force transients was not dependent on the relative content of MLC1f and MLC3f. Type IIA fibres containing fast and slow MLC isoforms were about 1.2 times slower than type IIA fibres containing only fast MLC isoforms. We conclude that while the cross-bridge kinetics is mainly determined by the MHC isoforms present, it is affected by fast and slow MLC isoforms but not by the relative content of MLC1f and MLC3f. Thus, the physiological role of fast and slow MLC isoforms in type IIA fibres is a fine-tuning of the cross-bridge kinetics.
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MESH Headings
- Analysis of Variance
- Animals
- Biomechanical Phenomena
- Blotting, Western
- Electrophoresis, Polyacrylamide Gel
- Muscle Contraction
- Muscle Fibers, Fast-Twitch/chemistry
- Muscle Fibers, Fast-Twitch/physiology
- Muscle Fibers, Skeletal/chemistry
- Muscle Fibers, Skeletal/physiology
- Muscle Fibers, Slow-Twitch/chemistry
- Muscle Fibers, Slow-Twitch/physiology
- Muscle, Skeletal/chemistry
- Muscle, Skeletal/physiology
- Myosin Heavy Chains/physiology
- Myosin Light Chains/chemistry
- Myosin Light Chains/physiology
- Protein Isoforms/analysis
- Protein Isoforms/chemistry
- Protein Isoforms/physiology
- Rabbits
- Rats
- Rats, Inbred F344
- Time Factors
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Affiliation(s)
- Oleg Andruchov
- Department of Cell Biology, University of Salzburg, Hellbrunnerstrasse 34, A-5020, Salzburg, Austria
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Bagni MA, Cecchi G, Colombini B. Crossbridge properties investigated by fast ramp stretching of activated frog muscle fibres. J Physiol 2005; 565:261-8. [PMID: 15774512 PMCID: PMC1464507 DOI: 10.1113/jphysiol.2005.085209] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Very fast ramp stretches at 9.5-33 sarcomere lengths s(-1) (l0 s(-1)) stretching speed, 16-25 nm per half-sarcomere (nm hs(-1)) amplitude were applied to activated intact frog muscle fibres at tetanus plateau, during the tetanus rise, during the isometric phase of relaxation and during isotonic shortening. Stretches produced an almost linear tension increase above the isometric level up to a peak, and fell to a lower value in spite of continued stretching, indicating that the fibre became suddenly very compliant. This suggests that peak tension (critical tension, P(c)) represents the tension at which crossbridges are forcibly detached by the stretch. The ratio of P(c) to the isometric tension at tetanus plateau (P0) was 2.37 +/- 0.12 (S.E.M.). This ratio did not change significantly at lower tension (P) during the tetanus rise but decreased with time during the relaxation and increased with speed during isotonic shortening. At tetanus plateau P(c) occurred when sarcomere elongation attained a critical length (L(c)) of 10.98 +/- 0.13 nm hs(-1), independently of the stretching speed. L(c) remained constant during the tetanus rise but decreased on the relaxation and increased during isotonic shortening. Length-clamp experiments on the relaxation showed that the lower values of P(c)/P ratio and L(c), were both due to the slow sarcomere stretching occurring during this phase. Our data show that P(c) can be used as a measure of crossbridge number, while L(c) is a measure of crossbridge mean extension. Accordingly, for a given tension, crossbridges on the isometric relaxation are fewer than during the rise, develop a greater individual force and have a greater mean extension, while during isotonic shortening crossbridges are in a greater number but develop a smaller individual force and have a smaller extension.
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Affiliation(s)
- M Angela Bagni
- Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, Italy
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36
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Smith DA, Sleep J. Mechanokinetics of rapid tension recovery in muscle: the Myosin working stroke is followed by a slower release of phosphate. Biophys J 2005; 87:442-56. [PMID: 15240478 PMCID: PMC1304365 DOI: 10.1529/biophysj.103.037788] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Crystallographic and biochemical evidence suggests that the myosin working stroke that generates force in muscle is accompanied by the release of inorganic phosphate (Pi), but the order and relative speed of these transitions is not firmly established. To address this problem, the theory of A. F. Huxley and R. M. Simmons for the length-step response is averaged over elastic strains imposed by filament structure and extended to include a Pi-release transition. Models of this kind are applied to existing tension-recovery data from length steps at different phosphate concentrations, and from phosphate jumps upon release of caged phosphate. This body of data is simulated by the model in which the force-generating event is followed by Pi release. A version in which the Pi-release transition is slow provides a better fit than a version with rapid Pi release and a slow transition preceding force generation. If Pi is released before force generation, the predicted rate of slow recovery increases with the size of the step, which is not observed. Some implications for theories of muscle contraction are discussed.
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Affiliation(s)
- David A Smith
- Department of Physiology, Monash University, Clayton, Victoria 3800, Australia.
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37
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Linari M, Reedy MK, Reedy MC, Lombardi V, Piazzesi G. Ca-activation and stretch-activation in insect flight muscle. Biophys J 2004; 87:1101-11. [PMID: 15298914 PMCID: PMC1304450 DOI: 10.1529/biophysj.103.037374] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/18/2003] [Accepted: 04/26/2004] [Indexed: 11/18/2022] Open
Abstract
Asynchronous insect flight muscle is specialized for myogenic oscillatory work, but can also produce isometric tetanic contraction. In skinned insect flight muscle fibers from Lethocerus, with sarcomere length monitored by a striation follower, we determined the relation between isometric force (F(0)) at serial increments of [Ca(2+)] and the additional active force recruited at each [Ca(2+)] by a stretch of approximately 12 nm per half-sarcomere (F(SA)). The isometric force-pCa relation shows that 1.5-2 units of pCa are necessary to raise isometric force from its threshold (pCa approximately 6.5) to its maximum (F(0,max)). The amplitude of F(SA) depends only on the preceding baseline level of isometric force, which must reach at least 0.05 F(0,max) to enable stretch-activation. F(SA) rises very steeply to its maximum as F(0) reaches approximately 0.2 F(0,max), then decreases as F(0) increases so as to produce a constant sum (F(0) + F(SA)) = F(max). Thus Ca- and stretch-activation are complementary pathways that trigger a common process of cross-bridge attachment and force production. We suggest that stretch-induced distortion of attached cross-bridges relieves the steric blocking by tropomyosin of additional binding sites on actin, thereby enabling maximum force even at low [Ca(2+)].
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MESH Headings
- Animals
- Calcium/pharmacology
- Cells, Cultured
- Dose-Response Relationship, Drug
- Flight, Animal/physiology
- Heteroptera/drug effects
- Heteroptera/physiology
- Isometric Contraction/drug effects
- Isometric Contraction/physiology
- Magnesium/pharmacology
- Mechanotransduction, Cellular/drug effects
- Mechanotransduction, Cellular/physiology
- Muscle Fibers, Skeletal/drug effects
- Muscle Fibers, Skeletal/physiology
- Muscle, Skeletal/drug effects
- Muscle, Skeletal/physiology
- Physical Stimulation/methods
- Stress, Mechanical
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Affiliation(s)
- Marco Linari
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze and Istituto Nazionale di Fisica della Materia, Firenze, Italy; and Duke University Medical Center, Department of Cellular Biology, Durham, North Carolina
| | - Michael K. Reedy
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze and Istituto Nazionale di Fisica della Materia, Firenze, Italy; and Duke University Medical Center, Department of Cellular Biology, Durham, North Carolina
| | - Mary C. Reedy
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze and Istituto Nazionale di Fisica della Materia, Firenze, Italy; and Duke University Medical Center, Department of Cellular Biology, Durham, North Carolina
| | - Vincenzo Lombardi
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze and Istituto Nazionale di Fisica della Materia, Firenze, Italy; and Duke University Medical Center, Department of Cellular Biology, Durham, North Carolina
| | - Gabriella Piazzesi
- Laboratorio di Fisiologia, Dipartimento di Biologia Animale e Genetica, Università degli Studi di Firenze and Istituto Nazionale di Fisica della Materia, Firenze, Italy; and Duke University Medical Center, Department of Cellular Biology, Durham, North Carolina
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38
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Callister RJ, Reinking RM, Stuart DG. Effects of fatigue on the catchlike property in a turtle hindlimb muscle. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2003; 189:857-66. [PMID: 14566421 DOI: 10.1007/s00359-003-0459-2] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/04/2003] [Revised: 08/06/2003] [Accepted: 09/02/2003] [Indexed: 10/26/2022]
Abstract
The purpose of this report was to test for the possibility that a catchlike, force-enhancing property, attributable to a particular stimulation pattern, could be evoked in non-mammalian turtle muscle, just as it has been shown in mammalian muscle. We tested for the presence of this property in dynamic lengthening and shortening contractions, as well as in the more commonly studied isometric contractions. A second aim was to note the effects of fatigue on the catchlike property, if the latter was present. The force response of the external gastrocnemius muscle in the adult turtle, Pseudemys ( Trachemys) scripta elegans, was compared for a control, constant-frequency 10 Hz, 1-s duration stimulation pattern using 0.1-ms pulses vs. the same pattern, but with two additional pulses within the first 100-ms interspike interval of the control stimulus train. This latter train produced a pronounced and prolonged enhancement of muscle force, which was attributed to a catchlike effect. It was greatly increased when the muscle was in a fatigued state. The extent of this force enhancement was significantly different for the three contraction types, and generally in the order: isometric>lengthening>shortening contraction. These differences were greater in fatigued vs. fresh muscle. Comparative aspects and potential mechanisms underlying the catchlike effect are discussed.
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Affiliation(s)
- R J Callister
- Discipline of Anatomy, School of Biomedical Sciences, Faculty of Health, University of Newcastle, Callaghan, NSW 2308, Australia
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39
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Piazzesi G, Reconditi M, Koubassova N, Decostre V, Linari M, Lucii L, Lombardi V. Temperature dependence of the force-generating process in single fibres from frog skeletal muscle. J Physiol 2003; 549:93-106. [PMID: 12665607 PMCID: PMC2342933 DOI: 10.1113/jphysiol.2002.038703] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Generation of force and shortening in striated muscle is due to the cyclic interactions of the globular portion (the head) of the myosin molecule, extending from the thick filament, with the actin filament. The work produced in each interaction is due to a conformational change (the working stroke) driven by the hydrolysis of ATP on the catalytic site of the myosin head. However, the precise mechanism and the size of the force and length step generated in one interaction are still under question. Here we reinvestigate the endothermic nature of the force-generating process by precisely determining, in tetanized intact frog muscle fibres under sarcomere length control, the effect of temperature on both isometric force and force response to length changes. We show that raising the temperature: (1) increases the force and the strain of the myosin heads attached in the isometric contraction by the same amount (approximately 70 %, from 2 to 17 degrees C); (2) increases the rate of quick force recovery following small length steps (range between -3 and 2 nm (half-sarcomere)-1) with a Q10 (between 2 and 12 degrees C) of 1.9 (releases) and 2.3 (stretches); (3) does not affect the maximum extent of filament sliding accounted for by the working stroke in the attached heads (10 nm (half-sarcomere)-1). These results indicate that in isometric conditions the structural change leading to force generation in the attached myosin heads can be modulated by temperature at the expense of the structural change responsible for the working stroke that drives filament sliding. The energy stored in the elasticity of the attached myosin heads at the plateau of the isometric tetanus increases with temperature, but even at high temperature this energy is only a fraction of the mechanical energy released by attached heads during filament sliding.
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Affiliation(s)
- G Piazzesi
- Dipartimento di Scienze Fisiologiche, Viale G. B. Morgagni, 63, I-50134 Firenze, Italy
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40
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Stehle R, Krüger M, Pfitzer G. Force kinetics and individual sarcomere dynamics in cardiac myofibrils after rapid ca(2+) changes. Biophys J 2002; 83:2152-61. [PMID: 12324432 PMCID: PMC1302303 DOI: 10.1016/s0006-3495(02)73975-1] [Citation(s) in RCA: 92] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Kinetics of force development and relaxation after rapid application and removal of Ca(2+) were measured by atomic force cantilevers on subcellular bundles of myofibrils prepared from guinea pig left ventricles. Changes in the structure of individual sarcomeres were simultaneously recorded by video microscopy. Upon Ca(2+) application, force developed with an exponential rate constant k(ACT) almost identical to k(TR), the rate constant of force redevelopment measured during steady-state Ca(2+) activation; this indicates that k(ACT) reflects isometric cross-bridge turnover kinetics. The kinetics of force relaxation after sudden Ca(2+) removal were markedly biphasic. An initial slow linear decline (rate constant k(LIN)) lasting for a time t(LIN) was abruptly followed by an ~20 times faster exponential decay (rate constant k(REL)). k(LIN) is similar to k(TR) measured at low activating [Ca(2+)], indicating that k(LIN) reflects isometric cross-bridge turnover kinetics under relaxed-like conditions (see also. Biophys. J. 83:2142-2151). Video microscopy revealed the following: invariably at t(LIN) a single sarcomere suddenly lengthened and returned to a relaxed-type structure. Originating from this sarcomere, structural relaxation propagated from one sarcomere to the next. Propagated sarcomeric relaxation, along with effects of stretch and P(i) on relaxation kinetics, supports an intersarcomeric chemomechanical coupling mechanism for rapid striated muscle relaxation in which cross-bridges conserve chemical energy by strain-induced rebinding of P(i).
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Affiliation(s)
- R Stehle
- Institute of Physiology, University Cologne, D-50931 Köln, Germany.
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41
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Bagni MA, Cecchi G, Colombini B, Colomo F. A non-cross-bridge stiffness in activated frog muscle fibers. Biophys J 2002; 82:3118-27. [PMID: 12023235 PMCID: PMC1302100 DOI: 10.1016/s0006-3495(02)75653-1] [Citation(s) in RCA: 101] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Force responses to fast ramp stretches of various amplitude and velocity, applied during tetanic contractions, were measured in single intact fibers from frog tibialis anterior muscle. Experiments were performed at 14 degrees C at approximately 2.1 microm sarcomere length on fibers bathed in Ringer's solution containing various concentrations of 2,3-butanedione monoxime (BDM) to greatly reduce the isometric tension. The fast tension transient produced by the stretch was followed by a period, lasting until relaxation, during which the tension remained constant to a value that greatly exceeded the isometric tension. The excess of tension was termed "static tension," and the ratio between the force and the accompanying sarcomere length change was termed "static stiffness." The static stiffness was independent of the active tension developed by the fiber, and independent of stretch amplitude and stretching velocity in the whole range tested; it increased with sarcomere length in the range 2.1-2.8 microm, to decrease again at longer lengths. Static stiffness increased well ahead of tension during the tetanus rise, and fell ahead of tension during relaxation. These results suggest that activation increased the stiffness of some sarcomeric structure(s) outside the cross-bridges.
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Affiliation(s)
- Maria A Bagni
- Dipartimento di Scienze Fisiologiche, Università degli Studi di Firenze, I-50134 Firenze, Italy.
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42
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Hopkins SC, Sabido-David C, van der Heide UA, Ferguson RE, Brandmeier BD, Dale RE, Kendrick-Jones J, Corrie JET, Trentham DR, Irving M, Goldman YE. Orientation changes of the myosin light chain domain during filament sliding in active and rigor muscle. J Mol Biol 2002; 318:1275-91. [PMID: 12083517 DOI: 10.1016/s0022-2836(02)00189-4] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
Structural changes in myosin power many types of cell motility including muscle contraction. Tilting of the myosin light chain domain (LCD) seems to be the final step in transducing the energy of ATP hydrolysis, amplifying small structural changes near the ATP binding site into nanometer-scale motions of the filaments. Here we used polarized fluorescence measurements from bifunctional rhodamine probes attached at known orientations in the LCD to describe the distribution of orientations of the LCD in active contraction and rigor. We applied rapid length steps to perturb the orientations of the population of myosin heads that are attached to actin, and thereby characterized the motions of these force-bearing myosin heads. During active contraction, this population is a small fraction of the total. When the filaments slide in the shortening direction in active contraction, the long axis of LCD tilts towards its nucleotide-free orientation with no significant twisting around this axis. In contrast, filament sliding in rigor produces coordinated tilting and twisting motions.
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Affiliation(s)
- Seth C Hopkins
- Pennsylvania Muscle Institute, University of Pennsylvania, Philadelphia 19104-6083, USA
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Mantovani M, Heglund NC, Cavagna GA. Energy transfer during stress relaxation of contracting frog muscle fibres. J Physiol 2001; 537:923-39. [PMID: 11744765 PMCID: PMC2279004 DOI: 10.1111/j.1469-7793.2001.00923.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022] Open
Abstract
1. A contracting muscle resists stretching with a force greater than the force it can exert at a constant length, T(o). If the muscle is kept active at the stretched length, the excess tension disappears, at first rapidly and then more slowly (stress relaxation). The present study is concerned with the first, fast tension decay. In particular, it is still debated if and to what extent the fast tension decay after a ramp stretch involves a conservation of the elastic energy stored during stretching into cross-bridge states of higher chemical energy. 2. Single muscle fibres of Rana temporaria and Rana esculenta were subjected to a short ramp stretch (approximately 15 nm per half-sarcomere at either 1.4 or 0.04 sarcomere lengths s(-1)) on the plateau of the force-length relation at temperatures of 4 and 14 degrees C. Immediately after the end of the stretch, or after discrete time intervals of fixed-end contraction and stress relaxation at the stretched length (Delta t(isom) = 0.5-300 ms), the fibre was released against a force ~T(o). Fibre and sarcomere stiffness during the elastic recoil to T(o) (phase 1) and the subsequent transient shortening against T(o) (phase 2), which is expression of the work enhancement by stretch, were measured after different Delta t(isom) and compared with the corresponding fast tension decay during Delta t(isom). 3. The amplitude of fast tension decay is large after the fast stretch, and small or nil after the slow stretch. Two exponential terms are necessary to fit the fast tension decay after the fast stretch at 4 degrees C, whereas one is sufficient in the other cases. The rate constant of the dominant exponential term (0.1-0.2 ms(-1) at 4 degrees C) increases with temperature with a temperature coefficient (Q(10)) of approximately 3. 4. After fast stretch, the fast tension decay during Delta t(isom) is accompanied in both species and at both temperatures by a corresponding increase in the amplitude of phase 2 shortening against T(o) after Delta t(isom): a maximum of approximately 5 nm per half-sarcomere is attained when the fast tension decay is almost complete, i.e. 30 ms after the stretch at 4 degrees C and 10 ms after the stretch at 14 degrees C. After slow stretch, when fast tension decay is small or nil, the increase in phase 2 shortening is negligible. 5. The increase in phase 2 work during fast tension decay (Delta W(out)) is a constant fraction of the elastic energy simultaneously set free by the recoil of the undamped elastic elements. 6. Delta W(out) is accompanied by a decrease in stiffness, indicating that it is not due to a greater number of cross-bridges. 7. It is concluded that, during the fast tension decay following a fast ramp stretch, a transfer of energy occurs from the undamped elastic elements to damped elements within the sarcomeres by a temperature-dependent mechanism with a dominant rate constant consistent with the theory proposed by A. F. Huxley and R. M. Simmons in 1971.
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Affiliation(s)
- M Mantovani
- Istituto di Fisiologia Umana, Università degli Studi di Milano, 20133 Milan, Italy
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Linari M, Lucii L, Reconditi M, Casoni ME, Amenitsch H, Bernstorff S, Piazzesi G, Lombardi V. A combined mechanical and X-ray diffraction study of stretch potentiation in single frog muscle fibres. J Physiol 2000; 526 Pt 3:589-96. [PMID: 10922010 PMCID: PMC2270031 DOI: 10.1111/j.1469-7793.2000.00589.x] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
The nature of the force (T) response during and after steady lengthening has been investigated in tetanized single muscle fibres from Rana temporaria (4 C; 2.15 micrometer sarcomere length) by determining both the intensity of the third order myosin meridional X-ray reflection (IM3) and the stiffness (e) of a selected population of sarcomeres within the fibre. With respect to the value at the isometric tetanus plateau (To), IM3 was depressed to 0.67 +/- 0.04 during steady lengthening at approximately 160 nm s(-1) (T approximately 1.7) and recovered to 0.86 +/- 0.05 during the 250 ms period of after-stretch potentiation following the rapid decay of force at the end of lengthening (T approximately 1.3); under the same conditions stiffness increased to 1.25 +/- 0.02 and to 1.12 +/- 0.03, respectively. After subtraction of the contribution of myofilaments to the half-sarcomere compliance, stiffness measurements indicated that (1) during lengthening the cross-bridge number rises to 1.8 times the original isometric value and the average degree of cross-bridge strain is similar to that induced by the force-generating process in isometric conditions (2.3 nm), and (2) after-stretch potentiation is explained by a residual larger cross-bridge number. Structural data are compatible with mechanical data if the axial dispersion of attached heads is doubled during steady lengthening and recovers half-way towards the original isometric value during after-stretch potentiation.
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Affiliation(s)
- M Linari
- Università di Firenze, 50134 Firenze, Italy, Austrian Academy of Sciences, 8010 Graz, Austria and Sincrotrone Trieste, 34012 Basovizza, Trieste, Italy
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Irving M, Piazzesi G, Lucii L, Sun YB, Harford JJ, Dobbie IM, Ferenczi MA, Reconditi M, Lombardi V. Conformation of the myosin motor during force generation in skeletal muscle. NATURE STRUCTURAL BIOLOGY 2000; 7:482-5. [PMID: 10881196 PMCID: PMC8397617 DOI: 10.1038/75890] [Citation(s) in RCA: 91] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Myosin motors drive muscle contraction, cytokinesis and cell locomotion, and members of the myosin superfamily have been implicated in an increasingly diverse range of cell functions. Myosin can displace a bound actin filament several nanometers in a single interaction. Crystallographic studies suggest that this 'working stroke' involves bending of the myosin head between its light chain and catalytic domains. Here we used X-ray fiber diffraction to test the crystallographic model and measure the interdomain bending during force generation in an intact single muscle fiber. The observed bending has two components: an elastic distortion and an active rotation that generates force. The average bend of the force-generating myosin heads in a muscle fiber is intermediate between those in crystal structures with different bound nucleotides, and the C-terminus of the head is displaced by 7 nm along the actin filament axis compared with the in vitro conformation seen in the absence of nucleotide.
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Affiliation(s)
- M Irving
- School of Biomedical Sciences, King's College London, London, UK.
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Piazzesi G, Koubassova N, Irving M, Lombardi V. On the working stroke elicited by steps in length and temperature. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 1999; 453:259-63; discussion 263-4. [PMID: 9889837 DOI: 10.1007/978-1-4684-6039-1_30] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/09/2023]
Abstract
Muscle contraction is generally thought to involve tilting of the light chain region of the myosin head. This could account for 5-10 nm of axial displacement as it moves from nearly perpendicular to the filament axis (the state at the beginning of the working stroke) to the rigor conformation (at the end of the working stroke). According to the kinetic model of Huxley and Simmons, the extent that a cross-bridge progresses through the working stroke depends on the mechanical conditions. A large tilting occurs only when the fibre is allowed to shorten. Evidence for such tilting was provided by the changes in intensity of the third myosin meridional reflection (M3) following a step release. However, there is little change in the M3 intensity when a force increase is elicited by a 10 degrees C temperature jump, and these results were interpreted to indicate that tilting is not the structural transition responsible for force generation. Here we present a simulation of the changes in the intensity of the M3 reflection elicited by step changes in either length or temperature, based on the atomic model of the actin-myosin head complex. The results show that the same set of assumptions for the motions associated with the working stroke can predict the response to both kinds of perturbation. The main difference is due to the larger extent of the working stroke elicited by the length step.
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Affiliation(s)
- G Piazzesi
- Università degli Studi di Firenze, Italy
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Dobbie I, Linari M, Piazzesi G, Reconditi M, Koubassova N, Ferenczi MA, Lombardi V, Irving M. Elastic bending and active tilting of myosin heads during muscle contraction. Nature 1998; 396:383-7. [PMID: 9845077 DOI: 10.1038/24647] [Citation(s) in RCA: 138] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
Muscle contraction is driven by a change in shape of the myosin head region that links the actin and myosin filaments. Tilting of the light-chain domain of the head with respect to its actin-bound catalytic domain is thought to be coupled to the ATPase cycle. Here, using X-ray diffraction and mechanical data from isolated muscle fibres, we characterize an elastic bending of the heads that is independent of the presence of ATP. Together, the tilting and bending motions can explain force generation in isometric muscle, when filament sliding is prevented. The elastic strain in the head is 2.0-2.7 nm under these conditions, contributing 40-50% of the compliance of the muscle sarcomere. We present an atomic model for changes in head conformation that accurately reproduces the changes in the X-ray diffraction pattern seen when rapid length changes are applied to muscle fibres both in active contraction and in the absence of ATP. The model predictions are relatively independent of which parts of the head are assumed to bend or tilt, but depend critically on the measured values of filament sliding and elastic strain.
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Affiliation(s)
- I Dobbie
- Randall Institute, King's College London, UK
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Abstract
Patients with severe heart failure are at high risk of sudden cardiac death. In the majority of these patients, sudden cardiac death is thought to be due to ventricular tachyarrhythmias. Alterations of the electric properties of single myocytes in heart failure may favor the occurrence of ventricular arrhythmias in these patients by inducing early or delayed afterdepolarizations. Mathematical models of the cellular action potential and its underlying ionic currents could help to elucidate possible arrhythmogenic mechanisms on a cellular level. In the present study, selected ionic currents based on human data are incorporated into a model of the ventricular action potential for the purpose of studying the cellular electrophysiological consequences of heart failure. Ionic currents that are not yet sufficiently characterized in human ventricular myocytes are adopted from the action potential model developed by Luo and Rudy (LR model). The main results obtained from this model are as follows: The action potential in ventricular myocytes from failing hearts is longer than in nonfailing control hearts. The major underlying mechanisms for this prolongation are the enhanced activity of the Na+-Ca2+ exchanger, the slowed diastolic decay of the [Ca2+]i transient, and the reduction of the inwardly rectifying K+ current and the Na+-K+ pump current in myocytes of failing hearts. Furthermore, the fast and slow components of the delayed rectifier K+ current (I(Kr) and I(Ks), respectively) are of utmost importance in determining repolarization of the human ventricular action potential. In contrast, the influence of the transient outward K+ current on APD is only small in both cell groups. Inhibition of I(Kr) promotes the development of early afterdepolarizations in failing, but not nonfailing, myocytes. Furthermore, spontaneous Ca2+ release from the sarcoplasmic reticulum triggers a premature action potential only in failing myocytes. This model of the ventricular action potential and its alterations in heart failure is intended to serve as a tool for investigating the effects of therapeutic interventions on the electric excitability of the human ventricular myocardium.
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Affiliation(s)
- L Priebe
- Department of Medicine III, University of Cologne, Germany
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Linari M, Dobbie I, Reconditi M, Koubassova N, Irving M, Piazzesi G, Lombardi V. The stiffness of skeletal muscle in isometric contraction and rigor: the fraction of myosin heads bound to actin. Biophys J 1998; 74:2459-73. [PMID: 9591672 PMCID: PMC1299588 DOI: 10.1016/s0006-3495(98)77954-8] [Citation(s) in RCA: 156] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
Step changes in length (between -3 and +5 nm per half-sarcomere) were imposed on isolated muscle fibers at the plateau of an isometric tetanus (tension T0) and on the same fibers in rigor after permeabilization of the sarcolemma, to determine stiffness of the half-sarcomere in the two conditions. To identify the contribution of actin filaments to the total half-sarcomere compliance (C), measurements were made at sarcomere lengths between 2.00 and 2.15 microm, where the number of myosin cross-bridges in the region of overlap between the myosin filament and the actin filament remains constant, and only the length of the nonoverlapped region of the actin filament changes with sarcomere length. At 2.1 microm sarcomere length, C was 3.9 nm T0(-1) in active isometric contraction and 2.6 nm T0(-1) in rigor. The actin filament compliance, estimated from the slope of the relation between C and sarcomere length, was 2.3 nm microm(-1) T0(-1). Recent x-ray diffraction experiments suggest that the myosin filament compliance is 1.3 nm microm(-1) T0(-1). With these values for filament compliance, the difference in half-sarcomere compliance between isometric contraction and rigor indicates that the fraction of myosin cross-bridges attached to actin in isometric contraction is not larger than 0.43, assuming that cross-bridge elasticity is the same in isometric contraction and rigor.
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Affiliation(s)
- M Linari
- Department of Physiological Sciences, University of Florence, Italy
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Daniel TL, Trimble AC, Chase PB. Compliant realignment of binding sites in muscle: transient behavior and mechanical tuning. Biophys J 1998; 74:1611-21. [PMID: 9545027 PMCID: PMC1299509 DOI: 10.1016/s0006-3495(98)77875-0] [Citation(s) in RCA: 123] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
The presence of compliance in the lattice of filaments in muscle raises a number of concerns about how one accounts for force generation in the context of the cross-bridge cycle--binding site motions and coupling between cross-bridges confound more traditional analyses. To explore these issues, we developed a spatially explicit, mechanochemical model of skeletal muscle contraction. With a simple three-state model of the cross-bridge cycle, we used a Monte Carlo simulation to compute the instantaneous balance of forces throughout the filament lattice, accounting for both thin and thick filament distortions in response to cross-bridge forces. This approach is compared to more traditional mass action kinetic models (in the form of coupled partial differential equations) that assume filament inextensibility. We also monitored instantaneous force generation, ATP utilization, and the dynamics of the cross-bridge cycle in simulations of step changes in length and variations in shortening velocity. Three critical results emerge from our analyses: 1) there is a significant realignment of actin-binding sites in response to cross-bridge forces, 2) this realignment recruits additional cross-bridge binding, and 3) we predict mechanical behaviors that are consistent with experimental results for velocity and length transients. Binding site realignment depends on the relative compliance of the filament lattice and cross-bridges, and within the measured range of these parameters, gives rise to a sharply tuned peak for force generation. Such mechanical tuning at the molecular level is the result of mechanical coupling between individual cross-bridges, mediated by thick filament deformations, and the resultant realignment of binding sites on the thin filament.
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Affiliation(s)
- T L Daniel
- Department of Zoology, University of Washington, Seattle 98195-1800, USA.
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