Tension-dependent changes of the intracellular Ca2+ transients in ferret ventricular muscles

Regulation of myocardial contraction as revealed by intracellular Ca2+ measurements using aequorin

Of the ions involved in myocardial function, Ca2+ is the most important. Ca2+ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation-contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca2+ concentration is regulated to elicit excitation-contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca2+ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca2+ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation-contraction coupling.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Work-loop contractions reveal that the afterload-dependent time course of cardiac Ca2+ transients is modulated by preload

Preload and afterload dictate the dynamics of the cyclical work-loop contraction that the heart undergoes in vivo. Cellular Ca²⁺ dynamics drive contraction, but the effects of afterload alone on the Ca²⁺ transient are inconclusive. To our knowledge, no study has investigated whether the putative afterload dependence of the Ca²⁺ transient is preload dependent. This study is designed to provide the first insight into the Ca²⁺ handling of cardiac trabeculae undergoing work-loop contractions, with the aim to examine whether the conflicting afterload dependency of the Ca²⁺ transient can be accounted for by considering preload under isometric and physiological work-loop contractions. Thus, we subjected ex vivo rat right-ventricular trabeculae, loaded with the fluorescent dye Fura-2, to work-loop contractions over a wide range of afterloads at two preloads while measuring stress, length changes, and Ca²⁺ transients. Work-loop control was implemented with a real-time Windkessel model to mimic the contraction patterns of the heart in vivo. We extracted a range of metrics from the measured steady-state twitch stress and Ca²⁺ transients, including the amplitudes, time courses, rates of rise, and integrals. Results show that parameters of stress were afterload and preload dependent. In contrast, the parameters associated with Ca²⁺ transients displayed a mixed dependence on afterload and preload. Most notably, its time course was afterload dependent, an effect augmented at the greater preload. This study reveals that the afterload dependence of cardiac Ca²⁺ transients is modulated by preload, which brings the study of Ca²⁺ transients during isometric contractions into question when aiming to understand physiological Ca²⁺ handling.NEW & NOTEWORTHY This study is the first examination of Ca²⁺ handling in trabeculae undergoing work-loop contractions. These data reveal that reducing preload diminishes the influence of afterload on the decay phase of the cardiac Ca²⁺ transient. This is significant as it reconciles inconsistencies in the literature regarding the influence of external loads on cardiac Ca²⁺ handling. Furthermore, these findings highlight discrepancies between Ca²⁺ handling during isometric and work-loop contractions in cardiac trabeculae operating at their optimal length.

Mechanotranduction Pathways in the Regulation of Mitochondrial Homeostasis in Cardiomyocytes

Mitochondria are one of the most important organelles in cardiomyocytes. Mitochondrial homeostasis is necessary for the maintenance of normal heart function. Mitochondria perform four major biological processes in cardiomyocytes: mitochondrial dynamics, metabolic regulation, Ca2+ handling, and redox generation. Additionally, the cardiovascular system is quite sensitive in responding to changes in mechanical stress from internal and external environments. Several mechanotransduction pathways are involved in regulating the physiological and pathophysiological status of cardiomyocytes. Typically, the extracellular matrix generates a stress-loading gradient, which can be sensed by sensors located in cellular membranes, including biophysical and biochemical sensors. In subsequent stages, stress stimulation would regulate the transcription of mitochondrial related genes through intracellular transduction pathways. Emerging evidence reveals that mechanotransduction pathways have greatly impacted the regulation of mitochondrial homeostasis. Excessive mechanical stress loading contributes to impairing mitochondrial function, leading to cardiac disorder. Therefore, the concept of restoring mitochondrial function by shutting down the excessive mechanotransduction pathways is a promising therapeutic strategy for cardiovascular diseases. Recently, viral and non-viral protocols have shown potentials in application of gene therapy. This review examines the biological process of mechanotransduction pathways in regulating mitochondrial function in response to mechanical stress during the development of cardiomyopathy and heart failure. We also summarize gene therapy delivery protocols to explore treatments based on mechanical stress-induced mitochondrial dysfunction, to provide new integrative insights into cardiovascular diseases.

Differentiating the effects of β-adrenergic stimulation and stretch on calcium and force dynamics using a novel electromechanical cardiomyocyte model

Cardiac electrophysiology and mechanics are strongly interconnected. Calcium is crucial in this complex interplay through its role in cellular electrophysiology and sarcomere contraction. We aim to differentiate the effects of acute β-adrenergic stimulation (β-ARS) and cardiomyocyte stretch (increased sarcomere length) on calcium-transient dynamics and force generation, using a novel computational model of cardiac electromechanics. We implemented a bidirectional coupling between the O'Hara-Rudy model of human ventricular electrophysiology and the MechChem model of sarcomere mechanics through the buffering of calcium by troponin. The coupled model was validated using experimental data from large mammals or human samples. Calcium transient and force were simulated for various degrees of β-ARS and initial sarcomere lengths. The model reproduced force-frequency, quick-release, and isotonic contraction experiments, validating the bidirectional electromechanical interactions. An increase in β-ARS increased the amplitudes of force (augmented inotropy) and calcium transient, and shortened both force and calcium-transient duration (lusitropy). An increase in sarcomere length increased force amplitude even more, but decreased calcium-transient amplitude and increased both force and calcium-transient duration. Finally, a gradient in relaxation along the thin filament may explain the nonmonotonic decay in cytosolic calcium observed with high tension. Using a novel coupled human electromechanical model, we identified differential effects of β-ARS and stretch on calcium and force. Stretch mostly contributed to increased force amplitude and β-ARS to the reduction of calcium and force duration. We showed that their combination, rather than individual contributions, is key to ensure force generation, rapid relaxation, and low diastolic calcium levels.NEW & NOTEWORTHY This work identifies the contribution of electrical and mechanical alterations to regulation of calcium and force under exercise-like conditions using a novel human electromechanical model integrating ventricular electrophysiology and sarcomere mechanics. By better understanding their individual and combined effects, this can uncover arrhythmogenic mechanisms in exercise-like situations. This publicly available model is a crucial step toward understanding the complex interplay between cardiac electrophysiology and mechanics to improve arrhythmia risk prediction and treatment.

Insights From Computational Modeling Into the Contribution of Mechano-Calcium Feedback on the Cardiac End-Systolic Force-Length Relationship

In experimental studies on cardiac tissue, the end-systolic force-length relation (ESFLR) has been shown to depend on the mode of contraction: isometric or isotonic. The isometric ESFLR is derived from isometric contractions spanning a range of muscle lengths while the isotonic ESFLR is derived from shortening contractions across a range of afterloads. The ESFLR of isotonic contractions consistently lies below its isometric counterpart. Despite the passing of over a hundred years since the first insight by Otto Frank, the mechanism(s) underlying this protocol-dependent difference in the ESFLR remain incompletely explained. Here, we investigate the role of mechano-calcium feedback in accounting for the difference between these two ESFLRs. Previous studies have compared the dynamics of isotonic contractions to those of a single isometric contraction at a length that produces maximum force, without considering isometric contractions at shorter muscle lengths. We used a mathematical model of cardiac excitation-contraction to simulate isometric and force-length work-loop contractions (the latter being the 1D equivalent of the whole-heart pressure-volume loop), and compared Ca²⁺ transients produced under equivalent force conditions. We found that the duration of the simulated Ca²⁺ transient increases with decreasing sarcomere length for isometric contractions, and increases with decreasing afterload for work-loop contractions. At any given force, the Ca²⁺ transient for an isometric contraction is wider than that during a work-loop contraction. By driving simulated work-loops with wider Ca²⁺ transients generated from isometric contractions, we show that the duration of muscle shortening was prolonged, thereby shifting the work-loop ESFLR toward the isometric ESFLR. These observations are explained by an increase in the rate of binding of Ca²⁺ to troponin-C with increasing force. However, the leftward shift of the work-loop ESFLR does not superimpose on the isometric ESFLR, leading us to conclude that while mechano-calcium feedback does indeed contribute to the difference between the two ESFLRs, it does not completely account for it.

Mechano-calcium and mechano-electric feedbacks in the human cardiomyocyte analyzed in a mathematical model

Experiments on animal hearts (rat, rabbit, guinea pig, etc.) have demonstrated that mechano-calcium feedback (MCF) and mechano-electric feedback (MEF) are very important for myocardial self-regulation because they adjust the cardiomyocyte contractile function to various mechanical loads and to mechanical interactions between heterogeneous myocardial segments in the ventricle walls. In in vitro experiments on these animals, MCF and MEF manifested themselves in several basic classical phenomena (e.g., load dependence, length dependence of isometric twitches, etc.), and in the respective responses of calcium transients and action potentials. However, it is extremely difficult to study simultaneously the electrical, calcium, and mechanical activities of the human heart muscle in vitro. Mathematical modeling is a useful tool for exploring these phenomena. We have developed a novel model to describe electromechanical coupling and mechano-electric feedbacks in the human cardiomyocyte. It combines the ‘ten Tusscher–Panfilov’ electrophysiological model of the human cardiomyocyte with our module of myocardium mechanical activity taken from the ‘Ekaterinburg–Oxford’ model and adjusted to human data. Using it, we simulated isometric and afterloaded twitches and effects of MCF and MEF on excitation–contraction coupling. MCF and MEF were found to affect significantly the duration of the calcium transient and action potential in the human cardiomyocyte model in response to both smaller afterloads as compared to bigger ones and various mechanical interventions applied during isometric and afterloaded twitches.

Functional Requirements of a Mathematical Model of Muscle Contraction

Muscle modeling has a long history. Models typically belong to the class of lumped models built around Hill's contractile element, or to crossbridge models at the ultrastructure level. Lumped models built on the contractile element are not sufficiently dynamic, since the muscle's contractile properties are set a priori by the force-velocity relation. We have shown that the force-velocity relation is not a unique descriptor of muscle's contractile state [1]. Alternatively, description of muscle as a time and length dependent force generator may be used to model muscle dynamics. This paper defines the functional requirements of a mathematical model of muscle contraction and evaluates how closely a lumped model based on ultrastructural dynamics resembles muscle. This single model was found able to describe isometric and isotonic contractions, the force-length relation, the force-velocity relation, and changes in contractile state and contraction rate. This approach may help link macro contractile properties, such as the force-velocity relation, to micro dynamics of crossbridge bonds. It may also be useful as a component of larger physiological models and simulations.

Mechanical regulation of gene expression in cardiac myocytes and fibroblasts

The intact heart undergoes complex and multiscale remodelling processes in response to altered mechanical cues. Remodelling of the myocardium is regulated by a combination of myocyte and non-myocyte responses to mechanosensitive pathways, which can alter gene expression and therefore function in these cells. Cellular mechanotransduction and its downstream effects on gene expression are initially compensatory mechanisms during adaptations to the altered mechanical environment, but under prolonged and abnormal loading conditions, they can become maladaptive, leading to impaired function and cardiac pathologies. In this Review, we summarize mechanoregulated pathways in cardiac myocytes and fibroblasts that lead to altered gene expression and cell remodelling under physiological and pathophysiological conditions. Developments in systems modelling of the networks that regulate gene expression in response to mechanical stimuli should improve integrative understanding of their roles in vivo and help to discover new combinations of drugs and device therapies targeting mechanosignalling in heart disease.

High tension in sarcomeres hinders myocardial relaxation: A computational study

Experiments have shown that the relaxation phase of cardiac sarcomeres during an isometric twitch is prolonged in muscles that reached a higher peak tension. However, the mechanism is not completely understood. We hypothesize that the binding of calcium to troponin is enhanced by the tension in the thin filament, thus contributing to the prolongation of contraction upon higher peak tension generation. To test this hypothesis, we developed a computational model of sarcomere mechanics that incorporates tension-dependence of calcium binding. The model was used to simulate isometric twitch experiments with time dependency in the form of a two-state cross-bridge cycle model and a transient intracellular calcium concentration. In the simulations, peak isometric twitch tension appeared to increase linearly by 51.1 KPa with sarcomere length from 1.9 μm to 2.2 μm. Experiments showed an increase of 47.3 KPa over the same range of sarcomere lengths. The duration of the twitch also increased with both sarcomere length and peak intracellular calcium concentration, likely to be induced by the inherently coupled increase of the peak tension in the thin filament. In the model simulations, the time to 50% relaxation (tR50) increased over the range of sarcomere lengths from 1.9 μm to 2.2 μm by 0.11s, comparable to the increased duration of 0.12s shown in experiments. Model simulated tR50 increased by 0.12s over the range of peak intracellular calcium concentrations from 0.87 μM to 1.45 μM. Our simulation results suggest that the prolongation of contraction at higher tension is a result of the tighter binding of Ca2+ to troponin in areas under higher tension, thus delaying the deactivation of the troponin.

Regulation of cardiac Ca2+ and ion channels by shear mechanotransduction

Cardiac contraction is controlled by a Ca²⁺ signaling sequence that includes L-type Ca²⁺ current-gated opening of Ca²⁺ release channels (ryanodine receptors) in the sarcoplasmic reticulum (SR). Local Ca²⁺ signaling in the atrium differs from that in the ventricle because atrial myocytes lack transverse tubules and have more abundant corbular SR. Myocardium is subjected to a variety of forces with each contraction, such as stretch, shear stress, and afterload, and adapts to those mechanical stresses. These mechanical stimuli increase in heart failure, hypertension, and valvular heart diseases that are clinically implicated in atrial fibrillation and stroke. In the present review, we describe distinct responses of atrial and ventricular myocytes to shear stress and compare them with other mechanical responses in the context of local and global Ca²⁺ signaling and ion channel regulation. Recent evidence suggests that shear mechanotransduction in cardiac myocytes involves activation of gap junction hemichannels, purinergic signaling, and generation of mitochondrial reactive oxygen species. Significant alterations in Ca²⁺ signaling and ionic currents by shear stress may be implicated in the pathogenesis of cardiac arrhythmia and failure.

Mechanosensitivity of microdomain calcium signalling in the heart

In cardiac myocytes, calcium (Ca²⁺) signalling is tightly controlled in dedicated microdomains. At the dyad, i.e. the narrow cleft between t-tubules and junctional sarcoplasmic reticulum (SR), many signalling pathways combine to control Ca²⁺-induced Ca²⁺ release during contraction. Local Ca²⁺ gradients also exist in regions where SR and mitochondria are in close contact to regulate energetic demands. Loss of microdomain structures, or dysregulation of local Ca²⁺ fluxes in cardiac disease, is often associated with oxidative stress, contractile dysfunction and arrhythmias. Ca²⁺ signalling at these microdomains is highly mechanosensitive. Recent work has demonstrated that increasing mechanical load triggers rapid local Ca²⁺ releases that are not reflected by changes in global Ca²⁺. Key mechanisms involve rapid mechanotransduction with reactive oxygen species or nitric oxide as primary signalling molecules targeting SR or mitochondria microdomains depending on the nature of the mechanical stimulus. This review summarizes the most recent insights in rapid Ca²⁺ microdomain mechanosensitivity and re-evaluates its (patho)physiological significance in the context of historical data on the macroscopic role of Ca²⁺ in acute force adaptation and mechanically-induced arrhythmias. We distinguish between preload and afterload mediated effects on local Ca²⁺ release, and highlight differences between atrial and ventricular myocytes. Finally, we provide an outlook for further investigation in chronic models of abnormal mechanics (eg post-myocardial infarction, atrial fibrillation), to identify the clinical significance of disturbed Ca²⁺ mechanosensitivity for arrhythmogenesis.

Cardiac thin filament regulation and the Frank-Starling mechanism

The heart has an intrinsic ability to increase systolic force in response to a rise in ventricular filling (the Frank–Starling law of the heart). It is widely accepted that the length dependence of myocardial activation underlies the Frank–Starling law of the heart. Recent advances in muscle physiology have enabled the identification of the factors involved in length-dependent activation, viz., titin (connectin)-based interfilament lattice spacing reduction and thin filament “on–off” regulation, with the former triggering length-dependent activation and the latter determining the number of myosin molecules recruited to thin filaments. Patients with a failing heart have demonstrated reduced exercise tolerance at least in part via depression of the Frank–Starling mechanism. Recent studies revealed that various mutations occur in the thin filament regulatory proteins, such as troponin, in the ventricular muscle of failing hearts, which consequently alter the Frank–Starling mechanism. In this article, we review the molecular mechanisms of length-dependent activation, and the influence of troponin mutations on the phenomenon.

Modeling muscle's nonlinear viscoelastic dynamics

Muscle's contractile properties are complicated by its viscoelastic properties. Failure of early viscoelastic muscle models led to Hill's force-velocity relation embodied as the contractile element. Adopting a particular force-velocity relation to describe muscle is neither easy, nor unique [1]. Time-varying elastance based models of the left ventricle have been popular since the idea was presented in 1969 [2]. This paper investigates adoption of the time-varying elastance concept to describe the viscoelastic properties of muscle. It will be shown that a time-varying elastance must be extended to a time, length, and velocity dependent elastance. Results show how a generalized force generator description of muscle [3] may be used to realistically model muscle's viscoelasticity.

Myocardial twitch duration and the dependence of oxygen consumption on pressure-volume area: experiments and modelling

We tested the proposition that linear length dependence of twitch duration underlies the well-characterised linear dependence of oxygen consumption (V(O(2)) ) on pressure–volume area (PVA) in the heart. By way of experimental simplification, we reduced the problem from three dimensions to one by substituting cardiac trabeculae for the classically investigated whole-heart. This allowed adoption of stress–length area (SLA) as a surrogate for PVA, and heat as a proxy for V(O(2)) . Heat and stress (force per cross-sectional area), at a range of muscle lengths and at both 1 mM and 2 mM [Ca(2+)](o), were recorded from continuously superfused rat right-ventricular trabeculae undergoing fixed-end contractions. The heat–SLA relations of trabeculae (reported here, for the first time) are linear. Twitch duration increases monotonically (but not strictly linearly) with muscle length. We probed the cellular mechanisms of this phenomenon by determining: (i) the length dependence of the duration of the Ca(2+) transient, (ii) the length dependence of the rate of force redevelopment following a length impulse (an index of Ca(2+) binding to troponin-C), (iii) the effect on the simulated time course of the twitch of progressive deletion of length and Ca(2+)-dependent mechanisms of crossbridge cooperativity, using a detailed mathematical model of the crossbridge cycle, and (iv) the conditions required to achieve these multiple length dependencies, using a greatly simplified model of twitch mechano-energetics. From the results of these four independent investigations, we infer that the linearity of the heat–SLA relation (and, by analogy, the V(O(2))–PVA relation) is remarkably robust in the face of departures from linearity of length-dependent twitch duration.

The interaction of Ca2+ with sarcomeric proteins: role in function and dysfunction of the heart

The hallmarks of the normal heartbeat are both rapid onset of contraction and rapid relaxation as well as an inotropic response to both increased end-diastolic volume and increased heart rate. At the microscopic level, Ca(2+) plays a crucial role in normal cardiac contraction. This paper reviews the cycle of Ca(2+) fluxes during the normal heartbeat, which underlie the coupling between excitation and contraction and permit a highly synchronized action of cardiac sarcomeres. Length dependence of the response of the regulatory sarcomeric proteins mediates the Frank-Starling Law of the heart. However, Ca(2+) transport may go astray in heart disease such as in congestive heart failure, and both jeopardize systole and diastole and triggering arrhythmias. The interaction between weak and strong segments in nonuniform cardiac muscle allows partial preservation of force of contraction but may further lead to mechanoelectric feedback or reverse excitation-contraction coupling mediating an early diastolic Ca(2+) transient caused by the rapid force decrease during the relaxation phase. These rapid force changes in nonuniform muscle may cause arrhythmogenic Ca(2+) waves to propagate by the activation of neighboring sarcoplasmic reticulum by diffusing Ca(2+) ions.

Titin and troponin: central players in the frank-starling mechanism of the heart

The basis of the Frank-Starling mechanism of the heart is the intrinsic ability of cardiac muscle to produce greater active force in response to stretch, a phenomenon known as length-dependent activation. A feedback mechanism transmitted from cross-bridge formation to troponin C to enhance Ca²⁺ binding has long been proposed to account for length-dependent activation. However, recent advances in muscle physiology research technologies have enabled the identification of other factors involved in length-dependent activation. The striated muscle sarcomere contains a third filament system composed of the giant elastic protein titin, which is responsible for most passive stiffness in the physiological sarcomere length range. Recent studies have revealed a significant coupling of active and passive forces in cardiac muscle, where titin-based passive force promotes cross-bridge recruitment, resulting in greater active force production in response to stretch. More currently, the focus has been placed on the troponin-based “on-off” switching of the thin filament state in the regulation of length-dependent activation. In this review, we discuss how myocardial length-dependent activation is coordinately regulated by sarcomere proteins.

Electromechanical coupling in the cardiac myocyte; stretch-arrhythmia feedback

The macroscopic hallmarks of the normal heartbeat are rapid onset of contraction and rapid relaxation and an inotropic response to both increased end diastolic volume and increased heart rate. At the microscopic level, the calcium ion (Ca(2+)) plays a crucial role in normal cardiac contraction. This paper reviews the cycle of Ca(2+) fluxes during the normal heartbeat, which underlie the coupling between excitation and contraction (ECC) and permit a highly synchronized action of cardiac sarcomeres. Length dependence of the response of the regulatory sarcomeric proteins mediates the Frank-Starling Law of the heart. However, Ca(2+) transport may go astray in heart disease and both jeopardize the exquisite mechanism of systole and diastole and triggering arrhythmias. The interplay between weakened and strong segments in nonuniform cardiac muscle may further lead to mechanoelectric feedback-or reverse excitation contraction coupling (RECC) mediating an early diastolic Ca(2+) transient caused by the rapid force decrease during the relaxation phase. These rapid force changes in nonuniform muscle may cause arrhythmogenic Ca(2+) waves to propagate by activation of neighbouring SR by diffusing Ca(2+) ions.

Regulatory mechanism of length-dependent activation in skinned porcine ventricular muscle: role of thin filament cooperative activation in the Frank-Starling relation

Cardiac sarcomeres produce greater active force in response to stretch, forming the basis of the Frank-Starling mechanism of the heart. The purpose of this study was to provide the systematic understanding of length-dependent activation by investigating experimentally and mathematically how the thin filament "on-off" switching mechanism is involved in its regulation. Porcine left ventricular muscles were skinned, and force measurements were performed at short (1.9 µm) and long (2.3 µm) sarcomere lengths. We found that 3 mM MgADP increased Ca(2+) sensitivity of force and the rate of rise of active force, consistent with the increase in thin filament cooperative activation. MgADP attenuated length-dependent activation with and without thin filament reconstitution with the fast skeletal troponin complex (sTn). Conversely, 20 mM of inorganic phosphate (Pi) decreased Ca(2+) sensitivity of force and the rate of rise of active force, consistent with the decrease in thin filament cooperative activation. Pi enhanced length-dependent activation with and without sTn reconstitution. Linear regression analysis revealed that the magnitude of length-dependent activation was inversely correlated with the rate of rise of active force. These results were quantitatively simulated by a model that incorporates the Ca(2+)-dependent on-off switching of the thin filament state and interfilament lattice spacing modulation. Our model analysis revealed that the cooperativity of the thin filament on-off switching, but not the Ca(2+)-binding ability, determines the magnitude of the Frank-Starling effect. These findings demonstrate that the Frank-Starling relation is strongly influenced by thin filament cooperative activation.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Large-scale models reveal the two-component mechanics of striated muscle

This paper provides a comprehensive explanation of striated muscle mechanics and contraction on the basis of filament rotations. Helical proteins, particularly the coiled-coils of tropomyosin, myosin and alpha-actinin, shorten their H-bonds cooperatively and produce torque and filament rotations when the Coulombic net-charge repulsion of their highly charged side-chains is diminished by interaction with ions. The classical "two-component model" of active muscle differentiated a "contractile component" which stretches the "series elastic component" during force production. The contractile components are the helically shaped thin filaments of muscle that shorten the sarcomeres by clockwise drilling into the myosin cross-bridges with torque decrease (= force-deficit). Muscle stretch means drawing out the thin filament helices off the cross-bridges under passive counterclockwise rotation with torque increase (= stretch activation). Since each thin filament is anchored by four elastic alpha-actinin Z-filaments (provided with force-regulating sites for Ca(2+) binding), the thin filament rotations change the torsional twist of the four Z-filaments as the "series elastic components". Large scale models simulate the changes of structure and force in the Z-band by the different Z-filament twisting stages A, B, C, D, E, F and G. Stage D corresponds to the isometric state. The basic phenomena of muscle physiology, i. e. latency relaxation, Fenn-effect, the force-velocity relation, the length-tension relation, unexplained energy, shortening heat, the Huxley-Simmons phases, etc. are explained and interpreted with the help of the model experiments.

Sarcomere mechanics in uniform and nonuniform cardiac muscle: a link between pump function and arrhythmias

Starling's law and the end-systolic pressure-volume relationship (ESPVR) reflect the effect of sarcomere length (SL) on the development of stress (sigma) and shortening by myocytes in the uniform ventricle. We show here that tetanic contractions of rat cardiac trabeculae exhibit a sigma-SL relationship at saturating [Ca2+] that depends on sarcomere geometry in a manner similar to that of skeletal sarcomeres and the existence of opposing forces in cardiac muscle shortened below slack length. The sigma-SL -[Ca2+](free) relationships (sigma-SL-Ca relationships) at submaximal [Ca2+] in intact and skinned trabeculae were similar, although the sensitivity for Ca2+ of intact muscle was higher. We analyzed the mechanisms underlying the sigma-SL-Ca relationship by using a kinetic model assuming that the rates of Tn-C Ca2+ binding and/or cross-bridge (XB) cycling are determined by either the SL, [Ca2+], or sigma. We analyzed the correlation between the model results and steady-state sigma measurements at varied SL at [Ca2+] from skinned rat cardiac trabeculae to test the hypotheses that the dominant feedback mechanism is SL-, sigma-, or [Ca2+]-dependent, and that the feedback mechanism regulates Tn-C Ca2+ affinity, XB kinetics, or the unitary XB force. The analysis strongly suggests that the feedback of the number of strong XBs to cardiac Tn-C Ca2+ affinity is the dominant mechanism regulating XB recruitment. Using this concept in a model of twitch-sigma accurately reproduced the sigma-SL-Ca relationship and the time courses of twitch sigma and the intracellular [Ca2+]i. The foregoing concept has equally important repercussions for the nonuniformly contracting heart, in which arrhythmogenic Ca2+ waves arise from weakened areas in the cardiac muscle. These Ca2+ waves can reversibly be induced with nonuniform excitation-contraction coupling (ECC) by the cycle of stretch and release in the border zone between the damaged and intact regions. Stimulus trains induced propagating Ca2+ waves and reversibly induced arrhythmias. We hypothesize that rapid force loss by the sarcomeres in the border zone during relaxation causes Ca2+ release from Tn-C and initiates Ca2+ waves propagated by the sarcoplasmic reticulum (SR). Modeling of the response of the cardiac twitch to rapid force changes using the feedback concept uniquely predicts the occurrence of [Ca2+]i transients as a result of accelerated Ca2+ dissociation from Tn-C. These results are consistent with the hypothesis that a force feedback to Ca2+ binding by Tn-C is responsible for Starling's law and the ESPVR in the uniform myocardium and leads to a surge of Ca2+ released by the myofilaments during relaxation in the nonuniform myocardium, which initiates arrhythmogenic propagating Ca2+ release by the SR.

Defining muscle elastance as a parameter

Functional descriptions of striated muscle are often based on the measured variables force and initial velocity of shortening, embodied as Hill's contractile element. The fundamental difficulty of describing the mechanical properties of muscle with a force-velocity relation that is set a priori, and the practical problem of the act of measurement changing muscle's force-velocity relation or elastance curve, are described. As an alternative, a new model of muscle contraction is presented, which characterizes muscle's contractile state with parameters, rather than variables. Muscle is treated as a force generator that is time, length, and velocity dependent. Muscle dynamics develop from a single equation based on the formation and relaxation of crossbridge bonds. This analytical function permits the calculation of muscle elastance via E(m)=[abstract: see text]. This new muscle model is defined independently from load properties, and muscle elastance is dynamic and reflects changing numbers of crossbridge bonds. This parameter is more representative of the mechanical properties of muscle than are variables such as muscle force and shortening velocity.

Superior logistic model for decay of Ca2+ transient and isometric relaxation force curve in rabbit and mouse papillary muscles

A decrease in myocardial intracellular calcium concentration ([Ca(2+)](i)) precedes relaxation, and a monoexponential function is typically used for fitting the decay of the Ca(2+) transient. However, a logistic function has been shown to be a better fit for the relaxation force curve, compared to the conventional monoexponential function. In the present study, we compared the logistic and monoexponential functions for fitting the [Ca(2+)](i) declines, which were measured using the aequorin method, and isometric relaxation force curves at 4 different onsets: the minimum time-derivative of [Ca(2+)](i) (d[Ca(2+)](i)/dt (min)) and force (dF/dt(min)), and the 10%, 20% and 30% lower [Ca(2+)](i) levels and forces over the data-sampling period in 7 isolated rabbit right ventricular and 15 isolated mouse left ventricular papillary muscles. Logistic functions were significantly superior for fitting the [Ca(2+)] (i) declines and relaxation force curves, compared to monoexponential functions. Changes in the normalized logistic [Ca(2+)] (i) decline and relaxation force time constants at the delayed onsets relative to their 100% values at d[Ca(2+)] (i)/dt(min) and dF/dt(min) were significantly smaller than the changes in the normalized monoexponential time constants. The ratio of the logistic relaxation force time constant relative to the logistic [Ca(2+)](i) decline time constant was significantly smaller in mouse than in rabbit. We conclude that the logistic function more reliably characterizes the [Ca(2+)](i) decline and relaxation force curve at any onset, irrespective of animal species. Simultaneous analyses using the logistic model for decay of the Ca(2+) transient and myocardial lusitropism might be a useful strategy for analysis of species-specific myocardial calcium handling.

Titin/connectin-based modulation of the Frank-Starling mechanism of the heart

The basis of the Frank-Starling mechanism of the heart is the increase in active force when muscle is stretched. Various findings have shown that muscle length, i.e., sarcomere length (SL), modulates activation of cardiac myofilaments at a given concentration of Ca2+ ([Ca2+]). This augmented Ca2+ activation with SL, commonly known as "length-dependent activation", is manifested as the leftward shift of the force-pCa (= -log [Ca2+]) relation as well as by the increase in maximal Ca2+ -activated force. Despite the numerous studies that have been undertaken, the molecular mechanism(s) of length-dependent activation is (are) still not fully understood. The giant sarcomere protein titin/connectin is the largest protein known to date. Titin/connectin is responsible for most passive force in vertebrate striated muscle and also functions as a molecular scaffold during myofibrillogenesis. Recent studies suggest that titin/connectin plays an important role in length-dependent activation by sensing stretch and promoting actomyosin interaction. Here we review and extend this previous work and focus on the mechanism by which titin/connectin might modulate actomyosin interaction.

The mechanoelectric feedback: a novel "calcium clamp" method, using tetanic contraction, for testing the role of the intracellular free calcium

Mechanical perturbations affect the membrane action potential, a phenomenon denoted as the mechanoelectric feedback (MEF), and may elicit cardiac arrhythmias. Two plausible mechanisms were suggested to explain this phenomenon: (i) stretch-activated channels (SACs) within the cell membrane and (ii) modulation of the action potential by the intracellular Ca(2+) (the Calcium hypothesis). The intracellular Ca(2+) varies mainly due to the effects of the mechanical perturbations on the affinity of troponin for calcium. The present study concentrates on the unique experimental methods that allow differentiating between the effects of SAC and Ca(2+) on the action potential. This is achieved by controlling the sarcomere lengths (SLs) independently of the intracellular Ca(2+) concentration, in the intact fiber. A dedicated experimental setup allowed simultaneous measurements of the membrane potential and the mechanical performance (Force and SL). The action potential was measured by voltage-sensitive dye (Di-4-ANEPPS). The SL was measured by laser diffraction technique and was controlled by a fast servomotor. The intracellular Ca(2+) was controlled (calcium clamp) by imposing stable tetanic contractions at various extracellular calcium concentrations ([Ca(2+)](0)s). Tetanus was obtained by 8 Hz stimulation in the presence of cyclopiazonic acid (CPA) (30 muM). Isolated trabeculae from a rat's right ventricle were studied at different SLs and [Ca(2+)](0)s. The experimental data strongly support the calcium hypothesis. Although the action potential duration (APD) decreases at longer SL, the [Ca(2+)](0) has a significantly larger effect on the APD. The APD decreases as the [Ca(2+)](0) increases. Understanding the underlying mechanism opens new research avenues for the development of therapeutic modalities for cardiac arrhythmias.

Derivation and Left Ventricular Pressure Phase Plane Based Validation of a Time Dependent Isometric Crossbridge Attachment Model

Huxley's crossbridge attachment model predicts tension (contractile force) development in isometric (fixed length) cells using constant attachment and detachment rates. Alternative models incorporating time-varying calcium concentrations are complex (coupled linear differential equations) and use time-dependent inputs (calcium, elastance, etc.) to model multiple states. We hypothesize that by incorporating the known significant rise and fall in intracellular calcium, via either an asymmetric damped function or a symmetric Gaussian function, into a time-varying, rather than constant, attachment rate function, the Huxley model prediction for tension (i.e., chamber pressure) in isovolumic (isometric) non-ejecting beats will improve. To test the hypothesis that the time-dependent model-predicted (TDM) pressure fits the in vivo isometric (isovolumic) LV pressure phase-plane (PPP) contour better than the constant attachment rate predicted pressure, we used the TDM to fit non-ejecting, premature ventricular contraction (PVC) PPP contours in 6 subjects. Conventional model fit was poor (relative error 74.0%+/-12.5%), while the asymmetric damped TDM rate function provided slight improvement relative to the conventional time-independent model (relative error 55.4%+/-9.8%). The symmetric Gaussian rate function TDM provided the best PPP fit to all non-ejecting beats tested (relative error 19.8%+/-4.8%). We conclude that approximating the lumped attachment rate via a time-varying, rather than constant, rate function generates a physiologically viable model of crossbridge behavior. The PPP provides the optimal arena for alternate mathematical formulation assessment of LVP contour prediction by time-dependent attachment rate functions and facilitates modeling of cardiac contraction and relaxation.

Cross-bridge-dependent change in Ca2+ sensitivity is involved in the negative inotropic effect of nifedipine in aequorin-injected ferret ventricular muscles

BACKGROUND

We hypothesized that the negative inotropic effect of nifedipine (Nif) on cardiac ventricular muscle is partly due to the cross-bridge-dependent decrease of Ca2+ sensitivity of the myofilaments as well as the decrease in Ca2+ influx.

METHOD AND RESULTS

We used aequorin-injected ferret papillary muscles and measured the slope of the extra-Ca(2+)-tension relation which expresses the change in the Ca2+ sensitivity through the feedback from the cross-bridges. Twitch tension was decreased significantly by 0.5 micromol/L Nif accompanying a significant reduction of the Ca2+ transient peak. When Nif (0.2-0.5 micromol/L) was added to the solution with 8 mmol/L Ca2+, the slope of the extra-Ca2+-tension relation became steeper in a concentration-dependent manner, which was similar to the change in the slope when the concentration of Ca2+ was decreased from 8 to 1 mmol/L in the absence of Nif. BAY-K 8644 (0.3 micromol/L), a dihydropyridine receptor agonist, showed the opposite effect on the slope of the extra-Ca2+-tension relation to that observed in Nif. However, 2,3-butanedione monoxime (3 mmol/L), an inhibitor of the active cross-bridges, antagonized the effect of BAY-K 8644.

CONCLUSION

Nif exerts its negative inotropic effect on cardiac muscle by suppressing Ca2+ binding to troponin C via the inhibition of the L-type Ca2+ channel, and by the cross-bridge-dependent decrease in the Ca2+ sensitivity, as in low extracellular Ca2+ concentration.

Spatial Nonuniformity of Contraction Causes Arrhythmogenic Ca2+Waves in Rat Cardiac Muscle

Landesberg and Sideman's four state model of the cardiac cross-bridge (XB) hypothesizes a feedback of force development to Ca(2+) binding by troponin C (TnC). We have further modeled this behavior and observed that the force (F)-Ca(2+) relationship as well as the F-sarcomere length (SL) relationship and the time course of F and Ca(2+) transients in cardiac muscle can be reproduced faithfully by a single effect of F on deformation of the TnC-Ca complex and, thereby, on the dissociation rate of Ca(2+). Furthermore, this feedback predicts that rapid decline of F in the activated sarcomere causes release of Ca(2+) from TnC-Ca(2+), which is sufficient to initiate arrhythmogenic Ca(2+) release from the sarcoplasmic reticulum (SR). This work investigated the initiation of Ca(2+) waves underlying triggered propagated contractions (TPCs) in rat cardiac trabeculae under conditions that simulate functional nonuniformity caused by mechanical or ischemic local damage of the myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small constant flow jet of solution that reduces excitation-contraction coupling in myocytes within that segment. Force was measured, and SL as well as [Ca(2+)](i) were measured regionally. When the jet contained caffeine, 2,3-butanedione monoxime or low-[Ca(2+)], muscle-twitch F decreased and the sarcomeres in the exposed segment were stretched by shortening the normal regions outside the jet. During relaxation, the sarcomeres in the exposed segment shortened rapidly. Short trains of stimulation at 2.5 Hz reproducibly caused Ca(2+) waves to rise from the borders exposed to the jet. Ca(2+) waves started during F relaxation of the last stimulated twitch and propagated into segments both inside and outside of the jet. Arrhythmias, in the form of nondriven rhythmic activity, were triggered when the amplitude of the Ca(2+) wave increased by raising [Ca(2+)](o). The arrhythmias disappeared when the muscle uniformity was restored by turning the jet off. These results show that nonuniform contraction can cause Ca(2+) waves underlying TPCs, and suggest that Ca(2+) dissociated from myofilaments plays an important role in the initiation of arrhythmogenic Ca(2+) waves.

Cross-Bridge-Dependent Change of the Ca2+ Sensitivity During Relaxation in Aequorin-Injected Tetanized Ferret Papillary Muscles

BACKGROUND The aim of the present study was to indicate the cross-bridge-dependent change in the Ca2+ affinity of troponin-C (TnC) during relaxation in an intact preparation, because the intracellular mechanism of relaxation is not fully understood, although several methods of evaluating global diastolic function have been reported.

METHODS AND RESULTS

The aequorin method was used with intact ferret papillary muscles and a tetanic contraction was induced by a repetitive electrical stimulation in the presence of ryanodine. The extra-Ca2+, the transient increase in the intracellular Ca2+ concentration in response to a rapid reduction in muscle length, which reflects the change in the Ca2+ affinity of TnC because of cross-bridge detachment, was measured, and the cross-bridge-dependent change in the Ca2+ affinity of TnC was estimated by observing the change in the slope of the extra-Ca2+ -tension relation. The extra-Ca2+ -tension relation measured during relaxation became steeper than that during contraction in all cases. The extra-Ca2+ -tension relation became steeper in the presence of 20 mmol/L caffeine during contraction in all cases.

CONCLUSION

During relaxation, the downstream-dependent change in the Ca2+ affinity of TnC was enhanced, compared with that during contraction, because of a decrease in the number of attached cross-bridges.

The alpha-helix, an overlooked molecular motor

At first sight the alpha-helix appears as a rigid scaffold braced by hydrogen bonds nearly parallel to the helix axis. Looked at more closely it turned out to be highly dynamic and able to transform chemical into mechanical energy. The hydrogen bonds are fairly weak and compliant bonds. Their length, usually between 0.267 and 0.291 nm (mean value, 0.28 nm), depends on the interaction of the side chains. The most important strong interaction is the electrostatic repelling force between equally charged side chains (Glu-, Asp-, Lys+, Arg+), well known by experiments with polyamino acids. In proteins with different amino acids, repelling forces between charged side chains work in the axial direction and stretch the hydrogen bonds. Extreme shortening of the hydrogen bonds occurs when ions, e.g., Ca2+, H+, or PO3-, are added and discharge side chains. This means a cooperative pitch decrease of the alpha-helix (pitch range between 0.52 and more than 0.55 nm; mean value, 0.54 nm). This pitch change is absolutely connected by steric reasons with torque generation and torsional rotations, as demonstrated by molecular and tubular alpha-helix models. Thus, charged alpha-helices are molecular motors propelled by the electrostatic energy of added ions. The motor effect is most striking with highly charged alpha-helical coiled coils, e.g., tropomyosin, myosin, and alpha-actinin that can rotate actin filaments by winding and unwinding. For example, the shortening of muscle depends on the sliding (drilling) motion of the Ca2+-activated helical actin filaments into the cross-bridges of the A-band. Here, models are presented for the in vitro sliding of actin filaments and for cytoplasmic streaming by winding and unwinding of myosin chains, and for membrane proteins that contain nonhelical domains between membrane-penetrating alpha-helices. They may transport molecules by the described torsional rotations if they perform supercoiling. Winding and supercoiling can lead to displacement of bound ions and to a feed-back-regulated oscillation between two different coiling stages E1 and E2 that explain "eversion". The models need the torque for 1-2 rotations. They explain active and passive transports, the driving-effects of ion gradients, ATP hydrolysis by unwinding, ATP synthesis by winding up of the supercoils, etc.

Spatial Nonuniformity of Excitation–Contraction Coupling Causes Arrhythmogenic Ca 2+ Waves in Rat Cardiac Muscle

Ca2+ waves underlying triggered propagated contractions (TPCs) are initiated in damaged regions in cardiac muscle and cause arrhythmias. We studied Ca2+ waves underlying TPCs in rat cardiac trabeculae under experimental conditions that simulate the functional nonuniformity caused by local mechanical or ischemic local damage of myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small jet of solution with a composition that reduces excitation-contraction coupling (ECC) in myocytes within that segment. The jet solution contained either caffeine (5 mmol/L), 2,3-butanedione monoxime (BDM; 20 mmol/L), or low Ca2+ concentration ([Ca2+]; 0.2 mmol/L). Force was measured with a silicon strain gauge and sarcomere length with laser diffraction techniques in 15 trabeculae. Simultaneously, [Ca2+]i was measured locally using epifluorescence of Fura-2. The jet of solution was applied perpendicularly to a small muscle region (200 to 300 microm) at constant flow. When the jet contained caffeine, BDM, or low [Ca2+], during the stimulated twitch, muscle-twitch force decreased and the sarcomeres in the exposed segment were stretched by shortening normal regions outside the jet. Typical protocols for TPC induction (7.5 s-2.5 Hz stimulus trains at 23 degrees C; [Ca2+]o=2.0 mmol/L) reproducibly generated Ca2+ waves that arose from the border between shortening and stretched regions. Such Ca2+ waves started during force-relaxation of the last stimulated twitch of the train and propagated (0.2 to 2.8 mm/sec) into segments both inside and outside of the jet. Arrhythmias, in the form of nondriven rhythmic activity, were induced when the amplitude of the Ca2+-wave was increased by raising [Ca2+]o. Arrhythmias disappeared rapidly when uniformity of ECC throughout the muscle was restored by turning the jet off. These results show, for the first time, that nonuniform ECC can cause Ca2+ waves underlying TPCs and suggest that Ca2+ dissociated from myofilaments plays an important role in the initiation of Ca2+ waves.

Evaluation of the cross-bridge-dependent change in the Ca2+ affinity of troponin C in aequorin-injected ferret ventricular muscles

Ca2+ affinity of cardiac troponin C (TnC) is regulated by the active cross-bridges (downstream-dependent mechanism). In the present study, we showed one of the methods to evaluate the downstream-dependent change in the Ca2+ affinity of TnC during contraction using the aequorin-injected ferret papillary muscle. For this purpose, the tension-dependent change in the extra-Ca2+ (a transient increase in the intracellular Ca2+ concentration ([Ca2+]i) in response to a quick length reduction) was measured under various conditions. We examined whether the regression line between the magnitude of tension reduction and the magnitude of the normalized extra-Ca2+ (the extra-Ca2+ was divided by [Ca2+]i immediately before length change) (the normalized extra-Ca2+-tension relation) in twitch contraction can be used for the estimation of the downstream-dependent change in the Ca2+ affinity of TnC. The normalized extra-Ca2+-tension relation became shallow by EMD 57033 (EMD) (one of the Ca2+ sensitizers) and by an increase in Ca2+ concentration in the solution ([Ca2+]o) in a concentration-dependent manner. However, 2,3-butanedione monoxime (BDM) (one of the desensitizers) antagonized the effects of EMD and higher [Ca2+]o in a concentration-dependent manner. These effects of EMD and BDM were also observed in the normalized extra-Ca2+-tension relation in tetanic contraction. The normalized extra-Ca2+-tension relation became steep by shortening the initial muscle length before contraction in tetanic contraction. Length-tension relation in twitch contraction was significantly shifted upward by higher [Ca2+]o and EMD, but BDM showed the opposite effects on them in a concentration-dependent manner. Thus, the downstream-dependent change in the Ca2+ affinity of TnC which physiologically functions in intact cardiac muscle can be evaluated using the normalized extra-Ca2+-tension relation.

Do stretch-induced changes in intracellular calcium modify the electrical activity of cardiac muscle?

Stretch of the myocardium influences the shape and amplitude of the intracellular Ca(2+)([Ca(2+)](i)) transient. Under isometric conditions stretch immediately increases myofilament Ca(2+) sensitivity, increasing force production and abbreviating the time course of the [Ca(2+)](i) transient (the rapid response). Conversely, muscle shortening can prolong the Ca(2+) transient by decreasing myofilament Ca(2+) sensitivity. During the cardiac cycle, increased ventricular dilation may increase myofilament Ca(2+) sensitivity during diastolic filling and the isovolumic phase of systole, but enhance the decrease in myofilament Ca(2+) sensitivity during the systolic shortening of the ejection phase. If stretch is maintained there is a gradual increase in the amplitude of the Ca(2+) transient and force production, which takes several minutes to develop fully (the slow response). The rapid and slow responses have been reported in whole hearts and single myocytes. Here we review stretch-induced changes in [Ca(2+)](i) and the underlying mechanisms. Myocardial stretch also modifies electrical activity and the opening of stretch-activated channels (SACs) is often used to explain this effect. However, the myocardium has many ionic currents that are regulated by [Ca(2+)](i) and in this review we discuss how stretch-induced changes in [Ca(2+)](i) can influence electrical activity via the modulation of these Ca(2+)-dependent currents. Our recent work in single ventricular myocytes has shown that axial stretch prolongs the action potential. This effect is sensitive to either SAC blockade by streptomycin or the buffering of [Ca(2+)](i) with BAPTA, suggesting that both SACs and [Ca(2+)](i) are important for the full effects of axial stretch on electrical activity to develop.

Load dependence of ventricular performance explained by model of calcium-myofilament interactions

Although a simple concept of load-independent behavior of the intact heart evolved from early studies of isolated, intact blood-perfused hearts, more recent studies showed that, as in isolated muscle, the mode of contraction (isovolumic vs. ejection) impacts on end-systolic elastance. The purpose of the present study was to test whether a four-state model of myofilament interactions with length-dependent rate constants could explain the complex contractile behavior of the intact, ejecting heart. Studies were performed in isolated, blood-perfused canine hearts with intracellular calcium transients measured by macroinjected aequorin. Measured calcium transients were used as the driving function for the model, and length-dependent rate constants yielding the highest concordance between measured and model-predicted midwall stress at different isovolumic volumes were determined. These length-dependent rate constants successfully predicted contractile behavior on ejecting contractions. This, along with additional model analysis, suggests that length-dependent changes in calcium binding affinity may not be an important factor contributing to load-dependent contractile performance in the intact heart under physiological conditions.

Stretch and quick release of rat cardiac trabeculae accelerates Ca2+ waves and triggered propagated contractions

Rapid shortening of active cardiac muscle [quick release (QR)] dissociates Ca2+ from myofilaments. We studied, using muscle stretches and QR, whether Ca2+ dissociation affects triggered propagated contractions (TPCs) and Ca2+ waves. The intracellular Ca2+ concentration was measured by a SIT camera in right ventricular trabeculae dissected from rat hearts loaded with fura 2 salt, force was measured by a silicon strain gauge, and sarcomere length was measured by laser diffraction while a servomotor controlled muscle length. TPCs (n = 27) were induced at 28 degrees C by stimulus trains (7.5 s at 2.65 +/- 0.13 Hz) at an extracellular Ca2+ concentration ([Ca2+]o) = 2.0 mM or with 10 microM Gd3+ at [Ca2+]o = 5.2 +/- 0.73 mM. QR during twitch relaxation after a 10% stretch for 100-200 ms reduced both the time between the last stimulus and the peak TPC (PeakTPC) and the time between the last stimulus and peak Ca2+ wave (PeakCW) and increased PeakTPC and PeakCW (n = 13) as well as the propagation velocity (Vprop; n = 8). Active force during stretch also increased Vprop (r = 0.84, n = 12, P < 0.01), but Gd3+ had no effect (n = 5). These results suggest that Ca2+ dissociation by QR during relaxation accelerates the initiation and propagation of Ca2+ waves.

Isoproterenol enhances myofilament Ca(2+) sensitivity during hypothermia in isolated guinea pig beating hearts

Isoproterenol is often required to treat acute left ventricular dysfunction during separation from cardiopulmonary bypass for cardiac surgery. We hypothesized that heart rate and intracellular Ca(2+) concentration ([Ca(2+)]i) homeostasis may be important factors when isoproterenol improves the cardiac function during hypothermia. Accordingly, we investigated the effect of isoproterenol on the cardiac functional variables, [Ca(2+)]i, and myofilament Ca(2+) sensitivity under spontaneous beating during hypothermia. Intact guinea pig hearts were perfused with a modified Krebs-Ringer solution (baseline) and Krebs-Ringer solution containing isoproterenol (1 nM) at 37 degrees C, 32 degrees C, and 27 degrees C while all cardiac variables and [Ca(2+)]i were recorded. Isoproterenol increased developed left ventricular pressure (LVP), maximum rate of increase in LVP, and coronary inflow at 27 degrees C, and it also increased heart rate and maximum rate of decrease in LVP at each temperature (P < 0.05). Isoproterenol produced a leftward shift of the curve of developed LVP as a function of available [Ca(2+)]i at 32 degrees C and 27 degrees C (P < 0.05), without changing available [Ca(2+)]i. Isoproterenol improves the cardiac function, especially systolic ventricular function, by enhancement of myofilament Ca(2+) sensitivity under spontaneous beating during hypothermia in intact guinea pig hearts.

IMPLICATIONS

Enhancement of myofilament Ca(2+) sensitivity is involved in the improvement of cardiac function by isoproterenol under spontaneous beating during hypothermia.

Heart rate modulates the slow enhancement of contraction due to sudden left ventricular dilation

In isovolumic blood-perfused dog hearts, left ventricular developed pressure (DP) was recorded while a sudden ventricular dilation was promoted at three heart rate (HR) levels: low (L: 52 +/- 1.7 beats/min), intermediate (M: 82 +/- 2.2 beats/min), and high (H: 117 +/- 3.5 beats/min). DP increased instantaneously with chamber expansion (Delta(1)DP), and another continuous increase occurred for several minutes (Delta(2)DP). HR elevation did not alter Delta(1)DP (32.8 +/- 1.6, 33.6 +/- 1.5, and 34.3 +/- 1.2 mmHg for L, M, and H, respectively), even though it intensified Delta(2)DP (17.3 +/- 0.9, 20.7 +/- 1.0, and 26.8 +/- 1.2 mmHg for L, M, and H, respectively), meaning that the treppe phenomenon enhances the length dependence of the contraction component related to changes in intracellular Ca(2+) concentration. Frequency increments reduced the half time of the slow response (82 +/- 3.6, 67 +/- 2.6, and 53 +/- 2.0 s for L, M, and H, respectively), while the number of beats included in half time increased (72 +/- 2.9, 95 +/- 2.9, and 111 +/- 3.2 beats for L, M, and H, respectively). HR modulation of the slow response suggests that L-type Ca(2+) channel currents and/or the Na(+)/Ca(2+) exchanger plays a relevant role in the stretch-triggered Ca(2+) gain when HR increases in the canine heart.

Morphine enhances myofilament CA(2+) sensitivity in intact guinea pig beating hearts

We investigated whether morphine alters intracellular Ca(2+) concentration ([Ca(2+)](i)), left ventricular pressure (LVP), and myofilament Ca(2+) sensitivity under physiologic conditions in intact guinea pig beating hearts and whether delta(1), delta(2), and kappa opioid stimulations are related to the direct cardiac effects of morphine. Transmural LV phasic [Ca(2+)](i) was measured from fluorescence signals at 385 nm and 456 nm. The Ca(2+) transients during each contraction were defined as available [Ca(2+)](i). The hearts were perfused with modified Krebs-Ringer solution containing morphine in the absence and presence of delta(1) (BNTX), delta(2) (NTB), and kappa (nor-BNI) antagonists, while developed LVP and available [Ca(2+)](i) were recorded. Morphine (1 microM) decreased available [Ca(2+)](i) by 44 +/- 12 nM without decreasing developed LVP at 2.5 mM of [CaCl(2)](e) (P < 0.05). Morphine (1 microM) caused a leftward shift in the curve of developed LVP as a function of available [Ca(2+)](i) (P < 0.05). BNTX (1 microM), but not nor-BNI (1 microM) or NTB (0.1 microM) blocked morphine (1 microM) effects to decrease available [Ca(2+)](i). Morphine decreases available [Ca(2+)](i) but not LVP, and it enhances myofilament Ca(2+) sensitivity under physiologic conditions at clinical concentrations in intact isolated beating guinea pig hearts. The delta(1) opioid stimulation modifies the effects of morphine on Ca(2+) transients and myofilament Ca(2+) sensitivity.

IMPLICATIONS

Morphine modifies myofilament Ca(2+) sensitivity and Ca(2+) transients in guinea pig hearts at concentrations that are clinically relevant.

Effect of transient stretch on intracellular Ca2+ during triggered propagated contractions in intact trabeculae

Transient stretch of cardiac muscle during a twitch contraction may dissociate Ca2+ from myofilaments into the cytosol at the moment of quick release of the muscle. We studied the effect of stretch and quick release of trabeculae on changes in intracellular Ca2+ ([Ca2+]i) during triggered propagated contractions (TPCs). Trabeculae were dissected from the right ventricle of 9 rat hearts. [Ca2+]i was measured using electrophoretically injected fura-2. Force was measured using a silicon strain gauge and sarcomere length was measured using laser diffraction techniques. Reproducible TPCs (n = 13) were induced by trains of electrical stimuli (378 ± 19 ms interval) for 7.5 s at [Ca2+]o of 2.0 mM (27.9 ± 0.2°C). The latency of the TPC force and the underlying increase in [Ca2+]i was calculated from the time (TimeF) between the last stimulus and the peak of TPC force (PeakF), or the time (TimeCa) between the last stimulus and the peak of the increase in [Ca2+]i during the TPCs (PeakCa). As a result of a 10% increase in muscle length for 150-200 ms during the last stimulated twitches, TimeF and TimeCa decreased and PeakF and PeakCa increased significantly (n = 13). In addition, transient stretch sometimes induced a twitch contraction subsequent to the accelerated TPC and its underlying increase in [Ca2+]i. These results suggest that Ca2+ binding and dissociation from the myofilaments by the stretch and quick release of muscle may modulate the TPC force and the underlying increases in [Ca2+]i and play an important role in the induction of arrhythmias.Key words: rat cardiac trabeculae, stretch, calcium transients.

Propofol inhibits Ca(2+) transients but not contraction in intact beating guinea pig hearts

We investigated whether propofol inhibits Ca(2+) transients and left ventricular pressure (LVP) in intact beating guinea pig hearts at clinical concentrations and whether an inhibition of Ca(2+) transients by propofol results from an impairment of sarcolemmal or of sarcoplasmic reticulum (SR) function. By using a Langendorff's preparation, transmural left ventricular phasic intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured by the fluorescence ratio of indo-1 emission at 385 nm and 456 nm and was calibrated to Ca(2+) transients (in nM). The Ca(2+) transients during each contraction were defined as available [Ca(2+)](i). Sixty hearts were perfused with modified Krebs-Ringer's solution containing lipid vehicle and propofol (1 and 10 microM) in the absence and presence of ryanodine, thapsigargin, and nifedipine, while developed LVP and available [Ca(2+)](i) were recorded. Propofol (10 microM) decreased available [Ca(2+)](i) by 11.0% +/- 1.3% without decreasing developed LVP (% of control, P < 0.05). Propofol (10 microM) caused a leftward shift in the curve of developed LVP as a function of available [Ca(2+)](i). Propofol (10 microM) with nifedipine (1 microM), but not with ryanodine (1 microM) or thapsigargin (1 microM), decreased available [Ca(2+)](i) by 15.5% +/- 1.7% (P < 0.05). Propofol decreases available [Ca(2+)](i) without decreasing cardiac contraction, and it enhances myofilament Ca(2+) sensitivity in intact beating hearts at clinical concentrations. The inhibition of available [Ca(2+)](i) by propofol may be mainly mediated by an impairment of sarcoplasmic reticulum Ca(2+) handling rather than the sarcolemmal L-type Ca(2+) current.

IMPLICATIONS

This is the first study of the effects of propofol on intracellular Ca(2+) concentration and myofilament Ca(2+) sensitivity under physiologic conditions in intact isolated beating guinea pig hearts.

Effect of troponin I phosphorylation by protein kinase A on length-dependence of tension activation in skinned cardiac muscle fibers

We examined the effect of troponin I (TnI) phosphorylation by cAMP-dependent protein kinase (PKA) on the length-dependent tension activation in skinned rat cardiac trabeculae. Increasing sarcomere length shifted the pCa (-log[Ca2+])-tension relation to the left. Treatment with PKA decreased the Ca2+ sensitivity of the myofilament and also decreased the length-dependent shift of the pCa-tension relation. Replacement of endogenous TnI with phosphorylated TnI directly demonstrated that TnI phosphorylation is responsible for the decreased length-dependence. When MgATP concentration was lowered in the absence of Ca2+, tension was elicited through rigorous cross-bridge-induced thin filament activation. Increasing sarcomere length shifted the pMgATP (-log[MgATP])-tension relation to the right, and either TnI phosphorylation or partial extraction of troponin C (TnC) abolished this length-dependent shift. We conclude that TnI phosphorylation by PKA attenuates the length-dependence of tension activation in cardiac muscle by decreasing the cross-bridge-dependent thin filament activation through a reduction of the interaction between TnI and TnC.

Effects of MgADP on length dependence of tension generation in skinned rat cardiac muscle

The effect of MgADP on the sarcomere length (SL) dependence of tension generation was investigated using skinned rat ventricular trabeculae. Increasing SL from 1.9 to 2.3 microm decreased the muscle width by approximately 11% and shifted the midpoint of the pCa-tension relationship (pCa(50)) leftward by about 0.2 pCa units. MgADP (0.1, 1, and 5 mmol/L) augmented maximal and submaximal Ca(2+)-activated tension and concomitantly diminished the SL-dependent shift of pCa(50) in a concentration-dependent manner. In contrast, pimobendan, a Ca(2+) sensitizer, which promotes Ca(2+) binding to troponin C (TnC), exhibited no effect on the SL-dependent shift of pCa(50), suggesting that TnC does not participate in the modulation of SL-dependent tension generation by MgADP. At a SL of 1. 9 microm, osmotic compression, produced by 5% wt/vol dextran (molecular weight approximately 464 000), reduced the muscle width by approximately 13% and shifted pCa(50) leftward to a similar degree as that observed when increasing SL to 2.3 microm. This favors the idea that a decrease in the interfilament lattice spacing is the primary mechanism for SL-dependent tension generation. MgADP (5 mmol/L) markedly attenuated the dextran-induced shift of pCa(50), and the degree of attenuation was similar to that observed in a study of varying SL. The actomyosin-ADP complex (AM.ADP) induced by exogenous MgADP has been reported to cooperatively promote myosin attachment to the thin filament. We hereby conclude that the increase in the number of force-generating crossbridges on a decrease in the lattice spacing is masked by the cooperative effect of AM.ADP, resulting in depressed SL-dependent tension generation.

Alterations in contractile properties and Ca2+ handling in streptozotocin-induced diabetic rat myocardium

The mechanisms of the slower time courses of Ca2+ transients (CaT) and contraction in diabetic (diabetes mellitus, DM) myocardium were studied. The aequorin method was applied to papillary muscles of streptozotocin-induced DM and control rats. The time courses of CaT and tension of twitch in DM were slower than those in control, although the magnitudes of the CaT and contraction were identical. The dependence of CaT decay time and relaxation time on developed tension in DM and control rats differed. The length-tension relation in twitch and the pCa-tension relation in tetanus were identical in the two groups. The magnitude of extra Ca2+ (transient increase in intracellular Ca2+ concentration induced by a quick release in tetanus) was identical in both groups. pCa-tension relations of skinned trabeculae at different sarcomere lengths were nearly identical. The cross-bridge cycling rate (CCR) in DM was slower than that in control. These results indicate that the tension-dependent change in the Ca2+ affinity of troponin C in DM myocardium functions as in control myocardium. The slower time courses of CaT and tension in DM myocardium are caused by slower Ca2+ uptake by the sarcoplasmic reticulum and the slower CCR.

Intracellular acidosis modulates the stretch-induced changes in E-C coupling of the rat atrium

By inducing a small reduction of the intracellular pH (0.18 units) with 20 mmol L-1 propionate we demonstrated that acidification changed the responses of isolated rat atria to stretch. Stretch (increase of the intra-atrial pressure) in normal pH increased the Ca2+ transients' amplitude (Indo-1 fluorescence) from 0.26 +/- 0.09 in 1 mmHg to 0.36 +/- 0.13 in 4 mmHg (P < 0.05, n=6), without affecting the diastolic [Ca2+]i level (n.s. n=6). The changes in Ca2+ balance during stretch were accompanied by a biphasic increase in the contraction force. Five minutes of continuous stretch increased the action potential duration (APD90%, P < 0.01, n=13) and decreased the APD15% (P < 0.001, n=13). During acidosis, the stretch-induced increase of the Ca2+ transient amplitude (0.4 +/- 0. 13 vs. 0.3 +/- 0.08, P < 0.05, n=6) was accompanied by the increase of the diastolic [Ca2+]i (1.16 +/- 0.07, P < 0.05, n=6) compared with non-acidotic control (1.06 +/- 0.06, n=6). Acidic intracellular pH also inhibited the stretch-induced changes in the action potentials (n=10) and slowed down the development of the contractile force during stretch. The results showed that acidosis modulates the mechanotransduction. It does this by interfering with the intracellular Ca2+ balance, inhibiting the Ca2+ extrusion mechanisms and reducing the Ca2+-buffering power of the cells. The physiological and pathological processes associated with stretch are therefore modulated by intracellular pH owing to its concerted effects on intracellular Ca2+ handling caused by a competitive inhibition of various Ca2+-binding molecules.