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Han JC, Pham T, Taberner AJ, Tran K. Cardiac "potential energy" estimation: ambiguous and subjective. J Appl Physiol (1985) 2024; 137:74-77. [PMID: 38450424 DOI: 10.1152/japplphysiol.00761.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2023] [Revised: 02/28/2024] [Accepted: 02/28/2024] [Indexed: 03/08/2024] Open
Affiliation(s)
- June-Chiew Han
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Toan Pham
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Andrew J Taberner
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
- Department of Engineering Science and Biomedical Engineering, The University of Auckland, Auckland, New Zealand
| | - Kenneth Tran
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
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Han JC, Pham T, Taberner AJ, Tran K. Last Word on Viewpoint: Cardiac "potential energy" estimation: ambiguous and subjective. J Appl Physiol (1985) 2024; 137:83-84. [PMID: 38981867 DOI: 10.1152/japplphysiol.00454.2024] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2024] [Accepted: 06/13/2024] [Indexed: 07/11/2024] Open
Affiliation(s)
- June-Chiew Han
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Toan Pham
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Andrew J Taberner
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
- Department of Engineering Science and Biomedical Engineering, The University of Auckland, Auckland, New Zealand
| | - Kenneth Tran
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
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Han JC, Pham T, Taberner AJ, Loiselle DS, Tran K. Resolving an inconsistency in the estimation of the energy for excitation of cardiac muscle contraction. Front Physiol 2023; 14:1269900. [PMID: 38028799 PMCID: PMC10656740 DOI: 10.3389/fphys.2023.1269900] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2023] [Accepted: 09/20/2023] [Indexed: 12/01/2023] Open
Abstract
In the excitation of muscle contraction, calcium ions interact with transmembrane transporters. This process is accompanied by energy consumption and heat liberation. To quantify this activation energy or heat in the heart or cardiac muscle, two non-pharmacological approaches can be used. In one approach using the "pressure-volume area" concept, the same estimate of activation energy is obtained regardless of the mode of contraction (either isovolumic/isometric or ejecting/shortening). In the other approach, an accurate estimate of activation energy is obtained only when the muscle contracts isometrically. If the contraction involves muscle shortening, then an additional component of heat associated with shortening is liberated, over and above that of activation. The present study thus examines the reconcilability of the two approaches by performing experiments on isolated muscles measuring contractile force and heat output. A framework was devised from the experimental data to allow us to replicate several mechanoenergetics results gleaned from the literature. From these replications, we conclude that the choice of initial muscle length (or ventricular volume) underlies the divergence of the two approaches in the estimation of activation energy when the mode of contraction involves shortening (ejection). At low initial muscle lengths, the heat of shortening is relatively small, which can lead to the misconception that activation energy is contraction mode independent. In fact, because cardiac muscle liberates heat of shortening when allowed to shorten, estimation of activation heat must be performed only under isometric (isovolumic) contractions. We thus recommend caution when estimating activation energy using the "pressure-volume area" concept.
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Affiliation(s)
- June-Chiew Han
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Toan Pham
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Andrew J. Taberner
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
- Department of Engineering Science and Biomedical Engineering, The University of Auckland, Auckland, New Zealand
| | - Denis S. Loiselle
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
- Department of Physiology, The University of Auckland, Auckland, New Zealand
| | - Kenneth Tran
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
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Han J, Taberner AJ, Loiselle DS, Tran K. Cardiac efficiency and Starling's Law of the Heart. J Physiol 2022; 600:4265-4285. [PMID: 35998082 PMCID: PMC9826111 DOI: 10.1113/jp283632] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2022] [Accepted: 08/18/2022] [Indexed: 01/11/2023] Open
Abstract
The formulation by Starling of The Law of the Heart states that 'the [mechanical] energy of contraction, however measured, is a function of the length of the muscle fibre'. Starling later also stated that 'the oxygen consumption of the isolated heart … is determined by its diastolic volume, and therefore by the initial length of its muscular fibres'. This phrasing has motivated us to extend Starling's Law of the Heart to include consideration of the efficiency of contraction. In this study, we assessed both mechanical efficiency and crossbridge efficiency by studying the heat output of isolated rat ventricular trabeculae performing force-length work-loops over ranges of preload and afterload. The combination of preload and afterload allowed us, using our modelling frameworks for the end-systolic zone and the heat-force zone, to simulate cases by recreating physiologically feasible loading conditions. We found that across all cases examined, both work output and change of enthalpy increased with initial muscle length; hence it can only be that the former increases more than the latter to yield increased mechanical efficiency. In contrast, crossbridge efficiency increased with initial muscle length in cases where the extent of muscle shortening varied greatly with preload. We conclude that the efficiency of cardiac contraction increases with increasing initial muscle length and preload. An implication of our conclusion is that the length-dependent activation mechanism underlying the cellular basis of Starling's Law of the Heart is an energetically favourable process that increases the efficiency of cardiac contraction. KEY POINTS: Ernest Starling in 1914 formulated the Law of the Heart to describe the mechanical property of cardiac muscle whereby force of contraction increases with muscle length. He subsequently, in 1927, showed that the oxygen consumption of the heart is also a function of the length of the muscle fibre, but left the field unclear as to whether cardiac efficiency follows the same dependence. A century later, the field has gained an improved understanding of the factors, including the distinct effects of preload and afterload, that affect cardiac efficiency. This understanding presents an opportunity for us to investigate the elusive length-dependence of cardiac efficiency. We found that, by simulating physiologically feasible loading conditions using a mechano-energetics framework, cardiac efficiency increased with initial muscle length. A broader physiological importance of our findings is that the underlying cellular basis of Starling's Law of the Heart is an energetically favourable process that yields increased efficiency.
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Affiliation(s)
- June‐Chiew Han
- Auckland Bioengineering InstituteUniversity of AucklandAucklandNew Zealand
| | - Andrew J. Taberner
- Auckland Bioengineering InstituteUniversity of AucklandAucklandNew Zealand,Department of Engineering ScienceUniversity of AucklandAucklandNew Zealand
| | - Denis S. Loiselle
- Auckland Bioengineering InstituteUniversity of AucklandAucklandNew Zealand,Department of PhysiologyUniversity of AucklandAucklandNew Zealand
| | - Kenneth Tran
- Auckland Bioengineering InstituteUniversity of AucklandAucklandNew Zealand
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5
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Garrett AS, Loiselle DS, Taberner AJ, Han JC. Slower shortening kinetics of cardiac muscle performing Windkessel work‑loops increases mechanical efficiency. Am J Physiol Heart Circ Physiol 2022; 323:H461-H474. [PMID: 35904884 DOI: 10.1152/ajpheart.00074.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Conventional experimental methods for studying cardiac muscle in vitro often do not expose the tissue preparations to a mechanical impedance that resembles the in vivo hemodynamic impedance dictated by the arterial system. That is, the afterload in work‑loop contraction is conventionally simplified to be constant throughout muscle shortening, and at a magnitude arbitrarily defined. This conventional afterload does not capture the time‑varying interaction between the left ventricle and the arterial system. We have developed a contraction protocol for isolated tissue experiments that allows the afterload to be described within a Windkessel framework that captures the mechanics of the large arteries. We aim to compare the energy expenditure of cardiac muscle undergoing the two contraction protocols: conventional versus Windkessel loading. Isolated rat left‑ventricular trabeculae were subjected to the two force-length work‑loop contractions. Mechanical work and heat liberation were assessed, and mechanical efficiency quantified, over wide ranges of afterloads or peripheral resistances. Both extent of shortening and heat output were unchanged between protocols, but peak shortening velocity was 39.0 % lower and peak work output was 21.8 % greater when muscles contracted against the Windkessel afterload than against the conventional isotonic afterload. The greater work led to a 25.2 % greater mechanical efficiency. Our findings demonstrate that the mechanoenergetic performance of cardiac muscles in vitro may have been previously constrained by the conventional, arbitrary, loading method. A Windkessel loading protocol, by contrast, unleashes more cardiac muscle mechanoenergetic potential, where the slower shortening increases efficiency in performing mechanical work.
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Affiliation(s)
- Amy S Garrett
- Auckland Bioengineering Institute, The University of Auckland, New Zealand
| | - Denis S Loiselle
- Auckland Bioengineering Institute, The University of Auckland, New Zealand.,Department of Physiology, The University of Auckland, New Zealand
| | - Andrew J Taberner
- Auckland Bioengineering Institute, The University of Auckland, New Zealand.,Department of Engineering Science, The University of Auckland, New Zealand
| | - June-Chiew Han
- Auckland Bioengineering Institute, The University of Auckland, New Zealand
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Fajardo G, Coronado M, Matthews M, Bernstein D. Mitochondrial Quality Control in the Heart: The Balance between Physiological and Pathological Stress. Biomedicines 2022; 10:biomedicines10061375. [PMID: 35740401 PMCID: PMC9220167 DOI: 10.3390/biomedicines10061375] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2022] [Revised: 06/02/2022] [Accepted: 06/06/2022] [Indexed: 12/12/2022] Open
Abstract
Alterations in mitochondrial function and morphology are critical adaptations to cardiovascular stress, working in concert in an attempt to restore organelle-level and cellular-level homeostasis. Processes that alter mitochondrial morphology include fission, fusion, mitophagy, and biogenesis, and these interact to maintain mitochondrial quality control. Not all cardiovascular stress is pathologic (e.g., ischemia, pressure overload, cardiotoxins), despite a wealth of studies to this effect. Physiological stress, such as that induced by aerobic exercise, can induce morphologic adaptations that share many common pathways with pathological stress, but in this case result in improved mitochondrial health. Developing a better understanding of the mechanisms underlying alterations in mitochondrial quality control under diverse cardiovascular stressors will aid in the development of pharmacologic interventions aimed at restoring cellular homeostasis.
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Affiliation(s)
- Giovanni Fajardo
- Department of Pediatrics and the Cardiovascular Institute, Stanford University, Stanford, CA 94305, USA;
| | | | - Melia Matthews
- Department of Biomedical and Biological Sciences, Cornell University, Ithaca, NY 14850, USA;
| | - Daniel Bernstein
- Department of Pediatrics and the Cardiovascular Institute, Stanford University, Stanford, CA 94305, USA;
- Correspondence: ; Tel.: +1-650-723-7913
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Chen Y, Li P, Peng Y, Xie X, Zhang Y, Jiang Y, Li T, Qin X, Li S, Yang H, Wu C, Zheng C, Zhu J, You F, Liu Y. Protective autophagy attenuates soft substrate-induced apoptosis through ROS/JNK signaling pathway in breast cancer cells. Free Radic Biol Med 2021; 172:590-603. [PMID: 34242793 DOI: 10.1016/j.freeradbiomed.2021.07.005] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/22/2021] [Accepted: 07/03/2021] [Indexed: 02/06/2023]
Abstract
Tumor microenvironments are characterized not only in terms of chemical composition, but also by physical properties such as stiffness, which influences morphology, proliferation, and fate of tumor cells. However, the underlying mechanisms between matrix stiffness and the apoptosis-autophagy balance remain largely unexplored. In this study, we cultured human breast cancer MDA-MB-231 cells on rigid (57 kPa), stiff (38 kPa) or soft (10 kPa) substrates and demonstrated that increasing autophagy levels and autophagic flux in the cells cultured on soft substrates partly attenuated soft substrate-induced apoptosis. Mechanistically, this protective autophagy is regulated by intracellular reactive oxygen species (ROS) accumulation, which triggers the downstream signals of JNK, Bcl-2 and Beclin-1. More importantly, soft substrate-induced activation of ROS/JNK signaling promotes cell apoptosis through the mitochondrial pathway, whereas it increases protective autophagy by suppressing the interaction of Bcl-2 and Beclin-1. Taken together, our data suggest that JNK is the mediator of soft substrate-induced breast cancer cell apoptosis and autophagy which is likely to be the mechanism that partly attenuates mitochondrial apoptosis. This study provides new insights into the molecular mechanism by which autophagy plays a protective role against soft substrate-induced apoptosis in human breast cancer cells.
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Affiliation(s)
- Yu Chen
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Ping Li
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Yueting Peng
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Xiaoxue Xie
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Yixi Zhang
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Ying Jiang
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Tingting Li
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Xiang Qin
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Shun Li
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Hong Yang
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Chunhui Wu
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China
| | - Chuan Zheng
- TCM Regulating Metabolic Diseases Key Laboratory of Sichuan Province, Hospital of Chengdu University of Traditional Chinese Medicine, No. 39 Shi-er-qiao Road, Chengdu, 610072, Sichuan, PR China
| | - Jie Zhu
- TCM Regulating Metabolic Diseases Key Laboratory of Sichuan Province, Hospital of Chengdu University of Traditional Chinese Medicine, No. 39 Shi-er-qiao Road, Chengdu, 610072, Sichuan, PR China
| | - Fengming You
- TCM Regulating Metabolic Diseases Key Laboratory of Sichuan Province, Hospital of Chengdu University of Traditional Chinese Medicine, No. 39 Shi-er-qiao Road, Chengdu, 610072, Sichuan, PR China
| | - Yiyao Liu
- Department of Biophysics, School of Life Science and Technology, University of Electronic Science and Technology of China, Chengdu, 610054, Sichuan, PR China; TCM Regulating Metabolic Diseases Key Laboratory of Sichuan Province, Hospital of Chengdu University of Traditional Chinese Medicine, No. 39 Shi-er-qiao Road, Chengdu, 610072, Sichuan, PR China.
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8
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Han JC, Tran K, Crossman DJ, Curl CL, Koutsifeli P, Neale JPH, Li X, Harrap SB, Taberner AJ, Delbridge LMD, Loiselle DS, Mellor KM. Cardiac mechanical efficiency is preserved in primary cardiac hypertrophy despite impaired mechanical function. J Gen Physiol 2021; 153:212441. [PMID: 34180944 PMCID: PMC8241536 DOI: 10.1085/jgp.202012841] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2020] [Accepted: 06/15/2021] [Indexed: 01/19/2023] Open
Abstract
Increased heart size is a major risk factor for heart failure and premature mortality. Although abnormal heart growth subsequent to hypertension often accompanies disturbances in mechano-energetics and cardiac efficiency, it remains uncertain whether hypertrophy is their primary driver. In this study, we aimed to investigate the direct association between cardiac hypertrophy and cardiac mechano-energetics using isolated left-ventricular trabeculae from a rat model of primary cardiac hypertrophy and its control. We evaluated energy expenditure (heat output) and mechanical performance (force length work production) simultaneously at a range of preloads and afterloads in a microcalorimeter, we determined energy expenditure related to cross-bridge cycling and Ca2+ cycling (activation heat), and we quantified energy efficiency. Rats with cardiac hypertrophy exhibited increased cardiomyocyte length and width. Their trabeculae showed mechanical impairment, evidenced by lower force production, extent and kinetics of shortening, and work output. Lower force was associated with lower energy expenditure related to Ca2+ cycling and to cross-bridge cycling. However, despite these changes, both mechanical and cross-bridge energy efficiency were unchanged. Our results show that cardiac hypertrophy is associated with impaired contractile performance and with preservation of energy efficiency. These findings provide direction for future investigations targeting metabolic and Ca2+ disturbances underlying cardiac mechanical and energetic impairment in primary cardiac hypertrophy.
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Affiliation(s)
- June-Chiew Han
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - Kenneth Tran
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - David J Crossman
- Department of Physiology, University of Auckland, Auckland, New Zealand
| | - Claire L Curl
- Department of Physiology, University of Melbourne, Melbourne, Australia
| | - Parisa Koutsifeli
- Department of Physiology, University of Auckland, Auckland, New Zealand
| | - Joshua P H Neale
- Department of Physiology, University of Auckland, Auckland, New Zealand
| | - Xun Li
- Department of Physiology, University of Auckland, Auckland, New Zealand
| | - Stephen B Harrap
- Department of Physiology, University of Melbourne, Melbourne, Australia
| | - Andrew J Taberner
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand.,Department of Engineering Science, University of Auckland, Auckland, New Zealand
| | - Lea M D Delbridge
- Department of Physiology, University of Melbourne, Melbourne, Australia
| | - Denis S Loiselle
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand.,Department of Physiology, University of Auckland, Auckland, New Zealand
| | - Kimberley M Mellor
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand.,Department of Physiology, University of Auckland, Auckland, New Zealand
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Mellor NG, Pham T, Tran K, Loiselle DS, Ward M, Taberner AJ, Crossman DJ, Han J. Disruption of transverse-tubular network reduces energy efficiency in cardiac muscle contraction. Acta Physiol (Oxf) 2021; 231:e13545. [PMID: 32757472 DOI: 10.1111/apha.13545] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2020] [Revised: 07/16/2020] [Accepted: 07/31/2020] [Indexed: 11/29/2022]
Abstract
AIM Altered organization of the transverse-tubular network is an early pathological event occurring even prior to the onset of heart failure. Such t-tubular remodelling disturbs the synchrony and signalling between membranous and intracellular ion channels, exchangers, receptors and ATPases essential in the dynamics of excitation-contraction coupling, leading to ionic abnormality and mechanical dysfunction in heart disease progression. In this study, we investigated whether a disrupted t-tubular network has a direct effect on cardiac mechano-energetics. Our aim was to understand the fundamental link between t-tubular remodelling and impaired energy metabolism, both of which are characteristics of heart failure. We thus studied healthy tissue preparations in which cellular processes are not altered by any disease event. METHODS We exploited the "formamide-detubulation" technique to acutely disrupt the t-tubular network in rat left-ventricular trabeculae. We assessed the energy utilization by cellular Ca2+ cycling and by crossbridge cycling, and quantified the change of energy efficiency following detubulation. For these measurements, trabeculae were mounted in a microcalorimeter where force and heat output were simultaneously measured. RESULTS Following structural disorganization from detubulation, muscle heat output associated with Ca2+ cycling was reduced, indicating impaired intracellular Ca2+ homeostasis. This led to reduced force production and heat output by crossbridge cycling. The reduction in force-length work was not paralleled by proportionate reduction in the heat output and, as such, energy efficiency was reduced. CONCLUSIONS These results reveal the direct energetic consequences of disrupted t-tubular network, linking the energy disturbance and the t-tubular remodelling typically observed in heart failure.
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Affiliation(s)
- Nicholas G. Mellor
- Auckland Bioengineering Institute The University of Auckland Auckland New Zealand
| | - Toan Pham
- Auckland Bioengineering Institute The University of Auckland Auckland New Zealand
| | - Kenneth Tran
- Auckland Bioengineering Institute The University of Auckland Auckland New Zealand
| | - Denis S. Loiselle
- Auckland Bioengineering Institute The University of Auckland Auckland New Zealand
- Department of Physiology The University of Auckland Auckland New Zealand
| | - Marie‐Louise Ward
- Department of Physiology The University of Auckland Auckland New Zealand
| | - Andrew J. Taberner
- Auckland Bioengineering Institute The University of Auckland Auckland New Zealand
- Department of Engineering Science The University of Auckland Auckland New Zealand
| | - David J. Crossman
- Department of Physiology The University of Auckland Auckland New Zealand
| | - June‐Chiew Han
- Auckland Bioengineering Institute The University of Auckland Auckland New Zealand
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10
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Guidry ME, Nickerson DP, Crampin EJ, Nash MP, Loiselle DS, Tran K. Insights From Computational Modeling Into the Contribution of Mechano-Calcium Feedback on the Cardiac End-Systolic Force-Length Relationship. Front Physiol 2020; 11:587. [PMID: 32547426 PMCID: PMC7273927 DOI: 10.3389/fphys.2020.00587] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2019] [Accepted: 05/11/2020] [Indexed: 11/23/2022] Open
Abstract
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.
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Affiliation(s)
- Megan E Guidry
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - David P Nickerson
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Edmund J Crampin
- Systems Biology Laboratory, School of Mathematics and Statistics, The University of Melbourne, Melbourne, VIC, Australia.,ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, School of Chemical and Biomedical Engineering, The University of Melbourne, Melbourne, VIC, Australia
| | - Martyn P Nash
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand.,Department of Engineering Science, The University of Auckland, Auckland, New Zealand
| | - Denis S Loiselle
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand.,Department of Physiology, The University of Auckland, Auckland, New Zealand
| | - Kenneth Tran
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
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