The Effects of Mechanical Preload on Transmural Differences in Mechano-Calcium-Electric Feedback in Single Cardiomyocytes: Experiments and Mathematical Models

The importance of mechanical conditions in the testing of excitation abnormalities in a population of electro-mechanical models of human ventricular cardiomyocytes

Background: Populations of in silico electrophysiological models of human cardiomyocytes represent natural variability in cell activity and are thoroughly calibrated and validated using experimental data from the human heart. The models have been shown to predict the effects of drugs and their pro-arrhythmic risks. However, excitation and contraction are known to be tightly coupled in the myocardium, with mechanical loads and stretching affecting both mechanics and excitation through mechanisms of mechano-calcium-electrical feedback. However, these couplings are not currently a focus of populations of cell models. Aim: We investigated the role of cardiomyocyte mechanical activity under different mechanical conditions in the generation, calibration, and validation of a population of electro-mechanical models of human cardiomyocytes. Methods: To generate a population, we assumed 11 input parameters of ionic currents and calcium dynamics in our recently developed TP + M model as varying within a wide range. A History matching algorithm was used to generate a non-implausible parameter space by calibrating the action potential and calcium transient biomarkers against experimental data and rejecting models with excitation abnormalities. The population was further calibrated using experimental data on human myocardial force characteristics and mechanical tests involving variations in preload and afterload. Models that passed the mechanical tests were validated with additional experimental data, including the effects of drugs with high or low pro-arrhythmic risk. Results: More than 10% of the models calibrated on electrophysiological data failed mechanical tests and were rejected from the population due to excitation abnormalities at reduced preload or afterload for cell contraction. The final population of accepted models yielded action potential, calcium transient, and force/shortening outputs consistent with experimental data. In agreement with experimental and clinical data, the models demonstrated a high frequency of excitation abnormalities in simulations of Dofetilide action on the ionic currents, in contrast to Verapamil. However, Verapamil showed a high frequency of failed contractions at high concentrations. Conclusion: Our results highlight the importance of considering mechanoelectric coupling in silico cardiomyocyte models. Mechanical tests allow a more thorough assessment of the effects of interventions on cardiac function, including drug testing.

Computational modeling of electromechanical coupling in human cardiomyocyte applied to study hypertrophic cardiomyopathy and its drug response

BACKGROUND AND OBJECTIVE

Knowledge of electromechanical coupling in cardiomyocyte and how it is influenced by various pathophysiological factors is fundamental to understanding the pathogenesis of myocardial disease and its response to medication, which is however hard to be thoroughly addressed by clinical/experimental studies due to technical limitations. At this point, computational modeling offers an alternative approach. The main objective of the study was to develop a computational model capable of simulating the process of electromechanical coupling and quantifying the roles of various factors in play in the human left ventricular cardiomyocyte.

METHODS

A new electrophysiological model was firstly built by combining several existing electrophysiological models and incorporating the mechanism of electrophysiological homeostasis, which was subsequently coupled to models representing the cross-bridge dynamics and active force generation during excitation-contraction coupling and the passive mechanical properties of cardiomyocyte to yield an integrative electromechanical model. Model parameters were calibrated or optimized based on a large amount of experimental data. The resulting model was applied to delineate the characteristics of electromechanical coupling and explore underlying determinant factors in hypertrophic cardiomyopathy (HCM) cardiomyocyte, as well as quantify their changes in response to different medications.

RESULTS

Model predictions captured the major electromechanical characteristics of cardiomyocyte under both normal physiological and HCM conditions. In comparison with normal cardiomyocyte, HCM cardiomyocyte suffered from systemic changes in both electrophysiological and mechanical variables. Numerical simulations of drug response revealed that Mavacamten and Metoprolol could both reduce the active contractility and alleviate calcium overload but had marked differential influences on many other electromechanical variables, which theoretically explained why the two drugs have differential therapeutic effects. In addition, our numerical experiments demonstrated the important role of compensatory ion transport in maintaining electrophysiological homeostasis and regulating cytoplasmic volume.

CONCLUSIONS

A sophisticated computational model has the advantage of providing quantitative and integrative insights for understanding the pathogenesis and drug responses of HCM or other myocardial diseases at the level of cardiomyocyte, and hence may contribute as a useful complement to clinical/experimental studies. The model may also be coupled to tissue- or organ-level models to strengthen the physiological implications of macro-scale numerical simulations.

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.

Irx5 and transient outward K+ currents contribute to transmural contractile heterogeneities in the mouse ventricle

Previous studies have established that fast transmural gradients of transient outward K⁺ current (I_to,f) correlate with regional differences in action potential (AP) profile and excitation-contraction coupling (ECC) with high I_to,f expression in the epimyocardium (EPI) being associated with short APs and low contractility and vice versa. Herein, we investigated the effects of disrupted I_to,f gradient on contractile properties using mouse models of Irx5 knockout (Irx5-KO) for selective I_to,f elevation in the endomyocardium (ENDO) of the left ventricle (LV) and Kcnd2 ablation (K_V4.2-KO) for selective I_to,freduction in the EPI. Irx5-KO mice exhibited decreased global LV contractility in association with reductions in cell shortening and Ca²⁺ transient amplitudes in isolated ENDO but not EPI cardiomyocytes. Moreover, transcriptional profiling revealed that the primary effect of Irx5 ablation on ECC-related genes was to increase I_to,f gene expression (i.e. Kcnd2 and Kcnip2) in the ENDO, but not the EPI. Indeed, K_V4.2-KO mice showed selective increases in cell shortening and Ca²⁺ transients in isolated EPI cardiomyocytes, leading to enhanced ventricular contractility and mice lacking both Irx5 and Kcnd2 displayed elevated ventricular contractility comparable to K_V4.2-KO mice. Our findings demonstrate that the transmural electromechanical heterogeneities in the healthy ventricles depend on the Irx5-dependent I_to,f gradients. These observations provide a useful framework for assessing the molecular mechanisms underlying the alterations in contractile heterogeneity seen in the diseased heart.

Type 1 Diabetes Impairs Cardiomyocyte Contractility in the Left and Right Ventricular Free Walls but Preserves It in the Interventricular Septum

Type 1 diabetes (T1D) leads to ischemic heart disease and diabetic cardiomyopathy. We tested the hypothesis that T1D differently affects the contractile function of the left and right ventricular free walls (LV, RV) and the interventricular septum (IS) using a rat model of alloxan-induced T1D. Single-myocyte mechanics and cytosolic Ca²⁺ concentration transients were studied on cardiomyocytes (CM) from LV, RV, and IS in the absence and presence of mechanical load. In addition, we analyzed the phosphorylation level of sarcomeric proteins and the characteristics of the actin-myosin interaction. T1D similarly affected the characteristics of actin-myosin interaction in all studied regions, decreasing the sliding velocity of native thin filaments over myosin in an in vitro motility assay and its Ca²⁺ sensitivity. A decrease in the thin-filament velocity was associated with increased expression of β-myosin heavy-chain isoform. However, changes in the mechanical function of single ventricular CM induced by T1D were different. T1D depressed the contractility of CM from LV and RV; it decreased the auxotonic tension amplitude and the slope of the active tension–length relationship. Nevertheless, the contractile function of CM from IS was principally preserved.

Optogenetic actuation in ChR2-transduced fibroblasts alter excitation-contraction coupling and mechano-electric feedback in coupled cardiomyocytes: a computational modeling study

With the help of the conventional electrical method and the growing optogenetic technology, cardiac fibroblasts (Fbs) have been verified to couple electrically with working myocytes and bring electrophysiological remodeling changes in them. The intrinsic properties of cardiac functional autoregulation represented by excitation-contraction coupling (ECC) and mechano-electric feedback (MEF) have also been extensively studied. However, the roles of optogenetic stimulation on the characteristics of ECC and MEF in cardiomyocytes (CMs) coupled with Fbs have been barely investigated. In this study, we proposed a combined model composed of three modules to explore these influences. Simulation results showed that (1) during ECC, an increased light duration (LD) strengthened the inflow of ChR2 current and prolonged action potential duration (APD), and extended durations of twitch and internal sarcomere deformation through the decreased dissociation of calcium with troponin C (CaTnC) complexes and the prolonged duration of Xb attachment-detachment; (2) during MEF, an increased LD was followed by a longer muscle twitch and deformation, and led to APD prolongation through the inward ChR2 current and its inward rectification kinetics, which far outweighed the effects of the delaying dissociation of CaTnC complexes and the prolonged reverse mode of Na⁺-Ca²⁺ exchange on AP shortening; (3) due to the ChR2 current's rectification feature, enhancing the light irradiance (LI) brought slight variations in peak or valley values of electrophysiological and mechanical parameters while did not change durations of AP and twitch and muscle deformation in both ECC and MEF. In conclusion, the inward ChR2 current and its inward rectification feature were found to affect significantly the durations of AP and twitch in both ECC and MEF. The roles of optogenetic actuation on both ECC and MEF should be considered in future cardiac computational optogenetics at the tissue and organ scale.

Differing effects of estrogen deficiency on the contractile function of atrial and ventricular myocardium

Estrogen deficiency has a significant influence on the excitation-contraction coupling in the ventricular myocardium but its impact on the atrial contractile function has not been studied. We have compared the effects of estrogen deficiency on the contractility and cytosolic Ca²⁺ transient of single cardiomyocytes isolated from the left atrium (LA) and the left ventricle (LV) of rats subjected to ovariectomy (OVX) or sham surgery (Sham). The characteristics of actin-myosin interaction were studied in an in vitro motility assay. We found that OVX decreased the contractility of LV single cardiomyocytes but increased that of LA myocytes. The disturbance of ventricular mechanical function may be explained by the acceleration of Ca²⁺ transient and reduced Ca²⁺ sensitivity of the actin-myosin interaction. The augmentation of LA contractility may be explained by accelerated cross-bridge kinetics and increased end-diastolic sarcomere length, which may lead to elevated tension in atrial cells due to the Frank-Starling mechanism.

Integrated Ca2+ flux and AFM force analysis in human iPSC-derived cardiomyocytes

We developed a new approach for combined analysis of calcium (Ca²⁺) handling and beating forces in contractile cardiomyocytes. We employed human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from dilated cardiomyopathy (DCM) patients carrying an inherited mutation in the sarcomeric protein troponin T (TnT), and isogenic TnT-KO iPSC-CMs generated via CRISPR/Cas9 gene editing. In these cells, Ca²⁺ handling as well as beating forces and -rates using single-cell atomic force microscopy (AFM) were assessed. We report impaired Ca²⁺ handling and reduced contractile force in DCM iPSC-CMs compared to healthy WT controls. TnT-KO iPSC-CMs display no contractile force or Ca²⁺ transients but generate Ca²⁺ sparks. We apply our analysis strategy to Ca²⁺ traces and AFM deflection recordings to reveal maximum rising rate, decay time, and duration of contraction with a multi-step background correction. Our method provides adaptive computing of signal peaks for different Ca²⁺ flux or force levels in iPSC-CMs, as well as analysis of Ca²⁺ sparks. Moreover, we report long-term measurements of contractile force dynamics on human iPSC-CMs. This approach enables deeper and more accurate profiling of disease-specific differences in cardiomyocyte contraction profiles using patient-derived iPSC-CMs.

In-silico human electro-mechanical ventricular modelling and simulation for drug-induced pro-arrhythmia and inotropic risk assessment

Human-based computational modelling and simulation are powerful tools to accelerate the mechanistic understanding of cardiac patho-physiology, and to develop and evaluate therapeutic interventions. The aim of this study is to calibrate and evaluate human ventricular electro-mechanical models for investigations on the effect of the electro-mechanical coupling and pharmacological action on human ventricular electrophysiology, calcium dynamics, and active contraction. The most recent models of human ventricular electrophysiology, excitation-contraction coupling, and active contraction were integrated, and the coupled models were calibrated using human experimental data. Simulations were then conducted using the coupled models to quantify the effects of electro-mechanical coupling and drug exposure on electrophysiology and force generation in virtual human ventricular cardiomyocytes and tissue. The resulting calibrated human electro-mechanical models yielded active tension, action potential, and calcium transient metrics that are in agreement with experiments for endocardial, epicardial, and mid-myocardial human samples. Simulation results correctly predicted the inotropic response of different multichannel action reference compounds and demonstrated that the electro-mechanical coupling improves the robustness of repolarisation under drug exposure compared to electrophysiology-only models. They also generated additional evidence to explain the partial mismatch between in-silico and in-vitro experiments on drug-induced electrophysiology changes. The human calibrated and evaluated modelling and simulation framework constructed in this study opens new avenues for future investigations into the complex interplay between the electrical and mechanical cardiac substrates, its modulation by pharmacological action, and its translation to tissue and organ models of cardiac patho-physiology.

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.