Contribution of mechanical factors to arrhythmogenesis in calcium overloaded cardiomyocytes: model predictions and experiments

Effect of strain-dependent conduction slowing on the re-entry formation and maintenance in cardiac muscle: 2D computer simulation

The effect of mechano-electrical feedback on re-entry formation and maintenance was studied using a model of myocardial electromechanics that accounts for two components of myocardial conductivity and delayed strain-dependent changes in membrane capacitance that causes a conduction slowing. Two scenarios were simulated in 2D numerical experiments: (i) propagation of an excitation-contraction wave beyond the edge of a nonconductive nonexcitable obstacle; (ii) circulation of a re-entry wave around a nonconductive nonexcitable obstacle. The simulations demonstrated that the delayed strain-dependent deceleration of the conduction waves promotes the detachment of the excitation-contraction waves from the sharp edge of an elongated obstacle and modulates the re-entry waves rotating around a compact obstacle. The data show that the mechano-electrical feedback, together with an increase in the stimulation frequency and an increase in the excitation threshold, is an arrhythmogenic factor that must be taken into account when analyzing the possibility of the re-entry formation.

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.

In silico analysis of the contribution of cardiomyocyte-fibroblast electromechanical interaction to the arrhythmia

Although fibroblasts are about 5–10 times smaller than cardiomyocytes, their number in the ventricle is about twice that of cardiomyocytes. The high density of fibroblasts in myocardial tissue leads to a noticeable effect of their electromechanical interaction with cardiomyocytes on the electrical and mechanical functions of the latter. Our work focuses on the analysis of the mechanisms of spontaneous electrical and mechanical activity of the fibroblast-coupled cardiomyocyte during its calcium overload, which occurs in a variety of pathologies, including acute ischemia. For this study, we developed a mathematical model of the electromechanical interaction between cardiomyocyte and fibroblasts and used it to simulate the impact of overloading cardiomyocytes. In contrast to modeling only the electrical interaction between cardiomyocyte and fibroblasts, the following new features emerge in simulations with the model that accounts for both electrical and mechanical coupling and mechano-electrical feedback loops in the interacting cells. First, the activity of mechanosensitive ion channels in the coupled fibroblasts depolarizes their resting potential. Second, this additional depolarization increases the resting potential of the coupled myocyte, thus augmenting its susceptibility to triggered activity. The triggered activity associated with the cardiomyocyte calcium overload manifests itself in the model either as early afterdepolarizations or as extrasystoles, i.e., extra action potentials and extra contractions. Analysis of the model simulations showed that mechanics contribute significantly to the proarrhythmic effects in the cardiomyocyte overloaded with calcium and coupled with fibroblasts, and that mechano-electrical feedback loops in both the cardiomyocyte and fibroblasts play a key role in this phenomenon.

The effects of the tropomyosin cardiomyopathy mutations on the calcium regulation of actin-myosin interaction in the atrium and ventricle differ

The molecular mechanisms of pathogenesis of atrial myopathy associated with hypertrophic (HCM) and dilated (DCM) mutations of sarcomeric proteins are still poorly understood. For this, one needs to investigate the effects of the mutations on actin-myosin interaction in the atria separately from ventricles. We compared the impact of the HCM and DCM mutations of tropomyosin (Tpm) on the calcium regulation of the thin filament interaction with atrial and ventricular myosin using an in vitro motility assay. We found that the mutations differently affect the calcium regulation of actin-myosin interaction in the atria and ventricles. The DCM E40K Tpm mutation significantly reduced the maximum sliding velocity of thin filaments with ventricular myosin and its Ca²⁺-sensitivity. With atrial myosin, its effects were less pronounced. The HCM I172T mutation reduced the Ca²⁺-sensitivity of the sliding velocity of filaments with ventricular myosin but increased it with the atrial one. The HCM L185R mutation did not affect actin-myosin interaction in the atria. The results indicate that the difference in the effects of Tpm mutations on the actin-myosin interaction in the atria and ventricles may be responsible for the difference in pathological changes in the atrial and ventricular myocardium.

The role of pacing rate in the modulation of mechano-induced immediate and delayed changes in the force and Ca-transient of cardiac muscle

Myocardial function is tuned by dynamic changes in length and load via mechano-calcium feedback. This regulation may be significantly affected by heart rhythm. We evaluated the mechano-induced modulation of contractility and Ca-transient (CaT) in the rat myocardium subjected to twitch-by-twitch shortening-re-lengthening (↓-↑) trains of different lengths (N = 1 … 720 cycles) at low (1 Hz) and near-physiological (3.5 Hz) pacing rates. Force/CaT characteristics were evaluated in the first post-train isometric twitch (immediate effect) and during slow changes (delayed maximal elevation/decrease) and compared with those of the pre-train twitch. The immediate inotropic effect was positive for N = 30 … 720 and negative for N = 1 … 20, while the delayed effect was always positive. The immediate and delayed inotropic effects were significantly higher at 3.5-Hz vs 1-Hz (P < 0.05). The prominent inotropism was accompanied by much smaller changes in the CaT diastolic level/amplitude. The shortening-re-lengthening train induced oscillations of the slow change in force at 3.5-Hz (always) and at 1-Hz (∼50% of muscles), which were dependent of the train length and independent of the pacing rate. We suggest that twitch-by-twitch shortening-re-lengthening of cardiac muscle decreases Ca²⁺ buffering by troponin C and elevates Ca²⁺ loading of the sarcoplasmic reticulum (SR); the latter cumulatively depends on the train length. A high pacing rate intensifies the cumulative transient shift in the SR Ca²⁺ loading, augmenting the post-train inotropic response and prolonging its recovery to the pre-train level. The pacing-dependent mechano-induced inotropic effects remain to be elucidated in the myocardium with impaired Ca handling.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Increased thin filament activation enhances alternans in human chronic atrial fibrillation

Action potential duration (APD) alternans (APD-ALT), defined as beat-to-beat oscillations in APD, has been proposed as an important clinical marker for chronic atrial fibrillation (cAF) risk when it occurs at pacing rates of 120-200 beats/min. Although the ionic mechanisms for occurrence of APD-ALT in human cAF at these clinically relevant rates have been investigated, little is known about the effects of myofilament protein kinetics on APD-ALT. Therefore, we used computer simulations of single cell function to explore whether remodeling in myofilament protein kinetics in human cAF alters the occurrence of APD-ALT and to uncover how these mechanisms are affected by sarcomere length and the degree of cAF-induced myofilament remodeling. Mechanistically based, bidirectionally coupled electromechanical models of human right and left atrial myocytes were constructed, incorporating both ionic and myofilament remodeling associated with cAF. By comparing results from our electromechanical model with those from the uncoupled ionic model, we found that intracellular Ca2+ concentration buffering of troponin C has a dampening effect on the magnitude of APD-ALT (APD-ANM) at slower rates (150 beats/min) due to the cooperativity between strongly bound cross-bridges and Ca2+-troponin C binding affinity. We also discovered that cAF-induced enhanced thin filament activation enhanced APD-ANM at these clinically relevant heart rates (150 beats/min). In addition, longer sarcomere lengths increased APD-ANM, suggesting that atrial stretch is an important modulator of APD-ALT. Together, these findings demonstrate that myofilament kinetics mechanisms play an important role in altering APD-ALT in human cAF. NEW & NOTEWORTHY Using a single cell simulation approach, we explored how myofilament protein kinetics alter the formation of alternans in action potential duration (APD) in human myocytes with chronic atrial fibrillation remodeling. We discovered that enhanced thin filament activation and longer sarcomere lengths increased the magnitude of APD alternans at clinically important pacing rates of 120-200 beats/min. Furthermore, we found that altered intracellular Ca2+ concentration buffering of troponin C has a dampening effect on the magnitude of APD alternans.

Transmural cellular heterogeneity in myocardial electromechanics

Myocardial heterogeneity is an attribute of the normal heart. We have developed integrative models of cardiomyocytes from the subendocardial (ENDO) and subepicardial (EPI) ventricular regions that take into account experimental data on specific regional features of intracellular electromechanical coupling in the guinea pig heart. The models adequately simulate experimental data on the differences in the action potential and contraction between the ENDO and EPI cells. The modeling results predict that heterogeneity in the parameters of calcium handling and myofilament mechanics in isolated ENDO and EPI cardiomyocytes are essential to produce the differences in Ca2+ transients and contraction profiles via cooperative mechanisms of mechano-calcium-electric feedback and may further slightly modulate transmural differences in the electrical properties between the cells. Simulation results predict that ENDO cells have greater sensitivity to changes in the mechanical load than EPI cells. These data are important for understanding the behavior of cardiomyocytes in the intact heart.

Investigations of Molecular Mechanisms of Actin-Myosin Interactions in Cardiac Muscle

The functional characteristics of cardiac muscle depend on the composition of protein isoforms in the cardiomyocyte contractile machinery. In the ventricular myocardium of mammals, several isoforms of contractile and regulatory proteins are expressed - two isoforms of myosin (V1 and V3) and three isoforms of tropomyosin chains (α, β, and κ). Expression of protein isoforms depends on the animal species, its age and hormonal status, and this can change with pathologies of the myocardium. Mutations in these proteins can lead to cardiomyopathies. The functional significance of the protein isoform composition has been studied mainly on intact hearts or on isolated preparations of myocardium, which could not provide a clear comprehension of the role of each particular isoform. Present-day experimental techniques such as an optical trap and in vitro motility assay make it possible to investigate the phenomena of interactions of contractile and regulatory proteins on the molecular level, thus avoiding effects associated with properties of a whole muscle or muscle tissue. These methods enable free combining of the isoforms to test the molecular mechanisms of their participation in the actin-myosin interaction. Using the optical trap and the in vitro motility assay, we have studied functional characteristics of the cardiac myosin isoforms, molecular mechanisms of the calcium-dependent regulation of actin-myosin interaction, and the role of myosin and tropomyosin isoforms in the cooperativity mechanisms in myocardium. The knowledge of molecular mechanisms underlying myocardial contractility and its regulation is necessary for comprehension of cardiac muscle functioning, its disorders in pathologies, and for development of approaches for their correction.

Mechano-electric heterogeneity of the myocardium as a paradigm of its function

Myocardial heterogeneity is well appreciated and widely documented, from sub-cellular to organ levels. This paper reviews significant achievements of the group, led by Professor Vladimir S. Markhasin, Russia, who was one of the pioneers in studying and interpreting the relevance of cardiac functional heterogeneity.

The cardiac muscle duplex as a method to study myocardial heterogeneity

This paper reviews the development and application of paired muscle preparations, called duplex, for the investigation of mechanisms and consequences of intra-myocardial electro-mechanical heterogeneity. We illustrate the utility of the underlying combined experimental and computational approach for conceptual development and integration of basic science insight with clinically relevant settings, using previously published and new data. Directions for further study are identified.

Na/K pump regulation of cardiac repolarization: insights from a systems biology approach

The sodium-potassium pump is widely recognized as the principal mechanism for active ion transport across the cellular membrane of cardiac tissue, being responsible for the creation and maintenance of the transarcolemmal sodium and potassium gradients, crucial for cardiac cell electrophysiology. Importantly, sodium-potassium pump activity is impaired in a number of major diseased conditions, including ischemia and heart failure. However, its subtle ways of action on cardiac electrophysiology, both directly through its electrogenic nature and indirectly via the regulation of cell homeostasis, make it hard to predict the electrophysiological consequences of reduced sodium-potassium pump activity in cardiac repolarization. In this review, we discuss how recent studies adopting the systems biology approach, through the integration of experimental and modeling methodologies, have identified the sodium-potassium pump as one of the most important ionic mechanisms in regulating key properties of cardiac repolarization and its rate dependence, from subcellular to whole organ levels. These include the role of the pump in the biphasic modulation of cellular repolarization and refractoriness, the rate control of intracellular sodium and calcium dynamics and therefore of the adaptation of repolarization to changes in heart rate, as well as its importance in regulating pro-arrhythmic substrates through modulation of dispersion of repolarization and restitution. Theoretical findings are consistent across a variety of cell types and species including human, and widely in agreement with experimental findings. The novel insights and hypotheses on the role of the pump in cardiac electrophysiology obtained through this integrative approach could eventually lead to novel therapeutic and diagnostic strategies.

Computational approaches to understand cardiac electrophysiology and arrhythmias

Cardiac rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. These impulses spread throughout the cardiac muscle to manifest as electrical waves in the whole heart. Regularity of electrical waves is critically important since they signal the heart muscle to contract, driving the primary function of the heart to act as a pump and deliver blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. For more than 50 years, mathematically based models of cardiac electrical activity have been used to improve understanding of basic mechanisms of normal and abnormal cardiac electrical function. Computer-based modeling approaches to understand cardiac activity are uniquely helpful because they allow for distillation of complex emergent behaviors into the key contributing components underlying them. Here we review the latest advances and novel concepts in the field as they relate to understanding the complex interplay between electrical, mechanical, structural, and genetic mechanisms during arrhythmia development at the level of ion channels, cells, and tissues. We also discuss the latest computational approaches to guiding arrhythmia therapy.

Cardiac myocytes and local signaling in nano-domains

It is well known that calcium-induced calcium-release in cardiac myocytes takes place in spatially restricted regions known as dyads, where discrete patches of junctional sarcoplasmic reticulum tightly associate with the t-tubule membrane. The dimensions of a dyad are so small that it contains only a few Ca²⁺ ions at any given time. Ca²⁺ signaling in the dyad is therefore noisy, and dominated by the Brownian motion of Ca²⁺ ions in a potential field. Remarkably, from this complexity emerges the integrated behavior of the myocyte in which, under normal conditions, precise control of Ca²⁺ release and muscle contraction is maintained over the life of the cell. This is but one example of how signal processing within the cardiac myocyte and other cells often occurs in small "nano-domains" where proteins and protein complexes interact at spatial dimensions on the order of ∼1-10 nm and at time-scales on the order of nanoseconds to perform the functions of the cell. In this article, we will review several examples of local signaling in nano-domains, how it contributes to the integrative behavior of the cardiac myocyte, and present computational methods for modeling signal processing within these domains across differing spatio-temporal scales.