Model Of Excitation-contraction Coupling Research Articles

Quantitative male-to-female translation of cardiac electrophysiological response to drugs.

Despite evidence that women are at higher risk of drug-induced Torsade de Pointes and sudden cardiac death, female sex is vastly underrepresented in cardiovascular research studies and clinical trials, thus limiting our fundamental understanding of sex-specific arrhythmia mechanisms and our ability to predict arrhythmia propensity. Indeed, we have shown that adoption of male-biased metrics can lead to a systematic underestimation of torsadogenic risk in women. To facilitate evaluation of the impact of sex differences on cardiomyocyte function and arrhythmogenesis, we developed quantitative tools that predict the electrophysiological response to drug administration in female cardiomyocytes starting from data collected in males. Using our mathematical models of excitation-contraction coupling in male and female human ventricular myocytes, we generated populations of models by randomly varying baseline model parameters. We collected action potential (AP) and Ca2+ transient (CaT) properties in each model variants and used multivariable regression to construct a “male-to-female translator”, i.e., a set of regression coefficients mapping male features onto female electrophysiology. We validated our translator against independent simulated data sets describing the effects of a large group of drugs on the baseline male and female models. Our results show high accuracy of prediction of drug-induced effects on female AP and CaT, where the average discrepancy between predicted and actual biomarkers falls within 1-2%. Next, we will use "male-to-female" translated features as inputs into our previously developed female-specific classifier of drug-induced torsadogenic risk to test the suitability of our translator for cardiac safety assessment in women. Classifier performance obtained with simulated data as input features will be used as a benchmark. Completion of this work will facilitate the translatability of male-biased experimental observations to female physiology and advance the characterization of sex differences in cardiac electrophysiology and drug cardiotoxicity.

Understanding Calmodulin Variants Affecting Calcium-Dependent Inactivation of L-Type Calcium Channels through Whole-Cell Simulation of the Cardiac Ventricular Myocyte.

Mutations in the calcium-sensing protein calmodulin (CaM) have been linked to two cardiac arrhythmia diseases, Long QT Syndrome 14 (LQT14) and Catecholaminergic Polymorphic Ventricular Tachycardia Type 4 (CPVT4), with varying degrees of severity. Functional characterization of the CaM mutants most strongly associated with LQT14 show a clear disruption of the calcium-dependent inactivation (CDI) of the L-Type calcium channel (LCC). CPVT4 mutants on the other hand are associated with changes in their affinity to the ryanodine receptor. In clinical studies, some variants have been associated with both CPVT4 and LQT15. This study uses simulations in a model for excitation-contraction coupling in the rat ventricular myocytes to understand how LQT14 variant might give the functional phenotype similar to CPVT4. Changing the CaM-dependent transition rate by a factor of 0.75 corresponding to the D96V variant and by a factor of 0.90 corresponding to the F142L or N98S variants, in a physiologically based stochastic model of the LCC prolonger, the action potential duration changed by a small amount in a cardiac myocyte but did not disrupt CICR at 1, 2, and 4 Hz. Under beta-adrenergic simulation abnormal excitation-contraction coupling was observed above 2 Hz pacing for the mutant CaM. The same conditions applied under beta-adrenergic stimulation led to the rapid onset of arrhythmia in the mutant CaM simulations. Simulations with the LQT14 mutations under the conditions of rapid pacing with beta-adrenergic stimulation drives the cardiac myocyte toward an arrhythmic state known as Ca2+ overload. These simulations provide a mechanistic link to a disease state for LQT14-associated mutations in CaM to yield a CPVT4 phenotype. The results show that small changes to the CaM-regulated inactivation of LCC promote arrhythmia and underscore the significance of CDI in proper heart function.

Computational Model of the Effect of Mitochondrial Dysfunction on Excitation–Contraction Coupling in Skeletal Muscle

It has become well established that mitochondria not only regulate myoplasmic calcium in skeletal muscle, but also use that calcium to stimulate oxidative phosphorylation (OXPHOS). While experimental approaches have allowed for imaging of mitochondrial calcium and membrane potentials in isolated fibers, capturing the role of mitochondria and the impact of mitochondrial impairments on excitation–contraction coupling (ECC) remains difficult to explore in intact muscle. Computational models have been widely used to examine the structure and function of skeletal muscle contraction; however, models of ECC to date lack communication between the myoplasm and mitochondria for regulating calcium and ATP during sustained contractions. To address this, a mathematical model of mitochondrial calcium handling and OXPHOS was integrated into a physiological model of ECC incorporating action potential propagation, calcium handling between the sarcoplasmic reticulum (SR) and the myoplasm, and crossbridge cycling. The model was used to examine the protective role of mitochondria during repeated stimulation and the impact of mitochondrial dysfunction on ECC resulting from progressive OXPHOS inhibition. Pathological myoplasmic calcium accumulation occurred through distinct mechanisms in the model in the case of either electron transport chain, F1F0 ATP synthase, or adenine nucleotide transporter impairments. To investigate the effect of each impairment on force, a model of calcium-stimulated apoptosis was utilized to capture dysfunction-induced reductions in muscle mass, driving whole muscle force loss. The model presented in this study can be used to examine the role of mitochondria in the regulation of calcium, ATP, and force generation during voluntary contraction.

Effects of extracellular potassium on calcium handling and force generation in a model of excitation-contraction coupling in skeletal muscle

It is well-established that extracellular potassium (Ko+) accumulation reduces muscle fiber excitability, however the effects of Ko+ on the excitation–contraction coupling (ECC) pathway are less understood. In vivo and in vitro studies following fatiguing stimulation protocols are limited in their ability to capture the effects of Ko+ on force production in combination with other simultaneously changing factors. To address this, a computational model of ECC for slow and fast twitch muscle is presented to explore the relative contributions of excitability-induced and metabolic-induced changes in force generation in response to increasing K+o. The model incorporates mechanisms previously unexplored in modelling studies, including the effects of extracellular calcium on excitability, calcium-dependent inhibition of calcium release, ATP-dependent ionic pumping, and the contribution of ATP hydrolysis to intracellular phosphate accumulation rate. The model was able to capture the frequency-dependent biphasic Force-K+o response observed experimentally. Force potentiation for moderately elevated K+o was driven by increased action potential duration, myoplasmic calcium potentiation, and phosphate accumulation rate, while attenuation of force at higher K+o was due to action potential failure resulting in reduced calcium release. These results suggest that altered calcium release and phosphate accumulation work together with elevated Ko+ to affect force during sustained contractions.

Modeling and simulation of excitation- contraction coupling of fast-twitch skeletal muscle fibers.

BACKGROUND: The current excitation-contraction coupling model of fast-twitch skeletal muscle fibers cannot completely simulate the excitation-contraction process.OBJECTIVE: To solve this problem, this study proposes an excitation-contraction model of fast-twitch skeletal muscle fibers based on the physiological structure and contractile properties of half-sarcomeres.METHODS: The model includes the action potential model of fast-twitch fiber membranes and transverse tubule membranes, the cycle model of in myofibril, the cross-bridge cycle model, and the fatigue model of metabolism.RESULTS: Finally, detailed analyses of the results from the simulation are conducted using the Simulink toolbox in MATLAB. Two conditions, non-coincidence and coincidence, are analyzed for both the thick and thin myofilaments.CONCLUSIONS: The simulation results of two groups of models are the same as the previous research results, which validates the accuracy of models.

Ryanodine Receptor Sensitivity Governs the Stability and Synchrony of Local Calcium Release during Cardiac Excitation-Contraction Coupling

Synchronous coordination of calcium-induced calcium release (CICR) across thousands of calcium release units (CRUs) within the cardiomyocyte is critical for normal excitation-contraction coupling (ECC). Here we utilize a novel “local control” mathematical model of ECC, designed to simulate a mouse cardiomyocyte, to investigate physiological and pathophysiological calcium (Ca2+) signaling. The model contains 20,000 independent CRUs each composed of 6 stochastically gated L-type Ca2+ channels (LCCs) in the transverse tubule membrane that are positioned across a local “dyadic subspace” from a junctional sarcoplasmic reticulum (JSR) compartment containing a cluster of 50 stochastically gated ryanodine receptors (RyR2s). Action potential depolarization of the sarcolemmal membrane activates LCC openings which elevate subspace [Ca2+] ([Ca2+]ds) across each of the cell's 20,000 CRUs. This rise in [Ca2+]ds promotes RyR2 opening to initiate the fundamental element of triggered Ca2+ release, the Ca2+ spark. The mechanistic design and true SR Ca2+ pump/leak balance displayed by our model allows us to quantify ECC fidelity and Ca2+ spark fidelity (the probability that a LCC opening or RyR2 opening induces a Ca2+ spark, respectively). We study the effect of excess SR Ca2+ leak in the context of “catecholaminergic polymorphic ventricular tachycardia (CPVT)” caused by the R33Q mutation in calsequestrin (CASQ2). CPVT is simulated by increasing RyR2 sensitivity to [Ca2+]i and reducing JSR Ca2+ buffering capacity to levels identified by prior studies of the CASQ2-R33Q “knock-in” mice. Our model effectively reproduces both normal and arrhythmogenic CPVT RyR2 [Ca2+]i sensitivity, local Ca2+ spark activity, and global Ca2+ signals. Under CPVT conditions enhanced RyR2 open probability is critical for the development of “unstable”, quiescent Ca2+ sparks that fail to terminate robustly as well as asynchronous systolic Ca2+ spark activity and increased diastolic SR Ca2+ leak.

Excitation-Metabolism Coupling in Mouse Heart

A beating heart must balance energy consumption with production. This balance requires cellular ATP production to respond dynamically to changes in cardiac output and energy demand. While it is well understood that mitochondrial oxidative phosphorylation serves as the primary pathway for ATP production in heart, the mechanisms that govern the regulation of mitochondrial metabolism remain unclear. Calcium (Ca2+), specifically Ca2+ in the mitochondrial matrix ([Ca2+]m), has been implicated as a likely signaling pathway for this regulation. [Ca2+]m has been suggested to regulate nearly every stage of mitochondrial metabolism including the activity of the tricarboxylic acid cycle (TCA) via Ca2+-sensitive dehydrogenases (DHOs), critical proteins in the electron transport chain (ETC), and even the F1/F0 ATP synthase itself. Here we expand our local control mathematical model of excitation-contraction coupling in mouse heart to investigate excitation-metabolism coupling. The model features mechanistic initiation and termination of Ca2+ sparks and cytosolic Ca2+ ([Ca2+]i) transients in a system that maintains sarcoplasmic reticulum (SR) Ca2+ pump/leak balance. Mitochondrial Ca2+ uptake and export is simulated using experimentally constrained formulations of the mitochondrial Ca2+ uniporter (MCU) and the mitochondrial sodium/Ca2+ exchanger (NCLX). The model employs key mechanisms of ATP consumption (i.e. SR Ca2+ ATPase and myosin ATPase) and buffering (phosphocreatine) as well as a Ca2+ dependent model for mitochondrial ATP generation. We investigated mitochondrial Ca2+ dynamics at physiological pacing frequencies for mouse heart and show that changes in ATP consumption can be translated to increased energy production through [Ca2+]m signals. Our model provides new insights into mitochondrial Ca2+ dynamics and how these [Ca2+]m signals may function to preserve energy homeostasis in the face of increased demand.

Calpain Inhibition Reduces Amplitude and Accelerates Decay of the Late Sodium Current in Ventricular Myocytes from Dogs with Chronic Heart Failure

Calpain is an intracellular Ca2+ -activated protease that is involved in numerous Ca2+ dependent regulation of protein function in many cell types. This paper tests a hypothesis that calpains are involved in Ca2+ -dependent increase of the late sodium current (INaL) in failing heart. Chronic heart failure (HF) was induced in 2 dogs by multiple coronary artery embolization. Using a conventional patch-clamp technique, the whole-cell INaL was recorded in enzymatically isolated ventricular cardiomyocytes (VCMs) in which INaL was activated by the presence of a higher (1μM) intracellular [Ca2+] in the patch pipette. Cell suspensions were exposed to a cell- permeant calpain inhibitor MDL-28170 for 1–2 h before INaL recordings. The numerical excitation-contraction coupling (ECC) model was used to evaluate electrophysiological effects of calpain inhibition in silico. MDL caused acceleration of INaL decay evaluated by the two-exponential fit (τ1 = 42±3.0 ms τ2 = 435±27 ms, n = 6, in MDL vs. τ1 = 52±2.1 ms τ2 = 605±26 control no vehicle, n = 11, and vs. τ1 = 52±2.8 ms τ2 = 583±37 ms n = 7, control with vehicle, P<0.05 ANOVA). MDL significantly reduced INaL density recorded at –30 mV (0.488±0.03, n = 12, in control no vehicle, 0.4502±0.0210, n = 9 in vehicle vs. 0.166±0.05pA/pF, n = 5, in MDL). Our measurements of current-voltage relationships demonstrated that the INaL density was decreased by MDL in a wide range of potentials, including that for the action potential plateau. At the same time the membrane potential dependency of the steady-state activation and inactivation remained unchanged in the MDL-treated VCMs. Our ECC model predicted that calpain inhibition greatly improves myocyte function by reducing the action potential duration and intracellular diastolic Ca2+ accumulation in the pulse train.ConclusionsCalpain inhibition reverses INaL changes in failing dog ventricular cardiomyocytes in the presence of high intracellular Ca2+. Specifically it decreases INaL density and accelerates INaL kinetics resulting in improvement of myocyte electrical response and Ca2+ handling as predicted by our in silico simulations.

Multiphysics model of a rat ventricular myocyte: a voltage-clamp study.

BackgroundThe objective of this study is to develop a comprehensive model of the electromechanical behavior of the rat ventricular myocyte to investigate the various factors influencing its contractile response.MethodsHere, we couple a model of Ca2 + dynamics described in our previous work, with a well-known model of contractile mechanics developed by Rice, Wang, Bers and de Tombe to develop a composite multiphysics model of excitation-contraction coupling. This comprehensive cell model is studied under voltage clamp (VC) conditions, since it allows to focus our study on the elaborate Ca2 + signaling system that controls the contractile mechanism.ResultsWe examine the role of various factors influencing cellular contractile response. In particular, direct factors such as the amount of activator Ca2 + available to trigger contraction and the type of mechanical load applied (resulting in isosarcometric, isometric or unloaded contraction) are investigated. We also study the impact of temperature (22 to 38°C) on myofilament contractile response. The critical role of myofilament Ca2 + sensitivity in modulating developed force is likewise studied, as is the indirect coupling of intracellular contractile mechanism with the plasma membrane via the Na + /Ca2 + exchanger (NCX). Finally, we demonstrate a key linear relationship between the rate of contraction and relaxation, which is shown here to be intrinsically coupled over the full range of physiological perturbations.ConclusionsExtensive testing of the composite model elucidates the importance of various direct and indirect modulatory influences on cellular twitch response with wide agreement with measured data on all accounts. Thus, the model provides mechanistic insights into whole-cell responses to a wide variety of testing approaches used in studies of cardiac myofilament contractility that have appeared in the literature over the past several decades.

Stochastic Simulation Assessing the Functional Changes Occurring during Heart Failure

Heart failure has a high prevalence in the general population and mortality rates that remain excessive. Heart failure is characterized by a the loss of the ability of the heart to pump blood and an increased propensity for fatal arrhythmias. Changes in excitation-contraction coupling occur in heart failure and involve the down regulation of sarcolemmal potassium channels, increased expression of the sodium-calcium exchanger, decreased SR calcium ATPase expression, as well are remodeling of the T-tubule network. These changes are aggravated by chronically increased beta-adrenergic stimulation which has been characterized as a maladaptive mechanism. In HF, a chronic hyperadrenerigic state results in chronic hyperphosphorlyation of the cardiac ryanodine receptor leading to increased sensitivity for calcium dependent channel activation. We have developed a computational model of excitation-contraction coupling (that improves upon Williams et al., 2011) based on the rat ventricular myocyte that includes the 20,000 calcium release units, each containing a cluster of 49 ryanodine receptors and 7 L-type calcium channels both of which gate stochastically. The model also includes the membrane currents that govern the action potential and the remaining components of calcium dynamics. The model is solved using our Ultra-fast Monte Carlo Algorithm and GPGPUs that enable this massive calculation to be solved efficiently. By implement changes in ion channel and transporter behavior in the model while also including the spatial re-organization of the TT system, we are able to examine how action potentials and Ca2+ signaling changes in HF. This work reveals how cellular Ca2+ instability arises and how late diastolic calcium release events (i.e. Ca2+ sparks) and increased diastolic leak underlie the propensity for development of arrhythmogenic calcium waves at the cellular level.

Post-transcriptional silencing of SCN1B and SCN2B genes modulates late sodium current in cardiac myocytes from normal dogs and dogs with chronic heart failure

The emerging paradigm for Na(+) current in heart failure (HF) is that its transient component (I(NaT)) responsible for the action potential (AP) upstroke is decreased, whereas the late component (I(NaL)) involved in AP plateau is augmented. Here we tested whether Na(v)β(1)- and Na(v)β(2)-subunits can modulate I(NaL) parameters in normal and failing ventricular cardiomyocytes (VCMs). Chronic HF was produced in nine dogs by multiple sequential coronary artery microembolizations, and six dogs served as a control. I(Na) and APs were measured by the whole cell and perforated patch-clamp in freshly isolated and cultured VCMs, respectively. I(NaL) was augmented with slower decay in HF VCMs compared with normal heart VCMs, and these properties remained unchanged within 5 days of culture. Post-transcriptional silencing SCN1B and SCN2B were achieved by virally delivered short interfering RNA (siRNA) specific to Na(v)β(1) and Na(v)β(2). The delivery and efficiency of siRNA were evaluated by green fluorescent protein expression, by the real-time RT-PCR, and Western blots, respectively. Five days after infection, the levels of mRNA and protein for Na(v)β(1) and Na(v)β(2) were reduced by >80%, but mRNA and protein of Na(v)1.5, as well as I(NaT), remained unchanged in HF VCMs. Na(v)β(1)-siRNA reduced I(NaL) density and accelerated I(NaL) two-exponential decay, whereas Na(v)β(2)-siRNA produced an opposite effect in VCMs from both normal and failing hearts. Physiological importance of the discovered I(NaL) modulation to affect AP shape and duration was illustrated both experimentally and by numerical simulations of a VCM excitation-contraction coupling model. We conclude that in myocytes of normal and failing dog hearts Na(v)β(1) and Na(v)β(2) exhibit oppositely directed modulation of I(NaL).

The Role of Reactive Oxygen Species in Contractile Dysfunction Following Ischemia/Reperfusion

Myocardial stunning occurs following ischemia-reperfusion, and is characterized by depressed contractility and elevated resting tension that is distinct from dysfunction associated with tissue damage or death. A wide variety of factors have been shown to contribute to post-ischemic myocardial dysfunction including cytosolic Ca2+ overload, acidosis, and the damaging effects of degradative enzymes and free radicals such as reactive oxygen species (ROS). These mechanisms act over a wide range of timescales. Intracellular Ca2+, Na+, and pH return to near pre-ischemic levels within a matter of minutes, while ROS production persists for up to three hours. The symptoms of myocardial stunning can persist even longer, indicative of a modified redox environment in the myocyte. Studies have revealed a number of sources of ROS production during reperfusion, but the mitochondria's role in regulating the balance of electron-carrying redox couples implicates cellular respiration as a critical source of ROS. In the modeling work presented here, we link a mathematical model of mitochondrial respiration and ROS production with a model of excitation-contraction coupling in the guinea pig myocyte. Experimentally determined ROS-dependent modulation of ryanodine receptors, sarcoplasmic reticulum (SR) Ca2+ ATPase, Na+-K+ ATPase and myofilaments is incorporated into our model. Using this integrated model, we show that contractile dysfunction associated with myocardial stunning persists after ROS-mediated alterations in protein function. ROS-related changes in Na+-K+ ATPase flux lead to altered intracellular Na+, which modulates diastolic intracellular Ca+ via the Na+-Ca2+ exchanger. Reduced SR Ca2+ ATPase activity and increased ryanodine receptor leak cause decreased SR Ca2+ load. Finally, ROS-related alteration of myofilament proteins reduces their Ca2+ sensitivity, which also contributes to decreased contractile force. Taken together, all of these mechanisms lead to contractile dysfunction caused by mitochondria-derived ROS.

Simulating the Electro-Mechanical Behavior of Skeletal Muscles

Computational models of the human body must be accurate enough to use in hypothesis testing or biological function analysis. This is possible only when such models use physiological information from different scales-such as the cell, tissue, and organ levels. This model of excitation-contraction coupling in skeletal muscles links cell-level electro-physiological behavior with organ-level biomechanical behavior.

Late sodium current contributes to diastolic cell Ca2+ accumulation in chronic heart failure

We elucidate the role of late Na+ current (INaL) for diastolic intracellular Ca2+ (DCa) accumulation in chronic heart failure (HF). HF was induced in 19 dogs by multiple coronary artery microembolizations; 6 normal dogs served as control. Ca2+ transients were recorded in field-paced (0.25 or 1.5 Hz) fluo-4-loaded ventricular myocytes (VM). INaL and action potentials were recorded by patch-clamp. Failing VM, but not normal VM, exhibited (1) prolonged action potentials and Ca2+ transients at 0.25 Hz, (2) substantial DCa accumulation at 1.5 Hz, and (3) spontaneous Ca2+ releases, which occurred after 1.5 Hz stimulation trains in ~31% cases. Selective INaL blocker ranolazine (10 microM) or the prototypical Na+ channel blocker tetrodotoxin (2 microM) reversibly improved function of failing VM. The DCa accumulation and the beneficial effect of INaL blockade were reproduced in silico using an excitation-contraction coupling model. We conclude that INaL contributes to diastolic Ca2+ accumulation and spontaneous Ca2+ release in HF.

Modeling Cardiac Action Potential Shortening Driven by Oxidative Stress-Induced Mitochondrial Oscillations in Guinea Pig Cardiomyocytes

Ischemia-induced shortening of the cardiac action potential and its heterogeneous recovery upon reperfusion are thought to set the stage for reentrant arrhythmias and sudden cardiac death. We have recently reported that the collapse of mitochondrial membrane potential (ΔΨ m) through a mechanism triggered by reactive oxygen species (ROS), coupled to the opening of sarcolemmal ATP-sensitive potassium (K ATP) channels, contributes to electrical dysfunction during ischemia-reperfusion. Here we present a computational model of excitation-contraction coupling linked to mitochondrial bioenergetics that incorporates mitochondrial ROS-induced ROS release with coupling between the mitochondrial energy state and electrical excitability mediated by the sarcolemmal K ATP current ( I K,ATP). Whole-cell model simulations demonstrate that increasing the fraction of oxygen diverted from the respiratory chain to ROS production triggers limit-cycle oscillations of ΔΨ m, redox potential, and mitochondrial respiration through the activation of a ROS-sensitive inner membrane anion channel. The periods of transient mitochondrial uncoupling decrease the cytosolic ATP/ADP ratio and activate I K,ATP, consequently shortening the cellular action potential duration and ultimately suppressing electrical excitability. The model simulates emergent behavior observed in cardiomyocytes subjected to metabolic stress and provides a new tool for examining how alterations in mitochondrial oxidative phosphorylation will impact the electrophysiological, contractile, and Ca 2+ handling properties of the cardiac cell. Moreover, the model is an important step toward building multiscale models that will permit investigation of the role of spatiotemporal heterogeneity of mitochondrial metabolism in the mechanisms of arrhythmogenesis and contractile dysfunction in cardiac muscle.

Effect of transmurally heterogeneous myocyte excitation–contraction coupling on canine left ventricular electromechanics

The excitation-contraction coupling properties of cardiac myocytes isolated from different regions of the mammalian left ventricular wall have been shown to vary considerably, with uncertain effects on ventricular function. We embedded a cell-level excitation-contraction coupling model with region-dependent parameters within a simple finite element model of left ventricular geometry to study effects of electromechanical heterogeneity on local myocardial mechanics and global haemodynamics. This model was compared with one in which heterogeneous myocyte parameters were assigned randomly throughout the mesh while preserving the total amount of each cell subtype. The two models displayed nearly identical transmural patterns of fibre and cross-fibre strains at end-systole, but showed clear differences in fibre strains at earlier points during systole. Haemodynamic function, including peak left ventricular pressure, maximal rate of left ventricular pressure development and stroke volume, were essentially identical in the two models. These results suggest that in the intact ventricle heterogeneously distributed myocyte subtypes primarily impact local deformation of the myocardium, and that these effects are greatest during early systole.

Model of Excitation-Contraction Coupling of Rat Neonatal Ventricular Myocytes

The neonatal rat ventricular myocyte culture is one of the most popular experimental cardiac cell models. To our knowledge, the excitation-contraction coupling (ECC) of these cells, i.e., the process linking the electrical activity to the cytosolic Ca 2+ transient and contraction, has not been previously analyzed, nor has it been presented as a complete system in detail. Neonatal cardiomyocytes are in the postnatal developmental stage, and therefore, the features of their ECC differ vastly from those of adult ventricular myocytes. We present the first complete analysis of ECC in these cells by characterizing experimentally the action potential and calcium signaling and developing the first mathematical model of ECC in neonatal cardiomyocytes that we know of. We show that in comparison to adult cardiomyocytes, neonatal cardiomyocytes have long action potentials, heterogeneous cytosolic Ca 2+ signals, weaker sarcoplasmic reticulum Ca 2+ handling, and stronger sarcolemmal Ca 2+ handling, with a significant contribution by the Na +/Ca 2+ exchanger. The developed model reproduces faithfully the ECC of rat neonatal cardiomyocytes with a novel description of spatial cytosolic [Ca 2+] signals. Simulations also demonstrate how an increase in the cell size (hypertrophy) affects the ECC in neonatal cardiomyocytes. This model of ECC in developing cardiomyocytes provides a platform for developing future models of cardiomyocytes at different developmental stages.

Calmodulin Mediates Differential Sensitivity of CaMKII and Calcineurin to Local Ca 2+ in Cardiac Myocytes

Calmodulin (CaM) mediates Ca-dependent regulation of numerous pathways in the heart, including CaM-dependent kinase (CaMKII) and calcineurin (CaN), yet the local Ca 2+ signals responsible for their selective activation are unclear. To assess when and where CaM, CaMKII, and CaN may be activated in the cardiac myocyte, we integrated new mechanistic computational models of CaM, CaMKII, and CaN with the Shannon-Bers model of excitation-contraction coupling in the rabbit ventricular myocyte. These models are validated with independent in vitro data. In the intact myocyte, model simulations predict that CaM is highly activated in the dyadic cleft during each beat, but not appreciably in the cytosol. CaMKII- δ C was almost insensitive to cytosolic Ca due to relatively low CaM affinity. Dyadic cleft CaMKII exhibits dynamic frequency-dependent responses to Ca, yet autophosphorylates only when local phosphatases are suppressed. In contrast, dyadic cleft CaN in beating myocytes is predicted to be constitutively active, whereas the extremely high affinity of CaN for CaM allows gradual integration of small cytosolic CaM signals. Reversing CaM affinities for CaMKII and CaN also reverses their characteristic local responses. Deactivation of both CaMKII and CaN seems dominated by Ca dissociation from the complex (versus Ca-CaM dissociation from the target). In summary, the different affinities of CaM for CaMKII and CaN determine their sensitivity to local Ca signals in cardiac myocytes.

A Dynamic Model of Excitation-Contraction Coupling during Acidosis in Cardiac Ventricular Myocytes

Acidosis in cardiac myocytes is a major factor in the reduced inotropy that occurs in the ischemic heart. During acidosis, diastolic calcium concentration and the amplitude of the calcium transient increase, while the strength of contraction decreases. This has been attributed to the inhibition by protons of calcium uptake and release by the sarcoplasmic reticulum, to a rise of intracellular sodium caused by activation of sodium-hydrogen exchange, decreased calcium binding affinity to Troponin-C, and direct effects on the contractile machinery. The relative contributions and concerted action of these effects are, however, difficult to establish experimentally. We have developed a mathematical model to examine altered calcium-handling mechanisms during acidosis. Each of the alterations was incorporated into a dynamical model of pH regulation and excitation-contraction coupling to predict the time courses of key ionic species during acidosis, in particular intracellular pH, sodium and the calcium transient, and contraction. This modeling study suggests that the most significant effects are elevated sodium, inhibition of sodium-calcium exchange, and the direct interaction of protons with the contractile machinery; and shows how the experimental data on these contributions can be reconciled to understand the overall effects of acidosis in the beating heart.

Development of the excitation-contraction coupling model using mathematical models of human ventricular cell

Development of the excitation-contraction coupling model using mathematical models of human ventricular cell