Cardiac resynchronization therapy improves altered Na channel gating in canine model of dyssynchronous heart failure.

BACKGROUND: Slowed Na⁺ current (INa) decay and enhanced late INa (INa-L) prolong the action potential duration (APD) and contribute to early afterdepolarizations. Cardiac resynchronization therapy (CRT) shortens APD compared with dyssynchronous heart failure (DHF); however, the role of altered Na⁺ channel gating in CRT remains unexplored. METHODS AND RESULTS: Adult dogs underwent left-bundle branch ablation and right atrial pacing (200 beats/min) for 6 weeks (DHF) or 3 weeks followed by 3 weeks of biventricular pacing at the same rate (CRT). INa and INa-L were measured in left ventricular myocytes from nonfailing, DHF, and CRT dogs. DHF shifted voltage-dependence of INa availability by -3 mV compared with nonfailing, enhanced intermediate inactivation, and slowed recovery from inactivation. CRT reversed the DHF-induced voltage shift of availability, partially reversed enhanced intermediate inactivation but did not affect DHF-induced slowed recovery. DHF markedly increased INa-L compared with nonfailing. CRT dramatically reduced DHF-induced enhanced INa-L, abbreviated the APD, and suppressed early afterdepolarizations. CRT was associated with a global reduction in phosphorylated Ca²âº/Calmodulin protein kinase II, which has distinct effects on inactivation of cardiac Na⁺ channels. In a canine AP model, alterations of INa-L are sufficient to reproduce the effects on APD observed in DHF and CRT myocytes. CONCLUSIONS: CRT improves DHF-induced alterations of Na⁺ channel function, especially suppression of INa-L, thus, abbreviating the APD and reducing the frequency of early afterdepolarizations. Changes in the levels of phosphorylated Ca²âº/Calmodulin protein kinase II suggest a molecular pathway for regulation of INa by biventricular pacing of the failing heart.

Computational modeling of interacting VEGF and soluble VEGF receptor concentration gradients.

Experimental data indicates that soluble vascular endothelial growth factor (VEGF) receptor 1 (sFlt-1) modulates the guidance cues provided to sprouting blood vessels by VEGF-A. To better delineate the role of sFlt-1 in VEGF signaling, we have developed an experimentally based computational model. This model describes dynamic spatial transport of VEGF, and its binding to receptors Flt-1 and Flk-1, in a mouse embryonic stem cell model of vessel morphogenesis. The model represents the local environment of a single blood vessel. Our simulations predict that blood vessel secretion of sFlt-1 and increased local sFlt-1 sequestration of VEGF results in decreased VEGF-Flk-1 levels on the sprout surface. In addition, the model predicts that sFlt-1 secretion increases the relative gradient of VEGF-Flk-1 along the sprout surface, which could alter endothelial cell perception of directionality cues. We also show that the proximity of neighboring sprouts may alter VEGF gradients, VEGF receptor binding, and the directionality of sprout growth. As sprout distances decrease, the probability that the sprouts will move in divergent directions increases. This model is a useful tool for determining how local sFlt-1 and VEGF gradients contribute to the spatial distribution of VEGF receptor binding, and can be used in conjunction with experimental data to explore how multi-cellular interactions and relationships between local growth factor gradients drive angiogenesis.

Integrative modeling of the cardiac ventricular myocyte.

Cardiac electrophysiology is a discipline with a rich 50-year history of experimental research coupled with integrative modeling which has enabled us to achieve a quantitative understanding of the relationships between molecular function and the integrated behavior of the cardiac myocyte in health and disease. In this paper, we review the development of integrative computational models of the cardiac myocyte. We begin with a historical overview of key cardiac cell models that helped shape the field. We then narrow our focus to models of the cardiac ventricular myocyte and describe these models in the context of their subcellular functional systems including dynamic models of voltage-gated ion channels, mitochondrial energy production, ATP-dependent and electrogenic membrane transporters, intracellular Ca dynamics, mechanical contraction, and regulatory signal transduction pathways. We describe key advances and limitations of the models as well as point to new directions for future modeling research. WIREs Syst Biol Med 2011 3 392-413 DOI: 10.1002/wsbm.122

CaMKII-dependent activation of late INa contributes to cellular arrhythmia in a model of the cardiac myocyte.

Cardiac voltage-gated Na(+) channels underlie membrane depolarization during the upstroke of the action potential (AP). These channels also exhibit a late, slowly-inactivating component of current (late I(Na)) that may be enhanced under pathological conditions such as heart failure, and may therefore promote AP prolongation and increase the likelihood of arrhythmia. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) functionally modifies Na(+) channels, however it remains unclear if the CaMKII-dependent changes in late I(Na) are a major contributor to cellular arrhythmias such as early after depolarizations (EADs). In this study we develop a model of I(Na), including CaMKII-dependent effects, based on experimental measurements. The Na(+) channel model is incorporated into a computational model of the whole myocyte which describes excitation-contraction coupling via stochastic simulation of individual Ca(2+) release sites. Simulations suggest that relatively small augmentation of late I(Na) is sufficient to significantly prolong APs and lead to the appearance of EADs.

Role of CaMKII in RyR leak, EC coupling and action potential duration: a computational model.

During heart failure, the ability of the sarcoplasmic reticulum (SR) to store Ca(2+) is severely impaired resulting in abnormal Ca(2+) cycling and excitation-contraction (EC) coupling. Recently, it has been proposed that "leaky" ryanodine receptors (RyRs) contribute to diminished Ca(2+) levels in the SR. Various groups have experimentally investigated the effects of RyR phosphorylation mediated by Ca(2+)/calmodulin-dependent kinase II (CaMKII) on RyR behavior. Some of these results are difficult to interpret since RyR gating is modulated by many external proteins and ions, including Ca(2+). Here, we present a mathematical model representing CaMKII-RyR interaction in the canine ventricular myocyte. This is an extension of our previous model which characterized CaMKII phosphorylation of L-type Ca(2+) channels (LCCs) in the cardiac dyad. In this model, it is assumed that upon phosphorylation, RyR Ca(2+)-sensitivity is increased. Individual RyR phosphorylation is modeled as a function of dyadic CaMKII activity, which is modulated by local Ca(2+) levels. The model is constrained by experimental measurements of Ca(2+) spark frequency and steady state RyR phosphorylation. It replicates steady state RyR (leak) fluxes in the range measured in experiments without the addition of a separate passive leak pathway. Simulation results suggest that under physiological conditions, CaMKII phosphorylation of LCCs ultimately has a greater effect on RyR flux as compared with RyR phosphorylation. We also show that phosphorylation of LCCs decreases EC coupling gain significantly and increases action potential duration. These results suggest that LCC phosphorylation sites may be a more effective target than RyR sites in modulating diastolic RyR flux.

CaMKII-induced shift in modal gating explains L-type Ca(2+) current facilitation: a modeling study.

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays an important role in L-type Ca(2+) channel (LCC) facilitation: the Ca(2+)-dependent augmentation of Ca(2+) current (I(CaL)) exhibited during rapid repeated depolarization. Multiple mechanisms may underlie facilitation, including an increased rate of recovery from Ca(2+)-dependent inactivation and a shift in modal gating distribution from mode 1, the dominant mode of LCC gating, to mode 2, a mode in which openings are prolonged. We hypothesized that the primary mechanism underlying facilitation is the shift in modal gating distribution resulting from CaMKII-mediated LCC phosphorylation. We developed a stochastic model describing the dynamic interactions among CaMKII, LCCs, and phosphatases as a function of dyadic Ca(2+) and calmodulin levels, and we incorporated it into an integrative model of the canine ventricular myocyte. The model reproduces behaviors at physiologic protein levels and allows for dynamic transition between modes, depending on the LCC phosphorylation state. Simulations showed that a CaMKII-dependent shift in LCC distribution toward mode 2 accounted for the I(CaL) positive staircase. Moreover, simulations demonstrated that experimentally observed changes in LCC inactivation and recovery kinetics may arise from modal gating shifts, rather than from changes in intrinsic inactivation properties. The model therefore serves as a powerful tool for interpreting I(CaL) experiments.