Heart failure (HF) is a chronic condition characterized by impaired contractility, metabolic imbalance, and increased propensity for arrhythmias. Ca/calmodulin-dependent protein kinase II (CaMKII) hyperactivation in HF increases intracellular Ca levels, thereby promoting Ca-dependent arrhythmias. Pathologic Na accumulation, also mediated by CaMKII phosphorylation of Na channels, is another hallmark of HF, and contributes to Ca overload and further increase in CaMKII activity. Cytosolic Na loading in HF also reduces mitochondrial Ca, which hampers the activation of Krebs cycle dehydrogenases resulting in pronounced oxidation of NADH to NAD+ during transitions of workload. Increased formation of reactive oxygen species (ROS) is further expected to promote CaMKII activation, thereby exacerbating the arrhythmia phenotype. To investigate quantitatively the role of these positive feedback loops in arrhythmogenesis in HF, we developed a biophysically detailed model of the rabbit ventricular myocyte in both healthy and failing conditions. Our model, developed and validated using experimental biomarkers observed in an established rabbit model of pressure- and volume overload-induced HF, integrates description of membrane electrophysiology, Na and Ca handling, CaMKII- and cAMP-dependent signaling, and mitochondrial energetic and ROS production. Our simulations confirmed the existence of the positive CaMKII-Na-Ca-ROS-CaMKII feedback loop, showing its role in the generation of early and delayed afterdepolarizations. Our data also suggest that CaMKII hyperactivation in HF and concomitant increase in β-adrenergic tone can further worsen its pro-arrhythmic impact. By using the new modeling framework to systematically assess the specific role of each node (or relationship) in this complex signaling network in promoting Ca and voltage instabilities, our analysis might reveal new anti-arrhythmic targets for HF patients.

Animal experimentation is key in the evaluation of cardiac efficacy and safety of novel therapeutic compounds. However, interspecies differences in the mechanisms regulating excitation-contraction coupling can limit the translation of experimental findings from animal models to human physiology and undermine the assessment of drugs’ efficacy and safety. Here, we built a suite of translators for quantitatively mapping electrophysiological responses in ventricular myocytes across species. We trained these statistical operators using a broad dataset obtained by simulating populations of our biophysically detailed computational models of action potential and Ca 2+ transient in mouse, rabbit, and human. We then tested our translators against experimental data describing the response to stimuli, such as ion channel block, change in beating rate, and β-adrenergic challenge. We demonstrate that this approach is well suited to predicting the effects of perturbations across different species or experimental conditions and suggest its integration into mechanistic studies and drug development pipelines.

Heart failure (HF) is a complex disease characterized by abnormal contraction, metabolic imbalance, and increased propensity for arrhythmias. Dysregulation of intracellular Na+ handling is a major (yet understudied) aspect of HF-induced remodeling of cardiac myocytes. Elevated late Na+ current (INaL) in HF prolongs the action potential (AP), thereby facilitating the development of arrhythmogenic early afterdepolarizations. Increased Na+ loading limits the ability of the Na+/Ca2+ exchanger to remove Ca2+, which along with the reduced sarcoplasmic reticulum (SR) Ca2+ uptake and increased diastolic SR Ca2+ leak leads to Ca2+ overload, thus contributing to diastolic dysfunction and triggered arrhythmias (i.e., via delayed afterdepolarizations). Increased Ca2+ signals enhance the activity of the Ca2+/calmodulin-dependent protein kinase II (CaMKII), which is upregulated and chronically active in HF and directly promotes INaL, diastolic Na+ influx and SR Ca2+ leak. To investigate quantitatively this vicious cycle of positive feedback in HF, we updated our computational model of the failing rabbit ventricular myocyte. We modified the main repolarizing and depolarizing currents to reproduce the HF-induced changes measured during AP-clamp experiments performed with physiologic Ca2+ handling ± CaMKII inhibition. We validated the cellular model using data describing the frequency-dependence of AP and Ca2+ transient properties assessed in normal condition and when various branches of the feedback loop are blocked. This updated model serves as a framework to investigate the role of the CaMKII-Na+-Ca2+-CaMKII feedback in promoting Ca2+ and AP instabilities. Analysis of the relative roles of the interacting components that form the feedback loop within the integrated AP-Ca2+ cycling-signaling model will allow the identification of the key relationships in the signaling network that could be targeted therapeutically to limit arrhythmias in HF.

Background: Heart failure (HF) is characterized by electrophysiological remodeling resulting in increased risk of cardiac arrhythmias. Previous reports suggest that elevated inward ionic currents in HF promote action potential (AP) prolongation, increased short-term variability of AP repolarization, and delayed afterdepolarizations. However, the underlying changes in late Na + current (I NaL ), L-type Ca 2+ current, and NCX (Na + /Ca 2+ exchanger) current are often measured in nonphysiological conditions (square-pulse voltage clamp, slow pacing rates, exogenous Ca 2+ buffers). Methods: We measured the major inward currents and their Ca 2+ - and β-adrenergic dependence under physiological AP clamp in rabbit ventricular myocytes in chronic pressure/volume overload–induced HF (versus age-matched control). Results: AP duration and short-term variability of AP repolarization were increased in HF, and importantly, inhibition of I NaL decreased both parameters to the control level. I NaL was slightly increased in HF versus control even when intracellular Ca 2+ was strongly buffered. But under physiological AP clamp with normal Ca 2+ cycling, I NaL was markedly upregulated in HF versus control (dependent largely on CaMKII [Ca 2+ /calmodulin-dependent protein kinase II] activity). β-Adrenergic stimulation (often elevated in HF) further enhanced I NaL . L-type Ca 2+ current was decreased in HF when Ca 2+ was buffered, but CaMKII-mediated Ca 2+ -dependent facilitation upregulated physiological L-type Ca 2+ current to the control level. Furthermore, L-type Ca 2+ current response to β-adrenergic stimulation was significantly attenuated in HF. Inward NCX current was upregulated at phase 3 of AP in HF when assessed by combining experimental data and computational modeling. Conclusions: Our results suggest that CaMKII-dependent upregulation of I NaL in HF significantly contributes to AP prolongation and increased short-term variability of AP repolarization, which may lead to increased arrhythmia propensity, and is further exacerbated by adrenergic stress.