Date of Award
Doctor of Philosophy (PhD)
In this work, a detailed multiscale computational model of Ca cycling and action potential in a ventricular myocyte is developed and used to study mechanisms of arrhythmias associated with mutations in the ryanodine receptor (RyR). The multiscale ventricular myocyte model reproduces experimentally observed Ca dynamics both at the local dyadic scale and at the global whole-cell scale, while also simulating the action potential (AP) shaped by membrane ionic currents. The model represents stochastic activation of L-type Ca channels (LCCs) and RyRs in the dyads and simulates random generation of Ca sparks both during excitation-contraction coupling (ECC) and during diastole. The kinetic model of RyR includes regulation of its openings by calsequestrin (CSQN). The model reproduces graded release and variable gain at the cellular level, i.e total sarcoplasmic reticulum (SR) release is a graded function of the amplitude of L-type calcium current (ICaL) and of voltage. The model was used to simulate whole-cell (macroscopic) consequences of changes in local (microscopic) dyadic properties associated with pathologies such as heart failure or impairment of CSQN function.
In addition, the model is used to study mechanisms of cellular arrhythmogenic behaviors associated with Ca handling abnormalities. Specifically, we study Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) associated with mutations in RyR. Results indicate that increased cytosolic Ca sensitivity of RyRs as a result of mutations, in combination with β-adrenergic stimulation (βAS), results in increased propensity for formation of spontaneous SR Ca release and Ca waves. The spontaneous release activates the electrogenic Na-Ca exchange current (INaCa) which can prolong the AP to cause reactivation of ICaL to form early afterdepolarizations (EADs) when it occurs during the phase-2 of AP. During diastole, the spontaneous Ca waves can activate INaCa generating delayed afterdepolarizations (DADs) which can then reach the threshold for activation of ICaL to form triggered activity (TA). The mechanistic basis for prevention of triggered arrhythmias in CPVT by the pharmacological agent flecainide was found to be due to its effect on mean open time of RyR rather than due to its effect on blocking the fast Na current (INa). However, such preventive action was not associated with abolition of Ca waves.
EADs and TA also occurred in a simulated hypoxic ventricular myocyte in the presence of βAS. These arrhythmogenic events were due to increased sensitivity of ICaL to βAS during hypoxic conditions.
M Nerbonne, Vitaly Klyachko, Daniel Moran