Mechanisms of Excitation and Remodeling of the Cardiac Action Potential in Two Model Systems.

Abstract

Differences in cardiac ionic currents underlie action potential duration (APD) heterogeneity and alterations of any current may be arrhythmogenic. Biophysical analysis of ionic currents is crucial in understanding electrophysiological mechanisms of regional electrical heterogeneity and underlying factors that promote arrhythmogenicity. The first project presented in this dissertation characterized, for the first time, the electrophysiology of the Purkinje system of the murine heart. Current-clamp analysis of Purkinje cells (PCs) demonstrated pacemaker activity and a prolonged plateau phase compared to ventricular myocytes (VMs). We investigated, using voltage-clamp, the major ionic currents underlying the action potential and automaticity in PCs and VMs. We observed hyperpolarization activated currents, which contribute to automaticity in PCs, but not VMs. PCs demonstrated differences in transient outward potassium currents, sodium current and T-type calcium current, which was not present in VMs. A computational model of the mouse PC was developed and simulations determined that unlike VMs, in PCs the presence of T-type calcium current was capable of prolonging APD. The second project of this dissertation investigated the remodeling of action potentials in atrial cells by free fatty acids (FAs), which has not been investigated in large mammals. This project used an ovine model to serve as a better surrogate of human cardiac structure, electrophysiology and metabolism. Current-clamp analysis of ovine left atrial (LA) myocytes exposed to each of the major FAs showed that only stearic acid (SA) altered LA APD at all values measured. Voltage-clamp recordings showed a ~60% and ~30% reduction of ICaL and ITO in SA-treated cells. Integration of the experimental data into a computational model of the human atrial action potential showed reduction of ICaL was sufficient to remodel LA APD. Reduction in ICaL in SA-treated cells was accompanied by disruption of transverse (t)-tubules, a membrane compartment in which calcium channels are predominantly located into microdomains, thus providing a novel mechanism of cellular remodeling by fatty acids. These two studies provide insight into ionic remodeling and importance of calcium currents in heterogeneity and alterations of the cardiac action potential.