Suppressing Early Afterdepolarizations in Cardiac Myocytes by Shaping a Modified Ca 2+ Conductance Under Dynamic Clamp

Abstract

Early afterdepolarizations (EADs) are highly arrhythmogenic alterations of the cardiac action potential (AP) characterized as transient reversals of repolarization during AP phase 2 or 3. Using the dynamic clamp technique, we combined mathematical modeling and electrophysiology in real-time to introduce a virtual L-type Ca current (ICa,L) with tunable biophysical properties in dissociated rabbit ventricular myocytes under current clamp. We sought to i) understand the etiology of EAD development and their dependence on ICa,L and ii) identify therapeutic strategies to suppress EADs by shaping a new L-type current. The native ICa,L was blocked with 20μM nifedipine and replaced by a “virtual” ICa,L. A modest 4mV leftward shift in the V1/2 of activation prolonged APD90 to 441±100ms inducing EADs in 25% of APs, while a 5mV leftward shift further prolonged APD90 to 1260±129ms, inducing EADs in 100% of APs thus revealing a striking dependence of EAD frequency on the ICa,L activation V1/2. To investigate the therapeutic potential of ICa,L modifications in suppressing EADs, we first induced a robust EAD regime by exposing cardiomyocytes either to hypokalemia (2.4 mM KCl) or oxidative stress (0.6mM H2O2, PCL=5s). EADs were completely abolished with 20μM nifedipine but promptly restored under dynamic clamp by a virtual Ca2+ current modeled as modified by H2O2. However, a 5mV rightward shift in the ICa,L V1/2 of activation dramatically reduced APD and EAD frequency from 100% to <5%. Shifting the V1/2 of the steady-state inactivation curve by 7mV towards more negative potentials completely abolished EADs. Both maneuvers reduced the ICa,L window current region and abolished EADs, while the predicted Ca transients were largely unaffected. These results support the hypothesis that EAD occurrence can be controlled by fine-tuning the biophysical properties of the L-type window current.