Tuning L-Type Ca2+ Current Properties to Suppress Early After depolarizations

Abstract

Early afterdepolarizations (EADs) are highly arrhythmogenic transient depolarizations occurring during phase 2 or 3 of the cardiac action potential. We recently reported that EADs are highly sensitive to minimal shifts (by 1-5 mV) in the half activation/inactivation potentials (V1/2) of the L-type Ca2+ current (ICa,L), such that V1/2 modifications which reduce the window current voltage range were highly effective in suppressing EADs (Madhvani et al., 2011). To better understand the underlying mechanisms, we have further explored the relevance of ICa,L biophysical parameters to EADs formation. We took advantage of the dynamic clamp technique, a hybrid experimental-computational system permitting real-time introduction of a programmable conductance into a myocyte under current clamp. A consistent EAD regime was achieved by pacing ventricular myocytes at 0.2 Hz in the presence of 600 μM H2O2. The native ICa,L was abolished with 20 μM nifedipine, and replaced with the dynamic clamp-generated ICa,L to recapitulate EADs. We found that altering the slope of the steady-state activation curve had profound effects on EAD take-off potential and amplitude. For z = 3.2 e0, EAD amplitude was 22±1.7 mV (take-off potential to peak). Steepening the voltage dependence of activation (z = 5.7 e0) increased the take-off potential and reduced EAD amplitude to 5.6±0.6 mV. Steepening the voltage dependence of activation beyond z = 5 e0 resulted in small voltage oscillations (<1 mV) without frank EADs. In contrast, making the voltage dependence shallower increased EAD amplitude, especially when combined with a reduction of the non-inactivating component of ICa,L (pedestal). Combined with our previous findings, our results show how multiple ICa,L parameters affect EAD formation, providing a template for the development of therapeutic interventions.