Dynamic clamping human and rabbit atrial calcium current: narrowing ICaL window abolishes early afterdepolarizations.

Abstract

Key points Early‐afterdepolarizations (EADs) are abnormal action potential oscillations and a known cause of cardiac arrhythmias. Ventricular EADs involve reactivation of a Ca2+ current (I CaL) in its ‘window region’ voltage range. However, electrical mechanisms of atrial EADs, a potential cause of atrial fibrillation, are poorly understood.Atrial cells were obtained from consenting patients undergoing heart surgery, as well as from rabbits. I CaL was blocked with nifedipine and then a hybrid patch clamp/mathematical‐modelling technique, ‘dynamic clamping’, was used to record action potentials at the same time as injecting an artificial, modifiable, I CaL (I CaL,D‐C).Progressively widening the I CaL,D‐C window region produced EADs of various types, dependent on window width. EAD production was strongest upon moving the activation (vs. inactivation) side of the window.EADs were then induced by a different method: increasing I CaL,D‐C amplitude and/or K+ channel‐blockade (4‐aminopyridine). Narrowing of the I CaL,D‐C window by ∼10 mV abolished these EADs.Atrial I CaL window narrowing is worthy of further testing as a potential anti‐atrial fibrillation drug mechanism. Atrial early‐afterdepolarizations (EADs) may contribute to atrial fibrillation (AF), perhaps involving reactivation of L‐type Ca2+ current (I CaL) in its window region voltage range. The present study aimed (i) to validate the dynamic clamp technique for modifying the I CaL contribution to atrial action potential (AP) waveform; (ii) to investigate the effects of widening the window I CaL on EAD‐propensity; and (iii) to test whether EADs from increased I CaL and AP duration are supressed by narrowing the window I CaL. I CaL and APs were recorded from rabbit and human atrial myocytes by whole‐cell‐patch clamp. During AP recording, I CaL was inhibited (3 µm nifedipine) and replaced by a dynamic clamp model current, I CaL,D‐C (tuned to native I CaL characteristics), computed in real‐time (every 50 µs) based on myocyte membrane potential. I CaL,D‐C‐injection restored the nifedipine‐suppressed AP plateau. Widening the window I CaL,D‐C, symmetrically by stepwise simultaneous equal shifts of half‐voltages (V 0.5) of I CaL,D‐C activation (negatively) and inactivation (positively), generated EADs (single, multiple or preceding repolarization failure) in a window width‐dependent manner, as well as AP alternans. A stronger EAD‐generating effect resulted from independently shifting activation V 0.5 (asymmetrical widening) than inactivation V 0.5; for example, a 15 mV activation shift produced EADs in nine of 17 (53%) human atrial myocytes vs. 0 of 18 from inactivation shift (P < 0.05). In 11 rabbit atrial myocytes in which EADs were generated either by increasing the conductance of normal window width I CaL,D‐C or subsequent 4‐aminopyridine (2 mm), window I CaL,D‐C narrowing (10 mV) abolished EADs of all types (P < 0.05). The present study validated the dynamic clamp for I CaL, which is novel in atrial cardiomyocytes, and showed that EADs of various types are generated by widening (particularly asymmetrically) the window I CaL, as well as abolished by narrowing it. Window I CaL narrowing is a potential therapeutic mechanism worth pursuing in the search for improved anti‐AF drugs.