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

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.