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

Key Points: Early-afterdepolarizations (EADs) are abnormal action potential oscillations and a known cause of cardiac arrhythmias. Ventricular EADs involve reactivation of a Ca current (I ) 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 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 (I ). Progressively widening the I 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 amplitude and/or K channel-blockade (4-aminopyridine). Narrowing of the I window by ∼10 mV abolished these EADs. Atrial I window narrowing is worthy of further testing as a potential anti-atrial fibrillation drug mechanism.

Abstract: Atrial early-afterdepolarizations (EADs) may contribute to atrial fibrillation (AF), perhaps involving reactivation of L-type Ca current (I ) in its window region voltage range. The present study aimed (i) to validate the dynamic clamp technique for modifying the I contribution to atrial action potential (AP) waveform; (ii) to investigate the effects of widening the window I on EAD-propensity; and (iii) to test whether EADs from increased I and AP duration are supressed by narrowing the window I . I and APs were recorded from rabbit and human atrial myocytes by whole-cell-patch clamp. During AP recording, I was inhibited (3 µm nifedipine) and replaced by a dynamic clamp model current, I (tuned to native I characteristics), computed in real-time (every 50 µs) based on myocyte membrane potential. I -injection restored the nifedipine-suppressed AP plateau. Widening the window I , symmetrically by stepwise simultaneous equal shifts of half-voltages (V ) of I 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 (asymmetrical widening) than inactivation V ; 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 or subsequent 4-aminopyridine (2 mm), window I narrowing (10 mV) abolished EADs of all types (P < 0.05). The present study validated the dynamic clamp for I , which is novel in atrial cardiomyocytes, and showed that EADs of various types are generated by widening (particularly asymmetrically) the window I , as well as abolished by narrowing it. Window I narrowing is a potential therapeutic mechanism worth pursuing in the search for improved anti-AF drugs.