Dynamic clamping human and rabbit atrial calcium current: narrowing ICaL window abolishes early afterdepolarizations
Authors: Kettlewell Sarah , Saxena Priyanka , Dempster John , Colman Michael A. , Myles Rachel C. , Smith Godfrey L. , Workman Antony J. 
Published: May 2019

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 (ICaL) 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. ICaL 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, ICaL (ICaL,D‐C).
Progressively widening the ICaL,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 ICaL,D‐C amplitude and/or K+ channel‐blockade (4‐aminopyridine). Narrowing of the ICaL,D‐C window by ∼10 mV abolished these EADs.
Atrial ICaL window narrowing is worthy of further testing as a potential anti‐atrial fibrillation drug mechanism.
AbstractAtrial early‐afterdepolarizations (EADs) may contribute to atrial fibrillation (AF), perhaps involving reactivation of L‐type Ca2+ current (ICaL) in its window region voltage range. The present study aimed (i) to validate the dynamic clamp technique for modifying the ICaL contribution to atrial action potential (AP) waveform; (ii) to investigate the effects of widening the window ICaL on EAD‐propensity; and (iii) to test whether EADs from increased ICaL and AP duration are supressed by narrowing the window ICaL. ICaL and APs were recorded from rabbit and human atrial myocytes by whole‐cell‐patch clamp. During AP recording, ICaL was inhibited (3 µm nifedipine) and replaced by a dynamic clamp model current, ICaL,D‐C (tuned to native ICaL characteristics), computed in real‐time (every 50 µs) based on myocyte membrane potential. ICaL,D‐C‐injection restored the nifedipine‐suppressed AP plateau. Widening the window ICaL,D‐C, symmetrically by stepwise simultaneous equal shifts of half‐voltages (V0.5) of ICaL,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 V0.5 (asymmetrical widening) than inactivation V0.5; 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 ICaL,D‐C or subsequent 4‐aminopyridine (2 mm), window ICaL,D‐C narrowing (10 mV) abolished EADs of all types (P < 0.05). The present study validated the dynamic clamp for ICaL, which is novel in atrial cardiomyocytes, and showed that EADs of various types are generated by widening (particularly asymmetrically) the window ICaL, as well as abolished by narrowing it. Window ICaL narrowing is a potential therapeutic mechanism worth pursuing in the search for improved anti‐AF drugs.