Differential responses of rabbit ventricular and atrial transient outward current (I(to)) to the I(to) modulator NS5806

Transient outward potassium current (I(to)) in the heart underlies phase 1 repolarization of cardiac action potentials and thereby affects excitation–contraction coupling. Small molecule activators of I(to) may therefore offer novel treatments for cardiac dysfunction, including heart failure and atrial fibrillation. NS5806 has been identified as a prototypic activator of canine I(to). This study investigated, for the first time, actions of NS5806 on rabbit atrial and ventricular I(to). Whole cell patch‐clamp recordings of I(to) and action potentials were made at physiological temperature from rabbit ventricular and atrial myocytes. 10 μmol/L NS5806 increased ventricular I(to) with a leftward shift in I(to) activation and accelerated restitution. At higher concentrations, stimulation of I(to) was followed by inhibition. The EC (50) for stimulation was 1.6 μmol/L and inhibition had an IC (50) of 40.7 μmol/L. NS5806 only inhibited atrial I(to) (IC (50) of 18 μmol/L) and produced a modest leftward shifts in I(to) activation and inactivation, without an effect on restitution. 10 μmol/L NS5806 shortened ventricular action potential duration (APD) at APD (20)‐APD (90) but prolonged atrial APD. NS5806 also reduced atrial AP upstroke and amplitude, consistent with an additional atrio‐selective effect on Na(+) channels. In contrast to NS5806, flecainide, which discriminates between Kv1.4 and 4.x channels, produced similar levels of inhibition of ventricular and atrial I(to). NS5806 discriminates between rabbit ventricular and atrial I(to,) with mixed activator and inhibitor actions on the former and inhibitor actions against the later. NS5806 may be of significant value for pharmacological interrogation of regional differences in native cardiac I(to).