Characteristics of a transient outward current (sensitive to 4-aminopyridine) in Ca2+-tolerant myocytes isolated from the rabbit atrioventricular node

 A transient outward current (Ito) has been observed in the atrioventricular node (AVN), but its characteristics in Ca-tolerant AVN myocytes have not been investigated previously. In this study, Ito was measured from Ca-tolerant rabbit AVN myocytes at 37°C, using the whole-cell patch-clamp technique. With interfering currents inhibited, 500-ms voltage-clamp pulses applied from –80 mV elicited Ito at potentials positive to –30 mV, which increased in magnitude with test potential amplitude. This current was completely blocked by external application of 5 mM 4-aminopyridine (4-AP). During a command pulse, Ito activated rapidly then inactivated with a bi-exponential time-course. Fast and slow time constants of current inactivation (τf and τs, respectively) showed voltage dependence. At 0 mV, τf was 14.5±2.7 ms and τs was 112.8±21.2 ms, whilst at +60 mV τf was 6.7±1.1 ms and τs was 63.7±9.2 ms (n=25). The steady-state inactivation relationship showed half-maximal inactivation at –33.8 mV (n=8). Re-activation of Ito after an inactivating pre-pulse showed a bi-exponential time-course of recovery: τ1 was 196±70 ms, and τ2 was 2707±1010 ms (n=6, at –80 mV). Repetitive application of voltage-clamp test pulses showed that Ito inactivation accumulated on repetitive stimulation, but reached a steady state rapidly for a given pulse frequency (0.2–1.0 Hz). AVN Ito was sensitive to the class 1 anti-arrhythmic flecainide (EC50 for peak current of 24 µM), which showed selectivity for the rapidly inactivating current component. Quinidine also inhibited Ito in a dose-dependent fashion, but did not affect the current time-course. Under voltage-clamp conditions, a simulated diastolic depolarisation from –70 to –45 mV did not significantly reduce Ito amplitude, and under current-clamp conditions 4-AP inhibited spontaneous action potentials. Although this is consistent with a significant role for Ito in shaping AVN activity, under the conditions of this study 4-AP also partially blocked the ”rapid” delayed rectifier current, IKr, and so the effects of 4-AP on action potentials could not be attributed exclusively to its effects on Ito.