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

A transient outward current (I-to) has been observed in the atrioventricular node AVN, but its characteristics in Ca-tolerant AVN myocytes have not been investigated previously. In this study, I-to was measured from Ca-tolerant rabbit AVN myocytes at 37 degrees C, using the whole-cell patch-clamp technique. With interfering currents inhibited, 500-ms voltage-clamp pulses applied from -80 mV elicited I-to 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, I-to activated rapidly then inactivated with a bi-exponential time-course. Fast and slow time constants of current inactivation ( tau(f) and tau(s), respectively) showed voltage dependence. At 0 mV, tau(f) was 14.5 +/- 2.7 ms and tau(s) was 112.8 +/- 21.2 ms, whilst at +60 mV tau(f) was 6.7 +/- 1.1 ms and tau(s) 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 I-to after an inactivating pre-pulse showed a bi-exponential time-course of recovery: tau(1) was 196 +/- 70 ms, and tau(2) was 2707 +/- 1010 ms (n = 6, at -80 mV). Repetitive application of voltage-clamp test pulses showed that I-to inactivation accumulated on repetitive stimulation, but reached a steady state rapidly for a given pulse frequency (0.2-1.0 Hz). AVN I-to was sensitive to the class 1 anti-arrhythmic flecainide (EC50 for peak current of 24 mu M), which showed selectivity for the rapidly inactivating current component. Quinidine also inhibited I-to 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 I-to amplitude, and under current-clamp conditions 4-AP inhibited spontaneous action potentials. Although this is consistent with a significant role for I-to in shaping AVN activity, under the conditions of this study 4-AP also partially blocked the "rapid" delayed rectifier current, I-Kr, and so the effects of 4-AP on action potentials could not be attributed exclusively to its effects on I-to.