title: Characteristics of a transient outward current (sensitive to 4-aminopyridine) in Ca2+-tolerant myocytes isolated from the rabbit atrioventricular node.
abstract: 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 degrees 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 (tauf and taus, respectively) showed voltage dependence. At 0 mV, tauf was 14.5+/-2.7 ms and taus was 112.8+/-21. 2 ms, whilst at +60 mV tauf was 6.7+/-1.1 ms and taus 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: tau1 was 196+/-70 ms, and tau2 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 microM), 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.
authors.affiliation: Department of Physiology and Cardiovascular Research Laboratories, School of Medical Sciences, University Walk, University of Bristol, Bristol, BS8 1TD, UK.
authors.forename: J S
authors.initials: JS
authors.lastname: Mitcheson
authors.forename: J C
authors.initials: JC
authors.lastname: Hancox
mesh_terms: D015761:4-Aminopyridine / Q000494:pharmacology*; D000200:Action Potentials / Q000187:drug effects; D000465:Algorithms; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D001283:Atrioventricular Node / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D004558:Electric Stimulation; D004594:Electrophysiology; D005424:Flecainide / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018408:Patch-Clamp Techniques; D011802:Quinidine / Q000494:pharmacology; D011817:Rabbits
chemical_list: D000889:Anti-Arrhythmia Agents; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D015761:4-Aminopyridine; D011802:Quinidine; D005424:Flecainide