4-Aminopyridine sensitive transient outward current in dog ventricular fibres

Abstract

(1) The depression of slow inward calcium current (I_Ca) induced by organic or inorganic inhibitors in voltage clamped dog ventricular preparations unmasks an early transient outward current (I_to). (2)I_to is depressed by 4-aminopyridine (1 mM) in a voltage dependent manner. (3)I_to appears in reponse to voltage steps above 40 mV (from holding voltage=resting voltage) and increases with raising the amplitude of clamp steps. (4) Within physiological range of membrane voltageI_to is smaller and decays several times faster thanI_Ca. Time course of the decline is approximately exponential (τ=25±6 ms at 80 mV above resting voltage). (5) Shifts of the holding voltage by 20 mV from the level of resting voltage alters the peak amplitude ofI_to. It is increased by hyperpolarization and reduced by depolarization. (6) The recovery ofI_to from inactivation at resting voltage was approximated by a single exponential. Time constant (390 ms) is about 15 times longer than the time constant of inactivation at 80 mV positive to the resting voltage.