The Effect of Stimulation Frequency on the Ionic Currents in Single Atrial Cells of the Rabbit

Abstract

In single atrial cells isolated from rabbit hearts the calcium current and [Caj-dependent transient outward current were recorded using the whole-cell clamp technique and the effect of stimulation frequency on these currents was investigated. Voltage dependent transient outward current, which contributes the initial, rapid repolarization phase of the action potential and is frequency-dependent, was also investigated. Increasing the stimulation frequency from O. 025 Hz to 1 Hz had no effect on the calcium current and [Caj-dependent transient outward current and greatly inhibited voltage-dependent transient outward current. The amplitude of voltage dependent transient outward current increased as the membrane potential became depolarized, its steady-state inactivation spans the voltage range -70 mV to -10 mVand steady-state activation curve -30 mV to 30 mV. Within the range of the resting membrane potential (at -70 mV), the voltage-dependent recovery time constant was 1. 3 s. The reversal potential was about -50 mV. Voltage-dependent transient outward current was inhibited by K-channel blockers and not inhibited by modulation of [Cali. From the above findings, it is concluded that due to the amplitude and voltage-dependent recovery time constant which were the basic mechanisms for frequency-dependency, the voltage- dependent transient outward current contributes the initial, rapid repolarization phase and changed the action potential configuration according to stimulation frequency in the rabbit atrium.