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
