A time- and voltage-dependent potassium current in the rabbit sinoatrial node cell.

Voltage clamp experiments were conducted in the rabbit sinoatrial node (S-A node) using the double microelectrode technique. When the membrane was repolarized to the resting potential after depolarizing test pulses, the outward current slowly decayed to the steady level (outward current tail). The magnitude of the outward current tail was a sigmoid function of the amplitude of the preceding depolarization. The degree of activation of this current varied from 0 at about -50 mV to 1 at about +20 mV. The time course of the current change was a simple exponential and was independent of the preceding depolarization. The reciprocal time constant appeared to be a U-shaped function of the membrane potential with a minimum value of about 3s-1 at -40 mV. The instantaneous current voltage relation was an inward-going rectifier, but showed no detectable negative slope. The reversal potential, obtained between 10 and 50 mM [K]o, decreased with a slope of 58 mV for a 10-fold increase in [K]o. These findings indicate that the outward current tail in the S-A node cell is attributable to a single component of K current (pacemaker current component). The pacemaker current component is mainly responsible for the slow diastolic depolarization.