A RAPIDLY ACTIVATING DELAYED RECTIFIER K+ CHANNEL IN RABBIT SINOATRIAL NODE CELLS

The single-channel current of the delayed rectifier K+ current (I-K) was recorded in rabbit sinoatrial node cells. In the cell-attached patch, depolarization from -70 mV to potentials more positive than -50 mV activated the I-K channel while repolarization deactivated it. The single-channel conductance was 7.8 pS for the outward current and 10.8 pS for the inward current (n = 6). The steady-state open probability (NPo) was maximum at around -30 mV and markedly decreased at more positive potentials. On repolarization from positive potentials, the channel was initially closed and then rapidly opened. The ensemble average showed an initial rise to a peak followed by the deactivation time course. Because the channel events were completely blocked by E-4031, the drug-sensitive component was examined in the whole cell current. The steady-state current-voltage relation of the drug-sensitive current showed a marked negative slope at potentials more positive than -10 mV. Upon repolarization, the drug-sensitive current initially increased (removal of inactivation) to the peak of the outward tail current, which was in agreement with the ensemble average of the single-channel current. We conclude that I-K in the sinoatrial node cells is largely composed of the rapidly activating I-K (I-K,I-r) channels and that the inward rectification of I-K,I-r, which is more marked than had been assumed in previous studies, is due to the decrease in NPo.