THE ROLE OF THE DELAYED RECTIFIER K+ CURRENT IN THE PACEMAKER ACTIVITY IN RABBIT SINOATRIAL NODE CELLS

The delayed rectifier K+ current (I-K) was examined in rabbit sinoatrial node cells using the patch-clamp technique. I-K was selectively blocked by an antiarrhythmic agent, E-4031, and it was found that the inward rectification was much stronger than previously believed in the whole-cell current study. At the single channel level, the I-K channel showed only a weak inward rectification (similar to 12 pS in 150 mM K+), while the steady-state open probability (P-o) showed a bell-shaped function against the membrane potentials. In the action-potential-clamp mode, I-K was very small at the peak of the action potential, and was similar to or larger than the net current in the repolarizing phase, indicating that I-K is essential for determining the maximum diastolic potential and following diastolic depolarization.