Relaxation of the ACh-induced potassium current in the rabbit sinoatrial node cell

Abstract

Voltage clamp experiments were carried out in order to study the mechanism of the ACh action in the rabbit S-A node cell. The following results were obtained: The reversal potential of the ACh-induced current behaved like a potassium electrode, confirming that the ACh-operated channels pass potassium ions selectively. On depolarizing voltage jumps the ACh-induced current showed an instantaneous peak from which the current decayed to a new steady level (relaxation). On hyperpolarizing voltage jumps the initial step change in current was followed by a gradual increase. The time course of the current change on voltage jumps was well fitted by a single exponential and the time constant became longer as the membrane potential was increased. The instantaneous I–V curve was linear while in the steady state the curve became flatter at low negative membrane potentials and steeper at high negative membrane potentials. The results suggest that ACh opens a specific potassium channel when the drug is bound to the muscarinic receptor. The opening and closing rate constants for this potassium channel depend on the membrane potential in such a way that on depolarizing voltage jumps the fraction of open channels gradually decreases and on hyperpolarization the fraction increases.