Single-channel analysis of a potassium inward rectifier in myocytes of newborn rat heart.

Unitary K+ currents in single cells isolated from ventricular muscle of newborn rat hearts were measured in response to different potentials and [K]o. The I/V curves were linear for potentials more negative than the zero-current voltage; especially in high [K]o (150 mM KCl), no clear outward currents could be detected indicating a drastic rectification in the inward direction. The channel is mainly selective to K+ but Na+ ions are also carried (PNa/PK = 0.056). The channel conductance is proportional to the square root of [K]o but Na+ ions seem to have a facilitatory effect on gamma K, the single-channel conductance. The channel activity, measured as Po, i.e. the probability to find the channel in open state, decreased as the membrane was hyperpolarized. This behavior was tentatively explained by an inactivation process as the membrane becomes more negative. The rate constants of the transitions between the different states were calculated according to a C-O-C model. A control of the gating process by permeant ion K+ was postulated, based on the increase of one of the rate constants from the closed to the open state with [K]o. Finally, the macroscopic I/V curves calculated from Po and delta i, the unit current, were found to be characteristic of a ion-blocked inward rectifier.