CARDIAC PURKINJE-FIBERS - [CA2+]I CONTROLS POTASSIUM PERMEABILITY VIA CONDUCTANCE COMPONENTS GK1 AND GK2

The pacemaker tail current of sheep Purkinje fibers became larger when CaCl2 was injected intracellularly by pressure; it was reduced or even abolished by injection of K-EGTA [ethylene glycol bis (.beta.-aminoethyl ether) tetraacetic acid]. The analysis of the [Ca2+]i-induced changes in the steady state activation (s.infin.) curve suggested that the effect could not be explained on the basis of internal screening of surface charges. The position of the s.infin.-curve on the potential axis remained almost unchanged after the injection of CaCl2 or K-EGTA. The vertical amplitude of the s.infin.-curve on the current axis, proportional to .hivin.gK2 [time dependent pacemaker conductance], was enlarged during CaCl2 injections and strongly diminished by K-EGTA injections. Ca injection displaced the fully deactivated part of the s.infin.-curve to larger outward current which corresponds to an increase in gK1 [time independent component]. EGTA injectin reduced gK1. The results correspond to a [Ca2+]i-sensitive K permeability. A possible interpretation is that [Ca2+]i controls Ca-binding near the inner mouth of a K channel, and that the binding is followed by a change in the channel''s configuration from a closed to an open state.