Decrease of K efflux and influx by external Cs ions in cardiac Purkinje and muscle cells.

Abstract

The effect of different external Cs concentrations on 42K efflux in nonstimulated cow Purkinje fibers and guinea-pig auricular preparations was studied in Tyrode solution containing varying amounts of external K (Ke). In some of these conditions also 42K influx and 137Cs influx were estimated. Resting membrane potentials in cow Purkinje fibers were measured by the standard microelectrode technique. Cs ions reduce K efflux and influx in a concentration dependent manner; the effect is due to an interaction of Cs ions with an external site of the membrane. The observed effects on K movement cannot be explained by the accompanying changes in membrane potential, but indicate an important reduction in K conductance. In the presence of 20 mM Cs, which removes most of the inward going rectification (Isenberg, 1976), a substantial K exchange persists and is sensitive to variations in external K concentration and to Cl substitution by acetylglycinate. A reduction of external Na ions does not modify the Cs effect. The results suggest that an important K movement occurs through the outward K rectifier. In the presence of Cs the preparation loses K in exchange for Cs; this Cs influx is substantially reduced by ouabain 10(-6) M and probably occurs through the active pump mechanism.