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

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.