Reduction of potassium permeability by chloride substitution in cardiac cells.

1. The efflux of radioactive K and Cl ions was measured in cow Purkinje fibres and ventricular preparations of cow, cat and frog. The effect of K and Cl was studied by changing the extracellular K concentration between zero and 54 mM, and by substituting Cl ions by acetylglycinate, isethionate, benzenesulphonate, propionate and nitrate. 2. In the absence of Cl the rate coefficient for 42K efflux showed a pronounced fall, which was more pronounced the higher the K concentration. This effect was not related to the change in membrane potential. The rate coefficient for 42K efflux increased in the presence of higher extracellular K concentrations. 3. 36Cl efflux increased in the presence of reduced as well as increased extracellular K concentrations. 4. The calculated permeability coefficient for K (PK) was maximal at 5-4 mM-K, decreased slightly at higher K concentrations, but fell markedly in K-free solutions, (to about 1/5 of the value in 5-4 mM-K). In Cl-free medium PK reduced to 0-67 of the value in the Cl medium, irrespective of the K concentration. 5. The calculated PCl was greater in K-free and 16-2 mM-K than in 5-4 mM-K. 6. The ratio PK/PCl showed important changes as a function of extracellular K concentration: the value was 5 in 5-4 mM-K and fell to 2 and 0-5 respectively in 16-2 and K-free solutions. 7. The results suggest that part of the changes in membrane resistance measured by electrical methods in Cl-free media is due to a simultaneous decrease in K conductance.