Effect of intracellular potassium upon the electrogenic pump of frog retinal pigment epithelium.

We have studied the hyperpolarizing, electrogenic pump located on the apical membrane of the retinal pigment epithelium (RPE) in an vitro preparation of bullfrog RPE-choroid. Changes in RPE [K+]i alter the current produced by this pump. Increasing [K+]o in the solution perfusing the basal membrane increases RPE [K+]i (measured with a K+-specific microelectrode), and also depolarizes the apical membrane. The depolarization is due to a decrease in electrogenic pump current flowing across the apical membrane resistance, since it is abolished when the pump is inhibited by apical ouabain, by cooling the tissue, or by 0 mM [K+]o outside the apical membrane. Removal of Cl- from the solution perfusing the basal membrane abolishes the K+-evoked apical depolarization by preventing the entry of K+ (as KCl) into the cell. We conclude that the increase in [K+]i causes the decrease in pump current. This result is consistent with the finding that [K+]i is a competitive inhibitor of the Na+ - K+ pump in red blood cells. It is possible that the light-evoked changes in [K+]o in the distal retina could alter RPE [K+]i, and thus could affect the pump from both sides of the apical membrane. Any change in pump current is likely to influence retinal function, since this pump helps to determine the composition of the photoreceptor extracellular space.