Potassium transport across the frog retinal pigment epithelium

Summary Previous experiments indicate that the apical membrane of the frog retinal pigment epithelium contains electrogenic NaK pumps. In the pressent experiments net potassium and rubidium transport across the epithelium was measured as a function of extracellular potassium (rubidium) concentration, [K]_o ([Rb]_o). The net rate of retina-to-choroid⁴²K(⁸⁶Rb) transport increased monotonically as [K]_o ([Rb]_o), increased from approximately 0.2 to 5mm on both sides of the tissue or on the apical (neural retinal) side of the tissue. No further increase was observed when [K]_o ([Rb]_o) was elevated to 10mm. Net sodium transport was also stimulated by elevating [K]_o. The net K transport was completely inhibited by 10^–4m ouabain in the solution bathing the apical membrane. Ouabain inhibited the unidirectional K flux in the direction of net flux but had not effect on the back-flux in the choroid-to-retina direction. The magnitude of the ouabain-inhibitable⁴²K(⁸⁶Rb) flux increased with [K]_o ([Rb]_o). These results show that the apical membrane NaK pumps play an important role in the net active transport of potassium (rubidium) across the epithelium. The [K]_o changes that modulate potassium transport coincide with the light-induced [K]_o changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium.