Potassium transport across the frog retinal pigment epithelium

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Abstract
Previous experiments indicate that the apical membrane of the frog retinal pigment epithelium contains electrogenic Na∶K 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 10−4 m 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 Na∶K 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.