| Abstract: | Previous experiments indicate that the apical membrane of the frog retinal pigment epithelium contains electrogenic Na:K pumps. In the present experiments net potassium and rubidium transport across the epithelium was measured as a function of extracellular potassium (rubidium) concentration, K]0 ( Rb]0). The net rate of retina-to-choroid 42K(86Rb) transport increased monotonically as K]0 ( Rb]0) increased from approximately 0.2 to 5 mM on both sides of the tissue or on the apical (neural retinal) side of the tissue. No further increase was observed when K]0 ( Rb]0) was elevated to 10 mM. Net sodium transport was also stimulated by elevating K]0. 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 no effect on the back-flux in the choroid-to-retina direction. The magnitude of the ouabain-inhibitable 42K(86Rb) flux increased with K]0 ( Rb]0). 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]0 changes that modulate potassium transport coincide with the light-induced K]0 changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium. |
