Response properties of the toad retinal pigment epithelium

Electrophysiologic responses were recorded in vitro from the retina of the toad, Bufo marinus. In the isolated retinal pigment epithelium (RPE)-choroid preparation, a decrease in the K+ concentration outside the apical membrane evoked a sequence of trans-epithelial (TEP) and membrane potential changes. A hyperpolarization originating at the RPE apical membrane that increased the TEP was followed by a delayed hyperpolarization originating at the basal membrane that decreased the TEP. This response sequence differed from the well-studied bullfrog RPE in exhibiting potentials generated at the basal membrane; it more closely resembled responses from the reptilian and avian RPE. The toad direct-current electroretinogram recorded from a neural retina-RPE-choroid preparation also differed from other amphibians and was more similar to responses recorded in reptiles, birds, and mammals. In this initial study, the electrophysiologic characteristics of the RPE's Na+/K+ pump were examined. Contrary to a previous report, ouabain, a specific inhibitor of the pump, depolarized the apical membrane and decreased the TEP. Ba2+ was used to block apical K+ conductance and unmasked a modulation of the Na+/K+ pump by subretinal K+. Decreasing the K+ concentration in the presence of Ba2+ depolarized the apical membrane and decreased the TEP. These responses were blocked by ouabain, indicating that the K+ concentration decrease slowed the Na+/K+ pump.