Effect of taurine on the isolated retinal pigment epithelium of the frog: electrophysiologic evidence for stimulation of an apical, electrogenic Na+-K+ pump.

Abstract

The apical surface of the retinal pigment epithelium (RPE) faces the neural retina whereas its basal surface faces the choroid. Taurine, which is necessary for normal vision, is released from the retina following light exposure and is actively transported from retina to choroid by the RPE. In these experiments, we have studied the effects of taurine on the electrical properties of the isolated RPE of the bullfrog, with a particular focus on the effects of taurine on the apical Na+-K+ pump. Acute exposure of the apical, but not basal, membrane of the RPE to taurine decreased the normally apical positive transepithelial potential (TEP). This TEP decrease was generated by a depolarization of the RPE apical membrane and did not occur when the apical bath contained sodium-free medium. With continued taurine exposure, the initial TEP decrease was sometimes followed by a recovery of the TEP toward baseline. This recovery was abolished by strophanthidin or ouabain, indicating involvement of the apical Na+-K+ pump. To further explore the effects of taurine on the Na+-K+ pump, barium was used to block apical K+ conductance and unmask a stimulation of the pump that is produced by increasing apical [K+]o. Under these conditions, increasing [K+]o hyperpolarized the apical membrane and increased TEP. Taurine reversibly doubled these responses, but did not change total epithelial resistance or the ratio of apical-to-basal membrane resistance, and ouabain abolished these responses. Collectively, these findings indicate the presence of an electrogenic Na+/taurine cotransport mechanism in the apical membrane of the bullfrog RPE. They also provide direct evidence that taurine produces a sodium-dependent increase in electrogenic pumping by the apical Na+-K+ pump.