Potassium modulation of taurine transport across the frog retinal pigment epithelium.

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Abstract
Net taurine transport across the frog retinal pigment epithelium-choroid was measured as a function of extracellular K concentration, [K+]O. The net rate of retina-to-choroid transport increased monotonically as [K+]O increased from 0.2 mM to 2 mM on the apical (neural retinal) side of the tissue. No further increase was observed when [K+]O was elevated to 5 mM. The [K+]O changes that modulate taurine transport approximate the light-induced [K+]O changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium. The taurine-K interaction was studied by using Rb as a substitute for K and measuring active Rb transport as a function of extracellular taurine concentration. An increase in apical taurine concentration, from 0.2 mM to 2 mM, produced a 3-fold increase in active Rb transport, retina to choroid. Net taurine transport can also be altered by relatively large, 55 mM, changes in [Na+]O. Apical ouabain, 10-4 M, inhibited active taurine, Rb, and K transport; in the case of taurine, this inhibition is mot likely due to a decrease in the Na electrochemical gradient. The apical membrane apparently contains a taurine, Na co-transport mechanism whose rate is modulated, indirectly, through the Na pump. This pump has previously been shown to be electrogenic and located on the apical membrane, and its rate is modulated, indirectly, by the taurine co-transport mechanism.

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