Transmembrane components of taurine flux across frog retinal pigment epithelium

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Abstract
In previous work from this laboratory, a net transepithelial flux of the amino acid taurine was measured across the in vitro frog retinal pigment epithelium. This flux was from retina to choroid and could be modulated by small (0.5 mM) changes in K+ concentration, by changes in taurine concentration, and by ouabain. In the present experiments we measured the unidirectional transmembrane fluxes across each of the two cell membranes, the apical membrane (facing the neural retina) and the basal membrane (facing the choroid) of the retinal pigment epithelium. In modified Ringer's solution containing 2mM taurine + 2mM K+, we found that the apparent outward permeability of the basal membrane, corrected for its actual area, was 26 times that of the apical membrane. As expected from the direction of net flux, the inward apical and outward basal fluxes dominated the transmembrane fluxes. When the apical Na+K+ pump was inhibited, the ratio of the apparent permeability of the basal membrane relative to the apical decreased from 26 to 4.4. The data are consistent with the previous suggestion of Na+:taurine co-transport into the retinal pigment epithelium across the apical membrane. The basal membrane response to ouabain and reduced K+ concentration suggests that a K+-dependent mechanism is responsible, at least in part, for the inward basal taurine flux.