Transport of acidic amino acids by the bovine pigment epithelium.

The regulation of acidic amino-acid transport across the retinal pigment epithelium is of particular interest since glutamate and possibly aspartate have been identified as putative neurotransmitters in the retina, at the level of the photoreceptor cell. The present study, designed to measure the rate of acidic amino-acid transport across the mammalian pigment epithelium (PE), shows that there is a net transport of both glutamate and aspartate in the retina to choroid direction (R-C), with the R-C unidirectional flux of glutamate being substantially larger than the corresponding aspartate flux. The R-C and C-R fluxes of glutamate were found to be inhibited by ouabain. Further investigations utilizing aspartate revealed that the fluxes in both directions were inhibited when ouabain was present on the retinal side of the tissue preparation. The R-C flux of glutamate was significantly reduced by lowered concentrations of Na+, K+ and Ca2+, whereas the C-R flux was diminished only by the reduced concentration Ca2+. The changes in K+ concentration which markedly altered the R-C flux of glutamate were within the range of light-induced changes of K+ which has been observed in the extracellular space of the photoreceptor cells. The transporting system appears to be relatively specific for the acidic amino acids; for aspartate was an effective competitive inhibitor of glutamate transport whereas basic (lysine) and neutral (leucine) amino acids were not. The directionality, ouabain sensitivity, ionic dependence and substrate specificity of the transmembrane fluxes tend to support the concept of active transport as a mechanism of acidic amino-acid removal from the neural retina.