The movement of organic solutes between the retina and pigment epithelium

The transport of several membrane precursors across the bullfrog retinal pigment epithelium (RPE)-choroid was studied in a modified Ussing chamber. The substrates choline, ethanolamine, and glucose did not exhibit a net flux across the tissue, while all amino acids tested showed a net flux in the retina to choroid direction. The 'sink' for a particular amino acid, leucine, was measured in bullfrog retinas in vitro. Decarboxylation of leucine was the major fate of the incorporated amino acid, while much less was directed toward protein synthesis. The subretinal fluid concentration of leucine was estimated to be in the order of 10 microM, approximately eight-fold less than in plasma, and consistent with levels in cerebrospinal fluid. The sink for leucine in the retina at 10 microM leucine is smaller than the fluxes across the RPE-choroid under conditions of an eight-fold gradient. This suggests that the RPE, not the retina, is responsible for controlling the concentrations of solutes in the subretinal fluid.