LIGHT-DEPENDENT HYDRATION OF THE SPACE SURROUNDING PHOTORECEPTORS IN CHICK RETINA

Purpose. To characterize the light-evoked increase in the volume (or h ydration) of the subretinal space (SRS) of chick retina and to investi gate its retinal pigment epithelial (RPE) origin. Methods. The authors perfused chick retina-RPE-choroid preparations with the extracellular space marker, tetramethylammonium (TMA(+)), and estimated its concent ration ([TMA(+)](0)) using double-barreled, ion-selective electrodes. They recorded dark-light changes in subretinal [TMA(+)](0) and studied the effects of drugs that suppress RPE ion transport and conductances . Results. The authors observed a slow, light-evoked decrease in subre tinal [TMA(+)](0), about 7% in amplitude, that slowly returned to base line and then increased at light OFF. It was 40% to 65% inhibited by b umetanide and furosemide, known to inhibit RPE apical membrane Na-K-Cl cotransport; 70% to 80% inhibited by DIDS, an inhibitor of RPE basal membrane chloride conductance, and 50% inhibited by 0.5 mM BaCl2, a bl ocker of the K+ conductance of the RPE apical membrane. Decreases in s ubretinal [TMA(+)], also were produced when the authors lowered [Cl-]( 0) or [K+](0) in the SRS by reducing concentrations in the retinal per fusate. The effect on [TMA(+)](0) was larger for [Cl-](0) than for [K](0) and was inhibited by bumetanide and DIDS. Conclusions. Changes in subretinal ion concentrations in light and darkness, most likely [K+] (0), modify SRS hydration by modulating RPE apical membrane Na-K-Cl co transport, as well as the conductive effluxes of K+ across the apical membrane and Cl- across the basal membrane. These changes depend princ ipally on the movement of water into or out of the RPE. In turn, subre tinal hydration modifies local ion concentrations and the composition of the interphotoreceptor matrix.