COTRANSPORT OF H-PROTEINS IN RETINAL-PIGMENT EPITHELIUM OF BULLFROG(,LACTATE AND H2O BY MEMBRANE)

1. The interaction between H+, lactate and H2O fluxes in the retinal m embrane of the pigment epithelium from bullfrog Rana catesbiana was st udied by means of ion-selective microelectrodes. 2. Changes in intrace llular pH and cell volume were recorded in response to abrupt changes in retinal solution concentration and/or osmolarity. 3. Two parallel p athways for water transport were identified across the retinal membran e, an osmotic one with a hydraulic water permeability of 3.2 x 10(-4) cm s(-1) (osmol l(-1))(-1) and one which depended on the presence of l actate.4. Addition of sodium lactate to the retinal solution caused ce ll shrinkages that were small compared with those produced by mannitol . The reflection coefficient for sodium lactate was 0.25. 5. Isosmotic replacement of Cl- with lactate caused an influx of water. Simultaneo us acidification of the retinal solution from pH 7.4 to 6.4 enhanced t he effect. The influx of water could proceed against osmotic gradients elicited by mannitol. 6. The interdependence of the fluxes of H+, lac tate and H2O can be described as cotransport: the fluxes had a fixed r atio of about 109 mmol of lactic acid per litre of water, the flux of one species mas able to energize the flux of the other two, and the fl uxes exhibited saturation for increasing driving forces. 7. The Gibbs equation gives an accurate quantitative description of these coupled f luxes.