Apical and basolateral membrane mechanisms that regulate pHi in bovine retinal pigment epithelium

pH regulation was studied in fresh explant bovine retinal pigment epithelium-choroid using the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and intracellular microelectrodes. Acid recovery was HCO3 dependent, inhibited by apical amiloride and apical or basal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and required apical and basal Na. Alkali recovery was HCO3 dependent and inhibitable by apical or basal DIDS. Three apical and two basolateral transporters were identified. Four contribute to acid extrusion, i.e., apical Na/H exchange, apical H-lactate cotransport, and apical Na-HCO3 cotransport and basolateral Na-HCO3 cotransport. At least two contribute to alkali extrusion, i.e., apical Na-HCO3 cotransport and a basolateral HCO3-dependent, DIDS-inhibitable mechanism, possibly Na-HCO3 cotransport, Cl/HCO3 exchange, or both. The apical Na-HCO3 cotransporter is electrogenic, carrying net negative charge inward. Basal Cl removal or addition of basal HCO3 caused HCO3- and Cl-dependent alkalinizations, respectively. Apical DIDS increased both responses. These cytosolic pH (pHi) regulatory mechanisms are so tightly coupled that changes in pHi can only occur after two or more of them are inhibited. In addition, these mechanisms help provide pathways for transport of Na and HCO3 across the retinal pigment epithelium between the blood and the distal retina.