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Rheogenic sodium‐bicarbonate co‐transport across the retinal membrane of the frog retinal pigment epithelium.

1. Na+ and HCO3‐ transport across the retinal membrane of the frog retinal pigment epithelium was studied by means of double‐barrelled Na(+)‐ and pH‐selective microelectrodes. Transient changes in the intracellular pH and in the intracellular Na+ activity were monitored in response to abrupt changes in the Na+ concentration and in the HCO3‐ concentration on the retinal side of the epithelium, and in response to transepithelial currents. 2. Removal of Na+ from the retinal side of the epithelium caused a depolarization of the membrane potential across the retinal membrane, a decrease in the intracellular Na+ activity and a decrease in the intracellular pH. 3. An increase in the HCO3‐ concentration on the retinal side of the epithelium from 27.5 to 50 mM caused a hyperpolarization of the membrane potential across the retinal membrane, an increase in the intracellular Na+ activity and an increase in the intracellular pH. 4. Passage of a transepithelial current of 30 microA in the choroid‐to‐retina direction caused an increase in the intracellular Na+ activity and an increase in the intracellular pH. 5. The data are interpreted as evidence for rheogenic co‐transport of Na+, HCO3‐ across the retinal membrane of the frog retinal pigment epithelium. 6. The transient changes described under 2, 3 and 4 above were blocked by 0.5 mM‐4‐acetamido‐4'‐isothiocyanostilbene‐2.2'‐disulphonic acid (SITS). The Na(+)‐ HCO3‐ co‐transport was not inhibited by 1 mM‐amiloride.