The bicarbonate ion pump in the endothelium which regulates the hydration of rabbit cornea.

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Abstract
The short-circuit current (SCC) and fluid transport across rabbit corneal endothelium were studied. The normal SCC of 27 .mu.A .cntdot. cm-2 was reduced to 19 .mu.A .cntdot. cm-2 in CO2-free Ringer, to 9 .mu.A .cntdot. cm-2 in HCO3--free Ringer and to zero in CO2 and HCO3--free Ringer. Carbonic anhydrase inhibitors reduced SCC from 27 to 19 .mu.A .cntdot. cm-2. Removal of exogenous CO2 caused no further reduction in SCC. In CO2-free Ringer, net exogenous HCO3- translocation was equal to SCC. In all cases studied, net fluid transport across the endothelium in open circuit was directly proportional to SCC. The endothelial pump which regulates corneal hydration apparently operates by pumping HCO3- inot the aqueous humor. Under physiological conditions, 2/3 of the substrate was supplied by exogenous HCO3- and 1/3 was supplied by conversion of exogenous CO2 by intracellular carbonic anhydrase. Metabolic CO2 did not participate significantly in the process, probably because it was at too low a concentration to compete effectively with exogenous CO2. Electron histochemical studies suggested that carbonic anhydrase is located immediately underneath the posterior membrane of the endothelium, across which the active HCO3- flux passes.