A chloride-activated Na+/HCO3- coupled transport activity in corneal endothelial membranes

Investigations of corneal endothelium were made to resolve the apparent con tradiction of the presence of sodium/bicarbonate cotransporter (NBC) in fre sh and cultured cells and NBC's reported absence in isolated plasma membran e vesicles. Gradient-driven ion fluxes into the vesicles were measured. Sho rt-term incubations (0-30 s) showed the presence of a bicarbonate-dependent inward sodium flux (BDSF), which was active when the insides of the vesicl es were preloaded with chloride ions. The BDSF was absent if chloride was p resent only externally to the vesicles, Chloride at concentrations between 30 and 40 mM inside the vesicle had its maximum effect on BDSF, Other anion s (acetate, thiocyanate, or gluconate) inside the vesicles did not mimic th e chloride effect. Associated with the net inward sodium flux was a net inw ard bicarbonate flux. Hill plots of sodium influx with respect to external bicarbonate concentrations indicated that the stoichiometry of the net tran sfer was 1.7 +/- 0.2 (mean +/- standard error, n = 5) bicarbonate ions for each sodium ion transported. There was no net chloride flux found across th e membrane vesicles. The finding of a novel chloride-activated NBC activity fully resolves the apparent contradiction between whole-cell and membrane vesicle preparations.