HCO3- transport in the toad lens epithelium is mediated by an electronegative Na(+)-dependent symport

Abstract

The pH-sensitive cell-entrapable dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) was used to continuously monitor epithelial intracellular pH (pHi) of intact toad lenses, enabling a description of a HCO3- transport mechanism that contributes to pHi homeostasis of this organ. In physiological medium, pH 7.40, the steady-state pHi was 7.48 +/- 0.03 (SE; n = 93). Induction of cell depolarization by either elevation of [K+] to 50 mM, addition of 0.2 mM quinidine, a K(+)-channel blocker, or addition of 0.1 mM Li+ ionophore that equalizes Na+ and K+ permeabilities elicited pHi increases (delta pHi = 0.18 +/- 0.02; P less than 0.0005; n = 13, for K+). These increases could be blocked or reverted by DIDS and were not affected by amiloride. Removal of Na+ induced an amiloride-insensitive acidification. pHi recovery seen upon Na+ reintroduction in the presence of amiloride was inhibited by DIDS. Despite the effects of DIDS on induced pHi changes, the agent did not affect control pHi. Elevation of medium HCO3- (pH to 7.7) produced a pHi increase followed by a spontaneous reversal. This increase was both DIDS and Na+ sensitive. pHi was not affected in any condition by removal (or addition) of Cl-, unless the lens was pretreated with the artificial Cl(-)-HCO3- exchanger tributyltin. Collectively, these results suggest that the primary mechanism for HCO3- movement across the lens epithelial membrane is an electronegative Na+ cotransporter and that this system is near equilibrium under normal physiological conditions.