Cl(-)-HCO3- exchange in rat renal basolateral membrane vesicles.

Pathways for HCO3- transport across the basolateral membrane were investigated using membrane vesicles isolated from rat renal cortex. The presence of Cl(-)-HCO3- exchange was assessed directly by 36Cl- tracer flux measurements and indirectly by determinations of acridine orange absorbance changes. Under 10% CO2/90% N2 the imposition of an outwardly directed HCO3- concentration gradient (pHo 6/pHi 7.5) stimulated Cl- uptake compared to Cl- uptake under 100% N2 in the presence of a pH gradient alone. Mediated exchange of Cl- for HCO3- was suggested by the HCO3- gradient-induced concentrative accumulation of intravesicular Cl-. Maneuvers designed to offset the development of ion-gradient-induced diffusion potentials had no significant effect on the magnitude of HCO3- gradient-driven Cl- uptake further suggesting chemical as opposed to electrical Cl(-)-HCO3- exchange coupling. Although basolateral membrane vesicle Cl- uptake was observed to be voltage sensitive, the DIDS insensitivity of the Cl- conductive pathway served to distinguish this mode of Cl- translocation from HCO3- gradient-driven Cl- uptake. No evidence for K+/Cl- cotransport was obtained. As determined by acridine orange absorbance measurements in the presence of an imposed pH gradient (pHo 7.5/pHi 6), a HCO3- dependent increase in the rate of intravesicular alkalinization was observed in response to an outwardly directed Cl- concentration gradient. The basolateral membrane vesicle origin of the observed Cl(-)-HCO3- exchange activity was verified by experiments performed with purified brush-border membrane vesicles. In contrast to our previous observations of the effect of Cl- on HCO3- gradient-driven Na+ uptake suggesting a basolateral membrane Na+-HCO3- for Cl- exchange mechanism, no effect of Na+ on Cl-HCO3- exchange was observed in the present study.