Cl −−HCO 3− exchange in rat renal basolateral membrane vesicles

Abstract

Pathways for HCO 3 − transport across the basolateral membrane were investigated using membrane vesicles isolated from rat renal cortex. The presence of Cl −HCO 3 − exchange was assessed directly by 36Cl − tracer flux measurements and indirectly by determinants of acridine orange absorbance changes. Under 10% CO 2/90% N 2 the imposition of an outwardly directed HCO 3 − concentration gradient (pH o 6/pH i 7.5) stimulated Cl − uptake compared to Cl − uptake under 100% N 2 in the presence of a pH gradient alone. Mediated exchange of Cl − for HCO 3 was suggested by the HCO 3 − 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 HCO 3 − gradient-driven Cl − uptake further suggesting chemical as opposed to electrical Cl−HCO 3 − 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 HCO 3 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 (pH o 7.5/pH i 6), a HCO 3 − 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 −−HCO 3 − 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 HCO 3 − gradient-driven Na + uptake suggesting a basolateral membrane Na +−HCO 3 − for Cl − exchange mechanism, no effect of Na + on Cl−HCO 3 − exchange was observed in the present study.