PH-Dependent transport of cadmium in rat renal brush border membrane vesicles: cadmium efflux via H +-antiport

Abstract

The effects of pH on the Cd efflux across the renal brush border membrane were investigated using isolated rat renal brush border membrane vesicles (BBMV). BBMV equilibrated at pH 5.5 or 7.5 (pH in) were mixed with an experimental buffer at pH 5.5 or 7.5 (pH out) containing CdCl 2. After the incubation with CdCl 2, BBMV were washed with a chelating agent, EGTA, to remove Cd bound to the outer surface of BBMV. The initial accumulation of Cd in BBMV incubated for 1, 3 or 5 min at pH in=5.5 and pH out=7.5 (outwardly directed H +-gradient) was significantly higher than that at pH in=pH out=7.5, but the equilibrated Cd accumulation incubated for over 15 min was almost the same. FCCP, a protonophore, diminished the increasing effect of the H +-gradient on the initial Cd accumulation, whereas valinomycine, a K + ionophore, did not. Kinetic analysis of the Cd accumulation incubated with different concentrations of CdCl 2 suggests that Cd is taken up into BBMV via the unsaturable component and saturable component and the outwardly directed H +-gradient increases the V max value of the saturable component. The BBMV were incubated with CdCl 2 at different temperatures under the outwardly directed H +-gradient, and the increase in Cd accumulation due to the gradient was analyzed by an Arrhenius plot. The plot of increased Cd accumulation was nonlinear, suggesting the involvement of a carrier-mediated process. These findings suggest that the outwardly directed H +-gradient increases the Cd efflux across the renal brush border membrane via H +-antiport system.