Calcium transport by the luminal membrane of distal tubule: II. Effect of pH, electrical potential and calcium channel inhibitors.

We studied the effect of pH, electrical potential and calcium channel inhibitors on calcium (Ca2+) uptake by the luminal membranes of distal nephron from rabbit kidney. Ca2+ uptake was measured using 45Ca and the rapid filtration technique. Uptake by the luminal membranes prepared from distal tubule suspensions were compared to the corresponding values obtained with membranes from proximal tubules. In the distal tubule experiments, Ca2+ transport was measured in the presence and the absence of Na+ in the incubation medium. As previously reported, Na+ inhibited Ca2+ uptake by the distal membrane vesicles. Increasing the pH of either the extravesicular or intravesicular media, or both, enhanced Ca2+ uptake by the distal membrane vesicles. We previously described two kinetics of Ca2+ transport by the luminal membrane of the distal nephron, with high and low affinities, respectively. In both the presence and absence of Na+, alkaline pH stimulated the low affinity-high velocity system by decreasing the Km Ca2+. In the presence of Na+, alkaline pH also increased the Vmax Ca2+ of the high affinity system with no significant changes in Km. These effects of pH were not related to a H+/Ca2+ exchange mechanism. In contrast, pH did not significantly influence Ca2+ transport through the proximal membrane vesicles. When an electrical potential gradient (inside negative) across the vesicles was created by either various anion gradients through the membranes, or by inducing K+ efflux with valinomycin, no relation could be detected between the electrical gradient and 0.5 mM Ca2+ uptake by the proximal or distal membranes.(ABSTRACT TRUNCATED AT 250 WORDS)