Characterization of ATP-dependent Ca2+ transport in the basolateral membrane vesicles from proximal and distal tubules of the rabbit kidney

Basolateral membrane vesicles were prepared from purified proximal and distal tubules of the rabbit kidney. The properties of the ATP-dependent Ca2+ transport were investigated. In both membranes, there was a high affinity, ATP-dependent Ca2+ transport system (Km = 0.1 microM). The optimal concentration of Mg2+ was 0.5 mM and the optimal concentration of ATP was 1 mM. The nucleotide specificity and pH dependence of the Ca2+ transport in both membranes were similar. In basolateral membrane vesicles, calmodulin had no effect on Ca2+ transport. However, in basolateral membrane vesicles depleted of calmodulin, exogenous calmodulin increased the Ca2+ transport by increasing maximal velocity. There were no major differences in the properties of the ATP-dependent Ca2+ transport system in these two membranes. These findings are discussed in relation to why parathyroid hormone differentially modulates Ca2+ transport in these two segments of the nephron.