Na-Ca exchange in renal tubular basolateral membranes.

We investigated the Na-dependent Ca transport in purified bovine luminal and basolateral renal tubular membranes. Na-dependent Ca uptake was observed in basolateral but not in luminal kidney tubular membranes. 45Ca uptake in basolateral membrane vesicles loaded with Na and suspended in K buffer was significantly greater than that observed in vesicles loaded with Na and suspended in Na buffer. The Na ionophore ETH-227 inhibited Na-dependent Ca uptake indicating that the Ca uptake was dependent on Na gradient. The Ca taken up in the presence of Na gradient could be released by the Ca ionophore A-23187 suggesting that Ca was accumulated inside the vesicles. In vesicles loaded with 45Ca, addition of Na to the media promoted Ca efflux. Methyl triphenylphosponium uptake and Ca uptake were significantly higher in the presence of a Na gradient as compared to those observed in the presence of other monovalent cation gradients, indicating the specificity for Na gradient and arguing against a calcium-activated sodium conductance pathway. The Na-dependent Ca uptake varied with intravesicular Na concentration with an apparent Km of 20-40 mM. The Km for Ca of the Na-dependent Ca uptake was 50 microM and the Vmax was 0.2 nmol/mg protein/min. The Na-dependent Ca uptake was inhibited by LaCl3, tetracaine and verapamil, unaffected by quinidine and amiloride, and slightly stimulated by chymotrypsin. These data demonstrate that the Na-dependent Ca transport by renal basolateral membranes is mediated by a Na-Ca exchange system and not by a calcium-activated sodium conductance pathway.