TRANSPORT CHARACTERISTICS OF A MURINE RENAL NA/P-I-COTRANSPORTER

A complementary deoxyribonucleic acid (cDNA) corresponding to a murine renal cortical Na/phosphate-(Na/P-i-) cotransporter was isolated and its transport properties characterized by electrophysiological techniques after expression in Xenopus laevis oocytes. A Na-dependent inward movement of positive charges (''short-circuit current'') was observed upon superfusion with P-i (and with arsenate). Increasing the Na concentration led to a sigmoidal elevation in P-i-induced short-circuit current; the apparent Michaelis constant, K-m, (around 40 mM Na) was increased by lowering the pH of the superfusate but was not influenced by altering the P-i concentration. Increasing the P-i (and arsenate) concentration led to a hyperbolic elevation in Na-dependent short-circuit current (apparent K-m for P-i at 100 mM Na was around 0.1 mM; apparent K-m for arsenate was around 1 mM); lowering the Na concentration decreased the apparent affinity for P-i. The P-i-induced short-circuit current was lower at more acidic pH values (at pH 6.3 it was about 50% of the value at pH 7.8); this pH dependence was similar if the P-i concentration was calculated in total, or if distinction was made between its mono- and divalent forms. Thus, the pH dependence of Na-dependent P-i transport (total P-i) may not be related primarily to a pH-dependent alteration in the availability of divalent P-i, but includes also a competitive interaction of Na with protons. The effect of P-i and Na concentration on the apparent K-m values for Na or P-i, respectively, provides evidence for an ordered interaction of ''cosubstrate'' (Na first) and ''substrate'' (P-i or arsenate second).