Transport characteristics of a murine renal Na/Pi

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).