Transport characteristics of a murine renal Na/Pi-cotransporter

A complementary deoxyribonucleic acid (cDNA) corresponding to a murine renal cortical Na/phosphate-(Na/Pi-) 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 Pi (and with arsenate). Increasing the Na concentration led to a sigmoidal elevation in Pi-induced short-circuit current; the apparent Michaelis constant, Km, (around 40 mM Na) was increased by lowering the pH of the superfusate but was not influenced by altering the Pi concentration. Increasing the Pi (and arsenate) concentration led to a hyperbolic elevation in Na-dependent short-circuit current (apparent Km for Pi at 100 mM Na was around 0.1 mM; apparent Km for arsenate was around 1 mM); lowering the Na concentration decreased the apparent affinity for Pi. The Pi-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 Pi concentration was calculated in total, or if distinction was made between its mono- and divalent forms. Thus, the pH dependence of Na-dependent Pi transport (total Pi) may not be related primarily to a pH-dependent alteration in the availability of divalent Pi, but includes also a competitive interaction of Na with protons. The effect of Pi and Na concentration on the apparent Km values for Na or Pi, respectively, provides evidence for an ordered interaction of "cosubstrate" (Na first) and "substrate" (Pi or arsenate second).