Sodium-dependent phosphate transport protein 2A
phosphate ion transport
Heavy metals inhibit Pi-induced currents through human brush-border NaPi-3 cotransporter in Xenopus oocytes.
Heavy metal intoxication with Hg2+, Pb2+ and Cd2+ commonly leads to phosphaturia. In this study, we examined the effects of these heavy metals on Pi-induced currents (Ip) through NaPi-3, the human renal cotransporter for Na+ and Pi. Hg2+ inhibited Ip in a dose- and time-dependent fashion. Hg2+ decreased the extrapolated maximal current but did not alter the apparent affinity for Pi. This inhibition was also observed with the membrane-permeable oxidizing agent 2,2'-dithio-bis(5-nitropyridine) (DTNP) but not with the membrane-impermeable 5,5'-dithiobis(2-nitrobenzoic acid). Hg(2+)- and DTNP-mediated inhibition of Ip was reversible only in the presence of the reducing agent 2,3-dihydroxybutane-1,4-dithiol. Cd2+ and Pb2+ also inhibited Ip. However, while CD2+ did not significantly alter the apparent affinity for Pi, the apparent concentration needed for half-maximal current (Km) for Pi was increased by Pb2+. In contrast to Hg2+, the inhibition of Ip by Cd2+ and Pb2+ was rapidly reversible upon washout. In the presence of the Na(+)-K(+)-adenosinetriphosphatase inhibitor ouabain, Ip was not reduced, and the effects of the heavy metals were maintained. In summary, the three heavy metals Hg2+, Cd2+, and Pb2+ inhibit Ip through the Na+/Pi cotransporter NaPi-3 by distinct mechanisms. Heavy metal-mediated inhibition of NaPi-3 may be responsible for the phosphaturia observed after intoxication with these compounds.