Electrogenic cotransport of Na+ and sulfate in Xenopus oocytes expressing the cloned Na+SO4(2-) transport protein NaSi-1.

The Na+/sulfate cotransporter cloned from rat kidney cortex (NaSi-1) has been expressed in oocytes of Xenopus laevis and subjected to electrophysiological analysis by current and voltage clamp methods. In current-clamped oocytes, superfusion with 1 mM sulfate resulted in a 12-mV depolarization of the cell membrane. Accordingly, in voltage-clamped oocytes sulfate induced an inward current IS, which was dependent on both the concentration of Na+ and sulfate in the superfusate. Half-maximal IS was observed at about 0.1 mM sulfate and 70 mM Na+. The Hill coefficients were 1 and 2.8 for sulfate and Na+, respectively. Thiosulfate and selenate created similar currents as sulfate with a similar Km. At saturating concentrations of thiosulfate and selenate, addition of sulfate could not induce an additive current. Phosphate (1 mM) did not inhibit sulfate-induced currents. Finally, IS was dependent on the holding potential being larger at more negative potentials. The results of this study strongly suggest an electrogenic cotransport of sulfate and Na+ with a stoichiometry of 1:3.