ELECTROGENIC COTRANSPORT OF NA-OOCYTES EXPRESSING THE CLONED NA+( ANDSULFATE IN XENOPUS)SO42- TRANSPORT PROTEIN NASI-1/

The Na+/sulfate cotransporter cloned from rat kidney cortex (NaSi-1) h as been expressed in oocytes of Xenopus laevis and subjected to electr ophysiological analysis by current and voltage clamp methods. In curre nt-clamped oocytes, superfusion with 1 mM sulfate resulted in a 12-mV depolarization of the cell membrane. Accordingly, in voltage-clamped o ocytes sulfate induced an inward current (I-S), which was dependent on both the concentration of Na+ and sulfate in the superfusate. Half-ma ximal I-S 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. Thiosu lfate and selenate created similar currents as sulfate with a similar K-m. At saturating concentrations of thiosulfate and selenate, additio n of sulfate could not induce an additive current. Phosphate (1 mM) di d not inhibit sulfate-induced currents. Finally, I-S was dependent on the holding potential being larger at more negative potentials. The re sults of this study strongly suggest an electrogenic cotransport of su lfate and Na+ with a stoichiometry of 1:3.