Sodium-dependent sulfate transport in renal outer cortical brush border membrane vesicles.

The transport of sulfate into outer cortical brush border membrane vesicles (early proximal tubule) was studied. Sulfate uptake was markedly enhanced by sodium and slightly enhanced by lithium but showed no stimulation by other monovalent cations tested. Kinetic analysis of the sodium-dependent component of sulfate flux indicated a single transport system obeying Michaelis-Menten kinetics. Studies of the dependence of sulfate flux on sodium concentration indicated that more than one sodium ion was involved in the sulfate transport event. Although sulfate influx was found to be stimulated by negative intravesicular potentials, evidence is presented that sodium/sulfate cotransport is nevertheless an electroneutral event presumably involving 2 Na+ per SO2-4. Under zero trans sodium conditions sodium-dependent sulfate influx was stimulated by the presence of intravesicular unlabeled sulfate (trans stimulation); however, a similar stimulation of sulfate efflux by the presence of unlabeled extravesicular sulfate was not observed. These latter results are consistent with an ordered binding scheme for the transporter, with sulfate binding before sodium on the extravesicular (urine) surface and dissociating before sodium on the intravesicular (cytosolic) surface.