Sulfate Transport in Rabbit Proximal Convoluted Tubules: Presence of Anion Exchange

These studies describe the characteristics of sulfate transport in proximal convoluted tubules from rabbit kidney. We measured absorptive and secretory fluxes of sulfate in isolated tubular segments perfused and bathed with fluids containing sulfate concentrations of 0.2-10 mM. At 2.0 mM, the sulfate flux in the absorptive direction averaged 4.76 +/- 0.50 and the secretory flux was 3.08 +/- 0.31 pmol . mm-1 . min-1. Ouabain 10(-5) M decreased each to approx. 1.15 pmol . mm-1 . min-1. Kinetic analysis of each unidirectional sulfate flux demonstrated saturation with increasing sulfate concentrations. Thiosulfate 2 mM in the bath inhibited both absorptive and secretory sulfate fluxes; thiosulfate in the perfusate inhibited only the absorptive flux. Similar results were obtained with 10(-6) M SITS in either bath or perfusate. Phosphate had no effect on sulfate transport. Each unidirectional sulfate flux was influenced by the sulfate concentration in the solution on the opposite side in a pattern consistent with the presence of an anion exchange mechanism. Anion exchange transport persisted at 22 degrees C when net sulfate transport was abolished. The data indicate that sulfate transport in the proximal convoluted tubule is bidirectional, independent of phosphate transport, and occurs via two forms of facilitated transport, one of which is an anion exchange mechanism.