pH gradient-stimulated sulfate transport by rabbit ileal brush-border membrane vesicles: evidence for SO4-OH exchange

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Abstract
In the presence of a pH gradient (7.7 inside, 5.5 outside), the initial velocity of SO4 uptake by rabbit ileal brush-border membrane (BBM) vesicles was markedly stimulated compared with uptake in the absence of a pH gradient. Under pH gradient conditions, SO4 was transiently accumulated at a concentration 13-fold higher than at equilibrium ("overshoot"). Superimposition of a HCO3 gradient did not further stimulate the initial velocity of SO4 uptake compared with a pH gradient alone. Evidence that this pH gradient-stimulated SO4 uptake represented SO4-OH exchange included 1) lack of sensitivity of SO4 transport to alterations of the membrane potential; 2) 85-95% inhibition of SO4 uptake by the anion exchange inhibitors DIDS and SITS; and 3) saturation kinetics (Km for SO4 = 0.475 .+-. 0.054 mM; Vmax = 4.1 .+-. 0.1 nmol SO4 .cntdot. mg prot-1 .cntdot. min-1). Sulfate did not inhibit pH gradient-stimulated 36Cl uptake, indicating that SO4-OH and Cl-HCO3(OH) are different exchangers. When BBM vesicles were compared with basolateral membrane (BLM) vesicles, pH gradient-stimulated SO4 uptake was found predominantly in the BBM preparation. Brush-border SO4-OH exchange was further localized by demonstrating Na-stimulated SO4 efflux from vesicles loaded under pH gradient conditions, suggesting that Na-SO4 cotransport and SO4-OH exchange are on the same BBM vesicles. In conclusion, a SO4-OH exchanger (or H-SO4 cotransporter) exists on the brush border of rabbit ileum which is distinct from the brush-border Cl-HCO3(OH) exchanger. This report raises the possibility that under certain circumstances Na-stimulated SO4 uptake may involve a dual-exchange mechanism (Na-H exchange and SO4-OH exchange).