Multispecific anion exchange in basolateral (sinusoidal) rat liver plasma membrane vesicles

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Abstract
The mechanisms and driving forces for hepatic uptake of sulfate were investigated in basolateral (sinusoidal) rat liver plasma membrane vesicles. A transmembrane pH difference (pH 8.0 inside, 6.0 outside) stimulated sulfate uptake above equilibrium (“overshoot”). This pH gradient-stimulated sulfate uptake was saturable with increasing concentrations of sulfate and could be inhibited by probenecid, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), carbonyl cyanide p-(trifluoromethoxy)-phenylhydrazone, and nigericin. At low buffer concentrations and pH 6.0 an inwardly directed sodium gradient also stimulated sulfate uptake. This sodium-dependent sulfate uptake could be inhibited by amiloride and DIDS, indicating indirect coupling of sodium and sulfate flux through concomitant sodium-proton and sulfate-hydroxyl exchange. Cisinhibition of initial pH gradient-stimulated sulfate uptake, as well as transstimulation of sulfate uptake under pH-equilibrated conditions (pH 7.5 inside and outside), were observed with sulfate, thiosulfate, oxalate, and succinate, but not with chloride, bicarbonate, acetate, lactate, pyruvate, p-aminohippurate, citrate, glutamate, aspartate, and taurocholate. Furthermore, cholate and sulfobromophthalein exhibited competitive inhibition of pH gradient-stimulated sulfate uptake. In addition, an inside-to-outside hydroxyl gradient also stimulated uptake of cholate and this pH gradient-sensitive portion of cholate uptake was inhibited by extravesicular sulfate. In contrast to basolateral membranes, no evidence for multispecific sulfate-hydroxyl exchange was found in canalicular plasma membrane vesicles.