Functional roles of Na+ and H+ in SO2-4 transport by rabbit ileal brush border membrane vesicles.

Abstract

Sulphate uptake by rabbit ileal brush border membrane vesicles was stimulated by a transmembrane sodium gradient [( Na+]o greater than [Na+]i), but not by a similar potassium gradient. 35SO4(-2) influx ( JSO4 oi ) from outside (o) to inside (i) these vesicles was a hyperbolic function of [SO4-2]o and the affinity constant for anion transport was strongly influenced by [Na+]o (100 mM Na+, KSO4 t = 0.52 mM SO4 -2; 10 mM Na+, KSO4 t = 4.32 mM SO4-2). JSO4 oi was a sigmoidal function of [Na+]o at pH 7.4 for both low (0.2 M) and high (4.0 mM) [SO4-2]o. The Na+-dependency of JSO4 oi was examined at pH 6.0, 7.4, and 8.0 (same pH inside and outside). At pH 6.0 and 7.4 sigmoidal Na+-dependent JSO4 oi exhibited nonlinear Eadie-Hofstee plots indicative of a transport mechanism capable of binding a variable number of sodium ions over the [Na+]o range used. Hill plots of anion transport under these conditions displayed slopes near unity at low [Na+]o and slopes approximating 2.0 at higher cation concentrations. At pH 8.0, Na+-dependent JSO4 oi was hyperbolic and showed linear Eadie-Hofstee and Hill plots, the latter with a single slope near 1.0. When a H+ gradient was imposed across the vesicle wall (pHi = 8.0, pHo = 6.0), Na+-dependent JSO4 oi was hyperbolic and significantly increased at each [Na+]o over values observed using bilateral pH 8.0. In contrast, a H+ gradient oriented in the opposite direction (pHi = 6.0, pHo = 8.0) led to Na+-dependent JSO4 oi that was sigmoidal and significantly lower at each [Na+]o than values found using bilateral pH 6.0. Electrogenicity of JSO4 oi at pH 8.0 for both high and low [Na+]o was demonstrated by using a valinomycin-induced transmembrane electrical potential difference. At pH 6.0, electrogenic JSO4 oi occurred only at low [Na+]o (5 mM); anion transfer was electroneutral at 50 mM Na+. A model is proposed for proton regulation of sodium sulphate cotransport where flux stoichiometry is controlled by [H+]i and sodium binding affinity is modified by [H+]o. Preliminary experiments with rabbit proximal tubular brush border membrane vesicles disclosed similar JSO4 oi kinetic properties and a common transport mechanism may occur in both tissues.