Functional roles of Na⁺ and H⁺ in SO ²⁻₄ transport by rabbit ileal brush border membrane vesicles

Summary

Sulphate uptake by rabbit ileal brush border membrane vesicles was stimulated by a transmembrane sodium gradient ([Na⁺] _o >[Na⁺] _i ), but not by a similar potassium gradient.³⁵SO ²⁻₄ influx (J ^SO4 _oi ) from outside (o) to inside (i) these vesicles was a hyperbolic function of [SO ²⁻₄ ] _o and the affinity constant for anion transport was strongly influenced by [Na⁺] _o (100mm Na⁺,K ^SO4 _t =0.52mm SO ²⁻₄ ; 10mm Na⁺,K ^SO4 _t =4.32mm SO ²⁻₄ ).J ^SO4 _t was a sigmoidal function of [Na⁺] _o at pH 7.4 for both low (0.2m) and high (4.0mm) [SO ²⁻₄ ] _o . The Na⁺-dependency ofJ ^SO4 _t 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⁺-dependentJ ^SO4 _t 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⁺-dependentJ ^SO4 _t 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 (pH _i =8.0, pH _o =6.0), Na⁺-dependentJ ^SO4 _t 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 (pH _i =6.0, pH _o =8.0) led to Na⁺-dependentJ ^SO4 _t that was sigmoidal and significantly lower at each [Na⁺] _o than values found using bilateral pH 6.0. Electrogenicity ofJ ^SO4 _t 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, electrogenicJ ^SO4 _t occurred only at low [Na⁺] _o (5mm); anion transfer was electroneutral at 50mm 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 similarJ ^SO4 _t kinetic properties and a common transport mechanism may occur in both tissues.