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Abstract

Uptake of SO₄²⁻ into brush-border membrane vesicles isolated from rat kindey cortex by a Ca²⁺-precipitation method was investigated by using a rapid-filtration technique. Uptake of SO₄²⁻ by the vesicles was osmotically sensitive and represented transport into an intra-vesicular space. Transport of SO₄²⁻ by brush-border membranes was stimulated in the presence of Na⁺, compared with the presence of K⁺ or other univalent cations. A typical ‘overshoot’ phenomenon was observed in the presence of an NaCl gradient (100mm-Na⁺ outside/zero mm-Na⁺ inside). Radioactive-SO₄²⁻ exchange was faster in the presence of Na⁺ than in the presence of K⁺. Addition of gramicidin-D, an ionophore for univalent cations, decreased the Na⁺-gradient-driven SO₄²⁻ uptake. SO₄²⁻ uptake was only saturable in the presence of Na+. Counter-transport of Na⁺-dependent SO4₄²⁻ transport was shown with MoO₄²⁻ and S₂O₂²⁻, but not with PO₄²⁻. Changing the electrical potential difference across the vesicle membrane by establishing different diffusion potentials (anion replacement; K⁺ gradient±valinomycin) was not able to alter Na+-dependent SO₄²⁻ uptake. The experiments indicate the presence of an electroneutral Na⁺/SO₄²⁻-co-transport system in brush-border membrane vesicles isolated from rat kidney cortex.