Contraluminal sulfate transport in the proximal tubule of the rat kidney

Abstract

In order to study contraluminal sulfate transport the influx rate of³⁵SO ²⁻₄ from the interstitium into cortical tubular cells has been determined. Preloading of the rat with sulfate augmented contraluminal³⁵SO ²⁻₄ influx; preperfusion with sulfate-free solutions diminished it. The contraluminal³⁵SO ²⁻₄ influx in sulfate-loaded animals followed two parameter kinetics (K _m 1.4 mmol/l,J _max 1.2 pmol·s⁻¹·cm⁻¹). The contraluminal³⁵SO ²⁻₄ influx (starting concentration 10 μmol/l) did not change when the K⁺ concentration was varied between 4 and 40 mmol/l and the Ca²⁺ concentration from zero to 3 mmol/l. Omission of Na⁺ from the perfusates augmented contraluminal³⁵SO ²⁻₄ influx markedly. The increase is larger at pH 6 than at pH 7.4. Changes of pH affect contraluminal³⁵SO ²⁻₄ influx only when the solutions are Na⁺- and K⁺-free. Under these conditions the³⁵SO ²⁻₄ influx decreased when the ambient pH was raised from pH 6.0 to pH 8.0. Thiosulfate, selenate, molybdate, oxalate, phosphate, arsenate, and bicarbonate exerted competitive inhibition, while formate, 2-oxoglutarate and paraaminohippurate showed a biphasic response: inhibition at 50 mmol/l, no inhibition at 150 mmol/l. Chloride and bicarbonate inhibited³⁵SO ²⁻₄ influx at 10 μmol/l³⁵SO ²⁻₄ , but augmented sulfate influx at 5 mmol/l³⁵SO ²⁻₄ concentration in rats not preloaded with sulfate. The data indicate the presence of a contraluminal sulfate transport system which is shared by a variety of inorganic and organic anions. The biphasic behaviour of some anions suggests parallel pathways leading to a cis-inhibition at small and trans-stimulation at high anion concentrations. Na⁺ and H⁺ may be cotransported or interact with the transport system at a modifier site.