Contraluminal sulfate transport in the proximal tubule of the rat kidney. I. Kinetics, effects of K⁺, Na⁺, Ca²⁺, H⁺ and anions

In order to study contraluminal sulfate transport the influx rate of 35SO2−4 from the interstitium into cortical tubular cells has been determined. Preloading of the rat with sulfate augmented contraluminal 35SO2−4 influx; preperfusion with sulfate-free solutions diminished it. The contraluminal 35SO2−4 influx in sulfate-loaded animals followed two parameter kinetics (Km 1.4 mmol/l, Jmax 1.2 pmol·s−1·cm−1). The contraluminal 35SO2−4 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 contraluminal 35SO2−4 influx markedly. The increase is larger at pH 6 than at pH 7.4. Changes of pH affect contraluminal 35SO2−4 influx only when the solutions are Na+- and K+-free. Under these conditions the 35SO2−4 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 inhibited 35SO2−4 influx at 10 μmol/l 35SO2−4, but augmented sulfate influx at 5 mmol/l 35SO2−4 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