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 35SO42- from the interstitium into cortical tubular cells has been determined. Preloading of the rat with sulfate augmented contraluminal 35SO42- influx; pre-perfusion with sulfate-free solutions diminished it. The contraluminal 35SO42- influx in sulfate-loaded animals followed two parameter kinetics (Km 1.4 mmol/l, Jmax 1.2 pmol X s-1 X cm-1). The contraluminal 35SO42- influx (starting concentration 10 mumol/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 35SO42- influx markedly. The increase is larger at pH 6 than at pH 7.4. Changes of pH affect contraluminal 35SO42- influx only when the solutions are Na+- and K+-free. Under these conditions the 35SO42- 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 35SO42- influx at 10 mumol/l 35SO42-, but augmented sulfate influx at 5 mmol/l 35SO42- 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.