Sulphate/molybdate interactions: in vivo and in vitro studies on the group VI oxyanion transport system in ovine renal tubule epithelial cells.

The study was undertaken to investigate the mechanism by which increased dietary sulphur compounds accelerate the excretion of molybdate in ruminants. Clearance studies show that [99Mo]molybdate and [35S]sulphate have similar behaviours in sheep; at plasma molybdate concentrations equal to or below the apparent t max the injection of sulphate accelerates the excretion of molybdate and vice-versa. The uptake of [35S]sulphate into brush-border membrane vesicles isolated from ovine kidney cortex by a Ca++ precipitation method was investigated by a rapid filtration technique. Transport of [35S]sulphate into the vesicles was stimulated in the presence of Na+ but not by K+. A typical "overshoot" phenomenon was observed in the presence of the Na+ gradient (initially 100 mM Na+ outside/zero mM Na+ inside). Both the rate and extent of accumulation was reduced by the presence of other group VI oxyanions, molybdate, selenate and tungstate and by an analogue thiosulphate, but not by other divalent anions such as phosphate or by pyruvate. Uptake was inhibited by HgCl2, N-ethyl-maleimide (NEM) and ouabain. The experiments indicate the presence of an electroneutral Na+/SO4- or group VI oxyanion co-transport system in ovine proximal tubule brush border membranes similar to that described in the rat. Sulphate/molybdate competition for sites in tubular reabsorption is another factor to be considered in the complex interactions of S and Mo compounds with copper in animal nutrition.