Inorganic sulfate and thiosulfate: transport and competition in renal tubules of the dog

In the renal tubules of the dog sulfate and thiosulfate are reabsorbed, and thiosulfate is secreted. Secretion of thiosulfate is maximally inhibited by carinamide at plasma levels as low as 20 µg/ml; sulfate transport is not affected. Sulfate and thiosulfate mutually inhibit each other's reabsorption. Simultaneous measurement of sulfate and thiosulfate reabsorption at varying concentration ratios of the two ions indicates that this mutual inhibition is of ‘competitive’ type. At normal sulfate levels, thiosulfate reabsorption increases in a semilogarithmic fashion with rising thiosulfate levels. By using a Lineweaver-Burk plot, thiosulfate Tm (transfer maximum) was estimated at 0.65 of sulfate Tm. Assuming this ratio to be identical with the ratio between the equilibrium constants for S2O3-carrier and SO4-carrier, theoretical reabsorption rates were calculated for thiosulfate and sulfate in each other's presence. The urinary excretion of thiosulfate at low plasma levels indicates that the site of tubular secretion is not distal to the site of reabsorption. Thiosulfate secretion is inhibited by sulfate. A common carrier is suggested for reabsorption of sulfate and thiosulfate and secretion of thiosulfate.