Renal tubular reabsorption of inorganic sulfate in the normal dog.

Inorganic sulfate is formed during the metabolism of the sulfur-containing amino acids. Although its exact functions are not well understood, the body nevertheless maintains a basic store of circulating sulfate at all times. Its plasma level, although low, is fairly constant, being maintained in the normal dog within limits of 1.2 and 1.8 millimols per liter. The maintenance of plasma concentration within these limits is brought about by renal mechanisms, an understanding of which is requisite for a complete picture -of the renal regulation of electrolyte balance. Since the sulfate ion has been shown to be completely filterable through the glomerular membrane (5,4), the stabilization of its plasma concentration depends, fundamentally, upon its tubular reabsorption, for upon the characteristics of this process depends the balance between its conservation and elimination by the kidneys. The urinary excretion of sulfate was studied in man by Hayman (5), who found that following the intravenous administration of sodium sulfate, the urine plasma concentration ratio of sulfate approached but never quite reached that of creatinine. This was cited as evidence that some sulfate was reabsorbed by the tubules. In the dog, Goudsmit, Power and Bollman (4) found that at elevated concentrations of sulfate in the plasma, the sulfate clearance approaches, as an asymptote, the creatinine clearance. They too inferred that some sulfate was reabsorbed by the tubules and although they did not analyze their data to prove their point, they indicated that the rate of reabsorption of sulfate was limited at high plasma-sulfate concentrations. More recently the problem was re-investigated in the dog by Schwartz, Smith and Winkler (15), who reached the opposite conclusion, namely, that the capacity to reabsorb sulfate increases directly with the quantity filtered through the glomeruli. Their experiments were complicated by wide variations in urine flow, rapid changes in serum sulfate concentration and progressively declining rates of glomerular filtration. These factors render their conclusions open. to question. The experiments presented below were performed in order to study the characteristics of the renal tubular reabsorption of inorganic sulfate in the normal dog. They indicate that the capacity to reabsorb sulfate is limited, and under normal conditions is unaffected by changes in the rate of reabsorption of other electrolytes. METHODS. Experiments were performed on two female mongrel dogs trained too lie with loose restraint on a eomfortable animal table. Urine collections were