Salicylate excretion in the rat; free flow micropuncture experiments.

As in other animals, the renal clearance of salicylate in the rat is greater when the urine is alkaline than when it is acid. In none of the present experiments was the rate of salicylate excretion greater than the rate of its glomerular filtration. Nevertheless, net secretion of salicylate was observed in the proximal tubules of alkalotic, nonalkalotic diuretic, and normal hydropenic animals when the plasma concentration of salicylate was below 150 microgram/ml. Alkalosis enhanced the salicylate content of proximal tubular fluid (approximately 30%). The enhancement is less than would be expected from published reports of proximal tubular fluid pH, if non-ionized salicylate were the only permeant form of the drug. In the absence of alkalosis and at high plasma concentrations, net reabsorption in the proximal tubule was observed. In some circumstances there is net reabsorption of salicylate between the end of the accessible portion of the proximal tubule and the beginning of the distal. Salicylate is reabsorbed from the distal tubule. There was only a small difference in distal reabsorptive rate between normal and alkalotic animals.