Renal clearance of salicylic acid and salicyluric acid in the rat and in the isolated perfused rat kidney.

The renal clearance of salicylate (SA) and salicyluric acid (SU) was studied after i.v. bolus injection of 5 mg/kg of SA to ureter cannulated rats and after administration of 1 to 20 mg of SA or 2.5 to 10 mg of SU to the isolated perfused rat kidney. Previous studies in this laboratory have shown that SA and SU are metabolically interconverted in the isolated perfused rat kidney. The renal clearance of SU was greater than the glomerular filtration rate after either SA or SU administration. SA renal clearance was much less than the glomerular filtration rate when SA was administered but was greater than the glomerular filtration rate when it was formed from SU. The data support the hypothesis that administered SA is cleared differently than SA formed from SU in the isolated perfused rat kidney, presumably due to rate-limiting diffusion of SA into the cell.