In vivo tracer studies of renal urea formation in the bullfrog

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Abstract
The fraction of the uri-nary urea which is formed from uric acid and from arginine within the kidney of Rana catesbiana was measured in vivo. A steady infusion of C14-labeled arginine, uric acid, or urea was given either systemically or through the renal portal vein, and urine from both ureters and systemic blood from the iliac artery were sampled serially. Urea specific activity was measured in all samples and uric acid or arginine specific activities were determined in the plasma samples. By isotopic-dilution calculations it was found that 8. 7% of the urinary urea is formed in the kidney; 7. 7% comes from arginine and 1% from uric acid. It was also shown that 4. 55% of the urinary urea is formed from purines in the whole organism, and that 30% of this occurs in the kidney. It was concluded that secreted urea is derived from blood perfusing the kidney and also from hydrolysis of precursors within the organ itself.

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