Handling of urea and related compounds by the renal tubules of the frog

¹ Departments of Zoology and Physiology, Duke University, Durham, North Carolina

A high arginase activity is found in the kidney of the adult frog Rana catesbeiana which actively secretes urea. Urea accumulates in the kidney in concentrations around seven to eight times the blood concentration. To test whether the kidney arginase causes significant formation of urea in tubular cells at normal plasma arginine levels, C¹⁴ urea, chemical urea, and creatinine clearances were measured. C¹⁴ urea and chemical urea clearances were identical, both being about seven times the GFR. 2,4-Dinitrophenol (DNP) inhibited the active secretion of urea, but C¹⁴ and chemical urea clearances remained identical to each other. When arginine was administered, urea was formed in the kidney and excreted in the urine at the rate of 35 µm/g kidney hr. The chemical urea clearance was then about twice the C¹⁴ urea clearance. DNP administered with arginine caused the C¹⁴ urea clearance to fall below the GFR but did not inhibit the formation of urea as shown by the chemical urea clearance being three times the GFR. Compounds chemically related to urea were handled in different ways by the frog kidney. Acetamide and methylurea were not secreted and did not accumulate in the kidney. DNP had little or no affect on their excretion. Thiourea was found to be actively secreted and to accumulate in the kidney. DNP inhibited thiourea secretion. It is concluded that urea is normally actively transported across the tubular wall and that formation of urea in the tubular cells is insignificant.

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