Intrarenal distribution of urea and related compounds: effects of nitrogen intake

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Abstract
Nitrogen and NaCl intake influence the excretion of urea. In rats maintained on a low-protein, high-salt intake the urea-to-inulin clearance ratio decreases to 0.01. In antidiuretic "low-protein" rats the urea concentrations in the inner medulla are higher than in the urine, whereas in rats on high-protein diet the urea concentration in the inner medulla is lower than in the urine. The specific activity of urea-C14 1 hr after subcutaneous injection is approximately the same in plasma, renal cortex, and outer and inner medulla. The intrarenal distributions of acetamide and methylurea show the same pattern and are influenced by nitrogen intake in the same way as urea, whereas thiourea concentrations in urine are always higher than in the papilla. It is concluded a) that urea formation cannot account for papilla urea concentrations higher than urine concentrations; b) that the handling of urea and related compounds in the collecting duct changes in a similar way with changes in the nitrogen and salt intake; and c) that in the rat urea, methylurea, and acetamide are either reabsorbed in the collecting duct against their concentration gradients or bound by the papilla tissue.

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