EFFECT OF UREA ON URINE CONCENTRATION IN THE RAT

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Abstract
Supplementary dietary urea has been shown to increase the maximal non-urea solute concentration of rats on a low protein diet from 1174 mOsm to 1484 mOsm. Analyses of renal tissue from these animals has revealed that rats on a low protein high salt diet can produce a urea concentration in the inner medulla that is 2 or 3 times that in the urine. Infusing normal rats with 15% mannitol at a rate sufficient to lower their urine urea concentration to less than 170 [mu]M/ml will also bring out the ability of the inner medulla of the rat kidney to concentrate urea. The ability of renal medullary tissue of the rat to concentrate urea implies that there is an active transport process for urea in the rat kidney and that the collecting duct of the rat, in contrast to the dog, is impermeable to urea.