The Relative Rates of Urea and Water Permeation in the Distal Nephron of the Intact Kidney*

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Abstract
Observations were made on the changes in urinary composition of the dog during the acute transition from water diuresis to antidiuresis, induced by vasopressin (ADH) injection. High doses of ADH, 300 mU, produce an initial rise in nonurea solute (NUS) concentration of the urine which is not associated with an increase in urea concentration. It appears that both water and urea permeate the distal nephron readily during this initial period. In contrast, after smaller amounts of ADH, 10 and 30 mU, the urea concentration rises in association with the early increase in NUS, or inulin, concentration. The ability of the epithelial membrane of the distal nephron to discriminate between urea and NUS appears to be a function of membrane permeability, and can be shown to be directly related to the dose of ADH. The apparent high degree of permeability to urea in the distal nephron, in the presence of maximal ADH activity, suggests that trapping of urea, by the medullary vascular countercurrent exchanger, may be the primary means by which a high urea concentration is attained within the distal collecting duct and medullary interstitium during the antidiuretic state.