An In Vitro Demonstration of Increased Collecting Tubular Permeability to Urea in the Presence of Vasopressin*

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Abstract
Using a technique of in vitro collecting tubular perfusion, we studied the effect of vasopressin on the movement of urea C14 and sucrose C14 out of the rat collecting tubule. Papillae exposed to vasopressin in vivo contained more water and more urea C14 afterin vitro perfusion than did paired controls. Similar differences were not obtained when saline without vasopressin, sucrose C14 instead of urea C14, or hypertonic instead of isotonic perfusate was used. Urea moves more readily across the collecting tubular'' wall in the presence of vasopressin and its movement is attributable in part to an increased permeability of the collecting tubular wall to urea and in part to solvent drag from the accompanying movement of water. A postulate is advanced to account for the ability of urea to enhance water reabsorption in the kidney of the rat and perhaps in other mammals.