A Saturable, Vasopressin-Sensitive Carrier for Urea and Acetamide in the Toad Bladder Epithelial Cell

The permeability of the toad bladder to a series of isotopically labeled nonelectrolytes was determined in the presence of 150 mM unlabeled acetamide. Under these conditions, overall bladder function was unimpaired, as shown by a normal response to vasopressin of short-circuit current and permeability coefficient of [³H]water,[¹⁴C]ethanol, and [¹⁴C]propionamide. The permeability of the bladder to isotopic acetamide and urea, however, was significantly depressed by unlabeled acetamide, in both the absence and presence of vasopressin. These experiments indicate a competition between unlabeled and isotopic species for binding sites, and show the existence of a saturable, vasopressin-sensitive carrier for urea and acetamide in the epithelial cell membrane.

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