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 [(3)H]water,[(14)C]ethanol, and [(14)C]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.