Efflux of sodium from isolated toad bladder

A method of simultaneously determining the rates of loss of Na 24 across the mucosal and serosal surfaces of the isolated toad bladder is described. The addition of vasopressin to the serosal medium causes an abrupt increase in the efflux of Na 24 across the mucosal boundary, with no significant change in the efflux across the serosal surface. Replacement of sodium in the mucosal medium with choline causes no change in the efflux of Na 24 across either the mucosal or serosal surfaces. The results indicate that the stimulation of active transport of sodium across the bladder after vasopressin is the result solely of an increase in the permeability to sodium of the mucosal surface. Vasopressin does not act directly on the active transport step at the serosal surface. In addition, the saturation kinetics which describe the process of sodium entry at the mucosal boundary are not the result of an exchange diffusion process, and appear not to be due to a sodium-induced change in the permeability of this surface to sodium.