Studies have been made on the isolated urinary bladder of the toad, Bufo marinus, in an attempt to evaluate gradients of chemical activity across the mucosal surfaces of the epithelial cells which would serve to maintain a net movement of sodium from the mucosal medium into the cells. The likelihood of such chemical gradients has been established by the demonstration of lower contents of sodium within the tissue, expressed as microequivalents per gram of tissue water, than of concentrations of sodium in the mucosal medium at all levels of the latter examined. The transepithelial transport of sodium and the sodium content of the tissue were found to increase rapidly with rise in concentration of sodium in the mucosal medium up to values of 30 to 60 meq per liter. Further increase in concentration of the medium above this value failed to induce further stimulation of sodium transport or increase in the sodium content of the tissue. Vasopressin increased the rate of transport of sodium at every concentration of sodium in the mucosal medium without altering this relationship. Although entry of sodium across the mucosal surface of the epithelial cells may be passive it is not by free diffusion but involves some considerable interaction with the mucosal surface of the bladder and constitutes the major determinant of the rate of transepithelial transport of sodium. Vasopressin acts to enhance this initial step in the transport of sodium.

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