Calcium Inhibition of the Action of Vasopressin on the Urinary Bladder of the Toad.

Earlier studies showed that exposure of the isolated urinary bladder of the toad to vasopressin produces striking increases in passive permeability to water and urea as well as increases in active transport of Na-(1-4). Bentley (5) observed that raising the concentration of Ca++ in the sero-sal bathing medium depressed the response to a 1 mU per ml concentration of vasopressin, as measured by the flow of water along a fixed os-motic gradient. In a later study, he reported that Ca++ failed to antagonize the action of vasopres-sin on active sodium transport (6). The nature of the antagonism was not explored further. A detailed study of Ca++ inhibition of the action of vasopressin in vitro should provide additional evidence bearing on the single site theory advanced by Koefoed-Johnson, Ussing, and Ander-sen on the basis of studies on anuran skin (7, 8). According to their hypothesis, transepithelial movement of water and small solutes is limited by a diffusion barrier and a porous barrier arranged in series. The neurohypophyseal hormones increase the pore diameter of the porous boundary, thus producing marked decreases in the resistances to the bulk flow of water and to Na+ transfer, as well as significant increases in the permeability coefficients of urea and related small nonelectrolytes. Leaf and his collaborators (3, 4, 9-11), in an extensive series of studies, also found that the pore theory can be applied to the action of vasopressin on the movement of * water and urea across the toad bladder. Bentley's findings, however, suggest that the effect of vaso-pressin on Na+ transport is not a simple consequence of increased porosity at the surface of the epithelial cells. Three possibilities are raised by these observations: a) Vasopressin may contain two active sites, one of which regulates water transport (subject to Ca++ inhibition) and the other Na+ transport (not subject to Ca++ inhibition); b) Na+ may enter the cell via a nonaqueous pathway that is also altered by vasopressin, while not subject to Ca++ antagonism; or c) vasopressin may have a separate action on the Na+ pump, and Ca++ has no effect on the pump site. Because of these possibilities, we elected to explore the modifying effects of Ca++ on the action of vasopressin in further detail and to examine these effects on all three parameters in the domain of vasopressin action, namely, resistance to osmotic flow of water , the permeability coefficient of urea, and …