45Ca fluxes in isolated toad bladder epithelial cells: effects of agents which alter water or sodium transport

Vasopressin enhances osmotic water flow and sodium transport across the toad urinary bladder by mechanisms involving cyclic AMP and calcium. It is believed that changes in intracellular calcium concentration or in its binding to membranes may in part mediate the effects of vasopressin. In addition, several agents which alter the response of the toad bladder to vasopressin may also act by altering cellular calcium metabolism. The effects of vasopressin and several agents which modify its effects in the toad bladder were studied on 45Ca fluxes in isolated epithelial cells from the toad bladder. Compartmental analysis of 45Ca exchange revealed three components. Vasopressin reduced the amount of calcium in the most rapidly exchanging pool from 1.67 +/- 0.20 to 0.86 +/- 0.12 nmol/mg of protein (P less than .025) and the most slowly exchanging pool from 2.72 +/- 0.26 to 1.90 +/- 0.34 nmol/mg of protein (P less than .001), while not affecting the intermediate pool. Theophylline, which mimics the natriferic and hydroosmotic effects of vasopressin, also mimicked the effects on 45Ca exchange by vasopressin. Exogenous cyclic AMP and the prostaglandin endoperoxide analog U46619, which mimic the hydroosmotic effect of vasopressin, also reduced the amount of calcium in the most slowly exchanging pool. Prostaglandin E1, which inhibits the hydroosmotic effect of vasopressin at the concentrations used increased the size of the most slowly exchanging pool. These studies suggest that prostaglandins and other agents which alter the effect of vasopressin in the isolated toad bladder may elicit their effects in part by influencing the calcium concentration at some critical site.