Verapamil inhibition of vasopressin-stimulated water flow: possible role of intracellular calcium.

The effects of the organic calcium antagonists verapamil and methoxyverapamil (D-600) on vasopressin-stimulated water flow in response to an osmotic gradient were investigated. When toad bladders were exposed to either verapamil or D-600 for 60 min, vasopressin-stimulated water flow was inhibited. In the presence of verapamil (100 microM), water flow decreased from 38 +/- 3 to 26 +/- 3 mg/min/hemibladder (P less than .05, n = 5) and in the presence of D-600 (100 microM) from 48 +/- 3 to 32 +/- 4 mg/min/hemibladder (P less than .02, n = 5). When preincubated with hemibladders for 90 min they also inhibited basal 45Ca uptake into isolated toad bladder epithelial cells (from 1.58 +/- 0.20 to 1.02 +/- 0.14 nmol/mg of protein per 5 min, P less than .05, n = 6). Addition of verapamil to the bathing media during recovery from a vasopressin stimulus to block reuptake of calcium into the epithelial cells inhibited water flow in response to a subsequent incubation with vasopressin to a greater extent than the inhibition of water flow when verapamil was present only before exposure to vasopressin. Inhibition was also greater in the presence of lower extracellular calcium concentrations. The results are consistent with the notion that the calcium antagonists inhibit vasopressin-stimulated water flow by virtue of depleting a critical pool of intracellular calcium rather than preventing the simultaneous entry of calcium from extracellular sites during the exposure to vasopressin.