Activation of the vasopressin-sensitive water permeability pathway in the toad bladder by N-ethyl maleimide.

Vasopressin stimulates transepithelial water flow in the toad urinary bladder. We report here that N-ethyl maleimide (NEM) (0.1 mM) produces a similar increase in osmotic water flow when applied to the mucosal surface of the tissue. NEM-induced water flow is sensitive to inhibitors of hormone-induced water flow, including serosal acidification, or exposure to quinidine or cytoskeleton-disruptive drugs. NEM-induced water flow is additive with that induced by a submaximal, but not a maximal, dose of vasopressin. The response to mucosal NEM is not reversed on removal of the reagent, but established NEM-induced water flow can be inhibited by serosal acidification or quinidine. Like vasopressin, mucosal NEM induces the appearance of fusion profiles and intramembranous particle aggregates (putative water channels) in the apical plasma membrane of the granular cells, and the incidence of particle aggregates correlates with water flow. NEM does not cause an increase in intracellular cAMP. Our data suggest that NEM stimulates transepithelial water flow by irreversibly activating cellular mechanisms normally triggered by vasopressin, hence causing the insertion of water channels.