Actin filaments and vasopressin-stimulated water flow in toad urinary bladder

Vasopressin increases the water permeability of the apical membrane of the granular epithelial cells of the toad urinary bladder. Cytochalasin B inhibits this action of the hormone, indicating that microfilaments may play a role in the water permeability response. We have extended previous functional studies with cytochalasin B and have demonstrated that dihydrocytochalasin B, a more specific inhibitor of actin filament elongation, similarly diminishes the hydrosmotic response to vasopressin. Biochemical studies of isolated epithelial cells indicate that an actin-like protein accounts for about 10% of the soluble protein of the epithelium. Morphological studies of whole toad bladders incubated with heavy meromyosin conclusively demonstrate that actin is a component of the epithelial cells and that actin-containing filaments are associated with both plasma membranes and cytoplasmic organelle membranes. Taken together, these findings provide strong, albeit indirect, evidence that actin microfilaments play a functional role in the hormone-induced increase in water permeability in the toad urinary bladder.