Evidence for involvement of microtubules in the action of vasopressin in toad urinary bladder

Summary

Colchicine, podophyllotoxin, and vinblastine have been found to inhibit the action of vasopressin on water movement in the toad urinary bladder. Tubulin is the major colchicine binding component of toad bladder epithelial cells, accounting for approximately 3.3% of the total cell protein. More than 99% of the tubulin is found in the soluble fraction after sonication, the remainder is in the particulate fraction. Similar to the characteristics of the binding of colchicine to tubulins from other sources, the binding of colchicine to toad bladder tubulin is temperature- and time-dependent, is inhibited competitively by podophyllotoxin (K _i =5.5×10⁻⁷ m), and has a binding constant of 1×10⁶ liters/mole at 37°. Binding activity decays according to first-order kinetics and is stabilized by vinblastine. The characteristics of the interactions of colchicine and podophyllotoxin with epithelial cell tubulinin vitro closely parallel the ability of these drugs to inhibit the response to vasopressinin vitro. These results, coupled with those of functional and morphological studies, support the view that the ability, of these drugs to affect vasopressin-induced water movement across toad bladder epithelial cells is related to the depolymerization of cytoplasmic microtubules.