To elucidate the effects of vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide (PACAP) on airway smooth muscle function, we studied rabbit isolated tracheal segments under isometric conditions in vitro. Addition of VIP and PACAP dose-dependently relaxed tracheal smooth muscle precontracted with acetylcholine, with the order potency being PACAP (1) > or = VIP (0.78), accompanied by the corresponding increase in intracellular cyclic AMP levels. The VIP- and PACAP-induced muscle relaxations were significantly inhibited by ouabain and K(+)-free medium. Incubation of tissues with VIP reduced the contractile responses to electrical field stimulation, whereas PACAP had no effect. These results suggest that VIP and PACAP may cause bronchodilation through activation of Na(+)-K(+)-ATPase and that VIP but not PACAP inhibits the release of acetylcholine from the cholinergic nerve terminals.