Autonomic interplay in VIP-induced tracheal smooth muscle relaxation

VIP-induced inhibitory responses of the guinea-pig tracheal pouch, an in vivo preparation designed to demonstrate non-noradrenergic non-cholinergic innervation, where determined in chloraloseurethane anaesthetized animals under control conditions, or pretreated with atropine, propranolol, both atropine and propranolol or atropine-propranolol with indomethacin. Of the five groups, the control group showed a significantly greater cumulative dose-response relaxation to VIP than did the treatment groups. Among the treatment groups, the propranolol pretreated animals showed significantly greater relaxation than the remaining treatments. The group that received atropine alone and the combination of atropine and propranolol showed the least relaxation to VIP. The relaxations in these two groups did not differ significantly. The significantly greater relaxation of the pouch to VIP in the propranolol treated group suggested that the non-noradrenergic inhibitory mechanism for VIP is dependent upon the existing smooth muscle tone of the pouch. Pretreatment of the guinea-pigs with indomethacin did not cause a significant change in the relaxation of the pouch due to VIP suggesting that VIP caused airway smooth muscle relaxation by a mechanism other than that of the release of bronchodilator prostaglandins.