The effects of parasympathectomy on serotonin-induced bronchoconstriction in the cat.

We studied the contribution of neural influences, in particular, the nonadrenergic, noncholinergic inhibitory system, on the modulation of airway responses to serotonin (5-HT) in the anesthetized and mechanically ventilated cat. Airway responses were assessed by measurement of pulmonary resistance (RL). Parasympathectomy (PS) caused a variable increase in bronchoconstriction in 9 of 10 cats (n = 10) that had received pretreatment intravenously with atropine (3 mg/kg) and propranolol (2 mg/kg); PS performed following pretreatment with phentolamine (2 mg/kg), propranolol, and atropine led to an increase in bronchoconstriction in only 2 of 6 cats. Dose-response curves to aerosolized 5-HT were performed in one group of cats (n = 5) before and 30 min after PS and in control group (n = 6). Both groups of animals were pretreated intravenously with atropine, propranolol, and phentolamine. The concentration of aerosolized 5-HT that was necessary to double pulmonary resistance (ED200RL) was significantly reduced from 0.117 to 0.062 mg/ml (p less than 0.01) by PS, in contrast to control cats in which ED200RL during 2 consecutive dose-response curves (0.076 and 0.087 mg/ml, respectively) did not change significantly. We conclude that there is neural modulation of airway responses to infused 5-HT through an alpha-adrenergic pathway and that an additional inhibitory nonadrenergic, noncholinergic neural influence is demonstrable in the response to aerosolized 5-HT.