Interaction between serotonin and efferent vagus nerves in dog lungs

We anesthetized, paralyzed, and ventilated 32 dogs. In 16 dogs we measured total pulmonary resistance (RL) during inhalation of acetylcholine (ACh), serotonin (5-HT), and histamine (Hist) aerosols. Cooling both cervical vagi reduced the bronchoconstriction caused by 5-HT 64% (P = 0.001), reduced Hist-induced bronchoconstriction 17% (P = 0.003), and did not significantly reduce bronchoconstriction due to ACh. In seven dogs, we ventilated both lungs separately through a double-lumen catheter. Application of 5-HT to one lung increased the transpulmonary pressure amplitude in the homolateral but not in the contralateral lung. Cooling the homolateral vagus reduced this response 32% (P = 0.02). In nine dogs, we stimulated the peripheral ends of both cut cervical vagi before and during aerosol application of ACh, 5-HT, and Hist. ACh and Hist increased baseline RL 97 and 134%, respectively, without increasing the effect of vagal stimulation. 5-HT increased baseline RL only 27% but greatly augmented the effect of vagal stimulation (mean increase, 271%, P = 0.001). We conclude that 5-HT acts to potentiate vagal effects on airway smooth muscle via the efferent vagal pathway.