Drazen. In vivo demonstration of nonadrenergic inhibitory innervation of the guinea pig trachea

ofautonomic nerves could excite nonadrenergic inhibi-tory motor pathways in the guinea pig respiratory system in vivo, we studied the effects of electrical stimulation of the cervical vagi and sympathetic nerve trunks on pressure changes (Pp) within an iso-lated, fluid-filled cervical tracheal segment which re-flected changes in trachealis muscle tone. We pre-served the innervation and circulation of the segment as evidenced by a rise in Pp with vagus nerve stimula-tion and a fall in Pp with intravenous isoproterenol. In five atropine-treated animals, stimulation of the cut vagi or sympathetic nerve trunks resulted in a mean fall in P, of 7.9 and 8.2 cm H2O, respectively. Treatment with propranolol attenuated the response to sympathetic stimulation but not vagal stimulation. To determine if these relaxation responses were mediated by an adrenergic or nonadrenergic mechanism, we studied an additional five animals that had been treated with 6-hydroxydopamine to destroy adrenergic nerve endings. In 6-hydroxydopamine, atropine, and propranolol-treated animals, sympathetic nerve stim-ulation decreased Pp only 0.65 cm H20, confirm-ing the elimination of adrenergic nerve influences, whereas vagus nerve stimulation decreased Pp 17.7 cm H20. After sectioning the recurrent laryngeal nerves, the mean decrease in Pp during vagus nerve stimulation was only 3.2 cm H20. These findings demonstrate th