Absence of nonadrenergic noncholinergic relaxation in the cat cervical trachea

Published in vivo experiments have not supported in vitro reports of the presence of nonadrenergic noncholinergic (NANC) inhibitory pathways in the cat trachea. We therefore examined these pathways, measuring tension in an innervated tracheal segment, flow resistance in more distal airways, and dynamic compliance, in 10 anesthetized mechanically ventilated cats. Initially, cervical vagal stimulation evoked contraction followed by relaxation of smooth muscle of trachea and lower airways; sympathetic stimulation evoked relaxation only. After muscarinic blockade and restoration of smooth muscle tone with 5-hydroxytryptamine (5-HT) applied topically to the tracheal mucosa, vagal stimulation did not affect tracheal segment tension, whereas sympathetic-evoked relaxation was preserved. Similar results were found when tone was restored with intravenous 5-HT, with vagal stimulation also decreasing resistance and increasing compliance. We conclude that NANC pathways are present in lower airways but not in the cervical trachea of the cat. We hypothesize that parasympathetic constriction of cat airway smooth muscle can occur without simultaneous NANC activation, whereas NANC activity occurs only in tandem with parasympathetic stimulation.