Evidence for differential reflex regulation of cholinergic and noncholinergic parasympathetic nerves innervating the airways

The hypothesis that cholinergic and nonadrenergic, noncholinergic parasympathetic nerves innervating the airways are subject to differential reflex regulation was addressed. Pronounced contractile and relaxant parasympathetic reflex responses could be evoked by intravenous histamine, laryngeal mucosal application of capsaicin, inhaled capsaicin, or electrical stimulation of the vagal afferent nerves projecting to the esophagus and abdominal viscera. These data suggest that activation of multiple vagal afferent nerve subtypes can initiate both cholinergic and noncholinergic parasympathetic reflexes in the airways. Conversely, hypoxia or activation of the diving response from the nose evoked only cholinergic contractile reflexes. All contractile and relaxant responses evoked by these stimuli were absent in vagotomized animals or in animals pretreated with the ganglionic blocker trimethaphan, confirming their reflex and parasympathetic nature. The data indicate that cholinergic and noncholinergic parasympathetic nerves regulating airway caliber in guinea pigs are comprised of two distinct parasympathetic pathways that are subject to differential reflex regulation. This previously unrecognized complexity of autonomic regulation of airway caliber has potentially important implications for the mechanisms of airways hyperresponsiveness.