Synergistic interactions between airway afferent nerve subtypes mediating reflex bronchospasm in guinea pigs

The hypothesis that airway afferent nerve subtypes act synergistically to initiate reflex bronchospasm in guinea pigs was addressed. Laryngeal mucosal application of capsaicin or bradykinin or the epithelial lipoxygenase metabolite 15(S)-hydroxyeicosatetraenoic acid evoked slowly developing but pronounced and sustained increases in tracheal cholinergic tone in situ. These reflexes were reversed by atropine and prevented by vagotomy, trimethaphan, or laryngeal denervation. Central nervous system-acting neurokinin receptor antagonists also abolished the reflexes without altering baseline cholinergic tone. Baseline tone was, however, reversed by disrupting pulmonary afferent innervation while preserving the innervation of the trachea and larynx. Surprisingly, selective pulmonary denervation also prevented the laryngeal capsaicin-induced tracheal reflexes, suggesting that laryngeal C-fibers act synergistically with continuously active intrapulmonary mechanoreceptors to initiate reflex bronchospasm. Indeed, reflex bronchospasm evoked by histamine was markedly potentiated by bradykinin, an effect mimicked by intracerebroventricular, but not intravenous, substance P. These data, as well as anatomic evidence for afferent nerve subtype convergence in the commissural nucleus of the solitary tract, suggest that airway nociceptors and mechanoreceptors may act synergistically to regulate airway tone.