Stimulation of C fibers by ammonia vapor triggers mucociliary defense reflex.

The neuropeptide substance P (SP) released from airway sensory C fibers accelerates mucociliary activity, and C fibers in the airways are stimulated by various irritants including ammonia (NH3) vapor. The short-term effects of NH3 vapor on mucociliary function in the in the maxillary sinus of rabbits anesthetized with urethane were investigated by a photoelectric technique. Challenges with 1.5 ml NH3 increased mucociliary activity dose-dependently, the maximal response being 26.6 +/- 1.6%. The increase appeared within 1.3 +/- 0.3 s after exposure. Atropine and hexamethonium decreased the effect of NH3, indicating that part of the response was mediated by cholinergic effector neurons, but a noncholinergic effect clearly remained. Pretreatment with large doses of capsaicin (13 mg i.a.) abolished the response, whereas the SP antagonist (D-Pro2, D-Trp7,9) SP inhibited the noncholinergic response. Challenges with NH3 vapor also decreased the respiratory rate. An identical response was noticed during injections with the C fiber stimulant capsaicin. Together these results indicate that NH3 vapor triggers a mucociliary protective reflex in the airways, involving capsaicin-sensitive C fibers. The recorded increase of mucociliary activity is probably due to the combined effect on the mucociliary system of both SP and acetylcholine released from the afferent and efferent part of the reflex arc, respectively.