Reflex apneic response evoked by laryngeal exposure to wood smoke in rats

Wood smoke is a type of potent inhaled irritants, but its irritant effects on the larynx are not fully understood. We studied the reflex respiratory responses evoked by laryngeal exposure to wood smoke in anesthetized Sprague-Dawley rats. Five ml of wood smoke were delivered into an isolated larynx at a constant flow rate of 1.4 ml/sec while animals breathed spontaneously via a trachea! tube. In each of the 30 rats tested, delivery of wood smoke immediately (within 1 s) triggered an apnea accompanied by hypertension. The apneic duration reached 6.9 ± 0.8 s or 1635 ±171 % (SB) (n = 30) of the mean baseline expiratory duration. Delivery of air or a gas mixture containing concentrations of O₂, CO₂, and CO matching those in wood smoke did not cause any detectable change in breathing pattern. After removal of smoke particulates, gas-phase smoke alone evoked an apneic response similar to that evoked by unfiltered smoke (n = 14). However, the smoke-induced apneic response was completely abolished by local application of anesthetic (lidocaine hydrochloride, 8%) to the mucosa of this airway segment (n = 8) or by denervation of superior laryngeal nerves (n = 30). These results suggest that laryngeal exposure to a small amount of wood smoke in rats evokes an apnea which may be a reflex resulting from a stimulation of laryngeal afférents by the gas phase of smoke.