Acute effects of cigarette smoke on breathing in rats: vagal and nonvagal mechanisms.

The acute ventilatory response to inhalation of cigarette smoke was studied in anesthetized Sprague-Dawley rats. Cigarette smoke (6 ml, 50%) generated by a machine was inhaled spontaneously via a tracheal cannula. Within the first two breaths of smoke inhalation, a slowing of respiration resulting from a prolonged expiratory duration (173 +/- 6% of the base line; n = 32) was elicited in 88% of the rats studied. This initial inhibitory effect on breathing was not affected either by an increase (410%) in the nicotine content of the cigarette smoke or by pretreatment with hexamethonium (33 mg/kg iv). However, bilateral vagotomy completely eliminated the initial ventilatory inhibition. Cooling both vagi to 5.1 degrees C blocked the reflex apneic response to lung inflation, but it did not abolish the inhibitory effect of smoke. After the initial response, a rapid shallow breathing pattern developed and reached its peak 5-12 breaths after inhalation of high-nicotine cigarette smoke; this delayed response could not be prevented by vagotomy and was undetectable after inhalation of low-nicotine smoke. We conclude that the initial inhibitory effect of smoke on breathing is mediated by vagal bronchopulmonary C-fiber afferents, which are stimulated by smoke constituents other than nicotine, whereas the delayed tachypneic response to smoke is caused by the absorbed nicotine.