Acute effect of cigarette smoke on breathing is attenuated by chronic smoking in rats

To determine whether the irritant effect of cigarette smoke on breathing was affected by chronic exposure to smoke, two groups of sixteen young male Sprague-Dawley rats were exposed daily to fresh cigarette smoke (treated group) or air (control group) for 4-8 wk. Acute ventilatory response to cigarette smoke (50% concn, 6 ml volume) inhaled via a tracheal cannula was then studied under anesthesia and compared between the two groups. Inhalation of either cigarette smoke or gas phase smoke induced an immediate and transient apnea or bradypnea that was completely abolished by bilateral vagotomy in both groups, suggesting the involvement of vagal bronchopulmonary afferents. However, the apneic response was markedly attenuated in the treated group. Inhalation of cigarette smoke, but not the gas phase smoke, also induced a delayed tachypneic response after the initial apnea. The tachypneic response was again significantly smaller in treated rats, and this difference between the two groups was eliminated after vagotomy. On the basis of these results, we suggest that a reduced stimulatory effect on vagal bronchopulmonary sensory receptors was responsible for the attenuated ventilatory responses to smoke observed in treated rats.