The stimulatory effect of nicotine on vagal pulmonary C-fibers in dogs

Our recent studies suggested that a nicotine-induced stimulation of afferent vagal C-fibers in the lungs was involved in eliciting the immediate cardiorespiratory responses to inhaling cigarette smoke. To examine this possibility, afferent impulses were recorded from vagal pulmonary C-fibers in 16 anesthetized, open-chest and artificially ventilated dogs, before and after four separate doses of nicotine (2.5, 5, 10 and 20 μg/kg) were injected into the right atrium. The base-line activity did not change after injection of isotonic saline. In contrast, nicotine stimulated 24 of 29 C-fibers: a burst of discharge was evoked immediately (1-2 sec) after the injection and usually lasted 3-8 sec. The peak responses of these pulmonary C-fibers to nicotine injections showed a dose-dependent relationship. In 17 C-fibers tested, the responses evoked by right atrial injection of 10 μg/kg of nicotine were similar to those evoked by delivery into the lungs of a single breath of smoke generated from cigarette with a high-nicotine content. Based upon these results, we conclude that nicotine alone stimulates vagal pulmonary C-fibers in a dose-dependent manner and this stimulant action of nicotine may play a part in eliciting the immediate reflex cardiorespiratory responses to inhalation of cigarette smoke.