Nicotine-induced airway smooth muscle contraction: neural mechanisms involving the airway epithelium. Functional and histologic studies in vitro.

To assess the mechanism of and the role of the epithelium in nicotine-induced bronchoconstriction in vitro, we performed a combined functional and histologic study. Functional study: We suspended tracheal strips or rings from 16 ferrets (1124 +/- 561 g, mean +/- SD) in organ baths. Alternate tracheal strips had their epithelium removed. Dose-response curves to acetylcholine (ACh) and nicotine were established for pairs of tissues with and without epithelium, each pair receiving only one dose of nicotine. Nicotine induced brief muscle contractions not exceeding 25% of the ACh-induced maximum. Contractions were blocked by hexamethonium and 10(-7) M atropine and were abolished or inhibited strongly by tetrodotoxin (TTX), suggesting the involvement of nicotinic neuronal and muscarinic smooth muscle receptors. Removal of the epithelium strongly inhibited contractions at concentrations of nicotine greater than 3 x 10(-5) M which completely removed any dose-response effect. ACh-induced contractions were unchanged, demonstrating smooth muscle integrity. We suggest that the removal of the epithelium attenuates nicotine-induced bronchoconstriction through the removal of nerves running in or close to the epithelium. Histologic study: In tracheae from 15 ferrets (8 male, 7 female), mean weight (+/- SD) 1288 (+/- 470) g, we examined 4 techniques of epithelium removal: (1) gentle scraping with a scalpel blade moved backwards (away from the cutting edge), (2) moving a Q-tip through the unopened tracheal tube without lateral pressure, and (3, 4) stroking the mucosa of opened tracheal segments with a Q-tip, exerting (3) light or (4) moderate pressure. All methods were equally (97%-100%) efficient in removing the epithelium but differed in the amount of damage caused to the basement membrane and/or submucosal tissue. Method (2) caused less damage to the basement membrane than the other methods but still removed almost one-third of it. The study showed that complete removal of the epithelium is at the expense of the submucosa and that a given result of "epithelium removal" is also attributable to removal of the neighboring subepithelial structures.