NICOTINE ENHANCES EXPRESSION OF THE ALPHA-3, ALPHA-4, ALPHA-5, AND ALPHA-7 NICOTINIC RECEPTORS MODULATING CALCIUM-METABOLISM AND REGULATINGADHESION AND MOTILITY OF RESPIRATORY EPITHELIAL-CELLS

The purpose of this study was to investigate the possibility of direct toxic effects of nicotine (Nic) on human bronchial epithelial cells ( BEC) suggested by our previous findings of functional nicotinic acetyl choline receptors (nAChRs) in the epithelial cells lining mucocutaneou s membranes. We now demonstrate for the first time that human and muri ne BEC both in vivo and in vitro express functional nAChRs, and that c lassic alpha 3, alpha 4, alpha 5 and alpha 7 subunits can contribute t o formation of these acetylcholine-gated ion channels, In human bronch ial and mouse lung tissues, and in cultures of human BEG, the nAChRs w ere visualized by subunit-specific antibodies on the cell membranes, p articularly at the sites of cell-to-cell contacts. The epithelial cell s of submucosal glands abundantly expressed alpha 7 nAChRs. Smoking si gnificantly (p<0.05) increased the relative numbers of nAChRs, and thi s effects could be reproduced in cultures of BEC exposed to 10 mu M Ni c. At a higher dose, Nic decreased the relative numbers of alpha 5-con taining nAChRs, suggesting a role for receptor desensitization. The fu nction of the nAChR channels expressed by BEC was demonstrated by biph asic increase in the concentrations of intracellular calcium ([Ca++](i )) in response to activation of the channel by Nic and fluctuations of [Ca++](i) due to channel blockade by mecamylamine (Mec). Long-term ex posure to milimolar concentrations of Nic resulted in a steady increas e of [Ca++](i), which may lead to cell damage. The biological roles of epithelial nAChRs apparently involve regulation of cell-to-cell commu nications, adhesion and motility, because Mec caused rapid and profoun d changes in these cell functions which were reversible by Nic. An ove r exposure of BEC to Nic, however, produced an antagonist-like effect, suggesting that the pathobiological effects of Nic toxicity might res ult from both activation of nAChR channels and nAChR desensitization. We conclude that medical consequences of smoking can be mediated by di rect toxic effects of inhaled Nic on the respiratory tissues wherein N ic specifically binds to and activates the nicotinic ion channels pres ent on the cell surfaces of BEG. We believe that outside the neural sy stem Nic interferes with functioning of non-neuronal cholinergic netwo rks by displacing from nAChR its natural ligand acetylcholine which ac ts as a local hormone or cytokine in a variety of non-neuronal locatio ns.