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JS Fedan and DG Frazer
Physiology Section, National Institute for Occupational Safety and Health, Morgantown, West Virginia.
The mechanisms by which the epithelium affects reactivity of guinea pig trachealis to agonists were examined using the isolated, perfused trachea preparation. Contractile agonists (acetylcholine, methacholine, carbachol or histamine) were more potent when applied to the serosal (extraluminal, EL) surface compared to the mucosal (intraluminal, IL) surface, and the IL maximum responses to these agents were smaller. In epithelium-denuded tracheae, IL reactivity to the agonists was increased to the EL level. Physostigmine (10(-7) M) increased the EL and IL potency of acetylcholine to that of carbachol (+/- epithelium), and elevated the IL acetylcholine maximum response (+ epithelium); the relative role of epithelial acetylcholinesterase could not be defined. Indomethacin (3 x 10(-6) M) increased, in an epithelium-dependent manner, the IL acetylcholine, carbachol and histamine maximum responses to the EL level. Phentolamine plus propranolol (both 10(-6) M) potentiated the IL maximum response to methacholine, Isoproterenol also was more potent extraluminally than intraluminally, and the EL and IL maximum responses were similar. IL isoproterenol reactivity was elevated to the EL level in rubbed tracheae. Corticosterone (5 x 10(-5) M) potentiated EL and IL responses to isoproterenol (+/- epithelium); the relative role of epithelial extraneuronal uptake could not be delineated. The epithelium reduces reactivity to mucosally applied drugs by acting as a diffusion barrier. In addition, responses to mucosally administered contractile agonists are inhibited by a physiological antagonism caused by modulatory prostanoids, catecholamines and, possibly, epithelium-derived relaxing factor.
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