Epithelial modulation of cholinergic responses in human tracheal smooth muscle

The aim of this study was to test the hypothesis that the increase in maximal responses to histamine, acetylcholine, and cholinergic electrical field stimulation and decreased relaxant responses to isoprenaline reported in asthmatic tracheal smooth muscle result from the epithelial damage observed in asthma. The effect of mechanical removal of the epithelium on contractile and relaxant responses was examined in normal human postmortem tracheal smooth muscle strips. The epithelium was removed from alternate tracheal strips obtained from 25 subjects within 14 h of sudden death from nonrespiratory causes. In paired samples, contractile cholinergic and inhibitory nonadrenergic, noncholinergic (i-NANC) neural responses to electrical field stimulation and responses to exogenous histamine, potassium chloride, theophylline, and isoprenaline were unaffected by removal of the epithelium. However, the maximal isometric tension (Tmax) induced by methacholine increased by 70.1 +/- 19.8% (mean +/- SE, p < 0.005, n = 9), without alteration in EC50. These data suggest that disruption of the epithelium is unlikely to be the explanation of the abnormalities observed in trachea in fatal asthma. Explanations of the increase in maximal response to methacholine following removal of the epithelium include loss of an epithelium-derived relaxant factor released via an epithelial muscarinic receptor or loss of a specific permeability or metabolic barrier imposed by the epithelium for methacholine.