the protective role of epitheliumderived nitric oxide in isolated bovine trachea airway epithelial cells from bovine airways can release relaxant factors such as nitric oxide no and prostaglandin e2 and the removal of airway epithelium results in an increased responsiveness of smooth muscle to spasmogen stimuli in this study we assessed whether or not epithelial no modulates the contractile response of bovine trachea in vitrocumulative concentrationresponse curves to acetylcholine ach histamine hist and 5hydroxytryptamine 5ht were obtained in both intact and epithelium denuded tracheal strips in the presence of indomethacin 10 micromin intact but not in epithelium denuded strips preincubation with the no synthase inhibitor lngnitroarginine methyl ester lname but not with dname shifted to the left the concentrationresponse curve to ach pd2 values in the absence and in the presence of lname were 34701 and 46001 respectively p005 and to hist pd2 in the absence and in the presence of lname 38901 and 45401 respectively p005 this effect was reversed by larginine 1mm but not by darginine the contractile response to 5ht was not affected by lname in either intact or epithelium denuded strips these data suggest that no is an epithelial relaxant factor modulating airway cholinergic and histaminergic contraction of bovine trachea and that the activation of the epithelial no synthase is a mediatorspecific process