Heterogeneity in the effects of epithelium removal in the canine bronchial tree

The effect of removing the airway epithelium on the responses of canine airways of decreasing diameter to contractile and relaxing agonists was explored. Three orders of canine bronchus were studied: second order (lobar bronchus), third order (segmental bronchus), and fourth order (subsegmental bronchus). Paired rings of tissue, with and without epithelium, were placed in organ chambers in physiological salt solution gassed with 95% O2-5% CO2 and maintained at 37 degrees C. For second- and third-order bronchi, epithelium removal caused significant left-ward shifts of the concentration-effect curves for 5-hydroxytryptamine, histamine, and acetylcholine. In fourth-order bronchi, there was no significant shift for any of the contractile agonists. Isoproterenol (during contractions evoked by acetylcholine) induced concentration-dependent relaxations that were significantly greater in bronchi with than in those without epithelium. This effect was most prominent in fourth-order bronchi. These results suggest that 1) the canine airway epithelium releases a relaxing factor, 2) in larger airways the major effect is reduction of contractile responses, and 3) in smaller airways the major effect is enhancement of relaxing responses.