Stuart-Smith, Karen

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.

Responses to arachidonate were examined in rings with and without epithelium of lobar, segmental, and subsegmental canine bronchi. Arachidonate evoked epithelium-dependent relaxations, which were less pronounced in subsegmental bronchi and abolished by indomethacin and meclofenamate. Nordihydroguairetic acid (NDGA) and nafazatrom reduced epithelium-dependent relaxations only in lobar but unmasked epithelium-independent relaxations to arachidonate in all bronchi. Prostaglandin E2 and prostacyclin relaxed all tissues similarly. In lobar bronchi without epithelium, basal release of prostaglandin E2 was reduced by indomethacin but unaffected by NDGA. Arachidonate augmented prostaglandin E2 release more in subsegmental than in lobar bronchi with epithelium; in bronchi without epithelium the rise was absent (lobar) or attenuated (subsegmental). Arachidonate augmented the release of 6-ketoprostaglandin F1 alpha more in lobar bronchi with than without epithelium; this was inhibited by indomethacin, but not NDGA. Thus arachidonate releases prostaglandin E2 (possibly produced by cyclooxygenase inaccessible to inhibitors and activated by lipoxygenase products) but not prostacyclin from the epithelium. Heterogeneity in response to arachidonate is not due to different sensitivity to, or production of, prostaglandins.

Experiments were conducted in canine bronchi to determine whether the effect of epithelium removal on relaxations was affected by the contractile agent used to induce active force and the degree of contraction. Pairs of fourth-order bronchi with and without epithelium were suspended in organ chambers in physiological salt solution (95% O2-5% CO2, 37 degrees C). The bronchi were contracted to the concentration of acetylcholine or 5-hydroxytryptamine that resulted in a contraction that was 40 or 80% of the response to 10(-4) M of the agonist (ED40 or ED80). Epithelium removal reduced relaxations to isoproterenol and sodium nitroprusside during contraction to the ED80 but not the ED40 of acetylcholine. Responses to forskolin were not affected. Bronchi were significantly more sensitive to relaxing agonists in the presence of 5-hydroxytryptamine; there was no effect on epithelium removal or level of contraction. Thus 1) the influence of epithelium on bronchial relaxation is greatest during high degrees of cholinergic tone and 2) experimental conditions influence the effect of epithelium removal on relaxation.