A possible role of airway epithelium in modulating hyperresponsiveness

1. In order to examine the role of airway epithelium in the responsiveness of smooth muscle in man, we measured the contractile responses to acetylcholine (ACh), histamine, and prostaglandin F2 alpha (PGF2 alpha) and the relaxation response to isoprenaline (Isop), in 48 bronchi obtained from 10 patients who received surgery. Responses were measured in the presence and absence of the epithelium. 2. Removal of epithelium (by rubbing the mucosa gently with forceps) significantly increased the contractile responses evoked by ACh, histamine and PGF2 alpha. 3. In contrast, removal of epithelium did not alter the relaxation response to Isop. 4. To clarify the mechanism underlying this epithelial inhibitory effect on smooth muscle contraction, we measured the contractile responses of dog trachea with the epithelium removed to increasing concentrations of ACh. After measuring the control response, we added about 0.1 g of the chopped epithelium in the organ chamber, and measured the response again. 5. After adding airway epithelium and incubating with tracheal strips, the contractile response of tracheal strips decreased significantly as compared to the control response. 6. These results show that airway epithelium possesses the ability to decrease the smooth muscle contraction to ACh, histamine and PGF2 alpha in man and dogs. 7. The mechanism of this inhibitory effect of the airway epithelium is not explained by a change in mechanical property of the airway nor the change in diffusion of these drugs to the smooth muscle across the epithelium. Thus, these results suggest that airway epithelium may have an important role in modulating smooth muscle tone, possibly by inactivation of these mediators, or by releasing an epithelium-derived relaxing factor.