Inhibition of airway smooth muscle contraction by airway epithelium in human bronchus]

To clarify the mechanism underlying the inhibitory effect of epithelial cells on smooth muscle contraction, we studied two types of human bronchial tissue preparations: (1) "acceptor" bronchial strip without epithelium, (2) "donor" bronchial ring with or without epithelium. We measured the contractile responses of acceptor bronchial strip surrounded by donor bronchial ring to increasing concentrations of acetylcholine (ACh). Removal of the epithelium of the donor bronchial rings significantly enhanced the contractions of the acceptor bronchial strips. Thus, airway epithelium decreases the airway smooth muscle contraction to ACh in human bronchus. The mechanism of this inhibitory effect of airway epithelium was not due to a change in mechanical property of the airway, nor to a change in diffusion path for the chemical mediators. These results suggest that human airway epithelium may have an important role in modulating airway smooth muscle tone, possibly by the release of an epithelium-derived relaxing factor.