Heterogeneity in epithelium-dependent responses.

The airway epithelium exerts a profound influence on the responsiveness of bronchial smooth muscle to both contracting and relaxing agents. This may be due to the release of an epithelium-derived factor or factors. There is a considerable heterogeneity in the effects of the epithelium between orders of bronchi, between species, and between pharmacologic agents. Such heterogeneity may reflect variations in the release and/or effect of the epithelium-derived relaxing factor(s). This report demonstrates that: (1) there is a basal and a stimulated release of the factor, (2) the prominence of different types of release varies between species, (3) the effect of the epithelium on relaxation of bronchial smooth muscle is greatest in the presence of high degrees of cholinergic tone, (4) the effects of the epithelium are not mediated via cyclic GMP, and (5) the epithelium-derived relaxing factor is not nitric oxide.