Role of the epithelium in opposing H2O2-induced modulation of acetylcholine-induced contractions in rabbit intrapulmonary bronchiole

1 The role played by the epithelium in H2O2-induced modulation of the mechanical responses induced by acetylcholine (ACh) in rabbit intrapulmonary bronchioles was investigated in epithelium-intact and -denuded strips. 2 When ACh (3 muM) was applied intermittently, H2O2 (30 muM) enhanced the ACh-induced contractions in epithelium-intact strips. In contrast, in epithelium-denuded strips H2O2 (30 muM) inhibited such contractions. At higher concentrations, H2O2 concentration-dependently attenuated the ACh-induced contractions in both epithelium-intact and -denuded strips, its action being more potent in the latter strips than in the former. 3 Diclofenac (a cyclo-oxygenase inhibitor; 3 muM) reduced the H2O2-induced enhancement of ACh contractions in epithelium-intact strips but had no effect on the H2O2-induced inhibition in epithelium-denuded strips. N-G-nitro-L-arginine did not alter the effect of H2O2 on ACh-induced contractions in epithelium-intact strips. 4 Catalase (500 u ml(-1)) completely blocked both H2O2-induced effects on ACh-contractions (enhancement and inhibition). Neither superoxide dismutase (200 u ml(-1)) nor deferoxamine (0.5 mM) had any effect on H2O2-induced inhibition in epithelium-denuded strips. 5 Aminotriazole (an inhibitor of catalase; 50 mM) significantly potentiated the H2O2-induced inhibition of ACh-contractions in epithelium-intact strips but not in epithelium-denuded strips. 6 The density ratio for catalase (epithelium-intact over -denuded strips) analysed by Western blot was about 2.1, suggesting that epithelium contains more catalase than smooth muscle. 7 It is concluded that in rabbit intrapulmonary bronchioles, H2O2 has dual actions on ACh-contractions. It is suggested that the epithelium may act as a powerful biochemical barrier via both the action of catalase (scavenging H2O2) and the release of bronchoconstrictor prostaglandins, thus attenuating the H2O2-induced modulation of ACh-contractions.