EFFECT OF EPITHELIUM REMOVAL AND OF ENKEPHALIN INHIBITION ON THE BRONCHOCONSTRICTOR RESPONSE TO 3 ENDOTHELINS OF THE HUMAN ISOLATED BRONCHUS

The three endothelins ET-1, ET-2 and ET-3 induced a potent contractile response in the human isolated bronchus with an intact epithelium, which proceeded on two different stages. The first stage was observed at low concentrations (high potency) (10(-12) to 10(-9) M) but corresponded to a low intrinsic activity (E(max) maximal effect induced by ACh 10(-3) M), the second stage appeared at higher concentrations and corresponded to higher intrinsic activity. The rank order of potency for the two stages of contractile activity was ET-1 > ET-2 = ET-3. Removal of the epithelium significantly enhanced the two stages of the contractile responses to the three endothelins and abolished the differences in potency and efficacy that were observed between ET-1, ET-2 and ET-3 when the epithelium was present. Phosphoramidon (10(-5) M), an enkephalinase inhibitor, was as potent as epithelium removal in enhancing the contractile responses to these agonists at low concentrations (first stage of contraction, 10(-16) to 10(-9) M). However, with high concentrations of endothelins (> 10(-9) M, second stage of contraction), phosphoramidon was less potent than epithelium removal in enhancing the contractile responses. In epithelium-denuded strips, preincubation with phosphoramidon did not further increase the maximal contractions induced by/or the potencies of ET-1, ET-2 or ET-3. After epithelium removal, responses to low doses of endothelins were attenuated by nicardipine (10(-6) M) whereas responses to high doses of the endothelins were not affected, as was also observed when the epithelium was present. In contrast, Bay K 8644 was unable to potentiate the responses to endothelins after epithelium removal. In conclusion, these results suggest that the epithelium of human bronchi exerts an inhibitory effect on the contractions induced by the three endothelins. This epithelial inhibitory effect could be attributed in part, though not completely, to an enkephalinase activity. Removal of the epithelium did not affect the action of nicardipine on endothelin-induced contractions and masked the potentiation by Bay K 8644 of contractile responses to low concentrations of the three endothelin isopeptides.