Mechanical and biochemical responses to endothelin-1 and endothelin-3 in bovine bronchial smooth muscle

Abstract

1. In this study, mechanical responses to endothelin-1 and endothelin-3 were examined in bovine bronchial smooth muscle. In addition, the involvement of phosphatidylinositol 4,5-bisphosphate hydrolysis (PIP2) in the responses to these peptides was assessed by measurement of inositol (1,4,5) trisphosphate (I(1,4,5)P3) production using a specific mass assay. 2. ET-1 evoked contractions of bovine bronchi which were concentration-dependent and initiated at between 10(-9) M and 10(-8) M. ET-1-evoked responses were unaffected by slight elevation of tone with potassium chloride (3 x 10(-2) M), methacholine (10(-6) M) or U46619 (10(-7) M). 3. Contractions to ET-1 were not altered by pre-incubation with atropine (10(-5) M), indomethacin (10(-5) M), nifedipine (10(-5) M), phosphoramidon (3.67 x 10(-5) M) or by removal of the epithelium. 4. ET-3 evoked small contractions which were not concentration-dependent. In the presence of phosphoramidon (3.67 x 10(-5) M) however, concentration-dependent contractions were obtained to ET-3 which were unaffected by atropine (10(-5) M) or by removal of the epithelium, but were significantly attenuated by indomethacin (10(-5) M). Nifedipine (10(-5) M) virtually abolished this response. 5. Both ET-1 and ET-3 (in the presence of phosphoramidon)-evoked contractions were significantly enhanced by the presence of the phorbol ester phorbol 12,13-dibutyrate (10(-8) M). Neither ET-1-, nor ET-3-mediated responses were antagonized by the protein kinase C (PKC) inhibitor, Ro 31-8220 (3 x 10(-9) - 3 x 10(-8) M). 6. ET-1 (3 x 10(-7) M) evoked a biphasic rise in levels of I(1,4,5)P3 which was unaltered by preincubation with atropine, whilst ET-3 (10(-10) - 3 x 10(-7) M) failed to alter levels of I(1,4,5)P3 at any time point examined, even in the presence of phosphoramidon (3.67 x 10(-5) M). 7. These results suggest that, in bovine bronchial smooth muscle, ET-l does not evoke contraction via cyclo-oxygenase metabolites, does not evoke release of the neurotransmitter substance acetylcholine, or require calcium influx via dihydropyridine-sensitive channels. ET-1 evokes 1(1,4,5)P3 production, but stimulation of protein kinase C may not be critical for the associated contraction. In contrast,ET-3-evoked contractions are partly mediated by cyclo-oxygenase metabolites. ET-3 does not stimulate PIP2 hydrolysis, nor activate PKC, but may, either directly or as a requirement of intermediates released in response to ET-3, rely upon extracellular calcium.