Mechanism of endothelin‐induced contraction in guinea‐pig trachea: comparison with rat aorta

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Abstract
Endothelin (1nm‐0.3 μm) produced a concentration‐dependent contraction of guinea‐pig epithelium‐containing (intact) trachea (EC50 = 30.9 nm). Endothelin was a less potent agonist than leukotriene D4 (LTD4; EC50 = 0.77 nm), but was more potent than carbachol (EC50 = 0.15 μm) or substance P (EC50 = 1.4 μm). Endothelin was a more potent contractile agent in rat endothelium‐denuded aorta (EC50 = 2.1 nm) than in guinea‐pig trachea. Endothelin‐induced contraction in guinea‐pig trachea was unaffected by mepyramine (10 μm), atropine (1 μm), SK&F 104353 (10 μm), a leukotriene receptor antgonist, or SQ 29,548 (1 μm), a thromboxane receptor antagonist. The contraction produced by 0.3 μm endothelin was potentiated by cyclo‐oxygenase inhibition with 5 μm indomethacin. Nicardipine (0.01 or 0.1 μm) or incubation in calcium‐free medium +0.1 mm EGTA for 30 min had a relatively minor or no effect on endothelin concentration‐response curves in guinea‐pig intact trachea, but markedly inhibited responses produced by endothelin in endothelium‐denuded aorta of the rat. Increasing the EGTA concentration in calcium‐free medium to 1 mm abolished endothelin‐induced contraction in guinea‐pig trachea. In guinea‐pig trachea, ryanodine (10 μm) produced a 2.1 fold shift to the right of endothelin concentration‐response curves and reduced the maximum response elicited by 0.3 μm endothelin. Staurosporine (0.01 μm and 0.1 μm), a protein kinase C inhibitor, was without effect on endothelin‐ or carbachol‐induced contraction in guinea‐pig trachea, but markedly inhibited the response produced by endothelin in rat aorta. Endothelin (3 nm‐0.3 μm) produced a concentration‐dependent stimulation of phosphatidylinositol (PI) turnover in guinea‐pig intact trachea, with an EC50 value of 45.9 nm. Removal of the epithilium markedly potentiated endothelin‐induced contraction in guinea‐pig trachea, producing a 4.7 fold leftward shift in endothelin concentration‐response curves and an increase in the contractile response elicited by 0.3 μm endothelin. These data indicate that endothelin is a potent agonist in guinea‐pig trachea whose response is markedly enhanced by removal of the airway epithelium. Endothelin‐induced contraction is not mediated to a marked extent by calcium influx via dihydropyridine‐sensitive calcium channels and does not involve the release of histamine, acetylcholine, leukotrienes or thromboxane. Rather, endothelin appears to produce contraction of guinea‐pig trachea via a direct action which involves stimulation of PI turnover and utilization of calcium from intracellular stores and, also, calcium influx via a pathway that is not sensitive to dihydropyridine calcium channel inhibitors. Endothelin‐induced contraction of rat aorta was more sensitive to the effects of incubation in Ca2+‐free medium, nicardipine or staurosporine, suggesting that differences exist in the relative mechanisms whereby endothelin produces contraction in different tissues.

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