Endothelin‐induced contractions of tracheal smooth muscle and identification of specific endothelin binding sites in the trachea of the rat

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Abstract
1 The presence of specific binding sites and the contractile activity of the novel peptide, endothelin have been investigated in rat trachea. 2 Endothelin (10−8–10−5m) induced long-lasting contraction of rat tracheal rings superfused with Krebs solution (EC50 5.4 × 10−6m). Contractions of the tissue to 10−6m endothelin were attenuated in Ca2+-free medium containing 0.1 mM EGTA but unaffected by nicardipine (10−7m). 3 After equilibration in Ca2+-free medium (without EGTA) a return to normal Ca2+ concentrations (2.5 mm), 30 min or 60 min following endothelin (10−6m), produced a sustained contraction of the tissue. 4 Specific binding sites for endothelin were identified on rat tracheal smooth muscle (KD 1.34 × 10−10m, maximal binding 1.2 fmol mm−2). Specific binding sites were also identified on nerve trunks. Endothelin binding was unaffected by co-incubation with nicardipine (10−7m) or verapamil (10−7m). 5 The discrepancy between the apparent KD for endothelin binding and the EC50 for endothelin-induced contraction suggests that the endothelin binding sites identified in this study may not be associated with the receptors mediating contraction. 6 These results indicate that endothelin binding sites are present on tracheal smooth muscle. The mechanism of endothelin-induced contraction, whilst being dependent on extracellular calcium, does not appear to involve binding to the dihydropyridine- or verapamil-sensitive sites on the voltage-dependent Ca2+ channel. Its long duration of action may be associated with a sustained increase in Ca2+ permeability.