Functional and binding characterization of endothelin receptors in human bronchus: evidence for a novel endothelin B receptor subtype?

Binding and functional studies were conducted to elucidate the receptor subtypes mediating contractions of human bronchus induced by endothelin (ET) receptor ligands. Binding experiments in human bronchial smooth muscle membrane preparations revealed the presence of ETA and ETB receptors in the ratio of approximately 40:60. In the presence of the combination of 1 microM BQ-123 (ETA receptor antagonist) and 1 microM S6c (ETB receptor agonist) or BQ-788 (ETB receptor antagonist) about 10 to 20% of [125I]-ET-1 binding remained. ET-1 (nonselective agonist), ET-3 (ETB receptor-preferring agonist), S6c, IRL 1620 or BQ-3020 (ETB receptor-selective agonists) potently contracted human bronchus. SB 209670 (10 microM) (ETA/ETB receptor antagonist) antagonized ET-1-induced contractions (pKB = 6.1), whereas, BQ-788 (3 microM), RES-701 (10 microM) or BQ-123 (3 microM) were without effect. The combination of BQ-788 (3 microM) and BQ-123 (3 microM) did not influence ET-1 concentration-response curves. Contractions elicited by IRL 1620 or BQ-3020, but not S6c or ET-3, were sensitive to inhibition by BQ-788 (0.03-3 microM). Based on the potent contractile effects of ETB receptor-selective agonists, and the lack of inhibitory effect of BQ-123, ET ligand-induced contractions in human bronchus appear to be mediated via an ETB receptor subtype(s). However, contractions induced by ET-1, ET-3 or S6c are not sensitive to classical ETB receptor antagonists such as BQ-788. Furthermore, a residual component (about 10-20%) of the binding of radiolabeled ET agonists is resistant to various ET ligands. Collectively, these data suggest the presence of a novel ETB receptor subtype which may mediate contraction induced by some ET ligands in human bronchus.