Suc-[Glu(9), Ala(11,15)]-endothelin-1 (8-21), IRL 1620, identifies two populations of ETB receptors in guinea-pig bronchus

1 The pharmacological properties of endothelin receptors (ETR) were investi gated in guinea-pig bronchus by comparing binding and functional results. 2 In binding assays, both the ETB agonists, endothelin-3 (ET-3) and N-suc-[ Glu(9), Ala(11,15)]ET-1(8-21) (IRL 1620), and the antagonist, N-cis-2,6-dim ethylpiperidinocarbonyl-L-gamma-methyllleucyl-D-1-methoxycarbonyltryptophan yl-D-norleucine (BQ 788), showed biphasic inhibition curves of [I-125]endot helin-1 (ET-1) binding to bronchus membranes prepared from intact or epithe lium-deprived tissue. IRL 1620 did not completely displace specifically [I- 125]-ET-1 bound to these tissue preparations. In the presence of the ETA-se lective antagonist, cyclo(-D-Trp-D-Asp-L-Pro-D-Val-L-Leu) (BQ 123, 1 mu M), IRL 1620 displacement curves were shallow but a complete inhibition was re ached at a concentration of 1 mu M. Both curves were better represented by two-site models. In addition, BQ 788 competition curves became monophasic w hen binding experiments were performed in the presence of 1 mu M BQ 123. Th e non-selective agonist, ET-1, and BQ 123 inhibited [I-125]-ET binding to b ronchus membranes in dose-dependent fashions with monophasic curves. 3 The contracting activity of IRL 1620 (0.55 nM-1.6 mu M) was tested on mul tiple-ring bronchial preparations pretreated with peptidase and cyclo-oxyge nase inhibitors. BQ 788 shifted IRL1620 concentration-response curves to th e right while BQ 123 did not influence bronchial responsiveness. In additio n, a potentiation of the maximal response to the agonist was observed in BQ 788 treated bronchial rings. This effect was abolished by tissue pretreatm ent with N-omega-nitro-L-argininemethylester (L-NAME) or epithelium removal but not by pretreatment with atropine or iberiotoxin. 4 Our results demonstrate that guinea-pig bronchus contains two populations of ETB receptors with different affinities for the ETB-selective agonist, IRL 1620. One ETB receptor population appears to activate bronchial muscle contraction while another on epithelial cells causes muscle relaxation thro ugh the release of nitric oxide (NO).