Comparison of endothelin _B (ET_B) receptors in rabbit isolated pulmonary artery and bronchus

Abstract

1 To explore potential differences between endothelin (ET) receptors in airway versus vascular smooth muscle from the same species, the ETB receptors mediating contractions produced by ET-1, ET-3 and the selective ETB ligands, sarafotoxin S6c (S6c) and BQ-3020, in rabbit bronchus and pulmonary artery were investigated by use of peptide and non-peptide ET receptor antagonists.2 In rabbit pulmonary artery SB 209670 (10 gM), a mixed ETA/ETB receptor antagonist, was a more potent antagonist of contractions produced by S6c (pKB=7.7; n=9; P<0.05), than those elicited by ET-1 (pKB= 6.7; n = 6) or ET-3 (pKB= 6.7; n = 5). BQ-788 (10 gM), an ETB receptor antagonist, inhibited responses produced by ET-3 (pKB= 5.1; n = 8), BQ-3020 (pKB= 5.2; n = 4) or S6c (pKB= 6.2; n = 9; P<0.05 compared to potency versus ET-3-or BQ-3020-induced contractions), but was without inhibitory effect on ET-1-induced contractions (n = 5). RES-701 (10 gM), another selective ETB receptor antagonist, was without effect on contractions produced by S6c (n = 4) or ET-l (n = 4), and potentiated ET-3-(n = 5) or BQ-3020-induced responses (n = 4).3 The combination of BQ-788 (10 gM) and BQ-123 (10 gM), an ETA-selective receptor antagonist, antagonized contractions produced by lower concentrations of ET-l (1 and 3 nM) in rabbit pulmonary artery, but was without effect on responses elicited by higher concentrations of ET-1 (n = 5). The combination of RES-701 (10 gM) and BQ-123 (10 gM) potentiated responses elicited by ET-1, producing a 3.7 fold shift to the left in the agonist concentration-response curve (n = 5).4 In rabbit bronchus SB 209670 (3 gM) had similar potency for antagonism of contractions produced by ET-1 (pKB=6.3; n=6), ET-3 (pKB=6.5; n=6) or S6c (pKB=6.1; n=8). BQ-788 (3 gM) was without effect on responses elicited by ET-1, ET-3 or S6c (n=6) but antagonized BQ-3020-induced contractions (pKB=6.4; n=4). RES-701 (3 gM) was without effect on contractions produced by S6c (n=6) or BQ-3020 (n=4), and potentiated rather than antagonized ET-1-or ET-3-induced responses (n=6), reflected by a significant (about 6 fold) shift to the left in ET-l or ET-3 concentration-response curves. The combination of BQ-788 (3 gM) and BQ-123 (3 gM) was without effect on contractions produced by ET-1 in rabbit bronchus (n=6). The combination of RES-701 (3 gM) and BQ-123 (3 gM) potentiated responses elicited by ET-1, producing a 5.2 fold shift to the left in the agonist concentration-response curve (n = 5).5 BQ-123 (3 or 10 gM), an ETA-selective receptor antagonist, was without effect on ET-1, ET-3 or S6c concentration-response curves (n=3-6) in rabbit pulmonary artery or rabbit bronchus. 6 These data indicate that contractions induced by ET-1, ET-3, S6c and BQ-3020 in rabbit pulmonary artery or rabbit bronchus appear to be mediated predominantly via stimulation of ETB receptors. However, the qualitative and quantitative differences in the relative profiles of the various structurally diverse peptide and non-peptide antagonists examined suggests that responses produced by th...

Differential modulation of endothelin ligand‐induced contraction in isolated tracheae from endothelin B (ETB) receptor knockout mice

“…However, in mouse trachea the inhibition of ET-1 concentration-response curves was markedly greater than obtained in human bronchus (Goldie et al, 1996a,b;Fukuroda et al, 1996). Furthermore, the functional potency of the nonpeptide ET B receptor antagonist, A192621, against Stx S6c-induced contractions of wildtype mouse trachea was signi®cantly (about 2-log units) greater than previously reported for the potent, peptide ET B receptor antagonist, BQ-788 in human and rabbit bronchus (Fukuroda et al, 1996;Hay et al, 1996;, despite the similar potencies of the two compounds from binding studies using the cloned human ET B receptor (unpublished observations). Collectively, these data suggest that there may exist marked species dierences in the potencies of the ET receptor antagonists.…”

“…Geometric mean EC 50 values (pD 2 s) were calculated from linear regression analyses of data. It is recognized that the ETs and related ligands may not interact with receptors in a classical manner which will result in a reversible competitive interaction between agonist, antagonist and receptor (Marsault et al, 1991;Waggoner et al, 1992;Hay et al, 1996). However, antagonist potencies were calculated assuming a classical competitive interaction.…”

“…There are several reports proposing the existence of additional ET receptor subtypes, in particular for the ET B receptor category (Bax & Saxena, 1994;Warner et al, 1993;Hay et al, 1996;Yoneyama et al, 1995). These hypotheses are based on binding and, in particular, contraction experiments exploring the eects of receptor antagonists against responses elicited by various ET agonists.…”

“…These hypotheses are based on binding and, in particular, contraction experiments exploring the eects of receptor antagonists against responses elicited by various ET agonists. For example, functional experiments have uncovered a limited or lack of eect of ET B receptor antagonists against ET B receptor-mediated responses in dierent tissues, including rabbit and human bronchi (Hay et al, 1996;. There are severe limitations in utilizing only pharmacological data for receptor classi®cation, with the need for supportive molecular biological, operational and structural information.…”

“…Furthermore, both receptor subtypes must be blocked before signi®cant inhibition of ET-1-evoked contraction is obtained, indicating marked functional reserve (Henry, 1993;Goldie et al, 1996a, b;Fukuroda et al, 1996). However, based upon pharmacological data using ET agonists and antagonists in various tissues, including the lung, it has been speculated that there may exist ET receptor subtypes in addition to the ET A and ET B receptor populations (Bax & Saxena, 1994;Warner et al, 1993;Hay et al, 1996;Yoneyama et al, 1995). This hypothesis appears to be based, to a large extent, on the insensitivity of ET ligand-induced contractions to the currently available ET receptor antagonists, and, thus far, has not been supported by molecular biological, structural or operational experiments.…”