Endothelin receptor subtype distribution predisposes coronary arteries to damage.

Several vasoactive drugs that lower blood pressure and increase heart rate induce regional cardiotoxicity in the dog, most frequently of right coronary arteries and right atrium. The basis for this selective damage is thought to result from local changes in vascular tone and blood flow. Administration of an endothelin receptor antagonist (ETRA, SB 209670) to dogs induced damage most frequent and severe in the right coronary artery and right atrium. Because site predisposition may correlate with distribution of vasoactive receptors, the objectives of this study were to map endothelin (ET) receptor distribution and density within regions of dog heart using both gene (mRNA) and protein expression endpoints for dog ET(A) and ET(B) receptors, and, additionally, correlate ET receptor subtype density with regional cardiac blood flow. A 10- to 15-mmHg reduction in mean arterial pressure with a concomitant increase in heart rate (10-20%), a six- and twofold increase in regional blood flow to the right and left atrium, respectively, and acute hemorrhage, medial necrosis, and inflammation were observed in the right coronary arteries and arteries of the right atrium after ETRA infusion for 5 days. Radioligand protein binding to quantify both ET receptors in normal dog heart indicated a twofold greater density of ET receptors in atrial regions versus ventricular regions. Importantly, ET receptor density in coronary arteries was markedly (about five- to sixfold) increased above that in atrial or ventricular tissues. ET receptor subtype characterization indicated ET(B) receptors were three times more prevalent in right coronary arteries compared to left coronary arteries and in situ hybridization confirmed localization of ET(B) in vascular smooth muscle. ET(A) receptor density was comparable in right and left coronary arteries. Quantitative real-time polymerase chain reaction for ET(A) and ET(B) receptor mRNA transcripts supported the site prevalence for message distribution. Consequently, the composite of protein and message expression profiles for ET(A) and ET(B) receptors indicated a disproportionate distribution of ET(B) receptors within right coronary artery of dog and this, along with functional measures of blood flow after ETRA infusion indicated a predisposition for exaggerated pharmacological responses and subsequent damage to right coronary arteries by ET and/or ETRAs.

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.