Characterization of the endothelin receptor selective agonist, BQ3020 and antagonists BQ123, FR139317, BQ788, 50235, ro462005 and bosentan in the heart

Abstract

1. In this study we used ligand binding techniques to determine the affinity and selectivity of endothelin receptor agonists and antagonists in human left ventricle which expresses both ET(A) and ET(B) receptors, and compared these results with cardiovascular tissues from rat and porcine hearts. 2. The linear tripeptide antagonist, FR139317 competed for [125I]-ET-1 binding to human left ventricle with over 200,000 fold selectivity for the ET(A) receptor (K(D)ET(A)= 1.20 ± 0.28 nM, K(D)ET(B) = 287 ± 93 μM) The ET(A)-selective non-peptide antagonist, 50235, competed with lower affinity and selectivity (K(D)ET(A) = 162 ± 61 nM, K(D)ET(B) = 171 ± 42 μM) in this tissue. BQ123 and FR139317 also showed high selectivity (greater than 20,000 fold) and affinity in rat (BQ123: K(D)ET(A) = 1.18 ± 0.16 nM, K(D)ET(B) = 1370 ± 1150 μM; FR139317: K(D)ET(A) = 2.28 ± 0.30 nM, K(D)ET(B) = 292 ± 114 μM) and pig heart (BQ123: K(D)ET(A) = 0.52 ± 0.05 nM, K(D)ET(B) = 70.4 ± 4.0 μM; FR139317: K(D)ET(A) = 2.17 ± 0.51 nM, K(D)ET(B) = 47.1 ± 5.7 μM) (n ≥ 3 individuals ± s.e.mean). 3. Although BQ3020 competed with over 1000 fold selectivity for the ET(B) subtype in human heart (K(D)ET(B) = 1.38 ± 0.72 nM, K(D)ET(A) = 2.04 ± 0.21 μM) the peptide inhibited only the binding of [125I]-ET-1 at concentrations greater than 100 nM in rat and porcine heart. This is in contrast to the data from the ETA-selective antagonists which indicated the presence of ET(B) sites in these tissues from animal hearts. 4. The peptide antagonist, BQ788, had a low, micromolar affinity (K(D) = 1.98 ± 0.13 μM) using human left ventricle and no significant selectivity for the human ET(B)-subtype in this tissue. 5. The non-peptide ET antagonists, Ro462005 (K(D) = 50.3 ± 9.5 μM) and bosentan (Ro470203; K(D) = 77.9 ± 7.9 nM) competed monophasically for [125I]-ET-1 binding sites in human left ventricle. 6. The results show that the ET(A) antagonists, BQ123 and FR139317, are highly selective for ET(A) receptors in all cardiac tissues tested, whereas BQ788 has a low affinity and no selectivity in this human tissue. Further we showed that there are species differences in the binding of BQ3020 to the ET(B) receptors in the hearts derived from human, rat and pig.