CHARACTERIZATION OF THE ENDOTHELIN RECEPTOR-SELECTIVE AGONIST, BQ3020AND ANTAGONISTS BQ123, FR139317, BQ788, ANTAGONIST-50235, RO462005 AND BOSENTAN IN THE HEART

1 In this study we used ligand binding techniques to determine the aff inity 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 [I-125]-ET-1 binding to human left ventricle with over 200,000 fol d selectivity for the ET(A) receptor (K-D ET(A) = 1.20 +/- 0.28 nM, K( D)ET(B) = 287 +/- 93 mu M). 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 mu M) in this tissue. BQ123 and FR1 39317 also showed high selectivity (greater than 20,000 fold) and affi nity in rat (BQ123: K(D)ET(A) = 1.18 +/- 0.16 nM, K(D)ET(B) = 1370 +/- 1150 mu M; FR139317: K(D)ET(A) = 2.28 +/- 0.30 nM, K(D)ET(B) = 292 +/ - 114 mu M) and pig heart (BQ123: K(D)ET(A) = 0.52 +/- 0.05 nM, K(D)ET (B) = 70.4 +/- 4.0 mu M; FR139317: K(D)ET(A) = 2.17 +/- 0.51 nM, K(D)E T(B) = 47.1 +/- 5.7 mu M) (n greater than or equal to 3 individuals +/ - s.e.mean). 3 Although BQ3020 competed with over 1000 fold selectivit y 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 mu M) the peptide inhibited only the binding of [I-125]-ET-1 at concentrations greater than 100 nM in rat and porc ine heart. This is in contrast to the data from the ETA-selective anta gonists which indicated the presence of ET(B) sites in these tissues f rom animal hearts. 4 The peptide antagonist, BQ788, had a low, micromo lar affinity (K-D = 1.98 +/- 0.13 mu M) 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 mu M) and bosentan (Ro470203; K-D = 77.9 +/- 7.9 nM) competed monophasicall y for [I-125]-ET-1 binding sites in human left ventricle. 6 The result s show that the ET(A) antagonists, BQ123 and FR139317, are highly sele ctive for ET(A) receptors in all cardiac tissues tested, whereas BQ788 has a low affinity and no selectivity in this human tissue. Further w e showed that there are species differences in the binding of BQ3020 t o the ET(B) receptors in the hearts derived from human, rat and pig.