Comparison of human ETA and ETB receptor signalling via G-protein and β-arrestin pathways

To determine the pharmacology of ETA- and ETB-mediated β-arrestin recruitment and compare this to established human pharmacology of these receptors to identify evidence for endothelin receptor biased signalling and pathway specific blockade by antagonists. The ability of ET-1, ET-2, ET-3, sarafotoxin 6b and sarafotoxin 6c to activate ETA and ETB-mediated β-arrestin recruitment was determined in CHO-K1 cells. Affinities were obtained for ETA selective (BQ123, sitaxentan, ambrisentan), ETB selective (BQ788) and mixed (bosentan) antagonists using ET-1 and compared to affinities obtained in competition experiments in human heart and by Schild analysis in human saphenous vein. Agonist dependence of affinities was compared for BQ123 and BQ788 in the ETA and ETB β-arrestin assays respectively. For β-arrestin recruitment, order of potency was as expected for the ETA (ET-1≥ET-2>>ET-3) and ETB (ET-1=ET-2=ET-3) receptors. However, at the ETA receptor sarafotoxin 6b and ET-3 were partial agonists. Antagonism of ET peptides by selective and mixed antagonists appeared non-competitive. BQ123, but not BQ788, exhibited agonist-dependent affinities. Bosentan was significantly more effective an inhibitor of β-arrestin recruitment mediated by ETA compared to the ETB receptor. In the ETA vasoconstrictor assay, ET-1, ET-2 and S6b were equipotent, full agonists and antagonists tested behaved in a competitive manner, although affinities were lower than predicted from the competition binding experiments in left ventricle. These data suggest that the pharmacology of ETA and ETB receptors linked to G-protein- and β-arrestin mediated responses was different and bosentan appeared to show bias, preferentially blocking ETA mediated β-arrestin recruitment.