title: Comparison of human ETA and ETB receptor signalling via G-protein and β-arrestin pathways.
abstract: OBJECTIVE
  To determine the pharmacology of ET(A)- and ET(B)-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.
  
  
  METHODS
  The ability of ET-1, ET-2, ET-3, sarafotoxin 6b and sarafotoxin 6c to activate ET(A) and ET(B)-mediated β-arrestin recruitment was determined in CHO-K1 cells. Affinities were obtained for ET(A) selective (BQ123, sitaxentan, ambrisentan), ET(B) 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 ET(A) and ET(B) β-arrestin assays respectively.
  
  
  RESULTS
  For β-arrestin recruitment, order of potency was as expected for the ET(A) (ET-1≥ET-2>>ET-3) and ET(B) (ET-1=ET-2=ET-3) receptors. However, at the ET(A) 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 ET(A) compared to the ET(B) receptor. In the ET(A) 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.
  
  
  CONCLUSIONS
  These data suggest that the pharmacology of ET(A) and ET(B) receptors linked to G-protein- and β-arrestin mediated responses was different and bosentan appeared to show bias, preferentially blocking ET(A) mediated β-arrestin recruitment.
authors.affiliation: Clinical Pharmacology Unit, University of Cambridge, Level 6 ACCI, Box 110 Addenbrooke's Hospital, Cambridge, CB2 0QQ, UK. jjm1003@medschl.cam.ac.uk
authors.forename: Janet J
authors.initials: JJ
authors.lastname: Maguire
authors.forename: Rhoda E
authors.initials: RE
authors.lastname: Kuc
authors.forename: Victoria R
authors.initials: VR
authors.lastname: Pell
authors.forename: Andrew
authors.initials: A
authors.lastname: Green
authors.forename: Mike
authors.initials: M
authors.lastname: Brown
authors.forename: Sanj
authors.initials: S
authors.lastname: Kumar
authors.forename: Tom
authors.initials: T
authors.lastname: Wehrman
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Quinn
authors.forename: Anthony P
authors.initials: AP
authors.lastname: Davenport
mesh_terms: D000818:Animals; D019390:Arrestins / Q000378:metabolism*; D000077300:Bosentan; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D065130:Endothelin A Receptor Antagonists; D065131:Endothelin B Receptor Antagonists; D019332:Endothelin-1 / Q000494:pharmacology; D019333:Endothelin-2 / Q000494:pharmacology; D019334:Endothelin-3 / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D006801:Humans; D044022:Receptor, Endothelin A / Q000819:agonists / Q000378:metabolism*; D044023:Receptor, Endothelin B / Q000819:agonists / Q000378:metabolism*; D015398:Signal Transduction; D013449:Sulfonamides / Q000378:metabolism; D014662:Vasoconstrictor Agents / Q000494:pharmacology; D014757:Viper Venoms / Q000494:pharmacology; D000071557:beta-Arrestins
chemical_list: D019390:Arrestins; D065130:Endothelin A Receptor Antagonists; D065131:Endothelin B Receptor Antagonists; D019332:Endothelin-1; D019333:Endothelin-2; D019334:Endothelin-3; D044022:Receptor, Endothelin A; D044023:Receptor, Endothelin B; D013449:Sulfonamides; D014662:Vasoconstrictor Agents; D014757:Viper Venoms; D000071557:beta-Arrestins; C057354:sarafotoxins s6; D019204:GTP-Binding Proteins; D000077300:Bosentan