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- Title
Binding of (-)-[<sup>3</sup>H]-CGP12177 at two sites in recombinant human β<sub>1</sub>-adrenoceptors and interaction with β-blockers.
- Authors
Joseph, Shirin S.; Lynham, James A.; Colledge, William H.; Kaumann, Alberto J.
- Abstract
To verify the hypothesis that the non-conventional partial agonist (-)-CGP12177 binds at two β1-adrenoceptor sites, human β1-adrenoceptors, expressed in CHO cells, were labelled with (-)-[3H]-CGP12177. We compared the binding affinity and antagonist potency of 12 clinically used β-blockers against the cyclic AMP-enhancing effects of (-)-isoprenaline and (-)-CGP12177. (-)-[3H]-CGP12177 bound to a high affinity site (H; KH=0.47 nM) and low affinity site (L); KL=235 nM). (-)-[3H]-CGP12177 dissociated from the β1-adrenoceptors with a fast component (koff=0.45 min-1), consistent with the L-site, and a slow component (koff=0.017–0.033 min-1), consistent with the H-site. (-)-Isoprenaline and (-)-CGP12177 caused 96-fold and 12-fold maximal increases in cyclic AMP levels with -logEC50M of 8.2 and 7.6. (-)-CGP12177 antagonised the effects of (-)-isoprenaline with a pKB of 9.9. The β-blockers antagonised the effects of (-)-isoprenaline more than the effects of (-)-CGP12177 with potency ratios: (-)-atenolol 1,000, (±)-metropolol 676, (-)-pindolol 631, (-)-timolol 589, (±)-carvedilol 204, (±)-oxprenolol 138, (±)-sotalol 132, (-)-propranolol 120, (±)-bisoprolol 95, (±)-alprenolol 81, (±)-nadolol 68 and (-)-bupranolol 56. In intact cells the binding constants of β-blockers, estimated from competition with 3–5 nM (-)-[3H]-CGP12177 (binding to the H-site), correlated with the corresponding affinities estimated from antagonism of the (-)-isoprenaline effects. We conclude that (-)-[3H]-CGP12177 binds at two sites in the recombinant β1-adrenoceptor. (-)-CGP12177 is an antagonist of catecholamine effects through the H-site and a non-conventional partial agonist through the L-site. β-blockers are more potent antagonists through the H-site than the L-site.
- Subjects
ADRENERGIC receptors; ANTIHYPERTENSIVE agents; ADRENERGIC beta blockers; BISOPROLOL; CATECHOLAMINES; BIOGENIC amines
- Publication
Naunyn-Schmiedeberg's Archives of Pharmacology, 2004, Vol 369, Issue 5, p525
- ISSN
0028-1298
- Publication type
Article
- DOI
10.1007/s00210-004-0884-y