Markedly reduced effects of (-)-isoprenaline but not of (-)-CGP12177 and unchanged affinity of β-blockers at Gly389-β 1-adrenoceptors compared to Arg389-β 1-adrenoceptors

Abstract

1. Substitution of arginine by glycine at position 389, a frequent β 1-adrenoceptor polymorphism, reduces adenylyl cyclase stimulation by (-)-isoprenaline. β 1-Adrenoceptors mediate the effects of catecholamines and nonconventional partial agonists ((-)-CGP12177) through different sites. We investigated the influence of the 389 polymorphism on β blocker affinity, as well as on the responses to (-)-isoprenaline and the nonconventional partial agonist (-)-CGP12177 on cyclic AMP levels in CHO cells expressing recombinant Arg389-β 1-adrenoceptors (101 fmol mg -1 protein) or Gly389-β 1-adrenoceptors (94 fmol mg -1). 2. The affinity of β-blockers and partial agonists, estimated from competition binding with (-)-[ 125I]-cyanopindolol, was not different for Arg389-β 1-adrenoceptors and Gly389-β 1-adrenoceptors. 3. The maximum cAMP increases by (-)-isoprenaline and (-)-CGP12177 at Gly389-β 1-adrenoceptors were reduced by 97 and 46%, but the potencies enhanced 2 and 0.5 log units, respectively, compared to Arg389-β 1-adrenoceptors. The intrinsic activity of (-)-CGP12177 with respect to the (-)-isoprenaline was 0.057 at Arg389-β 1-adrenoceptors and 1.05 at Gly389-β 1- adrenoceptors. 4. We confirm in intact CHO cells that responses to (-)-isoprenaline are markedly reduced at Gly389-β 1-adrenoceptors compared to Arg389-β 1-adrenoceptors. However, the 389 polymorphism reduces considerably less the agonist responses to (-)-CGP12177, indicating that coupling to G s protein is different for β 1-adrenoceptors activated by catecholamines than for receptors activated by nonconventional partial agonists. The affinity of β-blockers is conserved across the Arg389Gly polymorphism.