Agonist Actions of “β-Blockers” Provide Evidence for Two Agonist Activation Sites or Conformations of the Human β₁-Adrenoceptor

Abstract

Previous work with 4-

Deletion of β-adrenergic receptor 1, 2, or both leads to different bone phenotypes and response to mechanical stimulation

“…(22) The use of different doses of propranolol in these studies is critical due to the paradoxical and nonselective effects of propranolol on adrenergic receptors. (46) In our study, by selectively removing the b1-and b2-adrenergic receptors or both, we showed that b1-adrenergic signaling was the most important for cancellous and cortical responses to mechanical stimulation. Note, however, that we only found low expression levels of Adrb1 mRNA in osteoblasts from calvaria of WT mice.…”

Markedly reduced effects of (−)‐isoprenaline but not of (−)‐CGP12177 and unchanged affinity of β‐blockers at Gly389‐β1‐adrenoceptors compared to Arg389‐β1‐adrenoceptors

“…Through the high-affinity site b-blockers, including (À)-CGP12177, antagonize the effects of catecholamine. The low-affinity site usually appears to mediate the effects of NCPA (but see Baker et al, 2003), including those of (À)-CGP12177, and has low affinity for b-blockers (Kaumann, 2000;Konkar et al, 2000;Granneman, 2001;Lowe et al, 2002;Joseph et al, 2003). The two binding sites for (À)-[ 3 H]-CGP12177 (affinity ratio B500) were recently verified on recombinant Arg389-b 1 -adrenoceptors transfected at physiological density into CHO cells (Joseph et al, 2004).…”

Site of Action of β-Ligands at the Human β1-Adrenoceptor

“…CGP 12177 inhibits isoprenaline-induced ␤1-adrenoceptor responses as a high affinity neutral antagonist; however, at higher concentrations (i.e., with lower affinity) it produces an agonist response which is relatively resistant to antagonism by classical ␤-antagonists (Pak and Fishman, 1996;Konkar et al, 2000a,b;Lowe et al, 2002;Baker et al, 2003b). Further studies from knockout animals and transfected cell system receptors showed that both pharmacological profiles were dependent upon the presence of only the ␤1-adrenoceptor and not a further subtype of ␤-receptor, hence the current two-state receptor model (Pak and Fishman, 1996;Kaumann and Molenaar, 1997;Kaumann et al, 1998;Cohen et al, 2000;Konkar et al, 2000a,b;Granneman, 2001;Kaumann et al, 2001;Baker et al, 2003b). This is very different from the human ␤2-adrenoceptor, where CGP 12177 is a typical partial agonist (i.e., similar K D values) from binding (0.14 nM), partial antagonism (0.09 -0.17 nM), and agonism (EC 50 0.22 nM; Baker et al, 2002).…”

“…Furthermore, the plasma concentration of carvedilol (100 ng/ml ϭ 300 nM) used in human cardiovascular diseases is sufficient to activate this secondary site (Sawangkoon et al, 2000;Baker et al, 2003b). However, whereas the ␤1-low-affinity state appears to signal via G s -proteins in recombinant cell systems (Pak and Fishman, 1996;Konkar et al, 2000b;Baker et al, 2003b), this conformation may also couple to G i -proteins in rat heart and blood vessels (Kompa and Summers, 1999;Mallem et al, 2004).…”

“…Most classical antagonists and adrenaline, noradrenaline, isoprenaline, and dobutamine were found to preferentially act via the catecholamine site, whereas LY 362884 agonist actions occurred via the secondary site (Konkar et al, 2000b;Lowe et al, 2002;Joseph et al, 2004). Several "␤-antagonists" also have intrinsic agonist actions of their own (Lowe et al, 2002;Baker et al, 2003b). Some of these agonist responses were potently antagonized by neutral ␤1-antagonists (suggesting catecholamine site agonism, e.g., acebutolol), others were relatively resistant to antagonism (suggesting secondary site activation, e.g., carvedilol) although other compounds had agonist activity at both sites (alprenolol, pindolol;Baker et al, 2003b).…”