Evidence for two atypical conformations of beta-adrenoceptors and their interaction with Gi proteins

In this study, we investigated whether the responses of right atria from sinoaortic denervated rats to CGP12177 (4(3- t-butylamino-2-hydroxypropoxy benzidimidazole-2 one, hydrochloride)), isoprenaline and norepinephrine desensitized in parallel and whether CGP12177 interacted with distinct conformations of β-adrenoceptors. Right atria from rats 48 h after sinoaortic denervation were subsensitive to isoprenaline, norepinephrine and CGP12177. One week after sinoaortic denervation, the sensitivity to CGP12177 had recovered whereas the responses to isoprenaline and norepinephrine were still subsensitive, suggesting that the binding sites for these molecules showed independent behavior. In atria from 48 h sinoaortic-denervated rats, propranolol or 3 μM CGP20712A (2-hydroxy-5(2-((2-hydroxy-3-(4-((methyl-4-trifluormethyl)1H imidazole-2-yl)-phenoxypropyl) amino) ethoxy)-benzamide monomethane sulphonate)) blocked the responses to 10 nM–1 μM CGP12177 and steepened the curves. The concentration–response curves to CGP12177 in the presence of ICI118,551 ( erythro- dl-1(-methylindan-4-yloxy)-3-isopropylamino-butan-2-ol) were biphasic, suggesting that CGP12177 interacted with at least two conformations of β-adrenoceptors that showed negative cooperativism, one acting through β 2-adrenoceptor-Gi and the other via β 1-adrenoceptor-Gs. This hypothesis was confirmed in right atria from sinoaortic-denervated rats treated with pertussis toxin.