Characterization of M2 muscarinic receptor activation of different G protein subtypes

The M-2 muscarinic acetylcholine receptor (mAChR) expressed in insect cells (Spodoptera frugiperda, Sf9) using the baculovirus system formed active functional complexes with coexpressed G(i) as well as with G(o) proteins, while no complexes could be detected with internal G proteins. Comparison of the abilities of different muscarinic agonists and partial agonists to increase [S-35]GTPgammaS binding revealed no significant differences between M-2/G(i) and M-2/G(o) complexes neither with respect to affinities nor efficacies of the ligands studied. Coexpression with either G protein caused constitutive activity of the receptor amounting up to 66% of stimulable [S-35]GTPgammaS binding. Muscarinic antagonists, like atropine, scopolamine and N-methylscopolamine, behaved as inverse antagonists with potencies in good agreement with their binding affinities to the receptor. The results implicate that the functional reconstitution of M-2 muscarinic receptor with either G(i) or G(o) proteins in insect cells provides a valuable toot for screening of potencies as well as efficacies of agonists, partial agonists and inverse agonists at this receptor. (C) 2003 Elsevier Science Ltd. All rights reserved.