Tissue-dependent association of muscarinic acetylcholine receptors with guanine nucleotide-binding regulatory proteins.

The muscarinic acetylcholine receptors in heart and cerebellum form a stable association with guanine nucleotide-binding regulatory proteins (G proteins) in the presence of receptor agonists. This has been confirmed by purification of the muscarinic receptor-G protein complexes using an immunoprecipitation protocol. The isolated complexes were subjected to Western blotting to identify the G protein subunits present in the complexes. At saturating concentrations of carbachol, the muscarinic receptors in atrial membranes co-purified exclusively with Go, whereas in cerebellar and ventricular membranes an association with both Gi and Go was demonstrated. Further characterization of the G protein subunits allowed identification of the species of Gi alpha subunits present in the complexes of muscarinic receptor and G protein; in ventricle Gi alpha 2 was the only subtype present, whereas in cerebellum both Gi alpha 1 and Gi alpha 2 were present. These results demonstrate that a single muscarinic receptor subtype, depending on the tissue studied, is capable of interacting with more than one G protein subtype. The concentrations of agonist required to promote receptor-G protein association in atrial and ventricular membranes correlated with the high affinity component of receptor occupancy by agonist, as measured in equilibrium binding assays. Furthermore, incubation of cardiac membranes with saturating concentrations of pilocarpine or McN A343 resulted in reduced amounts of receptor-G protein complexes, compared with carbachol. Overall, our results suggest that the specificity of cellular effects of muscarinic agonists may relate, in part, to the selective interaction of receptor with G proteins.