DIFFERENTIAL SELECTIVITY OF M1 AND M3 TYPE MUSCARINIC ACETYLCHOLINE-RECEPTORS IN COUPLING WITH A G-PROTEIN GO1-ALPHA EXAMINED IN XENOPUS OOCYTES

The coupling specificities between muscarinic acetylcholine receptors (porcine M1 and M3 subtypes) and a heterotrimeric GTP binding protein (bovine Go1) were examined by analysing receptor-stimulated, phospholi pase C (PLC)-mediated chloride current responses in Xenopus oocyte exp ression system. Mi-stimulated responses were suppressed by Go1 alpha, and this suppression was reversed by expressing exogenous G beta and G gamma, whereas MS-stimulated responses were enhanced by Go1 alpha in the presence or absence of G beta and G gamma. Go1 alpha itself, when activated directly by intracellular fluoride injection, efficiently ac tivated PLC and stimulated chloride current responses. The results ind icate that M1 receptor does not couple with Go1, while M3 receptor act ivates Go1 effectively.