TRANSFECTED MUSCARINIC ACETYLCHOLINE-RECEPTORS SELECTIVELY COUPLE TO G(I)-TYPE G-PROTEINS AND G(Q 11)/

Regulation of effector functions by muscarinic acetylcholine receptor subtypes is mediated by pertussis toxin-sensitive and -insensitive G p roteins. In membranes from human embryonic kidney 293 cells transfecte d with mi, m2, and m3 muscarinic acetylcholine receptors, we detected the pertussis toxin-sensitive G proteins G(i1), G(i2), and G(i3) and t he pertussis toxin-insensitive G proteins G(q/11) and G(s). Subtype-sp ecific immunoprecipitation of G protein alpha subunits photolabeled wi th [alpha-P-32]GTP azidoanilide, in the absence and presence of carbac hol, revealed the selective coupling of activated muscarinic receptors to G protein subtypes. G(q/11) was activated via m2 and m3 receptors and G(i2) was activated via m2 receptors. All three receptor subtypes mediated the activation of G(i1) and G(i3). Effective activation of G( i1) and G(i3) via m1 and m3 receptors occurred only at high carbachol concentrations (EC(50) about 10-20 mu M), whereas carbachol with highe r potency (EC(50) about 1 mu M) induced activation of all G(i) subtype s via m2 receptors. Thus, coupling of muscarinic receptors and G prote in subtypes was principally selective; however, activation of distinct G protein subtypes by different muscarinic receptors occurred with di fferent efficacies.