Muscarinic acetylcholine receptor‐stimulated binding of guanosine 5′‐O‐(3‐thiotriphosphate) to guanine‐nucleotide‐binding proteins in cardiac membranes

Receptor‐regulated binding of the labeled GTP analog, guanosine 5′‐O‐(3‐thiotriphosphate) ([35S]GTP[S]), to guanine‐nucleotide‐binding proteins (G‐proteins) was studied in porcine atrial membranes enriched in muscarinic acetylcholine (mACh) receptors. Binding of [35S]GTP[S] to the membranes was not or only slightly affected by the cholinergic agonist, carbachol, unless a second nucleotide was simultaneously present in the binding assay. This additional nucleotide requirement was best fulfilled by GDP, being maximally effective at 0.1–1 μM. In contrast, the GDP analog, guanosine 5′‐O‐(2‐thiodiphosphate), could not replace GDP in promoting carbachol‐induced increase in [35S]GTP[S] binding. In addition to GDP, agonist‐induced stimulation of [35S]GTP[S] binding to porcine atrial membranes required the presence of Mg2+, being half‐maximally and maximally effective at about 30 μM and 300 μM, respectively. Addition of NaCl, which decreased control binding measured in the presence of GDP alone, had no effect on the maximal extent of agonist‐stimulated binding, but reduced the potency of carbachol in stimulating [35S]GTP[S] binding. Under optimal conditions, carbachol increased the binding of [35S]GTP[S] without apparent lag phase up to about 2.5‐fold, with half‐maximal and maximal increase being observed at 5–10 μM and 100 μM, respectively. The agonist‐induced stimulation was competitively antagonized by the mACh receptor antagonist, atropine. The number of GTP[S] binding sites under receptor control was two–three‐fold higher than the number of mACh receptors in the porcine atrial membranes used. Pretreatment of the membranes with pertussis toxin under conditions leading to 95% ADP‐ribosylation of the toxin‐sensitive G‐protein α‐subunits markedly reduced agonist‐stimulated [35S]GTP[S] binding, with, however, about 30% stimulation still remaining. The data presented indicate that agonist‐stimulated binding of [35S]GTP[S] to G‐proteins can be a sensitive assay for measuring receptor‐regulated G‐protein activation in native membranes and, furthermore, suggest that one agonist‐activated mACh receptor can activate two or three cardiac G‐proteins, being mainly members of the pertussis‐toxin‐sensitive G‐proteins.