title: Agonist-independent inhibition of G protein activation by muscarinic acetylcholine receptor antagonists in cardiac membranes.
abstract: Agonist activation of muscarinic acetylcholine (mACh) receptors in porcine atrial membranes stimulates binding of the GTP analog, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]), to membrane G proteins. In contrast, atropine as well as several other mACh receptor antagonists reduced the binding of GTP[S] below basal values, both in the absence and presence of the agonist carbachol, by a similar maximal extent (about 25%). Evidence is presented that this inhibitory action of atropine was not due to an antagonism of endogenous acetylcholine. Similar to agonist-induced stimulation, antagonist-induced inhibition of GTP[S] binding required the presence of GDP and Mg2+. On the other hand, addition of salts, e.g. NaCl, amplified agonist but reduced antagonist effects on GTP[S] binding. The data presented suggest that agonist-unliganded mACh receptors interact with and activate G proteins in native cardiac membranes and that antagonist binding induces a conformational change of the receptor, which then either does not interact with G proteins or which prevents GDP release from and subsequent GTP[S] binding to G proteins.
authors.affiliation: Pharmakologisches Institut der Universität Heidelberg, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Hilf
authors.forename: K H
authors.initials: KH
authors.lastname: Jakobs
mesh_terms: D000110:Acetylcholinesterase / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001711:Biotransformation / Q000187:drug effects; D002217:Carbachol / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D008566:Membranes / Q000378:metabolism; D018727:Muscarinic Antagonists; D009206:Myocardium / Q000378:metabolism*; D011485:Protein Binding; D011976:Receptors, Muscarinic / Q000187:drug effects; D013552:Swine
chemical_list: D018727:Muscarinic Antagonists; D011976:Receptors, Muscarinic; C034883:guanosine 5'-O-(2-thiotriphosphate); D016244:Guanosine 5'-O-(3-Thiotriphosphate); D001285:Atropine; D002217:Carbachol; D000110:Acetylcholinesterase; D019204:GTP-Binding Proteins; D008274:Magnesium