title: Selectivity of agonists for the active state of M1 to M4 muscarinic receptor subtypes.
abstract: We measured the intrinsic relative activity (RA(i)) of muscarinic agonists to detect possible selectivity for receptor subtypes and signaling pathways. RA(i) is a relative measure of the microscopic affinity constant of an agonist for the active state of a GPCR expressed relative to that of a standard agonist. First, we estimated RA(i) values for a panel of agonists acting at the M(4) muscarinic receptor coupled to three distinct G-protein pathways: G(i) inhibition of cAMP accumulation, G(s) stimulation of cAMP accumulation, and G alpha(15) stimulation of phosphoinositide hydrolysis. Our results show similar RA(i) values for each agonist, suggesting that the same active state of the M(4) receptor triggers the activation of the three G proteins. We also estimated RA(i) values for agonists across M(1) to M(4) muscarinic subtypes stably transfected in Chinese hamster ovary cells. Our results show selectivity of McN-A-343 [4-I-[3-chlorophenyl]carbamoyloxy)-2-butynyltrimethylammnonium chloride] for the M(1) and M(4) subtypes and selectivity of pilocarpine for the M(1) and M(3) subtypes. The other agonists tested lacked marked selectivity among M(1) to M(4) receptors. Finally, we estimated RA(i) values from published literature on M(1), M(2), and M(3) muscarinic responses and obtained results consistent with our own studies. Our results show that the RA(i) estimate is a useful receptor-dependent measure of agonist activity.
authors.affiliation: Department of Pharmacology, University of California, Irvine, California 92697-4625, USA.
authors.forename: Katherine W
authors.initials: KW
authors.lastname: Figueroa
authors.forename: Michael T
authors.initials: MT
authors.lastname: Griffin
authors.forename: Frederick J
authors.initials: FJ
authors.lastname: Ehlert
mesh_terms: D000818:Animals; D016466:CHO Cells; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D003412:Cricetulus; D000242:Cyclic AMP / Q000502:physiology; D006801:Humans; D007700:Kinetics; D043643:Receptor, Muscarinic M1 / Q000819:agonists* / Q000145:classification; D043585:Receptor, Muscarinic M2 / Q000819:agonists* / Q000145:classification; D043587:Receptor, Muscarinic M3 / Q000819:agonists* / Q000145:classification; D043588:Receptor, Muscarinic M4 / Q000819:agonists* / Q000145:classification; D043562:Receptors, G-Protein-Coupled / Q000502:physiology; D011994:Recombinant Proteins / Q000145:classification / Q000378:metabolism; D012044:Regression Analysis; D014162:Transfection
chemical_list: D043643:Receptor, Muscarinic M1; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; D043588:Receptor, Muscarinic M4; D043562:Receptors, G-Protein-Coupled; D011994:Recombinant Proteins; D005576:Colforsin; D000242:Cyclic AMP