Selectivity of Agonists for the Active State of M1 to M4 Muscarinic Receptor Subtypes

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Abstract
We measured the intrinsic relative activity (RAi) of muscarinic agonists to detect possible selectivity for receptor subtypes and signaling pathways. RAi 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 RAi values for a panel of agonists acting at the M4 muscarinic receptor coupled to three distinct G-protein pathways: Gi inhibition of cAMP accumulation, Gs stimulation of cAMP accumulation, and Gα15 stimulation of phosphoinositide hydrolysis. Our results show similar RAi values for each agonist, suggesting that the same active state of the M4 receptor triggers the activation of the three G proteins. We also estimated RAi values for agonists across M1 to M4 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 M1 and M4 subtypes and selectivity of pilocarpine for the M1 and M3 subtypes. The other agonists tested lacked marked selectivity among M1 to M4 receptors. Finally, we estimated RAi values from published literature on M1, M2, and M3 muscarinic responses and obtained results consistent with our own studies. Our results show that the RAi estimate is a useful receptor-dependent measure of agonist activity.

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