Selectivity of Agonists for the Active State of M1 to M4 Muscarinic Receptor Subtypes
Abstract
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 Gα 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-chlorophenylcarbamoyloxy)-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.