Affinity profiles of various muscarinic antagonists for cloned human muscarinic acetylcholine receptor (mAChR) subtypes and mAChRs in rat heart and submandibular gland

A family of five subtypes of muscarinic acetylcholine receptors (mAChR) has been identified based on their molecular structures and second signal transduction pathways. In the present study, we examined the antagonist binding profiles of 9 muscarinic antagonists (atropine, 4-DAMP, pirenzepine, oxybutynin, tiquizium, timepidium, propiverine, darifenacin and zamifenacin) for human muscarinic acetylcholine receptor subtypes (ml, mt, m3, m4 and ms) produced by using a baculovirus infection system in Sf9 insect cells, and rat tissue membrane preparations (heart and submandibular gland). In a scopolamine methyl chloride [N-methyl-H-3]- ([H-3]NMS) binding assay, pirenzepine and timepidium displayed the highest affinities for the mi and m2 subtypes, respectively, and both zamifenacin and darifenacin had the highest affinities for the nu subtype, although the selectivities among the five subtypes were less than 10-fold. Propiverine showed a slightly higher affinity for the ms subtype, whereas none of the drugs used in this study was uniquely selective for the m4 subtype. The binding affinities of muscarinic antagonists for rat heart and submandibular gland strong correlated with those for human cloned m2 and m3 subtypes, respectively. These data suggest that [3H]NMS binding studies using rat heart and submandibular gland might be useful methods which predict the affinities of test drugs for human muscarinic M2 and M3 receptor subtypes.