R-(+)-hyoscyamine: The first selective antagonist for guinea-pig uterus muscarinic receptor subtype

3‐α‐tropanyl 2‐(4‐Cl‐phenoxy)butyrate (SM 21): A Review of the Pharmacological Profile of a Novel Enhancer of Cholinergic Transmission

“…The affinity profile of SM 21 vs. MI (rabbit vas deferens, according to Eltze [23] and modified by Dei et al [18]), M2 (guinea pig atrium, according to Eltze et al [22] and modified by Dei et al [18]), M3 (guinea pig ileum, according Eltze and Figala [24]), and putative M4 receptors (prepuberal guinea pig uterus, according to Doje et al [20]) shows low M4/MI (1.9 times) and M2/MI (4.6 times) selectivity ratios as reported in Table 3. In this study, SM 21 selectivity was compared with that of the selective M4 antagonist R-(+)-hyoscyamine (34) and the selective M2 antagonist AFDX-116' (42). It is possible that a selectivity ratio of 4.6, even if small, may be high enough to enhance the pain threshold and to reverse amnesia as a consequence of ACh release.…”