Comparative pharmacology of recombinant human M3 and M5 muscarinic receptors expressed in CHO-K1 cells

1. Affinity estimates were obtained for several muscarinic antagonists against carbachol-stimulated [3H]-inositol phosphates accumulation in Chinese hamster ovary (CHO-KI) cells stably expressing either human muscarinic M3 or M5 receptor subtypes. The rationale for these studies was to generate a functional antagonist affinity profile for the M5 receptor subtype and compare this with that of the M3 receptor, in order to identify compounds which discriminate between these two subtypes. 2. The rank order of antagonist apparent affinities (pK(B)) at the muscarinic M5 receptor was atropine (8.7) > or =tolterodine (8.6) = 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 8.6)> darifenacin (7.7) > or =zamifenacin (7.6)>oxybutynin (6.6)= para-fluorohexahydrosiladifenidol (p-F-HHSiD, 6.6)>pirenzepine (6.4) > or = methoctramine (6.3)=himbacine (6.3)>AQ-RA 741 (6.1). 3. Antagonist apparent affinities for both receptor subtypes compare well with published binding affinity estimates. No antagonist displayed greater selectivity for the muscarinic M5 subtype over the M3 subtype, but himbacine, AQ-RA 741, p-F-HHSiD, darifenacin and oxybutynin displayed between 9- and 60 fold greater selectivity for the muscarinic M3 over the M5 subtype. 4. This study highlights the similarity in pharmacological profiles of M3 and M5 receptor subtypes and identifies five antagonists that may represent useful tools for discriminating between these two subtypes. Collectively, these data show that in the absence of a high affinity M5 selective antagonist, affinity data for a large range of antagonists is critical to define operationally the M5 receptor subtype.