Comparative pharmacology of recombinant human M<sub>3</sub> and M<sub>5</sub> muscarinic receptors expressed in CHO‐K1 cells

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<jats:p> <jats:list list-type="explicit-label"> <jats:list-item><jats:p>Affinity estimates were obtained for several muscarinic antagonists against carbachol‐stimulated [<jats:sup>3</jats:sup>H]‐inositol phosphates accumulation in Chinese hamster ovary (CHO‐K1) cells stably expressing either human muscarinic M<jats:sub>3</jats:sub> or M<jats:sub>5</jats:sub> receptor subtypes. The rationale for these studies was to generate a functional antagonist affinity profile for the M<jats:sub>5</jats:sub> receptor subtype and compare this with that of the M<jats:sub>3</jats:sub> receptor, in order to identify compounds which discriminate between these two subtypes.</jats:p></jats:list-item> <jats:list-item><jats:p>The rank order of antagonist apparent affinities (p<jats:italic>K</jats:italic><jats:sub>B</jats:sub>) at the muscarinic M<jats:sub>5</jats:sub> receptor was atropine (8.7)tolterodine (8.6)=4‐diphenylacetoxy‐N‐methylpiperidine (4‐DAMP, 8.6)>darifenacin (7.7)zamifenacin (7.6)>oxybutynin (6.6)=para‐fluorohexahydrosiladifenidol (p‐F‐HHSiD, 6.6)>pirenzepine (6.4)methoctramine (6.3)=himbacine (6.3)>AQ‐RA 741 (6.1).</jats:p></jats:list-item> <jats:list-item><jats:p>Antagonist apparent affinities for both receptor subtypes compare well with published binding affinity estimates. No antagonist displayed greater selectivity for the muscarinic M<jats:sub>5</jats:sub> subtype over the M<jats:sub>3</jats:sub> subtype, but himbacine, AQ‐RA 741, p‐F‐HHSiD, darifenacin and oxybutynin displayed between 9‐ and 60 fold greater selectivity for the muscarinic M<jats:sub>3</jats:sub> over the M<jats:sub>5</jats:sub> subtype.</jats:p></jats:list-item> <jats:list-item><jats:p>This study highlights the similarity in pharmacological profiles of M<jats:sub>3</jats:sub> and M<jats:sub>5</jats:sub> 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 M<jats:sub>5</jats:sub> selective antagonist, affinity data for a large range of antagonists is critical to define operationally the M<jats:sub>5</jats:sub> receptor subtype.</jats:p></jats:list-item> </jats:list> </jats:p><jats:p><jats:italic>British Journal of Pharmacology</jats:italic> (1999) <jats:bold>127</jats:bold>, 590–596; doi:<jats:ext-link xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" xlink:href="10.1038/sj.bjp.0702551">10.1038/sj.bjp.0702551</jats:ext-link></jats:p>

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