Down-regulation and desensitization of the muscarinic M₁ and M₂ receptors in transfected fibroblast B82 cells

Murine fibroblast cell lines stably transfected with the rat muscarinic m1 or m2 receptor genes were used to study the homologous regulation of the muscarinic M₁ or M₂ receptors. The cells were pretreated with the muscarinic receptor agonists, (+)-cismethyl-dioxolane, carbachol, 2,8-dimethyl-3-methylene-1-oxa-8-aza-spiro-[4,5]decane ((+) or (-)-YM796) or the muscarinic receptor antagonist atropine for up to 24 h. Our study has demonstrated that the muscarinic receptor nonselective full agonist, (+)-cismethyl-dioxolane, induced the down-regulation of both the muscarinic M₁ and the M₂ receptors in association with desensitization of the receptor-mediated functions. The muscarinic M₁ receptors are down-regulated without significant receptor internalization while the muscarinic M₂ receptors are more sensitive to down-regulation than the muscarinic M₁ receptors because of significant internalization of the muscarinic M₂ receptors in our system. The muscarinic M₁ receptor partial agonist, (-)-YM796 induced less down-regulation and no significant desensitization of the muscarinic M₁ receptors with no substantial effect on the muscarinic M₂ receptor density or function.