Pharmacologic comparison of selected agonists for the M₁ muscarinic receptor in transfected murine fibroblast cells (B82)

The radioligand binding and functional properties of 10 muscarinic agonists for the M₁ muscarinic receptors were characterized on the murine fibroblast B82 cells, which have been transfected with the m₁ gene. All of the muscarinic agonists completely inhibited [³H](-)methyl-3-quinuclidinyl benzilate binding to the M₁ muscarinic receptor in the transfected B82 cells. Their apparent inhibition constant values for agonist/[³H](-)methyl-3-quinuclidinyl benzilate inhibition experiments correlate well with their EC₅₀ values in stimulating phosphatidylinositide hydrolysis. Based on the maximal functional effects: (+)-cismethyl-dioxolane, oxotremorine-M, acetylcholine, carbachol and methacholine are most efficacious, McN-A-343 and arecoline are least efficacious, whereas the efficacies of oxotremorine and pilocarpine are intermediate. In addition, McN-A-343 inhibited carbachol-stimulated phosphatidylinositide hydrolysis. Spare receptors were detected for oxotremorine-M, methacholine and carbachol, but not the rest of the agonists, by comparing the receptor-occupancy curves with the concentration-response curves. These results suggest that the presence of a quaternary nitrogen (trimethylammonium group) within the structure of the agonist may be important for the expression of full agonist activity.