Application of transfected cell lines in studies of functional receptor subtype selectivity of muscarinic agonists.

Chinese hamster ovary cell lines which stably express individual subtypes of muscarinic acetylcholine receptors (m1-m5) were used to assess the potential selectivity of known muscarinic agonists in effecting coupling of different receptor subtypes to signal transduction mechanisms. Phosphoinositide hydrolysis was measured in m1-, m3- and m5-Chinese hamster ovary cells, whereas inhibition of forskolin-mediated cyclic AMP formation was measured upon the activation of m2 and m4 muscarinic receptors. The two muscarinic agonists pilocarpine and McN-A-343 were notably subtype selective on a functional basis. Pilocarpine was more efficacious in stimulating Phosphoinositide hydrolysis linked to m1 as compared to either m3 or m5 muscarinic receptors. On the other hand, McN-A-343 produced marked inhibition of cyclic AMP formation in m4-Chinese hamster ovary cells but only a small response at m2 receptors. The subtype selectivity of pilocarpine and McN-A-343 in these cases was not due to differences in the level of expression of muscarinic receptors in the various cell lines. In contrast, equalizing receptor number in pairs of cell lines masked apparent selectivities in other cases. Our data highlight the functional discrimination of pilocarpine and McN-A-343 among muscarinic receptor subtypes and emphasize the importance of using cell lines which express an equal number of receptors in the process of searching for subtype selective agonists.