Determination of dissociation constants and relative efficacies of some muscarinic agonists at nicotinic receptors.

Dissociation constant and relative efficacy values of six cholinergic agents endowed with a high muscarinic activity (carbachol, methylfurtrethonium, cis-2-methyl-4-trimetylammoniummethyl-1,3-dioxolane, cis-2-methyl-5-trimethylammoniummethyl-1,3-oxathiolane, muscarone, oxotremorine) have been determined on the frog rectus abdominis nicotinic receptors. The results obtained point out that: a) the nicotinic receptor reserve of the frog rectus abdominis is very little if any, b) there are no appreciable variations in the relative efficacy values of all the compounds tested, c) it is instead possible to establish a rank order of affinity. The comparison of the affinity values towards nicotinic receptors, herewith reported, and those towards ileal muscarinic receptors, previously reported could be useful to shed some light on the differences between muscarinic and nicotinic receptors.