Contraction of guinea-pig gallbladder : muscarinic M3 or M4 receptors?

Abstract The muscarinic receptor mediating contraction of the guinea-pig isolated gallbladder, currently being disputed to belong either to the M 3 or M 4 subtype, was characterized by subtype-preferring agonists and discriminating antagonists. Highly significant correlations of agonist potencies to contract the gallbladder, e.g., arecaidine propargyl ester, oxotremorine, 5-methylfurtrethonium>arecoline, arecaidine 2-butyne-1,4-diyl bisester>( R )-nipecotic acid ethyl ester>4-[[ N -(4-chlorophenyl)carbamyl]oxy]-2-butynyltrimethylammonium iodide (4-Cl-McN-A-343), ( S )-nipecotic acid ethyl ester>4-[[ N -(3-chlorophenyl)carbamoyl]oxy]-2-butynyltrimethylammonium chloride (McN-A-343) were found with muscarinic M 3 receptors mediating contraction of the guinea-pig ileum and vasodilation in rat perfused kidney. Functional affinities at guinea-pig gallbladder muscarinic receptors of antagonists known to distinguish between native or cloned muscarinic M 3 /m3 and M 4 /m4 receptors, e.g., himbacine, methoctramine, mefurtramine, tripitramine, idaverine, zamifenacin and 11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido(2,3-b)(1,4)benzodiazepin-6-one (AQ-RA 741), were consistent with those at guinea-pig ileal muscarinic M 3 receptors but not with published data at recently defined muscarinic M 4 receptors in rabbit anococcygeus muscle or at muscarinic M 1 and M 2 receptors in rabbit vas deferens. Antagonist affinities at guinea-pig gallbladder correlated also best with published binding data on native or cloned muscarinic M 3 /m3 receptors but not with those for muscarinic M 4 /m4 receptors. The agonist potencies and antagonist affinities suggest that smooth muscle contraction elicited by muscarinic stimuli in guinea-pig gallbladder is mediated by functional muscarinic M 3 receptors.