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

The muscarinic receptor mediating contraction of the guinea-pig isolated gallbladder, currently being disputed to belong either to the M3 or M4 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 M3 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 M3/m3 and M4/m4 receptors, e.g., himbacine, methoctramine, mefurtramine, tripitramine, idaverine, zamifenacin and 11-[[4-[4-(diethylamino)butyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyr ido(2,3-b)(1,4)benzodiazepin-6-one (AQ-RA 741), were consistent with those at guinea-pig ileal muscarinic M3 receptors but not with published data at recently defined muscarinic M4 receptors in rabbit anococcygeus muscle or at muscarinic M1 and M2 receptors in rabbit vas deferens. Antagonist affinities at guinea-pig gallbladder correlated also best with published binding data on native or cloned muscarinic M3/m3 receptors but not with those for muscarinic M4/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 M3 receptors.