Objective: Muscarinic receptors mediate contraction of the human gallbladder through unclear receptor subtypes. The aim of the present study was to characterize muscarinic acetylcholine receptors mediating contraction of the human gallbladder.
Materials and methods: Contraction of human gallbladder muscle strips caused by agonists carbachol and muscarine was measured and the inhibition of carbachol-induced contraction by muscarinic receptor antagonists was evaluated. Reverse transcription polymerase chain reaction was performed to determine the existence of muscarinic receptor subtypes.
Results: Carbachol and muscarine caused concentration-dependent contraction of gallbladder strips. Four receptor antagonists, including atropine, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), methoctramine, and pirenzepine, inhibited the carbachol-induced contraction. The relative inhibitory potency of these receptor antagonists was atropine > 4-DAMP > methoctramine > pirenzepine. The antagonist affinity estimates (pA(2) values) correlated with the known affinities at M(3), M(4), and M(5) muscarinic receptors. In addition, the M(4)-selective antagonist muscarinic toxin 3 did not inhibit and the M(5)-selective positive allosteric modulator VU0238429 did not potentiate carbachol-induced gallbladder contraction. This suggests that M(3) muscarinic receptors mediate the muscarinic response predominantly. The contractile response of carbachol was attenuated by the voltage-gated Ca(2+) channel inhibitor nifedipine and Rho-kinase inhibitor H-1152, but not affected by protein kinase C inhibitor chelerythrine. This implies the involvement of voltage-gated Ca(2+) channel and Rho kinase but not protein kinase C.
Conclusions: These results suggest a major role of M(3) muscarinic receptors mediating the human gallbladder contraction through voltage-gated Ca(2+) channels and Rho kinase. M(3)-selective muscarinic receptor antagonists could be of therapeutic importance in the treatment of biliary motility disorders.