Darifenacin reveals a functional role for M₄ muscarinic acetylcholine receptors in guinea pig gallbladder

Objective: Previous studies have revealed the presence of M₁ to M₄ muscarinic receptors in guinea pig gallbladder, with M ₃ and M₄ receptors being claimed to mediate contractions. Methods: In the present study, cumulative concentration-response curves to carbachol were constructed in the absence and presence of darifenacin, which has greater selectivity for M₃ over M₄ receptors, in guinea-pig gallbladder. Results: Darifenacin caused concentration-dependent dextral shifts of the carbachol curve, however, at 1 μM, it also caused a degree of insurmountable antagonism. Using a novel analytical approach, we obtained a pK_B estimate for darifenacin (7.51 ± 0.14) that is in excellent agreement with its affinity for M₂/M₄, but not M₃ receptors. Conclusion: Given our previous demonstration that M₂ receptors are unlikely to contribute to muscarinic contractions in this tissue, our current findings provide pharmacological evidence for a predominant role of M₄ muscarinic receptors in guinea pig gallbladder contractions.