M₁ and M₃ muscarinic receptors are involved in the release of urinary bladder-derived relaxant factor

We have investigated the muscarinic receptor subtype(s) mediating the release of urinary bladder-derived relaxant factor that is demonstrated by a coaxial bioassay system. Acetylcholine-induced relaxation of a precontracted anococcygeus muscle mounted within the bladder was considered as an evidence for the release of this factor. M₁-muscarinic agonist McN-A-343 and the cholinesterase inhibitor physostigmine also elicited relaxation responses in the coaxial bioassay besides acetylcholine. Acetylcholine-induced relaxation was antagonized by the subtype-selective muscarinic antagonists (pK_B): M₃-antagonist darifenacin (9.36 ± 0.11), M₃/M₁-antagonist 4-DAMP (9.30 ± 0.11), M₁-antagonist telenzepine (8.56 ± 0.21), M₄-antagonist tropicamide (6.63 ± 0.17) and M₂-antagonist AF-DX 116 (6.01 ± 0.21). The pK_B values of these antagonists have suggested that stimulation of M₁- and M₃-muscarinic receptors in the bladder wall mediates the release of urinary bladder-derived relaxant factor. In addition, McN-A-343, by activating the facilitatory M₁ receptors and physostigmine by inhibiting the acetylcholinesterase may induce the release of this factor through endogenous acetylcholine in the coaxial bioassay system.