D'Agostino G, Barbieri A, Chiossa E, Tonini M
Institute of Pharmacology, School of Pharmacy, University of Pavia, Pavia, Italy.
J Pharmacol Exp Ther. 1997 Nov;283(2):750-6.
A pharmacological analysis was carried out in the rat urinary bladder to assess the nature of muscarinic receptors subtypes functionally involved in the negative feedback mechanism regulating acetylcholine (ACh) secretion from postganglionic cholinergic nerve terminals and in smooth muscle contraction. Bladder strips were preincubated with 3H-choline, and the electrically evoked [3H]ACh release was detected simultaneously with contraction in the absence of acetylcholinesterase inhibitors. The effects were compared of seven muscarinic antagonists on [3H]ACh secretion (prejunctional effect) and muscle contraction (postjunctional effect). The rank order of postjunctional potencies (-log EC50) for the seven antagonists (atropine > 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) > hexahydrosiladiphenidol hydrochloride (HHSiD) > tripitramine > pirenzepine > AF DX-116 > methoctramine) as well as their postjunctional affinity estimates (pA2) are in keeping with the notion that muscarinic receptors responsible for bladder contraction belong to the M3 subtype. The M3 subtype-preferring 4-DAMP and HHSiD did not discriminate between prejunctional and postjunctional effects. The M2/M4 subtype-preferring antagonists tripitramine, methoctramine and AF-DX 116 were more potent in facilitating the evoked [3H]ACh release than in inhibiting the contractile response. The rank order of prejunctional potencies was atropine > 4-DAMP > tripitramine > HHSiD > methoctramine > AF-DX 116 > pirenzepine, indicating the involvement of M4 receptors. Furthermore, when potency relationship was determined by correlating prejunctional-log EC50 values with published constants for cloned and natives muscarinic receptor subtypes, the correlations were significant for both M4 and M5 subtypes, but the best correlation found (P < .001) was for the M4 subtype. These findings suggest that the negative feedback mechanism inhibiting the release of ACh in the rat urinary bladder is mediated by prejunctional autoreceptors of the M4 subtype.
在大鼠膀胱中进行了药理学分析，以评估参与调节节后胆碱能神经末梢乙酰胆碱（ACh）分泌的负反馈机制以及平滑肌收缩的毒蕈碱受体亚型的性质。将膀胱条用3H-胆碱预孵育，并在不存在乙酰胆碱酯酶抑制剂的情况下，在检测收缩的同时检测电诱发的[3H]ACh释放。比较了七种毒蕈碱拮抗剂对[3H]ACh分泌（节前效应）和肌肉收缩（节后效应）的影响。七种拮抗剂的节后效价（-log EC50）的排序（阿托品>4-二苯基乙酰氧基-N-甲基哌啶甲碘化物（4-DAMP）>盐酸六氢硅双苯酯（HHSiD）>曲匹拉明>哌仑西平>AF DX-116>甲奥克明）及其节后亲和力估计值（pA2）与负责膀胱收缩的毒蕈碱受体属于M3亚型的观点一致。偏好M3亚型的4-DAMP和HHSiD无法区分节前和节后效应。偏好M2/M4亚型的拮抗剂曲匹拉明、甲奥克明和AF-DX 116在促进诱发的[3H]ACh释放方面比抑制收缩反应更有效。节前效价的排序为阿托品>4-DAMP>曲匹拉明>HHSiD>甲奥克明>AF-DX 116>哌仑西平，表明M4受体的参与。此外，当通过将节前-log EC50值与已发表的克隆和天然毒蕈碱受体亚型的常数相关联来确定效价关系时，M4和M5亚型的相关性均显著，但发现的最佳相关性（P<.001）是M4亚型。这些发现表明，抑制大鼠膀胱中ACh释放的负反馈机制是由M4亚型的节前自身受体介导的。
