McKinney M, Miller J H, Gibson V A, Nickelson L, Aksoy S
Department of Pharmacology, Mayo Clinic Jacksonville, Florida 32224.
Mol Pharmacol. 1991 Dec;40(6):1014-22.
In this study the similarities and differences between the M2 and M4 subtypes in their recognition of agonists were explored. A CHO-K1 cell line transfected with the human m2 receptor was used as a homogeneous M2 tissue for comparison with two putative M4 systems (rat striatum and the N1E-115 mouse neuroblastoma cell line). The equilibrium binding dissociation constants and intrinsic efficacies for seven muscarinic agonists were determined for their stimulation of cyclic AMP inhibition via the M2 and M4 receptors. Partial receptor occlusion with propylbenzilylcholine mustard was used to determine binding constants for the more efficacious drugs and the reference agonist oxotremorine-M. The binding dissociation constants and relative efficacies for other agonists were then determined in reference to oxotremorine-M by a null method. For the M2 receptor the agonist binding dissociation constants ranged in potency from oxotremorine (1.5 microM) to bethanechol (171 microM), whereas relative efficacies varied from that of muscarine (relative efficacy = 0.9) to the value for McN-A343 (relative efficacy = 0.04). In general, most agonists bound with similar potencies to M2 and M4 receptors (Kd values within a factor of 2-3). However, oxotremorine bound to the N1E-115 and striatal M4 receptors about 3-fold and 10-fold less potently, respectively, than it did to the M2 receptor. Another exception was pilocarpine, which bound to the N1E-115 receptor (1.9 microM) with 8-fold and 12-fold higher potency than to the CHO-K1 M2 receptor and the striatal M4 receptor, respectively. Despite the low affinity of bethanechol for the M2 receptor, it was an efficacious agonist (maximal response equivalent to that of oxotremorine-M; relative efficacy = 0.6) at this subtype, whereas it was a partial agonist (60%) with lesser efficacy in the clonal M4 system. In contrast, McN-A343 and arecoline were significantly more efficacious at the two M4 receptors than they were at the M2 receptor. The M4 system in the rat striatum displayed some similarity to the N1E-115 M4 system, with regard to the efficacy ranking for certain agonists (arecoline greater than bethanechol greater than McN-A343 greater than or equal to pilocarpine). This rank order was different from the ranking of these four agonists in the M2 system, indicating that these two M4 receptors are more similar to each other in efficacy ranking than they are to the M2 receptor. However, the rat striatal and N1E-115 M4 receptor differed in their binding of oxotremorine and pilocarpine, indicating that these two M4 systems were not pharmacologically identical.(ABSTRACT TRUNCATED AT 400 WORDS)
在本研究中，探讨了M2和M4亚型在识别激动剂方面的异同。将转染了人m2受体的CHO-K1细胞系用作均质M2组织，与两种假定的M4系统（大鼠纹状体和N1E-115小鼠神经母细胞瘤细胞系）进行比较。测定了七种毒蕈碱激动剂通过M2和M4受体刺激环磷酸腺苷抑制的平衡结合解离常数和内在效能。使用丙基苯甲酰胆碱芥子气进行部分受体阻断，以确定更有效药物和参考激动剂氧化震颤素-M的结合常数。然后通过零方法参照氧化震颤素-M确定其他激动剂的结合解离常数和相对效能。对于M2受体，激动剂结合解离常数的效力范围从氧化震颤素（1.5微摩尔）到氨甲酰甲胆碱（171微摩尔），而相对效能从毒蕈碱的相对效能（相对效能=0.9）到McN-A343的值（相对效能=0.04）不等。一般来说，大多数激动剂与M2和M4受体的结合效力相似（解离常数Kd值相差2至3倍）。然而，氧化震颤素与N1E-115和纹状体M4受体的结合效力分别比与M2受体的结合效力低约3倍和10倍。另一个例外是毛果芸香碱，它与N1E-115受体（1.9微摩尔）的结合效力分别比与CHO-K1 M2受体和纹状体M4受体的结合效力高8倍和12倍。尽管氨甲酰甲胆碱对M2受体的亲和力较低，但它在该亚型中是一种有效的激动剂（最大反应等同于氧化震颤素-M；相对效能=0.6），而在克隆M4系统中它是一种部分激动剂（60%），效能较低。相比之下，McN-A343和槟榔碱在两种M4受体上的效力明显高于它们在M2受体上的效力。大鼠纹状体中的M4系统在某些激动剂的效能排序方面（槟榔碱>氨甲酰甲胆碱>McN-A343≥毛果芸香碱）与N1E-115 M4系统显示出一些相似性。这种排序与这四种激动剂在M2系统中的排序不同，表明这两种M4受体在效能排序上彼此比与M2受体更相似。然而，大鼠纹状体和N1E-115 M4受体在氧化震颤素和毛果芸香碱的结合方面存在差异，表明这两种M4系统在药理学上并不相同。（摘要截短至400字）
