Potter L T, Ferrendelli C A, Hanchett H E, Hollifield M A, Lorenzi M V
Department of Pharmacology, University of Miami School of Medicine, Florida 33133.
Mol Pharmacol. 1989 May;35(5):652-60.
Tetrahydroaminoacridine (THA) and a variety of other nonclassical antagonists of muscarine receptors were studied for their ability to bind to primary and allosteric sites on muscarine receptors in rabbit hippocampal membranes. Competition curves between 13 antagonists and 1 nM [3H]pirenzepine (Kd = 3 nM) were simple mass action curves, but THA produced steeper curves, indicating positive cooperativity. Nonetheless, THA inhibited the binding of low concentrations of [3H]pirenzepine, [3H]N-methylscopolamine, and [3H]oxotremorine-M to M1 receptors with similar IC50 values, indicating competition for primary sites. Antagonists were also compared for their ability to bind to allosteric sites and to slow the dissociation of [3H]pirenzepine from primary sites. THA was 6-8-fold more potent than verapamil, d-tubocurare, quinidine, and secoverine, the next most effective allosteric agents, and THA was more effective. McN-A-343, gallamine, pancuronium, and pirenzepine showed weaker allosteric effects. The large size and considerable rigidity of these compounds suggest large allosteric sites. The Hill coefficient for the allosteric effects of THA was 1.7, indicating more than one allosteric site. Solubilization of receptors did not alter steep inhibition curves between THA and [3H]quinuclidinyl benzilate or THA-induced slowing of the dissociation of this ligand. Hence, cooperative allosteric effects of THA are probably exerted on receptor monomers. Inhibition curves between THA and [3H]oxotremorine-M were not steep, and THA had no (allosteric) effect on the dissociation of this ligand from M1 or M2 receptors. Thus, the high affinity agonist conformation of muscarine receptors, once formed, may not bind THA readily. The present results indicate that compounds that can act allosterically may compete with acetylcholine for primary receptor sites but that allosteric effects of these drugs on muscarine receptors are not likely to be important clinically.
研究了四氢氨基吖啶（THA）和多种其他非经典毒蕈碱受体拮抗剂与兔海马膜中毒蕈碱受体上的主要位点和变构位点结合的能力。13种拮抗剂与1 nM [3H]哌仑西平（Kd = 3 nM）之间的竞争曲线为简单的质量作用曲线，但THA产生的曲线更陡，表明存在正协同效应。尽管如此，THA以相似的IC50值抑制低浓度的[3H]哌仑西平、[3H]N-甲基东莨菪碱和[3H]氧代震颤素-M与M1受体的结合，表明其对主要位点存在竞争。还比较了拮抗剂与变构位点结合以及减缓[3H]哌仑西平从主要位点解离的能力。THA的效力比维拉帕米、d-筒箭毒碱、奎尼丁和西可韦林（接下来最有效的变构剂）强6至8倍，且THA更有效。McN-A-343、加拉明、泮库溴铵和哌仑西平的变构效应较弱。这些化合物的大尺寸和相当的刚性表明存在大的变构位点。THA变构效应的希尔系数为1.7，表明存在不止一个变构位点。受体的增溶并未改变THA与[3H]喹核醇基苯甲酸酯之间的陡峭抑制曲线或THA诱导的该配体解离减缓。因此，THA的协同变构效应可能作用于受体单体。THA与[3H]氧代震颤素-M之间的抑制曲线不陡峭，且THA对该配体从M1或M2受体的解离没有（变构）作用。因此，一旦形成，毒蕈碱受体的高亲和力激动剂构象可能不易与THA结合。目前的结果表明，能够产生变构作用的化合物可能与乙酰胆碱竞争主要受体位点，但这些药物对毒蕈碱受体的变构效应在临床上可能并不重要。
