Dunlap J, Brown J H
Mol Pharmacol. 1983 Jul;24(1):15-22.
The binding of [3H]quinuclidinyl benzilate ([3H]QNB) to cardiac muscarinic receptors was inhibited not only by classical muscarinic antagonists but also by nicotinic blocking agents and inhibitors of acetylcholinesterase. Gallamine, pancuronium, ambenonium, and decamethonium were the most potent of these agents examined. All of the nicotinic antagonists with significant muscarinic receptor activity had two or three quaternary nitrogens, and the potency of a series of these compounds was a function of the distance between quaternary nitrogens. The effects of gallamine and pancuronium were studied in detail because these neuromuscular blocking agents showed heterogeneity in their binding to cardiac muscarinic receptors, whereas classical muscarinic antagonists such as QNB and atropine did not. Gallamine did not compete for all of the [3H]QNB binding sites on atrial membranes, but left at least 20% of [3H]QNB binding unaffected. Curves of pancuronium competition for [3H]QNB binding were shallow, consistent with two binding sites for pancuronium, with approximately 20% having low affinity. Additionally, in the presence of gallamine or pancuronium, [3H]QNB binding sites were no longer homogeneous, and Scatchard plots became nonlinear. Guanine nucleotides did not alter the effect of gallamine or pancuronium on [3H]QNB binding. Gallamine and pancuronium showed no agonist activity but, like atropine, completely antagonized muscarinic receptor-mediated inhibition of cyclic AMP formation. However, differences in the behavior of gallamine and atropine suggested that gallamine was not a purely competitive antagonist. Gallamine did not protect against receptor alkylation by propylbenzilylcholine mustard, and [3H]QNB dissociation was apparently slowed by gallamine. We interpret our data to suggest that gallamine not only competes for [3H]QNB binding sites, but also binds at a secondary site on the receptor, forming a ternary complex with [3H]QNB. Heterogeneity in ligand binding is proposed to result from the dual actions of gallamine and pancuronium as ligands at both primary and secondary sites on the cardiac muscarinic receptor.
[3H]喹核醇基苯甲酸酯（[3H]QNB）与心脏毒蕈碱受体的结合不仅受到经典毒蕈碱拮抗剂的抑制，还受到烟碱阻断剂和乙酰胆碱酯酶抑制剂的抑制。加拉明、泮库溴铵、氨苯铵和十烃季铵是所检测的这些药物中作用最强的。所有具有显著毒蕈碱受体活性的烟碱拮抗剂都有两个或三个季铵氮，并且一系列这些化合物的效力是季铵氮之间距离的函数。详细研究了加拉明和泮库溴铵的作用，因为这些神经肌肉阻断剂在与心脏毒蕈碱受体的结合中表现出异质性，而经典毒蕈碱拮抗剂如QNB和阿托品则没有。加拉明并不与心房膜上所有的[3H]QNB结合位点竞争，而是至少使20%的[3H]QNB结合不受影响。泮库溴铵竞争[3H]QNB结合的曲线较平缓，这与泮库溴铵有两个结合位点一致，其中约20%具有低亲和力。此外，在加拉明或泮库溴铵存在的情况下，[3H]QNB结合位点不再均匀，Scatchard图呈非线性。鸟嘌呤核苷酸不会改变加拉明或泮库溴铵对[3H]QNB结合的影响。加拉明和泮库溴铵没有激动剂活性，但与阿托品一样，能完全拮抗毒蕈碱受体介导的环磷酸腺苷形成的抑制作用。然而，加拉明和阿托品行为上的差异表明加拉明不是纯粹的竞争性拮抗剂。加拉明不能防止受体被丙基苯甲酰胆碱氮芥烷基化，并且加拉明显然减缓了[3H]QNB的解离。我们对数据的解释表明，加拉明不仅竞争[3H]QNB结合位点，并在受体的二级位点结合，与[3H]QNB形成三元复合物。配体结合的异质性被认为是加拉明和泮库溴铵作为心脏毒蕈碱受体一级和二级位点配体的双重作用导致的。
