el-Fakahany E E, Surichamorn W, Amrhein C L, Stenstrom S, Cioffi C L, Richelson E, McKinney M
Department of Pharmacology and Toxicology, University of Maryland School of Pharmacy, Baltimore.
J Pharmacol Exp Ther. 1988 Dec;247(3):934-40.
Pirenzepine selectively antagonized muscarinic receptor-mediated cyclic GMP formation in a noncompetitive fashion in mouse neuroblastoma cells (clone N1E-115). These effects of pirenzepine were time- and concentration-dependent and they were also reversible. Interestingly, whereas atropine elicited competitive antagonism of the cyclic GMP response at low concentrations, it also behaved like a noncompetitive antagonist at higher concentrations and its effects were partially reversible. Using additional approaches to study the mechanisms underlying this anomalous antagonistic profile of pirenzepine, we investigated whether this deviation from competition could be due to the short time of exposure to muscarinic agonists (30 sec) used in cyclic GMP measurements. Our data indicated that the mode of pirenzepine-induced antagonism of ligand binding to muscarinic receptors was different when assessed using nonequilibrium (30 sec) or equilibrium (1 hr) incubations. Thus, pirenzepine appeared to be noncompetitive and competitive under these two conditions, respectively. Furthermore, although pirenzepine blocked receptor-mediated phosphoinositide hydrolysis competitively when the response was measured at 20 min, it was clearly noncompetitive using 5-min incubations. Therefore, the noncompetitive antagonism by pirenzepine detected in cyclic GMP measurements might be only apparent and might be attributed, at least in part, to a lack of an equilibrium state under the specific conditions of these assays.
哌仑西平在小鼠神经母细胞瘤细胞（克隆N1E - 115）中以非竞争性方式选择性拮抗毒蕈碱受体介导的环鸟苷酸形成。哌仑西平的这些作用具有时间和浓度依赖性，且也是可逆的。有趣的是，虽然阿托品在低浓度时对环鸟苷酸反应产生竞争性拮抗作用，但在高浓度时其行为也类似于非竞争性拮抗剂，且其作用部分可逆。我们采用其他方法研究哌仑西平这种异常拮抗作用的潜在机制，调查了这种与竞争性的偏差是否可能是由于环鸟苷酸测量中使用的毒蕈碱激动剂暴露时间短（30秒）所致。我们的数据表明，当使用非平衡（30秒）或平衡（1小时）孵育评估时，哌仑西平诱导的配体与毒蕈碱受体结合拮抗作用的模式不同。因此，在这两种条件下，哌仑西平分别表现为非竞争性和竞争性。此外，虽然当在20分钟测量反应时哌仑西平竞争性阻断受体介导的磷酸肌醇水解，但使用5分钟孵育时它显然是非竞争性的。因此，在环鸟苷酸测量中检测到的哌仑西平的非竞争性拮抗作用可能只是表面现象，至少部分可归因于这些测定特定条件下缺乏平衡状态。
