Camara A L, Braga M F, Rocha E S, Santos M D, Cortes W S, Cintra W M, Aracava Y, Maelicke A, Albuguergue E X
Laboratory of Molecular Pharmacology II, Institute of Biophysics, Carlos Chagas Filho, UFRJ, Rio de Janeiro, Brazil.
Neurotoxicology. 1997;18(2):589-602.
Methamidophos (O,S-dimethyl phosphoroamidothiolate, Tamaron), an organophosphate (OP) anticholinesterase of limited toxicity, is widely used as an insecticide and acaricide. To provide additional insight into the molecular basis of its action, we have used electrophysiological and biochemical techniques to study the effects of methamidophos on the neuromuscular junction of rat and frog and on the central nervous system of rat. Methamidophos has a relatively weak inhibitory action on cholinesterases in rat diaphragm muscle, brain and hippocampal homogenates, with IC50 values on the order of 20-20 microM. An even weaker anticholinesterase activity was found in frog muscle homogenates, with the IC50 being above 300 microM. As further evidence of anticholinesterase activity, methamidophos (1-100 microM) was able to reverse the blockade by d-tubocurarine (0.5-0.7 microM) of neuromuscular transmission in rat phrenic nerve-hemidiaphragm preparations. Inhibition of cholinesterase activity by methamidophos was long lasting, which is consistent with the formation by the agent of a covalent bond with the enzyme's active serine residue. The action was also slowly reversible, which suggests spontaneous reactivation of the enzyme. electrophysiological studies at the rat neuromuscular junction showed that, due to its anticholinesterase activity, methamidophos increased the amplitude and prolonged the decay phase of nerve-evoked and spontaneous miniature end-plate potentials. In contrast to other OP compounds, e.g., paraoxon (Rocha et al., 1996a), methamidophos did not affect neurotransmitter release, nor did it interact directly with the muscle nicotinic acetylcholine receptor. Moreover, it contrast to paraoxon, methamidophos did not affect the whole-cell currents induced by application of acetylcholine, glutamate or gamma-aminobutyric acid recorded to cultured hippocampal neurons. Based on these data, methamidophos appears to have a selective effect on cholinesterase.
甲胺磷（O，S-二甲基硫代磷酰胺酯，稻丰散）是一种毒性有限的有机磷酸酯类抗胆碱酯酶剂，被广泛用作杀虫剂和杀螨剂。为了更深入了解其作用的分子基础，我们运用电生理和生化技术研究了甲胺磷对大鼠和青蛙神经肌肉接头以及大鼠中枢神经系统的影响。甲胺磷对大鼠膈肌、脑和海马匀浆中的胆碱酯酶具有相对较弱的抑制作用，半数抑制浓度（IC50）值约为20 - 20微摩尔。在青蛙肌肉匀浆中发现其抗胆碱酯酶活性更弱，IC50高于300微摩尔。作为抗胆碱酯酶活性的进一步证据，甲胺磷（1 - 100微摩尔）能够逆转筒箭毒碱（0.5 - 0.7微摩尔）对大鼠膈神经 - 半膈肌标本神经肌肉传递的阻断作用。甲胺磷对胆碱酯酶活性的抑制作用持久，这与该药剂与酶的活性丝氨酸残基形成共价键一致。其作用也是缓慢可逆的，这表明酶能自发重新激活。在大鼠神经肌肉接头处的电生理研究表明，由于其抗胆碱酯酶活性，甲胺磷增加了神经诱发和自发微小终板电位的幅度并延长了其衰减期。与其他有机磷酸酯类化合物如对氧磷（Rocha等人，1996a）不同，甲胺磷不影响神经递质释放，也不直接与肌肉烟碱型乙酰胆碱受体相互作用。此外，与对氧磷相反。甲胺磷不影响应用乙酰胆碱、谷氨酸或γ-氨基丁酸对培养海马神经元记录到的全细胞电流。基于这些数据，甲胺磷似乎对胆碱酯酶具有选择性作用。
