Abdallah E A, Jett D A, Eldefrawi M E, Eldefrawi A T
Department of Pharmacology and Experimental Therapeutics, University of Maryland, School of Medicine, Baltimore 21201.
J Biochem Toxicol. 1992 Summer;7(2):125-32. doi: 10.1002/jbt.2570070210.
The effects of the organophosphorus anticholinesterase paraoxon on the binding of radioactive ligands to the M3 subtype of the muscarinic receptor and receptor-coupled synthesis of second messengers in intact rat submaxillary gland (SMG) cells were investigated. The binding of [3H]quinuclidinyl benzilate ([3H]QNB) was most sensitive to atropine and the M3-specific antagonist 4-DAMP followed by pirenzepine and least sensitive to the cardioselective M2 antagonist AFDX116. This, and the binding characteristics of [3H]4-DAMP, confirmed that the muscarinic receptors in this preparation are of the M3 subtype. Activation of these muscarinic receptors by carbamylcholine (CBC) produced both stimulation of phosphoinositide (PI) hydrolysis and inhibition of cAMP synthesis, suggesting that this receptor subtype couples to both effector systems. Paraoxon (100 microM) reduced Bmax of [3H]4-DAMP binding from 27 +/- 4 to 13 +/- 3 fmol/mg protein with nonsignificant change in affinity, suggesting noncompetitive inhibition of binding by paraoxon. Like the agonist CBC, paraoxon inhibited the forskolin-induced cAMP formation in SMG cells with an EC50 of 200 nM, but paraoxon was greater than 500 fold more potent than CBC. However, while the inhibition by CBC was counteracted by 2 microM atropine, that by paraoxon was unaffected by up to 100 microM atropine. It suggested that this effect of paraoxon was not via binding to the muscarinic receptor. Paraoxon did not affect beta-adrenoreceptor function in the preparation, since it did not affect the 10 microM isoproterenol-induced cAMP synthesis, which was inhibited totally by 10 microM propranolol and partially by CBC. Paraoxon had a small but significant effect on CBC-stimulated PI metabolism in the SMG cells.(ABSTRACT TRUNCATED AT 250 WORDS)
研究了有机磷抗胆碱酯酶对氧磷对放射性配体与毒蕈碱受体M3亚型结合以及完整大鼠颌下腺（SMG）细胞中受体偶联的第二信使合成的影响。[3H]喹核醇基苯甲酸酯（[3H]QNB）的结合对阿托品和M3特异性拮抗剂4-DAMP最敏感，其次是哌仑西平，对心脏选择性M2拮抗剂AFDX116最不敏感。这一点以及[3H]4-DAMP的结合特性证实，该制剂中的毒蕈碱受体为M3亚型。氨甲酰胆碱（CBC）激活这些毒蕈碱受体既刺激了磷酸肌醇（PI）水解，又抑制了cAMP合成，表明该受体亚型与两个效应系统偶联。对氧磷（100 microM）使[3H]4-DAMP结合的Bmax从27±4降至13±3 fmol/mg蛋白，亲和力无显著变化，表明对氧磷对结合具有非竞争性抑制作用。与激动剂CBC一样，对氧磷抑制了forskolin诱导的SMG细胞中cAMP的形成，EC50为200 nM，但对氧磷的效力比CBC高500倍以上。然而，虽然CBC的抑制作用可被2 microM阿托品抵消，但对氧磷的抑制作用在高达100 microM阿托品时不受影响。这表明对氧磷的这种作用不是通过与毒蕈碱受体结合。对氧磷不影响该制剂中的β-肾上腺素能受体功能，因为它不影响10 microM异丙肾上腺素诱导的cAMP合成，后者被10 microM普萘洛尔完全抑制，被CBC部分抑制。对氧磷对SMG细胞中CBC刺激的PI代谢有轻微但显著的影响。（摘要截取自250字）
