Karanth S, Olivier K, Liu J, Pope C
Department of Physiological Sciences, Oklahoma State University, Stillwater, Oklahoma 74078, USA.
Toxicol Appl Pharmacol. 2001 Dec 15;177(3):247-55. doi: 10.1006/taap.2001.9312.
Organophosphorus insecticides (OPs) generally act through a common mechanism of toxicity initiated by inhibition of acetylcholinesterase (AChE). We studied the in vivo interactive toxicity of two common OPs, chlorpyrifos (CPF) and parathion (PS), in adult male rats. Dose-response studies estimated the acute oral LD1 values for the two OPs (CPF = 80 mg/kg po; PS = 4 mg/kg po) and these dosages or relative proportions were used to evaluate interactive toxicity. Three treatment strategies were evaluated: CPF followed by PS 4 h later (CPF-1st), PS followed by CPF 4 h later (PS-1st), and simultaneous (concurrent) exposures. Using LD1 dosages, rats in the CPF-1st and concurrent groups exhibited more cholinergic toxicity (i.e., salivation, lacrimation, urination, and diarrhea signs and involuntary movements) and higher lethality (7/8 and 6/8, respectively, beginning 1 h after PS) than those in the PS-1st group (2/8 lethality, beginning 3 days after CPF). Sequential exposures to lower dosages (CPF vs PS: 60 vs 3 mg/kg; 40 vs 2 mg/kg) led to more extensive neurotoxicity in the CPF-1st group compared to the other groups. Following lower dosages (40 vs 2 mg/kg), brain ChE inhibition was more extensive in the CPF-1st group at all time points (64-85%) and the concurrent group at 4 and 24 h after exposure (46-83%) compared to rats receiving PS first (7-48%). No differences were noted however, in plasma (71-93% inhibition) or liver (72-81%) cholinesterase activities nor were there group-related differences in plasma (50-60% inhibition) or liver (>85% inhibition) carboxylesterase activities. Incubation of liver samples with oxons in the presence or absence of calcium (i.e., 2 mM CaCl(2) or EGTA) prior to addition of ChE (striatal sample) substantially blocked ChE inhibition by CPO (IC50: without liver = 4 nM; liver + calcium = 279 nM; liver + EGTA = 48 nM) but had lesser effects on PO-mediated inhibition (IC50: without liver = 17 nM; liver + EGTA = 56 nM; liver + calcium = 57 nM). Liver homogenate from animals preexposed to PS substantially decreased ChE inhibition by CPO when calcium was included (IC50: +EGTA = 8 nM; +calcium = 225 nM), but liver homogenate from animals preexposed to CPF was ineffective at blocking PO-induced inhibition (IC50: +EGTA = 16 nM; +calcium = 16 nM). We conclude that prior inhibition of carboxylesterase activity impacts toxicity of subsequent exposure to PS more than CPF because of more active detoxification of CPO by A-esterase. Together, these findings indicate that interactive toxicity from combined exposures to two OP insecticides can be markedly influenced by the sequence of administration.
有机磷杀虫剂（OPs）通常通过抑制乙酰胆碱酯酶（AChE）引发的共同毒性机制起作用。我们研究了两种常见有机磷农药毒死蜱（CPF）和对硫磷（PS）对成年雄性大鼠的体内交互毒性。剂量反应研究估计了这两种有机磷农药的急性经口半数致死剂量（LD1值）（CPF = 80毫克/千克，经口；PS = 4毫克/千克，经口），并使用这些剂量或相对比例来评估交互毒性。评估了三种处理策略：先给予CPF，4小时后给予PS（CPF-1组）；先给予PS，4小时后给予CPF（PS-1组）；同时（并发）暴露。使用LD1剂量时，CPF-1组和并发组的大鼠比PS-1组的大鼠表现出更多的胆碱能毒性（即流涎、流泪、排尿和腹泻症状以及不自主运动）和更高的致死率（分别为7/8和6/8，在给予PS后1小时开始）（PS-1组致死率为2/8，在给予CPF后3天开始）。与其他组相比，依次给予较低剂量（CPF与PS：60对3毫克/千克；40对2毫克/千克）导致CPF-1组出现更广泛的神经毒性。给予较低剂量（40对2毫克/千克）后，与先给予PS的大鼠（7%-48%）相比，CPF-1组在所有时间点（64%-85%）以及并发组在暴露后4小时和24小时（46%-83%）脑胆碱酯酶抑制作用更广泛。然而，在血浆（71%-93%抑制）或肝脏（72%-81%）胆碱酯酶活性方面未观察到差异，在血浆（50%-60%抑制）或肝脏（>85%抑制）羧酸酯酶活性方面也未观察到与组相关的差异。在加入胆碱酯酶（纹状体样本）之前，将肝脏样本与氧磷在有或无钙（即2毫摩尔氯化钙或乙二醇双乙胺四乙酸）存在的情况下孵育，可显著阻断CPO对胆碱酯酶的抑制作用（半数抑制浓度：无肝脏 = 4纳摩尔；肝脏 + 钙 = 279纳摩尔；肝脏 + 乙二醇双乙胺四乙酸 = 48纳摩尔），但对PO介导的抑制作用影响较小（半数抑制浓度：无肝脏 = 17纳摩尔；肝脏 + 乙二醇双乙胺四乙酸 = 56纳摩尔；肝脏 + 钙 = 57纳摩尔）。预先暴露于PS的动物肝脏匀浆在加入钙时可显著降低CPO对胆碱酯酶的抑制作用（半数抑制浓度：+乙二醇双乙胺四乙酸 = 8纳摩尔；+钙 = 225纳摩尔），但预先暴露于CPF的动物肝脏匀浆在阻断PO诱导的抑制作用方面无效（半数抑制浓度：+乙二醇双乙胺四乙酸 = 16纳摩尔；+钙 = 16纳摩尔）。我们得出结论，由于A酯酶对CPO的解毒作用更强，预先抑制羧酸酯酶活性对随后暴露于PS的毒性影响比对CPF的影响更大。总之，这些发现表明，两种有机磷杀虫剂联合暴露的交互毒性可受到给药顺序的显著影响。
