山东省绿盲蝽田间种群对六种杀虫剂的敏感性监测

为了解山东地区绿盲蝽对常用杀虫剂的敏感性变化情况,于2010—2012年采用玻璃管药膜法监测山东聊城、菏泽、滨州、德州地区绿盲蝽田间种群对马拉硫磷、毒死蜱、灭多威、丁硫克百威、联苯菊酯和氟虫腈等6种杀虫剂的敏感性。结果显示,相对于2009年的监测数据,2010—2012 年各地绿盲蝽种群对不同杀虫剂表现出不同程度的敏感性变化,但相对毒力比值均小于10倍。其中对毒死蜱、联苯菊酯的敏感性均未降低,菏泽种群表现为敏感性增强,相对毒力比值小于1。2011—2012年德州种群对丁硫克百威、灭多威和氟虫腈的敏感性降低,相对毒力比值大于3倍,其它种群敏感性变化不大,相对毒力比值均小于3倍。3年间菏泽种群对马拉硫磷的敏感性变化不大,相对毒力比值小于3倍,其它种群敏感性均有所降低。因此,毒死蜱、马拉硫磷、联苯菊酯、丁硫克百威等仍是山东棉区防治绿盲蝽的有效药剂。     

O,O,S-Trimethyl phosphorothioate effects on immunocompetence

O,O,S-Trimethyl phosphorothioate (OOS), a contaminant of technical formulations of some organophosphorus pesticides, was found to be immunotoxic at subtoxic doses in female C57Bl/6 mice. Mice treated orally with acute doses of 10 mg/kg OOS show no overt toxic signs such as weight loss or malaise. In addition, the levels of serum cholinesterase was not decreased. Histopathologic investigation demonstrated no alterations in liver, lung, kidney, heart, skin, brain, spleen, or gut. The LD₅₀ for delayed toxicity was approximately 35 mg/kg. Despite the lack of general toxic changes at doses of 5–10 mg/kg OOS, specific immunotoxic changes were found. The humoral or cell-mediated immune response of splenocytes from mice treated with 10 mg/kg OOS to in vivo immunization was diminished with respect to control animals. Responses were measured in ex vivo assays. Cytotoxic T-lymphocyte (CTL) responses were assessed by alloimmunization with the tumor P815 followed by a ⁵¹Cr release assay done ex vivo with splenic lymphocytes. Humoral responses were assessed by immunization with sheep red blood cells followed by a Jerne plaque assay to determine anti-sheep red blood cell antibody. Both cellular and humoral responses could be stimulated in vitro using cells from OOS-pretreated, primed animals, thus indicating that no permanent cellular elterations had occurred.     

Evaluation of pesticide effects on humoral response to sheep erythrocytes and mouse hepatitis virus 3 by immunosorbent analysis

Effect of selected organochlorine, organophosphorus, and carbamate pesticides on the humoral immune IgM response was examined upon immunization of inbred C57B1/6 mice with neutral, polyvalent, T-dependent sheep erythrocytes (SRBC) and T-independent lipopolysaccharide (LPS). In addition, a pathogenic antigen, mouse hepatitis virus 3 (MHV3) was used for determination for the interaction of selected pesticides on the primary IgG immune response in a model of viral infection, of the genetically resistant A/J mouse strain. Single, sublethal doses of dieldrin, carbofuran, and matacil induced a marked immunosuppression of the humoral responses to both neutral and pathogenic antigens. The data showed that single, sublethal doses (0.4 ≤ LD₅₀ ≤ 0.6) of dieldrin, carbofuran, and matacil inhibited the number of SRBC-primed cells without any direct cytotoxic effect on the activated plasmocyte, as the titer of specific antibody measured by an enzyme-linked immunosorbent assay (ELISA) per activated cell, was constant. In contrast, exposure to malathion at 10–14 days prior to the assay increased the number of plaque-forming cells (PFC) and the anti-SRBC IgM and anti-MHV3 IgG antibody titer, suggesting therefore an increase in the humoral response to neutral and pathogenic antigens in C57B1/6 and A/J mice. Immunomodulation of the humoral IgM response by selected pesticides was shown to take place at a stage prior to antibody secretion from the activated cell, as the ELISA/PFC index was similar to the control value. The data obtained for dieldrin-induced inhibition of the humoral response to SRBC and LPS antigens suggest a mechanism of immunosuppression common for both T-dependent and T-independent antigens. Good correlations were obtained for the immunomodulatory effects of selected pesticides, as measured by PFC and ELISA, which encourages support for the latter technique in the immunotoxicological screening of pesticides.     

The relationship between brain levels of cismethrin and bioresmethrin in female rats and neurotoxic effects

The oral toxicity of 5-benzyl-3-furylmethyl-(1R, cis)-chrysanthemate (cismethrin) to female rats decreased as their environmental temperature was raised. Acute oral LD₅₀ values increased from 157 mg/kg at 4°C to 197 mg/kg at 20°C and to > 1000 mg/kg at 30°C. Cismethrin was much more toxic given intravenously when the LD₅₀ was 4.5 mg/kg. This value did not change at different environmental temperatures. Irrespective of the environmental temperature, or route of adminstration, following the respective LD₅₀'s cismethrin caused tremors in rats when brain levels of 0.5–1.0 μg/g were reached and, at death, brain concentrations were 3.9–5.1 μg/g. These results suggested that the accumulation of cismethrin by the brain could be used as a model for the nervous system as a whole. The isomeric 5-benzyl-3-furylmethyl-(1R, trans)-chrysanthemate (bioresmethrin) was about 50 times less toxic to rats than cismethrin. After an intravenous LD₅₀, tremors started when brain concentrations were 4–5 μg/g. At death, brain levels were 25–35 μg/g. Plasma esterases were about equally active in hydrolysing cismethrin and bioresmethrin, whereas liver microsomal esterases hydrolyzed bioresmethrin over 10 times more rapidly than cismethrin. It is suggested that the lower toxicity of bioresmethrin is not only due to its faster metabolism but to an intrinsically lower toxicity at the critical site of action in the nervous system.     

Toxicity of ddt and malathion to various larval stages of Mamestra brassicae L.

The toxicity of DDT and malathion to the larvae of Mamestra brassicae was determined following several methods of application. The toxicity (LD₅₀), expressed as μg insecticide per g of insect, did not change significantly between larval instars (a) when either insecticide was injected into fourth to sixth instars; (b) when DDT was applied in the food of fifth and sixth instars; or (c) when malathion was applied topically to second to sixth instars. Significant changes in toxicity were found between successive instars when DDT was applied topically, but there was no clear trend. When malathion was applied in the food, the fifth instars were more susceptible than the sixth instars; it was found that the former consumed a toxic dose of malathion at a greater rate, and that probably malathion was degraded in the gut at a slower rate. In a contact test, the first to third instars were far more susceptible than the later instars to malathion; with DDT this trend was much less marked. Uptake studies with [¹⁴C]malathion showed that differences in the contact toxicity of malathion between instars could be explained, at least partly, by the decline in uptake per unit weight with increasing larval size.     

The biochemical basis of malathion resistance in the sheep blowfly,Lucilia cuprina

The in vivo and in vitro metabolism of [¹⁴C]malathion was studied in susceptible (LS) and malathion resistant (RM) strains of the sheep blowfly, Lucilia cuprina (Wiedemann). No difference was found between strains in the penetration, excretion, storage, or inhibitory potency of the insecticide. However, RM degraded malathion to its α- and β-monocarboxylic acid metabolites more rapidly than LS, both in vivo and in vitro. This enhanced degradation of [¹⁴C]malathion occurred in vitro in both mitochondrial and microsomal fractions of resistant flies. Kinetic analysis revealed that these fractions degraded malathion by discrete mechanisms. The enzymes from the mitochondria of both strains had the same K_m, whereas the microsomal enzyme from the RM strain had a fivefold higher K_m than that from the LS strain. Studies of esterase activities and the effect of enzyme inhibitors showed that both the mitochondrial and microsomal resistance mechanisms were the result of enhanced carboxylesterase activity. It was concluded that increased carboxylesterase detoxification of malathion adequately explained the high level of malathion resistance in RM if rate-limiting factors such as cuticular penetration were taken into account.     

Cholinesterase inhibition by methamidophos and its subsequent reactivation

Methamidophos (O,S-dimethylphosphoramidothioate, Monitor) is an organophosphorus, cholinesterase-inhibiting insecticide. The rate constant (k_i) for inhibiting rat plasma cholinesterase (ChE) was 1.57 ± 0.03 × 10³M^?1 min^?1, for rat erythrocyte ChE was 8.86 ± 1.10 × 10³M^?1 min^?1, and for rat brain ChE was 6.58 ± 0.42 M^?1 min^?1. Brain and plasma cholinesterases spontaneously recovered from over 90% inhibition at 30 min to 50% inhibition in 4 and 14 hr, respectively. Pralidoxime increased the rate of reactivation in vitro. In vivo, rats poisoned with methamidophos exhibited signs of cholinergic stimulation. The LD₅₀ of ip methamidophos in male rats was 15 ± 0.7 mg/kg. Pralidoxime (60 mg/kg) and atropine (10 mg/kg) given with the methamidophos increased the LD₅₀ to 52 ± 4.9 mg/kg and 60 ± 0.4 mg/kg, respectively. In rats given 12.5 mg methamidophos (an LD₂₀), ChE activity was depressed 95 ± 12.5% in plasma, 92 ± 0.6% in stomach, and 88 ± 1% in brain at 1 hr after injection. At 48 hr after injection ChE activity had returned to 60% or more of control values in each of the tissues. Administration of a single dose of 60 mg/kg of pralidoxime along with methamidophos did not increase ChE activities at the times and places it was measured.     

Immunotoxicity of aminocarb : I. Comparative studies of sublethal exposure to aminocarb and dieldrin in mice

Immunotoxicity of a carbamate pesticide, aminocarb (Matacil), introduced orally in sublethal doses to C57B1/6 inbred mice, has been compared to the immunosupressive effects of the organochlorine pesticide, dieldrin (served as a positive control), in bacterial and viral infections. In vivo infection of pesticide-exposed mice with Salmonella typhimurium and mouse hepatitis virus 3 (MHV3) showed that two subsequent LD₅₀ doses of aminocarb did not decrease the resistance of animals to the pathogens, whereas exposure to dieldrin resulted in augmented mortality. In vitro studies showed that the spread of MHV3 virus infection and virus-induced cytopathic effects (cpe) were augmented in peritoneal macrophages after exposure to aminocarb, albeit to a much less extent than in the dieldrin group. Similarly, a decrease in the anti-MHV3 IgM serum antibody titer by aminocarb was less marked than in the dieldrin group. Alternatively, immunization of animals with a neutral antigen, sheep red blood cells (SRBC), showed a significant increase in the anti-SRBC humoral response 10 days after a single oral exposure to LD₅₀ aminocarb. The cellular immune response, determined by mixed lymphocyte reaction, was unaffected by sublethal aminocarb exposure. In addition, macrophage antigen processing of a single protein, avidin, was unaffected by aminocarb, contrary to the inhibition of avidin processing in macrophages from dieldrin-gavaged mice. The data do not indicate that immunotoxic properties are associated with aminocarb, and only slight effects with high sublethal doses of orally given aminocarb on macrophage susceptibility to the MHV3 viral pathogen were observed at a terminal phase of the disease.     

Studies on hydrolases in various house fly strains and their role in malathion resistance

Aliesterase, carboxylesterase, and phosphorotriester hydrolase activities in six house fly strains were studied in relation to malathion resistance. Selection of two susceptible strains with malathion for three generations resulted in an increase in both carboxylesterase activity and LD₅₀ of malathion, indicating that the increased detoxication by the enzyme was the major mechanism selected for malathion resistance. With the highly resistant strains, however, the carboxylesterase activity alone was not sufficient to explain the resistance level, and the involvement of additional mechanisms, including phosphorotriester hydrolase activity, was suggested. The E₁ strain, which had high phosphorotriester hydrolase activity but normal or low carboxylesterase activity, showed a moderate level, i.e., sevenfold resistance. Upon DEAE-cellulose chromatography, two or three esterase peaks were resolved from susceptible, moderately resistant, and highly resistant strains. The substrate specificity, the sensitivity to paraoxon inhibition, and the $\text{α}\text{β}$ ratio of malathion hydrolysis were studied for each esterase peak from the different strains. The results suggested the existence of multiple forms of esterases with overlapping substrate specificity in the house fly.     

Rapid in vitro screening assay for immunotoxic effects of organophosphorus and carbamate insecticides on the generation of cytotoxic T-lymphocyte responses

There has been an increasing need for rapid and easily interpreted techniques for the screening of possible immunotoxicants. Besides the obvious detrimental effects of exposure to immunosuppressive agents, the modulation of the immune system which results from exposure to these toxicants may be a sensitive index to the toxicologic effects of such agents. Other researchers have proposed assays to screen the effect of in vitro treatment with immunotoxicants on mitogenic and humoral immune responses. In this report, we have described an in vitro technique for screening the effect of immunotoxicants, in the presence and absence of a NADPH fortified liver postmitochondrial supernatant (S-9) from Arochlor 1254-treated rat, on another aspect of the mammalian immune system, the generation of a T-cell-mediated cytolytic (CTL) response. This enzyme system altered the effect of organophosphorus compounds on the generation of a CTL response. Malathion and fenitrothion were no longer suppressive following this pretreatment; however, ethyl and methyl parathion and fenthion were only partially detoxified. In contrast, the S-9 enzyme system did not alter the effect of carbamate pesticides, carbaryl and carbofuran, on the generation of CTL responses. This report describes the effects of these seven organophosphorus and carbamate pesticides on the generation of the CTL response. In addition, some of the in vivo data published on the immunomodulatory effects of these compounds were collated from the literature and a correlation between in vitro and in vivo studies was discussed.     

Steric,electronic, and polar parameters that affect the toxic actions of O-alkyl,O-phenyl phosphonothionate insecticides

The toxic action of a series of O-alkyl, O-substituted-phenyl alkyl- and aryl-phosphonates and phosphonothionates have been evaluated by correlating the linear free energy parameters for steric (E_s), electronic (σ), and polar ($\text{σ}{}^1$) effects with topical LD₅₀ to the house fly and oral LD₅₀ to the white mouse. In molecules free from major steric interactions with the reactive P atom, variations in these linear free energy parameters account for >90% of the variations in the LD₅₀ values, and the degree of correlation with LD₅₀ is at least as precise as that with the biomolecular rate constants for inhibition of the target-site enzyme acetylcholinesterase. The value of correlations of linear free energy parameters with LD₅₀ in understanding quantitative structure-activity relationships is illustrated.     

Effects of an impurity on malathion and its structural analog on rat liver carboxylesterases and erythrocyte esterases

The inhibitory effects on liver microsomal carboxylesterases and erythrocyte membrane esterases produced by an impurity of malathion was investigated. Treatment of rats with an impurity of malathion, O,O,S-trimethyl phosphorothioate (OOS-Me), and its structural analog O,O-dimethyl S-ethyl phosphorothioate (OOS-Et) inhibited liver microsomal malathion and phenthoate carboxylesterases. The inhibition lasted for at least 7 days following a single oral administration of OOS-Me. These treatments inhibited acetylcholinesterase (AChE) and (Na⁺ + K⁺)-dependent ATPase of erythrocyte membranes which persisted at least 3 days. OOS-Et was a more potent inhibitor of all the esterases examined than OOS-Me. Pretreatment of rats with a metabolic inducer, phenobarbital, or a metabolic inhibitor, piperonyl butoxide, had no effect on such inhibitory effects on liver microsomal carboxylesterases produced by OOS-Me or OOS-Et.     

Phenobarbital induction of permethrin detoxification and phenobarbital metabolism in susceptible and resistant strains of the beet armyworm Spodoptera exigua (Hübner)

The permethrin resistant strain (TR-strain) of the beet armyworm, Spodoptera exigua (Hübner), has 92.5-fold resistance to permethrin (at LD₅₀ level) compared to the permethrin susceptible strain (TS-strain). Bioassay involving permethrin mixed with piperonyl butoxide, an inhibitor of microsomal cytochrome P450s, significantly reduced the resistance ratio from 92.5- to 7.9-fold. However, S,S,S-tributylphosphorotrithioate and diethylmaleate which are inhibitors of esterases and glutathione S-transferase, respectively, did not affect the resistance level. These results indicate that the detoxification of permethrin in the TR-strain was primarily due to the cytochrome P450 monooxygenases. LD₅₀ for permethrin was increased to 4.5-fold by the pre-treatment of phenobarbital in the TS-strain. The effect of induction by phenobarbital was almost completely overcome by the piperonyl butoxide treatment. However, it was observed that phenobarbital treatment did not cause any change in the toxicity of permethrin to TR strain. Since this result deviated from the expectation that the metabolism of phenobarbital in the TR-strain should be greater than that in the TS-strain, it was deemed necessary to compare the metabolism of phenobarbital between the TS- and TR-strains. Comparison was made based on the concentration of phenobarbital in the hemolymph and whole body. The results showed no significant difference in phenobarbital treatment between the two strains used in this study suggesting the possibility that the induction system in TS-strain is different from the TR-strain.     

Biochemical and physiological investigations into the delayed toxicity produced by O,O,S-trimethyl phosphorothioate in rats

Oral administration of O,O,S-trimethyl phosphorothioate (OOS), an impurity in several technical organophosphorus insecticides, causes delayed toxicity in rats with death occurring up to 28 days after the treatment. The oral LD₅₀ was determined to be 60 mg/kg. The effect of a single nonlethal dose of OOS (20 mg/kg) on in vivo protein synthesis in different organs was determined by measurement of the incorporation of [¹⁴C]leucine at 6 hr to 28 days after treatment. As early as 6 hr after OOS treatment the incorporation of [¹⁴C]leucine into the liver, lung, thymus, kidney, and spleen was elevated and remained elevated for up to 7 days. With the exception of the lung, organ weights were significantly decreased during the same time period. On Day 28 after treatment, the amount of [¹⁴C]leucine incorporation had decreased to the control level in all of the organs studied. Treatment with OOS at 20 mg/kg caused a significant increase in hematocrit on Days 3,5, and 7, and as early as 6 hr after treatment at 60 mg/kg. The clinical biochemistry of plasma indicated that there was no significant change from control values in the glutamic pyruvic transaminase, glutamic oxalic transaminase, lactate dehydrogenase, or alkaline phosphatase activities with the 20 mg/kg dose. The analysis of the intermediary metabolites indicated that the redox state of cytosol was more reduced on Day 5, whereas that of mitochondria was not affected by OOS. Data obtained at selected times after oral administration of a 60 mg/kg dose of OOS and that obtained from animals starved for 3 days are also discussed.     

Changes in malathion resistance with age in Anopheles stephensi from Pakistan

Resistance to malathion in Anopheles stephensi from Pakistan was measured at intervals during the first week of adult life. LT₅₀ values for homozygous resistant females decreased four-fold during the first 7 days of adulthood. A decrease in resistance with age also occurred in heterozygotes; the LT₅₀ values of males and females fell sevenfold during the first 5 days of adulthood. The sensitivity to malathion of a susceptible strain increased with age. A biochemical basis for the declining resistance levels was investigated. Resistant and susceptible adults were homogenized at intervals during the first week of adulthood and soluble extracts were incubated with [¹⁴C]malathion. The rate of malathion metabolism to mono- and dicarboxylic acids was faster in resistant than in susceptible mosquitoes. The rate of malathion metabolism decreased with age in both strains. A decrease in carboxylesterase activity with age in resistant and susceptible mosquitoes is thus responsible for the increasing sensitivity to malathion. Implications for the monitoring of resistance in the field by diagnostic dosages and for the future use of malathion in mosquito control are discussed.     

Delayed acute toxicity of O,S,S-trimethyl phosphorodithioate and O,O,S-trimethyl phosphorothioate to the rat

The toxicity and LD₅₀ of O,S,S-trimethyl phosphorodithioate were reexamined in the rat. Animals treated orally (single dose) with this compound exhibited early cholinergic signs followed at approximately 5 hr by delayed toxic signs, with an LD₅₀ of 43 mg/kg. Contamination of O,S,S-trimethyl phosphorodithioate by as much as 5% (w/w) O,O,O-trimethyl phosphorothioate provided only limited antagonism to the dithioate's toxicity. In contrast, the addition of 5% O,O,S-trimethyl phosphorodithioate to O,O,S-trimethyl phosphorothioate gave protection against the toxic effects of the latter compound up to 80 mg/kg of toxicant. Pretreatment of rats with as little as 5% O,O,O-trimethyl phosphorothioate, 24 hr prior to treatment with 200 mg/kg O,O,S-trimethyl phosphorothioate, gave complete protection against the toxic effects of this compound. Conversely, administration of 10% (w/w) O,O,O-trimethyl phosphorothioate 4 or 24 hr after treatment with 60 or 80 mg/kg of O,O,S-trimethyl phosphorothioate provided only partial protection at 4 hr and no protection from the effects of the toxicant at 24 hr. The ability of O,O,O-trimethyl phosphorothioate to antagonize the toxicity of this compound depended markedly on the route of administration (oral, intravenous, or intraperitoneal). At 4 hr past treatment with toxicant, only oral administration of the antagonist provided full protection. Intraperitoneal and intravenous administration of antagonist 4 hr after treatment with toxicant were partially effective and completely ineffective, respectively, in halting the toxic effects of this compound.     

Effect of short-time malathion administration on glucose homeostasis in Wistar rat

The main objective of this study is to investigate the 24 h kinetic effect of acute administration of malathion, an organophosphorus insecticide, on glucose homeostasis in adult rats. A single dose of malathion (400 mg/kg, 1/5 of LD₅₀) was administered orally to rats, blood glucose was measured, liver and pancreas were removed to determine the level of hepatic glycogen, the activity of pancreatic acetylcholinesterase and butyrylcholinesterase, as well as the level of TBARs. Blood glucose increased in rats treated with a single dose of malathion. This effect was observed in the first hours of treatment, reached a 2.2 fold peak after 2 h, and then decreased after 4 h, followed by a decrease in the level of hepatic glycogen. The storage of glycogen starts from 6 to 12 h of administration. A decrease in cholinesterase activities was noted. The level of TBARs increased considerably in liver and pancreas. Results of this study can be explain by a stimulation of glycogenolysis and gluconeogenesis by liver, with a temporarily loss of endocrine functions of pancreas leading to hyperglycemia.     

Effect of passage through the gut of Eocanthecona furcellata on Spodoptera litura multiple nucleopolyhedrovirus infectivity and its subsequent dissemination

Infectivity of Spodoptera litura multiple nucleopolyhedrovirus (SpltMNPV) was compared between before (P0) and after (P1–P4) passage in subsequent generations through the gut of Eocanthecona furcellata. Viable virus was detected in E. furcellata feces up to 6 days after feeding on infected S. litura larvae. NPV mortality ranged between 93% and 10% when test larvae were exposed to polyhedra voided in feces collected after 1 and 6 days post-infected meal, respectively. The mean number of polyhedral occlusion bodies (POBs) in excreta and their infectivity (%) at all passages did not vary significantly. The comparison of observed LD₅₀ and ST₅₀ values among all passages did not reveal significant differences owing to their overlapping confidence limits. The gut-passed virus did not show a detrimental effect on survival rate, longevity, fecundity and percent egg hatchability of E. furcellata in the subsequent three generations. A field trial was also conducted to estimate virus dissemination through feces of predators that were fed upon prey infected with polyhedra before passage, after passage and healthy (control) prey and subsequently released on cabbage plants. An additional viral mortality up to the magnitude of 13–17% was noticed in the former two treatments. However, within these two treatments the viral mortality did not vary significantly. It was concluded that E. furcellata disseminated the virus through their feces into the ecosystem without any adverse effect to it and infectivity of the SpltMNPV is not altered after passage through the gut of the predator.     

The mechanism of resistance to lindane and hexadeuterated lindane in the Third Yumenoshima strain of house fly

The factors which cause lindane resistance in the Third Yumenoshima strain, a strain of house flies highly resistant to insecticides, were studied using hexadeuterated lindane. Hexadeuterated lindane has the same physicochemical properties as lindane, but the former is much less biodegradable than the latter. The LD₅₀ ratio of lindane to hexadeuterated lindane in this strain, deuterium isotope effect on LD₅₀ values, was larger than that in S_NAIDM, a susceptible (nonresistant) strain. The penetration rates of labeled and nonlabeled lindane through the insect cuticle were about the same for both strains. Thus, penetration rate does not cause resistance. The metabolic degradation of lindane in the resistant strain in vivo occurred much faster than in the susceptible strain. This was also the case for lindane degradation processes in vitro such as microsomal oxidation and glutathione conjugation. In both strains, significant isotope effects were observed in the degradation rates in vitro of labeled and nonlabeled lindane. Therefore, principal biodegradation and detoxication pathways should include reactions which cleave the CH bonds. When the much less biodegradable d₆ counterpart of lindane was applied to both strains, the susceptible strain became much more highly intoxicated than the other within 20 to 30 min. This indicates that a combination of both greater degradability and probably lower sensitivity at the action site are the main factors underlying resistance in the Third Yumenoshima strain.     

In vitro effects of malathion and O,O,S-trimethyl phosphorothioate on cytotoxic T-lymphocyte responses

Oral administration of O,O,S-trimethyl phosphorothioate (OOS), an impurity present in technical formulations of malathion, has been shown to be associated with a high incidence of pneumonia in rats and to be highly immunosuppressive in mice. Based on these findings, an in vitro model was established to study the effect of this and other organophosphorus compounds on murine cytotoxic T-lymphocyte (CTL) responses. The organophosphorus compounds were tested for their ability to block in vitro generation of CTL responses to alloantigen and/or the expression of these cytotoxic responses. Responses were generated in C57Bl/6 (H-2^b) spleen cells to mitomycin C-blocked P815 (H-2^d) tumor cells. The cytotoxicity of the cultured splenocytes to P815 target was measured using a 4-hr chromium release assay. These data demonstrated that malathion was able to block the ability of splenocytes to sensitize to P815 at concentrations as low as 25 μg/ml, but was not able to block the expression of cytotoxicity by mature killer T cells. The same was true for OOS which had been activated by preincubation with rat liver postmitochondrial supernatant (PMS). Activated OOS blocked the generation of CTL responses at concentrations as low as 75 μg/ml while having no effect on mature cytotoxic cells. In fact, both malathion and activated OOS were no longer able to suppress CTL responses if treatment was performed as early as 24 hr after exposure to antigen. Additionally, it was demonstrated that when malathion was preincubated with PMS it was no longer suppressive and that OOS without activation failed to suppress CTL responses.