Propoxur (PPx) exposure as hormonal disruptor and spermatocidal in rat model.

BACKGROUND: Propoxur is a carbamate insecticide widely used both in indoor and outdoor place to control insects. This present work was conducted to study the effect of exposure of propoxur (PPX) on hormonal and histological changes in the rat testes. METHODS: The control animals received distil water, while the treated animals received Propoxur (PPx) by inhalation every other day for one month (PPx-1) and two months (PPx-2) respectively. The animals were euthanized by cervical dislocation; blood sample was obtained for reproductive hormonal assay and the testes were excised following abdominal incision fixed in Bouin's fluid for histological observations. RESULTS: Significant decrease in the level of testosterone (TT) and increase in follicular stimulating hormone (FSH) and lutenizing hormone (LH) were observed in PPX treated groups alongside with the degenerative changes in the seminiferous tubules, complete loss of spermatogonia population, and the testicular basal membrane. There was no reversal of destruction 30 days after withdrawal of the insecticide, indicating a persistent effect. CONCLUSION: The exposure to PPX insecticide has obvious deleterious effects on rat testicular micro-structure and reproductive hormones, Therefore, inhalation of such insecticide should be limited with special care in handling to limit or minimize its hazards.

Determination of insecticides' lethal concentrations and metabolic enzyme levels in Triatoma dimidiata.

OBJECTIVE: The feasibility of the use of WHO impregnated paper and biochemical assays to determine lethal concentrations (LC50 and LC99) and insecticide metabolic enzyme levels of Triatoma dimidiata. MATERIALS AND METHODS: LC50 and LC99 were calculated with WHO papers impregnated at different concentrations of malathion, propoxur and deltamethrin; the percentage of insensitive acetylcholinesterase (iAChE); and the levels of esterases, glutathione S-transferases, and monooxygenases in laboratory nymphs of the first stage (5 to 7 days), were undertaken using the WHO biochemical assays. RESULTS: Respectively the LC50 and LC99 µg/cm2 obtained for malathion were 43.83 and 114.38, propoxur 4.71 and 19.29, and deltamethrin 5.80 and 40.46. A 30% of the population had an iAChE, and only a few individuals had high P450 and ß-eterase levels. CONCLUSIONS: Impregnated papers and biochemical tests developed by WHO for other insects, proved to be feasible methods in monitoring insecticide resistance and metabolic enzymes involved in T. dimidiata.

OBJETIVO: La factibilidad de usar los papeles impregnados y ensayos bioquímicos según la OMS para determinar concentraciones letales (CL50 y CL99) y niveles enzimáticos en la resistencia a insecticidas en Triatoma dimidiata. MATERIAL Y MÉTODOS: Se calcularon la CL50 y CL99 con papeles impregnados según la OMS a diferentes concentraciones de malatión, propoxur y deltametrina; el porcentaje de acetilcolinesterasa insensible (iAChE); y los niveles de esterasas, glutatión S-transferasas, y monooxigenasas en ninfas de laboratorio del estadio I (5-7 días) se determinaron usando los ensayos bioquímicos según la OMS. RESULTADOS: Se obtuvieron las CL50 y CL99 µg / cm2 respectivamente para malatión 43.83 y 114.38, propoxur 4.71 y 19.29, y deltametrina 5.80 y 40.46. Un 30% de las chinches tuvo iAChE, y sólo pocos individuos tuvieron niveles superiores de P450 y ß-eterasas. CONCLUSIONES: Los papeles impregnados y ensayos bioquímicos que describe la OMS para otros insectos demostraron ser métodos factibles para monitorear la resistencia a insecticidas y las enzimas metabólicas involucradas en T. dimidiata.

Toxic effects of new ethyl-carbamates on the morphology, mortality and acetylcholinesterase activity of Eisenia foetida.

The toxicity of the ixodicidal carbamates ethyl-4-bromophenyl carbamate (LQM 919), ethyl-4-chlorophenyl carbamate (LQM 996) and propoxur on Eisenia foetida adults was evaluated to estimate their ecotoxic potential. The earthworm mortality and weight loss produced by the three evaluated carbamates showed a concentration-dependent effect (p < 0.0001) in the contact filter paper test (CFPT). In the artificial soil test (AST), mortality increased in relation to the exposure time (p < 0.0001) and the concentration (p < 0.01) of the carbamates. Only the earthworms exposed in the CFPT showed morphological alterations. According to the LC50 obtained in the CFPT, the three carbamates were classified as very toxic and, according to the LC50 obtained in the AST, the three carbamates were classified as highly toxic for E. foetida. The values of ki and kd indicated that LQM 919 and LQM 996 are weak inhibitors with lower affinity for the acetylcholinesterase of E. foetida than that of propoxur. The concentrations in the CFPT and AST at which 100% mortality was observed in E. foetida were 64- and 4-fold higher, respectively, than the egg hatching inhibitory concentration 99% reported for ticks.

Propoxur-induced oxidative DNA damage in human peripheral blood mononuclear cells: protective effects of curcumin and α-tocopherol.

The present study enumerates the attenuating effects of curcumin and α-tocopherol against propoxur induced oxidative DNA damage in human peripheral blood mononuclear cells (PBMC). Cultured cells were isolated from peripheral blood of healthy volunteers, and were exposed to varying concentrations of propoxur (0-21 µg/ml) for 6, 12, and 24 h, and in combination with curcumin (9.2 µg/ml) or α-tocopherol (4.3 µg/ml) or both. Cytotoxic effect of propoxur was examined by MTT assay. The role of oxidative stress beneath the cytotoxicity of propoxur was evaluated by the measurement of reduced glutathione (GSH), malondialdehyde (MDA) and 8-hydroxy-2'-deoxyguanosine (8-OH-dG) levels in cell lysate. A concentration-dependent cell death, depletion of GSH, an increase in the level of both MDA and 8-OH-dG were observed. Co-treatment with curcumin or α-tocopherol significantly attenuates depleted GSH, decrease in MDA and 8-OH-dG levels in propoxur exposed cells (p < 0.05). The results of the present study provide experimental evidence of involvement of oxidative stress in propoxur-mediated genotoxicity in human PBMC and highlight the antioxidant role of curcumin and α-tocopherol following propoxur exposure.

Biochemical Responses in Freshwater Fish Exposed to Insecticide Propoxur.

Although designed to control pests selectively, there is some evidence that environmental contamination by pesticides increases risks for humans and wildlife. In the present study, we evaluated biomarkers of oxidative stress in Astyanax jacuhiensis exposed to (5, 15 and 30 µg L-1) of carbamate Propoxur (PPX) for 96 h. Glutathione S-transferase (GST) in liver and gills showed reduced activity in all PPX concentrations tested. Acetylcholinesterase (AChE) activities reduced in brain and muscle at concentrations 15 and 30 µg L-1 of PPX. Lipid peroxidation (LPO) and hydrogen peroxide (HP) had no significant differences. In the brain, protein carbonyl (PC) increased in all groups treated with PPX. Although PPX is a selective pesticide, it causes oxidative damage and enzyme alteration in fish. This study pointed out some biomarkers that could be used to assess effects of environmentally relevant concentrations of pesticides, and infer about studies using fish as bioindicator.

Protective effect of melatonin on propoxur-induced impairment of memory and oxidative stress in rats.

Propoxur (a carbamate pesticide) has been shown to adversely affect memory and induce oxidative stress on both acute and chronic exposure. This study was designed to explore the modulation of the effects of propoxur over cognitive function by melatonin (MEL). Cognitive function was assessed using step-down latency (SDL) on a passive avoidance apparatus, and transfer latency (TL) on an elevated plus maze. Oxidative stress was assessed by examining brain malondialdehyde (MDA) and reduced glutathione (GSH) levels and catalase (CAT) activity. A significant reduction in SDL and prolongation of TL was observed for the propoxur (10 mg/kg/d; p.o.) treated group at weeks 6 and 7 when compared with control. One week treatment with MEL (50 mg/kg/d; i.p.) antagonized the effect of propoxur on SDL, as well as TL. Propoxur produced a statistically significant increase in the brain MDA levels and decrease in the brain GSH levels and CAT activity. Treatment with MEL attenuated the effect of propoxur on oxidative stress. The results of the present study thus show that MEL has the potential to attenuate cognitive dysfunction and oxidative stress induced by toxicants like propoxur in the brain.

Combined toxicity of pesticide mixtures on green algae and photobacteria.

Different organisms have diverse responses to the same chemicals or mixtures. In this paper, we selected the green algae Chlorella pyrenoidosa (C. pyrenoidosa) and photobacteria Vibrio qinghaiensis sp.-Q67 (V. qinghaiensis) as target organisms and determined the toxicities of six pesticides, including three herbicides (simetryn, bromacil and hexazinone), two fungicides (dodine and metalaxyl) and one insecticide (propoxur), and their mixtures by using the microplate toxicity analysis. The toxicities of three herbicides to C. pyrenoidosa are much higher than those to V. qinghaiensis, and the toxicities of metalaxyl and propoxur to V. qinghaiensis are higher than those to C. pyrenoidosa, while the toxicity of dodine to C. pyrenoidosa is similar to those to V. qinghaiensis. Using the concentration addition as an additive reference model, the binary pesticide mixtures exhibited different toxicity interactions, i.e., displayed antagonism to C. pyrenoidosa but synergism to V. qinghaiensis. However, the toxicities of the multi-component mixtures of more than two components are additive and can be predicted by the concentration addition model.

(1)H NMR-based metabonomic profiling of rat serum and urine to characterize the subacute effects of carbamate insecticide propoxur.

Carbamate insecticide propoxur is widely used in agriculture and public health programs. To prevent adverse health effects arising from exposure to this insecticide, sensitive methods for detection of early stage organismal changes are necessary. We present here an integrative metabonomic approach to investigate toxic effects of pesticide in experimental animals. Results showed that propoxur even at low dose levels can induce oxidative stress, impair liver function, enhance ketogenesis and fatty acid ß-oxidation, and increase glycolysis, which contribute to the hepatotoxocity. These findings highlight the applicability of (1)H NMR spectroscopy and multivariate statistics in elucidating the toxic effects of propoxur.

Propoxur: a novel mechanism for insecticidal action and toxicity.

Propoxur is a carbamate insecticide that has recently attracted considerable attention as a possible treatment option for addressing the bedbug epidemic. The generally accepted mechanism of toxicity for propoxur involves the inhibition of ChE, as is the case for many agents in the category. Considerable research supports the concept that most physiologically active substances induce their effects through multi-faceted action. In this review, we provide evidence that ET--ROS--OS participate mechanistically in both the action and in human toxicity of pesticides, including propoxur. Propoxur is a catechol derivative that contains carbamate and isopropyl groups on the oxygens in its moiety. Metabolic studies with propoxur reveal hydrolysis of the carbamate and dealkylation of the isopropyl group to yield the parent catechol. In addition, nuclear hydroxylation produces a hydroquinone derivative. Both the catechol and this hydroquinone derivative are potentially able to undergo redox cycling with the corresponding quinone to produce ROS. It is primarily for these reasons that we believe propoxur may be similar to other classes of physiologically active compounds in producing effects through ET-ROS-OS. Generally, reactive ROS are generated by metabolic processes that yield ET entities, and this occurs with propoxur as well. Although ROS are commonly associated with toxicity, there is little recognition in the literature that they can also play a role in therapeutic action.

Metabonomic responses in rat urine following subacute exposure to propoxur.

Metabolic profiling of urine from pesticide-treated rats was investigated by the nuclear magnetic resonance (NMR)-based metabonomic strategy. Twenty-four-hour urine samples of rats were collected after administration with propoxur at doses of 0.85, 1.70, and 8.51 mg/kg, respectively, for 28 consecutive days. Liver tissue was fixed and the histopathological alterations were examined. The results showed that propoxur at high dose induced liver histopathological injury. Metabonomic analysis demonstrated that the levels of creatine and taurine markedly increased together with slight elevation of hippurate, glucose, and amino acids in low- and medium-dose groups. However, concentrations of urinary lactate, acetate, acetone, succinate, citrate, and 2-oxoglutarate increased in high-dose group. All these results suggested that propoxur could inhibit liver function through altering the energy and lipid metabolism. These data also supported the contention that the NMR-based metabonomic approach represents a promising new technology for the development of pesticide toxicity screening and mechanism exploration.

[Influence of joint exposure to diazinon, propoxur and bisphenol A on phagocytosis of RAW264.7 cell].

OBJECTIVE: To explore the toxicity of joint exposure to diazinon, propoxur and bisphenol A on phagocytosis. METHODS: Flow cytometer was employed to detect the influence of diazinon and bisphenol A, propoxur and bisphenol A in mixture (mixed according to ratio of IC(50)) on mouse macrophage RAW264.7 cells' function to phagocyte fluorescent microspheres, adopting the percentage of phagocytic cells (PP) and the phagocytic index (PI) as measurement indicators. The final concentrations of mixture of diazinon and bisphenol A were (0.4 + 0.1), (3.6 + 0.7), (36.2 + 7.2), (43.4 + 8.7), (52.1 + 10.4), (62.5 + 12.5), (75.0 + 15.0) µg/ml; while those of mixture of propoxur and bisphenol A were (0.2 + 2.0 × 10(-2)), (2.4 + 0.2), (23.7 + 2.0), (35.6 + 3.0), (53.3 + 4.4), (80.0 + 6.7), (120.0 + 10.0) µg/ml. Then based on the dose-response relationship, a 2 × 2 factorial design was then carried out among different doses of mixture with statistical significance to statistically evaluate the interaction between diazinon and bisphenol A, propoxur and bisphenol A. RESULTS: After the joint exposure, compared to the control group (PP = (23.6 ± 2.2)%; PI = 0.36 ± 0.03), any dose of the mixture of diazinon and bisphenol A ((52.1 + 10.4), (62.5 + 12.5), (75.0 + 15.0) µg/ml) could significantly increase the levels of PP ((29.0 ± 1.4)%, t = 3.89, P < 0.05; (30.2 ± 2.3)%, t = 4.74, P < 0.05; (35.0 ± 3.4)%, t = 8.21, P < 0.05) and PI (0.43 ± 0.03, t = 3.86, P < 0.05; 0.41 ± 0.02, t = 2.95, P < 0.05; 0.46 ± 0.03, t = 5.34, P < 0.05); while that of propoxur and bisphenol A ((35.6 + 3.0), (53.3 + 4.4), (80.0 + 6.7), (120.0 + 10.0) µg/ml) reduced the levels of PP ((20.6 ± 1.1)%, t = -3.00, P < 0.05; (20.2 ± 1.0)%, t = -3.42, P < 0.05; (19.4 ± 1.3)%, t = -4.23, P < 0.05; (18.8 ± 2.1)%, t = -4.81, P < 0.05) and PI (0.31 ± 0.01, t = -4.75, P < 0.05; 0.31 ± 0.01, t = -4.58, P < 0.05; 0.30 ± 0.01, t = -4.92, P < 0.05; 0.27 ± 0.02, t = -7.80, P < 0.05) on the contrary. The 2 × 2 factorial design was carried out between the mixture of diazinon (60.0 µg/ml; PP = (28.5 ± 3.4)%; PI = 0.49 ± 0.07) and bisphenol A (12.0 µg/ml; PP = (35.7 ± 2.7)%; PI = 0.67 ± 0.07), and the mixture of propoxur (48.0 µg/ml ; PP = (28.1 ± 2.2)%; PI = 0.48 ± 0.04) and bisphenol A (4.0 µg/ml; PP = (34.4 ± 2.7)%; PI = 0.59 ± 0.07). The mixture of diazinon and bisphenol A (PP = (30.4 ± 1.4)%, F(interaction) = 6.22, P < 0.05; PI = 0.53 ± 0.03, F(interaction) = 7.35, P < 0.05) and the mixture of propoxur and bisphenol A (PP = (27.5 ± 4.1)%, F(interaction) = 4.56, P < 0.05; PI = 0.46 ± 0.08, F(interaction) = 11.13, P < 0.05) both showed a significant antagonistic interaction on phagocytosis of RAW264.7 cell. CONCLUSION: It is suggested that the interactions between diazinon & bisphenol A and propoxur & bisphenol A both played the antagonistic role on phagocytic function of macrophages in vitro.

[Study of three methods on joint toxicity of diazinon, propoxur and bisphenol A on proliferation of mouse RAW264.7 cell].

OBJECTIVE: To investigate possible joint toxic effects of diazinon, propoxur and bisphenol A (BPA) on proliferation of RAW264.7 cells in vitro. METHODS: Cytotoxicity was assessed by MTT assay. The median inhibiting concentration values (IC50) and 95% confidence interval (CI) of diazinon, propoxur and BPA individually and in mixture (mixed according to ratio of IC50) were established by weighted probit method. The types of toxic interaction of diazinon & BPA and propoxur & BPA were assessed by three methods commonly used for binary mixtures, which were Additional Index Method, Equivalent Effect Curve Method and Logistic Regression Method. RESULTS: After 24-hour expoxure, the IC50 and 95% CI of diazinon, propoxur and BPA to RAW264.7 cells were 194.1 microg/ml (173.4 microg/ml-217.4 microg/ml), 448.4 mg/L (358.2 microg/ml-573.2 microg/ml) , and 37.5 microg/ml (35.3 microg/ml-39.9 microg/ml), respectively. Those of mixtures of diazinon & BPA and propoxur & BPA were 168.8 microg/ml (160.1 microg/ml-178.2 microg/ml) and 253.4 microg/ml (236.0-273.0 microg/ml). In the interaction assessment, three methods all demonstrated an antagonistic action of diazinon & BPA and an addition action of propoxur & BPA. CONCLUSION: It is concluded that the interaction of diazinon & BPA on proliferation of RAW264.7 cells is antagonistic, and that of propoxur & BPA is additive.

Evidence for inhibition of cholinesterases in insect and mammalian nervous systems by the insect repellent deet.

BACKGROUND: N,N-Diethyl-3-methylbenzamide (deet) remains the gold standard for insect repellents. About 200 million people use it every year and over 8 billion doses have been applied over the past 50 years. Despite the widespread and increased interest in the use of deet in public health programmes, controversies remain concerning both the identification of its target sites at the olfactory system and its mechanism of toxicity in insects, mammals and humans. Here, we investigated the molecular target site for deet and the consequences of its interactions with carbamate insecticides on the cholinergic system. RESULTS: By using toxicological, biochemical and electrophysiological techniques, we show that deet is not simply a behaviour-modifying chemical but that it also inhibits cholinesterase activity, in both insect and mammalian neuronal preparations. Deet is commonly used in combination with insecticides and we show that deet has the capacity to strengthen the toxicity of carbamates, a class of insecticides known to block acetylcholinesterase. CONCLUSION: These findings question the safety of deet, particularly in combination with other chemicals, and they highlight the importance of a multidisciplinary approach to the development of safer insect repellents for use in public health.

Propoxur-induced acetylcholine esterase inhibition and impairment of cognitive function: attenuation by Withania somnifera.

Propoxur (2-isopropoxyphenyl N-methylcarbamate) is widely used as an acaricide in agriculture and public health programs. Studies have shown that sub-chronic exposure to propoxur can cause oxidative stress and immuno-suppression in rats. Carbamates are also known to exhibit inhibitory effect on cholinesterase activity, which is directly related to their cholinergic effects. In the present study, the effect of Withania somnifera (Ashwagandha), a widely used herbal drug possessing anti-stress and immunomodulatory properties was studied on propoxur-induced acetylcholine esterase inhibition and impairment of cognitive function in rats. Male Wistar rats were divided into four groups. Group I was treated with olive oil and served as control. Group II was administered orally with propoxur (10 mg/kg b.wt.) in olive oil, group III received a combination of propoxur (10 mg/kg b.wt.) and W. somnifera (100 mg/kg b.wt.) suspension and group IV W. somnifera (100 mg/kg b.wt.) only. All animals were treated for 30 days. Cognitive behaviour was assessed by transfer latency using elevated plus maze. Blood and brain acetylcholine esterase (AChE) activity was also assessed. Oral administration of propoxur (10 mg/kg b.wt.) resulted in a significant reduction of brain and blood AChE activity. A significant prolongation of the acquisition as well as retention transfer latency was observed in propoxur-treated rats. Oral treatment of W. somnifera exerts protective effect and attenuates AChE inhibition and cognitive impairment caused by sub-chronic exposure to propoxur.

[Carcinogenic activity of the pesticide propoxur].

Wistar rats were fed propoxur in their diet at 0, 500, 3000, and 8000 ppm during throughout their life. The number of tumors was equal in the control and experimental groups. These were hemoblastoses and breast and uterine tumors. All tumors occurred spontaneously in the rats. A few experimental animals were found to have bladder epithelial hyperplasia that might be pretumorous; however, no bladder tumors were detected. It is concluded that the investigations revealed no carcinogenic activity of propoxur.

Evaluation of protective effect of bee pollen against propoxur toxicity in rat.

In this study, 28 Wistar female rats (200-250g) were used and divided into four equal groups. Group 1 was allocated as the control group. Groups 2-4 were administered 100mg/kg/bw/day bee pollen, 20mg/kg/bw/day propoxur, and 100mg/kg/bw/day bee pollen plus 20mg/kg/bw/day propoxur by gavage for 14 days, respectively. At the end of the 14th day, blood and tissues (the liver, kidney, brain, and heart) were collected from all animals. Oxidative stress markers (MDA, CAT, SOD, GSH-Px) and some other biochemical parameters (total protein, albumin, glucose, cholesterol, triglyceride, BUN, creatinine, uric acid, magnesium, sodium, potassium, chloride, total bilirubin, GGT, LDH, AST, ALT, and ALP) were analyzed. According to the data obtained, propoxur was determined to lead to negative changes in most of the biochemical parameters investigated and the administration of bee pollen was determined to alleviate these effects.

Effects of four carbamate compounds on antioxidant parameters.

The effect of four carbamates, aldicarb and its metabolites (aldicarb sulfone and aldicarb sulfoxide) and propoxur on glutathione content and the activity of the enzymes involved in the sulfur-redox cycle in the mammalian cellular model CHO-K1 cells after 24-h exposure were determined. Carbamate exposure resulted in a depletion of intracellular reduced glutathione (GSH) content, no change was observed in oxidized glutathione (GSSG) and a decrease in GSH/GSSG ratio was detected. After carbamates exposition a GSH/GSSG decreases in ranged from 12.44% to 21.35% of control was observed. Depletion of GSH levels was accompanied by the induction of glutathione reductase (GR) after 24h exposure with each of the four carbamates to CHO-K1 cells. After aldicarb sulfone, aldicarb sulfoxide, and propoxur exposure, glutathione peroxidase (GPx) activity increased in CHO-K1 cells by 198%, 32%, and 228% of control, respectively. After aldicarb sulfone and propoxur exposure, glutathione transferase (GST) activities increased by 49% and 230% of control, respectively. Due to the role played by GSH in preventing cytotoxicity via free-radical scavenging, results obtained suggest that high concentrations of aldicarb sulfone and propoxur closely resembling oxidative stress in CHO-K1 cells.

Time-Course Changes in Urine Metabolic Profiles of Rats Following 90-Day Exposure to Propoxur.

As a major kind of carbamate insecticide, propoxur plays an important role in agriculture, veterinary medicine, and public health. The acute toxicity of propoxur is mainly neurotoxicity due to the inhibition of cholinesterase. However, little is known regarding the toxicity of propoxur upon long-term exposure at low dose. In this study, Wistar rats were orally administrated with low dose (4.25 mg/kg body weight/day) for consecutive 90 days. And the urine samples in rats treated with propoxur for 30, 60, and 90 days were collected and analyzed by employing 1H NMR-based metabolomics approach. We found that propoxur caused significant changes in the urine metabolites, including taurine, creatinine, citrate, succinate, dimethylamine, and trimethylamine-N-oxide. And the alteration of the metabolites was getting more difference compared with that of the control as the exposure time extending. The present study not only indicated that the changed metabolites could be used as biomarkers of propoxur-induced toxicity but also suggested that the time-course alteration of the urine metabolomic profiles could reflect the progressive development of the toxicity following propoxur exposure.

Genotoxicity of low dose N-nitroso propoxur to human gastric cells.

Propoxur is among the most popular insect control agents in subtropical countries such as Taiwan. As a member of the N-methylcarbamate insecticide group, propoxur is notorious for its potential for conversion into highly genotoxic N-nitroso derivatives. Due to the fact that the stomach has been identified as the major target for N-nitroso N-methylcarbamates, this investigation used a human gastric cell line, SC-M1, in order to obtain results pertinent to the authentic adverse effects of this compound on human health. This report reveals that at dose levels inhibiting < or = 10% cell growth, a 2-h pulsed treatment of N-nitroso propoxur induced significant amounts of DNA damage. Most of the damaged DNA was repaired within 24 h after treatment removal, such that an outcome with a significant induction of chromosomal aberrations was not observed. Gene mutations and anchorage independence, on the other hand, were significantly induced by this same treatment. In conclusion, exposure to low doses of N-nitroso propoxur is not cytotoxic nor clastogenic, nevertheless, has the potential to increase genetic instability and, possibly as a result, to enhance the malignant potential of treated cells. We suggest that although the damaged DNA was repaired during the transient G2/M arrest period, it was probably not done in an appropriate way which would preserve the original genetic stability.

The delayed genotoxic effect of N-nitroso N-propoxur insecticide in mammalian cells.

The N-nitroso derivative of an extensively used insecticide, propoxur, consistently induced dose-responsive chromosome aberrations and sister-chromatid exchanges (SCEs) in Chinese hamster ovary (CHO-W8) cells. Further investigations indicated that post-treatment incubation with a regular 1.5-cell-cycle period did not offer an unbiased estimation of the genotoxicity of N-nitroso carbamate insecticides. The scale of chromosome aberration induction increased with extension of the post-treatment incubation period. Comparable phenomena were not found in CHO-AGT cells proficient for O(6)-methylguanine-DNA-methyltransferase. In CHO-W8 cells, pulsed-treatment of the insecticide in the 1st replication cycle showed higher SCE induction than in the 2nd cycle. Similar phenomenon was also found in SCE induced by N-nitroso derivatives from other carbamate insecticides including aldicarb, carbofuran and methomyl. Treated cells did not show significantly perturbed cell cycle progression until 12 h after treatment removal. Based on the above observations, the O(6)-methylguanine-DNA adduct is suggested to be the major lesion caused by the delayed genotoxic effect of N-methyl carbamate insecticides as described in this report.