Mevinphos induces developmental defects via inflammation, apoptosis, and altered MAPK and Akt signaling pathways in zebrafish

Mevinphos, an organophosphate insecticide, is widely used to control pests and enhance crop yield. Because of its high solubility, it can easily flow into water and threaten the aquatic environment, and it is known to be hazardous to non-target organisms. However, little is known about its developmental toxicity and the underlying toxic mechanisms. In this study, we utilized zebrafish, which is frequently used for toxicological research to estimate the toxicity in other aquatic organisms or vertebrates including humans, to elucidate the developmental defects induced by mevinphos. Here, we observed that mevinphos induced various phenotypical abnormalities, such as diminished eyes and head sizes, shortened body length, loss of swim bladder, and increased pericardiac edema. Also, exposure to mevinphos triggered inflammation, apoptosis, and DNA fragmentation in zebrafish larvae. In addition, MAPK and Akt signaling pathways, which control apoptosis, inflammation, and proper development of various organs, were also altered by the treatment of mevinphos. Furthermore, these factors induced various organ defects which were confirmed by various transgenic models. We identified neuronal toxicity through transgenic olig2:dsRed zebrafish, cardiovascular toxicity through transgenic fli1:eGFP zebrafish, and hepatotoxicity and pancreatic toxicity through transgenic lfabp:dsRed;elastase:GFP zebrafish. Overall, our results elucidated the developmental toxicities of mevinphos in zebrafish and provided the parameters for the assessment of toxicities in aquatic environments.

A method for quantitative risk appraisal for pesticide risk assessments

Pesticide risk assessments are fraught with uncertainties that arise from the process of estimating exposure to and toxicity of chemicals. Regulatory agencies resolve those uncertainties in a health-protective (conservative) manner, typically acknowledging only inter- and intraspecies uncertainties quantitatively. Other uncertainties may be acknowledged qualitatively, but those safety factors (SF) are not enumerated. Quantitative risk appraisal may be used to enumerate the multiplicative SF generated by conservative assumptions regarding uncertainties. The magnitude of SF derived from decision points dealing with historically unquantified uncertainty may far exceed explicit SF used to gauge acceptable margins of exposure (MoE). Examination of the basis for some previously unenumerated SF may justify potential changes in regulatory practices and policies. Using past risk assessments of 3 pesticides (mevinphos, parathion, and methyl iodide) for which the California Department of Pesticide Regulation found unacceptable risk as examples, the previously unquantified SF ranged from 47 to 1 × 10⁶ for scenarios involving handlers, reentry workers, and bystanders.

Alternative bioassay for the detection of saxitoxin using the speckled cockroach (Nauphoeta cinerea)

Paralytic shellfish poisoning (PSP) toxins produced by cyanobacteria pose a risk to public health as they occur in drinking water reservoirs and recreational lakes and accumulate in the food chain. One of these PSP toxins, saxitoxin (STX) is one of the most toxic nonprotein substances known. Accordingly, there is a requirement to monitor for these toxins. The standard bioassay used to detect these toxins is the mouse bioassay; however, its use is constrained by animal ethics guidelines and practical considerations. Reported here is the use of the globally distributed speckled cockroach Nauphoeta cinerea as a bioassay test organism for the selective detection of PSP toxicity of Anabaena circinalis aqueous extract and STX. N. cinerea was shown to be tolerant to pure cylindrospermopsin (CYN) and microcystin-LR (MC-LR) at doses 10-fold greater than mouse LD₅₀ values while being sensitive to STX. Similarly, N. cinerea was shown to be tolerant of toxin-containing aqueous extracts of Cylindrospermopsis raciborskii, Microcystis aeruginosa, and Nodularia spumigena while being sensitive to A. circinalis. Peak sensitivity to STX was 60 min postinjection with a KD₅₀ of 31.2 ng/g body weight. While this was approximately 3-fold less sensitive than the mouse bioassay, the insect test organism was around 34-fold smaller in mass than a mouse (20 g); thus one-tenth the amount of toxin in absolute quantity was required to reach an ED₅₀ level. The N. cinerea bioassay presents a selective test for PSP toxicity that is rapid, economical, efficient, and simple to perform.

MUSCARINIC RECEPTOR-INDEPENDENT ACTIVATION OF CYCLIC ADENOSINE MONOPHOSPHATE-DEPENDENT PROTEIN KINASE IN ROSTRAL VENTROLATERAL MEDULLA UNDERLIES THE SYMPATHOEXCITATORY PHASE OF CARDIOVASCULAR RESPONSES DURING MEVINPHOS INTOXICATION IN THE RAT

As inhibitors of acetylcholinesterase, clinical presentations of poisoning from organophosphate compounds are generally believed to entail overstimulation by the accumulated acetylcholine on muscarinic receptors at peripheral and central synapses. That some patients still yielded to acute organophosphate poisoning despite repeated dosing of atropine suggests that cellular mechanisms that are independent of muscarinic receptor activation may also be engaged in organophosphate poisoning. The present study was undertaken to test the hypothesis that muscarinic receptor-independent activation of cyclic adenosine monophosphate-dependent protein kinase A (PKA) in rostral ventrolateral medulla (RVLM), a medullary site where sympathetic vasomotor tone originates and where the organophosphate poison mevinphos (Mev) acts, is involved in the cardiovascular responses exhibited during organophosphate intoxication. In Sprague-Dawley rats, microinjection bilaterally of Mev (10 nmol) into the RVLM significantly augmented PKA activity in ventrolateral medulla that was not antagonized by coadministration of an equimolar concentration (1 nmol) of atropine or selective muscarinic receptor type M1 (pirenzepine), M2 (methoctramine), M3 (4-diphenyl-acetoxy-N-dimethylpiperidinium), or M4 (tropicamide) inhibitor. Comicroinjection of two selective PKA antagonists (100 pmol), N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide and (9R,10S,12S)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolol[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-1][1,6]benzodiazocine-10-carboxylic acid, significantly blunted the initial sympathoexcitatory cardiovascular response and the accompanying augmentation of nitric oxide synthase (NOS I) expression in the ventrolateral medulla exhibited during Mev intoxication; the secondary sympathoinhibitory phase and associated elevation in NOS II expression were unaffected. We conclude that whereas a muscarinic receptor-independent augmentation of PKA activity in the ventrolateral medulla was manifested throughout acute Mev intoxication, this activation was preferentially involved in the sympathoexcitatory phase by an upregulation of NOS I expression.

Heat Shock Protein 60 or 70 Activates Nitric-oxide Synthase (NOS) I- and Inhibits NOS II-associated Signaling and Depresses the Mitochondrial Apoptotic Cascade during Brain Stem Death

The cellular and molecular basis of brain stem death remains an enigma. As the origin of a "life-and-death" signal that reflects the progression toward brain stem death, the rostral ventrolateral medulla (RVLM) is a suitable neural substrate for mechanistic delineation of this phenomenon. Here, we evaluated the hypothesis that heat shock proteins (HSPs) play a neuroprotective role in the RVLM during brain stem death and delineated the underlying mechanisms, using a clinically relevant animal model that employed the organophosphate pesticide mevinphos (Mev) as the experimental insult. In Sprague-Dawley rats, proteomic, Western blot, and real-time PCR analyses demonstrated that Mev induced de novo synthesis of HSP60 or HSP70 in the RVLM without affecting HSP90 level. Loss-of-function manipulations of HSP60 or HSP70 in the RVLM using anti-serum or antisense oligonucleotide potentiated Mev-elicited cardiovascular depression alongside reduced nitric-oxide synthase (NOS) I/protein kinase G signaling, enhanced NOS II/peroxynitrite cascade, intensified nucleosomal DNA fragmentation, elevated cytoplasmic histone-associated DNA fragments or activated caspase-3, and augmented the cytochrome c/caspase-3 cascade of apoptotic signaling in the RVLM. Co-immunoprecipitation experiments further revealed a progressive increase in the complex formed between HSP60 and mitochondrial or cytosolic Bax or mitochondrial Bcl-2 during Mev intoxication, alongside a dissociation of the cytosolic HSP60-Bcl-2 complex. We conclude that HSP60 and HSP70 confer neuroprotection against Mev intoxication by ameliorating cardiovascular depression via an anti-apoptotic action in the RVLM. The possible underlying intracellular processes include enhancing NOS I/protein kinase G signaling and inhibiting the NOS II/peroxynitrite cascade. In addition, HSP60 exerts its effects against apoptosis by blunting Mev-induced activation of the Bax/cytochrome c/caspase-3 cascade.

Cholinergic-receptor-independent dysfunction of mitochondrial respiratory chain enzymes, reduced mitochondrial transmembrane potential and ATP depletion underlie necrotic cell death induced by the organophosphate poison mevinphos

Our current understanding of the nature of cell death that is associated with fatal organophosphate poisoning and the underlying cellular mechanisms is surprisingly limited. Taking advantage of the absence in an in vitro system of acetylcholinesterase, the pharmacological target of organophosphate compounds, the present study evaluated the hypothesis that the repertoire of cholinergic receptor-independent cellular events that underlie fatal organophosphate poisoning entails induction of mitochondrial dysfunction, followed by bioenergetic failure that leads to necrotic cell death because of ATP depletion. Pheochromocytoma PC12 cells incubated with the organophosphate pesticide mevinphos (0.4 or 4mumol) for 1 or 3h underwent a dose-related and time-dependent loss of cell viability that was not reversed by muscarinic (atropine) or nicotinic (mecamylamine) blockade. This was accompanied by depressed NADH cytochrome c reductase, succinate cytochrome c reductase or cytochrome c oxidase activity in the mitochondrial respiratory chain, reduced mitochondrial transmembrane potential, decreased ATP concentration, elevated ADP/ATP ratio, increased lactate dehydrogenase release and necrotic cell death. We conclude that Mev induces cholinergic receptor-independent necrotic cell death by depressing the activity of Complexes I to IV in the mitochondrial respiratory chain, eliciting reduction in mitochondrial transmembrane potential, depleting intracellular ATP contents and damaging cell membrane integrity.

Neuroprotective Role of Coenzyme Q10 against Dysfunction of Mitochondrial Respiratory Chain at Rostral Ventrolateral Medulla during Fatal Mevinphos Intoxication in the Rat

We evaluated the functional changes in the mitochondrial respiratory chain at the rostral ventrolateral medulla (RVLM), the medullary origin of sympathetic vasomotor tone, in an experimental model of fatal organophosphate poisoning using the insecticide mevinphos (Mev). We also investigated the neuroprotective role of coenzyme Q10 (CoQ10) in this process. Intravenous administration of Mev (1 mg/kg) in Sprague-Dawley rats maintained with propofol elicited an initial hypertension followed by hypotension, accompanied by bradycardia, with death ensuing within 10 min. Enzyme assay revealed a significant depression of the activity of nicotinamide adenine dinucleotide cytochrome c reductase, succinate cytochrome c reductase, and cytochrome c oxidase in the RVLM during this fatal Mev intoxication. ATP production also underwent a significant decrease. Pretreatment by microinjection bilaterally of CoQ10 (4 microg) into the RVLM significantly prevented mortality, antagonized the cardiovascular suppression, and reversed the depressed mitochondrial respiratory enzyme activities, or reduced ATP production in the RVLM induced during Mev intoxication. Our results indicated that dysfunction of mitochondrial respiratory chain and energy production at the RVLM takes place during fatal Mev intoxication. We further demonstrated that CoQ10 provides neuroprotection against Mev-induced cardiovascular depression and fatality through maintenance of activity of the key mitochondrial respiratory enzymes in the RVLM.

Neuroprotective role of heat shock protein 70 in the rostral ventrolateral medulla during acute mevinphos intoxication in the rat

Heat shock protein (HSP) is a family of highly conserved proteins that respond to stress and participate actively in cytoprotection. Within the HSP family, HSP70 is the major inducible member that confers protection against cell death. This study investigated whether HSP70 plays a neuroprotective role at the rostral ventrolateral medulla (RVLM), the origin of sympathetic neurogenic vasomotor tone in the medulla oblongata where the organophosphate insecticide mevinphos (Mev) acts to elicit cardiovascular toxicity. Experiments were carried out in adult male Sprague-Dawley rats that were maintained under propofol anesthesia. Intravenous administration of Mev (960 microg/kg) induced a significant increase in the HSP70 level in the ventrolateral medulla during phase I ('pro-life' phase), and returned to baseline during phase II ('pro-death' phase) Mev intoxication. Compared to artificial cerebrospinal fluid, normal mouse serum (1:20), or sense hsp70 oligonucleotide (50 pmol) pretreatment, microinjection of an anti-HSP70 antiserum (1:20) or an antisense hsp70 oligonucleotide (50 pmol) bilaterally into the RVLM significantly increased mortality, shortened the duration of phase I intoxication and augmented the induced hypotension in rats that received Mev (960 microg/ kg, i.v.). These results suggest that HSP70 induced in the RVLM during Mev intoxication provides neuroprotection against the organophosphate poison via prevention of cardiovascular depression.

DE NOVO SYNTHESIS OF UBIQUITIN CARBOXYL-TERMINAL HYDROLASE ISOZYME L1 IN ROSTRAL VENTROLATERAL MEDULLA IS CRUCIAL TO SURVIVAL DURING MEVINPHOS INTOXICATION

Ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCH-L1) is a deubiquitinating enzyme that is responsible for making ubiquitin, which is required to target proteins for degradation by the ubiquitin-proteasome pathway in neurons, available. We investigated whether UCH-L1 plays a neuroprotective role at the rostral ventrolateral medulla (RVLM), the origin of sympathetic neurogenic vasomotor tone in the medulla oblongata where the organophosphate insecticide mevinphos (Mev) acts to elicit cardiovascular toxicity. In Sprague-Dawley rats maintained under propofol anesthesia, Mev (960 microg/kg, i.v.) induced a parallel and progressive augmentation in UCH-L1 or ubiquitin expression at the ventrolateral medulla during the course of Mev intoxication. The increase in UCH-L1 level was significantly blunted on pretreatment with bilateral microinjection into the RVLM of a transcription inhibitor, actinomycin D (5 nmol), or a translation inhibitor, cycloheximide (20 nmol). Compared with aCSF or sense oligonucleotide (100 pmol) pretreatment, microinjection of an antisense uch-L1 oligonucleotide (100 pmol) bilaterally into the RVLM significantly increased mortality, reduced the duration of the "pro-life" phase, blunted the increase in ubiquitin expression in ventrolateral medulla, and augmented the induced hypotension in rats that received Mev. These findings suggest that de novo synthesis of UCH-L1, leading to an enhanced disassembly of ubiquitin-protein conjugates in the RVLM, is essential to maintenance of the "pro-life" phase of Mev intoxication via prevention of cardiovascular depression, leading to neuroprotection.

Differential contributions of NOS isoforms in the rostral ventrolateral medulla to cardiovascular responses associated with mevinphos intoxication in the rat

The organophosphate poison mevinphos (Mev) elicits cardiovascular responses via nitric oxide (NO) produced on activation of M2 muscarinic receptors (M2R) in the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates. This study further evaluated the contribution of nitric oxide synthase (NOS) isoforms at the RVLM to this process, using adult Sprague-Dawley rats. Bilateral co-microinjection into the RVLM of the selective NOS I inhibitor (250 pmol), 7-nitroindazole or N(omega)-propyl-L-arginine antagonized the initial sympathoexcitatory cardiovascular responses to Mev (10 nmol). Co-administration of a selective NOS II inhibitor, N6-(1-iminoethyl)-L-lysine (250 or 500 pmol) further enhanced these cardiovascular responses and reversed the secondary sympathoinhibitory actions of Mev. A potent NOS III inhibitor, N5-(1-iminoethyl)-L-ornithine (46 or 92 nmol) was ineffective. We also found that M2R co-localized only with NOS I- or NOS II-immunoreactive RVLM neurons. Furthermore, only NOS I or II in the ventrolateral medulla exhibited an elevation in mRNA or protein levels during the sympathoexcitatory phase, with further up-regulated synthesis of NOS II during the sympathoinhibitory phase of Mev intoxication. We conclude that whereas NOS III is not engaged, NO produced by NOS I and II in the RVLM plays, respectively, a sympathoexcitatory and sympathoinhibitory role in the cardiovascular responses during Mev intoxication.

DEPRESSION OF MITOCHONDRIAL RESPIRATORY ENZYME ACTIVITY IN ROSTRAL VENTROLATERAL MEDULLA DURING ACUTE MEVINPHOS INTOXICATION IN THE RAT

We investigated possible changes in bioenergetics at the rostral ventrolateral medulla (RVLM), a medullary site where sympathetic vasomotor tone originates and where the organophosphate poison mevinphos (Mev) acts to elicit cardiovascular intoxication. In Sprague-Dawley rats maintained under propofol anesthesia, microinjection bilaterally of Mev (10 nmol) into the RVLM induced progressive hypotension that was accompanied by an early augmentation (80-100 min post-Mev; Phase I), followed by a decrease (>100 min post-Mev; Phase II) in the power density of the vasomotor components (0-0.8 Hz) in systemic arterial pressure (SAP) signals. Enzyme assay revealed that local application of Mev into the RVLM also significantly and progressively depressed the activity of NADH cytochrome c reductase (marker for Complexes I and III) and cytochrome c oxidase (marker for Complex IV) in the mitochondrial respiratory chain of the RVLM, but not the heart. On the other hand, the activity of succinate cytochrome c reductase (marker for Complexes II and III) remained unaltered. Both the cardiovascular consequences and depression of mitochondrial respiratory chain enzymes elicited by Mev were significantly antagonized on comicroinjection of atropine (3.5 or 7 nmol) bilaterally into the RVLM. We conclude that Mev adversely effects cardiovascular control by acting as a cholinesterase inhibitor in the RVLM, whose neuronal activity is intimately related to the death process. The resulting accumulation of acetylcholine and prolonged activation of muscarinic receptors in the RVLM is manifested by a selective dysfunction of respiratory enzyme Complexes I and IV in the mitochondrial respiratory chain that underlies cardiovascular toxicity associated with organophosphate poisons such as Mev.

Engagement of inducible nitric oxide synthase at the rostral ventrolateral medulla during mevinphos intoxication in the rat

We evaluated the relationship between the toxicity induced by the organophosphate mevinphos (Mev) and inducible nitric oxide synthase (iNOS) in the rostral ventrolateral medulla (RVLM), the medullary origin of sympathetic neurogenic vasomotor tone. Adult Sprague-Dawley rats that were anesthetized and maintained with propofol were used. Laser scanning confocal microscopic analysis revealed colocalization of the M2 subtype of muscarinic receptors (M(2)R) and iNOS immunoreactivity in RVLM neurons. Comicroinjection bilaterally of Mev (10 nmol) and artificial cerebrospinal fluid (aCSF) into the RVLM elicited a progressive decline in systemic arterial pressure (SAP) and heart rate. This was accompanied during phase 1 Mev intoxication by an increase in the power density of the very high-frequency (VHF; 5-9 Hz), high-frequency (HF; 0.8-2.4 Hz), low-frequency (LF; 0.25- 0.8 Hz) and very low-frequency (VLF; 0-0.25 Hz) components of SAP signals. Phase 2 exhibited a reversal of the VHF and VLF power to control levels and a further reduction in the power density of both HF and LF components to below baseline. Hypotension and bradycardia promoted by Mev were significantly blunted on coadministration into the RVLM of the selective iNOS inhibitors S-methylisothiourea (250 pmol) or aminoguanidine (250 pmol). Not only was the augmented power density of HF and LF components during phase 1 Mev intoxication further enhanced, the reduced power of these two spectral components during phase 2 was appreciably antagonized. On the other hand, the temporal changes in VHF and VLF power were essentially the same as with coadministration of Mev and aCSF. We conclude that, as a cholinesterase inhibitor, Mev may induce toxicity via nitric oxide produced by iNOS on activation of the M(2)R by the accumulated acetylcholine in the RVLM.

[The effect of panpal prophylaxis on acetylcholinesterase activity in the blood, diaphragm and various parts of the brain in rats during treated and untreated poisoning with the organophosphorus insecticide phosdrine]]

In experiments on male rats the paper investigated the effect of pharmacological prophylaxis with Panpal (pyridostigmine in combination with benactyzine and trihexyphenidyl) on the activity of acetylcholinesterase in the whole blood, diaphragm, and selected parts of the brain (frontal cortex, pontomedular region, hippocampus, cerebellum) at hour 1 and 3 of untreated and treated (oxime HI-6 with atropine) intoxication with the organophosphorous insecticide phosdrine. Whereas in the CNS Panpal did not produce statistically significant changes in the activity of acetylcholinesterase in the course of untreated and treated phosdrine intoxication, in the blood and diaphragm Panpal markedly intensified phosdrine-induced inhibition of the acetylcholinesterase activity and, in addition, decreased the reactivating effect of the oxime HI-6. The data give evidence of the importance of the combination of the prophylactically used reversible acetylcholinesterase inhibitor pyridostigmine with anticholinergics, which could eliminate the consequences of a pyridostigmine-induced decrease in the activity of the enzyme in the periphery.

[Comparison of the effects of BI-6, a new asymmetric bipyridine oxime, with HI-6 oxime and obidoxime in combination with atropine on soman and fosdrine toxicity in mice]

The therapeutic efficacy of the new asymmetric bispyridinium oxime BI-6 against acute toxicity of the highly toxic organophosphate soman and the organophosphorus insecticide fosdrin by means of affecting the LD50 values of these noxiores substances was compared with the effect of the hitherto most perspective oxime HI-6 and the classic obidoxime always in combination with the identical dose of atropine. At the equimolar level the effect of oxime BI-6 against fosdrin completely equals the effects of both oximes HI-6 and obidoxime. The effect of oxime BI-6 against soman is even more marked than the effect of HI-6 but this difference is not statistically significant. On the other hand, at the equi-effective level, the effect of oxime BI-6 against soman is statistically significantly lower than the effect of HI-6, and against fosdrin it is even lower than the effect of both remaining oximes. The effects of the new oxime BI-6 equal, or slightly exceed the therapeutic effect of HI-6 but at the equimolar level only. At the equi-effective level which respects the toxicity of the oxime and is therefore more important for practical use, it is a therapeutically weaker reactivator of acetylcholinesterase than HI-6.

The efficacy of monopyridinium (2-PAAM, 2-PAEM) and bispyridinium (obidoxime, HI-6) oximes against mevinphos in mice

The efficacy of two new monopyridinium oximes (2-PAAM, 2-PAEM) and two bispyridinium oximes (obidoxime. HI-6) was tested in combination with atropine sulphate against acute poisoning with the organophosphorus insecticide mevinphos in mice. When mice were treated two min. after mevinphos poisoning, no significant differences in the therapeutic effect of tested oximes were observed. The oximes increased the 24 hr LD50 values of mevinphos about two times in comparison with the 24 hr LD50 values of mevinphos in mice protected with atropine sulphate alone and more than three times in comparison with non-treated intoxicated animals. On the other hand, both monopyridinium oximes were significantly more efficacious than HI-6 and as efficacious as obidoxime when they were administered 30 sec. after mevinphos poisoning. Both monopyridinium oximes and obidoxime increased the 24 hr values of mevinphos almost three times in comparison with the 24 hr values of mevinphos in mice protected with atropine sulphate alone and about twenty-five times in comparison with non-treated intoxicated animals, while the oxime HI-6 less than two times in comparison with the 24 hr values of mevinphos in mice protected with atropine sulphate alone and about fifteen times in comparison with non-treated intoxicated animals. Use of new monopyridinium oximes seems to be the improvement in the antidotal treatment of poisoning with organophosphorous insecticide mevinphos in comparison with HI-6 but not in comparison with obidoxime when oximes are used in equimolar doses.

[The effect of parasympatholytics on the therapeutic effectiveness of the oxime HI-6 against organophosphorus compounds (Soman, substance VX, Fosdrin) in mice]

BACKGROUND

Causal antidotal therapy of acute intoxications with organophosphorus compounds involving administration of the parasympatholytic and cholineesterase reactivator (oxime) has not been resolved so far satisfactorily despite knowledge of the basic mechanism of action of these noxious substances.

METHODS AND RESULTS

In experiments on mice the therapeutic effect of parasympatholytics atropine, benactyzine and biperidene (Akineton) combined with oxime HI-6 on the toxicity of highly toxic organophosphates soman and substance VX and the organophosphorus insecticide phosdrine was compared as regards their influence on the LD50 of these noxious substances during 24-hour survival of experimental animals. Two levels of antidotes were tested. These findings confirm that the LD50 value of untreated intoxication with all three organophosphorus compounds is most increased by oxime HI-6 combined with benactyzine regardless of the antidote dosage.

CONCLUSIONS

Oxime HI-6 is the most effective against highly toxic organophosphates and organophosphorus insecticides when combined with the centrally acting parasympatholytic benactyzine.

Risks from occupational and dietary exposure to mevinphos

Mevinphos (trade name, Phosdrin), a category 1 organophosphorus insecticide, has been used mainly as a cleanup pesticide for vegetable crops. A risk assessment for occupational and dietary exposure to mevinphos was initiated because of the high acute toxicity of the compound. Repetitive dosing with mevinphos did not cause any discernible histopathological effects in mice or rats, nor was it oncogenic in either species. The principal toxic effects of mevinphos, both short- and long term, were due to inhibition of cholinesterase activity. Consequently, potential adverse effects from short-term exposures were the primary concern. A human no-observed-effect level (0.025 mg/kg) for cholinergic signs was used as the regulatory basis for calculating margins of safety (MOSs) for potential acute dietary and short-term occupational exposures. Estimates of exposure to mixer/loaders, pilots, and flaggers associated with aerial application of mevinphos were based on passive dosimetry. Because no acceptable exposure studies for work tasks associated with ground application of mevinphos were available, surrogate data based on ground application of oxydemeton-methyl were used. Exposure estimates for field workers and harvesters relied on measured dislodgeable foliar residues of mevinphos and transfer factors generated from studies of other active ingredients. MOSs for mean acute occupational exposure of mixer/loader/applicators associated with ground application and of harvesters working in fruit trees were less than the value conventionally recommended to protect people from the toxic effects of mevinphos. MOSs for the 95th percentile of short-term worker exposure for all mixer/loader work categories associated with mevinphos application were also inadequate. Calculated MOSs for potential acute dietary exposure to measured residue levels of mevinphos were adequate for the various population subgroups. However, 25 of the USEPA tolerances for mevinphos on agricultural commodities were not adequate to protect for the toxic effects of mevinphos from theoretical acute dietary exposure to one or more population subgroups if commodities are consumed with residues at the tolerance level. When the mean short-term occupational exposures were combined with potential acute dietary exposure, the MOSs for mixer/loaders engaged in aerial applications, as well as ground applications, were inadequate to protect people from the toxic effects of mevinphos. As mitigation of the estimated excessive occupational exposures did not appear possible, both California and the USEPA were preparing to cancel registration of the product. However, an agreement was worked out between the manufacturer and the two agencies that ended production for domestic use but allowed existing stocks in the channels of trade to continue to be used for a limited period.

Use of the golden hamster in toxicology

The choice of an appropriate species and strain of laboratory animal is one of the toxicologist's major concerns. The biochemical, anatomical, and other characteristics of the golden hamster which make it a valuable though limited tool in toxicologic research were reviewed. The effects of organochloride insecticides on rats, mice, and hamsters were compared. These substances can be classified into 3 groups based on the hamster's sensitivity to acute toxic effects. The hamster is so insensitive to DDT and its immediate metabolites, DDD and DDE, that an LD50 cannot be established, whereas the hamster's sensitivity to other organochlorides varies when compared with mouse and rat sensitivity. The long-term effects of sub-lethal doses of organochloride insecticides on biochemical pathways were evaluated, particularly on liver function. The details of the induction of drug-metabolizing enzymes in the hamster and in the rat were also explored.