Subchronic and Chronic Inhalation Toxicity of Antimony Trioxide in the Rat

Fischer 344 rats were exposed by inhalation to Sb₂O₃ (antimonytrioxide) dust at exposure levels of 0, 0.25, 1.08, 4.92, and23.46 mg/m³ for 6 hr/day, 5 days/week for 13 weeks followedby a 27-week observation period. Subsequently, an inhalationon-cogenicity study was conducted at exposure levels of 0, 0.06,0.51, and 4.50 mg/m³ for 12 months followed by a 12-month observationperiod. The Sb₂O₃ in the subchronic study had a mass medianaerodynamic diameter (MMAD) of 3.05 ± 0.21 microns (mean± SD) with a geometric standard deviation (GSD) of 1.57± 0.06. In the chronic study, the MMAD was 3.76 ±0.84 and the GSD was 1.79 ± 0.32. Except for the eyes,no adverse clinical observations were attributed to Sb₂O₃ ineither study. In the subchronic study, corneal irregularitieswere seen after about 2 weeks of exposure and did not abateduring the observation period. In the chronic study, ophthalmoscopicevaluation at 24 months revealed a dose-related increase incataracts of 11, 24, 28, and 32% (both sexes combined) for eachgroup, respectively. Body weights were significantly lower (6%)than the control group's weights in the 23.46 mg/m³ males inthe subchronic study. These rats did not recover this weightduring the 27-week observation period. Body weights of the femalesin both studies and males in the chronic study were unaffected.There were no Sb₂O₃ effects on clinical chemistry or he-matologyin either study. Mean absolute and relative lung weights weresignificantly increased in the 4.92 and 23.46 mg/m³ groups inthe subchronic study. The 23.46 mg/m³ group's lung weights didnot recover to control levels during the 27-week observationperiod. Lung weights for rats in the chronic study were unaffected.Microscopic changes in the lungs in the subchronic and chronicstudy were limited to subacute-chronic interstitial inflammation,increased numbers of alveolar-in-traalveolar macrophages, foreignmaterial in the alveolar-in-traalveolar macrophages in the peribronchialand perivascular (chronic study only) lymphoid aggregates andin the peribronchial lymph nodes, granulomatous inflammation/granulomas,and fibrosis. In the chronic study, any observed neoplasms occurredwith comparable incidence among all groups and were within thehistorical range for controls. Clearance of Sb₂O₃ from the lungwas burden dependent and was reduced by 80/ in the 4.50 mg/m³group in the chronic study. The previously reported studies,which found Sb₂O₃ to be a carcinogen, were run at higher lungburdens. Under the exposure conditions of the current study,Sb₂O₃ was not a carcinogen.     

Hepatic Uptake and Metabolism of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and 2,3,7,8-Tetrachlorodibenzofuran

The pharmacokinetics of TCDD and related compounds is congener,dose, and species specific, with urinary and biliary excretionbeing dependent on the metabolism of these compounds. Isolatedhepatocytes and liver slices in suspension culture and hepaticmicrosomes were used as in vitro models to assess the hepaticuptake and metabolism of [³H]- and [¹⁴C]- TCDD and [³H]TCDF(0.01–1.0 µM) in control and induced (5 µgTCDD/kg, 3 days earlier) male Sprague-Dawley rats. TCDD pretreatment,with an increase in cytochromes P450 1A1 and 1A2 (CYP1 Al, CYP1A2),produced an increase in the hepatic uptake of TCDD, while noincrease in the hepatic uptake of TCDF was observed. The resultsare consistent with CYP1A2 serving as a hepatic binding proteinfor TCDD but not for TCDF. The rates of metabolism of TCDD andTCDF were directly proportional to their concentrations, indicatingthat the reaction follows first order kinetics at concentrationsfrom 0.01 to 1.0 µM. Very limited metabolism of TCDD andTCDF was observed in control rat liver (0.45 and 3.2 pmol/hr/ghepatocyte wet wt at 0.1 µm, respectively). TCDD inducedits own rate of metabolism about two- to fivefold at 1.0 µMbut no induction was observed at 0.01 and 0.1 µM. In contrast,TCDD markedly induced the rate of TCDF metabolism at all substrateconcentrations. While the results support the role of rat CYP1A1in TCDF metabolism, the data suggest that CYP1 Al or CYP1A2may not metabolize TCDD. These results also support the hypothesisthat the more rapid metabolism and excretion of TCDF accountsfor the relative resistance of the rat to the acute toxicityof TCDF. Comparative studies in rat and human liver microsomesfound that TCDF metabolism exhibited first order kinetics inboth species. Furthermore, the rate of TCDF metabo-lism in humanliver microsomes was similar to that of control rat liver microsomes.Together the results suggest that TCDF will be far more persistentin rats, and possibly humans, following exposure at low doseswhich do not significantly induce cytochrome P450 1A1 and/or1A2.     

Developmental Neurotoxicity: Evaluation of Testing Procedures with Methylazoxymethanol and Methylmercury

Testing procedures for identification of potential developmentalneurotoxicants were evaluated using two prototypical developmentalneurotoxicants, methylazoxymethanol (MAM) and methylmercury(MeHg). Evaluation of offspring of LongEvans rats incorporatedassessments of developmental toxicity, neurochemistry, histology,and behavior, with most testing being completed near weaning.A number of endpoints in the testing strategy were sensitiveto the effects of prenatal exposure to MAM [30 mg/kg on GestationDay (GD) 15]: (1) MAM caused reduced neonatal body weights butdid not effect viability or postnatal survivorship; (2) measurementof total and regional brain weight and histological analysisshowed that a number of regions, the cortex and hippocampusin particular, were affected by MAM exposure; (3) an assay forglial fibrillary acidic protein (GFAP) showed that the concentrationof this protein was significantly increased in the cortex andhippocampus of treated offspring; (4) a T-maze delayed-alternationprocedure indicated that MAM-treated pups were slower in theacquisition phase of the task relative to control pups; (5)motor activity testing revealed hyperactivity in treated offspringthat persisted into adulthood; and (6) acoustic startle proceduresrevealed reduced startle amplitudes in preweanlings. Few endpointswere significantly affected by prenatal MeHg exposure (1, 2,or 4 mg/kg on GD 6–15). High fetal and neonatal mortalityand lower neonatal body weights were detected at the highestdose of MeHg. Although minimal effects of MeHg may reflect arelative insensitivity of the test species and/or the test methods,the combined results from both chemicals suggest that some proceduresnot currently required in the developmental neurotoxicity guidelinemay be useful in hazard identification, and further evaluationwith other chemicals, species, strains, and/or exposure paradigmsmay be warranted.     

Effects of Dihydrotestosterone and Estradiol on Experimental IgA Nephropathy Induced by Vomitoxin

Ingestion of the trichothecene vomitoxin (VT) by mice induceseffects that mimic the common human glomerulonephritis, IgAnephropathy (IgAN). These include elevation of serum IgA, IgAimmune complexes, and mesangial IgA deposition. Based on previousobservations that male mice are more prone to VT-induced IgAN,the effects of castration of male and female B6C3F1 mice andsex hormone supplementation on several immunopathologic indicatorsof the disease were compared. In the first study, castratedand intact male and female mice were fed control AIN-76A dietor the same diet containing 10 ppm VT for 12 weeks. At Week12, all but the intact female group fed VT exhibited increasedserum IgA, with castrated female mice having greater levelsthan intact females. When microscopic hematuria was used asan indicator of disease severity in intact VT-fed mice, erythrocytecounts for males exceeded those for females at weeks 4 and 12.VT-fed, castrated females exhibited greater hematuria than intactcounterparts, whereas VT-fed, castrated males had lower urinaryerythrocyte counts than intact counterparts. In a second study,castrated male and female mice were implanted with controlledrelease pellets of placebo, 5-dihydrotestosterone (DHT), or17ß-estradiol (E₂) and then were fed either controldiet or a 10 ppm VT diet for 8 weeks. Castrated male and femalemice treated with VT and DHT pellet exhibited more severe hematuria,higher IgA levels, and greater mesangial IgA deposition thanmice exposed to the same diet with placebo or E₂ pellet at Week8. While VT-fed animals with an E₂ pellet exhibited greaterhematuria and mesangial IgA deposition at Week 8 than the placebogroups, their IgA levels were not significantly elevated overthose for VT-fed mice with a placebo pellet. Relative to twoother pathologic markers for IgAN, the aforementioned effectsin both studies were generally consistent with mesangial depositionof complement component C₃ but not IgG. The results suggestthat (1) enhanced male susceptibility to VT-induced IgAN maybe related to modulation by the biologically active androgenDHT and (2) while castration of females increased severity ofVT-induced IgAN, supplementation of castrated male or femalemice with E₂ did not reverse this effect but rather increaseddisease severity.     

Endocrine Modulation of Reproduction

The ability of foreign compounds to affect the functioning ofvarious endocrine systems is currently thought responsible fora wide variety of effects. The presentations in this Symposiumreviewed the evidence for and against the involvement of endocrinesystems in several different aspects of reproduction. The mechanismbehind the ability of a triazine herbicide to cause enhancedappearance of mammary tumors in one strain of female rats isreviewed by Stevens. The data suggest that enhanced aging, notdirect mammary modulation, is responsible. Dietary phytoestrogens,the mediators of their actions, their effects in various biologicalsystems, and the relationships between phytoestrogen producersand consumers are all provocatively and succinctly reviewedby Hughes. Kelce presents the strategy used to dissect the modeand mechanisms of action of a fungicide that opened a new awarenessin reproductive toxicology: the possibility of xenobiotics beingantiandrogens. Finally, to heighten our understanding of theinterplay among hormonal systems in vivo, Hess reviews the datathat show that androgens are not the only hormones importantin the development of the male reproductive system: the pituitaryis shown to play a critical role at specific stages of development.The breadth of these presentations, and the implications oftheir findings, should make us pause and realize how much thereis still to discover about the interaction between the reproductivesystem and anthropogenic compounds.     

Subchronic Effects of Dieldrin and Phenobarbital on Hepatic DNA Synthesis in Mice and Rats

Dieldrin, an organochlorine pesticide, has been shown to behepatocarcinogenic in mice but not rats. Phenobarbital, in contrast,induces hepatic tumors in both mice and rats. Previous studieshave shown that acute dietary exposure of rats or mice to eitherdieldrin or phenobarbital produces several liver changes, includingcentrilobular hypertrophy, induction of hepatic cytochrome P450,and increased liver weight. The present study examined the subchroniceffect of dieldrin (0.1, 1.0, 3.0, 10.0 mg dieldrin/kg diet)and phenobarbital (10, 50, 100, 500 mg phenobarbital/kg diet)on the induction of hepatic DNA synthesis and hepatocyte lethalityin male B6C3F1 mice and male F344 rats. Eight-week-old animalswere treated as above and evaluated for hepatic DNA synthesisafter 7, 14, 21, 28, and 90 days of continual treatment to dieldrinor phenobarbital. Maximal induction of hepatic DNA synthesisin mice was seen at the 14-, 21-, and 28-day sampling times.In rats, no significant increase in hepatic DNA synthesis orhepatocyte lethality was observed at any dose of dieldrin investigated.Phenobarbital produced a significant increase in hepatic DNAsynthesis in both rat and mouse liver following 7 days of treatment.The induction of DNA synthesis in rat liver was transient, withthe labeling index returning to control levels by 14 days oftreatment. In contrast, mice treated with phenobarbital showeda significant increase in hepatic DNA synthesis throughout thetreatment. In both mice and rats, dieldrin and phenobarbitalinduced hepatic DNA synthesis selectively in the centrilobularregion of the hepatic lobule. The lack of an increase in serumenzymes indicative of hepatic damage and the absence of liverhistopathology in mice or rats fed dieldrin or phenobarbitalindicate that the induction of DNA synthesis was not mediatedby a cytolethal, compensatory hyperplastic response, suggestinga mitogenic mechanism. Therefore, the species-specific inductionof hepatic DNA synthesis by either dieldrin or phenobarbitalcorrelated with the previously observed species-specific inductionof hepatic cancer by these two compounds.     

Chronic Dietary Toxicity/Oncogenicity Studies on 2,4-Dichlorophenoxyacetic Acid in Rodents

Forms of 2,4-dichlorophenoxyacetic acid (collectively knownas 2,4-D) are herbicides used to control a wide variety of broadleafand woody plants. Doses in the 2-year chronic/oncogenicity ratstudy were 0, 5, 75, and 150 mg/kg/day. The chronic toxicityparalleled subchronic findings, and a NOEL of 5 mg/kg/day wasestablished. A slight increase in astrocytomas observed (inmales only) at 45 mg/kg/day in a previously conducted chronicrat study was not confirmed in the present study at the highdose of 150 mg/kg/day. Doses in the 2-year mouse oncogenicitystudies were 0, 5, 150, and 300 mg/kg/day for females and 0,5, 62.5, and 125 mg/kg/day for males. No oncogenic effect wasnoted in the study. In summary, the findings of these studiesindicate low chronic toxicity of 2,4-D and the lack of oncogenicresponse to 2,4-D following chronic dietary exposure of 2,4-Din the rat and mouse.     

Molecular mechanisms of general anaesthesia

This review aims to give a balanced view of the various mechanisms which have been proposed to explain the phenomenon of general anaesthesia on both a molecular and whole animal level. An attempt is made to interrelate these and produce one cohesive model.     

A Two-Generation Reproduction Study in Rats Receiving Diets Containing Hexamethylenediamine

A Two-Generation Reproduction Study in Rats Receiving DietsContaining Hexamethylenediamine. SHORT, R. D., JOHANNSEN, F.R., AND SCHARDEIN, J. L. (1991). Fundam. Appl. Toxicol. 16,490–494. Rats received diets containing average dailydoses of 0, 50, 150, and 500 mg/kg/ day of hexamethylenediamineover two generations. Although no treatment-related mortalitywas observed in any of the groups, the weight gain of adultsand pups was slightly reduced in the high dose group. Whilethe litter size was also slightly reduced at birth in the highdose group, there was no adverse effect on survival during lactationin any of the treated groups. Thus, the dietary administrationof up to 150 mg/kg/day of hexamethylenediamine over two generationsdid not adversely affect reproduction or fertility in rats.