Toxicity Of 2,3,7,8-tetrachlorodibenzo-p-dioxin Toxicity Research Articles

Modulatory effect of l-glutamine on 2,3,7,8 tetrachlorodibenzo-p-dioxin-induced liver injury in rats

The aim of this study was to explore the effectiveness of L-glutamine (Gln) in alleviating the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in liver of rats. Rats were intraperitoneally administered Gln and TCDD doses daily for 21 days. In the liver of rats, the biochemical tests, pathological examination and micronucleus (MN) test were performed. TCDD significantly decreased the activities of antioxidant enzymes and serious pathological findings. Moreover, the rate of MNs in hepatocytes increased after treatment with dioxin. In rats treated with Gln alone, the MNs remained unchanged, but the ratio of glutathione (GSH) and the activity of glutathione peroxidase (GSH-Px) were significantly increased. Gln also prevented the suppression of GSH-Px (except for superoxide dismutase and catalase) and GSH in the livers of animals exposed to TCDD and displayed a strong protective effect against MNs. Thus, our findings for Gln might provide new insight into the development of therapeutic and preventive approaches in TCDD toxicity.

A Truncation in the Aryl Hydrocarbon Receptor of the CRL:WI(Han) Rat Does Not Affect the Developmental Toxicity of TCDD

The aryl hydrocarbon receptor (AhR) is required for the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and so the AhR of CRL:WI and CRL:WI(Han) rats was characterized. Western blot showed AhR proteins of approximately 110 and approximately 97 kDa in individual rats from both strains. The AhR cDNA from a CRL:WI(Han) rat with the approximately 110-kDa protein revealed a sequence that was identical to that of the CRL:WI and SD rat. However, cloning of the AhR from a rat with the approximately 97-kDa protein revealed a point mutation, and five variants encoding two C-terminally truncated variants of the AhR protein, arising from a point mutation in the intron/exon junction and consequent differential splicing. These C-terminally truncated variants were expressed and shown to give rise to a protein of approximately 97 kDa; the recombinant AhR bound TCDD with an affinity that was not statistically different from the full-length protein. A single-nucleotide polymorphism assay was developed, and showed that both alleles were represented in a Hardy-Weinberg equilibrium in samples of CRL:WI and CRL:WI(Han) populations; both alleles are abundant. Rats from two studies of TCDD developmental toxicity were genotyped, and the association with toxicity investigated using statistical analysis. There was no plausible evidence that the AhR allele had a significant effect on the toxic endpoints examined. These data show that the two AhR alleles are common in two strains of Wistar rat, and that the AhR alleles had no effect on TCDD-induced developmental toxicity in two independent studies.

Enhancement of Dioxin Toxicity with an Anti-Stress Drug, Carbenoxolone, in Mice

The effect of the carbenoxolone (CBX) on the subacute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied in C57BL/6J mice. The loss of body weight due to TCDD was enhanced by simultaneous treatment with CBX. In agreement with the change in body weight, CBX failed to improve TCDD-induced hepatic hypertrophy and thymic atrophy. Co-treatment of CBX with TCDD tended to cause hepatic hypertrophy more extensively than did TCDD alone, and combined treatment induced significant renal hypertrophy and splenic atrophy which were not seen in mice treated with TCDD or CBX alone. CBX had no effect on the TCDD-mediated induction of hepatic ethoxyresorufin O-deethylase activity, a marker for aryl hydrocarbon receptor (AhR)-linked gene expression. These results suggest that CBX enhances TCDD toxicity by a mechanism(s) distinct from activation of the AhR.

Interference by 2,3,7,8-tetrachlorodibenzo-p-dioxin with cultured mouse submandibular gland branching morphogenesis involves reduced epidermal growth factor receptor signaling

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to mouse embryonic teeth, sharing features of early development with salivary glands in common, involves enhanced apoptosis and depends on the expression of epidermal growth factor (EGF) receptor. EGF receptor signaling, on the other hand, is essential for salivary gland branching morphogenesis. To see if TCDD impairs salivary gland morphogenesis and if the impairment is associated with EGF receptor signaling, we cultured mouse (NMRI) E13 submandibular glands with TCDD or TCDD in combination with EGF or fibronectin (FN), both previously found to enhance branching morphogenesis. Explants were examined stereo-microscopically and processed to paraffin sections. TCDD exposure impaired epithelial branching and cleft formation, resulting in enlarged buds. The glands were smaller than normal. EGF and FN alone concentration-dependently stimulated or inhibited branching morphogenesis but when co-administered with TCDD, failed to compensate for its effect. TCDD induced cytochrome P4501A1 expression in the glandular epithelium, indicating activation of the aryl hydrocarbon receptor. TCDD somewhat increased epithelial apoptosis as observed by terminal deoxynucleotidyl transferase (TdT)-mediated nick end-labeling method but the increase could not be correlated with morphological changes. The frequency of proliferating cells was not altered. Corresponding to the reduced cleft sites in TCDD-exposed explants, FN immunoreactivity in the epithelium was reduced. The results show that TCDD, comparably with EGF and FN at morphogenesis-inhibiting concentrations, impaired salivary gland branching morphogenesis in vitro. Together with the failure of EGF and FN at morphogenesis-stimulating concentrations to compensate for the effect of TCDD this implies that TCDD toxicity to developing salivary gland involves reduced EGF receptor signaling.

Biochemical and molecular biological analysis of different responses to 2,3,7,8-tetrachlorodibenzo- p-dioxin in chick embryo heart and liver

We studied the mechanism of toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in the chick embryo, which is an organism highly sensitive to TCDD. TCDD was injected into egg yolks prior to embryogenesis, and eggs were incubated for 12 or 18 days. In TCDD-exposed embryos, we observed increased heart wet weight and change in the color of the liver, with abnormal fatty vesicle formation. To determine whether these effects were mediated by the aryl hydrocarbon receptor (AhR), we examined expression levels of AhR, CYP1A4, and CYP1A5. AhR was expressed continuously in the heart and liver during embryogenesis, whereas induction of CYP1A4 and CYP1A5 by TCDD was detected only in the liver. In situ hybridization study of tissue sections revealed induction of CYP1A4 in the abnormal liver tissue in which color change was not observed. To determine whether these different responses to TCDD depended on the cell type, primary cultures of chick hepatocytes and cardiac myocytes were established and 7-ethoxyresorufin- O-deethylase (EROD) activity was measured. Induction of EROD activity following exposure to TCDD was detected in hepatocytes but not in cardiac myocytes. Although the heart is a principal target organ for TCDD toxicity and AhR is expressed throughout embryogenesis, induction of CYP1A was not observed in the chick heart. Thus, we conclude that defects in the heart induced by exposure to TCDD occur via a different pathway than that occurring in the liver.

Biochemical and molecular biological analysis of different responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin in chick embryo heart and liver

We studied the mechanism of toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the chick embryo, which is an organism highly sensitive to TCDD. TCDD was injected into egg yolks prior to embryogenesis, and eggs were incubated for 12 or 18 days. In TCDD-exposed embryos, we observed increased heart wet weight and change in the color of the liver, with abnormal fatty vesicle formation. To determine whether these effects were mediated by the aryl hydrocarbon receptor (AhR), we examined expression levels of AhR, CYP1A4, and CYP1A5. AhR was expressed continuously in the heart and liver during embryogenesis, whereas induction of CYP1A4 and CYP1A5 by TCDD was detected only in the liver. In situ hybridization study of tissue sections revealed induction of CYP1A4 in the abnormal liver tissue in which color change was not observed. To determine whether these different responses to TCDD depended on the cell type, primary cultures of chick hepatocytes and cardiac myocytes were established and 7-ethoxyresorufin-O-deethylase (EROD) activity was measured. Induction of EROD activity following exposure to TCDD was detected in hepatocytes but not in cardiac myocytes. Although the heart is a principal target organ for TCDD toxicity and AhR is expressed throughout embryogenesis, induction of CYP1A was not observed in the chick heart. Thus, we conclude that defects in the heart induced by exposure to TCDD occur via a different pathway than that occurring in the liver.

Induction of cytochrome P450 1A is required for circulation failure and edema by 2,3,7,8-tetrachlorodibenzo- p-dioxin in zebrafish

The mechanism of toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) is thought to result from changes in gene expression via the aryl hydrocarbon receptor (AHR). The induction of cytochrome P450 1A (CYP1A) in various organs is a cardinal effect of TCDD. However, whether CYP1A is involved in endpoints of TCDD toxicity is controversial. We investigated the role of CYP1A in TCDD-induced developmental toxicities using gene knock-down with morpholino antisense oligos. Exposure of zebrafish embryos to TCDD, at concentrations eliciting the hallmark endpoints of developmental toxicity, induced CYP1A in the heart and vascular endothelium throughout the body. This induction by TCDD was markedly inhibited by morpholinos to zebrafish arylhydrocarbon receptor 2 (zfAHR2-MO) and to zebrafish CYP1A (zfCYP1A-MO). The zfAHR2-MO but not the zfCYP1A-MO inhibited zfCYP1A mRNA expression, indicating the specificities of these morpholinos. Injection of either zfAHR2-MO or zfCYP1A-MO blocked the representative signs of TCDD developmental toxicity in zebrafish, pericardial edema and trunk circulation failure. The morpholinos appeared do not affect normal development in TCDD-untreated embryos. These results suggest a mediatory role of zfCYP1A induction through zfAHR2 activation in causing circulation failure by TCDD in zebrafish. This is the first molecular evidence demonstrating an essential requirement for CYP1A induction in TCDD-evoked developmental toxicities in any vertebrate species.

Cloning and characterization of the zebrafish ( Danio rerio) aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) mediates the toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and related compounds in vertebrates. To further establish zebrafish as a vertebrate model to study the molecular mechanism of TCDD toxicity, we have isolated and characterized the cDNA encoding the zebrafish aryl hydrocarbon receptor (zfAhR2). Analysis of the deduced protein sequence revealed the 1027 amino acid protein is approximately 200 amino acids longer than previously isolated receptors. zfAhR2 is homologous to previously cloned PAS proteins within the basic helix-loop-helix and PAS domains. The C-terminal domain of zfAhR2 diverges from the mammalian AhR at position 420, and does not contain a Q-rich domain. zfAhR2 mRNA is first detected by Northern blot analysis at 24 h post fertilization, and expression increases throughout early development. Treatment of zebrafish embryos and zebrafish liver cells with graded doses of TCDD results in a dose-dependent increase in zfAhR2 mRNA. The time course for zfAhR2 and cytochrome P4501A mRNA induction by TCDD are similar. In vitro produced zfAhR2 protein dimerizes with the rainbow trout aryl hydrocarbon receptor nuclear translocator (rtARNTb) and binds dioxin response elements derived from the rainbow trout CYP1A gene. Finally, transient coexpression of zfAhR2 and rtARNTb in COS-7 cells results in a TCDD dose-related increase in transcription driven by the rainbow trout CYP1A promoter and enhancer.

Toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in embryos and larvae of the fathead minnow ( Pimephales promelas)

This study evaluates the TCDD toxicity in embryos and larvae of the fathead minnow based on body burden. For the fish embryos, the LOAEL with mild lesions was at a tissue dose of 40 pg TCDD/g. The LD 50 was 25,710 pg TCDD/g. Embryos exhibited gross lesions such as multifocal hemorrhages, edema, and lower jaw malformation. For the one-month-old larvae, the NOAEL was at 3,590 pg TCDD/g. The LD 50 was 70,915 pg TCDD/g. Wasting-type syndrome was observed at a tissue dose of 20,000 pg TCDD/g, and 100% mortality at 163,000 pg TCDD/g.

Comparisons of the effects of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin on chemically impacted and nonimpacted subpopulations of Fundulus heteroclitus: I. TCDD toxicity

AbstractA subpopulation of Fundulus heteroclitus inhabiting the chemically impacted estuary of Newark Bay, New Jersey, was found to be resistant to the toxicity of 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) as compared to a chemically non‐impacted subpopulation of F. heteroclitus in Tuckerton, New Jersey. The Newark/Raritan Bay estuary is a major site of TCDD contamination. The Newark F. heteroclitus embryos, when exposed to 12 to 100 parts per trillion (pptr) [3H]TCDD/acetone (nominal water concentrations) did not exhibit TCDD‐dependent lesions or death. In contrast, the Tuckerton embryos exhibited both TCDD concentration‐related lesions and death that was associated with the lesions. The Newark embryonic resistance to TCDD toxicity could not be attributed to a lesser absorption of [3H]TCDD as compared to the Tuckerton embryos. The Newark F. heteroclitus adult fish, when dermally treated with 0.30 to 60 ng/g [3H]TCDD/DMSO, did not exhibit TCDD treatment‐related increases in death, lesions, or liver‐to‐body‐weight ratios, whereas the Tuckerton adult fish did. The Newark fish did exhibit greater parasitic infestations of the liver, greater hepatic neoplasias, and greater liver‐to‐body‐weight ratios, all of which were independent of laboratory TCDD treatment, as compared to the Tuckerton fish. These hepatic effects in the Newark fish may be indicative of the chronic chemical exposure of this F. heteroclitus subpopulation.

Relationship between acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and disturbance of intermediary metabolism in the Long-Evans rat.

The aim of this study was to examine the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin, (TCDD) in a rat strain other than the Sprague-Dawley (S-D) rat, for which most of our data have been generated thus far. Doses for the biochemical study were selected based on an acute range-finding study, which indicated that Long-Evans (L-E) rats are somewhat less susceptible to TCDD toxicity than are S-D rats. Male L-E rats were dosed orally with 10, 20, 45, 67, 100 and 150 micrograms/kg TCDD. Body weight and feed intake were dose-dependently decreased prior to killing of the animals. Eight days after dosing, animals were killed and tryptophan, total T4 (TT4) and total T3 (TT3) levels were determined in serum, whereas the activities of ethoxy-resorufin-O-deethylase (EROD), phosphoenolpyruvate carboxykinase (PEPCK), gamma-glutamyl transpeptidase (gamma-GT) and tryptophan 2,3-dioxygenase (TdO) were measured in liver. EROD activity was fully induced at all doses studied, indicating that as in S-D rats, Ah-receptor-mediated effects do not seem to play any major role in the acute toxicity of TCDD in this rat strain either. Hepatic PEPCK activity was dose-dependently decreased in a similar dose range as in S-D rats, indicating inhibition of gluconeogenesis. Feed intake was dose-dependently decreased as a result of a dose-dependent elevation in serum tryptophan levels, which in turn were related to reduced liver TdO activity. Hepatic gamma-GT activity was also dose-dependently reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative developmental toxicity of 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD)

In mature animals, the acute toxic potency of TCDD exhibits up to a 5000-fold interspecies variability in the Hartley guinea pig, Sprague-Dawley rat, and Golden Syrian hamster. The embryotoxic and teratogenic potential of TCDD was evaluated in these species to assess whether there is a similar interspecies variability in the developmental toxicity of TCDD. TCDD was a potent developmental toxicant in all three species. Teratogenic activity, consisting of hydronephrosis, was only observed in the hamster. The results indicate that although there are species and tissue specific responses associated with the developmental toxicity and teratogenicity of TCDD, the toxic potency of TCDD is similar (within 10-fold) during prenatal development in these species.

Induction of ethoxyresorufin- O-deethylase and inhibiyion of glucocorticoid receptor binding in skin and liver of haired and hairless HRS/J mice by topically applied 2,3,7,8-tetrachlorodibenzo- p-dioxin

The biochemical changes associated with the toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) have been reported to include alterations in glucocorticoid and epidermal growth factor receptors and mixed function oxidase (MFO) induction. TCDD induces MFO activity in skin of both haired and hairless HRS/J mice. However, epidermal hyperplasia and hyperkeratosis are produced only in the skin of hairless mice. Therefore, since steroid and growth factor responses are implicated in cell proliferation and differentiation, these mice constitute a model system for assessing the possible roles of glucocorticoid and epidermal growth factor receptors in the toxicity of TCDD. The effect of dermal TCDD application (12 μg/kg in 100 μl acetone) on ethoxyresorufin- O-deethylase (EROD) activity, glucocorticoid receptor binding and epidermal growth factor receptors in liver and skin of hairless and haired mice was determined. No differences existed in the basal number of cytosolic glucocorticoid receptors (B max) or the apparent equilbrium binding constants ( K d) in control liver, dorsal skin and abdominal skin of male and female hairless mice and haired male mice. Seven days after topical application of TCDD, decreases of approximately 38% were observed in the hepatic B max of the glucocorticoid receptors in both haired and hairless mice. However, in dorsal skin, TCDD decreased BP max by approximately 40% in hairless mice but only 18% in haired mice. The dexamethasone-glucocorticoid receptor complex from both liver and skin of control and TCDD treated mice had similar sedimentation co-efficients in sucrose density gradients. TCDD had no effect on the K d of glucocorticoid receptors of skin or liver in haired and hairless mice. No difference was observed in the time-dependent increases in hepatic EROD activity between haired and hairless mice after dermal application of TCDD. However, the maximum induction of EROD activity in microsomes from the skin of haired mice was only 60% of the activity observed in hairless animals. The induction of EROD by TCDD did not correlate temporally with the decrease in glucocorticoid receptor binding.

Correlation between acute toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and total body fat content in mammals

Single oral 30-day LD 50s of 2,3,7,8-tetrachlorobibenzo- p-dioxin (TCDD) were correlated with total body fat (TBF) content in various species and strains of laboratory mammals. LD 50 values and TBF contents were either obtained from the leterature or determined by experiments. A log (LD 50) vs. log (TBF) plot yielded a highly significant linear regression equation ( r 2 = 0.834, P < 0.001, n = 20). It is suggested that this correlation exists for at least two reasons: (1) increasing TBF content in organisms represents an enhanced capacity to remove TCDD from the systemic circulation and (2) different TBF content reflects a differential role and regulation of fat metabolism for various organisms. Extrapolation of this correlation to man suggests that adult humans are among the less sensitive species to the acute toxicity of TCDD.

Remarkable residual alterations in responses to feeding regulatory challenges in Han/Wistar rats after recovery from the acute toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD)

Adult male Han/Wistar rats were treated with 1000 μg 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD)/kg body weight and allowed to restabilize their body weight at a lower level. Thereafter, their feeding or drinking responses were determined to the following ip challenges: NaCl (1 m, 10 ml/kg body weight); 2-deoxy- d-glucose (2DG; 400 mg/kg); sodium mercaptoacetate (MA; 800 μmol/kg); 2DG + MA (200 mg/kg + 400 μmol/kg); insulin (10 U/kg). In addition, the suppressive effects of naloxone (10 mg/kg), glucose (1.36 mg/kg) and fructose (1.36 mg/kg) on feed intake stimulated by 24-hr food deprivation were examined. After the restabilization, the body weights of TCDD-treated rats followed the course of body changes in control rats. The responses to NaCl were also similar in TCDD-treated and control rats. However, marked differences were observed in all other responses studied. Pretreatment with TCDD abolished 2DG-induced feeding, attenuated the effects of insulin and naloxone, caused an aberrant decreased in feed intake following MA, and resulted in hypersensitivity to the satiating effects of glucose and fructose. These data show that exposure to a high dose of TCDD leads to notable distortions in responses to metabolic challenges in Han/Wistar rats, which are present even when they have seemingly recovered from the acute toxicity. The results also indicate that the central nervous system plays a crucial role in TCDD toxicity, and suggest hypersensitivity to peripheral satiety signals coupled with hyporesponsiveness to metabolic cues of energy deficit to be important mechanisms in the pathogenesis of the wasting syndrome.

Alteration of the acute toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) by estradiol and tamoxifen

We have hypothesized that part of the toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) is mediated by interaction with the estrogen receptor complex. The experiments reported here investigate the interactions of TCDD with agonists and antagonists of the estrogen receptor. CD-1 female mice were observed for 2 months after treatment with various combinations of corn oil, estradiol, or tamoxifen, and/or TCDD in corn oil on 3 consecutive days. Estradiol had litttle effect on acute TCDD lethality but increased severity of TCDD-induced ascites and antagonized TCDD-induced uterine suppression. Severe liver damage did occur in TCDD and estradiol:TCDD treatment groups. Tamoxifen, a competetive inhibitor and a mixed agonist of the mouse estrogen receptor, antagonized the estrogenic effects of estradiol and estradiol: TCDD. Tamoxifen or tamoxifen: TCDD treatment greatly slowed body weight gain in comparison to controls and estrogen-treated animals. While the dose of tamoxifen used was otherwise non-toxic, tamoxifen greatly increased toxicity of TCDD as measured by time to death and percent lethality while having no effect on relative liver weight or relative uterine weight changes induced by TCDD. These findings are consistent with the hypothesis that a portion of the toxicity of TCDD is manifest through activity of the estrogen receptor complex.

Ah receptor in spleen of rodent and primate species: detection by binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin

In many species systemic toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is manifested by a generalized wasting syndrome accompanied by a variety of specific organ changes including atrophy of the thymus and spleen. TCDD toxicity in most tissues is thought to be mediated by the Ah receptor. Although the spleen is a prime target for TCDD toxicity, the possible presence of Ah receptor in the spleen has not previously been investigated. Specific binding of [3H]TCDD to Ah receptor in spleen cytosols was assessed by velocity sedimentation on sucrose gradients. Ah receptor was detected in spleen cytosols from adult Rhesus monkeys (mean +/- SEM, 36 +/- 8 fmol/mg cytosol protein), fetal Rhesus monkeys (9 +/- 6), Sprague-Dawley rats (20 +/- 5), C57BL/6J mice (18 +/- 2), New Zealand white rabbits (19 +/- 2), and Hartley guinea pigs (15 +/- 2). Ah receptor was not detectable in spleen cytosol from genetically "nonresponsive" DBA/2J mice or from Golden Syrian hamsters, a species resistant to toxicity of TCDD. Molecular properties of Ah receptor from spleen were similar to those of the receptor from liver of the same species. The high Ah receptor content in spleen cytosols from those species that are most susceptible to TCDD toxicity is consistent with the view that the Ah receptor mediates TCDD toxicity in spleen as well as in other tissues.

Corticosterone decreases toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in hypophysectomized rats.

Male Sprague-Dawley rats were hypophysectomized by an established surgical technique. Hypophysectomy aggravated the toxicity (mortality and mean time to death) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 micrograms/kg ip) when compared to sham-operated rats (100% mortality with 9 +/- 1 d mean time to death vs. 90% mortality with 32 +/- 6 d mean time to death, respectively). However, administration of corticosterone (25 micrograms/ml in drinking water) to hypophysectomized rats resulted in an attenuation of the toxicity (40-60% mortality with 40-90 d mean time to death) to a range of TCDD doses (125, 250, 500 micrograms/kg) much higher than the LD50 (about 60 micrograms/kg TCDD) in nonhypophysectomized rats (about 30 d mean time to death). Furthermore, thyroid hormone supplementation in hypophysectomized rats dosed with 125 micrograms/kg TCDD restored the toxicity of TCDD to approximately "normal." Based on these data it is concluded that one or more as yet unknown key factors that are important in the modulation of the toxicity of TCDD reside in the pituitary.

Effects of vitamin A and/or thyroidectomy on liver microsomal enzymes and their induction in 2,3,7,8-tetrachlorodibenzo- p-dioxin-treated rats

Vitamin A and thyroid hormone status have been shown previously to alter the toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) in rats. In the present study, we have examined the effects of a vitamin A-excess and a vitamin A-deficient diet on thyroid hormone levels, on selected drug-metabolizing enzymes in liver microsomes, and on their inducibility by TCDD in male Sprague-Dawley rats. Except for a slight increase in serum T 3 levels, none of these end points was affected by feeding rats the vitamin A-deficient diet. In contrast, excess dietary vitamin A caused a decrease in serum thyroxine (T 4) and triiodothyronine (T 3) levels, although the levels of T 3 remained in the euthyroid range (60–80 ng/dl). The concentration of liver microsomal cytochromes P-450 and b 5 and the basal activity of benzo[ a]pyrene hydroxylase and 7-ethoxyresorufin O-de-ethylase were unaffected by excess dietary vitamin A. This result is consistent with our previous observation that the basal activity of these enzymes is dependent more on T 3 than on T 4 levels. Vitamin A excess markedly suppressed the activity of liver microsomal UDP-glucuronosyl transferase toward 1-naphthol. However, no such enzyme suppression was observed in thyroidectomized rats. This suggests that the suppressive effect of vitamin A on UDP-glucuronosyl transferase activity may be dependent on T 3. Neither vitamin A nor thyroid status had any major effect on the inducibility of UDP- glucuronosyl transferase and cytochrome P-450-dependent enzyme activities by TCDD. However, vitamin A and TCDD had a nearly additive effect on suppression of serum T 4. It is concluded that liver microsomal enzyme induction is not associated with the modulatory effect of vitamin A and thyroid hormones on the toxicity of TCDD.

Composition of diet modifies toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in cold-adapted rats.

The effect of a high carbohydrate, high fat or high protein diet was studied on the acute toxicity of TCDD (125 micrograms/kg) in cold-adapted (4 +/- 1 degrees C) rats. Within 10 days after dosing, TCDD-treated rats fed a high carbohydrate or a high protein diet reduced their caloric intake by 25% whereas those fed a high fat diet consumed only 15% fewer kcal/MBS (metabolic body size). TCDD-treated rats fed a high protein diet lost body weight at the same rate as their pair-fed controls, whereas body weight loss in high fat-fed rats was significantly higher than in their pair-fed controls. In contrast, TCDD-treated rats fed a high carbohydrate diet effectively maintained their body weight in the 4 days immediately after TCDD dosage, whereas their pair-fed controls lost weight. Mortality in TCDD-treated animals was 100% irrespective of the diet; all pair-fed control rats (except one fed a high protein diet) were terminated on days corresponding to the spontaneous death of their TCDD-treated pairs. Mean time to 50% mortality and mean time to death were significantly longer in TCDD-treated rats fed a high carbohydrate diet in comparison with the other two TCDD-treated groups (p less than 0.05), although caloric intake was comparable. Serum triiodothyronine (T3) was reduced in TCDD-treated animals fed a high fat or a high carbohydrate diet but not in those fed a high protein diet; serum thyroxine (T4) was reduced in all the treated groups, irrespective of diet.(ABSTRACT TRUNCATED AT 250 WORDS)