An exact algorithm to find a maximum weight clique in a weighted undirected graph

We introduce a new algorithm MaxCliqueWeight for identifying a maximum weight clique in a weighted graph, and its variant MaxCliqueDynWeight with dynamically varying bounds. This algorithm uses an efficient branch-and-bound approach with a new weighted graph coloring algorithm that efficiently determines upper weight bounds for a maximum weighted clique in a graph. We evaluate our algorithm on random weighted graphs with node counts up to 10,000 and on standard DIMACS benchmark graphs used in a variety of research areas. Our findings reveal a remarkable improvement in computational speed when compared to existing algorithms, particularly evident in the case of high-density random graphs and DIMACS graphs, where our newly developed algorithm outperforms existing alternatives by several orders of magnitude. The newly developed algorithm and its variant are freely available to the broader research community at http://insilab.org/maxcliqueweight , paving the way for transformative applications in various research areas, including drug discovery.

Parallel-ProBiS: fast parallel algorithm for local structural comparison of protein structures and binding sites

The ProBiS algorithm performs a local structural comparison of the query protein surface against the nonredundant database of protein structures. It finds proteins that have binding sites in common with the query protein. Here, we present a new parallelized algorithm, Parallel-ProBiS, for detecting similar binding sites on clusters of computers. The obtained speedups of the parallel ProBiS scale almost ideally with the number of computing cores up to about 64 computing cores. Scaling is better for larger than for smaller query proteins. For a protein with almost 600 amino acids, the maximum speedup of 180 was achieved on two interconnected clusters with 248 computing cores. Source code of Parallel-ProBiS is available for download free for academic users at http://probis.cmm.ki.si/download.

Differences in binding of epidermal growth factor to liver membranes of TCDD-resistant and TCDD-sensitive rats after a single dose of TCDD

Epidermal growth factor (EGF) receptor has been implied as having a role in certain actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). After a single dose of TCDD, the receptor has been shown to be downregulated in several tissues including the liver. Two rat substrains, the Han/Wistar (Kuopio; H/W) rat and the Long-Evans (Turku AB; L-E) rat exhibit over a 1000-fold difference in their sensitivity to the lethal effect of TCDD. This large sensitivity difference was utilized in the current study to investigate whether or not a correlation exists between TCDD lethality and biochemical endpoints related to the hepatic EGF receptor. In the TCDD-sensitive L-E strain both the B(max) of the EGF receptor and the receptor protein as measured by Western blots, decreased dose and time dependently. Ten days after a lethal dose of TCDD (50 μg/kg), the downregulation was 80%. In the resistant H/W strain, two non-lethal doses were used (50 and 500 μg/kg), since the lethal dose is not known. These doses caused a downregulation already at 4 days after dosing, but no further decrease by day 10. The activity of phosphoenolpyruvate carboxykinase (PEPCK, the main gluconeogenetic enzyme in the liver and a proposed target of TCDD) decreased in H/W rats at least to the same extent as in L-E rats at both 4 and 10 days. It is concluded that EGF receptor downregulation is different in the two rat strains studied, despite the fact that a classical Ah receptor-regulated response (CYP1A1 induction) is similar. The results demonstrate that downregulation of the EGF receptor by TCDD is strain-dependent as well as dose- and time-dependent.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on tryptophan and glucose homeostasis in the most TCDD-susceptible and the most TCDD-resistant species, guinea pigs and hamsters

We have previously reported that in rats 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) lethality is associated (although not necessarily causally) with changes in brain serotonin (5-HT) metabolism. In the present study, we have examined whether this holds for other species by comparing the effect of TCDD in the most TCDD-susceptible and the most TCDD-resistant species, guinea pigs and hamsters, respectively. Body weight gain of guinea pigs exposed to TCDD (0.3-2.7 micrograms/kg) diminished dose dependently, while the effect was marginal in hamsters (900-4600 micrograms/kg). Brain 5-hydroxyindoleacetic acid (the main metabolite of brain 5-HT), brain tryptophan (the precursor amino acid of 5-HT), and plasma free and total tryptophan were not affected at any dose in guinea pigs. In contrast, 4 days after exposure, the levels of plasma free and total tryptophan were consistently increased in hamsters. These, as well as brain tryptophan, were still elevated 10 days after exposure. TCDD did not affect plasma glucose level in either species. Liver glycogen was decreased in a dose-dependent manner in TCDD-treated guinea pigs as well as in their pair-fed controls on day 10. There was no change in liver glycogen in hamsters. The activity of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase was only depressed in hamsters by all doses of TCDD. We conclude that changes in tryptophan metabolism or in carbohydrate homeostasis cannot explain the wide interspecies differences in susceptibility to the acute lethality of TCDD, although they may correlate with some aspects of its toxicity in certain species.

Reduced activity of tryptophan 2,3-dioxygenase in the liver of rats treated with chlorinated dibenzo-p-dioxins (CDDs): dose-responses and structure-activity relationship

The activity of tryptophan 2,3-dioxygenase (TdO) was measured in the livers of male Sprague-Dawley rats after acutely toxic doses (LD20-LD80) of chlorinated dibenzo-p-dioxins (CDDs) with 4 of the up to 7 chlorine substituents occupying the 2,3,7,8-positions. Treatment with toxic doses of CDDs results in voluntary feed refusal of rats. A corresponding involuntary reduction of feed intake in naive animals (pair-feeding) causes elevated levels of TdO activity. In the CDD treated rats, however, TdO activities were dose-dependently reduced. An LD80 reduced TdO activity to about 50% of the level found in the corresponding pair-fed animals. This decrease of TdO activity explains the dose-dependent increase of serum tryptophan, which in turn is the likely cause of voluntary feed refusal observed in CDD-treated rats. The activity of another enzyme which is regulated in a fashion very similar to that of TdO, viz., tyrosine aminotransferase (TAT), was consistently, but not dose-dependently, affected by treatment with CDDs.

Tissue distribution and toxicokinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin in rats after intravenous injection

Male Sprague-Dawley rats (240-290 g) received intravenously a nonlethal (9.25 micrograms/kg) or a lethal (72.7 micrograms/kg) dose of 14C-labeled 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administered as an emulsion. Animals were euthanized between 5 min and 16 days (lethal dose) or 32 days (nonlethal dose) after treatment. Tissue distribution was considered complete after 24 hr, as by this time radioactivity levels in white adipose tissue had reached a maximum. The highest levels of radioactivity were found in liver (5% of dose/g tissue), followed by white fat (1% of dose/g tissue); serum was lowest at 0.01% of dose/ml serum. Relatively high levels of radioactivity were also detected in most known target organs of TCDD toxicity, e.g., brown fat, adrenals, and thyroid. The pattern of organ distribution of TCDD was essentially the same after the lethal and the nonlethal dose, but did not follow a simple lipophilicity relationship, as levels in liver were higher than those in white fat, and those in brain were extremely low. A pool of TCDD in liposomes initially trapped in lung and spleen was redistributed within 24 hr mainly to liver and adipose tissue. Affinity of TCDD to storage fat seemed to play a more important role as a driving force for redistribution than did induction of cytochrome P450 1A2. The terminal slope of elimination of TCDD from tissues indicated a half-life of 16 days after the nonlethal dose. After the lethal dose radioactivity declined in all tissues for 2 to 8 days and then increased again, reflecting shrinking tissue volumes as well as remobilization of TCDD caused by the process of body mass wasting. Distribution data for 17 tissues and serum were subjected to regression analysis and resulted in up to two uptake phases and up to three elimination phases for a given tissue. After the nonlethal dose TCDD was mainly excreted via feces; combined urinary and fecal excretions occurred with a biological half-life of 16.3 +/- 3.0 days. Much longer half-lives were detected in white fat and skin. After the lethal dose, the fecal excretion of TCDD-derived radioactivity decreased after 8 days, and urinary excretion increased starting 12 days after dosing. Radioactivity in liver and white fat and the extractable portion in feces was mainly unchanged TCDD, as determined by thin-layer chromatography. Radioactivity in urine indicated the presence of a metabolite(s) of TCDD only.

A review of the relationship between acute toxicity (LC50) of gamma-hexachlorocyclohexane (gamma-HCH, Lindane) and total lipid content of different fish species

This paper provides an explanation for a 40-fold difference in the acute toxicity (LC50) of gamma-hexachlorocyclohexane (gamma-HCH, Lindane) in 14 different fish species, based on well recognized principles of toxicokinetics and toxicodynamics in combination with a compilation of data from the literature and some original data. The 48-h median lethal concentration (48-h LC50) of gamma-HCH in 14 fish species, belonging to 6 families, range from 22 to 900 micrograms/l. A significant positive linear relationship was found between lipid content (% of wet weight) and the 48-h LC50 of gamma-HCH in these fish species, revealing that the toxicity of gamma-HCH in various fish species is decreasing with increasing total lipid content. If median lethal concentrations are normalized for 1% lipid content, then the range of 48-h LC50s is reduced to between 18 and 32 micrograms/l. It is concluded that lipids of aquatic organisms can serve (among other functions) as a protective storage site against the toxic effects of gamma-HCH and, possibly, of other lipophilic, persistent organic chemicals which are bioconcentrated in body lipids. Therefore, in organisms with higher lipid content, a smaller fraction of a lipophilic chemical will reach target organs (liver, lung, central and peripheral nerves, etc.) to cause adverse effects. Results suggest that this correlation can be used to extrapolate the acute toxicity (48-h LC50) of gamma-HCH to other fish species if their lipid content is known. Furthermore, the data generated by extrapolation of this correlation could be useful in the environmental risk assessment of freshwater and marine organisms.

A pharmacodynamic model of triglyceride transport and deposition during feed deprivation or following treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the rat

A simplified model of fat metabolism was developed to predict fat synthesis, transport, deposition, and metabolism as a function of feed consumption, as a means of testing mechanisms proposed for the disruption in fat metabolism caused by TCDD. Rates of triglyceride (TG) synthesis and lipolysis were made directly dependent on daily average levels of feed consumption, which could be constant or follow other patterns. Seven compartments, including plasma very low density lipoprotein (VLDL) (1), plasma free fatty acid (FFA) (2), plasma VLDL remnants (3), lipoprotein lipase (LPL)-VLDL complex (4), liver TG (5), white adipose tissue (WAT) TG (6), and brown adipose tissue (BAT) TG (7) were required to obtain realistic rates of TG deposition in storage fat while maintaining normal levels of plasma TGs and FFA. The steady-state level of hepatic TG was very sensitive to the rate of storage fat lipolysis and hepatic VLDL export. Because plasma TG is taken up very rapidly by extra-hepatic tissues, reduction of WAT uptake rates had a greater effect on plasma TG than increasing rates of hepatic synthesis and VLDL export. Our results indicate that neither increased hepatic VLDL synthesis nor a simple inhibition of WAT LPL can, by themselves, explain the combined occurrence of hyperlipidemia and loss of fat observed in TCDD-treated animals. However, a TCDD-mediated enhancement of WAT TG lipolysis and consequential physiological responses in liver and BAT yield results compatible with experimental data.

Reduction of hepatic phosphoenolpyruvate carboxykinase (PEPCK) activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is due to decreased mRNA levels

We have previously shown that the rate of hepatic gluconeogenesis is reduced in TCDD-treated rats and that this decrease in carbohydrate production is associated with a dose-dependent reduction of the activity of PEPCK, the rate limiting enzyme of gluconeogenesis. This derailment of glucose metabolism has been suggested to be the critical lesion in acute TCDD toxicity. To further elucidate the mechanism of decreased PEPCK activity we performed Northern blot analyses using a cDNA probe complementary to a portion of the mRNA coding for PEPCK. We have demonstrated that 4 and 8 days after TCDD treatment (125 micrograms/kg, p.o.) liver PEPCK mRNA in Sprague-Dawley rats was decreased to very low levels as compared to vehicle-treated and pair-fed control animals. This decline of PEPCK mRNA was paralleled by decreased levels of PEPCK protein, as revealed by Western blot analyses and was accompanied by a reduction in the enzymatic activity of PEPCK. These results indicate that the decrease of PEPCK activity by TCDD is most likely the result of decreased expression of the PEPCK gene. These together with previous results also suggest that many of the physiological responses occurring in TCDD-treated animals (reduced feed intake, decreased insulin, increased corticosterone, increased glucagon and cAMP levels) which would normally stimulate PEPCK gene expression, are ineffective. Furthermore tryptophan 2,3-dioxygenase (TdO) activity, which is regulated in a very similar fashion to PEPCK activity, is also reduced after TCDD treatment, suggesting a common mechanism by which TCDD alters the regulation of these enzymes. P-450 1A1 mRNA and related EROD activity were maximally induced under the conditions of these experiments and represent a positive control for TCDD-related alterations of gene expression. However, because of differences in the dose-response characteristics of TCDD-induced reduction of PEPCK activity and induction of EROD activity an involvement of the Ah receptor in the reduction of PEPCK activity cannot be postulated.

Relative potency of chlorinated dibenzo-p-dioxins (CDDs) in acute, subchronic and chronic (carcinogenicity) toxicity studies: implications for risk assessment of chemical mixtures

This paper shows that the relative toxic potency of four chlorinated dibenzo-p-dioxins (CDDs) is similar in two species with different sensitivities (guinea pig, Sprague-Dawley rat). More importantly, it also demonstrates that the relative toxic potencies of these homologues are very similar for acute, subchronic and chronic dosing in the same species (rat). Furthermore, examination of different endpoints of toxicity (mortality, porphyria, carcinogenicity) suggests that the dose-responses for these diverse end-points after acute, subchronic, and chronic administration are very similar if not identical for tetra-CDD. Based on toxicokinetic and toxicodynamic considerations, a new, possibly generalizable rule (average tissue concentration x time = toxicity) is derived for CDDs. Implicit in the relative potency arguments of CDDs is the requirement of a practical threshold dose for all endpoints of toxicity including cancer.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture. Part I: Acute toxicity and toxic equivalency factors (TEFs)

There is presently no scientifically proven method to assess the toxicity of environmental samples containing complex mixtures of chlorinated dibenzo-p-dioxins (CDDs) of known composition. Their risk assessment is currently based on the interim concept of toxicity equivalency factors (TEFs), with the unproven assumption that all interactions of CDDs are additive. To address this problem we conducted acute toxicity studies with four different CDDs, viz 2,3,7,8-tetrachlorodibenzo-p-dioxin (tetra-CDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (penta-CDD), 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (hexa-CDD) and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (hepta-CDD), all containing chlorine substituents in the crucial 2,3,7,8-positions. The homologues, dissolved in corn oil/acetone, were administered to groups of five male Sprague Dawley rats at several doses (at least three) by gastric intubation. The obtained mortality data were employed to calculate the LD20,50 and 80 for each homologue. These data were subsequently used to prepare equipotent doses (expected mortality of 20, 50 and 80%) of a mixture containing all four homologues, each of them contributing one fourth of the toxicity, under the assumption of additive toxicity. The obtained LD50 value and (TEF) was for tetra-CDD 43 micrograms/kg (1), penta-CDD 206 micrograms/kg (0.2) hexa-CDD 887 micrograms/kg (0.05) and hepta-CDD 6325 micrograms/kg (0.007), respectively. The dose-response to the mixture confirmed the hypothesis of strict additivity in the acute toxicity of the four CDD homologues.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture. Part III: Structure-activity relationship with increased plasma tryptophan levels, but no relationship to hepatic ethoxyresorufin o-deethylase activity

Male Sprague-Dawley rats were treated with an LD20, an LD50, and an LD80 of 2,3,7,8-tetrachlorodibenzo-p-dioxin (tetra-CDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (penta-CDD), 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (hexa-CDD), 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (hepta-CDD), respectively, and a mixture of the four homologues where each CDD was represented at one-fourth its previously established LD20, LD50, and LD80, respectively. Plasma tryptophan levels, liver ethoxyresorufin O-deethylase (EROD) activities, and liver weights were determined at 2 and 8 days after treatment. Plasma tryptophan levels were dose-dependently elevated, particularly at 8 days after treatment, by as much as 75% over control levels. EROD activity in CDD-treated animals was induced 27- to 28-fold, as compared with vehicle-treated controls, but did not show any dose-response. Liver weights were also significantly increased by the CDD treatments, but the increase was not dose related. There was no correlation between plasma tryptophan levels, a biomarker of acute toxicity of CDDs, and EROD activity, a biomarker of arylhydrocarbon (Ah) receptor-mediated enzyme induction. It is concluded that the acute toxicity of CDDs, which correlates and shows perfect structure-activity relationship with reduced activities of key enzymes of intermediary metabolism, and the induction of enzymes by much lower doses of CDDs in the liver, have different mechanisms of action.

Comparative toxicity of four chlorinated dibenzo-p-dioxins (CDDs) and their mixture

Male Sprague-Dawley rats were treated with an LD20, LD50 and LD80 respectively, of tetra-, penta-, hexa-, hepta-CDD and a mixture of the four CDDs, all carrying chlorine substituents in the biologically crucial 2, 3, 7, and 8 positions. Specific activities of two key enzymes of gluconeogenesis, viz, phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase (PC), as well as the activity of the preneoplastic marker enzyme gamma-glutamyl transpeptidase (gamma-GT), were determined in livers of CDD-treated and ad libitum-fed control animals. PEPCK activity showed evidence for dose-related inhibition on the second day after dosing; PC activity was slightly reduced, whereas gamma-GT activity was dose-dependently inhibited. By 8 days after dosing PEPCK activities were dose-dependently decreased after administration of all four CDDs and their mixture. PC activities were significantly reduced, but no dose-response was evident. The activity of gamma-GT was dose-dependently inhibited, but only to a value of 25% below control activities. It is concluded that CDDs share a common mechanism of acute toxicity, viz, inhibition of glucocorticoid-dependent enzymes which results in a derailment of intermediary metabolism not compatible with survival of the animals.

Decontamination of human skin exposed to 2,3,7,8-tetrachlorodibenzo-p-Dioxin (TCDD) in vitro

Human post-mortem skin was exposed in vitro to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at 32 degrees C, under controlled humidity. In one-half of the samples, damage to the surface of the skin was simulated by stripping of the stratum corneum. After incubation with TCDD for 100 min, four different decontamination protocols were performed: (1) the sample was wiped with dry, adsorbent material (cotton balls); (2) a 10-min topical treatment with mineral oil was followed by dry wiping with cotton balls; (3) a 10-min topical treatment with mineral oil was followed by wiping with acetone-soaked cotton balls; and (4) the sample was washed with water and soap. After decontamination, skin samples were incubated (up to 300 min) again at 32 degrees C. One set of both intact and stripped TCDD-exposed skin samples was incubated for 300 min--absent decontamination--and was used as a control. Mineral oil treatment and acetone wipes, or water and soap, were effective in reducing (i.e., about two-fold) the amount of TCDD in the stratum corneum of intact skin. Mineral oil plus dry wipes reduced the amount of TCDD in the stratum corneum by about one-third, whereas dry wiping alone was ineffective. All protocols, however, were similarly effective in reducing the amount of TCDD in the epidermis and upper dermis; TCDD concentrations were decreased locally by factors of up to ten. In the lower dermis, a minimal effect of the decontamination procedures was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased hepatic phosphoenolpyruvate carboxykinase gene expression after 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment: implications for the acute toxicity of chlorinated dibenzo-p-dioxins in the rat

Decreased activity of the rate limiting gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK), has been recently suggested to be the critical lesion in the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We now show that other toxicologically relevant chlorinated dibenzo-p-dioxins (CDDs), with chlorine substituents in the crucial 2-,3-,7-, and 8-positions, exert the same effect on PEPCK activity. The doses required to cause this enzyme inhibition are within the acutely toxic range for each homologue, suggesting the same mechanism of action for these compounds. To further investigate the mechanism whereby dioxins decrease PEPCK activity, Northern analysis was performed using a cDNA probe complementary to a portion of the PEPCK mRNA. We could demonstrate that after TCDD treatment hepatic PEPCK mRNA was decreased by as much as 90% compared to pair-fed control animals (day 8 after dosing). This decrease in PEPCK mRNA was paralleled by a decrease of the amount of PEPCK protein and enzymatic activity. These results indicate that the physiological changes which occur in TCDD-treated animals (decreased feed consumption, low plasma insulin and elevated plasma corticosterone levels) which under normal conditions increase PEPCK gene expression and enzyme activity, are not effective in stimulating PEPCK synthesis in TCDD-treated animals.

Depletion of brain serotonin does not alter 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced starvation syndrome in the rat

We have previously reported a series of biological events in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-intoxicated rats which resulted in elevated brain serotonin (5-HT) levels, offering a possible explanation of the acute toxicity (reduced feed intake and death) in these animals. It was thus hypothesized that depletion of central 5-HT stores should alter the TCDD-induced starvation syndrome. Brain 5-HT was selectively depleted by intracerebroventricular infusions of the neurotoxin 5,7-dihydroxytrytamine (5,7-DHT). Subsequently the animals were given a lethal dose of TCDD. In rats treated with 5,7-DHT hypothalamic 5-HT was depleted up to 90% compared to control animals, yet TCDD induced the expected reduction of bodyweight and feed intake. These results suggest that although TCDD increases central 5-HT levels as a result of increased plasma tryptophan, this may not be the main cause for reduced feed intake and lethality in these animals.

Penetration, distribution and kinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin in human skin in vitro

The in vitro penetration of 3H-labeled 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) into human cadaver skin was studied at concentrations of 65 and 6.5 ng TCDD per cm2 of skin surface. Vehicles used were acetone to simulate exposure to TCDD as a dry material, and mineral oil to simulate exposure to TCDD in an oily medium. Penetration was performed for 30, 100, 300, and 1000 min in improved Franz cells. Skin was used either intact, or with stripped horny layer. Skin was sectioned along its natural layers and radioactivity determined in epidermis and dermis. TCDD did not readily penetrate into human skin in vitro. The vehicle of exposure to TCDD played an important role in dermal penetration. The rapidly evaporating acetone allowed TCDD to penetrate deeply into the loose surface lamellae of the horny layer, but then appeared to be poorly available for further penetration. Mineral oil as the vehicle, on the other hand, represented a lipophilic compartment which competed with lipophilic constituents of the stratum corneum for TCDD and hence slowed its penetration even more. The stratum corneum acted as a protective barrier, as its removal increased the amount of TCDD absorbed into layers of the skin. Hourly rates of absorption of TCDD per unit area of skin were calculated in two ways: a worst case scenario where TCDD absorbed into any layer of skin including the stratum corneum was used for regression analysis; and a physiological approach where only that amount of TCDD was considered absorbed which had penetrated beyond the epidermis into the region of dermal vascularization.(ABSTRACT TRUNCATED AT 250 WORDS)

Key enzymes of gluconeogenesis are dose-dependently reduced in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

Male Sprague-Dawley rats (240-245 g) were dosed ip with 5, 15, 25, or 125 micrograms/kg -,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in corn oil. Ad libitum-fed and pair-fed controls received vehicle (4 ml/kg) alone. Two or 8 days after dosing five rats of each group were sacrificed, their livers removed and assayed for the activities of three gluconeogenic enzymes [phosphoenol-pyruvate carboxykinase (PEPCK; EC 4.1.1.32), pyruvate carboxylase (PC; EC 6.4.1.1), and glucose-6-phosphatase (G-6-Pase, EC 3.13.9)], and one glycolytic enzyme [pyruvate kinase (PK; EC 2.7.1.40)] by established procedures. The activity of PK was not affected by TCDD at either time point. The activity of G-6-Pase tended to be decreased in TCDD-treated animals, as compared to pair-fed controls, but the decrease was variable without an apparent dose-response. The activity of PEPCK was significantly decreased 2 days after dosing, but a clear dose-response was apparent only at the 8-day time point. Maximum loss of activity at the highest dose was 56% below pair-fed control levels. PC activity was slightly decreased 2 days after TCDD treatment and displayed statistically significant, dose-dependent reduction by 8 days after dosing with a 49% loss of enzyme activity after the highest dose. It is concluded that inhibition of gluconeogenesis by TCDD previously demonstrated in vivo is probably due to decreased activities of PEPCK and PC. The data also support the prevailing view that PEPCK and PC are rate-determining enzymes in gluconeogenesis.

Reduced activities of key enzymes of gluconeogenesis as possible cause of acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats

Male Sprague--Dawley rats (350-375 g) were injected i.p. with TCDD (25 [sublethal dose] and 125 micrograms/kg [lethal dose], respectively, in corn oil/acetone), or vehicle only; vehicle-treated animals were pair-fed to their TCDD-treated counterparts. 1, 2, 4, 8, 16, and 32 days (28 days for lethal dose) thereafter, animals were sacrificed and activities of two key enzymes of gluconeogenesis determined in livers of rats. In livers of pair-fed rats both enzyme activities were little affected. In the livers of TCDD-treated animals the activity of phosphoenolpyruvate carboxykinase (PEPCK, EC 4.1.1.32) decreased rapidly, exhibiting significant losses by the 2nd day after treatment. Time course and extent of loss of PEPCK activity (about 50%) were similar after either dose. The activity of glucose-6-phosphatase (G-6-Pase, EC 3.1.3.9) decreased more slowly as a result of TCDD treatment; statistically significant losses were observed by 4 or 8 days after the lethal and sublethal dose, respectively. These results confirm the hypothesis that reduced in vivo rates of gluconeogenesis in TCDD-treated rats are due to decreased activities of gluconeogenic enzymes. In an additional set of experiments, rats were treated with 125 micrograms/kg TCDD, 25 micrograms/kg TCDD, or with vehicle alone. The 25 micrograms/kg or vehicle-treated rats were then pair-fed to rats dosed with 125 micrograms/kg of TCDD. Mean time to death and body weight loss at the time of death were essentially identical in all groups, lending additional support to the hypothesis that reduced feed intake is the major cause of TCDD-induced death in male Sprague--Dawley rats. Both appetite suppression and reduced total PEPCK activity in whole livers occurred in the same dose-ranges of TCDD, suggesting the possibility of a cause-effect relationship.

Is a serotonergic mechanism involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced appetite suppression in the Sprague-Dawley rat?

The major cause of TCDD-induced death in rats is a progressive voluntary feed refusal which has been correlated with reduced gluconeogenesis. Since centrally administered TCDD does not cause death or decreased feed intake in rats, the ability of TCDD to suppress appetite via peripheral mechanisms acting on the central nervous system was examined in two experimental models. First, it was found that the feed intake of rats on scheduled feeding cycles was not decreased by blood transfused from rats with TCDD-induced appetite suppression (8 days after a lethal dose of TCDD, i.p.). In contrast, a similar transfusion from normal, satiated rats did reduce feed intake of recipient rats by approximately 40%, suggesting that TCDD-treated rats are not satiated but rather that they are not hungry. In the second study tryptophan (the amino acid precursor of the neurotransmitter serotonin) was measured in the plasma and tryptophan, serotonin, norepinephrine and dopamine in the hypothalamus as well as dopamine and its metabolites in the striatum 4, 8, and 16 days after TCDD dosage (125 micrograms/kg, i.p.). Progressive time-dependent increases in tryptophan levels in plasma and brain were paralleled by increases in brain serotonin and 5-hydroxyindoleacetic acid (the primary metabolite of serotonin) in TCDD-treated rats. No changes were observed regarding the other biogenic amines. It is suggested based on these data and on substantial evidence from the published literature that a serotonergic mechanism may be involved in TCDD-induced feed intake reduction.

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

Single oral 30-day LD50s of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were correlated with total body fat (TBF) content in various species and strains of laboratory mammals. LD50 values and TBF contents were either obtained from the literature or determined by experiments. A log (LD50) vs. log (TBF) plot yielded a highly significant linear regression equation (r2 = 0.834, P less than 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.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced appetite suppression in the Sprague-Dawley rat is not a direct effect on feed intake regulation in the brain

The most striking sign of acute toxicity of TCDD in animals is a progressive reduction of feed intake, accompanied by loss of body weight eventually resulting in death. The mechanism(s) of this voluntary feed refusal is (are) not known but it is generally accepted that both centrally and peripherally (via feedback) acting anorectic agents exert their effect(s) in the hypothalamus. In this study direct administration into the lateral cerebral ventricle of rats resulted in much higher concentrations of TCDD in the hypothalamus and also in other regions of the brain than after a lethal intravenous (iv) injection. While rats injected iv displayed the expected cachectic syndrome, intracerebroventricularly (icv)-dosed animals ate and gained weight normally. These findings preclude the possibility of a direct effect of TCDD on appetite-regulating areas of the brain. Moreover, these results require the assumption that the appetite suppressive effect of TCDD is due to a (feedback) mechanism originating in the periphery.

Reduced gluconeogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

The effect of a usually lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 micrograms/kg) was studied on the conversion of 14C-alanine into 14C-glucose in male Sprague-Dawley rats by established procedures (determination of plasma alanine and blood glucose by enzymatic assays and isolation of 14C-alanine and 14C-glucose from whole blood by column chromatography). TCDD-treated rats converted significantly (p less than 0.05) less 14C-alanine into 14C-glucose than did their pair-fed or ad libitum-fed counterparts, indicating reduced gluconeogenesis as a result of TCDD treatment. This finding suggests that reduced gluconeogenesis in TCDD-treated rats contributed to the progressively developing, severe hypoglycemia observed in these animals. Corticosterone, a key hormone in gluconeogenesis, provides partial protection from TCDD-induced toxicity in hypophysectomized rats. Therefore, the conversion of 14C-alanine into 14C-glucose was also determined in hypophysectomized rats dosed with TCDD (125 micrograms/kg) and given corticosterone (25 micrograms/ml in drinking water). These rats also converted significantly (p less than 0.05) less 14C-alanine into 14C-glucose than did their pair-fed counterparts. However, in contrast to non-hypophysectomized TCDD-treated rats, these rats maintained marginal normoglycemia even at 64 days after dosing with TCDD, which suggests that the partial protective effect of corticosterone in hypophysectomized, TCDD-treated rats is unrelated to its effect on gluconeogenesis. The protection provided by corticosterone supplementation in TCDD toxicity is more likely due to reduced peripheral utilization of glucose enabling the animals to maintain marginal normoglycemia.

Target tissue morphology and serum biochemistry following 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in a TCDD-susceptible and a TCDD-resistant rat strain

The mode of action of the highly toxic environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is unknown. It was recently discovered that two strains of rat, Long-Evans (L-E) and Han/Wistar (H/W), differ widely in susceptibility to TCDD. Employing this strain divergence as a probe, the present study set out to assess the role of various biochemical and morphological effects in TCDD lethality. In the main experiment, the rats were treated once ip with 0,5,50, or (H/W) 500 micrograms/kg TCDD and killed 1 to 16 days postexposure. Several target organs were evaluated by light microscopy and a number of serum lipid and carbohydrate parameters as well as a few major regulatory hormones were analyzed. The results demonstrated that most alterations caused by TCDD were essentially similar in both strains. TCDD reduced circulating thyroxine to a slightly greater extent and more permanently in the sensitive L-E strain. Moreover, a highly significant interaction on thyroid-stimulating hormone was found among strain, dose, and time. Serum concentrations of corticosterone and free fatty acids were increased only in the L-E rats given 50 micrograms/kg TCDD, i.e., at an apparent LD100 dose level for this strain. Yet, the most striking interstrain difference was seen in the liver which was distinctly affected after Day 4 in L-E rats given 50 micrograms/kg TCDD but only marginally affected in rats from any H/W group. The lesion, while showing no necrotic cell changes, was suggestive of plasma membrane damage, possibly reflecting the production of free radicals. The relation of the findings to possible mechanisms of TCDD action is discussed.

A critical view of the mechanism(s) of toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Implications for human safety assessment

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been an important issue in occupational and environmental health for nearly two decades. During this period scientists have studied its possible impacts on exposed human populations. At the same time enormous efforts were made to elucidate the mechanism of TCDD action in various biological models. This paper provides a critical view of the advances made towards understanding the mechanism of TCDD action. Major topics discussed include the Ah-receptor hypothesis, TCDD as a thyroid hormone agonist, TCDD and vitamin A deficiency, TCDD's effect on receptor regulation, and its effect on intermediary metabolism including related hormonal responses. Although the exact mechanism of TCDD action is not yet known, more information is available on the toxicity of this compound than perhaps on that of any other substance. This wealth of information allows important conclusions regarding the assessment of acute, as well as of chronic, toxicities of TCDD for humans. There is no documented case of human death as a result of exposure to TCDD. It appears that humans are acutely less sensitive to TCDD than some animal species. The cause of TCDD-induced lethality in rats is a progressive lethal hypoglycemia due to inhibition of gluconeogenesis. Regulation of this metabolic pathway is quite different amongst species, although primates share great similarities. The assumption that the cause of TCDD-induced death in primates, in analogy to rats, is inhibition of gluconeogenesis would suggest that the acute toxicity of TCDD in humans would be in the range seen in rhesus monkeys (70-300 micrograms/kg). These values are about midway between the most (guinea pig) and least (hamster) sensitive species. TCDD is not a genotoxic agent and not an initiator, but promoter of tumor formation. There is considerable evidence that promotion of cancer, like any other chronic end point of toxicity, is a threshold-type biological process. Therefore, a linear extrapolation of the dose-response is an unnecessarily conservative approach in the safety assessment of TCDD. This paper, based on several studies with different end points of toxicity, supports the notion that 10 pg/kg/day of TCDD represent a safe lifetime exposure level for humans with regard to promotion of cancer, porphyria and chloracne.

Mode of metabolism is altered in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

Male Sprague-Dawley rats were fed either a high-fat (HF) or a high-carbohydrate (HC) diet and subsequently injected with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (125 micrograms/kg) or vehicle (pair-fed controls). In all TCDD-treated animals, a reduction in caloric intake was evident as early as 1 day after dosage. Respiratory quotients (RQ) were determined at 5-day intervals. Their pattern for the HC-fed but not for the HF-fed TCDD-treated rats was different from that of the corresponding pair-fed controls. After an initial parallel decrease the RQ values remained low for TCDD-treated rats whereas they increased again for pair-fed controls. Serum total thyroxine (T4) was significantly lower in TCDD-treated animals and this reduction was not influenced by the composition of the diet. Serum triiodothyronine (T3) was neither altered by diet nor by TCDD. Thymic atrophy was as severe in pair-fed as in TCDD-treated rats fed the HC diet but not in rats fed the HF diet. Our results suggest that TCDD-treated rats are in a different mode of metabolism from pair-fed rats and that this difference is related to gluconeogenesis.

Toxicity of 8-methoxypsoralen in cynomolgous monkeys (Macaca fascicularis)

Male and female cynomolgous monkeys were administered 0, 2, 6 or 18 mg/kg 8-methoxypsoralen (8-MOP) 3 times a week orally for 26 consecutive weeks. Dose-dependent emesis was the most sensitive indicator of 8-MOP toxicity. The lowest dose to elicit emesis was 3 x 6 mg/kg/week of 8-MOP. Among the histological findings proliferation of Kupffer cells was the only recurring observation. However, these finding as well as some hematological and serum electrolyte changes lacked a dose-response relationship. In the highest dosage group one female monkey was found in moribund condition on the 39th day of the study and was killed. Histopathological evidence indicated beginning shock as the cause of the rapidly deteriorating health of the monkey. Similar to effects in man and rats, 8-MOP displayed nonlinear pharmacokinetics in the cynomolgous monkey, saturation occurring between 3 x 2 and 3 x 6 mg/kg/week. Increased clearance of 8-MOP in the lowest dosage group after 26 test weeks was attributed to a combination of enzyme induction and saturable first pass effect. Since the plasma profile of 8-MOP at the lowest dose (3 x 2 mg/kg/week) in cynomolgous monkeys closely resembles that in humans after therapeutic doses (0.4-0.6 mg/kg) and because of other similarities (vomiting as earliest sign of toxicity, saturable first pass effect), it is reasonable to assume that chronic toxicity of 8-MOP as defined in this study is quite predictive for man.

Elevated plasma corticosterone levels and histopathology of the adrenals and thymuses in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

The relationship between thymic atrophy and plasma corticosterone levels was examined in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated, pair-fed and ad libitum-fed male Sprague-Dawley rats given a usually lethal (125 micrograms/kg) or non-lethal (25 micrograms/kg) dose of TCDD. At both dosages, corticosterone levels in TCDD-treated animals begun to rise as early as day 4 after treatment. At later time points corticosterone levels were 5-7 times higher in rats given the non-lethal dose, and 6-10 times higher in rats administered the lethal dose than the levels observed in ad libitum-fed controls. Corticosterone levels in control rats pair-fed to the lethal dose group (as a result of the severe reduction in feed intake) were similarly elevated as in TCDD-treated rats but this was not the case in pair-fed rats of the non-lethal TCDD dosage (due to an essentially unchanged feed intake). At both dosages, relative thymus weights of TCDD-treated rats started decreasing by day 4 and continued to decline for the most part of the study. Relative thymus weights of rats pair-fed to the non-lethal TCDD dosage were not different from ad libitum-fed rats. However, the decrease in relative thymus weights of rats pair-fed to the lethal TCDD dosage paralleled that of TCDD-treated rats with an apparent 8-day lag period. Morphologically, the thymus as well as the adrenal revealed differential changes in TCDD-treated rats from those observable in pair-fed rats. These results suggest that either TCDD exerts a direct effect on the thymus and the adrenals or it causes an additional stress (e.g., a metabolic stress) over and above the starvation stress, which may be responsible for the differential morphological changes in these glands.

Corticosterone modulates acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in male Sprague-Dawley rats

Bilateral adrenalectomy or adrenal demedullation was performed on male Sprague-Dawley rats by established surgical techniques. Subsequently, the dose-response (mortality and mean time to death) to TCDD was determined in adrenalectomized (10, 20, 40 micrograms/kg TCDD ip in 95:5 corn oil:acetone) or demedullated (15, 30, 60 micrograms/kg TCDD) rats. Adrenalectomy drastically increased mortality and greatly shortened mean time to death after dosing with TCDD. More importantly, adrenalectomized TCDD-treated rats died of hypoglycemic shock without losing much body weight. Conversely, adrenal demedullation had no effect on mortality or mean time to death caused by TCDD when compared to nondemedullated TCDD-treated controls. Thus, it was concluded that the factor(s) modulating the acute toxicity of TCDD resides in the adrenal cortex and not in the medulla. Administration of corticosterone (25 micrograms/ml in drinking water) to adrenalectomized rats returned the toxicity of TCDD to levels seen in nonadrenalectomized rats suggesting that this hormone is another key factor (in addition to the thyroid hormones) in the modulation of the acute toxicity of TCDD. Corticosterone supplementation (25, 50, or 100 micrograms/ml) to nonadrenalectomized rats, or to thyroidectomized-adrenalectomized rats (25 micrograms/ml), resulted in no additional beneficial effect indicating that a factor(s) other than thyroid hormones and corticosterone is also involved in the acute toxicity of TCDD.

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.

Elimination of [14C]heptachlor from body stores of lactating ewes treated with ovine growth hormone

Elimination of [14C]heptachlor from body burdens of sheep was measured using mature ewes nursing single offspring, and the influence of exogenous ovine growth hormone (oGH) on elimination was studied. Six ewes (62 +/- 2.5 kg BW) were dosed (i.p.) once with [14C]heptachlor (2.04 mg/kg Bw; .88 microCi/mg heptachlor) and three were treated additionally with oGH (oGH; 5 mg/d) for 21 d. Three additional ewes served as controls. Excreta were collected each day for 21 d. Milk and blood were collected every 3rd d until ewes were euthanized at d 21. 14C activity was measured in excreta, milk, blood and tissues. Total cumulative activity of [14C]heptachlor and(or) metabolites in excreta (21 d) did not differ (P greater than .20) in ewes given oGH (25 +/- 2%) vs none (23 +/- 2%). Milk yield and protein content were unaffected (P greater than .10) by oGH. Ewes given oGH eliminated 2.2 +/- 2% of total 14C dosage into milk during 21 d, whereas ewes untreated with oGH eliminated 1.3 +/- .2% (P less than .10); total 14C activity eliminated into milk plus excreta was similar for ewes given oGH or none (P greater than .10). For all six ewes, half-times (T1/2) for distribution and elimination of 14C activity (heptachlor and metabolites) were 1.5 d and 11.7 d, respectively. Blood concentrations of 14C activity during 21 d yielded elimination half-time as 23 d. Unlike bovines, which eliminate heptachlor slowly (T1/2 approximately 70 to 80 d) and mainly into milk fat, lactating ovines eliminated heptachlor and(or) metabolites mainly into excreta and about sixfold faster than bovines.

Some endocrine and morphological aspects of the acute toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Hormonal status was evaluated in TCDD-treated rats and in pair-fed and ad libitum-fed controls in order to separate hormonal changes resulting from the toxic insult of TCDD from those arising from progressive feed deprivation as it occurs in pair-fed controls. TCDD-treated rats received either a usually non-lethal (25 micrograms/kg) or a usually lethal (125 micrograms/kg) dose of TCDD whereas pair-fed and ad libitum-fed controls were given vehicle alone. Animals were terminated at predetermined time intervals and several hormones measured in serum or plasma. In addition, the morphology of the thyroid, pancreas, and pituitary was also examined. In both dosage groups, TCDD-treatment had the following effects: decreased TT4, FT4, insulin, and glucagon; mixed effects upon TT3, FT3, TSH, and GH. Pair-feeding to the non-lethal dose of TCDD had no effect on any of the hormones measured. Pair-feeding to the lethal dose of TCDD had the the following effects: slightly decreased TT4, FT4, TT3, TSH, and insulin; no effect on FT3 and glucagon. It is concluded that the endocrine status of TCDD-treated rats was different from that of pair-fed rats suggesting that some hormonal changes represent responses to an insult other than that due to starvation stress alone. A differential response between TCDD-treated and pair-fed rats was also observable morphologically in the corresponding endocrine glands indicating the importance of this additional control for morphologic observations in instances when reduced feed intake and body weight loss are prominent features of toxicity.

Disposition of xenobiotics: species differences

Species differences are a major obstacle in predicting toxicity of xenobiotics from one species to another. Species differences in toxicity of drugs and other chemicals may be due to differences in pharmacokinetics or pharmacodynamics. This paper illustrates the point that species differences in pharmacokinetics of xenobiotics may be the result of differences in any of the processes contributing to the disposition of a xenobiotic. This is an important point because biotransformation as a cause of species differences for the disposition of xenobiotics has been overemphasized in the past, whereas only scant attention has been paid to the other 3 major contributing processes to disposition, viz. absorption, distribution, and excretion. This brief overview presents a balanced examination of all 4 major processes (absorption, distribution, biotransformation and excretion) as they affect the pharmacokinetics of xenobiotics in various species.

Tissue-specific alterations of de novo fatty acid synthesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

De novo fatty acid synthesis was determined by the 3H2O method in numerous tissues and organs of TCDD-treated (125 micrograms/kg), pair-fed and free-fed male Sprague-Dawley rats to investigate if this important pathway of intermediary metabolism is altered by TCDD. Of the 12 tissues and organs examined, liver showed an increased, and interscapular brown adipose tissue (IBAT) a decreased de novo fatty acid synthesis when comparing TCDD-treated to pair-fed or free-fed control rats. De novo fatty acid synthesis was unaffected in other organs and tissues examined, with the exception that the concentration of 3H-fatty acids in plasma reflected the increased rate of synthesis seen in the liver of TCDD-treated animals. Increased de novo fatty acid synthesis in liver coincided with increased plasma triiodothyronine (T3) concentrations, whereas decreased de novo fatty acid synthesis in IBAT parallelled decreased plasma thyroxine (T4) levels. Thyroidectomy decreased de novo fatty acid synthesis, as expected, in both liver and IBAT. However, TCDD elicited no response in either of these organs in thyroidectomized rats. This finding suggests that changes observed in non-thyroidectomized rats are probably secondary effects. Indeed, known tissue-specific effects of T3 on liver and T4 on IBAT provide a likely explanation for the altered de novo fatty acid synthesis of these organs. It is suggested that increased de novo fatty acid synthesis in the liver of TCDD-treated rats might be responsible for the additional wasting away observable in these animals as compared to pair-fed controls.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on thermogenesis in brown adipose tissue of rats

Male Sprague-Dawley rats were treated with a usually lethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 125 micrograms/kg i.p. in corn oil) or with vehicle alone. Two, 4, and 8 days after treatment the temperature of interscapular brown adipose tissue (IBAT) was monitored during venous infusion of norepinephrine (480 ng/min) for 60 min. The temperature response was about 1.0-1.5 degrees C within 1 h in vehicle-treated, pair-fed and ad libitum-fed controls. In TCDD-treated animals, the response of IBAT decreased with time after TCDD dosage, amounting to only 0.3 +/- 0.1 degree C at 8 days after dosing (differences significant with respect to both controls, P less than 0.05). GDP binding to IBAT mitochondria (a measure of thermogenic capacity) was unchanged in all groups, indicating that the reduced thermogenic response was probably not caused by an impairment of the mitochondrial uncoupling process by TCDD.

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 T3 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 (T4) and triiodothyronine (T3) levels, although the levels of T3 remained in the euthyroid range (60-80 ng/dl). The concentration of liver microsomal cytochromes P-450 and b5 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 T3 than on T4 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 T3. 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 T4. 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)

Effect of a sublethal dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin on interscapular brown adipose tissue of rats

The effect of a sublethal dose (15 micrograms/kg) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was studied in selected tissues of male Sprague-Dawley rats by histological techniques 1, 3, 7 and 14 days after TCDD dosage. Histology of the heart, muscle, white adipose tissue, pancreas and the thyroid was unremarkable and that of the liver was found in agreement with previous reports. However, considerable changes were seen in interscapular brown adipose tissue (IBAT) of TCDD-treated rats. Initial accumulation followed by depletion of lipids, appearance of glycogen, cellular, mitochondrial and nuclear transformations were observed. In conjunction with other experiments it is concluded that a sublethal dose of TCDD alters fat and glucose metabolism in IBAT.

Dose-response and time course of hypothyroxinemia and hypoinsulinemia and characterization of insulin hypersensitivity in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

Male Sprague-Dawley rats were administered i.p. various doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in corn oil. At several time points thyroid stimulating hormone (TSH), total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3), reverse triiodothyronine (rT3) and insulin were determined in serum using radioimmunoassays, and glucose was measured by the glucose oxidase method. TSH and TT3 were not affected by any dose at any time point of measurement. TT4 and FT4 were decreased in a somewhat dose-dependent manner by days 2 to 4 after dosing. Return of TT4 and FT4 to normal values by day 32 after TCDD dosage also occurred in a dose-dependent manner, except in rats that died later. rT3 was also decreased at each dose level early and returned to normal levels in a somewhat dose-dependent fashion. Rats in the 2 highest dose groups became hypoinsulinemic and in the highest dose group also hypoglycemic by day 8 after dosing. Serum insulin and glucose remained suppressed in non-survivors of TCDD until death ensued. In survivors, serum insulin returned to normal values by day 32 after dosing. The hypoinsulinemic state was further characterized by hypersensitivity towards insulin, i.e. injection of an otherwise non-toxic dose of insulin 3 days after administration of 125 micrograms/kg TCDD was lethal to 80% of the rats within 24 h. Insulin hypersensitivity preceded both hypoglycemia and hypoinsulinemia. These findings suggest that hypothyroxinemia and hypoinsulinemia may be part of an adaptive process whereby rats attempt to diminish the toxic insult of TCDD.

Han/Wistar rats are exceptionally resistant to TCDD. I

Adult male Han/Wistar rats were administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) intraperitoneally at doses ranging from 125 to 1400 micrograms/kg and monitored for 39 to 48 days. Two rats succumbed in the course of the experiment: one in the group receiving 625 and one dosed 1000 micrograms/kg. Body weights of the animals decreased by 20 to 30% during the first 10 to 14 days and became stable thereafter. Feed consumption decreased to 1/3-1/2 of control levels by Day 4 (calculated per metabolic body mass) and returned gradually to starting values by about 4 weeks after dosing. Water intake displayed a triphasic pattern: at first it was slightly increased (Days 1 to 3), then reduced (on Days 4 to 12) and finally increased again throughout the remainder of the test period. The absolute and/or relative weights of thymus, testicles, ventral prostate and interscapular brown fat were significantly decreased at termination. These results indicate that the LD50-value for TCDD in the male, adult Han/Wistar rat is substantially above 1400 micrograms/kg, and that suppression of appetite is the principal phenomenon responsible for TCDD-induced body weight reduction.

Toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in cold-adapted rats

The toxicity of 60 micrograms/kg 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) given IP in corn oil/5% acetone was examined in male Sprague-Dawley rats adapted to 25 degrees C or 4 degrees C ambient temperature. Cold exposure significantly reduced mean time to death and tended to increase mortality. Body weight at the time of death was reduced at both ambient temperatures to about the same extent. Thus, the rate of body weight loss was about twice as fast in non-survivors at 4 degrees C than at 25 degrees C. There was a continuous decrease in feed intake of the non-survivors at 25 degrees C until death. However, no reduction in feed intake occurred in any of the rats at 4 degrees C ambient temperature. At 14 days after dosing all TCDD-dosed animals were hypothyroid in terms of T4 but essentially euthyroid in terms of T3. Oxygen consumption at 10 days after dosing was reduced to the same extent in all TCDD-dosed rats without regard to survival status. By day 20 after TCDD dosage, survivors increased their oxygen consumption at both ambient temperatures to nearly control levels whereas non-survivors were unable to do so. Body temperature of all animals remained within normal range except for the non-survivors, which showed reduced rectal temperature shortly before death.(ABSTRACT TRUNCATED AT 250 WORDS)

Histopathology of interscapular brown adipose tissue, thyroid, and pancreas in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats

The time course of histological changes was studied in rats lethally intoxicated (150 micrograms/kg) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In addition to TCDD-caused tissue damage described by others, the thyroid, pancreas, and interscapular brown adipose tissue (IBAT) were identified as tissues affected by TCDD. Because histological changes in the thyroid and pancreas occurred late (7 days after dosing), these effects are viewed as secondary due to altered hormonal homeostases. Both light and electron microscopic examination of IBAT identified this tissue as a target in TCDD toxicity. Histological changes in IBAT are characterized by three phases: (1) "fatty" IBAT (Days 1 to 3 after dosing); (2) fat depletion accompanied by glycogen accumulation (Days 4 to 7 after dosing); and (3) complete fat and glycogen depletion together with massive cellular damage (Days 8 to 14), particularly affecting the mitochondria. It is concluded that brown adipose tissue is a primary target in TCDD toxicity. It seems that destruction of brown adipose tissue by TCDD leads to an energy imbalance resulting in reduced oxygen consumption which forces animals to contribute a greater proportion of energy to the maintenance of their body temperature by anaerobic pathways. It is suggested that this less efficient energy utilization is the cause of a wasting syndrome.

Brown adipose tissue is a target tissue in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced toxicity

This paper demonstrates by electron microscopy that interscapular brown adipose tissue (IBAT) is a target tissue of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rats. Because of the known importance of IBAT in energy metabolism, it is suggested that brown adipose tissue may be a major target tissue in the toxicity of TCDD.

Reduced serum thyroid hormone levels in hexachlorobenzene-induced porphyria

The effect of feeding 0.1% hexachlorobenzene (HCB) for 55 days on mortality, body weight, urinary porphyrin excretion, serum thyroid hormones and induction of liver microsomal enzymes was studied in female Sprague-Dawley rats. This dosage regimen, followed by 42 days of a regular diet, resulted in 33% mortality with a mean time to death of 67 +/- 4 days. Body weight of survivors was not affected by dietary HCB, whereas non-survivors underwent a rapid weight loss (wasting) prior to death. At the end of the dosing period (day 55), rats fed the HCB diet exhibited an increase in the excretion of urinary porphyrins (4-fold) and a significant decrease in the levels of serum thyroxine (T4) and triiodothyronine (T3). When rats were returned to a regular diet the excretion of urinary porphyrins continued to rise (approx. 100 times higher than controls) and serum thyroid hormones remained suppressed. At the end of the experiment (day 97), the concentration of liver microsomal cytochrome P-450 and cytochrome bs and the activity of ethoxyresorufin-O-deethylase, pentoxyresorufin-O-dealkylase, aminopyrine-N-demethylase and UDP-glucuronosyl transferase were significantly induced, whereas the activity of NADPH-cytochrome c reductase and benzo[a]pyrene hydroxylase was not. Results demonstrate that HCB-induced lethality and porphyria occur by different mechanisms, reduced T4 and T3 serum levels accompany induction of porphyria by HCB, and induction of aryl hydrocarbon hydroxylase (with benzo[a]pyrene as substrate) is not a sensitive indicator of HCB exposure.

Effect of thyroid hormones on liver microsomal enzyme induction in rats exposed to 2,3,7,8,-tetrachlorodibenzo-p-dioxin

The effect of thyroidectomy and thyroid hormone replacement therapy on liver microsomal enzyme induction was studied in 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats (100 micrograms/kg). Treatment of non-thyroidectomized rats with TCDD had no effect on the concentration of liver microsomal cytochrome b5. In contrast, cytochrome b5 content was increased by TCDD treatment of thyroidectomized rats, regardless of replacement therapy with either T3 or T4. TCDD treatment increased the concentration of cytochrome P-450 (2-3-fold) and the activities of benzo[a]pyrene hydroxylase (4-7-fold), ethoxyresorufin O-de-ethylase (50-70-fold) and UDP-glucuronosyltransferase (5-7-fold) in non-thyroidectomized and thyroidectomized as well as thyroidectomized thyroid hormone treated rats; indicating the induction of these liver microsomal enzyme activities is independent of thyroid status. Because thyroid status alters the toxicity of TCDD but does not alter the ability of TCDD to induce microsomal enzymes, it appears that TCDD toxicity may not be directly related to microsomal enzyme induction.

Effect of partial jejunectomy and colectomy on the disposition of hexachlorobenzene in rats treated or not treated with hexadecane

The disposition of hexachlorobenzene (HCB) was studied in partially jejunectomized (middle section) or colectomized (excision of cecum and proximal colon) rats after iv or ip dosage (1.5 to 2.0 mg/kg). Excision of about 50% of the jejunum had no effect on body weight, feed intake, volume of urine, weight of feces, or urinary and fecal excretion of HCB as demonstrated by a comparison of sham-operated and jejunectomized animals. Similarly colectomy did not affect body weight, feed intake, volume of urine, or urinary excretion of HCB. However, the wet weight of feces was significantly higher and the amount of HCB in feces significantly lower in colectomized than in sham-operated rats. Hexadecane increased fecal excretion of HCB about two- to threefold without affecting its urinary excretion. The effect of jejunectomy and colectomy was similar in hexadecane-treated animals to that seen in untreated rats. Concentration of HCB in adipose tissue was significantly higher in colectomized rats than in sham-operated controls. Data represent in vivo evidence that the major site of nonbiliary, intestinal excretion of HCB is the large intestine.

Thyroid hormones modulate the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

These experiments examine the role of thyroxine (T4) and triiodothyronine (T3) on the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The first experiment is continuation of a study reported previously (Rozman et al., 1984). In this experiment, 60 male Sprague-Dawley rats were divided into 6 equal groups. Four groups of rats were thyroidectomized by 3 mCi Na131 l/kg rat. Five weeks later 2 of the thyroidectomized and 1 of the nonthyroidectomized groups of rats received ip 100 micrograms TCDD/kg body weight in corn oil/acetone, whereas 3 corresponding groups of rats served as vehicle controls. Two days after dosing and every 7 d thereafter, 1 thyroidectomized control group and 1 thyroidectomized TCDD-dosed group were given ip 105 micrograms T4/kg body weight. Mortality and body weight were monitored. The course of TCDD toxicity was similar in nonthyroidectomized and thyroidectomized T4-treated rats but was different in thyroidectomized animals without T4 replacement therapy. At d 90 after TCDD dosage, mortality was still lower and the mean time to death was increased (p less than 0.01) in this group of rats compared to nonthyroidectomized or thyroidectomized T4-treated rats. However, administration of T4 starting at d 91 after dosing with TCDD resulted within 2 wk in the same final mortality in thyroidectomized rats as in nonthyroidectomized or thyroidectomized T4-treated animals, indicating that thyroid hormones modulate the time course of the wasting syndrome but do not affect the ultimate mortality figure. Body weight loss was much slower in thyroidectomized (approximately 1 g/d) than in nonthyroidectomized or thyroidectomized T4-treated rats (approximately 8 g/d). In the second experiment the three vehicle control groups of the first experiment were used. Nonthyroidectomized vehicle controls and thyroidectomized T4-treated controls were maintained as before, whereas thyroidectomized controls received T3 at 5 micrograms/kg daily. One month later each rat was dosed with TCDD at 100 micrograms/kg in corn oil/acetone. Toxicity of TCDD was similar in nonthyroidectomized, thyroidectomized T4-treated, and thyroidectomized T3-treated rats as judged by mortality, body weight, and food intake, indicating no difference between T3 and T4 in the modulation of TCDD toxicity.

Intestinal excretion of toxic substances

Chemicals can be eliminated from body via feces by two major mechanisms, namely biliary and intestinal excretion. The relative importance of these processes in the elimination of a highly lipophilic xenobiotic such as hexachlorobenzene (HCB) has been studied. It has been demonstrated that fecal (90%) rather than urinary (10%) excretion is the major route of elimination of HCB in most species. It has been shown also that the bile of HCB dosed animals contained HCB metabolites only whereas fecal excretion consisted primarily of the parent compound. These findings suggested that the bile could not be the source of fecal HCB. Indeed, bile duct ligation in rats increased rather than decreased the fecal excretion of HCB. Experiments in rhesus monkeys with complete biliary bypass confirmed the conclusion that the source of fecal HCB is not the bile, suggesting that most of the fecal HCB originated from intestinal excretion. Exfoliation of intestinal epithelium and exudation across the intestinal mucosa are the two major nonbiliary mechanisms whereby xenobiotics can enter the intestinal lumen. The contribution of desquamation and exudation to fecal excretion of HCB was estimated in jejunectomized and hemicolectomized rats. Removal of 50% of the jejunum did not influence fecal excretion of HCB, whereas excision of 50% of the large intestine reduced it by 40%. These data suggest that the source of fecal HCB is nonbiliary, intestinal transfer (exudation) from blood into the intestinal contents, which occurs primarily in the large intestine. Fecal elimination of HCB is significantly enhanced by dietary treatments with mineral oil or hexadecane.(ABSTRACT TRUNCATED AT 250 WORDS)

Hexadecane increases the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): is brown adipose tissue the primary target in TCDD-induced wasting syndrome?

Addition of 5% hexadecane to the diet of rats increased fecal excretion of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) from 14 to 39% of an LD50 dose (60 micrograms/kg) during 10 days after dosing. This enhanced elimination did not result in reduced toxicity. On the contrary, the treatment has increased mortality from 60% in controls to 100% in hexadecane treated animals. Body weight changes were good indicators for predicting survival or nonsurvival after the LD50 dose but thymus weights were depressed without regard to survival status. The mechanism by which hexadecane potentiates the toxicity of TCDD is unknown but it is likely to be due to effects altering the disposition of TCDD. Based on similarities in the disposition of TCDD and hexachlorobenzene (HCB), it is suggested that the lethality causing target of TCDD is part of the peripheral compartment. The only site in the peripheral compartment that is compatible with the many thousand-fold species differences observed in TCDD toxicity is brown adipose tissue. The hypothesis is advanced that interaction between thyroid hormones and brown adipose tissue are responsible for the species differences in TCDD toxicity.