Thyroid status and thermogenesis in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Several key aspects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) toxicity resemble the effects of hypothyroidism, while in other ways the toxic responses are characteristics of hyperthyroidism. Whether thyroid dysfunction plays a role in TCDD toxicity remained unknown, however. We therefore determined the dose-related effects of TCDD treatment on plasma concentrations of L-thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3), and thyroid-stimulating hormone (TSH), and compared these changes with signs of TCDD toxicity. We also determined whether indices of functional thyroid status (and thermogenesis) were altered in response to TCDD treatment. Young adult male Sprague-Dawley rats were given single oral doses of TCDD (6.25-100 micrograms/kg) and evaluated 1 week later. Toxicity, measured by decreases in feed intake and body weight, ranged from minimal to severe. Plasma concentrations of T4 were greatly reduced at all doses tested, while T3 was increased in a dose-related fashion (up to 35%). TSH was elevated but was inversely proportional to dose. Thyroid histology was unremarkable, and TCCD treatment had little effect on the ability of rats to raise serum T4, T3, and TSH concentrations in response to acute cold stress. TCDD treatment caused a slight (8%) decrease in basal metabolic rate, yet comparable decreases were seen in pair-fed control animals. Thermogenesis, as measured by O2 consumption and colonic temperatures in rats exposed to various ambient temperatures, was only marginally affected. In summary, although thyroid hormone concentrations were markedly altered, rats given doses of TCDD sufficient to cause overt toxicity appeared to be essentially euthyroid. These results do not support proposals by other researchers that altered thyroid status is a major contributor to TCDD toxicity and/or a key response to TCDD exposure.     

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.     

Inhibition of intestinal glucose absorption in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Male Sprague-Dawley rats were injected with either 125 micrograms 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg or vehicle (pair-fed and ad libitum-fed controls). Transfer of water, electrolytes and D-glucose as well as fats of a tracer dose of the non-metabolizable radioactive marker 3-O-methyl-D-[U-14C]glucose was studied in isolated perfused jejunal segments 1, 2, 7, and 21 d after treatment (TCDD-treated and pair-fed control rats) and after 26 d in ad libitum-fed controls. TCDD-treated rats demonstrated reduced feed consumption and loss of body weight. Active intestinal absorption of glucose was significantly inhibited 30 and 22% compared to pair-fed controls, respectively 2 and 7 d after TCDD treatment. After 21 d the inhibition (14%) was less significant. There were no differences in glucose transfer between severely starved pair-fed controls (body weights 370 +/- 26 g) and ad libitum-fed rats (body weights 512 +/- 15 g). Water absorption and transfer of sodium and calcium was not influenced by TCDD treatment. However, a significant increase of potassium transfer was observed in parallel with impaired glucose absorption. The uptake of 3-O-methylglucose into mucosal tissue was not impaired, whereas the transfer to the serosal side was significantly inhibited by 30-60% compared to pair-fed as well as ad libitum-fed animals from day 2 until the end of the experiment. These results suggest that TCDD is involved in an inhibition of glucose transport at the basolateral membrane.     

Circadian alterations in prolactin, corticosterone, and thyroid hormone levels and down-regulation of prolactin receptor activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Studies were initiated to determine whether 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects circadian rhythms of serum prolactin (PRL), corticosterone, thyroxine (T4), and triiodothyronine (T3) in male Sprague-Dawley rats. In addition, the effects of TCDD on PRL receptor activity, as assessed by the ability of PRL to induce ornithine decarboxylase (ODC), were determined. The earliest effect detected following TCDD administration was a significant decrease in the serum PRL concentration compared with that of pair-fed controls within 4 hr (p less than 0.05). This was followed by a significant decrease in serum T4 by 6 hr (p less than 0.05). By 8 hr the serum peak of corticosterone was shifted to 2 hr later in the TCDD-treated rats. This temporal sequence of hormonal changes suggests that the earlier alteration in PRL may be involved in the later alterations in the concentrations of serum T4 and corticosterone. The serum PRL concentration 7 days after TCDD administration was significantly higher (p less than 0.05) in TCDD-treated animals compared with that in pair-fed controls (mean of 20.5 +/- 3.7 vs 13.6 +/- 1.8 ng/ml serum, p less than 0.05, respectively). The elevation of ODC activity in response to PRL, 2 days after TCDD, was decreased in the order of thymus greater than adrenal greater than spleen greater than heart greater than kidney greater than liver. By 7 days, liver ODC activity in response to PRL was only 12% that detected in pair-fed controls. Liver ODC activity in response to dexamethasone and aminophylline was decreased to 25 and 22% of pair-fed controls, respectively, by 7 days after TCDD administration. However, in kidney, TCDD-treated rats had an increased ODC response to aminophylline to 191% of pair-fed controls by Day 7. These results suggest that the ability of TCDD to alter receptor coupling or the receptor number for diverse hormones may play a role in TCDD toxicity.     

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.     

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.     

Effects of thyroid hormones on the induction of cleft palate by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in C57BL/6N mice

The induction of cleft palate by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administered with thyroid hormones triiodothyronine (T3) or thyroxine (T4) was investigated in C57BL/6N mice. Timed-pregnant mice were treated with vehicle, TCDD, T3, T4, TCDD plus T3, or TCDD plus T4 on Days 10 to 13 of gestation. No cleft palates were observed in any control fetuses in this study, nor have there been any cleft palates in 1193 fetuses or 154 control litters in the past 24 months. TCDD (3 micrograms/kg/day) caused about 8% cleft palates per litter, while T3 (120, 240, 480 micrograms/kg/day) and T4 (625, 1250, 2500 micrograms/kg/day) resulted in no more than 1.2% cleft palates per litter in any of the treatment groups and the incidence was not dose related. The combination of TCDD (3 micrograms/kg/day) plus T3 at 120, 240, and 480 micrograms/kg/day resulted in 15.9, 20.6, and 31.4% cleft palates per litter, respectively. TCDD plus T4 at 625, 1250 and 2500 micrograms/kg/day caused 15.1, 22.9, and 27.2% cleft palates per litter. No cleft palates were observed when large doses of T3 were given in combination with T4. These data demonstrated that coadministration of T3 or T4 with TCDD increased the incidence of cleft palate to incidences greater than expected from the separate administration of the hormones plus TCDD.     

Increased permeability of the rat biliary tree by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment and protection by hepatoactive agents

Male Sprague-Dawley rats were treated ip on Day 0 with 0, 20, 50, or 100 micrograms/kg of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and biliary tree permeability was evaluated on Days 2, 4, 7, 10, 14, or 20 by segmented retrograde intrabiliary injection of [3H]sucrose or [14C]mannitol. Seven days after 100 micrograms/kg TCDD, the percentage recovery in bile of both [3H]sucrose (73.9 +/- 4.2 vs 27.6 +/- 7.6, control vs TCDD, means +/- SE) and [14C]mannitol (22.7 +/- 2.2 vs 12.1 +/- 2.2) was decreased, demonstrating that the permeabilities of both the intracellular (canalicular) and paracellular pathways were increased. Seven days after 50 micrograms/kg TCDD, the recovery of [3H]sucrose was decreased (73.5 +/- 5.4 vs 39.0 +/- 2.8) but the recovery of [14C]mannitol was not (25.5 +/- 1.5 vs 22.9 +/- 1.9). Thus, an increase in paracellular permeability is obtained at a lower dose of TCDD. In rats treated with 100 micrograms/kg TCDD on Day 0, co-treatment with chlordecone (15 mg/kg/day on Days 2-6) or thyroxine (50 micrograms/kg on Day 2) had no effect. Pregnenolone-16 alpha-carbonitrile (75 mg/kg/day on Days 4-6) treatment increased [14C]mannitol recovery in both TCDD and control groups; its effect must not be specific. Methimazole given in drinking water (0.5%) on Days -7 through 7 reversed the increased permeability effects of TCDD (100 micrograms/kg) without affecting the biliary tree permeability ([14C]mannitol recovery) of control animals.     

Studies on the role of lipid peroxidation in the acute toxicity of TCDD in rats

Lipid peroxidation has been shown to be enhanced following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), but its role in TCDD toxicity is unclear. The present study was undertaken to further elucidate the relations between lipid peroxidation and TCDD lethality. A time course and dose-response experiment in Long-Evans (L-E; LD50 ca. 10 micrograms/kg) and Han/Wistar (H/W; LD50 greater than 3000 micrograms/kg) rats showed that hepatic lipid peroxidation, measured as the amount of thiobarbituric acid-reactive substances (TBA-RS), was induced by TCDD dose-dependently in L-E, but not in H/W rats. Hepatic glutathione peroxidase activity was suppressed in much the same manner in both strains. Lipid peroxidation correlated with body weight loss in L-E rats alone. When 500 micrograms/kg of TCDD was given to L-E rats, lipid peroxidation increased about 3-fold on Day 11 in the liver, while no change was seen in cardiac or renal TBA-RS. The pair-fed controls did not survive the 11-day test period and exhibited gastrointestinal hemorrhages. At 6 days, liver atrophy and elevated (over 2-fold) TBA-RS values were recorded in pair-fed controls but not in their TCDD-treated counterparts. TCDD decreased hepatic glutathione peroxidase activity by almost 50% at 6 days, while pair-feeding was without effect. Liver morphology was different between TCDD-treated and pair-fed rats. Moreover, the livers of TCDD-treated L-E rats contained much higher concentrations of probably peripheral fat-derived fatty acids than did the livers of pair-fed or ad libitum control rats. Restricted feeding over 6 days induced hepatic lipid peroxidation more in H/W than in L-E rats. Endotoxin increased liver TBA levels similarly in both strains having an additive effect with high doses of TCDD in H/W rats. Added as a 0.5% concentration in chow, butylated hydroxyanisole (BHA), but not ethoxyquin, tended to increase survival rate and time in L-E rats exposed to 20 micrograms/kg of TCDD; at 50 micrograms/kg the only survivor was again in the BHA group. However, neither antioxidant had any effect on initial body weight loss. It is concluded that lipid peroxidation mainly arises as a secondary phenomenon in TCDD toxicity, is not the cause of the typical histopathological liver lesion, but may contribute to lethality.     

Digestible energy and efficiency of feed utilization in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Treatment of male rats (300 to 325 g) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 15 or 50 micrograms/kg) caused dose-dependent reductions in body weight, feed and water intakes, and fecal output. Urine output, however, was not altered by TCDD. Fecal energy loss, as a percentage of daily feed energy intake (kcal/day), was similar in control and TCDD-treated rats as was the percentage of feed energy absorbed by the gastrointestinal tract, i.e., digestible energy. These findings dispel the long-standing proposal that a gross malabsorption syndrome is responsible for weight loss in TCDD-treated rats and place greater emphasis on hypophagia as the reason for weight loss. In support of a central role for hypophagia, it was found that control rats pair-fed to rats treated with a sublethal dose of TCDD (15 micrograms/kg) lost almost the same amount of weight. However, from 15 to 50 days post-treatment, the pair-fed animals consistently maintained their weight at a 10- to 15-g higher level than age-matched TCDD-treated rats. To determine why this weight difference occurs, the efficiency of feed utilization from Day 30 to 45 post-treatment in ad libitum fed control and TCDD-treated rats (15 micrograms/kg) that were maintaining different levels of body weight was compared. First, daily feed intakes of TCDD-treated and control rats were determined from Day 25 to 30 post-treatment. Second, weights of both groups were lowered by reducing feed intake in two successive 5-day periods to 50 and 10% of the respective ad libitum level. Third, on Days 40 to 45, both groups were refed their prereduction level of intake but reduced in proportion to the intervening loss in metabolic tissue mass. At each level of feed energy reduction, weight losses observed in the TCDD-treated and control rats were equivalent. Furthermore, although given only prerestriction amounts of feed that were indexed to their reduced metabolic body size (body wt kg 0.75), both TCDD-treated and control rats gained weight rapidly and at similar rates during the refeeding period. Thus, rats treated with a sublethal dose of TCDD displayed normal efficiency of feed utilization but did so at a subnormal level of weight. That is, just like control rats, TCDD-treated rats increased their efficiency of feed utilization (weight gain/feed intake) but only when their body weight was caused to fall below the lower weight maintenance level determined by the TCDD dose administered.(ABSTRACT TRUNCATED AT 400 WORDS)     

Hepatic indices of thyroid status in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

The functional thyroid status of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated rats is unknown. Therefore, activities of certain thyroid-responsive enzymes were examined in the livers of adult male Sprague-Dawley rats 1 week after treatment with TCDD (6.25, 25 or 100 micrograms/kg). Activity of the thyroid-responsive flavin L-glycerol-3-phosphate dehydrogenase (per mg mitochondrial protein) was decreased slightly in livers of TCDD-treated rats, while that of succinate dehydrogenase remained unchanged. In contrast, activities (per mg supernatant protein) of three thyroid-responsive NADP-dependent cytosolic enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, were increased by TCDD treatment in a dose-dependent manner. Lactate dehydrogenase (activity per mg supernatant protein) was also augmented slightly 1 week after TCDD administration. Liver mass was increased by TCDD treatment in a dose-dependent manner, but DNA content per liver was similar at all doses examined. Total hepatic protein, expressed per liver or mg hepatic DNA, was increased in TCDD-treated rats when compared to their pair-fed counterparts. The decreased activity of the mitochondrial L-glycerol-3-phosphate dehydrogenase, in contrast to the increased activities of the supernatant enzymes, malic enzyme, glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, is not consistent with a shift in functional thyroid status following TCDD treatment.     

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.     

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 μg/kg) diminished dose dependently, while the effect was marginal in hamsters (900–4600 μg/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. Received: 12 January 1995 / Accepted: 27 February 1995     

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.     

UDP-glucuronosyltransferase inducers reduce thyroid hormone levels in rats by an extrathyroidal mechanism.

As many microsomal enzyme inducers have been shown to reduce thyroid hormone levels, this study was conducted to determine if this reduction is produced by directly blocking the synthesis of thyroid hormones, or by indirectly increasing the biotransformation and deactivation of thyroxine (T4) by microsomal enzymes. Surgically thyroidectomized male rats received thyroid hormone replacement therapy by implanted osmotic minipumps, resulting in T4 and T3 serum levels that were similar to those observed in euthyroid controls. Three days after minipump implantation (Day 0), rats were fed diets containing four UDP-glucuronosyltransferase (UDP-GT) inducers: phenobarbital (PB), 3-methylcholanthrene (3MC), pregnenolone-16 alpha-carbonitrile (PCN), or polychlorinated biphenyls (PCB) for 10 days. PB, 3MC, and PCN reduced total (Days 3-10) and free (Days 7-10) T4 serum concentrations 30-50%, whereas PCB produced a 70-75% reduction in total and free serum T4 (Days 3-10). Treatment with PB, PCN, and PCB decreased levels of total T3 (Days 7-10). UDP-GT activity toward T4 was increased by PB, 3MC, PCN, and PCB 270, 400, 570, and 660%, respectively, and was found to correlate with serum T4 levels (total and free). These results demonstrate that reduction of thyroid hormone levels by microsomal enzyme inducers is produced in part by an extrathyroidal mechanism, quite possibly an increase in T4 glucuronidation.     

Altered regulation of adrenal steroidogenesis in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated rats

A single treatment of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (50 micrograms/kg) produced two distinct effects on adrenal steroidogenesis in rats 13 days post-treatment. In unstressed rats, the very low corticosterone levels early in the light phase (AM) increased 4-fold relative to ad libitum-fed control (ALC) rats, but the peak level of corticosterone that is seen late in the light phase (PM) decreased up to 40% relative to ALC rats. The AM stimulation was also observed in rats pair-fed to compensate for the diminished feed intake of TCDD-treated animals, indicating that the change results from nutritional deprivation. The PM suppression, however, was not observed in pair-fed rats. In rats given a lower dose of TCDD (15 micrograms/kg), there was no AM stimulation, whereas the suppression of the PM diurnal peak of corticosterone was retained. Plasma adrenocorticotropin (ACTH) levels and adrenal size were not changed by these treatments, indicating that TCDD affects adrenal responsiveness. TCDD did not, however, have a significant effect on corticosterone secretion in rats receiving high doses of ACTH. In control animals, the availability of cholesterol to cytochrome P-450scc limits the rate of steroidogenesis. While the specific content of the cytochrome was unaffected by TCDD, cholesterol turnover by this enzyme appeared to be affected following TCDD treatment, as evidenced by small increases in the mitochondrial levels of free cholesterol, reactive cholesterol, and in the proportion of P-450scc complexed with cholesterol relative to both ad libitum- and pair-fed controls. This accumulation of mitochondrial cholesterol following TCDD treatment is consistent with an inhibition of cholesterol metabolism at cytochrome P-450scc in vivo that is removed upon isolation of the mitochondria. These TCDD-induced increases were enhanced substantially in ACTH-stimulated rats, probably because ACTH enhances cholesterol influx into the mitochondria. Normally, substrate availability is rate limiting in cholesterol side-chain cleavage, and the AM stimulation of steroidogenesis by TCDD may result from such increased cholesterol transfer. The inhibition of cholesterol side-chain cleavage resulting from TCDD treatment may, however, only become rate limiting for corticosterone synthesis when cholesterol transfer is more substantially activated, as for peak PM secretion.     

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.     

Acute neurobehavioural effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in Han/Wistar rats.

The neurobehavioural effects of a single non-lethal dose (1000 micrograms/kg intraperitoneally) of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were assessed in young male Han/Wistar rats, highly resistant to acute lethality of TCDD. TCDD decreased body weight significantly compared with ad libitum fed controls. TCDD did not change the behaviour or the motility of rats in the open field test 8 days after the treatment nor did it affect the spontaneous motor activity up to 27 days after the exposure. In the elevated plus-maze test for anxiety, TCDD-treated rats did not differ from either ad libitum fed controls or pair-fed controls. In the 24-hr passive avoidance test, the learning of TCDD-treated rats did not differ significantly from that of ad libitum fed controls or pair-fed controls from 8 hr to 16 days after the treatment. TCDD did not affect the motor coordination or the maintenance of balance on the rotating rod but it impaired them slightly in the elevated horizontal bridge test 16 hr after exposure. It did not affect nociception in the hot plate test 16 hr or 8 days after the injection. The results suggest that a single sublethal dose of TCDD does not alter markedly the general behaviour of Han/Wistar rats, in contrast to its striking effect on feeding behaviour which results in a marked decrease in body weight gain.     

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin treatment on mechanical function of the rat heart

Mechanical responses of isolated atria to (-)-isoproterenol and activities of myocardial pyruvate kinase and citrate synthase, enzymes involved in energy metabolism, were assessed in rats 7 days after 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment. Basal tension development by electrically paced left atria was significantly elevated by all doses of TCDD (6.25, 25, or 100 micrograms/kg) when compared to that of vehicle-treated rats with unlimited access to feed. The basal rate of spontaneously beating right atria was significantly depressed in rats receiving 100 micrograms/kg TCDD. In left atria from rats treated with 100 micrograms/kg TCDD, maximal inotropic responses to (-)-isoproterenol and 1-methyl-3-isobutylxanthine were enhanced to the same degree. Right and left atria from rats receiving 100 micrograms/kg TCDD had an increased sensitivity to the chronotropic and inotropic effects of (-)-isoproterenol, respectively. The augmented atrial effects caused by TCDD were not secondary to loss of body weight because pair-fed animals that lost the same amount of weight did not display the responses. The ratio of heart ventricular mass to body weight and the activities of pyruvate kinase and citrate synthase in homogenates of heart ventricular muscle were not affected by TCDD treatment. Thus, overtly toxic doses of TCDD in the rat did not depress mechanical function of the heart.     

Relationship of Alterations in Energy Metabolism to Hypophagia in Rats Treated with 2,3,7,8-Tetrachlorodibenzo-p-dioxin

Relationship of Alterations in Energy Metabolism to Hypophagiain Rats Treated with 2,3,7,8-Tetrachlorodibenzo-p-dioxin. POTTER,C. L., MENAHAN, L. A., AND PETERSON, R. E. (1986). Fundam. Appl.Toxicol. 6, 89–97. Efficiency of energy utilization wasevaluated temporally in 2,3,7,8-tetrachlorodibenzo-p-dioxin(TCDD, 50 µg/kg)-treated male Sprague—Dawley rats(275–300 g), their pair-fed counterparts, and a groupwith ad libitum access to ground feed. TCDD treated rats exhibiteda progressive reduction in feed intake and body weight. Theweight loss of vehicle-treated rats, pair-fed to the TCDD-treatedgroup, was comparable to that found in rats receiving TCDD.Following treatment, rats administered TCDD were as efficientin absorbing feed energy from the gut as control rats. Thiswas evidenced by similar relative digestible energy values inTCDD-treated rats, their pair-fed partners, and a group withad libitum access to feed. Equivalent decreases in oxygen consumptionand carbon dioxide production in TCDD-treated rats and theirpair-fed counterparts, relative to rats with ad libitum accessto feed, suggested that the decrease in both of these parametersin TCDD-treated rats was secondary to hypophagia and/or weightloss. Decline of respiratory quotient (RQ) to almost 0.7 inboth TCDD-treated rats and their pair-fed counterparts is indicativeof fat combustion. By Day 17 post-treatment, RQ increased significantlyin the TCDD-treated and pair-fed groups possibly due to a limitationin the availability of lipid stores. Also, TCDD-treated ratsand their pair-fed partners diminished their water intake toa similar extent without reducing urine output. Likewise, urinaryexcretion of both energy and urea was decreased to the sameextent in rats treated with TCDD as it was in their pair-fedcounterparts. However, TCDD-treated rats tended to excrete moreurinary ammonia than their pair-fed partners on Days 10 and16 post-treatment. Thus, TCDD treatment does result in a reductionof caloric intake in the rat, yet efficiency of energy utilizationis preserved.