Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Hepatocyte damage induced by carbon tetrachloride: inhibited lipoprotein secretion and changed lipoprotein composition

Changes of lipoprotein secretion and composition in response to CCl4 treatment were studied in monolayer cultures of rat primary hepatocytes. (1) CCl4 decreased secretion of very low density lipoproteins (VLDL) by about 85%, while high density lipoprotein (HDL) secretion was less affected (about 40%). The effect was concentration-dependent. (2) CCl4 significantly inhibited secretion of VLDL- and HDL-associated triglycerides and cholesterol esters. VLDL- and HDL-associated cholesterol was not affected, while secretion of phospholipids was increased. (3) Hepatocytes secreted the apolipoproteins B48, B100, E, C, and A-I. CCl4 reduced secretion of apoproteins associated with VLDL by almost 20%, and by about 75% when associated with HDL. The de novo synthesis of apolipoproteins was attenuated by CCl4. (4) CCl4 caused variations in the apolipoprotein composition in VLDL and HDL. CCl4 intoxication of the liver affected the morphology and/or function of the lipoproteins, which drastically impaired their ability to act as transport vehicles for lipids from the liver to the circulation.

Mechanism of carbon tetrachloride-induced hepatotoxicity. Hepatocellular damage by reactive carbon tetrachloride metabolites

CCl4-induced liver damage was modeled in monolayer cultures of rat primary hepatocytes with a focus on involvement of covalent binding of CCl4 metabolites to cell components and/or peroxidative damage as the cause of injury. (1) Covalent binding of 14C-labeled metabolites was detected in hepatocytes immediately after exposure to CCl4. (2) Low oxygen partial pressure increased the reductive metabolism of CCl4 and thus covalent binding. (3) [14C]-CCl4 was bound to lipids and to proteins throughout subcellular fractions. Binding occurred preferentially to triacylglycerols and phospholipids, with phosphatidylcholine containing the highest amount of label. (4) The lipid peroxidation potency of CCl4 revealed subtle differences compared to other peroxidative substances, viz., ADP-Fe3+ and cumol hydroperoxide, respectively. (5) CCl4, but not the other peroxidative substances, decreased the rate of triacylglycerol secretion as very low density lipoproteins. (6) The anti-oxidant vitamin E (alpha-tocopherol) blocked lipid peroxidation, but not covalent binding, and secretion of lipoproteins remained inhibited. (7) The radical scavenger piperonyl butoxide prevented CCl4-induced lipid peroxidation as well as covalent binding of CCl4 metabolites to cell components, and also restored lipoprotein metabolism. The results confirm that covalent binding of the CCl3* radical to cell components initiates the inhibition of lipoprotein secretion and thus steatosis, whereas reaction with oxygen, to form CCl3-OO*, initiates lipid peroxidation. The two processes are independent of each other, and the extent to which either process occurs depends on partial oxygen pressure. The former process may result in adduct formation and, ultimately, cancer initiation, whereas the latter results in loss of calcium homeostasis and, ultimately, apoptosis and cell death.

Pathogenesis of carbon tetrachloride-induced hepatocyte injury bioactivation of CCI4 by cytochrome P450 and effects on lipid homeostasis

The CCl4-induced development of liver damage was studied in monolayer cultures of primary rat hepatocytes: (1) CCl4 caused accumulation of triglycerides in hepatocytes following cytochrome P450 induction with beta-naphthoflavone or metyrapone. Ethanol or a high dose of insulin plus triiodothyronine had the same effect. (2) CCl4 increased the synthesis of fatty acids and triglycerides and the rate of lipid esterification. Cholesterol and phospholipid synthesis from acetate was also increased. (3) CCl4 reduced beta-oxidation of fatty acids as assessed by CO2-release and ketone body formation. Hydrolysis of triglycerides was also reduced. (4) The content of unsaturated fatty acids in microsomal lipids was decreased by almost 50% after incubation with CCl4, while saturated fatty acids increased slightly. (5) CCl4 exerted a pronounced inhibitory effect on the exocytosis of macromolecules (albumin), but did not affect secretion of bile acids from hepatocytes.

In vivo and in vitro studies on the regulatory link between 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase in rat liver

The activities of 3-hydroxy-3-methylglutaryl CoA reductase (HMGCoA reductase; EC 1.1.1.34), rate-limiting enzyme of cholesterol biosynthesis, and cholesterol 7 alpha-hydroxylase (EC 1.14.13.17), key enzyme of the neutral bile acid synthesis pathway, were measured in the microsomal fraction of rat liver and in rat liver cells to investigate the coordinate regulation of the two pathways. Both enzyme activities exhibited the same diurnal rhythm and responded in a coordinate fashion to fasting or bile acid-feeding (decrease) and to cholestyramine-feeding (increase). Cholesterol-feeding decreased the activity of HMGCoA reductase, increased that of cholesterol 7 alpha-hydroxylase, and concomitantly increased free cholesterol in microsomes. In an ex vivo setting using primary hepatocytes from animals fed a high cholesterol diet the activity of HMGCoA reductase was initially low and that of cholesterol 7 alpha-hydroxylase was elevated. Release of cholesterol into the medium with ongoing incubation caused HMGCoA reductase activity to increase, and that of cholesterol 7 alpha-hydroxylase to decline. Incubation of hepatocytes with a cholesterol-containing lipoprotein fraction stimulated the activity of cholesterol 7 alpha-hydroxylase, but left HMGCoA reductase activity unaffected. The results confirm the idea of a joint regulation of the two key enzymes of cholesterol metabolism in response to the levels of substrate and metabolites, and support the notion that with respect to bile acid and cholesterol levels, respectively, regulation of HMGCoA reductase activity may be secondary to that of cholesterol 7 alpha-hydroxylase. The in vitro studies supply evidence that the effects of cholesterol and bile acid excess or deficiency are direct and do not involve accessory changes of hormone levels or mediators.

Heteroclitic immunization induces tumor immunity

In tumor transplantation models in mice, cytotoxic T lymphocytes (CTLs) are typically the primary effector cells. CTLs recognize major histocompatibility complex (MHC) class I-associated peptides expressed by tumors, leading to tumor rejection. Peptides presented by cancer cells can originate from viral proteins, normal self-proteins regulated during differentiation, or altered proteins derived from genetic alterations. However, many tumor peptides recognized by CTLs are poor immunogens, unable to induce activation and differentiation of effector CTLs. We used MHC binding motifs and the knowledge of class I:peptide:TCR structure to design heteroclitic CTL vaccines that exploit the expression of poorly immunogenic tumor peptides. The in vivo potency of this approach was demonstrated using viral and self-(differentiation) antigens as models. First, a synthetic variant of a viral antigen was expressed as a tumor antigen, and heteroclitic immunization with peptides and DNA was used to protect against tumor challenge and elicit regression of 3-d tumors. Second, a peptide from a relevant self-antigen of the tyrosinase family expressed by melanoma cells was used to design a heteroclitic peptide vaccine that successfully induced tumor protection. These results establish the in vivo applicability of heteroclitic immunization against tumors, including immunity to poorly immunogenic self-proteins.

Tumor immunity and autoimmunity induced by immunization with homologous DNA

The immune system can recognize self antigens expressed by cancer cells. Differentiation antigens are prototypes of these self antigens, being expressed by cancer cells and their normal cell counterparts. The tyrosinase family proteins are well characterized differentiation antigens recognized by antibodies and T cells of patients with melanoma. However, immune tolerance may prevent immunity directed against these antigens. Immunity to the brown locus protein, gp75/ tyrosinase-related protein-1, was investigated in a syngeneic mouse model. C57BL/6 mice, which are tolerant to gp75, generated autoantibodies against gp75 after immunization with DNA encoding human gp75 but not syngeneic mouse gp75. Priming with human gp75 DNA broke tolerance to mouse gp75. Immunity against mouse gp75 provided significant tumor protection. Manifestations of autoimmunity were observed, characterized by coat depigmentation. Rejection of tumor challenge required CD4(+) and NK1.1(+) cells and Fc receptor gamma-chain, but depigmentation did not require these components. Thus, immunization with homologous DNA broke tolerance against mouse gp75, possibly by providing help from CD4(+) T cells. Mechanisms required for tumor protection were not necessary for autoimmunity, demonstrating that tumor immunity can be uncoupled from autoimmune manifestations.

Co-expression of transforming growth factor beta and interferon tau during peri-implantation period in the ewe

The transforming growth factor beta (TGFbeta) family is known to control cell migration, growth, differentiation, function and regulation of extracellular matrix, all of which are required for the process of implantation. Expression of TGFbeta by the conceptus and endometrium was studied during the period of implantation in the ewe. A total of thirty-four ewes were hysterectomized on day 12, 14, 16, 18 or 20 of pregnancy (day 0 = day of estrus). Conceptus (200 mg wet weight) and endometrial (300 mg wet weight) tissues were cultured in vitro in 7 and 10 ml Eagle's minimal essential medium, respectively. The culture media were subjected to a bioassay to determine concentrations of TGFbeta. Conceptus culture media (CCM) were also analyzed for contents of ovine interferon-tau (oIFNr), low molecular weight acidic protein, produced by the trophectoderm between days 8 and 21 of pregnancy. Whole uteri including conceptus(es) and conceptuses (day 16) only were fixed and subjected to immunohistochemical and in situ hybridization studies. Levels of oIFNr produced by conceptuses were the highest on day 16 at 4.4 microg/ml. Concentrations of TGFbeta in day 12, 14, 16, 18 and 20 CCM were 38+/-19, 102+/-56, 862+/-152, 728+/-191 and 336+/-106 pg/ml, respectively, and approximately 90% of TGFbeta activity in CCM was due to TGFbeta1 whereas less than 10% was due to TGFbeta3 based on neutralization with TGFbeta subtype-specific antibodies. Immunohistochemical studies revealed that day 16 conceptuses displayed major staining for TGFbeta1, no beta2 staining and minor staining for beta3. In situ hybridization studies also revealed that day 16 trophectoderm possessed most TGFbeta1 mRNA while day 14 trophectoderm and day 20 chorion/amnion displayed weaker staining for TGFbeta1 mRNA. TGFbeta in day 12, 14, 16, 18 and day 20 endometrial culture media was 156+/-37, 129+/-33, 49+/-22, 62+/-23 and 179+/-40 pg/ml, respectively, and approximately 65% and 35% of the activities were due to TGFbeta1 and beta2, respectively. These results indicate that TGFbeta production by the conceptus coincides with the time when oIFNtau production starts to decline. These observations support the postulate that TGFbeta may play an important role in implantation in the ovine species.

Polychlorinated biphenyls affect the activities of gluconeogenic and lipogenic enzymes in rat liver: is there an interference with regulatory hormone actions?

1. The effects of dietary polychlorinated biphenyls (PCBs) (30-2000 ppm) on activities of gluconeogenic (phosphoenolpyruvate carboxykinase-PEPCK, and fructose 1,6-bisphosphatase-FdPase) and lipogenic enzymes (fatty acid synthase-FAS, ATP citrate lyase-ACL, malic enzyme-ME, glucose 6-phosphate dehydrogenase-G6PDH, and 6-phosphogluconate dehydrogenase-PGDH) were studied in livers of the female Sprague-Dawley and Wistar rat. 2. PCB amounts accumulating in the liver reflected the extent of dietary exposure. The Wistar strain was more sensitive to PCBs than the Sprague-Dawley strain. Of the Clophentype PCBs those containing 60 and 64% chlorine displayed the most pronounced effects. 3. Activities of gluconeogenic enzymes (PEPCK and FdPase) were dose-dependently decreased by PCBs, PEPCK being considerably more sensitive. This decrease was also found under conditions where the activity of PEPCK was induced (administration of adrenalin, glucagon or cAMP, feeding high protein diets, starvation). 4. Activities of lipogenic enzymes were induced by PCBs. The increase was much greater with ME, G6PDH and PGDH (up to 10-fold) than with FAS and ACL (approximately 2-fold). PCB effects were dose-dependent, but transient. 5. In cultured hepatocytes basal activities of lipogenic enzymes were induced by PCBs in the absence of hormones. With saturating levels of insulin or triiodothyronine, enzyme activities were also induced, but addition of PCBs resulted in an additive effect. 6. These results suggest that in the female rat PCBs can mimic the actions of certain hormones by affecting either hormone levels, hormone receptor systems or regulatory systems.

The enzyme inducers 3-methylcholanthrene and phenobarbital affect the activities of glucocorticoid hormone-regulated enzymes in rat liver and kidney

3-Methylcholanthrene, an inducer of P448-type cytochromes (mostly 1A1 and 1A2), and phenobarbital, an inducer of P450-type cytochromes (mostly 2B1 and 2B2), are prototypical for the actions of many xenobiotics. They cause endocrine disruption by affecting, among others, steroid hormone levels. Rats were treated with single bolus doses of 3-methylcholanthrene or phenobarbital, and enzyme activities that are controlled by glucocorticoids were measured in liver and kidney. The activities of the cytosolic enzymes L-alanine aminotransferase, indoleamine 2,3-dioxygenase (L-tryptophan pyrrolase), phosphoenolpyruvate carboxykinase, L-serine dehydratase and L-tyrosine aminotransferase were affected in a similar fashion: an initial activity reduction followed by two overshoots of activity 1 and 2 days after dosing. 3-Hydroxy-3-methylglutaryl coenzyme A reductase, the microsomal key enzyme of sterol synthesis, responded with a temporary reduction of activity only and evidently lost its diurnal rhythm. The time course of these changes is most likely caused by a combination of sub-physiological levels of glucocorticoids plus changes of other regulatory hormones elicited by feed intake, postprandial state, etc. A possible role for a combined action of the arylhydrocarbon (Ah) and glucocorticoid receptors in the effects of 3-methylcholanthrene is also suggested.

Priming for T-cell-mediated rejection of established tumors by cutaneous DNA immunization

DNA immunization has been shown to elicit both antibody and CTL responses against antigens expressed by infectious organisms. Because CTL responses have been implicated in rejection of cancer, we investigated whether DNA immunization by particle bombardment using a gene gun could induce CTL responses that were capable of rejecting tumors in mice. DNA immunization by particle bombardment using genes encoding beta-galactosidase and ovalbumin primed mice to generate CTLs in two genetic backgrounds (DBA/2 and C57BL/6 strains, respectively). DNA immunization was more potent in inducing CTLs than immunization with an optimized regimen of ovalbumin peptide plus immune adjuvant. Immunity induced by DNA immunization protected mice against s.c. challenge with tumors expressing the beta-galactosidase antigen. Tumors were rejected even when DNA immunization was started 3 or 7 days after tumor challenge as tumors were becoming established. Tumor rejection required CD8(+) T cells, confirming a role for CTLs in vivo. These studies show that DNA immunization by particle bombardment can efficiently induce CTL responses that are capable of rejecting even established tumors.

The toxicity of brominated and mixed-halogenated dibenzo-p-dioxins and dibenzofurans: an overview

Brominated dibenzo-p-dioxins and dibenzofurans can be formed under laboratory conditions by pyrolysis of flame retardants based on polybrominated biphenyls and biphenyl ethers. Their occurrence in the environment, however, is due to combustion processes such as municipal waste incineration and internal combustion engines. As these processes generally take place in the presence of an excess of chlorine, predominantly mixed brominated and chlorinated compounds have been identified so far in environmental samples. Brominated dibenzo-p-dioxins or dibenzofurans bind to the cytosolic Ah receptor about as avidly as their chlorinated congeners and induce hepatic microsomal enzymes with comparable potency. The same holds true for mixed brominated-chlorinated compounds. Gross pathologic symptoms-hypothyroidism, thymic atrophy, wasting of body mass, lethality-also occur at doses that, on a molar concentration basis, are virtually identical to those seen with the chlorinated compounds. Their potency to induce malformations in mice following prenatal exposure is equivalent to that of chlorinated dibenzo-p-dioxins and dibenzofurans. Possible activities as (co)carcinogens and endocrine disrupters have not been evaluated, but are likely to exist. Considering the overall similarity in action of chlorinated and brominated dibenzo-p-dioxins and dibenzofurans, environmental and health assessments should be based on molar body burdens without discrimination for the nature of the halogen.

Nutritional regulation of the activities of lipogenic enzymes of rat liver and brown adipose tissue

Nutrition-induced effects on the activity of enzymes of lipogenesis, fatty acid synthase (FAS: EC 2.3.1.85), ATP citrate lyase (ACL: EC 4.1.3.8), malic enzyme (ME; EC 1.1.1.40), glucose-6-phosphate dehydrogenase (G6PDH: EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (PGDH; EC 1.1.1.44) were investigated in liver and interscapular brown adipose tissue (BAT) of rats. The lipogenic enzymes could be grouped into two categories according to their response to dietary manipulations: FAS and ACL, both key enzymes of lipogenesis, responded fast and strongly to dietary manipulations. ME, G6PDH and PGDH, enzymes which also contribute to metabolic pathways other than lipogenesis, responded in a more sustained and less pronounced fashion. Feed deprivation caused the specific activities of lipogenic enzymes to decline several-fold. Refeeding of previously fasted (up to 3 days) animals increased the activities dramatically (10-to 25-fold) to far above pre-fasting levels ("overshoot"). Repetition of the fasting/refeeding regimen increasingly impaired the ability of both tissues to synthesize overshooting enzyme activities in the subsequent refeeding period. The fasting-induced decline of the activities was prevented when sugars were provided to the animals via drinking water. The sugars displayed different effectivities: sucrose = glucose > fructose > maltose > > lactose. Sugars as the sole nutrient after fasting were also able to induce overshooting enzyme activities. Again, activities of FAS and ACL responded in a more pronounced fashion than the other three enzymes. Transition from feeding one diet to feeding a new diet of different composition led to adaptation of the lipogenic enzyme activities to levels characteristic for the new diet. Replacing a low-carbohydrate with a high-carbohydrate diet proceeded with major alterations of enzyme activities. This process of attaining a new level took up to 20 days and involved pronounced oscillations of the specific activities. In contrast, when a high-carbohydrate diet was replaced with another diet. particular one high in fat, transition to new enzyme activities was completed within 2-3 days and proceeded without oscillations. All dietary manipulations caused more pronounced responses in young (35d-old) than in adult (180d-old) animals.

The response of rat serum lipids to diets of varying composition or contaminated with organochlorine pesticides

The effects of different diets (high carbohydrate, high protein, high fat) and diets contaminated with polychlorinated biphenyls (PCBs) and/or gamma-hexachlorocyclohexane (lindane) on the levels of serum triglycerides, cholesterol and phospholipids were investigated in Wistar rats. Serum triglyceride levels differed significantly among the diets, while those of cholesterol and phospholipids were much less affected by the diet composition. A change in diet composition resulted in a gradual adaptation to the lipid levels characteristic of the new diet with major variations including oscillations. There was, however, no specific component of a diet that could be associated with any specific change in serum lipids. While feed deprivation decreased the serum lipids (40-65% in 3 days), refeeding the starved animals caused pronounced increases of the lipids that were different among the diets. The response of the triglyceride levels was the strongest (up to 10 times the starvation levels) followed by those of the phospholipids (4-fold) and cholesterol (2.5-fold). Response of the triglyceride levels peaked within 1 or 2 days of refeeding, whereas those of cholesterol and phospholipids took 4 days to reach the maximum. Feeding PCB-contaminated diets increased the serum lipids in a dose-dependent manner (15-250 ppm). Higher PCB concentrations were increasingly inhibitory (350 ppm) or overtly toxic (> 400 ppm). Elevated lipids returned to the starting levels immediately after peaking (triglycerides) or only after several days (cholesterol, phospholipids) but with an earlier onset at lower PCB concentrations. Refeeding starved animals with PCB-contaminated diets also increased the serum lipids dose-dependently. Feeding lindane-containing diets (50-150 ppm) as well as refeeding animals with lindane diets resulted in a considerable increase of the triglyceride levels, while cholesterol and phospholipids increased much less. Higher lindane concentrations (250 ppm) were inhibitory. The outcome on serum lipid levels on feeding diets contaminated with both PCBs and lindane was basically additive.

Toxicokinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin in female Sprague-Dawley rats including placental and lactational transfer to fetuses and neonates

The toxicokinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in virgin female Sprague-Dawley (S-D) rats, the effects of pregnancy, parturition, and lactation on the distribution and/or redistribution of TCDD, and placental and lactational transfer to fetuses and neonates were investigated. Doses of 5.6 micrograms/kg of 14C-labeled TCDD were given i.v. either to virgin rats or to pregnant rats on Day 18 of gestation and 1 day postparturition, respectively. Virgin females were terminated on Day 1, 2, 4, 8, 16, or 32, pregnant rats on Day 1, 2, 4, or 8, after dosing to collect tissues. Two groups of neonates, which were born either to TCDD-treated or nontreated dams were cross-fostered beginning on the first day after birth to simulate exposure to TCDD either by lactational transfer only or by both placental and lactational transfer. Serum and 18 different tissues were collected from virgin rats to evaluate the kinetic profile of TCDD. Serum and tissue samples from liver, kidney, brown, and white adipose tissue were collected from pregnant and postparturition rats. Liver samples from fetuses and neonates were obtained on Gestational Days 19 and 20, or postnatally on Days 1 and 5. TCDD equivalents were calculated from measurement of radioactivity. The results show that the profile of TCDD distribution in virgin female rats was similar to that in male rats but that the concentration of TCDD in most tissues was higher in females than in males.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between toxicity and effects on intermediary metabolism in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated male C57BL/6J and DBA/2J mice

Male mice were treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) by gavage. C57BL/6J (C57) mice received 0.03 to 235 micrograms/kg, DBA/2J (DBA) mice 1 to 3295 micrograms/kg. On Day 8 after dosing blood was collected, and livers and kidneys were removed. Body weights and feed intake were not much affected until Day 8 after exposure. Hepatomegaly developed at doses above 3 and 97.5 micrograms/kg in C57 and DBA mice, respectively. Ethoxyresorufin O-deethylase activity was induced in liver with an ED50 of 1.1 and 16 micrograms/kg and in kidney with an ED50 of 65 and 380 micrograms/kg in C57 and DBA mice, respectively. The activity of phosphoenolpyruvate carboxykinase (PEPCK) in livers of both mouse strains was reduced over the entire dose range, displaying a plateau in the dose response at the onset of acute toxicity of TCDD. This enzyme activity was decreased by as much as 80% at the respective lethal doses. PEPCK activity in kidney was not affected. Glucose-6-phosphatase activity (G-6-Pase) in liver was altered only in the lethal dose range with a maximum reduction of about 50%. Serum glucose concentration was reduced over the entire dose range, but the reduction was significant only at doses in which G-6-Pase activity was affected, reaching levels as low as 3 mmol/liter in DBA mice. Tryptophan 2,3-dioxygenase activity was not lowered at any dose of TCDD in either mouse strain, and no increase in serum tryptophan levels was observed. Serum levels of thyroxine (T4) and triiodothyronine (T3) were dose dependently decreased over most of the dose range administered, with T3 levels exactly paralleling T4 levels in both mouse strains. It is concluded that TCDD causes acute toxicity in male C57 and DBA mice by a severe reduction of gluconeogenesis, but, in contrast to rats, it does not affect tryptophan homeostasis. Following administration of TCDD serum T3 levels in the mouse appear to correlate with T4 levels, whereas in the rat they are independent of each other.

Incorporation of first-order uptake rate constants from simple mammillary models into blood-flow limited physiological pharmacokinetic models via extraction efficiencies

Incorporation of First-Order Uptake Rate Constants from Simple Mammillary Models into Blood-Flow Limited Physiological Pharmacokinetic Models via Extraction Efficiencies. W. L. Roth, L. W. D. Weber, and K. Rozman (1995). Pharm. Res. 263-269. First-order rate constants obtained from classical pharmacokinetic models correspond to mammillary systems in which all of the blood (or plasma) is assumed to be located in a central compartment. In such models the rate at which chemicals are transported out of this pool and into another compartment is the product of the mass of chemical in the central compartment multiplied by a rate constant, which is not limited in magnitude by the blood flow, or the rate at which chemicals from the blood are delivered to the peripheral compartment. Most of the physiologically-based models published to date dispense with some of the information available from mammillary models by assuming that all of the chemical delivered by the flow of blood rapidly equilibrates and can be taken up by the tissue under the control of a "partition coefficient" (Rij = Cj/Ci). We show that the partition coefficient alone does not retain the uptake rate (kji) information available from a classical mammillary model, but that the uptake rate information can be incorporated via unitless extraction efficiency parameters, epsilon j.

The effect of gamma-hexachlorocyclohexane (lindane) on the activities of liver lipogenic enzymes and on serum lipids in rats

The effect of dietary gamma-hexachlorocyclohexane (lindane) (50-350 ppm, 0.17-1.19 mumol/kg chow) on the activity of enzymes of lipogenesis, viz., fatty acid synthase (FAS; EC 2.3.1.85), citrate cleavage enzyme (CCE; EC 4.1.3.8), malic enzyme (ME; EC 1.1.1.40), glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (PGDH; EC 1.1.1.44), and on serum lipid levels, was investigated in livers of 35-day-old male Wistar rats. Lindane (150 ppm) caused a substantial decline of enzyme activities within the first 24 h of treatment. The decrease was transient, however, and enzyme activities subsequently recovered despite continuation of lindane feeding. The recovery of enzyme activities was comparatively fast in the case of ME, G6PDH and PGDH, but very slow with FAS and CCE. Activities of lipogenic enzymes decrease when animals are starved, and increase much beyond prestarvation levels upon subsequent refeeding. Lindane in the refeeding diet blunted this overshoot of FAS and CCE activities in a dose-dependent manner. In contrast, activities of Me, G6PDH and PGDH responded to low dietary lindane concentrations with a substantial stimulation of the increase of activity, whereas at high lindane concentrations the overshoot was inhibited. According to their responses to lindane exposure, liver lipogenic enzymes could be grouped into 2 categories with FAS and CCE representing one and ME, G6PDH and PGDH representing the other group. Polychlorinated biphenyls (PCBs) in the diet caused basically opposite changes of the activities of the lipogenic enzymes. Co-administration of lindane and PCBs resulted in an apparent cancellation of effects, suggesting that lindane and PCBs affect fatty acid synthesis at opposite points.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipogenic enzymes of rat liver and adipose tissue. Dietary variations and effect of polychlorinated biphenyls

The lipogenic enzymes fatty acid synthase (FAS; EC 2.3.1.85), citrate cleavage enzyme (CCE; EC 4.1.3.8), malic enzyme (ME; EC 1.1.1.40), glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (PGDH; EC 1.1.1.44) were investigated in liver and in brown adipose tissue (BAT) of Wistar rats under various dietary conditions and in the presence of 15 to 250 ppm (approximately 0.045-0.75 mumol/kg chow) polychlorinated biphenyls (PCBs). In response to refeeding starved animals, enzyme activities in both tissues increased to above normal levels and thereafter exhibited pronounced oscillations of their activities. The extent of increase depended on the carbohydrate and fat content of the diet. The lipogenic enzymes could be grouped in two categories according to their sensitivity to dietary carbohydrate: FAS and CCE responded faster to smaller changes in dietary composition, while ME, G6PDH and PGDH required larger changes and more time to respond. Diet-induced alterations of enzyme activities were of the same order of magnitude in liver and BAT. They were age-dependent, being more pronounced in young animals. Independent of the type of dietary manipulations, activities changed in a coordinate fashion, i.e., the changes of the activities of all 5 enzymes occurred at similar ratios to each other with an identical time course. Feeding PCB-containing diets resulted in a considerable increase of the activities of the lipogenic enzymes in liver, which was significantly greater with ME, G6PDH and PGDH. The effect was dose-dependent but transient. In liver the response to PCB feeding was identical in male and female animals, whereas in BAT lipogenic activities increased in females, but decreased in males. Refeeding starved animals with a PCB-containing diet led to an additional stimulation of the normal refeeding-induced increase of the enzyme activities in liver and BAT. This PCB-induced increase was 2-fold for FAS and CCE, but up to 15-fold for the other enzymes. All PCB-induced effects were significantly less pronounced in old than in young animals. In primary hepatocytes activities increased in hormone-free medium in the presence of PCBs. While activity was induced in insuline- and triiodothyronine-containing medium, this increase was significantly greater with PCBs present.

A pharmacodynamically responsive model of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) transfer between liver and fat at low and high doses

We have constructed a pharmacokinetic model for TCDD in the rat, emphasizing its transfer between plasma, lipid, and cytoplasmic compartments of the liver, white adipose tissue (WAT), and brown adipose tissue (BAT). Volumes of the lipid pools are controlled by a submodel of triglyceride (TG) metabolism and transport that responds via a receptor for TCDD in WAT cytoplasm with a Kd of about 2 nM. This submodel, and one for cytochrome P450IA2 induction, allowed us to simulate binding of TCDD to the induced P450IA2 binding sites at low doses (1 ng/kg to 10 micrograms/kg) independently of the decreased feed intake and hyperlipidemia associated with higher doses (20 to 120 micrograms/kg). In low-dose simulations, the induction of cytochrome P450IA2 binding sites for TCDD dominated the redistribution of TCDD between WAT and liver. When simulations were performed using a partitioning model with constant gastrointestinal flows, increased WAT lipolysis driven by reduced feed intake (from 10 to 120 micrograms/kg) is predicted to result in a decreased WAT volume and a sharp drop in the mass of WAT-associated TCDD, while initially increasing the levels of TCDD in liver. However, the observed concentration of TCDD in WAT increased in rats treated with a high dose (72 micrograms/kg) of TCDD. The rise in tissue concentrations could not be explained without incorporating decreased intestinal flows into the gastrointestinal absorption process, which increased the resorption of TCDD. TCDD concentrations in tissue increased only when the relative tissue volumes decreased more rapidly than the whole-body TCDD elimination rate.

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.

The penetration of 2,3,7,8-tetrachlorodibenzo-p-dioxin into viable and non-viable porcine skin in vitro

Freshly harvested, full thickness porcine skin was kept metabolically viable at 4 degrees C in a minimal essential medium for at least 48 h, as judged by the formation of lactate or 14CO2 from 14C-labeled glucose. In vitro topical exposure to the environmental contaminant, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 65 ng/cm2) for up to 1000 min did not affect the viability of skin. The penetration and distribution of TCDD into porcine skin was studied in an in vitro system under a variety of conditions, such as viability status, different vehicles or concentrations, or artificial removal of the stratum corneum. Loss of viability of the skin increased the rate of penetration of TCDD by about 60%. Removal of the stratum corneum to mimic lesioned skin increased the rate of dermal penetration of TCDD about 3-fold. The use of acetone as the vehicle, simulating dermal exposure to TCDD as a dust or from a volatile solvent, resulted in higher rates of penetration than the use of mineral oil as the vehicle, which simulates the situation of industrial accidents. The percentage of dose absorbed was independent of the dose of TCDD (65 or 6.5 ng/cm2) administered to the surface of skin. Rates of dermal penetration of TCDD ranged form 14 to 985 pg/cm2 skin per h, or 0.2-1.5% of the dose/h, depending on the conditions of exposure. These rates of penetration are comparable with results obtained by others in several other species, with both in vitro and in vivo systems including human skin in vitro. Full thickness porcine skin, viable or previously frozen, is therefore a valid in vitro model to estimate dermal penetration of TCDD in humans.

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 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.

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.

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.

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.

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.

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.

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.

Metabolism and distribution of [14C]glucose in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Male Sprague-Dawley rats were given a single, usually lethal, dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 125 micrograms/kg ip in corn oil), or vehicle alone. Twenty-four hours after ip administration of TCDD the animals received an ip injection of 14C-labeled glucose, and the time course and amount of exhalation of 14CO2 were monitored for 8 h continuously and once daily for 20 min for the subsequent 5 d. TCDD treatment reduced the amount of 14CO2 exhaled within 8 h after the injection of [14C]glucose by 33%, as compared to pair-fed controls. Blood levels of radioactivity were affected by TCDD accordingly. No particular organ appeared to act as a sink for the radioactivity not exhaled during these 8 h by the treated animals. TCDD (125 micrograms/kg) induced significant changes in the disposition of radioactivity in heart and brown adipose tissue between 25 and 125 min after the iv injection of [14C]glucose. The areas under the curve of [14C]glucose-derived radioactivity were the same after either iv or ip injection in the blood of TCDD-treated rats, allowing a direct comparison of experiments with iv or ip injection of [14C]glucose. The half-lives of radioactivity in the exhaled air and in feces of treated animals were greatly elevated during the 5 d following administration of [14C]glucose. These results indicate that TCDD induces in rats, within 24 h after dosing, alterations in the metabolism of glucose that preceded changes in insulin homeostasis, because hypoglycemia and hypoinsulinemia in rats do not occur until about a week after TCDD treatment. Since overt signs of acute toxicity (reduced feed intake and body weight loss) are also not noticeable until several days after a lethal dose of TCDD, it is probable that this earlier disturbance of glucose metabolism is part of the biological changes that result in wasting away and eventually in death.

The postnatal pattern of ornithine decarboxylase activity reveals a disparity of rat brain regeneration capacity after prenatal X-ray or 5-azacytidine treatment

Pregnant Wistar rats were treated on the 15th day of gestation either with 1.4 Gy X-radiation, or with 2 X 2.5 mg 5-azacytidine per kg body weight. X-irradiation caused negligible mortality among the offspring, despite of a 35% reduction of brain weights. The course of brain ornithine decarboxylase activity exhibited two breaches within 5 days after birth, each followed by recovery to control levels. After 5-azacytidine treatment brain weights were reduced by 16% only, but two thirds of the young died within a short time after birth. During three days following birth, the activity of ornithine decarboxylase in the brains of the young animals split into two ranges, a high one at control level and a low one at about one fifth of control level. As the ratio of brains with low to those with high enzyme activities correlated with the rate of postnatal mortality, the splitting of early postnatal enzyme activities was interpreted in terms of a nothing-or-all-law: beyond a certain amount of 5-azacytidine incorporated into brain DNA, gene expression was impaired to an extent not compatible with the survival of the animals.

A practicable variant of the ion exchange method for the radiometric estimation of ornithine decarboxylase activity

A known ornithine decarboxylase assay working with ion exchange separation of [3H]ornithine and [3H]putrescine has been revised. The assay can be performed in disposable 1.5 ml vessels with a total of four pipetting steps. The separation of enzyme substrate and product, respectively, requires 3 h per 50 samples. The detection limit is about 50 pmoles [3H]putrescine formed.

The influence of prenatal diazepam treatment on the postnatal pattern of mouse brain Na, K-ATPase activity

Pregnant mice were treated with 1 and 5 mg/kg/day diazepam, respectively, during five days of the second and of the last third of gestation. The activity of Na, K-ATPase was determined in the brains of their offspring. Diazepam, 5 mg/kg/day, caused changes in the developmental patterns of the enzyme activity which did not become evident before 16 days of age. As expected from the low doses of diazepam applied the effect on enzyme activity was moderate. It may, however, provide a biochemical correlate of altered behaviour.

Developmental patterns of three X chromosomal enzyme activities in the brains of female and male mice. Lack of sex-dependent reaction to a fractionated prenatal x-ray dose

The developmental profiles of the activities of the X chromosome-linked enzymes alpha-galactosidase, hypoxanthine guanine phosphoribosyltransferase, and glucose-6-phosphate dehydrogenase were studied in the brains of mice. X chromosome inactivation, which takes place at an early stage of development, led to identical enzyme activities in females and males between the 15th day of gestation and the 64th day after birth. The enzyme activities were also studied after an X-ray dose of 3 times 1.05 Gy, delivered on gestational days 11, 12 and 13. Treatment with this dose, but not with 3 times 0.95 Gy or 1.15 Gy, respectively, was followed by the death of predominantly female offspring within 2 days after birth. The assumption that the reason for this might be a reactivation of inactive female X chromosomal genes in brain cells was tested in the present experiment. There were marked radiation-induced alterations of the activities of the above-mentioned enzymes. However, sex differences of these enzyme activities were not found after the irradiation treatment. A participation of X chromosome reactivation in the elevated mortality of female offspring is therefore unlikely.

Distribution of polychlorobiphenyls in pregnant mice and fetuses on various days of gestation

14C-2,4',5-trichlorobiphenyl (test series A) and a laboratory mixture of 14C-PCBs, which in addition to 2,4',5-trichlorobiphenyl, contained dichlorobiphenyl and other trichlorobiphenyl isomers (test series B), were administered orally to pregnant mice on the 13th, 15th and 17th day post conceptionem (p.c.), respectively. The development of the fetuses was accompanied by increasing activity concentrations and changes in the distribution patterns of the radioactive PCBs (and/or metabolites). In either test series, one day after administration the 18-day old fetus showed approximately a 2.5 times higher activity concentration than the fetus of an earlier developmental stage (14th day p.c.). Considering the weight gain, the quantity of activity per fetus increased almost tenfold. PCBs were concentrated only in the intestine of the terminal fetus, probably because of an increased intake of PCB-containing amniotic fluid. The affinity of the PCBs, especially of the 2,4',5 trichlorobiphenyl, to lipids, and the increasing lipid content of the fetal tissue may offer an additional explantation of the increase in activity concentration during the fetal development.