Iodothyronine metabolism in rat liver homogenates

Restorative effects of gallic acid against sub-chronic hepatic toxicity of co-exposure to zinc oxide nanoparticles and arsenic trioxide in male rats

Background and objectives

This study aimed to assess the effect of zinc oxide nanoparticles (ZNPs) and/or arsenic trioxide (ATO) exposure on the liver of adult male Sprague Dawley rats. Moreover, the probable ameliorative impact of gallic acid (GA) against ZNPs and ATO-induced hepatotoxicity and the possible underlying mechanisms were evaluated.

Methods

Sixty male Sprague Dawley rats were distributed into six groups. The 1^st and 2^nd groups were orally given distilled water (1 ml/kg) and 20 mg GA/kg b. wt, respectively. The 3^rd and 4^th groups were orally given 100 mg ZNPs/kg b. wt and 8 mg ATO/kg b. wt, respectively. The 5^th group was co-administered ZNPs and ATO at the doses mentioned above. The last one was co-administered ZNPs, ATO, and GA at the earlier described doses. All tested compounds were orally given once a day for 60 successive days. Then, serum levels of alkaline phosphatase (ALP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), total, direct, indirect bilirubin, triglycerides, total cholesterol, HDL, VLDL, and LDL were estimated. The hepatic content of malondialdehyde (MDA), superoxide dismutase (SOD), and glutathione peroxidase (GPx) was evaluated. Moreover, Bcl-2 and Bax's reactive proteins were immunohistochemically detected, and Zn and As residual patterns in hepatic tissues were assessed.

Results

ZNPs, ATO, and ZNPs+ATO-exposed rats showed significantly (P < 0.001) elevated serum AST (219%, 233%, and 333%), ALT (300%, 400%, and 475%), ALP (169%, 205%, and 294%), and total bilirubin (42%, 68%, and 109%) compared to the control ones. On the other hand, a significantly (P < 0.001) declined SOD (58%, 49%, and 43%) and GPx (70%, 63%, and 56%) but increased MDA (133%, 150%, and 224%) was recorded in the hepatic tissues of ZNPs, ATO, and ZNPs+ATO exposed rats, respectively, relative to the control rats. Moreover, the hepatic tissues of the ZNPs, ATO, and ZNPs+ATO exposed rats showed a significant (P < 0.001) decrease in Bcl-2 (28%, 33%, and 23%) but elevation in Bax (217%, 267%, and 236%) immunoreactivities compared to the control rats. These findings were consistent with the microscopic alterations in the hepatic architecture and accumulation of Zn and As. Furthermore, a notable hyperlipidemic condition was recorded following ZNPs and/or ATO exposure. On the contrary, GA notably reduced hepatic enzymes compared to ZNPs+ATO-exposed rats. Additionally, GA markedly improved ZNPs+ATO-afforded liver tissue damage and apoptotic events.

Conclusion

Overall, GA oral dosing significantly mitigated the negative effects of ZNPs and ATO on the liver by improving the antioxidant defense system and controlling apoptotic changes.

Identification of the Hepatic Metabolites of Flumazenil and their Kinetic Application in Neuroimaging

Studies of the neurobiological causes of anxiety disorders have suggested that the γ-aminobutyric acid (GABA) system increases synaptic concentrations and enhances the affinity of GABA_A (type A) receptors for benzodiazepine ligands. Flumazenil antagonizes the benzodiazepine-binding site of the GABA/benzodiazepine receptor (BZR) complex in the central nervous system (CNS). The investigation of flumazenil metabolites using liquid chromatography (LC)-tandem mass spectrometry will provide a complete understanding of the in vivo metabolism of flumazenil and accelerate radiopharmaceutical inspection and registration. The main goal of this study was to investigate the use of reversed-phase high performance liquid chromatography (PR-HPLC), coupled with electrospray ionization triple-quadrupole tandem mass spectrometry (ESI-QqQ MS), to identify flumazenil and its metabolites in the hepatic matrix. Carrier-free nucleophilic fluorination with an automatic synthesizer for [¹⁸F]flumazenil, combined with nano-positron emission tomography (NanoPET)/computed tomography (CT) imaging, was used to predict the biodistribution in normal rats. The study showed that 50% of the flumazenil was biotransformed by the rat liver homogenate in 60 min, whereas one metabolite (M1) was a methyl transesterification product of flumazenil. In the rat liver microsomal system, two metabolites were identified (M2 and M3), as their carboxylic acid and hydroxylated ethyl ester forms between 10 and 120 min, respectively. A total of 10-30 min post-injection of [¹⁸F]flumazenil showed an immediate decreased in the distribution ratio observed in the plasma. Nevertheless, a higher ratio of the complete [¹⁸F]flumazenil compound could be used for subsequent animal studies. [¹⁸F] According to in vivo nanoPET/CT imaging and ex vivo biodistribution assays, flumazenil also showed significant effects on GABA_A receptor availability in the amygdala, prefrontal cortex, cortex, and hippocampus in the rat brain, indicating the formation of metabolites. We reported the completion of the biotransformation of flumazenil by the hepatic system, as well as [¹⁸F]flumazenil's potential as an ideal ligand and PET agent for the determination of the GABA_A/BZR complex for multiplex neurological syndromes at the clinical stage.

Hepatic Energy Metabolism under the Local Control of the Thyroid Hormone System

The energy homeostasis of the organism is orchestrated by a complex interplay of energy substrate shuttling, breakdown, storage, and distribution. Many of these processes are interconnected via the liver. Thyroid hormones (TH) are well known to provide signals for the regulation of energy homeostasis through direct gene regulation via their nuclear receptors acting as transcription factors. In this comprehensive review, we summarize the effects of nutritional intervention like fasting and diets on the TH system. In parallel, we detail direct effects of TH in liver metabolic pathways with regards to glucose, lipid, and cholesterol metabolism. This overview on hepatic effects of TH provides the basis for understanding the complex regulatory network and its translational potential with regards to currently discussed treatment options of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) involving TH mimetics.

The palliative effect of mulberry leaf and olive leaf ethanolic extracts on hepatic CYP2E1 and caspase-3 immunoexpression and oxidative damage induced by paracetamol in male rats

This study investigated the possible protective role of mulberry leaf (MLE) and olive leaf (OLE) ethanolic extracts against paracetamol (PTL)-induced liver injury in rats compared to silymarin as a reference drug. Initially, MLE and OLE were characterized using gas chromatography-mass spectrometry (GC/MS). Then, forty male Sprague Dawley rats were divided into five groups: the negative control group orally received distilled water for 35 days, the PTL-treated group (PTG) received 500 mg PTL/kg b. wt. for 7 days, the MLE-treated group (MLTG) received 400 mg MLE/kg b. wt., the OLE-treated group (OLTG) received 400 mg OLE/kg b. wt., and the silymarin-treated group (STG) received 100 mg silymarin/kg b. wt. The last three groups received the treatment for 28 days, then PTL for 7 days. The GC-MS characterization revealed that MLE comprised 19 constituents dominated by ethyl linoleate, phytol, hexadecanoic acid, ethyl ester, and squalene. Moreover, OLE comprised 30 components, and the major components were 11-eicosenoic acid, oleic acid, phytol, and à-tetralone. MLE and OLE significantly corrected the PTL-induced normocytic normochromic anemia, leukocytosis, hypercholesterolemia, and hypoproteinemia. Moreover, the MLE and OLE pretreatment considerably suppressed the PTL-induced increment in serum levels of hepatic enzymes, including alkaline phosphatase, alanine aminotransferase, and aspartate aminotransferase. Furthermore, the PTL-induced depletion in antioxidant enzymes, including glutathione peroxidase, superoxide dismutase, and catalase, and the rise in hepatic malondialdehyde content were significantly reversed by the MLE and OLE pretreatment. Besides, MLE and OLE pretreatment significantly protected the hepatic tissue against PTL-induced DNA damage, pathological perturbations, and increased caspase 3 and CYP2E1 immunoexpression. Of note, OLTG showed better enhancement of most indices rather than MLTG. Conclusively, these findings imply that OLE, with its antioxidant and antiapoptotic capabilities, is superior to MLE in protecting against PTL-induced liver injury.

Thymoquinone supplementation mitigates arsenic-induced cytotoxic and genotoxic alterations in rat liver

Arsenic, a widespread environmental toxin, produces multiple organ toxicity, including hepatotoxicity. Thymoquinone (TQ) is known to restore liver functions in several hepatic injury models. This study aims to assess the mitigative potential of TQ against sodium arsenate (NaAs)-induced cytotoxic and genotoxic alterations in the liver. Rats were randomly distributed to control, NaAs, TQ, and NaAs+TQ groups. NaAs+TQ and TQ group of rats were pre-treated with TQ (1.5 mg/kg bwt, orally) for 14 days, and the treatment was further continued for 30 days, with and without NaAs treatment (5 mg/kg bwt, orally), respectively. The deleterious histological alterations in the liver of arsenic intoxicated animals were accompanied by an upsurge in the activities of serum ALT and AST, the diagnostic indicators of liver injury. NaAs caused pronounced alterations in the activities of membrane marker and carbohydrate metabolic enzymes and the enzymatic and non-enzymatic components of hepatic antioxidant defense. Significant hepatocyte DNA damage and hepatic arsenic accumulation were also observed in arsenic-exposed rats. TQ supplementation alleviated these adverse alterations and improved the overall hepatic metabolic and antioxidant status in NaAs-administered rats. Prevention of oxidative injury could be the key mechanism of TQ-elicited protective effects. TQ may have an excellent scope as a dietary supplement in the management of arsenic-induced hepatic pathophysiology.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Reactivity of the Thyroid System to Short-Term Stress in Wistar Rats with Visceral Obesity and Restricted Social Activity

The obesity problem requires a study of its pathophysiological consequences affecting hormonal regulation and organism’s reactivity to extreme exposures. The study was aimed first to examine the effect of a high-calorie diet and social isolation, as well as their combination for 4 months, on the development of obesity, its metabolic and behavioral sequelae, features of the thyroid status, while at the second stage, to assess the reaction of hormonal indices of the thyroid status to short-term stress in rats. The experiments were carried out on male Wistar rats and at the first stage focused on the effects of a high-calorie diet and social isolation, as well as their combinations for 4 months. At the end of the experiment, behavioral reactions, metabolic syndrome indices, thyroid status, and cortisol levels were evaluated. At the second stage, the animals were exposed to short-term acute stress, and the shifts in the hormonal indices were recorded one hour later versus the initial background. A high-calorie diet led to the development of metabolic syndrome, signs of depression, increased thyroid-stimulating hormone (TSH), thyroxine and triiodothyronine serum levels, as well as iodothyronine deiodinase type 1 (D1) activity, in the rat liver. At the same time, there was a decrease in thyroperoxidase activity and an increase in thyroid levels of triglycerides and malondialdehyde. The physiological response to stress in the control rat group included an increase in cortisol and TSH serum levels, however, against the background of a high-calorie diet, no cortisol release into the bloodstream was recorded. Social isolation did not alter normal reactivity of the adrenal cortex, but reduced TSH release in response to acute stress, since the initial level of this hormone was slightly elevated against the background of chronic social isolation stress. Thus, excessive nutrition and the deficit of social activities in male Wistar rats led to significant changes in the organism’s reactivity to acute stress.

Oral Nigella sativa oil administration alleviates arsenic-induced redox imbalance, DNA damage, and metabolic and histological alterations in rat liver

Arsenic, an omnipresent environmental contaminant, is regarded as a potent hepatotoxin. Nigella sativa oil (NSO) consumption has been shown to improve hepatic functions in various in vivo models of acute hepatic injury. The present study evaluates the protective efficacy of NSO against sodium arsenate (As)-induced deleterious alterations in the liver. Male Wistar rats were divided into four groups, namely, control, As, NSO, and AsNSO. After pre-treating rats in AsNSO and NSO groups with NSO (2 mL/kg bwt, orally) for 14 days, NSO treatment was further extended for 30 days, with and without As treatment (5 mg/kg bwt, orally), respectively. As induced an upsurge in serum ALT and AST activities indicating liver injury, as also confirmed by the histopathological findings. As caused significant alterations in the activities of membrane marker enzymes and carbohydrate metabolic enzymes, and in the vital components of antioxidant defense system. Marked DNA damage and hepatic arsenic accumulation were also observed in As-treated rats. Oral NSO administration ameliorated these deleterious alterations and improved overall hepatic antioxidant and metabolic status in As-treated rats. Prevention of oxidative damage could be the underlying mechanism of NSO-mediated protective effects. The results suggest that NSO could be a useful dietary supplement in the management of arsenic hepatotoxicity.

Screening the ToxCast Phase 1, Phase 2, and e1k Chemical Libraries for Inhibitors of Iodothyronine Deiodinases

Deiodinase enzymes play an essential role in converting thyroid hormones between active and inactive forms by deiodinating the pro-hormone thyroxine (T4) to the active hormone triiodothyronine (T3) and modifying T4 and T3 to inactive forms. Chemical inhibition of deiodinase activity has been identified as an important endpoint to include in screening chemicals for thyroid hormone disruption. To address the lack of data regarding chemicals that inhibit the deiodinase enzymes, we developed robust in vitro assays that utilized human deiodinase types 1, 2, and 3 and screened over 1800 unique chemicals from the U.S. EPA's ToxCast phase 1_v2, phase 2, and e1k libraries. Initial testing at a single concentration identified 411 putative deiodinase inhibitors that produced inhibition of 20% or greater in at least 1 of the 3 deiodinase assays, including chemicals that have not previously been shown to inhibit deiodinases. Of these, 228 chemicals produced enzyme inhibition of 50% or greater; these chemicals were further tested in concentration-response to determine relative potency. Comparisons across these deiodinase assays identified 81 chemicals that produced selective inhibition, with 50% inhibition or greater of only 1 of the deiodinases. This set of 3 deiodinase inhibition assays provides a significant contribution toward expanding the limited number of in vitro assays used to identify chemicals with the potential to interfere with thyroid hormone homeostasis. In addition, these results set the groundwork for development and evaluation of structure-activity relationships for deiodinase inhibition, and inform targeted selection of chemicals for further testing to identify adverse outcomes of deiodinase inhibition.

Reduced Skeletal Muscle Protein Turnover and Thyroid Hormone Metabolism in Adaptive Thermogenesis That Facilitates Body Fat Recovery During Weight Regain

Objective: The recovery of body composition after weight loss is characterized by an accelerated rate of fat recovery (preferential catch-up fat) resulting partly from an adaptive suppression of thermogenesis. Although the skeletal muscle has been implicated as an effector site for such thrifty (energy conservation) metabolism driving catch-up fat, the underlying mechanisms remain to be elucidated. We test here the hypothesis that this thrifty metabolism driving catch-up fat could reside in a reduced rate of protein turnover (an energetically costly "futile" cycle) and in altered local thyroid hormone metabolism in skeletal muscle. Methods: Using a validated rat model of semistarvation-refeeding in which catch-up fat is driven solely by suppressed thermogenesis, we measured after 1 week of refeeding in refed and control animals the following: (i) in-vivo rates of protein synthesis in hindlimb skeletal muscles using the flooding dose technique of ¹³C-labeled valine incorporation in muscle protein, (ii) ex-vivo muscle assay of net formation of thyroid hormone tri-iodothyronine (T3) from precursor hormone thyroxine (T4), and (iii) protein expression of skeletal muscle deiodinases (type 1, 2, and 3). Results: We show that after 1 week of calorie-controlled refeeding, the fractional protein synthesis rate was lower in skeletal muscles of refed animals than in controls (by 30-35%, p < 0.01) despite no between-group differences in the rate of skeletal muscle growth or whole-body protein deposition-thereby underscoring concomitant reductions in both protein synthesis and protein degradation rates in skeletal muscles of refed animals compared to controls. These differences in skeletal muscle protein turnover during catch-up fat were found to be independent of muscle type and fiber composition, and were associated with a slower net formation of muscle T3 from precursor hormone T4, together with increases in muscle protein expression of deiodinases which convert T4 and T3 to inactive forms. Conclusions: These results suggest that diminished skeletal muscle protein turnover, together with altered local muscle metabolism of thyroid hormones leading to diminished intracellular T3 availability, are features of the thrifty metabolism that drives the rapid restoration of the fat reserves during weight regain after caloric restriction.

Thyroid dysfunction in children with leukemia over the first year after hematopoietic stem cell transplantation

Background Thyroid function in children with leukemia during the first year after hematopoietic stem cell transplantation (HSCT) was investigated. Methods The medical records of 186 subjects [111 boys and 75 girls; lymphoid=75, myeloid=111; median age at HSCT was 10.7 (0.8-21.8) years old] were reviewed retrospectively. Results In children with leukemia, T3 decreased at 1 month (p<0.001) and recovered 9 months to the levels before HSCT. TSH decreased at 1 month (p<0.001), recovered at 3 months and increased at 12 months (p<0.001) to the levels before HSCT. The incidence of euthyroid sick syndrome (ESS, 23.2%, 15.5%, 5.9%, 5.2%, 3.9%, p for trend <0.001) decreased and subclinical hypothyroidism (SH, 0%, 3.9%, 14.8%, 22.1%, 21.3%, p for trend <0.001) increased at 1, 3, 6, 9 and 12 months after HSCT. Out of 55 patients developing ESS during 3 months after HSCT, 54 recovered to normal thyroid function within 5 months without medication. Among the total 186 subjects, 21 patients have been treated with levothyroxine. Both height and weight standard deviation scores continued to decrease over 1 year after HSCT. Conclusions In children with leukemia, one-quarter had ESS at 1 month and one-fifth had SH at 12 months and continued growth impairments were observed during 1 year after HSCT. Most of the ESS patients recovered to normal within 5 months without medication. More long-term follow-up of thyroid function and growth in children with leukemia after HSCT is crucial.

Design of Radioiodinated Pharmaceuticals: Structural Features Affecting Metabolic Stability towards in Vivo Deiodination

Radioiodinated pharmaceuticals are convenient tracers for clinical and research investigations because of the relatively long half-lives of radioactive iodine isotopes (i.e., 123I, 124I, and 131I) and the ease of their chemical insertion. Their application in radionuclide imaging and therapy may, however, be hampered by poor in vivo stability of the C-I bond. After an overview of the use of iodine in biology and nuclear medicine, we present here a survey of the catabolic pathways for iodinated xenobiotics, including their biodistribution, accumulation, and biostability. We summarize successful rational improvements in the biostability and conclude with general guidelines for the design of stable radioiodinated pharmaceuticals. It appears to be necessary to consider the whole molecule, rather than the radioiodinated fragment alone. Iodine radionuclides are generally retained in vivo on sp2 carbon atoms in iodoarenes and iodovinyl moieties, but not in iodinated heterocycles or on sp3 carbon atoms. Iodoarene substituents also have an influence, with increased in vivo deiodination in the cases of iodophenols and iodoanilines, whereas methoxylation and difluorination improve biostability.

Elevated Serum Tetrac in Graves Disease: Potential Pathogenic Role in Thyroid-Associated Ophthalmopathy

CONTEXT

The sources and biological impact of 3,3',5,5' tetraiodothyroacetic acid (TA4) are uncertain. CD34+ fibrocytes express several proteins involved in the production of thyroid hormones. They infiltrate the orbit in Graves disease (GD), an autoimmune process known as thyroid-associated ophthalmopathy. It appears that the thyrotropin receptor plays an important role in the pathogenesis of thyroid-associated ophthalmopathy.

OBJECTIVE

To quantify levels of TA4 in healthy participants and those with GD, determine whether fibrocytes generate this thyroid hormone analogue, and determine whether TA4 influences the actions of thyroid-stimulating hormone and thyroid-stimulating immunoglobulins in orbital fibroblasts.

DESIGN/SETTING/PARTICIPANTS

Patients with GD and healthy donors in an academic medical center clinical practice were recruited.

MAIN OUTCOME MEASURES

Liquid chromatography-tandem mass spectrometry, autoradiography, real-time polymerase chain reaction, hyaluronan immunoassay.

RESULTS

Serum levels of TA4 are elevated in GD. TA4 levels are positively correlated with those of thyroxine and negatively correlated with serum levels of triiodothyronine. Several cell types in culture generate TA4 from ambient thyroxine, including fibrocytes, HELA cells, human Müller stem cells, and retinal pigmented epithelial cells. Propylthiouracil inhibits TA4 generation. TA4 enhances the induction by thyrotropin and thyroid-stimulating immunoglobulins of several participants in the pathogenesis of thyroid-associated ophthalmopathy, including interleukin 6, hyaluronan synthase 1, prostaglandin endoperoxide H synthase 2, and haluronan production.

CONCLUSION

TA4 may be ubiquitously generated in many tissues and enhances the biological impact of thyrotropin and thyroid-stimulating immunoglobulins in orbital connective tissue. These findings may identify a physiologically important determinant of extrathyroidal thyroid-stimulating hormone action.

Identifying reference chemicals for thyroid bioactivity screening

Reference chemicals were selected based on thyroid bioactivity in 'Tier 1' screening assays used by the U.S. EPA's Endocrine Disruptor Screening Program. Active reference chemicals had significant effects on thyroid-responsive endpoints in the amphibian metamorphosis assay, and the male and female pubertal rat assays. In the absence of thyroid weight or histopathological effects, additional published studies providing mechanistic data on thyroid activity were required for active chemicals. Inactive reference chemicals had no significant effects on thyroid-responsive endpoints in Tier 1 assays, or in amphibian or rodent studies from several online databases. The 34 reference chemicals (29 active and five inactive) will be useful for performance-based validation of alternative, high throughput screening assays for thyroid bioactivity.

Perinatal Iron Deficiency-Induced Hypothyroxinemia Impairs Early Brain Development Regardless of Normal Iron Levels in the Neonatal Brain

BACKGROUND

Both perinatal hypothyroxinemia and perinatal iron deficiency (ID) are associated with poor neurodevelopment in offspring. Iron is an important component of thyroid peroxidase, a key enzyme in the synthesis of thyroid hormone. The authors' previous study demonstrated that perinatal ID can lead to maternal hypothyroxinemia during pregnancy. The goal of this study was to determine whether perinatal ID-associated hypothyroxinemia can cause brain defects prior to neonatal brain iron depletion.

METHODS

Two rat models were established to imitate the two common types of maternal ID (mild ID with anemia [ID + A] and ID without anemia [ID - A]), and iron limitation was initiated two weeks before pregnancy. Maternal and neonatal thyroid hormones in serum were analyzed at postnatal day (P) 0 and P10. Neonatal thyroid hormone, as well as mRNA expression of some thyroid hormone-responsive genes in the cerebral cortex and hippocampus, were measured at P10. Serum iron and brain iron concentrations were analyzed by inductively coupled plasma mass spectrometry. Liver iron concentration was determined using graphite furnace atomic absorption spectroscopy. Hemoglobin was analyzed with an automated blood coagulation analyzer. Surface righting reflex and vibrissae-evoked forelimb placing were measured to assess the sensorimotor behaviors.

RESULTS

It was found that pre-pregnant mild ID resulted in maternal hypothyroxinemia, which lasted from gestation day 13 to P10. Pre-pregnant mild ID decreased the neonatal brain total triiodothyronine level at P10. Consistent with a low total triiodothyronine level, the mRNA expression of some thyroid hormone-responsive genes (Mbp, RC3, and Srg1) were significantly reduced in the neonatal cerebral cortex and hippocampus in both ID rat models at P10. Furthermore, ID rat pups at P10 showed retarded sensorimotor skills. No significant difference was found between the control and the ID pups in terms of iron concentrations in the neonatal brain at P10.

CONCLUSIONS

This study demonstrates that perinatal ID-associated hypothyroxinemia is sufficient to impair early brain development, regardless of whether the neonatal brain iron level is normal, and monitoring thyroid hormone level is indicated in ID pregnant women.

New insights into the structure and mechanism of iodothyronine deiodinases

Iodothyronine deiodinases are a family of enzymes that remove specific iodine atoms from one of the two aromatic rings in thyroid hormones (THs). They thereby fine-tune local TH concentrations and cellular TH signaling. Deiodinases catalyze a remarkable biochemical reaction, i.e., the reductive elimination of a halogenide from an aromatic ring. In metazoans, deiodinases depend on the rare amino acid selenocysteine. The recent solution of the first experimental structure of a deiodinase catalytic domain allowed for a reappraisal of the many mechanistic and mutagenesis data that had been accumulated over more than 30 years. Hence, the structure generates new impetus for research directed at understanding catalytic mechanism, substrate specificity, and regulation of deiodinases. This review will focus on structural and mechanistic aspects of iodothyronine deiodinases and briefly compare these enzymes with dehalogenases, which catalyze related reactions. A general mechanism for the selenium-dependent deiodinase reaction will be described, which integrates the mouse deiodinase 3 crystal structure and biochemical studies. We will summarize further, sometimes isoform-specific molecular features of deiodinase catalysis and regulation, and we will then discuss available compounds for modulating deiodinase activity for therapeutic purposes.

Caloric restriction induces energy-sparing alterations in skeletal muscle contraction, fiber composition and local thyroid hormone metabolism that persist during catch-up fat upon refeeding

Weight regain after caloric restriction results in accelerated fat storage in adipose tissue. This catch-up fat phenomenon is postulated to result partly from suppressed skeletal muscle thermogenesis, but the underlying mechanisms are elusive. We investigated whether the reduced rate of skeletal muscle contraction-relaxation cycle that occurs after caloric restriction persists during weight recovery and could contribute to catch-up fat. Using a rat model of semistarvation-refeeding, in which fat recovery is driven by suppressed thermogenesis, we show that contraction and relaxation of leg muscles are slower after both semistarvation and refeeding. These effects are associated with (i) higher expression of muscle deiodinase type 3 (DIO3), which inactivates tri-iodothyronine (T₃), and lower expression of T₃-activating enzyme, deiodinase type 2 (DIO2), (ii) slower net formation of T₃ from its T₄ precursor in muscles, and (iii) accumulation of slow fibers at the expense of fast fibers. These semistarvation-induced changes persisted during recovery and correlated with impaired expression of transcription factors involved in slow-twitch muscle development. We conclude that diminished muscle thermogenesis following caloric restriction results from reduced muscle T₃ levels, alteration in muscle-specific transcription factors, and fast-to-slow fiber shift causing slower contractility. These energy-sparing effects persist during weight recovery and contribute to catch-up fat.

Effect of maternal iodine supplementation on thyroid function and birth outcome in goiter endemic areas

OBJECTIVE

The study was undertaken to examine the clinical and endocrine parameters of thyroid in a total of 460 pregnant women belonging to non-goiter areas (group 1; n = 156) and endemic areas without (group 2; n = 154) and with iodine supplementation (group 3; n = 150), and their respective newborns.

METHODS

Women of group 3 with visible goiter were administered two capsules of iodized oil orally each containing 200 mg of iodine, from weeks 6--8 of pregnancy. Blood samples were obtained from all groups during each trimester, at parturition (umbilical cord blood) and after delivery. Serum triiodothyronine (T3), thyroxine (T4) and thyroid stimulating hormone (TSH) levels were measured by specific enzyme immunoassays.

RESULTS

In group 2, serum T4 concentrations were low while T3 and TSH levels were high which showed hypothyroidism in the women of endemic areas. Goiter size decreased in most of the subjects who received a single dose of iodized oil and resulted in increase in serum concentrations of thyroid hormones; whereas, TSH levels decreased. Iodine supplementation also resulted in raised T4 and low TSH levels in the cord blood of neonates. During the course of study, two abortions, three still births and one cretin were reported in group 2; none was reported in group 3; and one still birth was reported in group 1.

CONCLUSIONS

The oral administration of a single dose of iodized oil is capable of correcting iodine deficiency both clinically and endocrinologically in mothers and neonates. Iodine supplementation has the potential to positively impact the birth weight of newborns.

High performance liquid chromatography-tandem mass spectrometry method for ex vivo metabolic studies of a rhenium-labeled radiopharmaceutical for liver cancer

The radio-isotope rhenium-labeled N-[2-(triphenylmethyl)thioethyl]-3-aza-19-ethyloxycarbonyl-3-[2-(triphenylmethyl)thioethyl] octadecanoate) ligand (188Re-MN-16ET) is a novel therapeutic agent under preclinical evaluation for hepatoma. A reversed-phase high performance liquid chromatography coupled with a tandem mass spectrometric analysis method and diode array detector (DAD) involving a T type splitter was developed to characterize this pharmaceutical in rat liver tissue solution and determine its biotransformation rate. The separation was accomplished on a C18 column (chromolith silica, 4.6 mm x 100 mm) using an acetonitrile-ammonium acetate buffer gradient as the mobile phase. The detection was achieved by DAD set at 250nm and tandem mass spectrometry using electrospray ionization in the positive ion mode. Re-MN-16ET displayed a retention time of 23.2 min and a transition ion pair corresponding to m/z677 --> 631 for multiple reaction monitoring. Its biotransformation reaction in rat liver homogenate proceeded for 90 min in a 37°C water bath. The characterization was conducted using aliquots that were extracted and concentrated from the reaction mixture for various incubation times. Re-MN-16ET exhibited a biotransformation half-life (t1/2) of 8-9 min in liver tissue solution and was almost completely exhausted after 90 min. Two of its metabolites, consisting of the Re-labeled carboxylic acid derivative, predominately, and its corresponding demetallized disulfide ligand were found in the liver homogenate, providing a metabolism pathway for the radio-pharmaceutical.

Triiodothyronine induces lipid oxidation and mitochondrial biogenesis in rat Harderian gland

The rat Harderian gland (HG) is an orbital gland producing a copious lipid secretion. Recent studies indicate that its secretory activity is regulated by thyroid hormones. In this study, we found that both isoforms of the thyroid hormone receptor (Trα (Thra) and Trβ (Thrb)) are expressed in rat HGs. Although Thra is expressed at a higher level, only Thrb is regulated by triiodothyronine (T3). Because T3 induces an increase in lipid metabolism in rat HGs, we investigated the effects of an animal's thyroid state on the expression levels of carnitine palmitoyltransferase-1A (Cpt1a) and carnitine palmitoyltransferase-1B (Cpt1b) and acyl-CoA oxidase (Acox1) (rate-limiting enzymes in mitochondrial and peroxisomal fatty acid oxidation respectively), as well as on the mitochondrial compartment, thereby correlating mitochondrial activity and biogenesis with morphological analysis. We found that hypothyroidism decreased the expression of Cpt1b and Acox1 mRNA, whereas the administration of T3 to hypothyroid rats increased transcript levels. Respiratory parameters and catalase protein levels provided further evidence that T3 modulates mitochondrial and peroxisomal activities. Furthermore, in hypothyroid rat HGs, the mitochondrial number and their total area decreased with respect to the controls, whereas the average area of the individual mitochondrion did not change. However, the average area of the individual mitochondrion was reduced by ∼50% in hypothyroid T3-treated HGs, and the mitochondrial number and the total area of the mitochondrial compartment increased. The mitochondrial morphometric data correlated well with the molecular results. Indeed, hypothyroid status did not modify the expression of mitochondrial biogenesis genes such as Ppargc1a, Nrf1 and Tfam, whereas T3 treatment increased the expression level of these genes.

Body condition and endocrine profiles of Steller sea lion (Eumetopias jubatus) pups during the early postnatal period

Body condition indices have been useful in assessing the health of domestic and free ranging populations of terrestrial mammals. Given the high energy demand and rapid growth during the postnatal period of Steller sea lion (Eumetopias jubatus) (SSL) pups, body condition was expected to be related to concentrations of a suite of hormones (cortisol, aldosterone, thyroxine, triiodothyronine, leptin) previously associated with changes in body mass and composition in developing pinnipeds. Blood samples were collected from free ranging SSL pups of known ages and sex. A body condition index (BCI) previously developed for SSL pups based on a mass and length relationship was applied to 61 SSL pups ranging in age from 5 to 38days old. BCI was not related to pup age. Overall, male pups were larger than females and older pups were larger than younger pups. Aldosterone was negatively correlated with BCI only in female pups, whereas no relationship was observed between aldosterone and BCI in males. Further, male pups had higher aldosterone concentrations than females. Concentrations of cortisol, total and free thyroxine (T4), and total triiodothyronine (T3) decreased when regressed against the elapsed time between researchers' arrival on the rookery and removal of pup from the holding corral for blood collection. While the overall variation attributed to the rookery disturbance was low (r(2)<0.293), it may be of significance for future studies on free ranging pinnipeds. This study adds to the current knowledge of the postnatal changes in free ranging SSL pups by providing ranges of the BCI and several hormone concentrations from an apparently stable population.

Identification of iopanoic acid as substrate of type 1 deiodinase by a novel nonradioactive iodide-release assay

Enzymatic 5'- and 5-deiodination are key reactions for local and systemic activation and inactivation of iodothyronines and thyronamines. Expression of the three deiodinase (DIO) isoenzymes is regulated by a number of parameters, including thyroid status, genotype, micronutrient availability, and disease-related signaling. In addition, DIO are potential targets of pharmacological as well as environmentally derived substances, which might affect their enzymatic activity (endocrine disruptors). With the classical DIO activity assay, testing depends on the availability of radioactively labeled substrates (e.g. (125)I-rT(3)) to monitor the release of radioactive iodide. Recently, liquid chromatography-tandem mass spectrometry was described as an alternative method apparently resolving this limitation. However, it has a high demand in technical equipment and analytical routine and is limited in sample number by considerable measuring time. We therefore combined the classical deiodination assay with an easily accessible photometric method taking advantage of the Sandell-Kolthoff reaction for measuring iodide release. In brief, iodine works as a catalyst within this redox reaction between Ce(4+) and As(3+) leading to an acceleration of destaining. Furthermore, the protocol was adapted to minimize handling effort and time consumption. Because this method is not dependent on radioactivity, it expands the substrate spectrum of the classical method. Suitability of this assay was tested with tissue samples from animal experiments (hepatic Dio1 activity in hypo- and hyperthyroid mice) and established DIO inhibitors. As a new but not unexpected finding, the alleged inhibitor iopanoic acid turned out to be a DIO substrate. This finding was confirmed by liquid chromatography-tandem mass spectrometry, and its potential clinical impact requires further studies.

Nonthyroidal illness syndrome in children

Neuroendocrine changes in the hypothalamic-pituitary-thyroid axis during critical illness result in nonthyroidal illness syndrome (NTIS) characterized by abnormal thyrotropin (TSH) and thyroid hormone levels. Studies looking at the natural history of neuroendocrine changes during critical illness have revealed the presence of NTIS. NTIS has been described in a variety of patient settings. Many studies have tried to uncover the pathophysiology behind NTIS and several theories are proposed. Whether NTIS requires treatment or intervention is still controversial and the results of the treatment studies are arguably mixed. Whether implicitly stated or not, the underlying purpose of all the natural history, pathophysiology, or treatment studies is to determine whether NTIS is adaptive or maladaptive. Some studies have illustrated a correlation between illness severity and the degree of NTIS but a cause and effect relationship is still elusive. The human studies can be divided between those with either adult or pediatric subjects, with much less data available in the latter. This review examines the available literature on NTIS with an emphasis on the pediatric literature.

Effect of iron repletion and correction of iron deficiency on thyroid function in iron-deficient Iranian adolescent girls

The aim of this study was to determine whether iron supplementation in iron-deficient adolescent girls would improve thyroid function. A double-blind clinical trial was performed in a region in southern I.R. Iran. A total of 103 iron deficient participants were chosen. In all, 94 participants successfully completed this study. Participants were randomly assigned to one of two groups and treated with a 300 mg ferrous sulfate 5 times/week (n = 47) and placebo 5 times/week (n = 47) for 12 weeks. Blood samples were collected and assayed for hemoglobin, hematocrit, serum ferritin, iron, total iron binding capacity (TIBC), Thyroid stimulating hormone (TSH), total thyroxine (TT4), total triiodothyronine (TT3), free thyroid hormones (FT4 and FT3), triiodothyronine resin uptake (T3RU), reverse triiodothyronine (rT3), selenium and albumin concentrations. Statistical analysis was performed with parametric and non-parametric methods as appropriate. Data analysis revealed a significant increase in TT4, TT3, T3RU and a significant decrease in rT3 concentration in comparison to initial values in iron treated group (12%, p<0.001; 3.5%, p<0.001; 16%, p<0.05 and 47%, p<0.001, respectively). At 12 week there were significant differences between control and placebo in TT4, TT3, T3RU and rT3 concentrations (9.9 vs 8.4 microg dL(-1), 145.2 vs 130.4 microg dL(-1), 32.5 vs 28.4% and 23 vs 41 microg dL(-1), respectively, all p<0.001). Alterations in FT3 and TSH concentration were not significant, but concentration of FT4 revealed a significant difference between the beginning and the end of the study in iron treated group (10.3 vs 11.4, p<0.001). Iron supplementation improves some indices of thyroid function in iron-deficient adolescent girls.

Cellular and molecular basis of deiodinase-regulated thyroid hormone signaling

The iodothyronine deiodinases initiate or terminate thyroid hormone action and therefore are critical for the biological effects mediated by thyroid hormone. Over the years, research has focused on their role in preserving serum levels of the biologically active molecule T(3) during iodine deficiency. More recently, a fascinating new role of these enzymes has been unveiled. The activating deiodinase (D2) and the inactivating deiodinase (D3) can locally increase or decrease thyroid hormone signaling in a tissue- and temporal-specific fashion, independent of changes in thyroid hormone serum concentrations. This mechanism is particularly relevant because deiodinase expression can be modulated by a wide variety of endogenous signaling molecules such as sonic hedgehog, nuclear factor-kappaB, growth factors, bile acids, hypoxia-inducible factor-1alpha, as well as a growing number of xenobiotic substances. In light of these findings, it seems clear that deiodinases play a much broader role than once thought, with great ramifications for the control of thyroid hormone signaling during vertebrate development and metamorphosis, as well as injury response, tissue repair, hypothalamic function, and energy homeostasis in adults.

Development of a validated liquid chromatography/tandem mass spectrometry method for the distinction of thyronine and thyronamine constitutional isomers and for the identification of new deiodinase substrates

Thyronines (THs) and thyronamines (TAMs) are two groups of endogenous iodine-containing signaling molecules whose representatives differ from each other only regarding the number and/or the position of the iodine atoms. Both groups of compounds are substrates of three deiodinase isozymes, which catalyze the sequential reductive removal of iodine from the respective precursor molecule. In this study, a novel analytical method applying liquid chromatography/tandem mass spectrometry (LC-MS/MS) was developed. This method permitted the unequivocal, simultaneous identification and quantification of all THs and TAMs in the same biological sample. Furthermore, a liquid-liquid extraction procedure permitting the concurrent isolation of all THs and TAMs from biological matrices, namely deiodinase (Dio) reaction mixtures, was established. Method validation experiments with extracted TH and TAM analytes demonstrated that the method was selective, devoid of matrix effects, sensitive, linear over a wide range of analyte concentrations and robust in terms of reproducible recoveries, process efficiencies as well as intra-assay and inter-assay stability parameters. The method was applied to study the deiodination reactions of iodinated THs catalyzed by the three deiodinase isozymes. With the HPLC protocol developed herein, sufficient chromatographic separation of all constitutional TH and TAM isomers was achieved. Accordingly, the position of each iodine atom removed from a TH substrate in a Dio-catalyzed reaction was backtracked unequivocally. While several established deiodination reactions were verified, two as yet unknown reactions, namely the phenolic ring deiodination of 3',5'-diiodothyronine (3',5'-T2) by Dio2 and the tyrosyl ring deiodination of 3-monoiodothyronine (3-T1) by Dio3, were newly identified.

Epididymis expresses the highest 5'-deiodinase activity in the male reproductive system: kinetic characterization, distribution, and hormonal regulation

We characterized the enzymes that catalyze the deiodination of T(4) to T(3) in the male reproductive tract. Testis, epididymis (EPI), seminal vesicles, prostate, bulbourethral glands, spermatozoa, and semen were taken from sexually mature rats (300 g). Iodothyronine 5'-deiodinase (5'-D) activity was quantified by the radiolabeled-iodide-release method. 5'-D activity was 10-fold higher in EPI and semen than in the rest of the tissues. In EPI, semen, and prostate, the enzymatic activity was completely inhibited by 1 mm 6-n-propyl-2-thiouracil, whereas in the other tissues the inhibition was partial (50%). The high susceptibility to 6-n-propyl-2-thiouracil inhibition, a ping-pong kinetic pattern, and low cofactor (Michaelis Menten constant for dithiothreitol=0.7 mm) and high substrate (Michaelis Menten constant for reverse T(3)=0.4 microm) requirements indicate that EPI 5'-D corresponds to type 1 deiodinase (D1). Real-time RT-PCR amplification of D1 mRNA in this tissue confirms this conclusion. The highest EPI D1 expression occurred at the onset of puberty and sexual maturity, and in the adult, this activity was more abundant in corpus and caput than in the caudal region. EPI D1 expression was elevated under conditions of hyperthyroidism and with addition of 17beta-estradiol. Our data also showed a direct association between D1 and a functional epididymis marker, the neutral alpha-glucosidase enzyme, suggesting that local generation of T(3) could be associated with the development and function of EPI and/or spermatozoa maturation. Further studies are necessary to analyze the possible physiological relevance of 5'-D in the male reproductive system.

Morphological and biochemical changes in the Harderian gland of hypothyroid rats

The secretory activity of the Harderian gland (HG) is influenced by both exogenous (such as light and temperature) and endogenous (such as prolactin, thyroid hormones and steroid hormones) factors, which vary among species. In the present study, the effects of hypothyroidism on the rat HG were examined at morphological and biochemical levels. The decrease in cytoplasmic lipoproteic vacuoles and the increase in mucosubstance secretion in the acinar lumina were the most notable histological effects elicited by hypothyroidism. The release of all granules with nuclei and cellular debris suggested the occurrence of holocrine secretion. Electron microscopy revealed in the glandular cells of hypothyroid rat an increased condensation of chromatin in the nuclei, mitochondria with decreased cristae and vacuolisation, decreased glycogen granules, autophagic vacuoles, and lipofuscins in the cytoplasm. TUNEL reaction indicated DNA fragmentation in hypothyroid HG, indicative of an underlying apoptotic process. Translocation of cytochrome c from mitochondria to cytosol strongly supported this hypothesis. In conclusion, these findings indicate that thyroid hormones play a pivotal role in preserving the structural integrity of the rat HG and, hence, its secretory activity.

Mitochondrial energy metabolism in a model of undernutrition induced by dexamethasone

The present investigation was undertaken to evaluate whether mitochondrial energy metabolism is altered in a model of malnutrition induced by dexamethasone (DEX) treatment (1·5mg/kg per d for 5d). The gastrocnemius and liver mitochondria were isolated from DEX-treated, pair-fed (PF) and control (CON) rats. Body weight was reduced significantly more in the DEX-treated group (−16%) than in the PF group (−9%). DEX treatment increased liver mass (+59%v.PF, +23%v. CON) and decreased gastrocnemius mass. Moreover, in DEX-treated rats, liver mitochondria had an increased rate of non-phosphorylative O2consumption with all substrates (approximately +42%). There was no difference in enzymatic complex activities in liver mitochondria between rat groups. Collectively, these results suggest an increased proton leak and/or redox slipping in the liver mitochondria of DEX-treated rats. In addition, DEX decreased the thermodynamic coupling and efficiency of oxidative phosphorylation. We therefore suggest that this increase in the proton leak and/or redox slip in the liver is responsible for the decrease in the thermodynamic efficiency of energy conversion. In contrast, none of the variables of energy metabolism determined in gastrocnemius mitochondria was altered by DEX treatment. Therefore, it appears that DEX specifically affects mitochondrial energy metabolism in the liver.

The Influence of Iron Status on Iodine Utilization and Thyroid Function

Despite significant progress, deficiencies of iron and iodine remain major public health problems affecting > or =30% of the global population. These deficiencies often coexist in children. Recent studies have demonstrated that a high prevalence of iron deficiency among children in areas of endemic goiter may reduce the effectiveness of iodized salt programs. These findings argue strongly for improving iron status in areas of overlapping deficiency, not only to combat anemia but also to increase the efficacy of iodine prophylaxis. The dual fortification of salt with iodine and iron may prove to be an effective and sustainable method to accomplish these important goals.

Effects of administration of iron, iodine and simultaneous iron-plus-iodine on the thyroid hormone profile in iron-deficient adolescent Iranian girls

OBJECTIVE

To investigate whether iron supplementation can improve thyroid hormone function in iron-deficient adolescent girls.

DESIGN

A double-blind randomized intervention study.

SETTING

The study was performed from 2002 through 2003 in the Islamic Republic of Iran.

SUBJECTS

103 iron-deficient non-anaemic girls who fulfilled all inclusion criteria were included, and 94 subjects successfully completed the study.

INTERVENTIONS

Patients were randomly assigned to one of four groups and treated with a single oral dose of 190 mg iodine plus 300 mg ferrous sulphate 5 times/week (n=24), 300 mg ferrous sulphate 5 times/week (n=23), a single oral dose of 190 mg iodine (n=25), or a placebo (n=22) for 12 weeks.

RESULTS

All groups were comparable at baseline. After the intervention, there was a significant increase in ferritin and transferrin saturation in the iron+iodine group (17.6 vs 8.7 microg/dl, and 18.8 vs 7.2%, respectively, P<0.001 for both) and in the iron group (P<0.001 for both). Urinary iodine doubled in the iron+iodine group and in the iodine group (P<0.001 for both). Thyroid indices tT4, tT3 and T3RU increased and reverse RT3 decreased in the iron+iodine group (10 vs 8.9 microg/dl, P< 0.001; 143 vs 138 microg/dl, P<0.05; 32.3 vs 28.4%, P<0.001 and 24.8 vs 44.2 ng/dl, P<0.001, respectively) and in the iron group. These two groups did not differ for any of the four indices, but both differed significantly from the iodine and placebo groups.

CONCLUSIONS

Our results indicate that improvement of iron status was accompanied by an improvement in some indices of thyroid hormones.

SPONSORSHIP

This study was supported by the Dean of Research Affairs of the Tehran University of Medical Sciences.

Unique regulation of thyroid hormone metabolism during fasting in the house musk shrew (Suncus murinus, Insectivora: Soricidae)

The active hormone, 3,3',5-triiodothyronine (T3) is derived from thyroxine (T4) by the action of iodothyronine 5'-deiodinases (5'-D). By now two types of 5'-D have been identified; Type 1 (D1) and type 2 (D2). A relative contribution of these isotypes to the circulating T3 levels in the human remains to be determined whereas a number of reports indicate that, under physiological conditions, D1 plays a major role in maintaining circulating T3 levels in rodents. In both human and rodents, sickness and starvation reduce serum T3 concentration mainly through decrease in D1 activity. Recently, we found that the house musk shrew (Suncus murinus, Insectivora: Soricidae) has a different tissue distribution of D1 activity. Because compared to rodents D1 activity in the shrew was found only in liver at a much reduced level, D2 rather than D1 may play a role in the maintenance of serum T3. Therefore, we questioned how D1 and D2 activities change in fasted shrews and how these changes affect circulating thyroid hormone levels. We thus starved shrews for 24, 48 or 72 h and measured changes in serum concentration of T3, T4, and 3,3',5'-triiodothyronine (reverse T3, rT3) and D1 activities as well as its mRNA expression in liver. D2 activities were also measured in brown adipose tissue (BAT) and cerebral cortex of shrews. Unlike in human and rodents, T3 levels in shrews remained constant during fasting while T4 levels tended to decrease, resulting in an increase in its T3/T4 ratio. On the other hand, changes in rT3 levels were similar to those in human and rodents, being elevated with fasting. D1 mRNA and its activity were significantly reduced in the liver whereas D2 activities in BAT and cerebral cortex were increased by fasting. These results indicated that fasting in shrews also reduced hepatic D1 activity but it did not affect circulating T3 levels. The increased T3/T4 ratio together with increased D2 activity in BAT and cerebral cortex with fasting suggest that D2 rather than D1 is responsible for the maintenance of T3 levels in the house musk shrew.

Uncoupling of oxidative phosphorylation prevents the urinary alcohol level cycling caused by feeding ethanol continuously at a constant rate

The mechanism of the UAL cycle in the intragastric feeding model of alcoholic liver disease in the rat was investigated by administering dinitrophenol (DNP) with ethanol in the diet. The question was: is the rate of oxidative phosphorylation fluxuation essential for the cycle to occur? The question has been partially answered by showing that rotenone, which inhibits complex I, blocks the cycle by preventing the generation of NAD from NADH. This would inhibit ATP generation from complex I but would not affect oxidative phosphorylation by complex 2 and 3. Since the rate of O2 consumption is normal at the troughs of the cycle and decreases at the peaks of the cycle and the levels of ATP are reduced at the peaks of the cycle, it is likely that the rate of oxidative phosphorylation also cycles. Since 2-4 dinitrophenol (DNP) uncouples oxidative phosphorylation, it was anticipated that feeding it with ethanol would prevent the cycle from occurring. This proved to be the case. In addition, DNP caused energy wasting and prevented the increase in serum alanine aminotranspeptidase caused by ethanol feeding, probably by preventing the hypoxia which occurs at the peaks of the cycle.

Age-dependent adaptation of the liver thyroid status and recovery of serum levels and hepatic insulin-like growth factor-I expression in neonatal and adult diabetic rats

The effect of treatment with thyroxine (T(4)) on the hepatic deiodinase (5'D-I) activity and triiodothyronine (T(3)) content and on insulin-like growth factor-I (IGF-I) secretion and mRNA hepatic expression were studied in neonatal and adult diabetic (D) rats and compared with 4 thyroidectomized (Tx) groups: neonatal and adult Tx rats treated or not with T(4). Serum T(3) and T(4) decreased by 92% in both Tx populations and by 80% to 70% in D adults according to the severity of diabetes: -70 mg/kg body weight (BW) (D(70)) or 50 mg/kg BW (D(50)) of streptozotocin (STZ) injected, whereas only a 30% to 33% decrease was found in D neonates. A similar decrease of liver 5'D-I activity and T(3) concentrations was found in neonatal and adult Tx rats, whereas a significant reduction in those parameters was observed only in adult diabetics, either D(70) or D(50), but not in D neonates. Serum levels and liver mRNA expression of IGF-I determined by ribonuclease protection assay, plasma and pituitary growth hormone (GH), plasma insulin, and glycemia were also measured in both D populations. A decrease in circulating IGF-I, previously reported for Tx adult rats, was also found in both D populations. T(4) treatment recovered IGF-I and liver T(3) in both Tx groups and D neonates, but not in D adults. These results show an age-dependent adaptation of the liver thyroid economy in diabetes, as hepatic 5'D-I does not respond to diabetes in neonates and IGF-I is insensitive to T(4) treatment in adult diabetics and suggest a positive correlation between hepatic T(3) content and IGF-I expression in conditions of diabetes and Tx.

Acute decrease in circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis

In thyrotoxicosis there is an impaired GH response to GHRH, normal GH responsiveness to GHRP-6 and lack of synergistic GH response after simultaneous administration of both peptides. We have previously shown that the GHRH-induced GH release in these patients increases after an acute reduction of circulating T3 values with administration of iopanoic acid, a compound that inhibits peripheral conversion of T4 to T3. We have now studied the effect of a decrease in serum T3 levels on the GH response to GHRP-6 (1 microg/kg) plus GHRH (100 microg) in 9 hyperthyroid patients before and after 15 days of treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (600 mg/day). Nine normal subjects were also studied. In all hyperthyroid patients iopanoic acid induced a rapid decrease and normalisation of serum T3 levels. In these subjects peak GH (microg/l; mean +/- SE) and AUC (microg/l x 120 min) values after GHRP-6 plus GHRH were significantly higher on day 15 compared to pretreatment values (peak, 18.3 +/- 3.0 vs 13.4 +/- 1.9; AUC, 1227.9 +/- 212.9 vs 968.5 +/- 160.4; p<0.05). Despite the significant enhancement of the GH responsiveness to GHRP-6 plus GHRH after treatment with iopanoic acid, this response remained significantly blunted when compared to controls both in terms of peak GH (18.3 +/- 3.0 vs 83.7 +/- 15.2; p<0.05) and AUC values (1227.9 +/- 212.9 vs 4956.5 +/- 889.3; p<0.05). In conclusion, our results show that an acute decrease of circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis. This could suggest that circulating T3 does not have a major role in the mechanisms involved in the synergistic effect of these peptides.

The impact of iron and selenium deficiencies on iodine and thyroid metabolism: biochemistry and relevance to public health

Several minerals and trace elements are essential for normal thyroid hormone metabolism, e.g., iodine, iron, selenium, and zinc. Coexisting deficiencies of these elements can impair thyroid function. Iron deficiency impairs thyroid hormone synthesis by reducing activity of heme-dependent thyroid peroxidase. Iron-deficiency anemia blunts and iron supplementation improves the efficacy of iodine supplementation. Combined selenium and iodine deficiency leads to myxedematous cretinism. The normal thyroid gland retains high selenium concentrations even under conditions of inadequate selenium supply and expresses many of the known selenocysteine-containing proteins. Among these selenoproteins are the glutathione peroxidase, deiodinase, and thioredoxine reductase families of enzymes. Adequate selenium nutrition supports efficient thyroid hormone synthesis and metabolism and protects the thyroid gland from damage by excessive iodide exposure. In regions of combined severe iodine and selenium deficiency, normalization of iodine supply is mandatory before initiation of selenium supplementation in order to prevent hypothyroidism. Selenium deficiency and disturbed thyroid hormone economy may develop under conditions of special dietary regimens such as long-term total parenteral nutrition, phenylketonuria diet, cystic fibrosis, or may be the result of imbalanced nutrition in children, elderly people, or sick patients.

Changes in thyroid hormone levels in chicken liver during fasting and refeeding

In chickens, fasting results in increased plasma thyroxine (T(4)) levels and decreased plasma 3,5,3'-triiodothyronine (T(3)) levels. Refeeding, in turn, restores normal plasma T(3) and T(4) levels. The liver is an important tissue for the regulation of circulating thyroid hormone levels. Previous studies demonstrated that the increase in hepatic type III deiodinase in fasted chickens plays a role in the decrease of plasma T(3). Another factor that could be important is the level of T(4) and T(3) uptake by the liver. In mammals, caloric restriction is known to diminish transport of T(4) and T(3) into tissues. The present study examines whether this is also the case in chicken. Four-week-old chickens were subjected to a 24-h starvation period followed by refeeding. Blood and liver samples were collected at the start of refeeding and at different times of refeeding. Thyroid hormone levels were measured directly in plasma and in tissues following extraction. The results demonstrate that intrahepatic T(4) levels are increased and T(3) levels are decreased in fasted compared to ad libitum fed chickens. The parallel changes in plasma and hepatic T(3) and T(4) content demonstrate that T(4) availability in liver tissue is not diminished during fasting, suggesting that in chicken thyroid hormone uptake by the liver is not affected by nutritional status.

The use of oral radiographic contrast agents in the management of hyperthyroidism

Oral iodinated radiographic contrast agents such as ipodate and iopanoic acid form an important part of the armamentarium used to treat hyperthyroidism. They rapidly and dramatically reduce serum triiodothyronine (T3) levels by inhibiting conversion of thyroxine (T4) to T3 in the periphery and by blocking secretion from the thyroid. Potential risks from the large iodine load resulting from their use limit their widespread applicability. In addition, they are ineffective when used alone on a long-term basis. However, these agents may be especially useful in treating thyrotoxic patients preoperatively, in neonatal Graves' disease, in massive levothyroxine ingestion, and when other conventional antithyroid drugs are unsuccessful or contraindicated.

Effect of endotoxin challenge on hepatic 5'-deiodinase activity in cattle

Thyroid status is compromised in a variety of acute and chronic infections and toxin-mediated disease states. Conversion of thyroxine (T4) into the metabolically active hormone, triiodothyronine (T3), is catalyzed by 5'-deiodinase (5'D). Our objective was to determine the effect of endotoxin (LPS) challenge with and without L-arginine (Arg) infusion on hepatic activity of 5'D and plasma concentrations of T4 and T3. In a 2 x 2 factorial, beef heifers (275-310 kg b.wt.) were fed low (8% CP; 6.5 kg/d) or high (14% CP; 7.2 kg/d) isocaloric protein diets (1.96 Mcal/kg DM) for 10 d before LPS challenge. L-Arginine in saline (0.5 g/kg b.wt.) or saline alone was infused i.v. throughout an 8 hr period starting 2 hr before bolus LPS injection (Escherichia coli, 055: B5; 0.2 microg/kg; i.v.). Blood samples were collected at -2, 0, 3, 6, 12, and 24 hr relative to LPS injection. Liver samples were obtained 20 hr before, and then 6 and 24 hr after LPS challenge using a biopsy needle. Plasma T4 and T3 concentrations were not affected by dietary CP or Arg. Compared with levels at 0 hr, LPS challenge decreased plasma T4 (P < 0.01) and T3 (P < 0.001), respectively, 8.4% and 28.9% at 6 hr and 19.7% and 31.3% at 24 hr. Consistent with these changes, the T3:T4 ratio was lower than that at 0 hr (P < 0.001) 22.0% at 6 hr and 13.5% at 24 hr. Hepatic 5'D activities 20 hr before LPS injection were 2.80 +/- 0.11 nmol I- x hr(-1) x mg protein(-1) and decreased 24 hr after LPS, respectively, 45.4% (P < 0.01) and 17.6% (P < 0.05) in saline- and Arg-infused heifers. The results indicate that mild LPS challenge in cattle inhibits hepatic generation of T3 and decreases plasma concentrations of thyroid hormones. The data also suggest that the impact of LPS on 5'D activity in liver can be altered by Arg supplementation.

Iopanoic acid-induced decrease of circulating T3 causes a significant increase in GH responsiveness to GH releasing hormone in thyrotoxic patients

OBJECTIVE

Thyroid hormones participate in GH synthesis and secretion, and an impaired GH response to many pharmacological stimuli, including GH releasing hormone (GHRH), has been found in thyrotoxicosis. Although the mechanisms involved in this process have not been fully elucidated, there is evidence that thyroid hormones could act at both hypothalamic and pituitary levels. There are no data in the literature about the effect of an acute reduction of circulating T3 levels on GH secretion in hyperthyroidism. The GH responsiveness to GHRH was therefore evaluated in a group of hyperthyroid patients during short-term treatment with iopanoic acid. Iopanoic acid is a compound that induces a rapid decrease in serum T3 levels, mainly by inhibition of peripheral conversion of T4 to T3. To the authors' knowledge, there is no evidence of a direct effect of iopanoic acid on GH secretion.

DESIGN

Hyperthyroid patients were submitted to a GHRH test (100 microg, i.v.) before (day 0), and on days 4, 7 and 15 after oral treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (200 mg every 8 h). A group of normal control subjects was also submitted to a single GHRH test (100 microg, i.v.).

PATIENTS

Nine patients with thyrotoxicosis (eight women, one man), with a mean age of 34 years, were studied. All patients had high serum levels of total T3 and total T4, and suppressed TSH levels. None of them had taken any medication for at least 3 months before the study. The patients were compared with a group of nine control subjects (five women, four men) with a mean age of 31 years.

MEASUREMENTS

GH and TSH were measured by immunofluorometric assays. Total T3, total T4 and IGF-I were determined by radioimmunoassay. Albumin levels were measured by a colorimetric method.

RESULTS

Iopanoic acid induced a rapid and maintained decrease in serum T3 concentrations, with a significant reduction on days 4, 7 and 15 compared with pre-treatment values. In hyperthyroidism, peak GH levels (mean +/- SE mU/l) after GHRH were significantly higher on day 15 (24.4 +/- 3.8) than those observed on days 0 (14.2 +/- 1.6), 4 (15.2 +/- 3.0) and 7 (19.6 +/- 5.0). There was a 79% increase in this response on day 15 compared with the pre-treatment period. Hyperthyroid patients had a blunted GH response to GHRH on days 0, 4 and 7 in comparison with control subjects. However, on day 15, no differences were observed between the area under the curve (mean +/- SE mU/l.120 min) in thyrotoxic patients (1770 +/- 306) and in the control group (3300 +/- 816). IGF-I and albumin levels did not change during iopanoic acid administration.

CONCLUSIONS

The results show that an acute reduction in serum T3 levels elicits an increase in GH responsiveness to GHRH in hyperthyroidism. Although the mechanisms involved in this process are still unknown, it is possible that T3 influences GH responsiveness to GHRH via hypothalamic somatostatin release. Alternatively, T3 could have a direct effect at the pituitary somatotroph, modulating GHRH intracellular pathways.

Alterations in the enzyme activity and protein contents of protein disulfide isomerase in rat tissues during fasting and refeeding

Protein disulfide isomerase (PDI) is an enzyme that participates in the formation of disulfide bonds. It is also known to be the subunits of some enzymes and the membrane-associated thyroid hormone-binding protein. In this study, we measured the quantitative distribution of PDI protein in rat tissues and examined the relationship between protein level and enzyme activity in PDI during fasting and refeeding. Western blotting with specific anti-PDI antiserum detected the PDI protein band of 55 kd. Among several tissues, liver contained the largest amount of PDI protein, followed by kidney and fat, in which one-third to one-fourth of the hepatic PDI protein existed. The PDI protein band was also detected in heart and muscle. Fasting for 3 days decreased PDI protein levels in rat liver by 40%; control levels were recovered after 3 days of refeeding. The same change was observed in kidney. PDI activity, measured by the scrambled ribonuclease method, did not show the parallel alteration to PDI protein level in liver and kidney. Isomerase activity decreased to 50% of control values during fasting, but did not recover by refeeding. Thyroidal status did not affect either PDI protein level or isomerase activity. These findings show that fasting and refeeding affect PDI protein and enzyme activity, and that PDI protein level does not always reflect PDI activity.

Anterior pituitary function during critical illness and dopamine treatment

OBJECTIVE

To summarize the available data on anterior pituitary function in critical illness and to focus on the endocrine effects of dopamine infusion. The analogy with anterior pituitary function in the elderly is highlighted, and the potential importance of these observations for recovery from critical illness is discussed.

DATA SOURCES

Computerized search of published research and reference list review.

STUDY SELECTION

Review of 178 citations. Included are seven original studies on the effect of dopamine on pituitary function in adult and pediatric critical illness performed by the authors.

DATA EXTRACTION

Studies on the endocrinology of illness, chronic stress, aging, and dopamine, or on the clinical importance of endocrine changes.

DATA SYNTHESIS

The different pituitary axes are important determinants of normal anabolism and immune function. Continuously increased serum cortisol concentrations, insulin resistance, blunted prolactin release, and attenuated pulsatility of growth hormone and luteinizing hormone secretory patterns, as well as multiple anomalies in the thyroid axis, characterize the endocrine profile of prolonged critical illness. Dopamine, a natural catecholamine with hypophysiotropic properties, which has been used for more than two decades as an inotropic and vasoactive drug in intensive care, suppresses the circulating concentrations of all anterior pituitary-dependent hormones, except for cortisol. Available evidence suggests that the major effect of dopamine administration on the endocrine system is unlikely to be beneficial for the threatened metabolic and immunologic homeostasis of the severely ill patient. This pattern of hypopituitarism induced by chronic, severe illness and exogenous dopamine administration is reminiscent of the hormonal profiles obtained in experimental models of chronic stress, suggesting that endogenous dopamine may play a role in the endocrine and metabolic response to critical illness.

CONCLUSIONS

The dopamine-induced or aggravated pituitary dysfunction in critical illness warrants caution with prolonged infusion of this catecholamine as a so-called supportive agent, particularly in early life. The potential of combined hormonal therapy to improve the metabolic and immune status of the critically ill patient deserves thorough investigation.

Fuel utilization by early newborn brain is preserved under congenital hypothyroidism in the rat

Mental retardation associated with hypothyroidism may be caused by impairment of brain ketone body-metabolizing enzymes during the suckling period. However, much evidence suggests that, immediately after delivery, lactate, instead of ketone bodies or glucose, may be the best substrate for the brain. In this work, we have studied the effect of experimentally induced congenital hypothyroidism on the rate of lactate, glucose, and 3-hydroxybutyrate utilization in early neonatal brain slices. Methimazole (MMI) administration to the mothers caused a 5.4- and 1.7-fold decrease in neonatal plasma concentrations of L-thyroxine (T4) and 3,5,3'-triiodo-L-thyronine (T3), respectively. Propylthiouracil (PTU) administration to the mothers caused a 7.3- and > 2-fold decrease in plasma T4 and T3 concentrations, respectively. MMI-induced hypothyroidism did not significantly modify the rate of lactate, glucose, or 3-hydroxybutyrate oxidation to CO2 and their incorporation into lipids by the neonatal brain. However, PTU-induced hypothyroidism decreased the rate of lactate and glucose oxidation to CO2 and their incorporation into lipids by 17% (p < 0.05). 3-Hydroxybutyrate utilization was not modified by this treatment. Separation by HPLC of the lipids revealed that PTU-mediated inhibition of lipid synthesis from lactate and glucose may be accounted for by specific inhibition of the rate of sterol synthesis (15%, p < 0.05), whereas the rate of phospholipid synthesis was unaffected. These results suggest that the early newborn may develop mechanisms aimed at avoiding the possible brain damage caused by the inhibition of lipid synthesis brought about by mild neonatal hypothyroidism.

Calorigenic effect of diiodothyronines in the rat

1. In hypothyroid rats, we determined the effects of administration of different doses of 3,3',5-triiodo-L-thyronine (T3), 3,3'-diiodo-L-thyronine (3,3'-T2) and 3,5-diiodo-L-thyronine (3,5-T2) ("T2 isomers' refers specifically to these latter two isomers throughout this paper) on resting metabolism (RM) and on the oxidative capacity (measured as cytochrome oxidase activity) of tissues that are metabolically very active. 2. The T2 isomers induced a dose-dependent calorigenic effect when injected I.P. into hypothyroid rats. The increase in RM was already evident at a dose of 2.5 micrograms (100 g body wt)(-1), and the greatest effect was observed at the highest dose, 10 micrograms (100 g body wt)(-1), when RM reached a value not significantly different from that of the euthyroid controls (1.92 +/- 0.08 and 1.93 +/- 0.13 (1 O2) kg(-1) h(-1) for 3,5'-T2, respectively, vs. 2.1 +/- 0.12 (1 O2) kg(-1) h(-1) for euthyroid controls). T3 administration restored RM to normal euthyroid values, even at a dose of 2.5 micrograms (100 g body wt)(-1). 3. The effect of T2 isomers on RM was paralleled by an increase in the oxidative capacity of tissues that are metabolically very active (liver, skeletal muscle, brown adipose tissue (BAT) and heart). The increases were between 33% (liver + 3,3'-T2) and 63% (muscle + 3,3'-T2). By contrast, T3 induced its greatest effect on the liver, with a smaller effect on skeletal muscle, but no significant stimulation in heart and BAT, whatever the dose. 4. These results suggest that T8 isomers might be mediators of the direct thyroid hormone regulation of energy metabolism.

Different regulation of thyroid hormone transport in liver and pituitary: its possible role in the maintenance of low T3 production during nonthyroidal illness and fasting in man

Nonthyroidal illness (NTI) and fasting in man are characterized by a low serum concentration of T3 and an increased serum concentration of rT3. Since the serum level of T3 is one of the most important factors that determine the metabolic rate, the low serum T3 during NTI or fasting results in reduction of the energy consumption of the body. This can be regarded as an adaptive mechanism to save energy, and thus to conserve protein and to protect organ function. The low serum T3 concentration should preferentially be maintained until recovery from illness or adequate calorie supply. This implies that the low serum T3 should not result in a rise in serum TSH. We postulate that different regulation of thyroid hormone transport into the relevant tissues, i.e., liver and pituitary, may play a role in maintenance of the low T3 production during NTI and fasting. This hypothesis is further elaborated in this paper by comparing (i) the properties of the thyroid hormone uptake mechanism in rat and human hepatocytes, perfused rat liver, and rat anterior pituitary cells, and (ii) the effects of fasting and conditions that mimic NTI on thyroid hormone transport in the same preparations. In addition, the consequences of changes in thyroid hormone transport and peripheral thyroid hormone metabolism during fasting and NTI for the serum level of rT3 and for TSH secretion are discussed. The data are compatible with the existence of different transport systems for thyroid hormone in liver and pituitary. We suggest that these different thyroid hormone carriers allow tissue-specific regulation of the intracellular availability of T3.

Thyroid hormones homeostasis in rats refed after short-term and prolonged fasting

Effects of starvation on thyroid hormone homeostasis were usually determined after 2 or more days of fasting, however, both in man and in rodents, natural feeding cycles comprise far shorter fasting periods. Therefore serum levels of T4, FT4, T3, FT3 and rT3 were measured in rats refed chow diet for 1, 4, 8 or 24 hours after 14 or 48 hours of starvation. Both short-term (14 h) and long-term fasting (48 h) decreased body weight and serum glucose level. Short-term fast decreased serum FT3 and did not change serum levels of T4, FT4, T3 and rT3. Total T3 and reverse T3 increased after one and 4 hours, free T3 after 4 hours and total T4 after 4 and 8 hours of refeeding. Percent of FT3 did not change after short-term fast, declined after 1 and 4 hours of refeeding, and normalised thereafter. Prolonged starvation (48 h) decreased serum T4, T3, FT3 and % FT3 with no changes in FT4 and rT3. After 24 hours of refeeding only FT3 and % FT3 returned to control levels while total T4 and total T3 were still diminished, and reverse T3 levels did not change. The results suggest that the length of preceding fasting period may strongly influence thyroid hormone homeostasis during fasted-to-fed transition.