Relationship between resting metabolism and hepatic metabolism: effect of hypothyroidism and 24 hours fasting.

In this paper, we have examined the relationship between the changes in the resting metabolic rate (RMR) and the changes in hepatic metabolism induced by hypothyroidism and 24 h fasting. The results show that hypothyroidism induces a significant decrease in RMR, while 24 h fasting reduces RMR in euthyroid but not in hypothyroid rats. We have also measured oxygen consumption in isolated hepatocytes from euthyroid and hypothyroid rats, fed or fasted for 24 h. The results show that hypothyroidism is characterized by reduced hepatic oxygen consumption. On the other hand, 24 h fasting induces an increase in oxygen consumption in both euthyroid and hypothyroid rat liver cells, although the respiratory rates of hypothyroid rats were lower than those of euthyroid rats. The above findings, as a whole, show that hypothyroidism and 24 h fasting have similar effects on RMR but opposite effects on hepatic metabolism. In addition, a normal thyroid state does not appear to be necessary for the observed changes in hepatic metabolism due to 24 h fasting.     

Cecal microbial transplantation attenuates hyperthyroid-induced thermogenesis in Mongolian gerbils

Endothermic mammals have a high energy cost to maintain a stable and high body temperature (T_b, around 37°C). Thyroid hormones are a major regulator for energy metabolism and T_b. The gut microbiota is involved in modulating host energy metabolism. However, whether the interaction between the gut microbiota and thyroid hormones is involved in metabolic and thermal regulations is unclear. We hypothesized that thyroid hormones via an interaction with gut microbiota orchestrate host thermogenesis and T_b. l -thyroxine-induced hyperthyroid Mongolian gerbils (Meriones unguiculatus) increased resting metabolic rate (RMR) and T_b, whereas Methimazole-induced hypothyroid animals decreased RMR. Both hypothyroid and hyperthyroid animals differed significantly in faecal bacterial community. Hyperthyroidism increased the relative abundance of pathogenic bacteria, such as Helicobacter and Rikenella, and decreased abundance of beneficial bacteria Butyricimonas and Parabacteroides, accompanied by reduced total bile acids and short-chain fatty acids. Furthermore, the hyperthyroid gerbils transplanted with the microbiota from control donors increased type 2 deiodinase (DIO2) expression in the liver and showed a greater rate of decline of both serum T3 and T4 levels and, consequently, a more rapid recovery of normal RMR and T_b. These findings indicate that thyroid hormones regulate thermogenesis depending on gut microbiota and colonization with normal microbiota by caecal microbial transplantation attenuates hyperthyroid-induced thermogenesis. This work reveals the functional consequences of the gut microbiota-thyroid axis in controlling host metabolic physiology and T_b in endotherms.     

长爪沙鼠的贮食行为与其个性和代谢水平之间的关系

贮食行为是动物应对食物资源的季节性和不可预测性变化的一种适应性生存策略。在群居性贮食动物中,同胎个体常表现出不同的贮食水平,而关于贮食行为与动物的个性和代谢水平之间的关系尚缺乏研究。本文以长爪沙鼠（Meriones unguiculatus）为对象,在筛选出具有高贮食和低贮食行为特征个体的基础上,比较了两组动物的个性特征（勇敢行为和探索能力）、静止代谢率、血清甲状腺激素水平、贮食期间的运动距离和贮食后的平均每日代谢率等。结果发现：高、低贮食长爪沙鼠的数量各占49%（22/45）和47%（21/45）,两组动物之间的个性特征、静止代谢率和血清甲状腺激素均没有显著差异。在贮食期间高贮食个体的运动距离显著高于低贮食个体,且在停止贮食后,高贮食个体的平均每日代谢率显著低于低贮食个体。这些结果表明,在室内条件下,长爪沙鼠的贮食量高低与个性和静止代谢率无关,但高贮食个体会在停止贮食后降低其总能量消耗,以补偿贮食过程中的高能量代价。     

Effect of thyroid state on cytosolic free calcium in resting and electrically stimulated cardiac myocytes

The effects of the thyroid state on the cytosolic free Ca2+ concentration, [Ca2+]i, of resting and K+-depolarized cardiomyocytes were studied using the fluorescent Ca2+ indicator fura2. The mean resting [Ca2+]i in euthyroid myocytes (89 +/- 8 nM) was not significantly different from that in hyperthyroid myocytes (100 +/- 14 nM). The resting O2-consumption rate was identical for both groups when expressed per mg protein, but a 35% higher value was observed in the hyperthyroid group when expressed per cell on account of the cellular hypertrophy induced by thyroid hormone. Potassium induced depolarization (50 mM [K+]0) raised the level of [Ca2+]i by 50% in both groups. When ATP-coupled respiration was blocked with oligomycin, the 50 mM K+-induced rise in [Ca2+]i was accompanied in both groups by a 40% rise in glycolytic activity as inferred from measurement of lactate production. Ca2+-fluorescence transients were recorded from electrically stimulated myocytes of euthyroid, hyperthyroid and hypothyroid rats. The time taken to reach peak fluorescence (TPL) and that to 50% decay of peak fluorescence (RL0.5) decreased in the direction hypothyroid----hyperthyroid, indicating an increase in Ca2+ fluxes in the same direction. Isoproterenol (1 microM) enhanced the peak Ca2+ fluorescence in electrically stimulated hypothyroid and euthyroid myocytes but not in hyperthyroid myocytes. Both the TPL and RL0.5 were decreased by isoproterenol in euthyroid, but more so in hypothyroid myocytes. None of these parameters were influenced by isoproterenol in the hyperthyroid group. We conclude that (1) thyroid hormone increases neither the O2-consumption rate nor the level of [Ca2+]i of resting cardiomyocytes and (2) the effects of the beta-receptor-agonist isoproterenol on Ca2+ transients of electrically stimulated myocytes, are inversely related to the documented changes in beta-receptor density in heart tissue occurring with alterations in the thyroid state.     

Influence of physical activity and dietary restraint on resting energy expenditure in young nonobese females

An understanding of the physiological and behavioral determinants of resting energy requirements is important to nutritional considerations in females. We examined the influence of endurance training and self-reported dietary restraint on resting metabolic rate and fasting plasma hormones in 44 nonobese females characterized for body composition, maximal aerobic power (VO2 max), and daily energy intake. To examine the association of metabolic rate and dietary restraint with hormonal status, fasting plasma levels of insulin, glucose, and thyroid hormones (total and free fractions of thyroxine and triiodothyronine) were determined. In univariate analysis, resting metabolic rate (kcal.min-1) was positively related to VO2 max (L.min-1) (r = 0.54; p less than 0.01). This relationship, however, was partially dependent on body size, since fat-free mass was also related to resting metabolic rate (r = 0.42; p less than 0.01) and VO2 max (L.min-1) (r = 0.75; p less than 0.01). After controlling for fat-free weight using partial correlation analysis, the relation between RMR and VO2 max was weaker but controlling for fat-free weight using partial correlation analysis, the relation between RMR and VO2 max was weaker but still significant (partial r = 0.38; p less than 0.05). On the other hand, high levels of dietary restraint were associated with higher levels of body fat (r = 0.31; p less than 0.05) and a lower resting metabolic rate (r = -0.29; p = 0.07). These associations persisted after control for differences in fat-free mass. Total energy intake as well as total and free levels of triiodothyronine were not related to resting metabolic rate or level of dietary restraint.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of in vivo TSH secretion by leptin

Leptin, the product of the ob gene, is a hormone secreted by adipocytes that regulates food intake and energy expenditure. The hypothalamus-pituitary-thyroid axis is markedly influenced by the metabolic status, being suppressed during food deprivation.The aim of the present study was to assess whether leptin can act as a metabolic signal connecting the adipose tissue with the pituitary-thyroid axis. We studied the effect of leptin administration (10 microg, i.c.v.) on spontaneous TSH secretion and TSH responses to TRH in euthyroid and hypothyroid food-deprived rats. Spontaneous TSH secretion was assessed over 6 h with samples taken every 7 min. Administration of leptin to food-deprived euthyroid rats led to a reversal of the inhibitory effect exerted by fasting on spontaneous TSH secretion. This stimulatory effect of leptin on spontaneous TSH appears to be dependent on the thyroid status since it could not be observed in hypothyroid rats. This data suggests that blunted spontaneous TSH secretion in food-deprived rats is a functional and reversible state, and that the decreased leptin concentrations could be the primary event responsible for the suppression of the hypothalamic-pituitary-thyroid-axis in food-deprived rats.     

Lactose synthetase activity in mouse mammary glands is controlled by thyroid hormones

Epithelial cells in explants from the mammary glands of euthyroid mature virgin mice are proliferatively dormant. They must undergo DNA synthesis and traverse the cell cycle in vitro before they are able to differentiate fully in response to insulin, hydrocortisone, and prolactin, and synthesize enzymatically active alpha-lactalbumin (measured as lactose synthetase activity). In contrast, glands from hyperthyroid mature virgin mice do not require DNA synthesis in vitro to differentiate. Explants from the euthyroid virgin tissue overcome their dependence on DNA synthesis when 10(-9) M 3,5,3'-triiodo-L-thyronine is added directly to the cultures in addition to the other three hormones. Explants from involuted mammary glands from euthyroid primiparous mice do not require DNA synthesis in vitro to make the milk protein even though they, like explants from mature euthyroid virgin tissue, are proliferatively dormant and do not contain detectable lactose synthetase activity in vivo. Glands from primiparous animals made mildly hypothyroid by ingestion of 0.1% thiouracil in drinking water during 7 wk of involution remain morphologically indistinguishable from glands of their euthyroid counterparts. However, explants from the glands of these hypothyroid animals revert to a state of dependence on DNA synthesis to differentiate functionally. These observations suggest that the dependence on DNA synthesis and cell cycle traversal for hormonal induction of lactose synthetase activity in the mouse mammary gland is controlled by thyroid hormones.     

Modulation by thyroid status of cyclic AMP-dependent and Ca2+-dependent mechanisms of hormone action in rat liver cells. Possible involvement of two different transduction mechanisms in alpha 1-adrenergic action

The actions of hormones which are associated to cAMP-dependent and calcium-dependent mechanisms of signal transduction were studied in hepatocytes obtained from rats with different thyroid states. In cells from euthyroid and hyperthyroid rats, the metabolic actions of epinephrine were mediated mainly through alpha 1-adrenoceptors; beta-adrenoceptors seem to be functionally unimportant. In contrast, both alpha 1- and beta-adrenoceptors mediate the actions of epinephrine in hepatocytes from hypothyroid animals. Phosphatidylinositol labeling was strongly stimulated by epinephrine, vasopressin and angiotensin II in cells from eu-, hyper- or hypothyroid rats. However, metabolic responsiveness to vasopressin and angiotensin II was markedly impaired in the hypothyroid state. The glycogenolytic response to the calcium ionophore A-23187 was also impaired, suggesting that hepatocytes from hypothyroid rats are less sensitive to calcium signalling. The persistence of alpha 1-adrenergic responsiveness in the hypothyroid state suggests that the mechanism of signal transduction for alpha 1-adrenergic amines is not identical to that of the vasopressor peptides. alpha 1-Adrenergic stimulation of cyclic AMP accumulation was not detected in cells from hypothyroid rats. These data suggest that factors besides calcium and besides cAMP are probably involved in alpha 1-adrenergic actions. Metabolic responses to glucagon and to the cAMP analogue dibutyryl cAMP were not markedly changed during hypothyroidism, although cAMP accumulation produced by glucagon and beta-adrenergic agonists was enhanced. In hyperthyroidism, cell responsiveness to epinephrine, vasopressin, angiotensin II and glucagon was decreased, but sensitivity to cAMP was not markedly altered. The factors involved in this hyposensitivity to hormones during hyperthyroidism are unclear.     

Thyroid status is a key modulator of tumor oxygenation: implication for radiation therapy

In normal tissues, thyroid hormones play a major role in the metabolic activity and oxygen consumption of cells. Because the rate of oxygen consumption is a key factor in the response of tumors to radiation, we hypothesized that thyroid hormones may affect the metabolic activity of tumor cells and hence modulate the response to cytotoxic treatments. We measured the influence of thyroid status on the tumor microenvironment in experimental tumors. Hypothyroidism and hyperthyroidism were generated in mice by chronic treatment with propyl thiouracil and l-thyroxine. Thyroid status significantly modified tumor pO(2) as measured with EPR oximetry. Mechanistically, this was the result of the profound changes in oxygen consumption rates. Thyroid status was associated with a significant change in tumor radiosensitivity since the regrowth delay was increased in hypothyroid mice compared to euthyroid mice, an effect that was abolished when temporarily clamped tumors were irradiated. This study provides unique insights into the impact of modulating tumor oxygen consumption and could have implications in the management of cancer patients with thyroid disorders.     

Poly(ADP-ribose) polymerase is affected early by thyroid state during liver regeneration in rats

Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme involved in DNA synthesis, DNA repair, and cell replication and transformation, also plays a role in the early steps of liver regeneration induced by partial hepatectomy (PH). PARP and DNA topoisomerase I (Topo I) activities and de novo DNA synthesis were studied during liver regeneration in rats with altered thyroid state. Hepatic PARP activity, evaluated as [(32)P]NAD incorporated into isolated liver nuclei, was inhibited in hyperthyroid rats and increased in hypothyroid animals. In both euthyroid and hyperthyroid rats PARP activity was rapidly stimulated, peaking 6 h after PH. In hypothyroid animals, an early decrease in activity was found, at a minimum of 6 h after PH, followed by an early onset of DNA synthesis. An inverse relationship between PARP and Topo I activities was a shared feature among euthyroid, hypothyroid, and hyperthyroid rats. Together these data show that, in replicating hepatocytes, thyroid hormones exert a regulatory role on PARP activity, which reflects the control of a number of nuclear proteins involved in DNA metabolism.     

Upregulation of Slc39a10 gene expression in response to thyroid hormones in intestine and kidney

A novel zinc transporter has been purified and cloned from rat renal brush border membrane. This transporter was designated as Zip10 encoded by Slc39a10 gene and characterized as zinc importer. Present study documents the impact of thyroid hormones on the expression of Zip10 encoded by Slc39a10 gene in rat model of hypo and hyperthyroidism. Serum T(3) and T(4) levels were reduced significantly in hypothyroid rats whereas these levels were significantly elevated in hyperthyroid rats as compared to euthyroid rats thereby confirming the validity of the model. Kinetic studies revealed a significant increase in the initial and equilibrium uptake of Zn(++) in both intestinal and renal BBMV of hyperthyroid rats in comparison to hypothyroid and euthyroid rats. By RT-PCR, Slc39a10 mRNA expression was found to be significantly decreased in hypothyroid and increased in hyperthyroid as compared to euthyroid rats. These findings are in conformity with the immunofluorescence studies that revealed markedly higher fluorescence intensity at periphery of both intestinal and renal cells isolated from hyperthyroid rats as compared to hypothyroid and euthyroid rats. Higher expression of Zip10 protein in hyperthyroid group was also confirmed by western blot. These findings suggest that expression of zinc transporter protein Zip10 (Slc39a10) in intestine and kidney is positively regulated by thyroid hormones.     

Epidermal growth factor receptor ontogeny in mice with congenital hypothyroidism

To assess the effect of endogenous thyroid hormone on hepatic EGF receptors in developing mice we measured EGF binding to plasma membrane receptors in liver and brain of mice with congenital hypothyroidism and in euthyroid controls at 20, 30 and 40 days of age. At 20 days hepatic EGF receptor binding was low in both hypothyroid and control animals. Between 20 and 30 days the hepatic binding increased dramatically in the euthyroid animals, an increase that was greater in males than females. The increase in binding was due to an increase in the high affinity receptor population. Among hypothyroid animals there were no changes in hepatic EGF receptor binding with increasing age. In cerebral cortex EGF binding was similar in euthyroid and hypothyroid animals at all ages. These results suggest that thyroxine has regulatory effects on the postnatal ontogeny of hepatic EGF receptors.     

Hormonal stimulation of cyclic AMP accumulation and glycogen phosphorylase activity in calcium-depleted hepatocytes from euthyroid and hypothyroid rats.

Activation of glycogen phosphorylase by hormones was examined in hepatocytes isolated from euthyroid and hypothyroid female rats and incubated by Ca2+-free buffer containing 1 mM-EGTA. Basal glycogen phosphorylase activity was decreased in Ca2+-free buffer. However, the activation of hepatocyte glycogen phosphorylase, in the absence of extracellular Ca2+, in response to adrenaline, glucagon or phenylephrine was slightly lower, whereas that by vasopressin was abolished. The activation of glycogen phosphorylase by phenylephrine, adrenaline or isoproterenol (isoprenaline) in hepatocytes from euthyroid rats incubated in the absence of Ca2+ was not accompanied by any detectable increase in total cyclic AMP. The log-dose/response curves for activation of phosphorylase by phenylephrine or low concentrations of adrenaline were the same in hepatocytes from hypothyroid as compared wit euthyroid rats, whereas the response to isoproterenol was greater in hepatocytes from hypothyroid rats. However, the increases in total cyclic AMP accumulation caused by adrenaline or isoproterenol were greater in hepatocytes from hypothyroid rats than in hepatocytes from euthyroid rats. The increases in cyclic AMP accumulation caused by adrenaline or isoproterenol in Ca2+-depleted hepatocytes from hypothyroid rats were blocked by propranolol, a beta-adrenergic antagonist. In contrast, propranolol was only partially effective asan inhibitor of the activation of glycogen phosphorylase by phenylephrine or adrenaline in hepatocytes from hypothyroid rats and ineffective on phosphorylase activation in cells from euthyroid rats. These data indicate that the alpha-adrenergic activation of glycogen phosphorylase is not affected by the absence of extracellular Ca2+, and the extent to which total cyclic AMP was increased by adrenergic amines did not correlate with glycogen phosphorylase activation.     

Effect of propranolol on the metabolism of thyroid hormones

The effect of propranolol on thyroxine (T4) and 3,5',3'-triiodothyronine (T3) plasma concentrations, fractional disappearance rates (k), metabolic clearance rates (MCR) and catabolic rates has been investigated in young pigs. The animals were examined in the period 18-24 h after feeding. Plasma concentrations of T3 were lower after treatment with propranolol, but T4 was not significantly affected. The k value of T4 was decreased by propranolol but that for T3 was unaffected. The MCR of T4 and the catabolic rates of both hormones were reduced by propranolol. The reduction in metabolic rate after propranolol is thus probably related to its action on thyroid hormone metabolism.     

Influence of thyroid hormone on the in vitro translational activity of specific mRNAs in the rat heart

The influence of thyroid hormone on the translational activity of specific cardiac mRNA was determined by in vitro translation of RNA isolated from the heart of normal, hypothyroid, and 3,3',5-triiodo-L-thyronine-injected hypothyroid rats. Proteins synthesized in vitro in the presence of [35S]methionine were separated by two-dimensional gel electrophoresis and quantitated by a novel scanning procedure using digital matrix photometry. A total of 421 translational products were detected by fluorography and changes in the predominance of 12 of these were influenced by the thyroid state of the animals. The relative predominance of 8 species was increased in euthyroid animals, whereas 4 translational products were increased in hypothyroid animals. The majority of these thyroid hormone-related alterations occurred in spot pairs of similar molecular weights, but slightly different isoelectric points. In contrast, the relative predominance of mRNAs coding for the major contractile proteins, light chain 1, light chain 2, tropomyosin, actin, and myosin heavy chain was not altered by the thyroid status of the animals. The relative levels of these abundant mRNA species remained unaltered in spite of a thyroid hormone-related increase in total RNA levels. In vivo effects of thyroid hormone on cardiac RNA levels are complex. In addition to a general increase in total RNA and mRNA levels, increases or attenuations in the predominance of a small number of specific mRNA species are observed when euthyroid and hypothyroid animals are compared.     

D-propranolol and DL-propranolol both decrease conversion of L-thyroxine to L-triiodothyronine.

The effects of propranolol (DL-propranolol) and D-propranolol on thyroid hormone metabolism were studied in six euthyroid volunteers receiving L-thyroxine (T4) and six hypothyroid patients receiving T4 replacement. D-propranolol as well as propranolol decreased L-triiodothyronine (T3) concentrations and the ratio of T3 to T4 in the euthyroid subjects, and D-propranolol decreased these variables in the subjects with hypothyroidism (propranolol was not given to this group). It is concluded from this study and from parallel invitro investigations that the effect of propranolol on the conversion of T4 to T3 is unrelated to its beta-adrenergic blocking activity, and that at low therapeutic doses propranolol may exert appreciable "membrane-stabilising" effects in vivo.     

The mechanism of the increase in mitochondrial proton permeability induced by thyroid hormones.

Three possible mechanisms by which different levels of thyroid hormones in rats might cause the observed sevenfold change in the apparent proton permeability of the inner membrane of isolated liver mitochondria were investigated. (a) Cytochrome c oxidase was isolated from the livers of hypothyroid, euthyroid and hyperthyroid rats and incorporated into liposomes made with soya phospholipids. There was no difference between the proton current/voltage curves of the three types of vesicles. The hormonal effects, therefore, were not an inherent property of the enzymes, and were not due to different coupling of electron flow through the enzyme to proton transport. (b) The surface area of the mitochondrial inner membrane was shown by three different assays to be greater by a factor of between two and three in mitochondria from hyperthyroid animals than in mitochondria from hypothyroid animals; euthyroid controls were intermediate. This difference in surface area of the inner membrane explains less than half of the difference in apparent proton permeability. (c) The proton permeability of liposomes prepared from phospholipids extracted from mitochondrial inner membranes of hyperthyroid rats was three times greater than the proton permeability of those from hypothyroid rats; euthyroid controls were intermediate. This suggests, first, that the proton permeability of the phospholipid bilayer is an important component of the proton permeability in intact mitochondria and, second, thyroid hormone-induced changes in the bilayer are a major part of the mechanism of increased proton permeability. Such changes may be due to the known differences in fatty acid composition of mitochondrial phospholipids in different thyroid states. Thus we have identified two mechanisms by which thyroid hormone levels in rats change proton flux/mass protein in isolated liver mitochondria: a change in the area of the inner membrane/mass protein and a change in the intrinsic permeability of the phospholipid bilayer.     

On the thyroid hormone-induced increase in respiratory capacity of isolated rat hepatocytes.

The respiratory capacities of hepatocytes, derived from hypothyroid, euthyroid and hyperthyroid rats, have been compared by measuring rates of oxygen uptake and by titrating components of the respiratory chain with specific inhibitors. Thyroid hormone increased the maximal rate of substrate-stimulated respiration and also increased the degree of ionophore-stimulated oxygen uptake. In titration experiments, similar concentrations of oligomycin or antimycin were required for maximal inhibition of respiration regardless of thyroid state, suggesting that the changes in respiratory capacity were not the result of variation in the amounts of ATP synthase or cytochrome b. However, less rotenone was required for maximal inhibition of respiration in the hypothyroid state than in cells from euthyroid or hyperthyroid rats, implying that hepatocytes from hypothyroid animals contain less NADH dehydrogenase. The concentration of carboxyatractyloside necessary for maximal inhibition of respiration was 100 microM in hepatocytes from hypothyroid rats, but 200 microM and 300 microM in hepatocytes from euthyroid and hyperthyroid rats, respectively, indicating a possible correlation between levels of thyroid hormone and the amount or activity of adenine nucleotide translocase. The increased capacity for coupled respiration in response to thyroid hormone is not associated with an increase in the components of the electron transport chain or ATP synthase, but correlates with an increased activity of adenine nucleotide translocase.