Thyroid hormone action at the cell level (second of two parts)

A Historical Reflection on Scientific Advances in Understanding Thyroid Hormone Action

Background: Thyroid hormone (TH) has actions in every tissue of the body and is essential for normal development, as well as having important actions in the adult. The earliest markers of TH action that were identified and monitored clinically, even before TH could be measured in serum, included oxygen consumption, basal metabolic rate, serum cholesterol, and deep tendon reflex time. Cellular, rodent, amphibian, zebrafish, and human models have been used to study TH action. Summary: Early studies of the mechanism of TH action focused on saturable-specific triiodothyronine (T3) nuclear binding and direct actions of T3 that altered protein expression. Additional effects of TH were recognized on mitochondria, stimulation of ion transport, especially the sodium potassium ATPase, augmentation of adrenergic signaling, role as a neurotransmitter, and direct plasma membrane effects. The cloning of the thyroid hormone receptor (THR) genes in 1986 and report of the THR crystal structure in 1995 produced rapid progress in understanding the mechanism of TH nuclear action, as well as the development of modified THR ligands. These findings revealed nuances of TH signaling, including the role of nuclear receptor coactivators and corepressors, repression of positively stimulated genes by the unliganded receptor, THR isoform-specific actions of TRα (THRA) and TRβ (THRB), and THR binding DNA as a heterodimer with retinoid-x-receptor (RXR) for genes positively regulated by TH. The identification of genetic disorders of TH transport and signaling, especially Resistance to Thyroid Hormone (RTH) and monocarboxylate transporter 8 (Mct8) defects, has been highly informative with respect to the mechanism of TH action. Conclusions: The impact of THR isoform, post-translational modifications, receptor cofactors, DNA response element, and selective TH tissue uptake, on TH action, have clinical implications for diagnosing and treating thyroid disease. Additionally, these findings have led to the development of novel TH and TH analogue therapies for metabolic, neurological, and cardiovascular diseases.

Effect of Thyroid Hormones onFlavin-containing Monooxygenase Activity in Co-cultured Adult Rat Hepatocytes

The regulation of flavin-containing monooxygenase (FMO) by thyroid hormones was examined under well defined in vitro conditions using adult male rat hepatocytes co-cultured with rat liver epithelial cells of primitive biliary origin. Serum free medium was used to avoid interferences from foetal bovine serum. The effect of thyroxine (T4) and its major metabolite l-triiodothyronine (T3) on FMO activity was estimated spectrophotometrically by measuring the rate of methimazole oxygenation. The highest non-cytotoxic doses of T3 and T4 that could be used in co-cultures were determined by measuring both lactate dehydrogenase leakage into the medium and microsomal protein content of the hepatocytes as a function of culture time. In addition, hormonal responsiveness of the in vitro system used was confirmed by malic enzyme activity measurements. Administration of 10 mum T3 or T4 was found to cause a significant decrease in FMO activity and content, suggesting a suppressive role of both hormones on the regulation of FMO activity in male rats.

Long-term, high intake of vitamin C decreases size and increases quantity of liver mitochondria in Guinea-pigs

Microsomal cytochrome P450 is an important enzyme involved in drug metabolism and bioactivation in the liver. In guinea-pigs, the specific amount of cytochrome P450 depends on dietary vitamin C intake. Short-term vitamin C treatment can induce non-genomic effects on hepatocytes, such as activating mitochondrial respiration without changing cytochrome concentration. The aim of this investigation was to elucidate the long-term effects of a low or high vitamin C diet on the quantity and size of mitochondria in the hepatocytes of guinea-pigs. After 6 - 8 weeks of vitamin C feeding, the group receiving a low vitamin C content in their food (10 mg/100 g food) had significantly less mitochondria, but of a larger size, than the group receiving a high vitamin C content (680 mg/100 g food). This animal model investigation demonstrated that treatment with long-term vitamin C can induce morphological changes in the mitochondria of hepatocytes.

GABA uptake is inhibited by thyroid hormones: implications for depression

Studies of the effects of thyroid hormones on the uptake of neurotransmitters by homogenates of rat cerebral cortex have revealed a significant competitive inhibition of neuronal uptake of [3H]GABA by thyroid hormones (T3 greater than T4 greater than rT3). The IC50 for inhibition of GABA uptake by T3 was estimated at 4 microM and that of T4 at 11 microns. GABA uptake in homogenates of cerebral cortex from hypothyroid rats was significantly enhanced over that of controls; however, uptake in tissues from hyperthyroid rats was not significantly diminished.

Reduced metabolic efficiency of skeletal muscle energetics in hyperthyroid patients evidenced quantitatively by in vivo phosphorus-31 magnetic resonance spectroscopy

Skeletal muscle energetics of seven hyperthyroid patients were investigated throughout a rest-exercise-recovery protocol using phosphorus-31 magnetic resonance spectroscopy (31P MRS) to quantitatively document in vivo the metabolic bases of impaired muscle performance in hyperthyroidism. The contributions of the main pathways of adenosine triphosphate (ATP) synthesis to energy production and proton efflux were measured and compared with results from normal muscle. At rest, a reduced concentration of phosphocreatine (PCr) was calculated for hyperthyroid patients when compared with controls, whereas pH and concentrations of inorganic phosphate (Pi) and phosphomonoesters (PME) were not different from controls. During exercise, the analysis of changes in pH and PCr concentration demonstrated that (1) at the onset of exercise, the magnitude of glycolysis activation is significantly larger for patients, resulting in a marked pH decrease; (2) the energy cost of exercise is higher for patients as compared with controls performing the same amount of work; and (3) both anaerobic and aerobic pathways are significantly more activated in the hyperthyroid group throughout the 3 minutes of exercise. During recovery, the rates of proton efflux and PCr resynthesis were similar in both groups, excluding any alteration in oxidative function and proton handling as a cause of initial glycolytic hyperactivation. The increased energy cost measured for patients during exercise evidences an increased need for energy, which is (1) probably linked to the existence of additional ATP-consuming mechanism(s), and (2) supported by hyperactivation of both aerobic and anaerobic pathways. These findings imply that, all things equal, a hyperthyroid muscle requires more energy to function than normal, and as a result is potentially more fatiguable.

Natural sex steroids and their xenobiotic analogs in animal production: growth, carcass quality, pharmacokinetics, metabolism, mode of action, residues, methods, and epidemiology

Natural and xenobiotic compounds having sex-related actions have long been used for growth promotion and various changes in carcass quality in meat animals. The first compounds used were synthetic estrogens; however, later on a whole battery of compounds having androgenic, and progestogenic actions have also been involved. In surveying the effects of these compounds in meat-producing animals, it became clear that these drugs increase the growth rate of the treated animals and bring about changes in the carcass that are generally characterized by lower fat content and more lean mass. Extensive studies undertaken in various countries, including the European Economic Community (EEC), have shown that if used according to good husbandry practices, the meat from treated animals does not have excessive amounts of residues compared with the endogenous amount of steroid production in the animals in question and also in human beings. The banning of these compounds in the European community brought a new phenomenon of illegal or black market cocktails. These mixtures of anabolic steroids are injected into the body of the animals rather than implanted in the ears, which is the normal practice in countries where they have not yet been banned. Several screening and confirmatory methods are now available for monitoring programs. However, these programs need excessive resources in terms of manpower, funds, and proper legislation, which in underdeveloped countries is questionable, particularly in the absence of strong scientific evidence for the exercise.

The direct effects of 3,5,3'-triiodo-L-thyronine (T3) on myocyte contractile processes. Insights into mechanisms of action

Administration of 3,5,3'-triiodo-L-thyronine (T3) has recently been suggested to acutely improve left ventricular performance. However, the cellular and molecular mechanisms responsible for this improvement in left ventricular function with T3 remained unknown. Accordingly, the present study examined the direct effects of T3 administration on myocyte contractile function and the sarcolemmal systems that might potentially contribute to these effects. In isolated porcine left ventricular myocytes (n = 81), velocity of shortening increased in the presence of 80 pmol/L T3 compared with that in untreated myocytes (117.0 +/- 5.0 versus 77.3 +/- 3.3 microns/sec, p < 0.05). In a separate series of experiments (n = 29), myocyte velocity of shortening increased in the presence of both T3 and beta-adrenergic receptor stimulation (25 nmol/L isoproterenol) to greater than that with beta-adrenergic receptor stimulation alone (274.3 +/- 16.9 versus 203.7 +/- 16.2 microns/sec, p < 0.05). Cyclic adenosine monophosphate generation was next examined in isolated myocyte preparations (n = 9). In the presence of T3, no significant increase in cyclic-adenosine monophosphate generation was observed compared with that in untreated myocytes (39.1 +/- 8.3 versus 24.7 +/- 5.8 fmols/myocyte, p = 0.17). However, in the presence of both T3 and beta-adrenergic receptor stimulation, cyclic-adenosine monophosphate generation increased significantly to greater than that with beta-adrenergic receptor stimulation alone (224.4 +/- 61.1 versus 120.1 +/- 35.5 fmoles/myocyte, p < 0.05). Because cyclic-adenosine monophosphate modulates intracellular Ca2+ processes, L-type Ca+2 channel current (patch clamp methods; -picoamp/picofarad, n = 15) and peak intracellular Ca+2 levels (fura 2 ionic measurement, n = 47) were next measured. In the presence of T3, a shift in the activation voltage at peak L-type Ca+2 channel current was observed from baseline (5.5 +/- 1.4 versus 9.0 +/- 1.0 mV, p < 0.05). Furthermore, in the presence of both T3 and beta-adrenergic receptor stimulation, peak L-type Ca+2 channel current (8.9 +/- 0.7 versus 6.3 +/- 1.0 mV, p < 0.05) and peak intracellular Ca+2 levels (189.9 +/- 8.4 versus 171.7 +/- 8.3 nmol/L, p < 0.05) increased compared with values obtained with beta-adrenergic receptor stimulation alone. Important findings from the present study were twofold: (1) T3 improved myocyte contractile processes through a cyclic-adenosine monophosphate-independent mechanism and (2) T3 potentiated the effects of beta-adrenergic receptor stimulation transduction by increasing cyclic-adenosine monophosphate production, L-type Ca+2 channel current, and Ca+2 availability to the myocyte contractile apparatus.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothyroidism affects the expression of the beta-F1-ATPase gene and limits mitochondrial proliferation in rat liver at all stages of development

In order to analyze the role of thyroid hormones in mitochondrial biogenesis, we have studied the expression pattern of the beta subunit of the mitochondrial ATP-synthase complex in liver and in isolated mitochondria during postnatal development of hypothyroid rats. Chemically induced hypothyroidism promoted a significant reduction in body and liver masses at all stages of development. Furthermore, plasma 3,5,3'-triiodo-L-thyronine (T3) and 3,5,3',5'-tetraiodo-L-thyronine (T4) concentrations were significantly reduced in hypothyroid animals when compared to euthyroid animals. Remarkably, steady-state beta-F1-ATPase mRNA levels in livers of hypothyroid animals showed an approximately 50% reduction when compared to age-matched euthyroid rats at all stages of development. The relative amounts of beta-F1-ATPase protein determined in isolated mitochondria of 1-day-old and adult hypothyroid animals were similar to those determined in mitochondria of age-matched euthyroids, indicating that hypothyroidism does not affect organelle differentiation in the liver of suckling and adult rats. In contrast, the relative amount of beta-F1-ATPase protein in liver homogenates varied (0-30% reduction) due to the hypothyroid condition during development. These findings suggest the existence of compensatory mechanisms operating at the translational and/or post-translational levels which promote proliferation of mitochondria in the hypothyroid liver. However, when the liver mass was considered, hypothyroidism significantly reduced overall mitochondrial proliferation in rat liver. Interestingly, the effects of thyroid hormones on the biogenesis of the ATP synthase complex at latter stages of development provide an example in which the hypothyroid condition limits the expression of the nuclear-encoded gene with no apparent effect on the expression of the mitochondrial-encoded genes (ATP synthase subunits 6-8).

Thyroxine pretreatment and halothane administration alter Ca2+ transport and transmembrane potential in rat liver mitochondria. An additional mechanism for halothane-induced liver damage in the hyperthyroid rat model

Male rats pretreated with thyroid hormones and exposed to halothane in non-hypoxic conditions develop acute liver damage. In order to investigate the mechanisms leading to liver damage in this animal model, the effects of thyroxine (T4) pretreatment and halothane administration on Ca2+ transport and transmembrane potential were studied in isolated rat liver mitochondria. Five-day T4-pretreatment reduced the mitochondrial Ca2+ loading capacity and increased the rate of Ca2+ cycling across the mitochondrial membrane. Halothane administration further increased Ca2+ cycling and produced a time- and dose-dependent loss of transmembrane potential which was more pronounced in mitochondria from T4-pretreated rats than in euthyroid animals. When mitochondria from T4-pretreated rats were incubated in the presence of the Ca2+ chelator EGTA, membrane potential was well preserved. In contrast, when Ca2+ concentration in the extramitochondrial medium was increased, mitochondria deenergization occurred earlier. These findings confirm that alterations in Ca2+ transport and mitochondrial function can be interrelated events and suggest that a Ca(2+)-dependent, halothane-induced loss of transmembrane potential could participate in generating acute liver damage in hyperthyroid rats exposed to halothane in non-hypoxic conditions.

Respiratory activity of isolated liver Mitochondria following Trypanosoma congolense infection in rabbits: the role of thyroxine

1. The effect of trypanosome infection on rabbit liver mitochondrial oxidative phosphorylation was investigated, with and without thyroxine replacement. 2. State 3 respiration, respiratory control ratio (RCR) and ADP/O ratio were significantly reduced in mitochondria from trypanosome-infected animals whereas there was no change in state 4 respiration. 3. State 3 respiration, RCR and ADP/O ratio were not significantly altered in trypanosome-infected animals given thyroxine replacement therapy. 4. Trypanosome infection leads to impairment of mitochondrial integrity, apparently through lowered thyroxine levels. Replacement of thyroxine therefore sustains optimal mitochondrial respiratory activity.

Treatment of Raynaud's phenomenon with triiodothyronine corrects co-existent autonomic dysfunction: preliminary findings

Cardiovascular autonomic function was assessed in 9 subjects with Raynaud's phenomenon. The underlying diseases were systemic lupus erythematosus (n = 5), systemic sclerosis (n = 3) and rheumatoid arthritis (n = 1). Five standard non-invasive tests, 3 of heart rate and 2 of blood pressure, were employed. Compared with age and sex matched controls (n = 25), the number of values abnormal was 24 of 45 (53%) overall and between one and 4 (median, 2) individually. Significant differences were present for 3 tests, two of heart rate and one of blood pressure. The subjects were given triiodothyronine, 60 to 80 micrograms per day, for vasospastic attacks. Autonomic function was reassessed between weeks 4 and 9 (9 subjects) and between weeks 12 and 18 (8 subjects) after introduction of triiodothyronine. Test results showed a considerable improvement. At the second reassessment, the number of values abnormal was now 5 of 40 (12.5%) overall and nil (n = 4) or one (n = 4) individually. Significant differences remained for one heart rate test only. Adverse side effects to triiodothyronine occurred in a single subject and were readily controlled. Evidence of somatic neuropathy was present electrophysiologically in all 9 subjects and clinically in 8. Triiodothyronine may have corrected autonomic dysfunction by increasing blood flow to ischaemic peripheral nerves or by acting on the autonomic system more directly. Further study of triiodothyronine in autonomic insufficiency appears merited.

Effects of antiepileptic drugs on hormones

A hormone is an intrinsic substance carried via the blood to a target organ which is then functionally stimulated. Similar to extrinsically administered medications, the metabolism and function of the hormones may be altered by antiepileptic drugs (AEDs). The proposed mechanisms are (a) enhanced metabolism (natural steroids, synthetic steroids, e.g., decadron and birth control pills, thyroxine, and vitamin D3), (b) altered protein bonding (thyroxine, sex hormones), (c) impaired release into the systemic circulation (calcitonin, insulin, vitamin K clotting factors) and (d) altered end-organ effect. The AEDs most likely to interact with hormones are barbiturates, carbamazepine, and phenytoin.

Effect of L-thyroxine (LT4) and D-thyroxine (DT4) on cardiac function and high-energy phosphate metabolism: a 31P NMR study

31P NMR spectroscopy was used to monitor the cardiac energy metabolism in hypothyroid rat hearts. Differential alterations in phosphocreatine and inorganic phosphate levels were observed upon treatment of hypothyroid animals with DT4 and LT4, while both agents were equipotent in reducing cholesterol. These results show potential for NMR spectroscopy as a technique to determine therapeutic selectivity.

Thyroid function in brain-dead donors

Twenty brain-dead potential organ donors were studied prospectively to establish thyroid function. Two or three consecutive blood samples were obtained during brain death. Seven times a sample was available before brain death occurred. Free triiodothyronine (FT3) fell in most patients (80%). Very low (less than 1.6 pmol/l) and subnormal levels (between 2 and 3 pmol/l) were found in 65% and 15% of the patients, respectively. Serum reverse total triiodothyronine (rT3) was inversely correlated with FT3. Free thyroxine (FT4) was less often decreased (mean 14.68 +/- 1.42 pmol/l) and 35% of the patients had normal levels. Mean thyroid stimulating hormone (TSH) remained normal (0.71 +/- 0.15 microU/ml). The study of consecutive samples during brain death did not show a constant, progressive decrease in hormonal levels. There is no statistical difference between values observed before and after brain death. No correlation was found between FT3 levels and hemodynamic data or immediate allograft function. The pattern of thyroid function in these patients was typical of the sick euthyroid syndrome with a low T3 or low T3 and low T4 serum levels. This syndrome usually does not need to be treated. However, many experiment findings and some clinical data argue in favor of T3 therapy in donors and possibly in recipients. The dosage regimen must be adjusted to be effective without causing harm to multiorgan donors before it can be widely used. It remains to be proved that low FT3 serum indicates low intracellular FT3 and worse metabolic function in clinical conditions.

Rapid stimulation of hepatic oxygen consumption by 3,5-di-iodo-L-thyronine

Tri-iodothyronine (T3) and thyroxine (T4) as well as 3,5-di-iodothyronine (T2) stimulated O2 consumption by isolated perfused livers from hypothyroid rats at a concentration as low as 1 pM by about 30% within 90 min. Application of T2 resulted in a faster stimulation than with application of T3 or T4. Inhibition of iodothyronine monodeiodinase by propylthiouracil, thereby blocking the degradation of T4 to T3 and of T3 to T2, demonstrated that only T2 is the active hormone for the rapid stimulation of hepatic O2 consumption: T3 and T4 lost all of their stimulative activity, whereas T2 was as potent as in the absence of propylthiouracil. Perfusion experiments with thyroid-hormone analogues confirmed the specificity of the T2 effect. The nucleus is unlikely to contribute to the rapid T2 effect, as can be deduced from perfusion experiments with cycloheximide and lack of induction of malic enzyme by T2. In conclusion, a new scheme of regulation of mitochondrial activity is proposed: T2 acts rapidly and directly via a mitochondrial pathway, whereas T3 exerts its long-term action indirectly by induction of specific enzymes.

Insulin resistance in Graves' disease: a quantitative in-vivo evaluation

Hyperthyroidism is considered to be an insulin-resistant state, but a quantitative evaluation of some action of insulin is still lacking. We performed euglycaemic clamp at about 350 and 7000 pmol l-1 plasma insulin concentration in combination with the 3H-glucose infusion in 12 patients with Graves' disease and in 12 matched controls. Fasting plasma insulin (126 +/- 6.5 vs. 77.5 +/- 5.7 pmol l-1; P less than 0.001), C-peptide (502 +/- 36 vs. 363 +/- 41 pmol l-1; P less than 0.001) and glucagon (47 +/- 3.3 vs. 33.3 +/- 3 pmol l-1; P less than 0.01) were significantly higher in hyperthyroids than in euthyroids. Basal hepatic glucose production was significantly higher in hyperthyroids than in euthyroids (18.3 +/- 1.4 vs. 9.2 +/- 0.5 mumol l-1; P less than 0.0001), and its suppression during physiological hyperinsulinaemia was only 50% in hyperthyroids. Glucose utilization and suppression of lipolysis were normally stimulated by insulin. All parameters altered during hyperthyroidism were normalized during methimazole-induced euthyroidism. We conclude that insulin resistance involves mainly glucose rather than lipid and is selective at the hepatic level.

Insulin binding to monocytes and in vivo peripheral insulin sensitivity are normal in Graves' disease

Insulin resistance in hyperthyroidism seems to depend on increased glucose production rather than on decreased glucose utilization. A decreased insulin binding on different target cells has been reported in patients in whom an in vivo evaluation of peripheral insulin sensitivity was lacking. In 20 patients with Graves' disease (6 males, 14 females), aged 40.0 +/- 2.0 yr, BMI 23.7 +/- 0.7, and in 20 well-matched controls we performed the following tests: 75 g o.G.T.T., euglycemic-hyperinsulinemic clamp at 50 microU/ml combined with D-[3-3H] glucose infusion in tracer amounts, in vitro insulin binding on circulating monocytes. Fasting plasma glucose values were similar in the two groups, whereas plasma insulin values were significantly higher in hyperthyroids (21.4 +/- 2.5 vs 10.6 +/- 0.6 microU/ml, p less than 0.01). The values of peripheral glucose utilization (5.61 +/- 0.24 vs 6.01 +/- 0.22 mg/kg-min) and of total specific insulin binding (4.07 +/- 0.20 vs 4.39 +/- 0.23% bound to 10(7) cells/ml) were not significantly different in the two groups. These results indicate that in vitro and in vivo data, when recorded in the same patients, are concordant to confirm a normal peripheral tissue sensitivity to insulin in Graves' disease.

The effect of thyroidectomy in the fetal sheep on lung liquid reabsorption induced by adrenaline or cyclic AMP

1. In fetal sheep at 113-120 days' gestation, thyroidectomy was performed and tracheal, arterial and venous catheters inserted. Following a recovery period experiments were performed from 120-145 days to measure changes in lung liquid secretion or its absorption in response to I.V. adrenaline infusion or to introduction of dibuteryl cyclic AMP into lung liquid. The results were compared with those previously obtained in non-thyroidectomized fetuses. 2. Plasma levels of thyroid hormones in non-thyroidectomized fetuses confirmed the pattern found by previous workers. In thyroidectomized fetuses the levels of thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) were very low except in one fetus which showed biochemical evidence of thyroid regeneration towards the end of gestation. 3. In thyroidectomized fetuses the normal response to adrenaline infusion (diminution of reversal of lung liquid secretion) was profoundly suppressed and very little gestational maturation in this response took place, except in the one fetus with evidence of thyroid regeneration in which a normal reabsorptive response developed in late gestation. 4. In thyroidectomized fetuses, the normal response to dibuteryl cyclic AMP was greatly reduced and its increase with gestation which normally parallels that seen during adrenaline infusion did not take place.

3,3',5-L-triiodothyronine but not L-thyroxine can block the induction of tyrosinase by imidazole in cultured B16 melanoma cells

3,3',5-L-triiodothyronine (T3) can inhibit tyrosinase activity in B16/C3 melanoma cells in culture and block the induction of that enzyme by imidazole (Endocrinol. 119, 2118, 1986). The current study examined the effect of thyroxine (T4) on tyrosinase activity. Proliferating cultures were exposed to either T3 (0-500 nM) or T4 (0-50,000 nM) in addition to imidazole (10 mM). Imidazole induced enzyme activity by approximately 3-fold and T3 could block that induction with a maximal inhibition occurring at 10 nM. Even at the highest concentrations used, T4 had no effect on tyrosinase activity. These studies disclose that T4, the tetraiodinated parent compound from which T3 is derived, is devoid of intrinsic biological activity in this T3 responsive system.

Biochemical and functional effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the heart of female rats

Biochemical, functional and morphologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the hearts of female rats were examined. Six days after the treatment of rats with TCDD, the blood pressures and resting heart rates were significantly less than in control animals. Treated animals were also less responsive to the effects of the beta-1 agonist, (-)isoproterenol. No histopathologic changes were observed in the heart although extensive centrilobular necrosis occurred in the liver after TCDD administration. Serum levels of thyroxine were 66% less than in control animals. Marked lipid peroxidation was produced in the liver with small but significant increases occurring in the heart. TCDD administration had no effect on catalase activity in the heart, but produced a 20% decrease in superoxide dismutase activity relative to control animals. The effects of TCDD on cardiac function do not appear to be due to a direct action of the xenobiotic on the heart but possibly to a down-regulation of beta-receptors in the heart as a result of the hypothyroid state.

Influence of thyroid hormones and testosterone on the activities of hepatic mitochondrial enzymes in the Indian garden lizard, Calotes versicolor

Thyroidectomy and castration in Calotes versicolor significantly decreased the activities of hepatic mitochondrial cytochrome oxidase, alpha-glycerophosphate dehydrogenase (alpha-GPDH), and succinate dehydrogenase (SDH) when compared to sham-operated controls. Administration of thyroid hormones in thyroidectomized lizards and testosterone in castrated specimens stimulated the activities of all three enzymes studied. Chloramphenicol, when injected with thyroxine prevented the hormone-stimulated activities of cytochrome oxidase and SDH, while actinomycin D and chloramphenicol, when administered along with testosterone propionate (low dose) prevented the testosterone-stimulated activities of cytochrome oxidase and alpha-GPDH.

Evidence for ADP-ribosylation in the mechanism of rapid thyroid hormone control of mitochondria

Triiodothyronine in vitro at concentrations between 10(-13) and 10(-11) M very rapidly activates oxidative phosphorylation in hypothyroid rat liver mitochondria. Comparing the concentrations of hormone with estimates of the amounts of respiratory chain components present suggests that this activation may involve an amplification mechanism. Here we present evidence that while no changes in phosphorylation were detected following hormone administration, nicotinamide, an inhibitor of mono ADP-ribosylation reported to occur rapidly and reversibly in mitochondria, prevented activation by hormone. Moreover incubation with nicotinamide of euthyroid mitochondria and derived intact inner membrane vesicles revealed lowered ADP/O ratios under the same conditions as shown by hypothyroid preparations. While this lesion could be reversed simply by washing the intact mitochondria, the membrane vesicles required triiodothyronine addition.

The influence of nanomolar calcium ions and physiological levels of thyroid hormone on oxidative phosphorylation in rat liver mitochondria. A possible signal amplification control mechanism

Using different conditions mitochondria from hypothyroid rats can show both unchanged ADP/O ratios and lowered ADP/O ratios without evidence of uncoupling when compared with euthyroid controls. Raising the free Ca2+ concentration to around 25 nM progressively lowered the ADP/O ratio in hypothyroid but not in euthyroid mitochondria. Ruthenium Red did not alter this behaviour and further increasing the Ca2+ concentration to levels below those which stimulate State 3 respiration had no additional effect. Measurements of the free Ca2+ concentration in the mitochondrial suspending medium using a Quin 2 fluorescence assay showed that the mitochondria did not buffer the free Ca2+ at these low concentrations. At 25 nM-free Ca2+, addition of 10-13) M-T3 to hypothyroid mitochondria produced an immediate and significant increase in the ADP/O ratio without altering the free Ca2+ concentration. The hormone effect was maximal by 10(-11) M. The concentration of ATP synthetase can be estimated to lie at about 10 nM in these experiments. Hence it appears possible that a substantial amplification of the hormone signal may have taken place. Comparison with binding studies suggests that T3 may have been maximally stimulating when somewhat less than half its receptor sites had been filled. The possible mechanisms by which this receptor mediated alteration of the ADP/O ratio might be achieved are discussed.

Evidence of a direct action of triiodothyronine (T3) on the cell membrane of GH3 cells: an electrophysiological approach

Electrophysiological experiments demonstrate that triiodothyronine (T3) exerts a direct effect on the membrane of a strain of cultured rat pituitary tumor cells, GH3/B6. These cells respond to pressure application of T3 (2-5 nl, concentration 1 X 10(-10) M) with an increase in the membrane resistance (Rm) and a hyperpolarization. Spontaneously firing cells become silent.

The rapid alteration by tri-iodo-L-thyronine in vivo of both the ADP/O ratio and the apparent H+/O ratio in hypothyroid-rat liver mitochondria

Mitochondria from the livers of thyroidectomized rats have a lowered ADP/O ratio, which can be restored to normal within 15 min after intravenous injection of a near-physiological dose of tri-iodothyronine. Thyroidectomy lowered the measured delta pH, which appears to be compensated by a rise (not statistically significant) in the membrane potential, so that the protonmotive force is unaltered. A simple simulation technique is described for use in estimating H+/O ratios by the oxygen-pulse technique, which circumvents the problem that this ratio can be seriously underestimated because of re-uptake of protons from the bulk phase by the mitochondria before their expulsion is complete. By this procedure the H+/O ratio of hypothyroid mitochondria is shown to be lowered by the same factor as the ADP/O ratio, and both these ratios are very rapidly restored in parallel by hormone administration. Although these findings could be consistent with a proposal that tri-iodothyronine rapidly modulates by some mechanism the efficiency of the respiratory-chain-linked proton pumps, the kinetic properties of the proton exchange suggest that the bulk-phase protons measured may not reflect faithfully those that drive the ATP synthetase.

Direct in vitro action of thyroid hormones on mitochondrial RNA-polymerase

The authors show the direct in vitro action of thyroid hormones on RNA-polymerase activity in rat liver mitochondria. 3,5,3' L-triiodothyronine (L-T3) and 3,5,3',5' L-tetraiodothyronine (L-T4) stimulate mitochondrial RNA synthesis without either increasing the permeability of preswollen mitochondria or stimulating the synthesis of the triphosphate ribonucleotides (NTP's). Thyroid hormones do not directly depress mitochondrial RNA hydrolysis. Studies carried out with structural analogues of thyroid hormones indicate the structural specifications of the regulating system of the mitochondrial RNA-polymerase. L-T3 and L-T4 are also effective 'in vitro' on mitochondria obtained from animals undergoing different hormonal and dietary treatments, with the exceptions of those fed with a hypoprotein diet. Thus, the authors suggest the possible intervention of a specific mitochondrial receptor for L-T3 and L-T4.

In vitro effects of thyroid hormones on red blood cell Ca++-dependent ATPase activity

Thyroid hormones (TH) have been shown to exert a direct stimulatory effect on the Ca++-dependent ATPase from human and other mammalian erythrocytes. In this in vitro system, T4 has been shown to be more effective than T3. In the present study, TH effects on Ca++-dependent ATPase were investigated, using rabbit and human erythrocyte membranes, after preincubation with 10(-10) M T4, in the presence or in the absence of exogenous calmodulin (CaM) (5.10(-12) M to 5.10(-9) M). Ca++-dependent ATPase activity was measured as inorganic phosphate (Pi) release from 1 mM ATP. The results showed that basal Ca++-dependent ATPase activity in rabbits was moderately increased by T4 (1.44 +/- 0.05 vs 1.32 +/- 0.04 mumol Pi/mg protein/90 min, mean +/- SE; p less than 0.05). The time course of Pi release did not show any stimulatory effect of T4 during the first hour of incubation. The effect of T4 became apparent, however, 1 h after the addition of ATP (delta T4: 15%). With human membranes, T4 induced a relative stimulation of the Ca++-dependent ATPase of 8-10% (p less than 0.05) in experimental conditions where the enzyme was not maximally stimulated by CaM (delta CaM over basal activity: 5-40%). In conditions of high CaM stimulation (delta CaM: 50-320%), T4 had no effect. These results confirm that Ca++-dependent ATPase activity is increased by T4. The effect of T4 is small, and appears as a late event during incubation with ATP. Stimulation by T4 is expressed in states of low enzyme activation by CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of thyroid hormone on intracellular Ca2+ mobilization by noradrenaline and vasopressin in relation to glycogenolysis in rat liver

The relation between Ca2+ efflux, Ca2+ mobilization from mitochondria and glycogenolysis was studied in perfused euthyroid and hypothyroid rat livers stimulated by Ca2+-mobilizing hormones. Ca2+ efflux, induced by noradrenaline (1 microM) in the absence or presence of DL-propranolol (10 microM) from livers perfused with medium containing a low concentration of Ca2+ (approx. 24 microM), was decreased by more than 50% in hypothyroidism. This correlated with an equal decrease of the fractional mobilization of mitochondrial Ca2+, which could account for 65% of the difference between the net amounts of Ca2+ expelled from the euthyroid and hypothyroid livers. With vasopressin (10 nM) similar results were found, suggesting that hypothyroidism has a general effect on mobilization of internal Ca2+. In normal Ca2+ medium (1300 microM), however, the effect of vasopressin on net Ca2+ fluxes and phosphorylase activation was not impaired in hypothyroidism, indicating that Ca2+ mobilization from the mitochondria in this case plays a minor role in phosphorylase activation. The alpha 1-adrenergic responses of Ca2+ efflux, phosphorylase activation and glucose output, glucose-6-phosphatase activity and oxygen consumption in hypothyroid rat liver were completely restored by in vivo T3 injections (0.5 micrograms per 100 g body weight, daily during 3 days). Perfusion with T3 (100 pM) during 19 min did not influence hypothyroid rat liver oxygen consumption and alpha 1-receptor-mediated Ca2+ efflux. However, this in vitro T3 treatment showed a completely recovered alpha 1-adrenergic response of phosphorylase and a partly restored glucose-6-phosphatase activity and glucose output. The results indicate that thyroid hormones may control alpha 1-adrenergic stimulation of glycogenolysis by at least two mechanisms, i.e., a long-term action on Ca2+ mobilization, and a short-term action on separate stages of the glycogenolytic process.

Thyroxine-induced differential mortality of cleft lip mouse embryos: dose- and time-response studies of the A/WySn strain

Pregnant A/WySn mice, 20 to 30% of whose offspring have spontaneous cleft lip, were treated with thyroxine. Following treatment, cleft lip and normal embryos died, but cleft lip embryos died at a higher rate. The increased liability of cleft lip embryos to thyroxine-induced death was considered as a possible experimental route to identify the basic genetic defect that causes cleft lip. A time-response study indicated that cleft lip embryos responded more than normals following treatment on any of days 7 to 12 of gestation, that there is no sharply defined critical period, and that normal and cleft lip embryos do not differ in time of maximum sensitivity. A dose-response study showed linear responses of normal and cleft lip embryos on a probit-log dose scale, with a common slope and LD50's of 1.9 and 1.3 mg respectively. These dose-response properties indicate that normal and cleft lip embryos are probably killed by the same mechanism, but differ in dosage tolerance. That is, they differ quantitatively, not qualitatively. Thyroxine did not significantly change the cleft lip frequency, and the difference between normal and cleft lip embryos that leads to cleft lip itself is therefore not in the same pathway as that which leads to thyroxine-induced death. A hypothetical example of the defect basic to both pathways is presented.

The translation system of rat heart muscle mitochondria is stimulated following treatment with L-triiodothyronine

In vitro translation of mitochondrial translation products by mitochondria isolated from the myocardia of rats injected with L-triiodothyronine daily for 5 days was compared with those of euthyroid animals. The incorporation of 35S-methionine into proteins was greater by heart mitochondria isolated from T3-treated rats than by those isolated from euthyroid animals. This increase is due to a stimulation of mitochondrial protein synthesis rather than being the result of a reduction in the rate of protein degradation or an increase in the specific radioactivity of the amino acid pool. The results also established that the polypeptide profile of mitochondrial translation products is the same in heart mitochondria isolated from euthyroid and T3-treated animals. The relative increase in the rates of synthesis of mitochondrial translation products is non-uniform. These results suggest that the hormone acts by inducing a general but non-uniform increase in the activity of the mitochondrial transcription and/or translation system(s).

Hyperthyroid heart disease

The importance of cardiovascular system involvement in hyperthyroidism has been recognized for many years. In the middle-aged and elderly patient, often with mild but prolonged elevation of plasma thyroid hormones, symptoms and signs of heart failure and complicating atrial fibrillation may dominate the clinical picture and mask the more classical endocrine manifestations of the disease. Pitfalls in diagnosis and the importance of early recognition and treatment are discussed. Despite experimental evidence for a short-term inotropic action of thyroid hormone excess, clinical data support the existence of a reversible cardiomyopathy in hyperthyroidism with impaired contractile reserve. Enhanced myocardial performance at rest primarily reflects the peripheral actions of thyroid hormone excess. Most, if not all, of the cardiac abnormalities return to normal once a euthyroid state has been achieved, although atrial fibrillation may persist in a minority. Optimum treatment requires rapid and definitive antithyroid therapy, usually using a large dose of radio-iodine, and rapid control of heart failure. Systemic anticoagulation is indicated in the presence of atrial fibrillation and should be continued until sinus rhythm has been present for at least three months, either spontaneously or after cardioversion.

Thyroid hormones reversibly suppress somatostatin secretion and immunoreactivity in cultured neocortical cells

Thyroid hormone effects on brain somatostatin-like immunoreactivity (SRIF-LI) were studied in an in vitro model system. Serum was removed from the nutrient culture medium of fetal day-18 rat cerebral cortex cells maintained in primary, long-term, dispersed monolayer culture. Chronic administration of either T3 or T4 in serum-free medium was associated with suppressed release of SRIF-LI into the culture medium (36-43 h accumulation), cell content of peptide and acute release in response to potassium-induced depolarization. Suppression was dose-dependent with an IC50 of less than 1 nM for T3. The most dramatic effects were observed for K+-induced release. Thirty-five to 50% suppression was typically observed with T3 at a near maximum dose (3 nM). Reverse T3 and diiodotyrosine were less potent and effective than T3. TRIAC and diiodothyronine also possessed significant suppressive activity. T3 suppression of release depended on duration of pretreatment. Administered for less than 16 h, T3 failed to significantly suppress K+-induced release, but significant suppression was observed for pretreatment periods of 16 h or longer. Indirect fluorescent immunohistochemical examination revealed a reduction in the number of cells positively stained for SRIF-LI in T3-treated dishes relative to controls. Upon removal of T3 and subsequent recovery in serum supplemented medium for 24 h, T3-treated and control cells exhibited similar levels of SRIF-LI release and cell content. T3-treated and control cells incorporated [3H] leucine into trichloracetic acid precipitable counts to similar extents. Dexamethasone and several sex steroids failed to modify the effects of T3 and did not independently influence SRIF-LI levels. Acute cycloheximide administration did not reverse T3 effects. The data indicate that primary brain cell cultures may be useful models to examine direct peripheral hormone actions on nervous tissue. Thyroid hormones suppress SRIF-LI levels in a dose, time and structure-dependent manner, which appears to be reversible. The findings are consistent with a possible integration of peripheral hormone and brain peptide physiology.

The role of thyroid hormones in the control of energy expenditure

Thyroid hormones have a direct effect on the basal or resting metabolic rate in man and a permissive effect on the adaptive thermogenesis of small animals, while altering the energy expended in exercise to the extent that patients with thyroid disorders exercise to a greater or lesser degree. The physiological concepts of energy expenditure need to be seen in the context of a new method for measuring 'thyroid thermogenesis'. Thyroid hormones seem, in evolutionary terms, to have developed a thermogenic role during the transition from poikilothermy to homeothermy; they are responsible for the increased heat production required for homeotherms to maintain body temperature above that of the environment. The potential mechanisms responsible for thyroid hormone-controlled energy expenditure are complex. Uncoupled oxidative phosphorylation is probably not responsible for thyroid hormone-controlled thermogenesis except in the special case of brown adipose tissue thermogenesis, where thyroid hormones act permissively. The concept that increased ATP generation must be coupled to ATP utilization needs to be linked with the idea that thyroid hormone-controlled thermogenesis must be through inefficient pathways of metabolism. Several of these potentially important pathways of intermediary metabolism in thyroid hormone-controlled thermogenesis can now be defined and measured, but their role in the regulation of nutritionally induced alterations in thyroid status and thermogenesis remains to be explored.

Effects of thyroxine treatment on contractions of soleus muscles of anaesthetized cats

The effects of chronic thyroxine treatment on cat soleus muscle contractions were studied. Maximum twitch tension, contraction time, half relaxation time and tension-time integral of maximal twitches of the soleus muscles of thyroxine treated cats were significantly decreased. Consequently, there was a decrease in tension and degree of fusion of incomplete tetanic contractions of the soleus muscle. The maximum tetanic tension was not statistically significantly changed, suggesting that the effects may be due to a decrease in the duration of the active state of the muscle. Isoprenaline given intravenously during incomplete tetanic contractions of the soleus muscle caused a statistically significant depression of tension in the control group but not in the thyroxine treated group. This further suggests reduction in the duration of the active state of soleus muscles of thyroxine treated cats. Propranolol injected chronically with thyroxine reversed or prevented the depression of tension caused by thyroxine treatment, suggesting the involvement of beta-adrenoceptors in these effects. The decrease in tension and degree of fusion during incomplete tetanic contractions of the thyroxine treated soleus could be responsible, at least partly, for the muscle weakness and tremor of thyrotoxicosis. Cyclic AMP may possibly be the mediator of these effects.

Short-term control of mitochondrial adenine nucleotide translocator by thyroid hormone

An improved simple technique for measuring adenine nucleotide translocator activity at low medium substrate concentrations is described. Confirming previous reports, thyroidectomy was shown to lead to lowered translocator activity in rat liver mitochondria. The rapidly exchangeable portion of the matrix nucleotide also decreased in hypothyroid preparations even though the total nucleotides increased substantially. The apparent Km of translocator for ADP increased from 2.8 to 6.2 microM in hypothyroid preparations: Mg2+ ions raised this to about 20 microM. All of these changes in adenine nucleotide translocation were entirely reversed by 15 min after a single intravenous near-physiological dose of triiodothyronine.

Some theoretical questions of mechanisms of thyroid hormone action

The effects of iodothyronines, the suggested mechanisms of their action, the methodical difficulties of determining receptors of thyroid hormones, the significance of iodothyronine formation during hormone action and some stereochemical aspects of the receptor--thyroid hormone interaction have been reviewed and a new model of the thyroid hormone action has been developed. It is suggested that the thyroid hormones may interact with more than one protein and nucleic acid at the same time and in this way "help" the interaction of macromolecules, thus catalysing and modulating biochemical processes of diverse character.

Thyroid hormone regulation of heme oxidation in the liver

The effects of 3,5,3'-triiodothyronine (T3) on heme oxygenase (EC 1.14.99.3) activity and cytochrome P-450 content in liver were examined in thyroidectomized rats. T3, when administered for 5 days at a dose of 6 micrograms/100 g of body weight, stimulated basal heme oxygenase activity approximately equal to 2-fold compared to diluent-treated animals. The induction of heme oxygenase by cobalt heme also was enhanced approximately equal to 3-fold in T3-treated animals. T3 treatment lowered cytochrome P-450 content by approximately equal to 50% and potentiated the depletion of this heme protein after cobalt heme administration. Reverse T3 had no effect either on cytochrome P-450 content or on heme oxygenase activity in liver. The time course of response to a single dose of T3 (50 micrograms/100 g of body weight) revealed that both basal and cobalt heme-induced heme oxygenase activity peaked at 48 hr and that cytochrome P-450 content declined to approximately equal to 40% of controls at 96 hr. Examination of microsomal proteins by polyacrylamide gel electrophoresis after T3 treatment disclosed that major bands in the Mr approximately equal to 50,000-55,000 region were diminished. The administration of T3 together with SKF-525A, a compound known to complex with the heme prosthetic group of cytochrome P-450, resulted in partial preservation of these proteins. These data indicate that thyroid hormone can regulate heme oxygenase activity and concomitantly can lower cytochrome P-450 content in liver. The hormone also can act in a synergistic fashion to enhance the response of hepatic heme oxygenase to a chemical inducer of the enzyme. Thyroid status thus may be a potentially significant determinant of the rate of heme oxidation in the liver.

Regulation of glycosaminoglycan synthesis by thyroid hormone in vitro

Human skin fibroblasts synthesize and accumulate glycosaminoglycans (GAG). Recently, we reported that fibroblasts incubated in thyroid hormone-deficient media accumulate more GAG than do cultures incubated in the same media enriched with 0.1 muM triiodothyronine (T(3)) (1981. Endocrinology. 108: 2397). The current study characterizes that enhanced accumulation. Confluent cultures were maintained in thyroid hormone-deficient media without or with added T(3), labeled with [(3)H]acetate and analyzed for total [(3)H]GAG and [(3)H]hyaluronic acid content. Addition of T(3) to thyroid hormone-depleted media consistently inhibited the incorporation of [(3)H]acetate into GAG by 28-60% in fibroblast cultures from four different normal human donors. Maximal inhibitory effect was observed within 3 d after hormone addition at concentrations > 1 nM. 73% of the maximal inhibitory effect was observed in the presence of physiologic concentrations of T(3) (0.16 nM total T(3) or 1.4 pM free T(3)). The following observations indicated that T(3) inhibition of [(3)H]GAG accumulation is most likely due to a decrease in GAG synthesis rather than to changes in the acetate pool or GAG degradation: (a) Addition of 0, 100, 500, and 2,500 muM unlabeled acetate progressively decreased [(3)H]acetate incorporation into GAG, up to 80%, without altering the further inhibitory effect of T(3) (35-40%); (b). A similar effect of T(3) on GAG (32% inhibition) was observed using [(3)H]glucosamine as substrate; (c) T(3) decreased hyaluronate synthetase activity by 32%; and (d) There was no effect of T(3) on GAG degradation in a pulse-chase experiment. The effect of T(3) on [(3)H]GAG accumulation appears to be quite specific, since the hormone had no effect on the incorporation of [(3)H]leucine into trichloroacetic acid-precipitable material.Thus, thyroid hormone inhibits GAG accumulation in a dose-, time-dependent, and reversible manner. This inhibition is apparently due to specific effects on the rate of macromolecular synthesis.

Specific binding sites for the triiodothyronine in the plasma membrane of rat thymocytes. Correlation with biochemical responses

As a prerequisite to studies of whether the plasma membrane of the rat thymocyte contains specific, saturable binding sites for the thyroid hormone 3,5,3'-triiodothyronine (T(3)), a method was developed for the isolation of a plasma membrane fraction from these cells. As judged from both electron microscopic and marker enzyme studies, the fraction was composed principally of plasma membrane vesicles, was free of nuclear contaminants, and was only slightly contaminated with other subcellular components. At 37 degrees C and pH 7.4, binding of [(125)I]T(3) by the fresh membrane preparation was rapid, reaching a maximum at 5 min and then declining with time, so that by 60 min binding was virtually nil. Decreased binding with time was due to a loss of functional binding sites, but did not reflect desensitization, since the decrease in binding activity with time was independent of the presence or absence of T(3). Scatchard analysis of saturation studies revealed the presence of two binding sites, one with an apparent dissociation constant (K(d)) of 0.95 nM and a maximum capacity of 5.3 x 10(10) sites/100 mug protein, and the other with an apparent K(d) of 25 nM and a binding capacity of 1.4 x 10(12) sites/100 mug protein. Measurement of the ability of several thyronine analogues to inhibit the binding of [(125)I]T(3) revealed the following rank order of potency: l-T(3) > l-T(4) > d-T(3) = d-T(4) > l-3,5-T(2) > rT(3) > d,l-thyronine. Binding of T(3) was inhibited by the omission of calcium from the medium or by the addition of the beta adrenergic antagonist alprenolol. As judged from studies of the lower affinity binding site, these manipulations decreased the affinity, but not the number, of binding sites for T(3). The relative potencies of thyronine analogues to inhibit the binding of [(125)I]T(3) were generally parallel to their previously reported potencies in stimulating the uptake of the sugar analogue 2-deoxy-glucose (2-DG) in intact rat thymocytes in vitro. Further, the inhibition of T(3)-binding produced by l-alprenolol or by excluding calcium from the medium resembled the previously reported inhibition that these manipulations produce with respect to T(3)-induced enhancement of 2-DG uptake. These findings suggest that the binding sites for T(3) present in the plasma membrane of rat thymocytes act as functional receptors linked to the stimulation of 2-DG uptake that T(3) induces in these cells.