Effects of thyroid states on the Cori cycle,glucose-alanine cycle,and futile cycling of glucose metabolism in rats

The effect of thyroid status on glucose recycling was measured in intact rats by comparing the fates of differently labeled [³H]- and [¹⁴C]glucose. Glucose recycling at the level of three-carbon compounds (i.e., Cori and glucose-alanine cycles) was measured by comparing the rates of turnover of [6-³H]- and [6-¹⁴C]glucose in the same animal. The rate of recycling increased (33–110%) in hyperthyroid rats and decreased (22–30%) in hypothyroid (thyroidectomized) rats. The relative importance of the Cori and glucose-alanine cycles was measured by analyzing the labeled glycolytic intermediates after the injection of labeled glucose; and by measuring the rate of glucose production from the infused labeled lactate and alanine. The results showed that the rate of the Cori cycle is much greater than the glucose-alanine cycle in rats. Substrate cycling at the level of glucokinase-glucose-6-phosphatase was measured by comparing the rates of turnover of [2-³H]- and [6-³H]glucose; and phosphofructokinase-fructose bisphosphatase was measured by comparing the rates of turnover of [3-³H]- and [6-³H]glucose. These cycles were also affected by thyroid states of the animals. The rate of the phosphofructokinase-fructose bisphosphatase cycle increased threefold in hyperthyroid rats and decreased by about half in hypothyroid rats. The glucokinase-glucose-6-phosphatase substrate cycle occurred at the rate of nearly 2 μmol/min/100 g body wt in the hyperthyroid, fasted rats; it was not detectable in hypo- or euthyroid rats. The contribution of the energy released by these cycles to thyroid thermogenesis was discussed. Effects of thyroid states on glucose metabolism in perfused muscles were also studied. There is an apparent shift in the source of energy for oxidation in the hyperthyroid rat. The ratio of lactate production to glucose uptake was significantly elevated in the hyperthyroid rats. This change predisposes for increased glucose recycling in hyperthyroid rats to avoid lactate accumulation and acidosis.     

Metabolism of glucose in hyper- and hypo-thyroid rats in vivo. Relation of catecholamine actions to thyroid activity in controlling glucose turnover.

1. In euthyroid rats, treatment with reserpine of 6-hydroxydopamine, which deprived neuronal terminals of catecholamines, resulted in increases in rates and rate coefficients for blood glucose turnover in the starved states as determined by decay of [U-14C,6-3H]-glucose. Conversely, the injection of adrenaline or noradrenaline into starved euthyroid rats caused a marked decrease in rate coeeficients for glucose turnover. There was no change in the percentage glucose recycling under these conditions. 2. Adrenaline and noradrenaline caused more pronounced hyperglycaemia in hyperthyroid than in euthyroid rats owing to the greater activation of hepatic glucose production. 3. The increase in glucose turnover characteristics of hyperthyroidism was observed even after treatment with an alpha- or beta-adrenergic antagonist, showing the insignificant role of the balance between alpha- and beta-adrenergic receptors in the thyroid-dependent metabolic changes. 4. Rate coefficients for glucose turnover were not affected by reserpine treatment or catecholamine injections when rats had been rendered hyperthyroid. 5. Thus catecholamines are direct determinants of glucose-turnover rates in the starved state, and depend to some extent on the prevailing thyroid state.     

Increased glucose transporter (GLUT4) protein expression in hyperthyroidism

We have studied skeletal muscle glucose uptake by perfused hindquarter preparations from rats treated with thyroxine. Basal glucose uptake (in the absence of insulin) was approximately 2 fold higher in muscle of hyperthyroid rats compared to controls. Insulin (10(-7) M) stimulated glucose uptake 4.0 and 6.8 fold in the 10 day and 30 day controls rats, respectively. Maximal glucose uptake (10(-7) M insulin) was not different in control and hyperthyroid rats and thus insulin responsiveness in the hyperthyroid animals was reduced to 2.5 fold stimulation. The abundance of the insulin-sensitive glucose transporter protein (muscle/fat, GLUT-4), measured by Western blot analysis using polyclonal antisera, was higher in skeletal muscle from both groups of hyperthyroid rats. These studies indicate that thyroid hormones increase basal glucose uptake in skeletal muscle and this is due, at least in part, to an increment of GLUT-4 isoform. Increased expression of muscle glucose transporter proteins may be responsible for the increased peripheral glucose utilization seen in hyperthyroidism.     

Nighttime immunoreactive somatostatin content of the median eminence in hypo- and hyperthyroid rats

The median eminence content of immunoreactive somatostatin (IRS) was measured by radioimmunoassay in 107 male albino rats, who were either hypothyroid after surgical thyroidectomy (N = 38), hyperthyroid following a subcutaneous implant of 5 mg of L-thyroxine (N = 36), or otherwise untreated (N = 33). Thyroid function was assessed by determining plasma levels of T4 and TSH from trunk blood obtained at the time of decapitation. Subgroups of animals from the 3 groups were killed either before (1800 hr), during (2200, 0200, 0400 hr), or after the dark portion of their 14:10 LD photoperiod. Although no changes in median eminence IRS content were found throughout the period of study within any of the 3 groups, hypothyroid animals (297.23 +/- 13.47 ng per ME; 620.41 +/- 58 ng IRS/mg protein) had a significantly lower median eminence IRS concentration than untreated rats (355.86 +/- 16.55 ng of IRS per ME, P less than 0.01; 906.86 +/- 96.38 ng IRS/mg protein, P less than 0.05) and hyperthyroid animals (384.12 +/- 14.67 ng per ME, P less than 0.001; 874.1 +/- 104.5 ng IRS@mg protein, P less than 0.05). It is concluded, that the feedback of thyroid hormones on the hypothalamic-pituitary axis during thyroid hormone excess in vivo, contrary to what occurs in hypothyroidism, is probably independent of hypothalamic somatostatin.     

Reduced serum acylated ghrelin levels in patients with hyperthyroidism

BACKGROUND AND OBJECTIVE: Recent studies have revealed that circulating ghrelin levels seem to play a role in energy homeostasis. The effect of hyperthyroidism on ghrelin levels is not fully known. METHODS: Serum levels of ghrelin and its relationship with insulin resistance were evaluated in 48 patients with hyperthyroidism and 43 euthyroid healthy controls. Thyroid hormones, insulin, glucose, ghrelin levels and lipid parameters were measured in all subjects. Insulin sensitivity was determined using the homeostasis model assessment. RESULTS: Serum ghrelin levels were significantly decreased in hyperthyroid patients than in controls (32.5 +/- 23.3 vs. 54.1 +/- 35.5 pg/ml, p < 0.001). Circulating ghrelin levels significantly correlated with age (r = -0.26, p = 0.01), fasting glucose (r = -0.21, p = 0.01), free triiodothyronine (r = -0.18, p = 0.04), free thyroxine (r = -0.23, p = 0.02) and thyroid stimulating hormone (r = 0.21, p = 0.04), but not with blood pressure, body mass index, lipid parameters, insulin and homeostasis model assessment (p > 0.05). Multiple regression analysis revealed glucose level to be the most important predictor of circulating ghrelin level. CONCLUSION: These results indicate that hyperthyroidism has effect on serum ghrelin levels. Further studies are needed for the exact mechanism.     

Plasma Levels of Resistin and Ghrelin Before and After Treatment in Patients With Hyperthyroidism

ObjectiveTo investigate ghrelin and resistin concentrations in patients with hyperthyroidism before and after restoration to a euthyroid state and to correlate the 2 peptides with anthropometric and insulin resistance parameters.MethodsThe study included hyperthyroid patients and euthyroid healthy participants as a control group. Hyperthyroid patients were evaluated at the start of the study and after normalization of thyroid function with appropriate antithyroid drugs. Anthropometric parameters, insulin resistance parameters (fasting blood glucose, fasting insulin, and homeostasis model assessment-insulin resistance), thyroid function tests, and measurement of ghrelin and resistin were assessed in patients and control participants.ResultsThe study included 40 hyperthyroid patients (32 women and 8 men, aged 26-42 years) and 30 euthyroid healthy participants (20 women and 10 men, aged 25-43 years) as a control group. In hyperthyroid patients, serum resistin levels and insulin resistance parameters werehigher and plasma ghrelin levels were lower than in control participants (P <.001), and all normalized after treatment. Ghrelin levels were correlated only with insulin resistance parameters, but no correlations with any anthropometric or laboratory data were found. Resistin levels did not correlate with any clinical or laboratory data of hyperthyroid patients.ConclusionIn hyperthyroid patients, resistin was increased and ghrelin was decreased, they were not related to anthropometric parameters, and they normalized after treatment of hyperthyroidism. (Endocr Pract. 2012;18:376-381)     

Body, heart, thyroid gland and skeletal muscle weight changes in rats with altered thyroid status

In the present paper we describe changes of anatomical parameters in inbred Lewis strain rats, namely their body weight, body weight gain per week, absolute and relative heart, thyroid gland and skeletal muscle weights, that are assumed to reflect experimentally altered thyroid status. The hyperthyroid state was induced by DL-thyroxine or Na 3,3',5-triiodo-L-thyronine, while methimazole was employed for inducing hypothyroidism. We have found that when compared to euthyroid rats, hypothyroidism resulted in a significantly lower body weight gain, absolute and relative heart weight and, in contrast, in a significant increase of absolute and relative thyroid gland weight. On the other hand, hyperthyroidism led to a significant increase of absolute and relative heart weight and to a significant reduction of absolute and relative thyroid gland weight. However, the body mass was not significantly altered in hyperthyroidism as compared with euthyroid rats. We conclude that our protocol leads to chronic hyper- or hypothyroidism as demonstrated by body, heart and thyroid gland weight changes. These anatomical data can thus be utilized as supplemental criteria for the assessment of the thyroid state of experimental rats.     

Effect of long-term exposure to low dose gamma-irradiation on the rat thyroid status

The effect of long-term exposure to low-dose external radiation on the rat thyroid status was studied. The experiments were carried out on Wistar female rats. The single doses absorbed were 0.1; 0.25; 0.5 Gy. The rats were irradiated 20 times (5 days x 4 weeks). The animals were decapitated after 1, 30 and 180 days following the last irradiation. Blood serum was assayed for content of thyroxin (T4) and triiodothyronine (T3) radioimmunologically. The liver was spectrophotometrically assayed for thyroid-induced NADP-malatedehydrogenase (NADP-MDH). It was shown that the long-term 0.5-Gy irradiation of the animals induced a decrease in blood T4 and T3 concentrations 1.34-1.71-fold and 1.24-1.43-fold after 1, 30 and 180 days, respectively. The T3 level was diminished most pronouncedly after 1 day, whereas that of T4--after 30 days following the exposure. With the doses of 0.1 and 0.25 Gy absorbed, the T4 and T3 concentration remained unchanged throughout all the periods studied. The activity of NADP-MDH was decreased 1.55-2.46-fold in all the experimental animals, and it was held decreased after 180 days (1.43-1.50-fold) in 0.25- and 0.5-Gy-irradiated groups, which indicates a disturbance in thyroid hormone metabolism in rats exposed chronically to low-dose radiation. After 180 days, the experimental animals experienced an elevation of thyroid gland weight on 15-20%. The thyroid status disturbance seemed to be due to both inhibited T4 and T3 biosynthesis in thyroid and disturbed hormone peripheral metabolism under radiation exposure.     

Effects of thyroid state on the expression of hepatic thyroid hormone transporters in rats

Liver uptake of thyroxine (T4) is mediated by transporters and is rate limiting for hepatic 3,3',5-triiodothyronine (T3) production. We investigated whether hepatic mRNA for T4 transporters is regulated by thyroid state using Xenopus laevis oocytes as an expression system. Because X. laevis oocytes show high endogenous uptake of T4, T4 sulfamate (T4NS) was used as an alternative ligand for the hepatic T4 transporters. Oocytes were injected with 23 ng liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats, and after 3-4 days uptake was determined by incubation of injected and uninjected oocytes for 1 h at 25 degrees C or for 4 h at 18 degrees C with 10 nM [125I]T4NS. Expression of type I deiodinase (D1), which is regulated by thyroid state, was studied in the oocytes as an internal control. Uptake of T4NS showed similar approximately fourfold increases after injection of liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats. A similar lack of effect of thyroid state was observed using reverse T3 as ligand. In contrast, D1 activity induced by liver mRNA from hyperthyroid and hypothyroid rats in the oocytes was 2.4-fold higher and 2.7-fold lower, respectively, compared with euthyroid rats. Studies have shown that uptake of iodothyronines in rat liver is mediated in part by several organic anion transporters, such as the Na+/taurocholate-cotransporting polypeptide (rNTCP) and the Na-independent organic anion-transporting polypeptide (rOATP1). Therefore, the effects of thyroid state on rNTCP, rOATP1, and D1 mRNA levels in rat liver were also determined. Northern analysis showed no differences in rNTCP or rOATP1 mRNA levels between hyperthyroid and hypothyroid rats, whereas D1 mRNA levels varied widely as expected. These results suggest little effect of thyroid state on the levels of mRNA coding for T4 transporters in rat liver, including rNTCP and rOATP1. However, they do not exclude regulation of hepatic T4 transporters by thyroid hormone at the translational and posttranslational level.     

Role of the hypophysis in thyroid regeneration after partial thyroidectomy.

The role of the hypophysis in thyroid regeneration was investigated by measuring the mitotic activity of the thyroid remnant in hemithyroidectomized rats as well as the blood levels of thyroid hormone at various time-intervals after hemithyroidectomy. Mitotic activity underwent a significant increase to reach a peak (a 5- to 8- fold increase) 2 days after hemithyroidectomy. The thyroid hormone level in blood was lower than in controls. Histologically, the thyroid gland showed signs of an elevated rate of functional activity, as indicated by losses of colloid and cell hypertrophy. In a second approach, the mitotic activity of the thyroid remnant was estimated in hypophysectomized and in thyroxine treated rats. Both hypophysectomy and thyroxine injection prevented occurrence of the mitotic peak at 2 days. The regeneration of the thyroid after hemithyroidectomy, as it occurred in the present work, may be explained by a release of thyroid stimulating hormone from the pituitary, brought about by the low level of circulating thyroid hormone, itself resulting from a loss of thyroid tissue.     

Acute effects of leptin on 5'-deiodinases are modulated by thyroid state of fed rats.

Leptin has been shown to modulate deiodinase type 1 (D1) and type 2 (D2) enzymes responsible for thyroxine (T4) to triiodothyronine (T3) conversion. Previously, it was demonstrated that a single injection of leptin in euthyroid fed rats rapidly increased liver, pituitary, and thyroid D1 activity, and simultaneously decreased brown adipose tissue (BAT) and hypothalamic D2 activity. We have now examined D1 and D2 activities, two hours after a single subcutaneous injection of leptin (8 microg/100 g BW) into hypo- and hyperthyroid rats. In hypothyroid rats, leptin did not modify pituitary, liver and thyroid D1, and thyroid D2 activity, while pituitary D2 was decreased by 41% (p<0.05) and hypothalamic D2 showed a 1.5-fold increase. In hyperthyroid rats, thyroid and pituitary D1, and pituitary and hypothalamic D2 were not affected by leptin injection, while liver D1 showed a 42% decrease (p<0.05). BAT D2 was decreased by leptin injection both in hypo- and hyperthyroid states (42 and 48% reduction, p<0.001). Serum TH and TSH showed the expected variations of hypo- and hyperthyroid state, and leptin had no effect. Serum insulin was lower in hypothyroid than in hyperthyroid rats and remained unchanged after leptin. Therefore, acute effects of leptin on D1 and D2 activity, expect for BAT D2, were abolished or modified by altered thyroid state, in a tissue-specific manner, showing an IN VIVO interplay of thyroid hormones and leptin in deiodinase regulation.     

Nociceptive reaction-related state of glial intermediate filaments in the brain of rats with hyperfunction of the thyroid gland: An ontogenetic aspect

In experiments on Wistar rats of two age groups (5 weeks old and mature, 5 months old), we studied the content and polypeptide composition of glial fibrillary acidic protein (GFAP) in the cerebral cortex, hippocampus, thalamus, and brainstem under conditions of experimentally induced hyperfunction of the thyroid gland, as well as of combination of hyperthyroidism with the pain model (consequences of laparotomy). The hyperthyroid state was created by administration of L-thyroxine (initial dose, 10 μg/day) with food for 2 weeks; the dose was increased daily by 10 μg. At the end of experiment, we measured the level of thyroxine in the blood serum using an immunoenzymatic technique. We found that in the hyperthyroid state 5-week-old rats showed a significant increase in the content of filamentous GFAP fraction in the hippocampus, while when this influence was combined with the postoperation pain syndrome, an increase was observed in the brainstem (by 18 to 27%). Results of immunoblotting demonstrated that degraded polypeptides with a molecular mass of 47 to 45 kdalton were manifested under these conditions, which is indicative of intensification of cytoskeletal rearrangement of astroglia. In mature rats under the same conditions, we observed a drop in the level of insoluble polypeptides of intermediate filaments in astrocytes in the thalamus, hippocampus, and cerebral cortex. Neirofiziologiya/Neurophysiology, Vol. 38, No. 4, pp. 280–286, July–August, 2006.     

Radioiodine (131I) therapy of Graves' disease: use of the new high resolutional ultrasonic scanner for the determination of the accurate weight of the thyroid gland

In this series, eighteen patients with Graves' disease were treated with 8000 rads (80 Gy) of radioiodine (131I), using the new high resolutional ultrasonic scanner for the determination of the accurate weight of the thyroid gland. The mean dose of radioiodine administered orally was 4.6 +/- 3.0 mCi (170.2 +/- 110.0 MBq) and 133.7 +/- 44.6 microCi/g (4.95 +/- 1.65 MBq). At one year after treatment, twelve of eighteen patients (66.7%) became euthyroid, five (27.8%) remained hyperthyroid and one (5.6%) became hypothyroid. Analysis of various factors which may be related to the effect of radioiodine therapy revealed that the weight of the thyroid gland in the hyperthyroid and euthyroid groups was significantly different (61.7 +/- 33.5 g vs. 25.1 +/- 9.1 g, p less than 0.05). Furthermore, all patients with larger glands (more than sixty grams) remained hyperthyroid, while the incidence of euthyroidism was as high as 80% in patients with smaller glands (less than forty grams). Although the number of patients studied was small, these results indicate that a larger thyroid gland requires a larger radioiodine dose per gram of tissue than a smaller gland, suggesting that the therapeutic radiation dose should be graded according to the gland size even when the gland size is accurately estimated by ultrasound. Further study is required to determine the appropriate radiation dose graded according to the gland size.     

Effect of modification of thyroid function on cholesterol 7 alpha-hydroxylation in rat liver.

Hepatic activities of cholesterol synthesis and cholesterol 7 alpha-hydroxylation were determined in hyper- and hypo-thyroid rats after oral administration of glucose or cholesterol. Increases in activities of both cholesterol synthesis and cholesterol 7 alpha-hydroxylation induced by glucose administration were enhanced by pretreatment with thyroid powder but suppressed by pretreatment with thiouracil. The enhancement of 7 alpha-hydroxylation was produced by a relatively small amount of thyroid powder, but high doses were required to increase cholesterol synthesis. On the other hand, the suppression of 7 alpha-hydroxylation was brought about by a low dose of thiouracil, but high doses were required to decrease cholesterol synthesis. Although cholesterol synthesis increased similarly in both hypo- and hyper-thyroid rats after glucose administration, hydroxylase activity in hypothyroid rats began to increase more slowly and was always lower than that in hyperthyroid rats. Thus it is concluded that cholesterol 7 alpha-hydroxylase activity is more sensitive to thyroid function than are activities of cholesterol-synthetic enzymes. When exogenous cholesterol was given, hypothyroid rats showed a larger increase in serum cholesterol concentration than hyperthyroid rats, and there was an inverse relationship between serum cholesterol concentrations and hepatic cholesterol 7 alpha-hydroxylase activities.     

Metabolism of glucose in hyper- and hypo-thyroid rats in vivo. Glucose-turnover values and futile-cycle activities obtained with 14C- and 3H-labelled glucose.

1. A trace amount of glucose labelled with 14C uniformly and with 3H at position 2, 3 or 6 was injected intravenously into starved rats to measure the turnover rate of blood glucose. 2. Reliable estimates were made based on the semilogarithmic plot of specific radioactivity of the glucose contained in whole blood samples taken from the tail vein. 3. Glucose turned over more rapidly in hyperthyroid and more slowly in hypothyroid than in euthyroid rats. The percentage contribution of glucose recycling (determined from the difference in replacement rates between [U-14C]glucose and [6-3H]glucose) to the glucose utilization increased on induction of hyperthyroidism. 4. Futile cycles between glucose and glucose 6-phosphate (determined from the difference between replacement rates of [2-3H]glucose and [6-3H]glucose) were activated and inactivated by induction of hyperthyroid and hypothyroid states respectively. 5. The hepatic content of glycogen was much lower in hyper- and hypo-thyroid than in euthyroid rats. The enhanced glucose production in hyperthyroid rats resulted from not only activationof hepatic gluconeogenesis but also diversion of the final product of gluconeogenesis from liver glycogen to blood glucose. In hypothyroidism, the inhibition of gluconeogensis led to suppression of both glucose production and glycogenesis in the liver.     

The involvement of calmodulin in the secretion of calcitonin from the porcine thyroid gland

The effect of the calmodulin antagonist W7 has been studied in the pig in vivo by measuring directly the secretion rate of calcitonin (CT) in the thyroid venous blood after surgical isolation of the thyroid and subsequent perfusion of the gland in situ. Over the concentration range of W7 9-100 mumol/l in the perfusing plasma there was a significant increase in CT secretion rate associated with the addition of W7 to the perfusing blood. There was no significant change in the perfusing plasma calcium concentration. It is suggested that calmodulin plays an important role in calcium homeostasis within the porcine thyroid C-cell.     

Thyroxine and triiodothyronine influence cultured throid cells: alterations of morphology and mucosubstances.

Further experimentation with the steer thyroid cell line indicates that the formation of "follicles" is enhanced by exposure to 8.9 X 10(-7) M thyroxine (T4) or triiodothyronine (T3) for 96 hr. The production of mucosubstances by the cultured thyroid cells is also increased by treating with T4 or T3, effects demonstrable after staining with PAS-alcian blue at pH 2.5. It is suggested that these in vitro effects bear on morphological organization and synthetic activity of the thyroid gland as well as on autoregulation and intraglandular homeostasis that may occur in situ.     

Reparative growth in the rat thyroid

This study was designed to investigate the mitotic response to wounding in the rat thyroid. The spatial distribution of mitotic activity 48 hr after incision of the thyroid isthmus, or mere exposure of the gland (sham-operation), was assessed using a stathmokinetic technique. Incision resulted in a 66-fold increase over normal in metaphase index adjacent to the wound, falling over 2 mm to a stable 13-fold elevation. Sham-operation produced a smaller response with a complete return to normal levels over 1-1 X 5 mm. The results demonstrate that there is a dramatic localized mitotic response to wounding in the thyroid together with a smaller generalized response. Further, the response to sham-operation indicates that thyroid follicular cells respond to a diffusible 'wound hormone'. We suggest that this may be a major mechanism mediating reparative growth in this gland.     

Regulation by thyroid hormone of the synthesis of a cytosolic thyroid hormone binding protein during liver regeneration.

To understand the regulation by thyroid hormone, 3,3',5-triiodo-L-thyronine (T3), of the synthesis of a cytosolic thyroid hormone binding protein (p58-M2) during liver regeneration, the synthesis of p58-M2 was evaluated. The synthesis of p58-M2 was measured by metabolic labeling of primary cultures derived from the regenerating liver of euthyroid, hypo- or hyperthyroid rats. During regeneration, the increase in the liver/body weight ratio is approximately 25% higher in hyper- than in hypothyroid rats. However, T3 has no effect on the rate of overall liver regeneration observed in four days. In mature liver, T3 increased the synthesis of p58-M2 by approximately 2.5-fold. During regeneration, however, the change in the synthesis of p58-M2 varied with the thyroid status. In euthyroid rats, the synthesis of p58-M2 continued to increase up to 2-fold during liver regeneration. In hyperthyroid rats, after an initial increase by 1.5-fold on day 1, the synthesis of p58-M2 subsequently declined during regeneration. In hypothyroid rats, the synthesis of p58-M2 remained virtually unchanged during regeneration. These results indicate that T3 regulates the synthesis of p58-M2 in mature and regenerating liver.