Effects of hypo- and hyper-thyroidism on liver composition, blood glucose, ketone bodies and insulin in the male rat

Glucose metabolism before and after radioiodine therapy of a patient with Graves' disease: Assessment by continuous glucose monitoring

Hyperthyroidism causes impaired glucose tolerance, insulin resistance (IR) and insulin secretion. However, the glucose variability affected by thyroid dysfunction remains unclear. Glucose variability was assessed by continuous glucose monitoring (CGM) in a non-diabetic patient with Graves' disease (GD), to the best of our knowledge, for the first time. A 28-year-old man with GD, who had been taking methimazole for 4 years, was treated with radioiodine on August 17th 2016. Although the patient exhibited normal glycated hemoglobin (HbA1c; 5.3%) and blood glucose values during the oral glucose tolerance test (OGTT; fasting and 120 min blood glucose were 5.38 and 6.39 mmol/l, respectively) before radioiodine therapy, CGM exhibited high 24 h mean glucose and nocturnal hyperglycemia. An increased fasting insulin level, suppressed levels of blood glucagon and high homeostatic model assessment of IR were also observed. The disordered glucose metabolism improved as soon as the patient's thyroid function turned to hypothyroidism 4 months after radioiodine therapy. The glucose intolerance in patients with hyperthyroidism, missed by the OGTT and HbA1c tests, may be more common than anticipated.

Toxicities Associated with 1-month Treatment with Propylthiouracil (PTU) and Methimazole (MMI) in Male Rats

Thionamides such as propylthiouracil (PTU) and methimazole (MMI) have been used for more than 50 years to treat the more common causes of thyrotoxicosis/hyperthyroidism such as Graves’ disease. Serious adverse effects associated with thionamides in humans include idiosyncratic liver damage, agranulocytosis, aplastic anemia, and vasculitis. Both prospective and retrospective clinical studies with these drugs have failed to identify predictive biomarker for these adverse effects. To assess whether rat is a good model for predicting drug-related adverse events in the liver and in the bone marrow, we conducted a comprehensive study in male rats with multiple doses of PTU and MMI. As expected, euthyroid animals became hypothyroid along with several secondary changes associated with hypothyroidism. There were slight reductions in red blood cell parameters along with some marginal effects on the bone marrow elements. However, there was no evidence of significant neutropenia and liver injury in both PTU-treated and MMI-treated cohorts. MMI-related effects were noted in the seminiferous tubules of the testes. Overall, 1-month daily treatment of euthyroid rats with PTU or MMI resulted in hypothyroidism, minor bone marrow effects, and several secondary effects associated with hypothyroidism, but without any evidence of adverse effects reported in humans including liver injury and agranulocytosis.

Global metabolic effects of glycerol kinase overexpression in rat hepatoma cells

Glycerol kinase has several diverse activities in mammalian cells. Glycerol kinase deficiency is a complex, single-gene, inborn error of metabolism wherein no genotype-phenotype correlation has been established. Since glycerol kinase has been suggested to exhibit additional activities than glycerol phosphorylation, expression level perturbation in this enzyme may affect cellular physiology globally. To investigate this possibility, we conducted metabolic investigations of wild-type and two glycerol kinase-overexpressing H4IIE rat hepatoma cell lines constructed in this study. The glycerol kinase-overexpressing cell lines exhibited a significantly higher consumption of carbon sources per cell, suggesting excess carbon expenditure. Furthermore, we quantified intracellular metabolic fluxes by employing stable isotope 13C labeling with a mathematically designed substrate mixture, gas chromatography-mass spectrometry, and comprehensive isotopomer balancing. This flux analysis revealed that the pentose phosphate pathway flux in the glycerol kinase-overexpressing cell lines was 2-fold higher than that in the wild-type, in addition to subtler flux changes in other pathways of carbohydrate metabolism. Furthermore, the activity and transcript level of the lipogenic enzyme glucose-6-phosphate dehydrogenase, the rate-limiting enzyme of the pentose phosphate pathway, were also about 2-fold higher than that of the wild-type; these data corroborate the flux analysis results. This study shows that glycerol kinase affects carbon metabolism globally, possibly through its additional functions, and highlights glycerol kinase's multifaceted role in cellular physiology.

Non-invasive measurements of myocardial carbon metabolism using in vivo 13C NMR spectroscopy

Despite their prime role in maintaining contractile performance, myocardial substrate uptake, substrate preference and metabolism are difficult to assess non-invasively. The objective of the present work was to extend the scope of cardiac 13C nuclear magnetic resonance (NMR) spectroscopy to the in vivo situation ('closed-chest model') and to quantitatively appraise myocardial metabolism in vivo. For this purpose, overnight-fasted Sprague-Dawley rats received intravenous infusions of non-radioactive 13C-labeled glucose, 3-hydroxybutyrate, and acetate as markers for glycolysis, metabolism of ketone bodies and direct incorporation into tricarboxylic acid (TCA) cycle, respectively. In vivo 13C NMR spectra (at 7 T) were acquired from the myocardium with a time resolution of 6 min. At the end of the infusion experiments, tissue extracts were prepared and further analyzed by high-resolution 13C NMR spectroscopy in order to corroborate the findings obtained in vivo. Accordingly, 3-hydroxybutyrate and acetate were rapidly extracted by the myocardium and supplied 42 +/- 6 and 53 +/- 9% of the acetyl-CoA for TCA cycle operation, whereas glucose, although also well extracted, did not contribute to myocardial oxidative metabolism. Myocardial TCA cycle turnover (V(TCA)) in vivo was estimated at 1.34 +/- 0.07 micromol/min/g wet weight, myocardial oxygen consumption (MVO2) at 2.95 +/- 0.16 micromol/min/g wet weight, exchange rate between alpha-ketoglutarate and glutamate (V(x)) at 1.22 +/- 0.08 micromol/min/g wet weight and rate of glutamine synthesis (V(gln)) at 0.14 +/- 0.02 micromol/min/g wet weight. The substantial synthesis of myocardial glutamine is in contrast to experiments with isolated and saline perfused hearts. In conclusion, it is demonstrated that 13C NMR spectroscopy of the heart in intact rats is feasible and provides new quantitative insight into myocardial substrate uptake, preference and metabolism in vivo.

Thyroid hormone and dehydroepiandrosterone permit gluconeogenic hormone responses in hepatocytes

The importance of the sn-glycerol- 3-phosphate (G-3-P) electron transfer shuttle in hormonal regulation of gluconeogenesis was examined in hepatocytes from rats with decreased mitochondrial G-3-P dehydrogenase activity (thyroidectomized) or increased G-3-P dehydrogenase activity [triiodothyronine (T(3)) or dehydroepiandrosterone (DHEA) treated]. Rates of glucose formation from 10 mM lactate, 10 mM pyruvate, or 2.5 mM dihydroxyacetone were somewhat less in hypothyroid cells than in cells from normal rats but gluconeogenic responses to calcium addition and to norepinephrine (NE), glucagon (G), or vasopressin (VP) were similar to the responses observed in cells from normal rats. However, with 2. 5 mM glycerol or 2.5 mM sorbitol, substrates that must be oxidized in the cytosol before conversion to glucose, basal gluconeogenesis was not appreciably altered by hypothyroidism but responses to calcium and to the calcium-mobilizing hormones were abolished. Injecting thyroidectomized rats with T(3) 2 days before preparing the hepatocytes greatly enhanced gluconeogenesis from glyc erol and restored the response to Ca(2+) and gluconeogenic hormones. Feeding dehydroepiandrosterone for 6 days depressed gluconeogenesis from lactate or pyruvate but substantially increased glucose production from glycerol in euthyroid cells and restored responses to Ca(2+) in hypothyroid cells metabolizing glycerol. Euthyroid cells metabolizing glycerol or sorbitol use the G-3-P and malate/aspartate shuttles to oxidize excess NADH generated in the cytosol. The transaminase inhibitor aminooxyacetate (AOA) decreased gluconeogenesis from glycerol 40%, but had little effect on responses to Ca(2+) and NE. However, in hypothyroid cells, with minimal G-3-P dehydrogenase, AOA decreased gluconeogenesis from glycerol more than 90%. Thus, the basal rate of gluconeogenesis from glycerol in the euthyroid cells is only partly dependent on electron transport from cytosol to mitochondria via the malate/aspartate shuttle and almost completely dependent in the hypothyroid state, and the hormone enhancement of the rate in euthyroid cells involves primarily the G-3-P cycle. These data are consistent with Ca(2+) being mobilized by gluconeogenic hormones and G-3-P dehydrogenase being activated by Ca(2+) so as to permit it to transfer reducing equivalents from the cytosol to the mitochondria.

Early changes in plasma glucagon and growth hormone response to oral glucose in experimental hyperthyroidism

The mechanisms underlying deterioration of glucose tolerance associated with hyperthyroidism are not completely understood. Increases in glucagon and growth hormone (GH) secretion have been previously found in hyperthyroid subjects, and could play a crucial role in this phenomenon. However, studies have not yet established the time sequence of changes in plasma glucose on the one hand and glucagon and GH on the other. To assess the early effects of thyroid hormone excess on glucose tolerance and plasma concentrations of the main glucoregulatory hormones, 12 nondiabetic euthyroid subjects underwent an oral glucose tolerance test (OGTT) before and after triiodothyronine ([T3] 120 micrograms/d) was administered for 10 days. Plasma levels of glucose, insulin, glucagon, and GH were determined at fasting and after the glucose load. T3 administration caused a marked increase in serum T3 (8.8 +/- 0.6 v 2.0 +/- 0.1 nmol/L), with clinical and biochemical signs of thyrotoxicosis. During the treatment, plasma glucose significantly increased both at fasting and after the glucose load (basal, 5.3 +/- 0.1 v 4.9 +/- 0.2 mmol/L, P < .05; area under the curve [AUC] for OGTT, 7.7 +/- 0.3 v 6.7 +/- 0.4 mmol/L min, P < .01) without any change in plasma insulin levels. After T3 administration, plasma glucagon levels were lower than at baseline (basal, 92 +/- 7 v 148 +/- 35 ng/L; AUC, 74 +/- 6 v 98 +/- 16 ng/L.min, P < .05), showing an appropriate reduction by the increased glucose levels. Conversely, plasma GH showed impaired suppression by hyperglycemia (AUC, 1.2 +/- 0.3 v 0.7 +/- 0.2 microgram/L.min, P < .05). In conclusion, thyroid hormone excess rapidly impairs glucose tolerance. Altered secretion of GH is an early event in thyrotoxicosis accompanying the onset of hyperglycemia, whereas plasma glucagon is appropriately suppressed by the increased plasma glucose levels. Thus, GH but not glucagon may contribute to the early hyperglycemic effect of thyrotoxicosis.

Hepatic regulation of pancreatic alpha-cell function

The direct feedback regulation between the endocrine gland and its target organ is an expected biological relationship. However, such a phenomenon is far from being well established in the case of the endocrine pancreas and its major target organ, the liver, especially since plasma glucose has been established as the prime regulator. In this perspective, I have reexamined the feedback regulation between plasma glucose and glucagon secretion by the pancreatic alpha cell. Surprisingly, available data in the literature appear to document a frequent breakdown of this well-established interdependence between plasma glucose and pancreatic alpha cells, as reflected by a sustained elevation of plasma glucagon levels in several physiologic and pathologic states with concurrent euglycemia or hyperglycemia. Moreover, normal or low glucagon concentrations in the presence of fasting hypoglycemia in patients with insulinoma or non-islet cell tumors secreting insulin-like peptides and in patients with hepatic glycogen storage disorders may enhance our hypothesis that plasma glucose level may not be the major regulator of glucagon secretion. Extensive data in the literature show that hyperglucagonemic states are characterized by a unique metabolic environment, namely hepatic glycogen depletion. Similarly, hepatic glycogen stores are abundant in the presence of normal or low glucagon concentrations. These findings imply a distinct relationship between hepatic glycogen content and plasma glucagon level.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diet and feeding time on daily variations in plasma insulin, hepatic c-AMP and other metabolites in a teleost fish,Dicentrarchus labrax L

Juvenile sea bass, 1.5 years old, of mixed sex, held on long photoperiods were fed early on the photoperiod and late on the photoperiod, using different diets. Fish fed natural diets showed a daily rhythmicity of plasma insulin, liver c-AMP, plasma glucose, liver glycogen and muscle glycogen content, however, fish fed a commercial diet did not show this daily rhythmicity except for plasma insulin levels. In addition, these fish had significantly lower levels of plasma insulin, liver c-AMP and plasma glycerol than the group fed on the natural diets at similar feeding times. The time of feeding also induced different rhythmicity patterns in hormones and metabolites as well as a significant change in their mean levels. These facts are discussed in relation with the pre-feeding activity and increased appetite exhibited by the fish fed late on the photoperiod and with their implications on fish culture.

Defective pancreatic alpha and beta cell secretion in thyrotoxicosis

Pancreatic alpha and beta cell hormone secretion was studied in 11 patients with thyrotoxicosis before and in 7 patients after thyroid function was normalized with either prophylthiouracil or methimazole and propranolol (R). All had IV arginine and IV glucose infusions. Forty control subjects had IV arginine; 21 had IV glucose tests. After arginine, untreated patient had blunted serum insulin at both 15 and 30 minutes (p less than 0.05, p less than 0.001) compared to control subjects, blunted glucagon at 30 minutes (p less than 0.05) and blunted glucose at both 15 and 30 minutes (p less than 0.001, p less than 0.01) compared to control subjects. After glucose, untreated patients had lower nadir glucagon than in the studies with both arginine and glucose infusions. These data document blunted glucagon, suppressed glucose and insulin peaks after arginine in thyrotoxicosis, indicate that both alpha and beta cell hormone secretion may be abnormal, and that the preferential abnormality follows protein rather than carbohydrate loading.

Diabetes-induced alterations in the translational activity of specific messenger ribonucleic acids isolated from rat hearts

During diabetes mellitus, total proteins and ribonucleic acids are significantly decreased in the rat heart, and these parameters can be increased by insulin administration. To determine whether all ribonucleic acids are equally sensitive to insulin, we examined the influence of this hormone on individual translatable ribonucleic acids. Cardiac ribonucleic acid prepared from control, untreated, and insulin-treated diabetic animals was translated in vitro in the presence of [35S]methionine. The radiolabeled peptides were separated by two-dimensional gel electrophoresis and were analyzed by fluorometry. We found that diabetes induces both qualitative and quantitative changes in the predominance of a few specific translatable messenger ribonucleic acid species. The translation of 11 messenger ribonucleic acid species was significantly decreased and that of eight messenger ribonucleic acid species was significantly increased in diabetic preparation. Twelve of the 19 translation products were quantified by digital matrix photometry: three labeled peptides were observed only when cardiac ribonucleic acid from diabetic animals was added to the cell-free translation system, four new peptides appeared when cardiac ribonucleic acid from control animals was added, and although the remaining five peptides were translated in vitro after either control or diabetic ribonucleic acid was added, their relative predominance was altered 2- to 200-fold. When translation products coded for by messenger ribonucleic acids prepared from either diabetic or hypothyroid hearts were compared, we found that most of the alterations induced by diabetes were also induced by hypothyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of streptozotocin-diabetes and L-thyroxine treatment on TSH and GH, and circulating glucose and glycerol in thyroidectomized rats

The response to exogenous thyroxine in thyroidectomized rats made diabetic by treatment with streptozotocin was greatly impaired, as shown by their growth retardation and the lack of increase in plasma GH and pituitary GH and TSH concentrations. Insulin administration partially compensated for these endocrine alterations in diabetic thyroidectomized rats. When these animals received enough exogenous thyroxine to normalize their plasma PBI and TSH levels, insulin administration did not decrease their augmented glucose and glycerol concentrations. These findings show the permissive action between thyroid hormones and insulin although some effects of the former counteract those of insulin.

13C NMR study of gluconeogenesis from labeled alanine in hepatocytes from euthyroid and hyperthyroid rats

Metabolism of [3-13C]alanine in the presence and absence of beta-hydroxybutyrate or ethanol has been followed at 25 degrees C by 13C NMR at 90.5 MHz in primary hepatocytes from untreated rats and rats treated with triiodothyronine and not allowed to eat for 24 hr. The phosphoenolpyruvate/pyruvate futile cycle was followed in situ by comparing the concentration of 13C at the scrambled alanine C2 position with that at glucose C5. In the absence of ethanol, the flux through pyruvate kinase was 60% of the gluconeogenic flux in hepatocytes from hyperthyroid rats, compared with 25% in the controls. Incubation with ethanol reduced the pyruvate kinase flux in the hyperthyroid state to that measured in the controls. Under all conditions, the relative concentration of label at the aspartate C2 and C3 sites was 1:2, whereas at the corresponding carbons in glutamate, randomization was almost complete. These observations, which require flux of unscrambled label into aspartate, are consistent with intramitochondrial synthesis of aspartate only if there is incomplete mixing of the intramitochondrial oxaloacetate pool. The 13C enrichment measured in the ketone bodies is increased by the presence of exogenous beta-hydroxybutyrate. The greater labeling that we observe at C2 of beta-hydroxybutyrate compared with C4 under this condition is explained by the flow through 3-hydroxy-3-methylglutaryl-coenzyme A synthase.

Thyroid-hormone effects on ornithine decarboxylase

We examined thyroidectomized, normal and hyperthyroid rats and found that ornithine decarboxylase activity was directly correlated with thyroid functional state in heart and liver and unaffected in brain, testes and spleen, phenomena that correlate with the known effect of thyroid hormone on protein synthesis.

Carbohydrate metabolism in hypothyroid myopathy

The carbohydrate metabolism in hypothyroid patients was investigated. After an overnight fast, the blood glucose level was 24% lower and the blood lactate level was 35% lower in the untreated hypothyroid patients than that observed in the treated hypothyroid patients or in the normal subjects. There was no difference in the blood alanine or plasma free fatty acid values between the subject groups. Skeletal muscle biopsied from two hypothyroid patients with marked myopathy showed normal glycogen content, 0.83%-0.86% (normal 1.06%), but reduced activity of acid maltase, 32-50 nmoles/min/g (normal 97). Forearm ischemic stimulation applied to hypothyroid patients failed to elevate the level of lactate. The results are compatible with impaired glycogenolysis from the skeletal muscle, which may be a contributory factor in the myopathy in hypothyroidism.