Administration Of Triiodothyronine Research Articles

Regulation of the activity of 3-hydroxy-3-methylglutaryl coenzyme a reductase by insulin

In hypophysectomized - diabetic rats full restoration of hepatic HMG-CoA reductase activity required administration of both triiodothyronine and insulin. Giving triiodothyronine alone resulted in full restoration of reductase mRNA and immunoreactive protein but not of enzyme activity. Increasing the ratio of dithiothreitol to microsomal protein resulted in full HMG-CoA reductase activity. With 5mM glutathione, HMG-CoA reductase activity was 5 to 10 fold higher in non-diabetic animals. The data suggest that insulin acts by increasing the portion of HMG-CoA reductase present in its active free sulfhydryl form.

Changes in bone and muscle constituents during dieting for obesity

The effects of dietary carbohydrate (CHO) content and of tri-iodothyronine (T3) administration on calcium, zinc and phosphate balances and on urinary hydroxyproline output were examined in eight obese subjects undergoing therapy with low energy (2741-3301 kJ/day), high calcium (28.9-35.1 mmol/day) diets. Calcium balances were generally positive at the high levels of intake used, but significantly more calcium was retained when dietary CHO intake (as proportion of total energy intake) was increased. Zinc balances were more positive when the proportion of dietary CHO was high and correlated both with nitrogen balance and with daytime insulin concentration. Urinary hydroxyproline output generally increased with dieting irrespective of dietary CHO content. Administration of T3 (30-80 micrograms/day) markedly increased zinc loss without affecting calcium balance.

Precocious induction of hepatic glucokinase and malic enzyme in artificially reared rat pups fed a high-carbohydrate diet

Glucokinase and NADP:malate dehydrogenase (malic enzyme) first appear in liver when rat pups are weaned from milk which is high in fat to lab chow which is high in carbohydrate. To examine the influence of diet during the early neonatal period, before developmental changes in the circulating concentrations of thyroid and adrenocortical hormones occur, high-carbohydrate formula (56% of calories from carbohydrate), isocaloric and isonitrogenous with rat milk, was intermittently infused via gastrostomy starting on the second day of life. Pups had no further access to their dams. Body weights attained by these pups were at least 90% of those attained by mother-fed pups, which served as controls. In artificially reared rats fed the high-carbohydrate formula, on Day 4, glucokinase and malic enzyme were 30 and 18% of adult activity, respectively; on Day 10, glucokinase and malic enzyme were 71 and 96% of adult activity, respectively. On Days 4 and 10 glucose-6-phosphate dehydrogenase was elevated four- to fivefold in pups fed the high-carbohydrate formula compared to mother-fed pups. A second isocaloric formula, with 22% of calories from carbohydrate but low in protein, resulted in intermediate levels of all three enzymes on Day 10. Pups fed the high-carbohydrate formula has plasma insulin concentrations four- to fivefold greater than mother-fed pups on both Days 4 and 10. Triiodothyronine administration (1 μg/g body wt) on Day 1 enhanced the induction of malic enzyme but not glucokinase on Day 4 in pups fed the high-carbohydrate formula. The results demonstrate that neonatal rat liver is competent to respond to high carbohydrate intake by induction of glucokinase and malic enzyme.

Increased phenylalanine incorporation in regenerating skeletal muscle grafts.

Skeletal muscle regenerates following grafting, but little is known about protein synthesis and its regulation during regeneration. We determined the sequence of changes in protein synthesis in rat extensor digitorum longus (EDL) muscle by the measurement of phenylalanine (Phe) incorporation into muscle protein at various times after grafting. Compared with control EDL, Phe incorporation in grafts doubled in 1 day, was four- to eight-fold greater from days 2 to 10 after grafting, and then subsided. Tissue mass (wet weight) increased rapidly from days 7 to 20 in EDL grafts. The maximal increase in protein synthesis occurred 7-10 days after grafting, whether or not the nerve was left intact. Autoradiography indicated that incorporated radioactivity was associated with regenerating muscle fibers on day 10. Deficiencies of insulin, pituitary or testicular hormones, or chronic in vivo administration of insulin, growth hormone, testosterone, or tri-iodothyronine did not substantially alter the elevation in incorporation of the Phe into muscle protein 10 days after grafting. The breakdown of EDL protein, measured in vitro simultaneously with protein synthesis, was increased five-fold, and overall protein degradation was elevated six-fold 10 days after grafting. These findings indicate that Phe incorporation is rapidly elevated following grafting of the EDL, and that by days 7-10 reflects synthesis in regenerating muscle fibers. The increase in protein synthesis associated with muscle regeneration at this time appears to be independent of innervation and anabolic hormones.

A freeze-fracture study of extensor digitorum longus and soleus muscle fibers from thyrotoxic rats

Freeze-fracture methods were used to study the sarcoplasmic reticulum and surface membranes in muscles from rats after chronic administration of triiodothyronine (150 μg/kg daily, for 1 to 20 days). The major effect of the hormone on the sarcoplasmic reticulum was to increase the numbers of indentations in the terminal cisternae in parallel with an increase in the speed of the isometric twitch. The indentations increased from 7.3 ± 0.2 to 10.6 ± 0.1 (mean ± 1 SEM)/μm of terminal cisternae in the fast-twitch extensor digitorum longus (EDL) and from 0.9 ± 0.1 to 4.4 ± 0.1/μm in slow-twitch soleus fibers. The increase in indentation density in both types of muscle occurred within 10 days of the commencement of hormone injection. During the same period there was a small reduction in the density of intramembrane particles in the plasmalemma and a significant increase in the number of caveolae, from 14.6 ± 0.25 to 20.4 ± 0.3/μm 2 in EDL fibers, and from 22.9 ± 0.3 to 28.6 ± 0.3/μm 2 in soleus. The increase in caveolae density was coincident with an increase in the area of T-tubule membrane. The results provide further evidence (a) that the indentations in the terminal cisternae play a functional role in muscle activation and (b) that the caveolae are the surface openings of transverse tubules.

Stimulation of mouse liver glutathione S-transferase activity in propylthiouracil-treated mice in vivo by tri-iodothyronine.

Female C57Bl/6J mice were given drinking water containing 0.05% propylthiouracil to induce a hypothyroid condition. Mitochondrial glycerol-3-phosphate dehydrogenase activity, used as an index of hypothyroidism, was 57.1 +/- 4.5 and 29.4 +/- 3.8 nmol/min per mg of protein for control and propylthiouracil-treated animals respectively. Administration of tri-iodothyronine resulted in an approx. 4.5-fold increase in dehydrogenase activity in propylthiouracil-treated animals. A dose-dependent increase in hepatic GSH S-transferase activity in propylthiouracil-treated animals was observed at tri-iodothyronine concentrations ranging from 2 to 200 micrograms/100 g body wt. This increase in transferase activity was seen only when 1,2-epoxy-3-(p-nitrophenoxy)propane was used as substrate for the transferase. Transferase activity with 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene as substrate was decreased by tri-iodothyronine. Administration of actinomycin D (75 micrograms/100 g body wt.) inhibited the tri-iodothyronine induction of transferase activity. Results of these studies strongly suggest that tri-iodothyronine administration markedly affected the activities of GSH S-transferase by inducing a specific isoenzyme of GSH S-transferase and suppressing other isoenzymic activities.

Effects of administration of tri-iodothyronine on the response of cardiac and renal pyruvate dehydrogenase complex to starvation for 48 h

Effects of administration of tri-iodothyronine (T3) on activities of cardiac and renal pyruvate dehydrogenase complex (active form, PDHa) were investigated. In fed rats, T3 treatment did not affect cardiac or renal PDHa activity, although blood non-esterified fatty acid and ketone-body concentrations were increased. Starvation (48 h) of both control and T3-treated rats resulted in similar increases in the steady-state concentrations of fatty acids and ketone bodies, but inactivation of cardiac and renal pyruvate dehydrogenase complex activities was diminished by T3 treatment. Inhibition of lipolysis increased renal and cardiac PDHa in control but not in T3-treated 48 h-starved rats, despite decreased fatty acid and ketone-body concentrations in both groups. The results suggest that hyperthyroidism influences the response of cardiac and renal PDHa activities to starvation through changes in the metabolism of lipid fuels in these tissues.

Endocrine regulation of sex differences in hepatic histidase activity.

Hepatic histidase activity in adult female rats is twice that in adult male rats. Hypophysectomy and thyroidectomy result in a significant increase in hepatic histidase activities in males, but not in females. This effect on histidase is reversed by the exogenous administration of tri-iodothyronine, but not by ectopic pituitary glands or purified pituitary hormones.

Sites of thyroid hormone action on Na?K-ATPase along the rabbit nephron

Although Ismail-Beigi and Edelman demonstrated in 1971 that thyroid hormones control the activity of Na-K-ATPase in the mammalian kidney, the actual site of this regulation inside the organ was not located. We therefore decided to study the relationship between thyroid hormones and Na-K-ATPase activity in individual nephron segments obtained by microdissection of collagenase-treated rabbit kidneys. For this purpose, the changes in the activity and number of catalytic sites of Na-K-ATPase in response to thyroidectomy or triiodothyronine administration were examined. Eight to 12 days after thyroidectomy, Na-K-ATPase activity had dropped by 40 to 80% in the convoluted and straight portions of the proximal tubules, and in the cortical and outer medullary collecting tubules, but not in the thick ascending limbs of Henle's loops or distal convoluted tubules. The apparent number of catalytic sites for Na-K-ATPase, as measured by specific binding of 3H-ouabain, decreased in parallel with Na-K-ATPase activity, and therefore this enzyme's specific activity was not altered. Fourty eight hours after injection of thyroidectomized animals with a single dose of either 100 or 500 micrograms/kg triiodothyronine, Na-K-ATPase activity in target segments was restored to the level measured in control animals. These effects of thyroid hormone were specific for Na-K-ATPase, since the activity of adenylate cyclase, another marker of the basolateral membrane, was not altered by thyroidectomy. The results obtained indicate that triiodothyronine controls Na-K-ATPase activity in specific nephron segments, by altering the number of this enzyme's catalytic sites.

Effects of altered thyroid status on beta-adrenergic actions on skeletal muscle glycogen metabolism.

The effects of hypothyroidism on glycogen metabolism in rat skeletal muscle were studied using the perfused rat hindlimb preparation. Three weeks after propylthiouracil treatment, serum thyroxine was undetectable and muscle glycogen and Glc-6-P were decreased. Basal and epinephrine-stimulated phosphorylase a and phosphorylase b kinase activities were also significantly reduced, as were epinephrine-stimulated cAMP accumulation and cAMP-dependent protein kinase activity. Conversely, basal and epinephrine-stimulated glycogen synthase I activities were significantly higher while the Ka of the enzyme for Glc-6-P was lower in hypothyroid animals. Propylthiouracil-treated rats also had increased phosphoprotein phosphatase activities towards phosphorylase and glycogen synthase and decreased activity of phosphatase inhibitor 1. beta-Adrenergic receptor binding and basal and epinephrine-stimulated adenylate cyclase activities were reduced in muscle particulate fractions from hypothyroid rats. Administration of triiodothyronine to rats for 3 days after 3 weeks of propylthiouracil treatment restored the altered metabolic parameters to normal. It is proposed that the decreased beta-adrenergic responsiveness of the enzymes of glycogen metabolism in hypothyroid rat skeletal muscle is due to increased activity of phosphoprotein phosphatases and to reduced beta-adrenergic receptors and adenylate cyclase activity.

Effect of thyroid hormone excess on action, secretion, and metabolism of insulin in humans.

To determine the effect of thyroid hormone excess on insulin secretion, metabolism and action in humans, we examined intravenous glucose tolerance, glucose-induced insulin secretion, insulin clearance, monocyte insulin receptor binding, and the dose-response characteristics for the effects of insulin on glucose production, uptake, oxidation, and nonoxidative disposal in 10 normal volunteers for 14 days before and after oral administration of triiodothyronine (T3) in doses that increased plasma T3 to levels observed in spontaneous thyrotoxicosis (P less than 0.001). After T3 postabsorptive plasma glucose (P less than 0.05) and insulin (P less than 0.05) both increased; intravenous glucose tolerance was unaffected, but plasma insulin responses were increased (P less than 0.01); basal glucose production, uptake, and oxidation all increased (all P less than 0.05), whereas nonoxidative glucose disposal was unaffected (P = NS); monocyte insulin receptor binding increased (P less than 0.01) due to increased receptor affinity (P less than 0.05); and receptor number was not significantly altered (P = NS). Insulin clearance was increased. Insulin-induced suppression of glucose production was impaired (Km 22 +/- 3 vs. 37 +/- 7 microU/ml, P less than 0.02); maximal insulin-induced glucose uptake (10.7 +/- 0.6 vs. 13.0 +/- 0.9 mg X kg-1 X min-1, P less than 0.001) and oxidation (3.41 +/- 0.30 vs. 5.34 +/- 0.59 mg X kg-1 X min-1, P less than 0.001) were increased without a significant change in Km. However, submaximal rates of nonoxidative glucose disposal and glucose uptake were inappropriately low for the increased insulin receptor binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Different alterations of nuclear triiodothyronine receptor capacity in liver and kidney induced by starvation and triiodothyronine administration.

Many studies have shown alterations in the number of nuclear triiodothyronine receptor (NT3R) under pathophysiologic situations. Most of these studies were performed on the rat liver and it is not known whether NT3R in different tissues exhibits an alteration similar to that in the liver. We compared the change of nuclear receptor capacity for T3 in the liver and kidney during starvation and after T3 injection. Fasting for 72 h decreased maximal binding capacity (Cmax) in the rat liver receptor to 67% of the control, while it did not significantly change Cmax in the kidney. These changes in Cmax were parallel to those of nuclear protein concentrations in both tissues. Daily sc injection of T3 (20 micrograms/100 g body weight) for 3 days also caused the different alteration of Cmax in the liver and kidney. After T3, hepatic NT3R increased to 182% of the control, but renal NT3R increased only to 136%. Association constants were the same in all groups. These results show that changes of NT3R capacity under some conditions vary in different tissues.

Pure α-subunit producing tumor derived from a thyrotropic tumor: Impaired regulation of a-subunit and its mRNA by thyroid hormone

We have recently described a mouse pituitary tumor line which produces only the α-subunit of the glycoprotein hormones. This tumor line may be a useful animal model to study autonomous pituitary tumors which secrete only α-subunit. Our pure α-subunit producing tumor was derived from a thyrotropic tumor which secreted intact TSH as well as free α-subunit. Our current studies compare the regulation of α-subunit biosynthesis in a conventional thyrotropic tumor and the α-subunit producing tumor. Thyroxine or triiodothyronine administration to mice bearing the a-subunit producing tumor resulted in no change in plasma α-subunit concentration, and a 10–19% reduction in tumor α-subunit mRNA concentration that was not statistically significant. In contrast, thyroxine administration to mice bearing the thyrotropic tumor resulted in an 81% reduction in plasma α-subunit concentration, and a 75% reduction in tumor α-subunit mRNA concentration (P < 0.01). Other studies using a cDNA specific for thyrotropin-β (TSHβ) failed to detect TSHβ mRNA in the α-subunit producing tumor, while TSHβ mRNA was easily detected in the conventional thyrotropic tumor. We conclude that during the development of the a-subunit producing tumor from a thyrotropic tumor, loss of TSHβ mRNA was also associated with an impaired capacity for thyroid hormone to decrease concentrations of α-subunit mRNA.

Familial Euthyroid Hyperthyroxinemia Secondary to Pituitary and Peripheral Resistance to Thyroid Hormones

Among 18 family members, representing four generations studied, a familial form of euthyroid hyperthyroxinemia was found in 6. The increased serum total thyroxine value in all hyperthyroxinemic subjects could not be explained by abnormalities in binding proteins. Five of the six patients had a goiter, and all had increased concentrations of triiodothyronine and free thyroxine without symptoms or signs of hyperthyroidism. Basal serum levels of thyroid-stimulating hormone (thyrotropin) were normal in all six; in the four who were tested, these levels responded normally to thyrotropin-releasing hormone (TRH). The normal suppression of basal and TRH-stimulated thyrotropin increase after administration of triiodothyronine did not occur. These patients seem to have resistance of peripheral and pituitary tissues to the actions of thyroid hormones. Family studies revealed that the disorder appeared as a new sporadic mutation and was consistent with an autosomal-dominant mode of inheritance. This disorder, an important example of euthyroid hyperthyroxinemia, should not be confused with Graves' disease.

Aldosterone and plasma renin activity in hyperthyroid rats: effects of propranolol and propylthiouracil.

Variations in renin- angiotensin-aldosterone system (RAAS) in experimentally induced hyperthyroidism were studied. The changes observed in the RAAS in these conditions, evaluated through the plasma renin activity (PRA), are parallel to serum aldosterone concentration (AC). An increase in PRA and AC is produced following the administration of triiodothyronine, possibly through elevated adrenergic activity: beta-blocker, propranolol, returned the PRA and AC to normal. The active hormone is seen to be triiodothyronine, confirmed by using the antithyroid preparation, propylthiouracil, to inhibit the conversion of thyroxine. Propylthiouracil administration to hyperthyroid animals lowers the PRA and AC.

Effect of inositol and tri-iodothyronine on the hormonal responsiveness of hepatocytes obtained from partially hepatectomized rats.

Hepatocytes obtained from animals partially hepatectomized (72 h before the experiment) have a diminished responsiveness to alpha 1-adrenergic amines, vasopressin, angiotensin and glucagon and an increased responsiveness to beta-adrenergic amines. Administration of inositol or tri-iodothyronine to the hepatectomized animals induced a recovery in the hepatocyte responsiveness to the Ca2+-dependent hormones and abolished that to beta-adrenergic amines; the response to glucagon was not improved.

Triiodothyronine increases splanchnic release and peripheral uptake of glucose in healthy humans

To examine the effect of experimental hyperthyroidism induced by triiodothyronine (T3) administration on glucose utilization and glucose-stimulated insulin secretion, euglycemic insulin clamp studies (A), hyperglycemic clamp studies (B), and estimation of splanchnic glucose metabolism after an oral glucose load by means of the hepatic venous catheter technique (C) were performed in healthy volunteers whose serum T3 concentration had been increased from 1.1 +/- 0.02 (SE) to 3.5 +/- 0.2 ng/ml. At plasma insulin concentrations of 77 +/- 6 microU/ml and maintenance of euglycemia, insulin-mediated glucose metabolism (6.8 +/- 0.4 mg X kg-1 X min-1) was not significantly altered following T3 exposure (7.2 +/- 0.7) in study A. When plasma glucose was raised and maintained at 7 mmol/l above basal, glucose uptake (10.6 +/- 0.7 mg X kg-1 X min-1) was increased to 14.6 +/- 2.0 mg X kg-1 X min-1 (P less than 0.02) following T3 administration in study B. In study C, basal splanchnic glucose output (104 +/- 10 mg/min) was markedly augmented when preceded by T3 exposure (167 +/- 17 mg/min; P less than 0.005). After a 75-g glucose load, splanchnic glucose output was enhanced by T3 by 30% (43.4 +/- 2.2 vs. 56.9 +/- 6.6 g/150 min; P less than 0.05), whereas arterial plasma concentrations of glucose, insulin, and C-peptide were not significantly different. It is concluded that in healthy humans, exposure to elevated plasma concentrations of T3 increases the amount of glucose to be released from the splanchnic bed. Glucose tolerance, however, remains unaltered, because peripheral glucose uptake is increased at the same time.

Thyrotropin controls transcription of the thyroglobulin gene.

The availability of rat thyroglobulin cDNA clones was exploited to study the regulation of thyroglobulin gene transcription by thyrotropin (TSH). Groups of rats were subjected to treatments leading to reduction or increase in the rat serum TSH (rTSH) levels. Thyroid gland nuclei were isolated, incubated in vitro in the presence of 32P-labeled uridine triphosphate, and thyroglobulin transcripts were quantitated by hybridization to immobilized rat thyroglobulin cDNA clones. Transcription of the thyroglobulin gene was found to be very active in thyroid nuclei from control animals. It represented about 10% of total RNA polymerase II activity. Chronic hyperstimulation of the thyroid glands with endogenous rTSH was achieved in rats treated with the goitrogen propylthiouracil. No significant increase of thyroglobulin gene transcription could be measured in thyroid nuclei from these animals. On the contrary, a dramatic decrease in thyroglobulin gene transcription was observed in those animals in which endogenous rTSH levels had been suppressed by hypophysectomy or by the administration of triiodothyronine. Injection of exogenous bovine TSH in such animals readily restored transcriptional activity of the gene. Our results identify transcription as an important regulatory step involved in TSH action. They suggest that normal TSH levels induce close to maximal expression of the thyroglobulin gene but that continuous presence of TSH is required in order to maintain the gene in an activated state.

Oral triiodothyronine administration lowers plasma fibronectin levels in humans

It has been shown that both serum triiodothyronine (T 3) and plasma fibronectin values decline with fasting and increase with treatment of diabetic ketoacidosis in a paralles manner. 13 To evaluate the mechanism responsible for these changes, we examined the effects of both oral T 3 administration and fasting in six healthy, adult subjects. Initial plasma fibronectin values were normal in four subjects (group 1) and decreased in two subjects (group 2). Initial serum T 3 and thyroxine (T 4) values were normal in both groups. Despite a substantial rise in serum T 3 values with oral T 3 administration, plasma fibronectin fell in group 1 subjects. Fasting caused a significant decline in serum T 3 levels, but only a small further decline of plasma fibronectin concentrations. Serum T 3 levels rose after 1 week of refeeding, whereas plasma fibronectin levels in group 1 did not return toward normal. Plasma fibronectin values did not change in group 2 subjects with oral T 3, fasting, or refeeding. In conclusion, there is no simple cause-and-effect relationship between previously observed changes in plasma fibronectin and serum T 3 concentrations. High doses of oral T 3 will lower plasma fibronectin levels in subjects with initially normal plasma fibronectin values and a normal caloric intake.

Effects of thyroid status on presynaptic alpha 2-adrenoceptor function and beta-adrenoceptor binding in the rat brain.

The effect of thyroid status on noradrenergic synaptic function in the mature rat brain was examined by measuring presynaptic alpha 2- and post-synaptic beta-adrenoceptors. Repeated triiodothyronine (T3) administration to rats (100 micrograms/kg X 14 days: hyperthyroid) caused an 18% increase in striatal beta-adrenoceptors as shown by [3H]-dihydroalprenolol binding with no change in membranes from cerebral cortex or hypothalamus. In contrast, hypothyroidism (propylthiouracil, PTU X 14 days) produced significant 12% and 30% reductions in striatal and hypothalamic beta-adrenoceptors respectively with no change in the cerebral cortex. Presynaptic alpha 2-adrenoceptor function was measured in the two dysthyroid states using the clonidine-induced hypoactivity model. Experimental hyperthyroidism increased the degree of clonidine-induced hypoactivity, and suggests increased presynaptic alpha 2-adrenoceptor function compared with control rats, whereas hypothyroidism suppressed presynaptic alpha 2-adrenoceptor function. These results show firstly that changes in thyroid status in the mature rat may produce homeostatic alterations at central noradrenergic synapses as reflected by changes in pre- and post-synaptic adrenoceptor function. Secondly, there appear to be T3-induced changes in beta-adrenoceptors in the striatum where changes in dopaminergic neuronal activity have previously been demonstrated.