Thyroxine Stimulation of Amino Acid Incorporation into Protein

Differential expression of key enzymes of energy metabolism in preneoplastic and neoplastic rat liver lesions induced by N-nitrosomorpholine and dehydroepiandrosterone

Preneoplastic liver foci and neoplasms of different morphological phenotypes were induced in rats with N-nitrosomorpholine (NNM; 120 mg/l in drinking water for 7 weeks) and the peroxisome proliferator dehydroepiandrosterone (DHEA; 0.6% in the diet for up to 84 weeks). Preneoplastic glycogen storage foci (GSF) occurred mainly upon treatment with NNM, and amphophilic cell foci (APF) were mainly observed in rats treated with DHEA alone or in combination with NNM. The 2 types of lesions belong to 2 different cellular lineages, the glycogenotic/basophilic lineage and the amphophilic lineage, which are characterized by distinct patterns of alterations in key enzymes of energy metabolism. Whereas in GSF enzymes of glucose metabolizing pathways were modified (increase in glucose-6-phosphate dehydrogenase and pyruvate kinase, decrease in glucose-6-phosphatase), APF mainly demonstrated alterations in mitochondrial enzymes (increase in cytochrome c oxidase, succinate dehydrogenase and glycerol-3-phosphate dehydrogenase) and, to a lower extent, in peroxisomal enzymes (increase in peroxisomal hydratase and acyl-CoA oxidase). The alterations in enzyme expression reflect an insulinomimetic effect in GSF and a thyromimetic effect in APF. Neoplasms resulting from APF show a more differentiated phenotype than those arising from GSF. We suggest that the different and in many aspects opposite effects of the 2 carcinogens on key enzymes of distinct pathways of energy metabolism modulate the process of neoplastic liver cell transformation and result in phenotypically different preneoplasias and neoplasias reflecting different cellular lineages.

Thyroid hormone in neural rescue

Serum thyroxine (T4), triiodothyronine (T3), and reverse triiodothyronine (rT3) were followed for 24 h in dogs resuscitated following 9 min of controlled cardiac arrest (CA). Total T4, free T4, total T3, and free T3 decreased, while reverse T3 was elevated in the 24 h following resuscitation. Similar changes occurred with only 30 sec of CA. Levothyroxine sodium (L-T4) post-CA (7.5 micrograms/kg/h = CA + 7.5 or 15 micrograms/kg/h = CA + 15) increased total T4, free T4, and total T3. Free T3 decreased in the CA + 7.5 group but did not fall in CA + 15 group. Neurologic function improved significantly by 6 through 24 h (p < 0.05). Follow-up studies infusing T3 or rT3 failed to improve neurologic outcome. Systemic oxygen consumption (VO2) and delivery was assessed in a separate group of seven dogs that received a pre-CA L-T4 infusion of 15 micrograms/kg/h for 1.5 h and L-T4 infusion for 6 h afterward while controls (n = 7) received saline. Systemic VO2, VCO2, and RQ were calculated from blood contents and cardiac output and serum levels of circulating TSH, T4, FT4, T3, FT3, and rT3 were measured before L-T4 and periodically over 6 h. L-T4 maintained significantly higher T4, FT4, T3, FT3, rT3, VO2, and cardiac output compared to controls. No change in canine TSH was detected. Rapid and dramatic decreases in thyroid hormones following resuscitation indicate a significant acute serum hypothyroid state that may benefit from L-T4 treatment. L-T4 enhances systemic oxygen consumption and delivery and these changes may contribute to L-T4's neural protective effect.

Acute administration of T3 or rT3 failed to improve outcome following resuscitation from cardiac arrest in dogs

Documentation of profound changes in serum thyroid hormone concentrations associated with cardiac arrest and resuscitation, as well as other acute emergencies, have spurred evaluation of possible therapeutic thyroid hormone administration. Acute and significant, this state, characterized by abnormally low serum thyroid hormone concentrations, may indicate selective thyroid replacement therapy. In a previous investigation, post-resuscitation infusion of levothyroxine sodium (L-T4) to normalize serum 3,5,3'-triiodothyronine (T3) concentrations was associated with significant reduction of neurologic deficit caused by severe global cerebral ischemia. Since L-T4 has been reported to act directly or via one of its metabolites, most likely T3, this most active form of thyroid hormone was tested. When L-T4 reduced the neurologic deficit, an increase in 3,3',5'-triiodothyronine (rT3) was also observed. This study therefore determined whether a post-resuscitation treatment with either T3 (n = 8) or rT3 (n = 8) provided protection against global cerebral ischemia comparable to that of L-T4. Global cerebral ischemia was achieved with 9 min of ventricular fibrillation. Following resuscitation, one of three solutions (saline group as a control) was infused for 24 h at rates that reproduced the normal serum T3 concentrations or the rT3 concentrations achieved previously during the L-T4 therapy. The successful elevation of T3 and mimicking rT3 concentrations was assessed and confirmed by radioimmunoassay (RIA). In addition, TSH levels were measured by a novel RIA specific for canine thyroid-stimulating hormone (cTSH). Neurologic deficit was assessed with a well-standardized neurologic deficit examination. In contrast to previous studies using L-T4 infusion, no significant reduction of neurologic deficit was observed. Serum thyroid hormone changes confirmed previously described decreases and in no case did changes in cTSH appear causal in these changes. Thus, we concluded that L-T4 may offer a therapeutic advantage over T3 or rT3.

Effects of mast cell inhibition on plasma and aortic content of lipids, hexosamine, and hydroxyproline in rats. Role of thyroid gland

Twenty-one-day-old rats were treated with mast cell inhibitors for four weeks. Some of the rats were thyroidectomized in the beginning of the experiment. Mast cell impairment decreased plasma cholesterol and increased plasma triglyceride levels; in aorta, there was a decrease in total protein, hexosamine and hydroxyproline levels, while triglyceride concentration increased and total cholesterol did not change. In thyroidectomized rats, mast cell impairment led to decreased plasma cholesterol and aortic triglyceride content, increased aortic hexosamine and hydroxyproline levels and did not change plasma triglycerides, aortic total cholesterol and total protein content. It was concluded that some effects of mast cell inhibition on arterial wall metabolism are mediated through the thyroid gland.

Studies on the range of tissue protein synthesis in pigs: the effect of thyroid hormones

The effects of thyroid hormones on the range of tissue protein synthesis in growing pigs using the constant infusion technique with [14C]lysine were studied. 2. During a 6 h infusion, samples were taken from blood and, at the end of the infusion, from liver, pancreas, stomach, small and large intestines, kidney cortex, kidney medulla, muscle and skin. 3. Lower relative specific radioactivities of free leucine and lysine in several tissues were observed in the hormone-treated group than in the untreated one. 4. The range of protein synthesis rate and the daily amount of protein synthesized in tissues was higher in all tissues after application of thyroid hormones. 5. Assuming that the organs analysed represented 70% of the total trichloroacetic acid-precipitable protein of the pig, the estimated range of daily protein synthesis was 251-490 and 312-880 g in untreated and hormone-treated pigs respectively.

Effect of thyroid hormones on the in vivo protein synthesis in rats

The studies were made in order to estimate the rate of tissue protein synthesis in euthyroid, hyperthyroid and hypothyroid rats (110 g body weight). Hyperthyroidism was simulated by daily oral application of 4,44 micrograms thyroxine (T4) and 0,89 micrograms triiodothyronine (T3)/100 g body weight. Hypothyroidism was induced by oral application of 17,8 mg of methylthiouracil (MTU) per day and 100 g body weight. In both cases the treatment lasted for 7 days. The concentration of both hormones in blood serum was twice that of the control group in the T3 + T4 treated group. The MTU application produced a decrease in T3 and T4 concentration to levels below the lower detection limits of the methods used. The fractional rate of tissue protein synthesis was estimated using the constant intravenous infusion technique with L-14C-leucine and L-14C-lysine simultaneously. The range of the fractional rate of protein synthesis was found to be higher in hyperthyroid than in control animals in all organs under study (liver, small intestine, large intestine, gastrocnemius muscle and pancreas). Protein synthesis was stimulated to the greatest extent in muscle (4,5--5,1% . d-1 in controls vs, 5,5--6,7% . d-1 in hyperthyroid animals). The influence of hypothyroidism on the rate of protein synthesis was uncertain.

The influence of hyperthyroidism and hypothyroidism on the wound healing of experimental myocardial infarction in the rat

In hyperthyroid and hypothyroid rats myocardial infarction was produced by coronary artery ligature. At different times following the ligature the animals were given 3H-thymidine or 3H-proline. The following parameters were determined: the number of DNA- and tropocollagen-synthesizing connective-tissue cells at the infarction border and at the infarction site; the mean silver-grain density above the nuclei or cells; the duration of one cell cycle; and the number of mitoses. The labelling and mitotic indices as well as the percentage of the standard deviation from the mean values were estimated. The following results were obtained: 1. In hyperthyroidism wound healing is accelerated. The number of DNA-synthesizing connective-tissue cells is increased. The cell-cycles are shorter. The release of tropocollagen by the fibroblasts takes place at a strikingly fast rate. 2. In hypothyroidism wound healing is delayed. The 3H-thymidine and mitotic indices are lowered. The release of tropocollagen is slowed down. The granulation tissue contains localized accumulations of mucopolysaccharides.

Biochemistry of hyperthyroidism and hypothyroidism

The thyroid hormones act directly on mitochondria, and thereby control the transformation of the energy derived from oxidations into a form utilizable by the cell. Through their direct actions on mitochondria, the hormones also control indirectly the rate of protein synthesis and thereby the amount of oxidative apparatus in the cell. A rationale for the effects of thyroid hormone excess or deficiency is based upon studies of the mechanism of thyroid hormone action. In hypothyroidism, slow fuel consumption leads to a low output of utilizable energy. In hyperthyroidism, rapid fuel consumption leads to a high energy output, but as efficiency decreases, the utilizable energy produced decreases. Many of the chemical and physical features of these diseases can be reduced to changes in available energy.

Determinants of thyroxine-induced cardiac hypertrophy in mice

Thyroxine was administered to mice to determine whether the cardiac hypertrophy which accompanies experimental thyrotoxicosis is a result of circulatory changes produced by thyroxine, or the result of a hormonal effect on myocardial growth.

When mice were fed both reserpine and thyroxine they developed as much cardiac hypertrophy as those fed thyroxine alone, although the circulatory changes associated with thyrotoxicosis were greatly reduced by reserpine treatment. In addition, when mice were injected with synthetic l -thyroxine, the rate of cardiac protein synthesis was increased before any definite circulatory changes were observed.

The results suggest an effect of thyroid hormone on cardiac protein synthesis that is independent of the effects of the hormone on the general circulation.

Effect of dietary protein level and thyroxine on vitamin A depletion from liver in chicks

1. The effect of dietary protein level and supplementation with thyroxine and thiouracil on depletion of liver stores of vitamin A, on the percentage of liver vitamin A in the alcohol form and on the plasma level of vitamin A and protein has been studied in chicks. In an experiment with cockerels the relation was investigated between the vitamin A level of plasma and the electrophoretic pattern of its proteins.

2. Liver vitamin A depletion was considerably reduced by lowering the dietary protein level.

3. Protein malnutrition lowered the percentage of vitamin A found in the liver in the alcohol form.

4. Depletion of vitamin A from the liver was related to the percentage of vitamin A in the alcohol form.

5. Thyroxine increased both vitamin A depletion from the liver and the percentage of vitamin A in the alcohol form, whereas thiouracil had the opposite effect.

6. Plasma vitamin A, and plasma protein contents were significantly lowered in cockerels receiving a 5% protein diet. The drop in plasma protein level was essentially caused by a reduction of the albumin fraction. No significant change in the levels of globulins was observed even after a 4-week period of protein malnutrition.

7. When the dietary protein level was restored to normal both the albumin and vitamin A levels in plasma increased.

Thyroxine: Effects on Amino Acid Incorporation into Protein in vivo

Treatment of rats with L-thyroxine increases the incorporation in vivo of radioactive amino acids into protein of liver, kidney, and heart, but not of spleen, testis, or brain. The distribution of the effect among the organs is the same as that observed in thyroxine stimulation of oxidative metabolism. Thus, stimulation of protein synthesis seems to be a physiological action of the thyroid hormone.

Reversal of thyroxine-induced hypermetabolism by puromycin

Previous studies have demonstrated that in addition to its effects on metabolic rate, thyroxine stimulates protein biosynthesis. The administration of puromycin, a drug which blocks protein synthesis and, therefore, the thyroxine effect on protein synthesis, acutely reverses the hypermetabolism induced in rats by prior administration of thyroxine and restores the oxygen consumption of the thyrotoxic rats to the euthyroid level. The results suggest that a larger fraction of the total body basal oxygen consumption in hyperthyroidism is related to the process of protein synthesis than in the euthyroid state and that the calorigenic effect of thyroxine is secondary to its effect on protein synthesis.