Dose response of protein turnover in rat skeletal muscle to triiodothyronine treatment

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

Interactions between Growth of Muscle and Stature: Mechanisms Involved and Their Nutritional Sensitivity to Dietary Protein: The Protein-Stat Revisited

Childhood growth and its sensitivity to dietary protein is reviewed within a Protein-Stat model of growth regulation. The coordination of growth of muscle and stature is a combination of genetic programming, and of two-way mechanical interactions involving the mechanotransduction of muscle growth through stretching by bone length growth, the core Protein-Stat feature, and the strengthening of bone through muscle contraction via the mechanostat. Thus, growth in bone length is the initiating event and this is always observed. Endocrine and cellular mechanisms of growth in stature are reviewed in terms of the growth hormone-insulin like growth factor-1 (GH-IGF-1) and thyroid axes and the sex hormones, which together mediate endochondral ossification in the growth plate and bone lengthening. Cellular mechanisms of muscle growth during development are then reviewed identifying (a) the difficulties posed by the need to maintain its ultrastructure during myofibre hypertrophy within the extracellular matrix and the concept of muscle as concentric "bags" allowing growth to be conceived as bag enlargement and filling, (b) the cellular and molecular mechanisms involved in the mechanotransduction of satellite and mesenchymal stromal cells, to enable both connective tissue remodelling and provision of new myonuclei to aid myofibre hypertrophy and (c) the implications of myofibre hypertrophy for protein turnover within the myonuclear domain. Experimental data from rodent and avian animal models illustrate likely changes in DNA domain size and protein turnover during developmental and stretch-induced muscle growth and between different muscle fibre types. Growth of muscle in male rats during adulthood suggests that "bag enlargement" is achieved mainly through the action of mesenchymal stromal cells. Current understanding of the nutritional regulation of protein deposition in muscle, deriving from experimental studies in animals and human adults, is reviewed, identifying regulation by amino acids, insulin and myofibre volume changes acting to increase both ribosomal capacity and efficiency of muscle protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) and the phenomenon of a "bag-full" inhibitory signal has been identified in human skeletal muscle. The final section deals with the nutritional sensitivity of growth of muscle and stature to dietary protein in children. Growth in length/height as a function of dietary protein intake is described in the context of the breastfed child as the normative growth model, and the "Early Protein Hypothesis" linking high protein intakes in infancy to later adiposity. The extensive paediatric studies on serum IGF-1 and child growth are reviewed but their clinical relevance is of limited value for understanding growth regulation; a role in energy metabolism and homeostasis, acting with insulin to mediate adiposity, is probably more important. Information on the influence of dietary protein on muscle mass per se as opposed to lean body mass is limited but suggests that increased protein intake in children is unable to promote muscle growth in excess of that linked to genotypic growth in length/height. One possible exception is milk protein intake, which cohort and cross-cultural studies suggest can increase height and associated muscle growth, although such effects have yet to be demonstrated by randomised controlled trials.

Nutrition, infection and stunting: the roles of deficiencies of individual nutrients and foods, and of inflammation, as determinants of reduced linear growth of children

The regulation of linear growth by nutritional and inflammatory influences is examined in terms of growth-plate endochondral ossification, in order to better understand stunted growth in children. Linear growth is controlled by complex genetic, physiological, and nutrient-sensitive endocrine/paracrine/autocrine mediated molecular signalling mechanisms, possibly including sleep adequacy through its influence on growth hormone secretion. Inflammation, which accompanies most infections and environmental enteric dysfunction, inhibits endochondral ossification through the action of mediators including proinflammatory cytokines, the activin A-follistatin system, glucocorticoids and fibroblast growth factor 21 (FGF21). In animal models linear growth is particularly sensitive to dietary protein as well as Zn intake, which act through insulin, insulin-like growth factor-1 (IGF-1) and its binding proteins, triiodothyronine, amino acids and Zn2+ to stimulate growth-plate protein and proteoglycan synthesis and cell cycle progression, actions which are blocked by corticosteroids and inflammatory cytokines. Observational human studies indicate stunting to be associated with nutritionally poor, mainly plant-based diets. Intervention studies provide some support for deficiencies of energy, protein, Zn and iodine and for multiple micronutrient deficiencies, at least during pregnancy. Of the animal-source foods, only milk has been specifically and repeatedly shown to exert an important influence on linear growth in both undernourished and well-nourished children. However, inflammation, caused by infections, environmental enteric dysfunction, which may be widespread in the absence of clean water, adequate sanitation and hygiene (WASH), and endogenous inflammation associated with excess adiposity, in each case contributes to stunting, and may explain why nutritional interventions are often unsuccessful. Current interventions to reduce stunting are targeting WASH as well as nutrition.

Neonatal hyper- and hypothyroidism alter the myoglobin gene expression program in adulthood

Myoglobin acts as an oxygen store and a reactive oxygen species acceptor in muscles. We examined myoglobin mRNA in rat cardiac ventricle and skeletal muscles during the first 42 days of life and the impact of transient neonatal hypo- and hyperthyroidism on the myoglobin gene expression pattern. Cardiac ventricle and skeletal muscles of Wistar rats at 7-42 days of life were quickly removed, and myoglobin mRNA was determined by Northern blot analysis. Rats were treated with propylthiouracil (5-10 mg/100 g) and triiodothyronine (0.5-50 µg/100 g) for 5, 15, or 30 days after birth to induce hypo- and hyperthyroidism and euthanized either just after treatment or at 90 days. During postnatal (P) days 7-28, the ventricle myoglobin mRNA remained unchanged, but it gradually increased in skeletal muscle (12-fold). Triiodothyronine treatment, from days P0-P5, increased the skeletal muscle myoglobin mRNA 1.5- to 4.5-fold; a 2.5-fold increase was observed in ventricle muscle, but only when triiodothyronine treatment was extended to day P15. Conversely, hypothyroidism at P5 markedly decreased (60%) ventricular myoglobin mRNA. Moreover, transient hyperthyroidism in the neonatal period increased ventricle myoglobin mRNA (2-fold), and decreased heart rate (5%), fast muscle myoglobin mRNA (30%) and body weight (20%) in adulthood. Transient hypothyroidism in the neonatal period also permanently decreased fast muscle myoglobin mRNA (30%) and body weight (14%). These results indicated that changes in triiodothyronine supply in the neonatal period alter the myoglobin expression program in ventricle and skeletal muscle, leading to specific physiological repercussions and alterations in other parameters in adulthood.

Hypothyroid state does not protect but delays neuronal death in the hippocampal CA1 region following transient cerebral ischemia: focus on oxidative stress and gliosis

We investigated protective effects of hypothyroidism on delayed neuronal death, gliosis, lipid peroxidation and Cu,Zn-superoxide dismutase (SOD1) in the gerbil hippocampal CA1 region (CA1) after 5 min of transient cerebral ischemia. The hypothyroidism was induced by 0.025% methimazole treatment. Free triiodothyronine and thyroxine levels were markedly decreased in the hypothyroid group. Four days after ischemia/reperfusion, only a few NeuN-immunoreactive (+) neurons were detected in the CA1 of euthyroid-ischemia (eu-ischemia) group; however, at this time point, the number of NeuN(+) neurons was significantly higher in the hypothyroid-ischemia (hypo-ischemia) group than in the eu-ischemia group. At 5 days postischemia, NeuN(+) neurons were significantly decreased in the hypo-ischemia group: The number of NeuN(+) neurons in this group was similar to that in the eu-ischemia group. Activations of GFAP(+) astrocytes and Iba-1(+) microglia in the CA1 were higher in the eu-ischemia group 3 and 4 days after ischemia/reperfusion. At 5 days postischemia, the activations of both the glial cells in the CA1 were similar between the two groups. 4-Hydroxy-2-nonenal (HNE), a marker for lipid peroxidation, immunoreactivity in the eu-ischemia group was higher than in the hypo-ischemia group; at 5 days postischemia, the immunoreactivity was similar between the two groups. In contrast, SOD1 level was lower in the CA1 of the eu-ischemia group. These results suggest that hypothyroid state does not protect against delayed neuronal death but only delays the neuronal death in the hippocampal CA1 region after transient cerebral ischemia by reducing lipid peroxidation and increasing SOD1.

Metabolic control and future opportunities for growth regulation

The last 10 years have seen a significant expansion in the scope of attempts to manipulate the growth of animals (Buttery, Lindsay and Haynes, 1986). The expansion of interest has been driven by a number of factors, both economic and theoretical. At the economic level the need to develop energetically and economically efficient strategies of animal production has been coupled with a renewed awareness of the implications for human health of excessive intakes of saturated fats. Emphasis then has switched from the maximization of weight gain as an end in itself towards a need to promote protein deposition at any given intake and, at the same time, to reduce the fat content of meat and meat products. These twin objectives might be achieved by one of three strategies: the promotion of protein deposition alone, because at any given rate of weight gain this will tend to minimize the rate of fat deposition (the so-called repartitioning effect); the reduction of fat gain (an approach that has received particularly close attention by those concerned primarily with human obesity); or ideally the simultaneous promotion of protein accretion and depression of that of fat.

Dietary thyroid hormone improves growth and muscle protein accumulation of black-boned chickens

1. The objective of the study was to examine the effect of dietary thyroid hormone on performance and muscle protein accumulation of black-boned chickens. 2. A total of 720 1-d-old birds was housed in 24 pens and assigned to 4 diets containing triiodothyronine (T3) at 0, 0.1, 0.3 or 0.5 mg/kg. 3. The trial was split into starter (0 to 4 weeks) and grower (5 to 8 weeks) phases. At the beginning of the grower phase, each pen was split into two; one subgroup was fed basal diet (T3-) and the other continued with T3-added diet (T3+). 4. Dietary T3 at 0.1 mg/kg improved average daily body weight gain until week 6 but retarded it thereafter. However, T3 at 0.5 mg/kg depressed growth throughout the whole period. During weeks 7 to 8, T3 retarded growth, whereas growth recovered when T3 was withdrawn from the diet. 5. Birds fed the diet containing 0.1 mg/kg during weeks 0 to 4 and those receiving basal diet during weeks 5 to 8 showed the greatest thigh muscle growth, but this growth was depressed when they were fed the 0.5 mg/kg diet during weeks 0 to 8. 6. Serum T3 concentration was increased, while thyroxine (T4) concentration was decreased by dietary T3 treatment at 2 weeks old. At 4 weeks old, T3 and T4 concentrations decreased. At 6 and 8 weeks old, among T3-treated groups the serum T3 concentration of the T3- group was higher than that of the T3+ group. 7. In conclusion, dietary T3 improved weight gain and muscle growth of black-boned chickens before week 6, whereas supplementation depressed growth after 6 weeks. Optimum dietary T3 concentrations showed a carry-over effect on both body weight gain and thigh muscle growth.

Unexpected Peripheral Markers of Thyroid Function in a Patient with a Novel Mutation of the MCT8 Thyroid Hormone Transporter Gene

The specific thyroid hormone transporter, MCT8, located on the X chromosome, has led to the identification a novel syndrome. The objective is to relate phenotype with several tissue-specific thyroid functions. A 1-year-old boy, who had severe psychological damage and low serum T4, had received l-T4 for 3 months. At admission, body length was normal but weight was low. Off therapy, serum TSH was mildly elevated, serum T4 and free T4 were low, and serum T3 and free T3 were high. Direct sequencing of the MCT8 gene revealed a single nucleotide change that resulted in a novel nonsense mutation at codon 261 (Q261X) in exon 3. Since serum T3 was high, peripheral markers of hyperthyroidism were looked for. Bone age was advanced, despite the presence of malnutrition and low T4. Serum SHBG, a marker of thyroid hormone action in liver, was markedly elevated. Markers of skeletal muscle catabolism, ammonemia and lactic acid, were found to be elevated. The phenotype of MCT 8 mutation might be explained by differences in the entry of thyroid hormones into different cells. In the presence of an inactive MCT8 transporter, the high blood T3 levels might not be enough to prevent brain damage early in life, while they seem to be able to induce a postnatal state of peripheral hyperthyroidism in other tissues, such as liver, bone and skeletal muscle.

Pancytopenia in untreated patients with Graves' disease

Severe pancytopenia is a rare but severe complication of thyrotoxicosis. In this report, we describe four patients with Graves' disease who presented with pancytopenia at diagnosis. Methimazole (30-40 mg/d) or propylthiouracil (400 mg/d) restored normal hematopoiesis in three of the patients. The remaining patient evolved to aplastic anemia under therapy with methimazole (60 mg/d), but had an increased peripheral blood count that almost reached normal values after radioiodinetherapy and standard immunosuppressive treatment with antithymocyte globulin (700 mg/d, intravenous infusion for 5 days), oral cyclosporin (400 mg/d), prednisone (30-60 mg/d), and granulocyte colony-stimulating factor (150 microg subcutaneous injection, 3 times per week). We conclude that: (1) a hematologic evaluation of all patients with Graves' disease should be performed before administering antithyroid drugs, (2) antithyroid drugs may be administered to patients with pancytopenia and bone marrow hypercellularity but a reevaluation of the bone marrow must be done if there is no recovery of the peripheral blood cell count when euthyroidism state is achieved, (3) standard immunosuppressive treatment of aplastic anemia caused by antithyroid drugs restores normal hematopoiesis, and (4) a thyroid evaluation of patients with pancytopenia should be done, even though no related symptoms are found.

Decreased cellular uptake and metabolism in Allan-Herndon-Dudley syndrome (AHDS) due to a novel mutation in the MCT8 thyroid hormone transporter

We report a novel 1 bp deletion (c.1834delC) in the MCT8 gene in a large Brazilian family with Allan-Herndon-Dudley syndrome (AHDS), an X linked condition characterised by severe mental retardation and neurological dysfunction. The c.1834delC segregates with the disease in this family and it was not present in 100 control chromosomes, further confirming its pathogenicity. This mutation causes a frameshift and the inclusion of 64 additional amino acids in the C-terminal region of the protein. Pathogenic mutations in the MCT8 gene, which encodes a thyroid hormone transporter, results in elevated serum triiodothyronine (T3) levels, which were confirmed in four affected males of this family, while normal levels were found among obligate carriers. Through in vitro functional assays, we showed that this mutation decreases cellular T3 uptake and intracellular T3 metabolism. Therefore, the severe neurological defects present in the patients are due not only to deficiency of intracellular T3, but also to altered metabolism of T3 in central neurones. In addition, the severe muscle hypoplasia observed in most AHDS patients may be a consequence of high serum T3 levels.

Triiodothyronine but not thyroxine accelerates myofibrillar proteolysis via ATP production in cultured muscle cells

These experiments were done to clarify that the differential effects of thyroxine (T(4)) and triiodothyronine (T(3)) on skeletal muscle protein turnover are caused by their roles on ATP production. Primary cultured chick muscle cells were treated with a physiological level of T(4) (60 ng/ml), T(3) (12 ng/ml), or ATP (0.5 mM) for 6 days and the protein content, ATP production, proteasome activity, and myofibrillar protein breakdown were measured. The protein content measured as an index of cell growth was not affected by T(4), T(3), or ATP. The cellular ATP level was increased by T(3) and ATP, but not by T(4). Proteasome activity and N(tau)-methylhistidine (MeHis) release measured as an index of myofiblillar protein breakdown was also increased by T(3) and ATP, but not by T(4). These results indicate that T(3) but not T(4) increases ATP production followed by an increase in proteasome activity, and thus stimulates myofibrillar proteolysis.

Ischemic injury and repair process after transection in hypothyroid rat muscles

Hindlimb ischemia for 4 h, followed by reperfusion, resulted in necrosis of most soleus muscle in euthyroid rats, whereas only slight damage occurred in hypothyroid rats. Muscle repair after transection of the tibialis anterior muscle of hypothyroid rats showed delayed debris removal, initial retardation of myotube formation, and a higher incidence of aberrant sarcomeres in newly formed muscle fibers by electron microscopy. The protective mechanism against ischemia in hypothyroid muscles can probably be attributed to decreased degradation of high-energy phosphates, reduced formation of substrates for xanthine oxidase during ischemia, and attenuated generation of harmful oxygen free radicals during reperfusion. Initial delay of myotube formation seems to reflect retarded proliferation of muscle precursor cells. Prolonged occurrence of aberrant sarcomeres in hypothyroidism is perhaps due to a delay or imbalance in the synthesis of proteins that assemble sarcomeres. These findings demonstrate the significant roles of thyroid hormones in ischemic injury and muscle repair.

Experimental hyperthyroidism causes inactivation of the branched-chain alpha-ketoacid dehydrogenase complex in rat liver

Hyperthyroidism induced by 3-day treatment of rats with thyroid hormone (T(3); 3,5,3'-triiodothyronine) at 0.1 or 1 mg/kg body wt/day resulted in a reduced activity state (% of enzyme in its active, dephosphorylated state) of the hepatic branched-chain alpha-ketoacid dehydrogenase (BCKDH) complex. One treatment with 0.1 mg T(3)/kg body wt caused a significant effect on the activity state of BCKDH complex after 24 h, indicating that the reduction of the activity state was triggered by the first administration of T(3). Hyperthyroidism also caused a stable increase in BCKDH kinase activity, the enzyme responsible for phosphorylation and inactivation of the BCKDH complex, suggesting that T(3) caused inactivation of the BCKDH complex by induction of its kinase. Western blot analysis also revealed increased amounts of BCKDH kinase protein in response to hyperthyroidism. No change in the plasma levels of branched-chain alpha-keto acids was observed in T(3)-treated rats, arguing against an involvement of these known regulators of BCKDH kinase activity. Inactivation of the hepatic BCKDH complex as a consequence of overexpression of its kinase may save the essential branched-chain amino acids for protein synthesis during hyperthyroidism.

Changes in satellite cell content and myosin isoforms in low-frequency-stimulated fast muscle of hypothyroid rat

Chronic low-frequency stimulation was used to study the effects of enhanced contractile activity on satellite cell content and myosin isoform expression in extensor digitorum longus muscles from hypothyroid rats. As verified by immunohistochemical staining for desmin, vimentin, and myosin heavy chain (MHC) isoforms and by histological analysis, stimulation induced a transformation of existing fast fibers toward slower fibers without signs of fiber deterioration or regeneration. Immunohistochemically detected increases in MHC I and MHC IIa isoforms, as well as reduced numbers of fibers expressing the faster MHC isoforms, mirrored the rearrangement of the thick-filament composition. These changes, especially the upregulation of MHC IIa, were accompanied by an induction of developmental MHC isoforms in the transforming adult fibers. Satellite cell content rose 2.6-, 3.0-, and 3.7-fold over that of corresponding controls (P < 0.05 in all cases) in 5-, 10-, and 20-day-stimulated muscles, respectively. Hypothyroidism alone had no effect on satellite cell content but resulted in a significant reduction in fiber size. The relative satellite cell contents increased (P < 0.05) from 3.8% in euthyroid control muscles to 7.9, 11.5, and 13.8% in the 5-, 10-, and 20-day-stimulated hypothyroid muscles, respectively. In 20-day-stimulated muscles, the relative satellite cell content reached an almost twofold higher level than that of normal slow-twitch soleus muscle. This increase occurred concomitantly with a rise in myonuclear density, most probably because of the fusion of satellite cells with existing fibers.

Comparison of the effects of thyroxine and triiodothyronine on protein turnover and apoptosis in primary chick muscle cell cultures

Primary chick muscle cells were treated with physiological level of thyroxine (T4) or triiodothyronine (T3) to examine the effects of the hormones on growth, protein turnover, and apoptosis of the cells. Creatine kinase activity, as an index of differentiation, was increased by both T4 and T3. Even when the conversion from T4 to T3 was blocked by iopanoic acid, T4 increased creatine kinase activity. The rate of protein degradation estimated from [3H] tyrosine release was increased by T3 but not by T4. DNA cleavage and fragmentation, as indices of apoptosis, were induced by T3 but not by T4. These results show that T4 stimulates cell differentiation but not protein degradation and apoptosis in primary chick muscle cells, while all events are stimulated by T3.

Lowering of plasma triiodothyronine level and sympathetic activity does not alter hypoalbuminaemia in rats fed on a diet of low protein concentration

Previous studies have described high plasma triiodothyronine (T3) concentrations and sympathetic activity in rats fed on low-protein diets. The present investigation examined how the nutritional status of rats fed on a low-protein diet was affected when these hormonal changes were reduced by drug administration. The low-protein diet (LP group) prevented growth, reduced plasma albumin levels, elevated plasma T3 concentration, and increased both the weight of the interscapular brown adipose tissue (BAT) and the activity of BAT cytochrome c oxidase (EC 1.9.3.1). Lowering the plasma T3 concentration (with carbimazole; CA group) elevated the plasma insulin concentration, promoted a small increase in the plasma albumin concentration and caused weight gain in comparison with the LP group. Reduction of sympathetic activity (with alpha-methyl-p-tyrosine; MT group) promoted a small elevation in plasma albumin concentration accompanied by a diminished T3 concentration, BAT weight, and an increase in fat deposition in relation to LP rats. In a second experiment, simultaneous lowering of the plasma T3 concentration and sympathetic activity (CA/MT group) resulted in weight gain associated with elevated plasma insulin concentration and fat deposition and a marked reduction in BAT cytochrome c oxidase activity. However no change in the hypoalbuminaemia was observed. The results of the present study suggest that in spite of the previously described increase in metabolic rate in fed on a diet with low-protein concentration when compared with controls, the mechanisms involved in the control of BAT activity and fat deposition seem to be independent of those which cause liver protein depletion and hypoalbuminaemia.

Role of N-acetylglutamate turnover in urea synthesis by rats treated with the thyroid hormone

We determined whether the synthesis and degradation of N-acetylglutamate would regulate urea synthesis when the thyroid status was manipulated. Experiments were done on three groups of rats, each being given 6-propyl-2-thiouracil (PTU, a thyroid inhibitor) without a triiodothyronine (T3) treatment, treated with PTU + T3, or receiving neither PTU nor T3 (control). The plasma concentration and urinary excretion of urea, the liver concentration of N-acetylglutamate, and the liver N-acetylglutamate synthesis in rats given PTU alone were each significantly higher than in the control rats. Compared with the control rats, the liver N-acetylglutamate degradation was significantly lower in those rats given PTU without the T3 treatment. Treatment of the PTU-treated rats with T3 reversed the effects of PTU to the values of the control rats. N-Acetylglutamate synthesis in the liver was closely correlated with the excretion of urea, and inverse correlation between the liver N-acetylglutamate degradation and urea excretion was found. These results suggest that the greater synthesis and lower degradation of N-acetylglutamate in the hypothyroid (PTU alone) rats would be likely to increase the hepatic concentration of this compound and stimulate urea synthesis.

The hormonal control of protein metabolism

While all the hormones described have regulatory effects on the rates of protein synthesis and breakdown there is a complex interaction between them in this control process. Insulin, GH and IGF-I play a dominant role in the day-to-day regulation of protein metabolism. In humans insulin appears to act primarily to inhibit proteolysis while GH stimulates protein synthesis. In the post-absorptive state IGF-I has acute insulin-like effects on proteolysis but in the fed state, or when substrate is provided for protein synthesis in the form of an amino acid infusion, IGF-I has been shown to stimulate protein synthesis. Growth hormone and testosterone have an important role during growth but continue to be required to maintain body protein during adulthood. Thyroid hormones are also required for normal growth and development. The hormones glucagon, glucocorticoids and adrenaline are all increased in catabolic states and may work in concert to increase protein breakdown in muscle tissue and to increase amino acid uptake in liver for gluconeogenesis. While increased glucocorticoids result in reduced muscle mass the effects of glucagon may be predominantly in the liver resulting in increased uptake of amino acids. In contrast to the catabolic effect of adrenaline on glucose and lipid metabolism, studies to date suggest that adrenaline may have an anti-catabolic effect on protein metabolism. Despite this adrenaline increases the production of the gluconeogenic amino acid alanine by muscle and its uptake by the splanchnic bed. There is considerable interest in the use of anabolic hormones, either alone or in combination, in the treatment of catabolic states. GH combined with insulin has been shown to improve whole-body and skeletal muscle kinetics while GH combined with IGF-I has a greater positive effect on protein metabolism in catabolic states than either hormone alone. If catabolic states are to be treated successfully a greater understanding of the role of the catabolic hormones in these states and the possible treatment of these states with anabolic hormones is required.

Response of leucine metabolism to hyperinsulinemia under amino acid replacement in experimental hyperthyroidism

We investigated the responsiveness of protein metabolism to insulin as a mediator of the protein catabolic response to hyperthyroidism in humans. Six healthy volunteers were studied in a postabsorptive state before and after oral intake of thyroid hormones (2 micrograms.kg-1.day-1 L-thyroxine for 6 wk along with 1 microgram.kg-1.day-1 triiodothyronine for the last 2 wk). Insulin was infused at 7.14 nmol.kg-1.min-1 for 140 min under euglycemic and eukalemic clamps. An appropriate amino acid infusion was used to blunt insulin-induced hypoaminoacidemia. Leucine kinetics were assessed using a primed continuous infusion of L-[1-13C]leucine. Hyperthyroidism induced a significant increase (P < 0.05) in leucine endogenous appearance rate (a reflection of proteolysis; 2.15 +/- 0.06 vs. 1.76 +/- 0.03 mumol.kg-1.min-1 in the control state), oxidation (0.54 +/- 0.04 vs. 0.47 +/- 0.07), and nonoxidative disposal (a reflection of protein synthesis; 1.80 +/- 0.06 vs. 1.45 +/- 0.06). Insulin lowered proteolysis. Further hyperthyroidism improved the ability of insulin to inhibit proteolysis, whether considered as an absolute decrease (-0.57 +/- 0.02 vs. -0.45 +/- 0.05 mumol.kg-1.min-1, P < 0.05) or related to insulinemia [1.59 +/- 0.11 vs. 1.01 +/- 0.08 mumol leucine.kg-1.min-1/(nmol insulin/l), P < 0.05]. Insulin also moderately (but significantly P < 0.05) lowered protein synthesis in both control and hyperthyroid states. These changes in insulin action may provide a mechanism to save body protein during hyperthyroidism.

Differential regulation of the expression of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase by thyroid hormone and insulin-like growth factor-I in the L6 muscle cell line

The aim of this study was to investigate the mechanism(s) underlying the thyroid-hormone (L-tri-iodothyronine, T3)-induced elevation of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase (SERCA1) levels in L6 myotubes and the potentiating effect of insulin-like growth factor-I (IGF-I) [Muller, van Hardeveld, Simonides and van Rijn (1991) Biochem. J. 275, 35-40]. T3 increased the SERCA1 protein level (per microgram of DNA) by 160%. The concomitant increase in the SERCA1 mRNA level was somewhat higher (240%). IGF-I also increased SERCA1 protein (110%) and mRNA levels (50%), whereas IGF-I + T3 increased SERCA1 protein and mRNA levels by 410% and 380% respectively. These SERCA1 mRNA analyses show that the more-than-additive action of T3 and IGF-I on SERCA1 expression is, at least in part, pre-translational in nature. Further studies showed that the half-life of SERCA1 protein in L6 cells (17.5 h) was not altered by T3. In contrast, IGF-I prolonged the half-life of SERCA1 protein 1.5-1.9-fold, which may contribute to the disproportional increase in SERCA1 protein content compared with mRNA by IGF-I. Measurements of SERCA1 mRNA half-life (as determined by actinomycin D chase) showed no difference from the control values (15.5 h) in the presence of T3 or IGF-I alone. When T3 and IGF-I were both present, the SERCA1 mRNA half-life was prolonged 2-fold. No significant effects of T3 and IGF-I were observed on the half-life of total protein (37.4 h) and total RNA (37.0 h). The absence of an effect of T3 on SERCA1 protein and mRNA stability, when it was present alone, suggested transcriptional regulation, which was confirmed by nuclear run-on experiments, showing a 3-fold increase in transcription frequency of the SERCA1 gene by T3. We conclude that the synergistic stimulating effects of T3 and IGF-I on SERCA1 expression are the result of both transcriptional and post-transcriptional regulation. T3 acts primarily at the transcriptional level by increasing the transcription frequency of the SERCA1 gene, whereas IGF-I seems to act predominantly at post-transcriptional levels by enhancing SERCA1 protein and mRNA stability, the latter, however, only in the presence of T3.

Thyroid-induced changes in the growth of the liver, kidney, and diaphragm of neonatal rats

Eu-, hypo- and hyper-thyroid rats were studied 12 days postpartum. Hypothyroidism was induced by administering propylthiouracil (PTU) via the mother's drinking water between late gestation and throughout lactation. This procedure effectively blocked the normal early postnatal surge of T3 and T4. In contrast, hyperthyroidism was induced in the young pups by daily injections of T4 from day 3 postpartum. The effects of these experimental manipulations of thyroid status on the rates of protein turnover and growth of the liver, kidney, and diaphragm were studied and compared with measurements made on appropriate euthyroid control tissues. Tissue rates of protein synthesis were decreased in response to hypothyroidism with consequent growth retardation of all three tissues and the whole animal. In contrast, the three body tissues responded very differently to the induction of hyperthyroidism. Hepatic rates of protein synthesis and growth were completely unaffected by thyroid excess. The response of the diaphragm was essentially the reverse of that seen with hypothyroidism, i.e., the enhanced rates of protein synthesis and protein degradation leading to muscle hypertrophy. The rates of protein turnover in the kidney were also increased, but unlike the diaphragm the net result was renal atrophy. Clearly, thyroid hormones influence the normal rapid growth of the neonate and its individual tissues. However, beyond a certain concentration the threshold of responsiveness to these hormones seems to vary between individual tissues.

Relationships between the thyroid and somatotropic axes in steers. II: Effects of thyroid status on plasma concentrations of insulin-like growth factor I (IGF-I) and the IGF-I response to growth hormone

Three studies assessed the effect of thyroid status on regulation of plasma IGF-I in cattle. First, four Angus-Hereford steers (av wt 345 kg) were fed 4 mg/d propylthiouracil daily for 35 d. With continued feeding of PTU steers were sequentially injected with thyroxine (T4, 5 mg/d, IM for 5 d) followed by triiodothyronine (T3, 2 mg/d, IM for 5 d). An injection of bovine pituitary growth hormone (GH, 0.1 mg/kg, IM) was given to each steer on day 35 of PTU, day 5 of T4 and again on day 5 of T3. PTU alone increased plasma thyroid stimulating hormone (TSH), decreased plasma T4 and T3 but had no influence on IGF-I. T3, but not T4, lowered plasma TSH, IGF-I and the IGF-I response to GH (P < .05). Next, twelve bull calves (av wht 167 kg) were divided equally into two groups. A control group was injected daily for five d with buffered saline; the experimental group was concurrently treated with T3 (5 mg/d, sc) for five d. Beginning the sixth day, all calves were injected with GH (0.1 mg/kg, IM daily) for three d with the respective buffer or T3 treatments continuing. Plasma IGF-I was depressed 29% by T3. The incremental area under the three-d response curve was less (P < .03) in T3 cattle. A growth trial was conducted in which twenty-four Angus x Hereford steers were injected daily with T3 (2 mg/kg, bi-daily x 56 d) or implanted with Synovex-S (S) in a 2 x 2 factorial arrangement. Synovex increased empty body protein gain (EBPG) and plasma IGF-I 15.5 and 27.9% (P < .01), respectively; T3 decreased EBPG and plasma IGF-I 13.9 and 15.1% (P < .07), respectively, in steers which maintained suppression in plasma TSH. The data support the conclusion that elevated T3 decreases plasma IGF-I, in part, through a diminished GH-responsiveness and anabolic treatments such as S can reverse the effects of excess T3.

Effects of thyroxine on the expression of alcohol dehydrogenase in rat liver and kidney

We studied the effect of thyroxine on alcohol dehydrogenase activity, immunoreactive protein levels and messenger RNA levels in the livers of thyroidectomized and sham-operated male rats. Effects on kidney alcohol dehydrogenase activity were also examined. Sham-operated rats injected with 100 micrograms thyroxine/kg/day, which induced hyperthyroidism, showed a 30% decrease in liver and a 40% decrease in kidney alcohol dehydrogenase activity compared with sham-operated rats injected with vehicle. Hypothyroid rats exhibited a 1.5-fold increase in alcohol dehydrogenase activity in liver and kidney compared with thyroidectomized rats injected with a replacement dose of 20 micrograms thyroxine/kg/day. We saw a twofold and a 2.5-fold higher level of alcohol dehydrogenase activity in liver and kidney, respectively, of hypothyroid rats compared with hyperthyroid rats. These effects were not accounted for by nutritional differences; daily food intake did not differ between groups. Immunoreactive protein levels as seen on Western blots varied in the same direction as enzyme activity. Northern-blot analysis showed higher levels of liver alcohol dehydrogenase messenger RNA in hypothyroid rats compared with euthyroid rats. These studies show that liver alcohol dehydrogenase activity and protein levels are modulated by thyroxine at pathophysiologically relevant levels and that this effect is not due to changes in food intake; kidney alcohol dehydrogenase activity is regulated in parallel. The change in alcohol dehydrogenase activity appears to be controlled in part by pretranslational mechanisms in hypothyroid animals and by posttranslational mechanisms in hyperthyroid animals.

Comparison of the effects of cassava (Manihot esculenta Crantz) organic cyanide and inorganic cyanide on muscle and bone development in a Nigerian breed of dog

Effects of cassava (Manihot esculenta Crantz)-borne organic cyanide and inorganic cyanide in the form of sodium cyanide on bone and muscle development were investigated in eighteen dogs of Nigerian breed. After 16 weeks of stabilization in the laboratory from the time of purchase when the dogs were fed on the same diet, they were randomly assigned to three experimental groups of six dogs each. The control group was fed on rice while the other two groups were fed on either cassava (gari) or rice plus cyanide. The three diets were made isoenergetic and isonitrogenous by varying the quantity of meat incorporated into them. The results obtained after 14 weeks of feeding the respective diets indicated that there was retardation of muscle development in the gari-fed dogs. This may have resulted from gluconeogenesis from muscle protein associated with suppression of production of insulin by the pancreas in this group. The results indicated also that the effects of inorganic dietary cyanides on muscle development were different. Both forms of dietary cyanides, however, had no adverse effect on bone development.

Insulin- and thyroid hormone-independent adaptation of myofibrillar proteolysis to glucocorticoids

Myofibrillar protein breakdown in skeletal muscle progresses through two distinct phases in response to chronic glucocorticoid administration in the rat, i.e., an early phase lasting 4-5 days, during which proteolysis increases followed by a later phase during which proteolysis decreases. The possible involvement of insulin and the iodothyronines in this phenomenon has now been examined. Diabetic, thyroidectomized, and normal rats were treated with corticosteroid for 10-11 days, and at timed intervals muscle proteolysis was evaluated by measuring the release of 3-methyl-L-histidine (3-MH) and tyrosine from the perfused hindquarter as well as the excretion of 3-MH in the urine. Corticosterone (CTC) administration to normal rats increased plasma insulin, whereas plasma 3,5,3'-triiodothyronine responded with an early rise followed by a fall after 4-5 days. However, the biphasic response of myofibrillar proteolysis to chronic glucocorticoid treatment was not abolished in CTC-treated diabetic or thyroidectomized rats. CTC treatment increased release of tyrosine by perfused muscle of diabetic rats but, unlike 3-MH release, did not diminish later. Thus the adaptation of myofibrillar proteolysis to chronic glucocorticoid treatment appears to be independent of insulin and thyroid hormones. However, insulin may play a role in curtailing glucocorticoid-induced breakdown of nonmyofibrillar proteins.

Effects of cimaterol on muscle protein metabolism and its actions in hypothyroid and hyperthyroid rats

Objectives were to examine mechanisms underlying anabolic actions of cimaterol in skeletal muscle and to evaluate cimaterol's actions in hypothyroid and hyperthyroid rats. In the first study growing rats were fed either a control diet or a diet containing cimaterol for 10 days. In a second study sham-thyroidectomized and thyroidectomized (Tx) rats were assigned to one of 5 treatments: control (sham-Tx), Tx, Tx supplemented with cimaterol, Tx injected with triiodothyronine (T3), and Tx rats injected with T3 and supplemented with cimaterol. Effects of treatments on growth, muscle weights and urinary NT-methylhistidine (NMH) excretion were evaluated in both trials. Muscle was also collected for determinations of DNA, RNA, protein and activities of several proteolytic enzymes. Cimaterol caused muscle hypertrophy and increased urinary NMH excretion. Hence, anabolic actions of cimaterol may result from an increase in myofibrillar protein synthesis which exceeds changes in myofibrillar protein degradation. Urinary NMH excretion was reduced by thyroidectomy and increased in hyperthyroid rats and both hypothyroidism and hyperthyroidism were characterized by myopathy. Cimaterol increased muscle weights in hypothyroid but not in hyperthyroid rats. Therefore, cimaterol's anabolic properties are thyroid hormone-independent and antagonized by excess thyroid hormone. Anabolic actions of cimaterol in hypothyroid rat muscle were attributed to an action on protein synthesis because urinary NMH excretion was not affected by cimaterol but muscle RNA concentration was increased. Activities of cathepsins B, D and L and neutral proteinase were dose-related to thyroid status, however, were unrelated to cimaterol-dependent perturbations in NMH excretion. Control of muscle protein balance by thyroid hormones may involve regulation of these enzymes; however, control of muscle protein degradation by cimaterol is likely directed towards other proteolytic mechanisms or to mechanisms which alter susceptibility of myofibrillar proteins to degradation.

Effect of dietary thyroxine on growth and muscle protein metabolism in broiler chickens

The effects of thyroxine (T4) on growth, feed efficiency, breast muscle (Musculus pectoralis profundus) weight, liver weight, abdominal fat content, and rates of muscle protein synthesis (Ks) and breakdown (Kd) estimated by N tau-methylhistidine excretion were investigated in broiler chickens of both sexes. Dietary inclusion of .4 ppm T4 resulted in a less abdominal fat content in female chickens and a higher muscle protein synthesis rate in male chickens. Administration of T4 at the level of 1.2 ppm produced higher feed efficiency in both sexes and greater muscle weight and muscle protein synthesis rate in male chickens. This treatment also produced less abdominal fat content in female chickens. Administration of T4 at the level of 3.6 ppm produced lower body weight gain, breast muscle weight, liver weight, and abdominal fat content in both sexes and both higher protein synthesis and breakdown in skeletal muscle in male chickens. These experiments demonstrated that the abdominal fat content could be successfully reduced by dietary inclusion of T4, especially in female broiler chickens. The findings in the present investigation suggested that the improved feed efficiency and muscle growth were largely accomplished by the increase in muscle protein synthesis induced by the dietary inclusion of thyroxine.

Muscle glutamine concentration and protein turnover in vivo in malnutrition and in endotoxemia

A comparison of the changes in the concentration of glutamine [Gln] in skeletal muscle in a variety of catabolic states with the attendant changes in rates of protein synthesis and degradation indicates a number of substantial correlations which provide insight into both the way in which [Gln] is regulated in muscle and possible regulatory influences of [Gln] on protein balance. There is a striking direct correlation between [Gln] and the rate of protein synthesis in the whole data set. Further examination of this relationship in protein deficiency shows that the changes in [Gln] correlate mainly with the reductions in ribosomal concentration (RNA/protein) and with the decrease in the rate of protein degradation. Because the fall in [Gln] in protein deficiency is also correlated with the decrease in free T3 concentrations, it is suggested that in this case the correlations of [Gln] with rates of protein turnover may be incidental, reflecting thyroidal influences on both protein turnover and glutamine transport. In contrast, in endotoxemia the changes in [Gln] were highly correlated with the ribosomal activity, kRNA, and in this case [Gln] was inversely correlated with the rate of protein degradation. Similar correlated changes occur in starvation and in response to glucocorticoids, and it is suggested that the reductions in [Gln] in endotoxemia could be causally related to the development of insulin resistance and the inhibition of the translational phase of protein synthesis which occurs in these circumstances. The mechanism of the reduction in [Gln] and any linked inhibition of protein synthesis is unknown, but it is shown to be independent of prostaglandin production.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of muscle protein turnover: possible implications for modifying the responses to trauma and nutrient intake

The physiological control of muscle protein balance has been reviewed. In addition to trauma, fasting and reduced activity have been shown to cause muscle protein loss through changes in synthesis and breakdown. Many of the effects of these states are mediated by alterations in the concentrations of insulin, glucagon, steroids and catecholamines. Branched-chain amino acids also appear to have specific effects in improving protein synthesis. Recently, prostaglandins have been identified as having a central role as mediators in the control of protein metabolism by many hormones and pathological states. Identification of factors which control muscle protein synthesis leads to the possibility that the metabolic response to illness and injury and its attendant muscle protein loss could be open to pharmacological manipulation. Inhibition of prostaglandin synthesis by non-steroidal anti-inflammatory drugs can improve muscle protein turnover, but their clinical usefulness may be limited by side-effects. Hormonal manipulation may offer the possibility of abolishing the metabolic response. For example, inhibition of adrenal secretion in surgical patients by spinal anaesthesia appears to modify many of the metabolic effects of injury. A variety of other treatments have been used to minimize the metabolic derangements of injury. Some of these have considerable potential, but as yet clinical benefits from their use have not been positively identified. It is likely that a pharmacological approach to the nutritional disorders of stress and injury will prove to be of major interest in the future.

Effect of low ambient temperature on protein turnover and heat production in chicks

1. Effect of low ambient temperature on protein turnover in the liver and whole body was investigated in chicks together with the contribution of protein synthesis to the total heat production. 2. Both protein synthesis and degradation in the whole body were increased, the latter to a larger extent, at low ambient temperature (LT, 22 degrees C) compared with adequate temperature (AT, 30 degrees C). Liver protein synthesis was not significantly altered by the temperature treatment. 3. The total heat production of LT group was as high as 160% of the AT group. 4. The increased heat production due to enhanced whole-body protein synthesis accounted for only 1.4% of the heat increment in thermogenesis at low ambient temperature, suggesting that protein synthesis would contribute little, if any, to cold-induced thermogenesis in chicks.

Two-week stimulation or blockade of the sympathetic nervous system in man: influence on body weight, body composition, and twenty four-hour energy expenditure

Seven lean healthy young men were studied for 6 weeks during exposure to pharmacologic inhibition or stimulation of the sympathetic nervous system. For a period of 2 weeks their beta-adrenergic receptors were either blocked with propranolol hydrochloride (160 mg/d) or stimulated with terbutaline sulphate (15 mg/d). After a further 2 weeks of placebo administration (500 mg lactose/d), the subjects crossed over to the drug they had not been taking at the beginning of the experiment for another 14 days. During the last five days of each 2-week period, the subjects consumed a weight-maintaining diet, composed of 12% protein, 48% carbohydrate, and 40% fat. They consumed exactly the same menus on the same days during the subsequent study periods. Body weight and physical activity were measured every day for 6 weeks. Daily heart rate and nitrogen excretion were measured continuously for days at the end of each 2-week period, the last two days of which were spent in a respiration chamber where energy expenditure and a variety of metabolic parameters were measured. In the respiration chamber on the propranolol, placebo, and terbutaline treatments, respectively, significant differences were observed in mean daily heart rate (65 +/- 3, 75 +/- 4, and 84 +/- 4 beats/min), mean sleeping heart rate (51 +/- 2, 56 +/- 3, and 62 +/- 3 beats/min), nitrogen excretion (13.6 +/- 0.7, 12.6 +/- 0.6, and 11.9 +/- 0.6 g/d), fat oxidation (+1,045 +/- 95, +1,243 +/- 148, and +1,278 +/- 84 kcal/d) and thyroid hormones (12.0 +/- 0.7, 15.7 +/- 0.9, and 17.2 +/- 1.0 T3/T4 ratio).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of graded doses of tri-iodothyronine on ventricular myosin ATPase activity and isomyosin profile in young and old rats

Ventricular myosin ATPase activity, V1 isomyosin content and serum T3 (tri-iodothyronine) values decrease with age in male Fischer 344 rats. To determine if the age decrement in ATPase activity and V1 isomyosin content are caused by decreased T3 levels or an age-related decrease in V1 isomyosin induction by T3, 3-, 12- and 24-month-old male Fischer 344 rats were given constant T3 infusions by osmotic minipump. Rats at all ages were given 0.75, 5 and 15 micrograms(/100 g per 24 h) doses of T3, whereas 12- and 24-month-old rats were given an additional 0.4 microgram dose. In control rats, T3 levels decreased from 97 +/- 2.7 at 3 months to 75 +/- 4.7 ng/100 ml at 24 months. Likewise, Ca2+-activated myosin ATPase activity decreased from 1.04 +/- 0.05 to 0.68 +/- 0.05 mumol of Pi/min per mg of protein, and the relative proportion of V1 of isomyosin decreased from 90 +/- 4.0 to 26 +/- 2.0%. The lowest (0.4 microgram) T3 dose, which was sufficient to restore T3 levels in 24-month-old animals to 3-month control values, abolished the age decrement in myosin ATPase activity and markedly increased the proportion of V1 isomyosin present in the ventricle. These findings indicate that the senescent ventricle responds readily to small doses of T3 and strongly suggest that the age decrement in serum T3 levels is sufficient to contribute to the age-related decrease in myosin ATPase activity and V1 isomyosin content. Since these parameters correlate with ventricular contractility, the age decrement in T3 levels may also contribute to the decreased ventricular contractility and cardiac output observed in senescent rats.

Studies on the possible role of thyroid hormone in altered muscle protein turnover during sepsis

Five days after thyroidectomy (Tx) or sham-Tx in young male Sprague-Dawley rats, sepsis was induced by cecal ligation and puncture (CLP). Control animals underwent laparotomy and manipulation of the cecum without ligation or puncture. Sixteen hours after CLP or laparotomy, protein synthesis and degradation were measured in incubated extensor digitorum longus (EDL) and soleus (SOL) muscles by determining rate of 14C-phenylalanine incorporation into protein and tyrosine release into incubation medium, respectively. Triiodothyronine (T3) was measured in serum and muscle tissue. Protein synthesis was reduced by 39% and 22% in EDL and SOL, respectively, 16 hours after CLP in sham-Tx rats. The response to sepsis of protein synthesis was abolished in Tx rats. Protein breakdown was increased by 113% and 68% in EDL and SOL, respectively, 16 hours after CLP in sham-Tx animals. The increase in muscle proteolysis during sepsis was blunted in hypothyroid animals and was 42% and 49% in EDL and SOL, respectively. T3 in serum was reduced by sepsis, both in Tx and sham-Tx rats. T3 in muscle, however, was maintained or increased during sepsis. Abolished or blunted response of muscle protein turnover after CLP in hypothyroid animals may reflect a role of thyroid hormones in altered muscle protein metabolism during sepsis. Reduced serum levels of T3, but maintained or increased muscle concentrations of the hormone, suggests that increased T3 uptake by muscle may be one mechanism of low T3 syndrome in sepsis, further supporting the concept of a role for thyroid hormone in metabolic alterations in muscle during sepsis.

Regulation of Ca2+-dependent protein turnover in skeletal muscle by thyroxine

Dantrolene, an agent that inhibits Ca2+ mobilization, improved protein balance in skeletal muscle, as thyroid status was increased, by altering rates of protein synthesis and degradation. Thyroxine (T4) caused increases in protein degradation that were blocked by leupeptin, a proteinase inhibitor previously shown to inhibit Ca2+-dependent non-lysosomal proteolysis in these muscles. In addition, T4 abolished sensitivity to the lysosomotropic agent methylamine and the autophagy inhibitor 3-methyladenine, suggesting that T4 inhibits autophagic/lysosomal proteolysis.

Synergism of triiodothyronine and corticosterone on muscle protein breakdown

The concerted effect of triiodothyronine (T3) and corticosterone on muscle protein synthesis and breakdown was studied. Thyroidectomized young male rats were treated with T3 (1.5 microgram/100 g body weight per day), corticosterone (10 mg/100 g body weight per day) and both T3 and corticosterone for 4 days. On the 3rd day of the experiment urine was collected to measure N tau-methylhistidine excretion as an index of muscle protein breakdown. On the last day of the experiment, the rates of protein synthesis in skeletal muscles were measured by the large-dose [3H]phenylalanine method. N tau-Methylhistidine excretion was slightly increased by T3 treatment and it was increased about 3-times by corticosterone treatment. When both T3 and corticosterone were administered, it was increased about 6-fold. The rate of muscle protein breakdown calculated from the difference between the rate of protein synthesis and the growth rate was consistent with these findings. The rate of muscle protein synthesis was increased by T3, and it was decreased by corticosterone. The rate was the same as that of the thyroidectomized control group when the animals were given T3 and corticosterone, showing that T3 restrained the inhibiting effect of corticosterone on muscle protein synthesis. The results indicate that a physiological level of T3 enhances the catabolic action of pharmacological doses of glucocorticoids on muscle protein breakdown.

Ontogenesis of triiodothyronine nuclear receptors in three skeletal muscles in male and female chicks

The ontogenesis of nuclear triiodothyronine receptors was determined in the pectoralis (alpha W fibers: fast contracting, glycolytic metabolism), adductor brevis (alpha R fibers: fast contracting, oxido-glycolytic metabolism) and adductor magnus (beta R fibers: slow contracting, oxidative metabolism), muscles of male and female chickens at 18 days in ovo and 0, 6, and 30 days ex ovo. In the fast muscles (adductor brevis and pectoralis major), the T3 receptor number decreases from the 18th day of incubation to hatching or the 6th day after hatching, respectively, and then increases. In the slow muscle (adductor magnus), the T3 receptor number increases from 18 days in ovo to hatching and then decreases. At 30 days after hatching, the number of T3 receptors was higher in the fast muscles than in the slow one. A comparison of the two fast muscles reveals that the number of T3 receptors was more elevated in the pectoralis (glycolytic) fibers than in the adductor brevis (oxido-glycolytic) fibers. An overall test of significance showed a higher number of T3 nuclear receptors in muscles from females than from males.

The effects of endotoxaemia on protein metabolism in skeletal muscle and liver of fed and fasted rats

The response of muscle and liver protein metabolism to either a single or three successive daily injections of an endotoxin (Escherichia coli lipopolysaccharide, serotype 0127 B8; 1 mg/ml, 0.3 mg/100 g body wt.) was studied in vivo in the fed rat, and at 24 and 30 h after endotoxin treatment during fasting. In the fed rats there was a catabolic response in muscle, owing to a 60-100% increase in muscle protein degradation rate, and a 52% fall in the synthesis rate. Although there was a 20% decrease in food intake, the decrease in protein synthesis was to some extent independent of this, since rats treated with endotoxin and fasted also showed a lower rate of muscle protein synthesis, which was in excess of the decrease caused by fasting alone. The mechanism of this decreased protein synthesis involved decreased translational activity, since in both fed and fasted rats there was a decreased rate of synthesis per unit of RNA. This occurred despite the fact that insulin concentrations were either maintained or increased, in the fasted rats, to those observed in fed rats. In the liver total protein mass was increased in the fed rats by 16% at 24 h, and the fractional synthesis rate at that time was increased by 35%. In rats fasted after endotoxin treatment the liver protein mass was not decreased as it was in the control fasted rats, and the fractional synthesis rate was increased by 22%. In both cases the increased synthesis rate reflected an elevated hepatic RNA concentration. The extent of this increase in hepatic protein synthesis was sufficient at one point to compensate for the fall in estimated muscle protein synthesis, so that the sum total in the two tissues was maintained.

The effect of hypothyroidism on sarcoplasmic reticulum in fast-twitch muscle of the rat

The effects of hypothyroidism on the Ca2+-transport capabilities of fast-twitch muscle (m. gastrocnemius) of the rat were studied in whole-muscle homogenate and isolated sarcoplasmic reticulum. Hypothyroidism did not affect the percentage recovery and the vesicle composition of the sarcoplasmic reticulum fraction, the total lipid and phospholipid-to-protein ratios and the protein composition (both qualitative and quantitative). Also the Ca2+-loading capacity of purified sarcoplasmic reticulum, in the presence of oxalate, and the Ca2+ and pH dependence of both the uptake reaction and the coupled ATPase activity were unchanged. However, the homogenate Ca2+-loading capacity and the Ca2+-uptake activity were depressed, as was the yield of purified sarcoplasmic reticulum. The results indicate a 31% reduction of the entire sarcoplasmic reticulum membrane system per volume of muscle. Ca2+/ATP coupling ratios, determined in purified sarcoplasmic reticulum vesicles by measurement of initial rates of net Ca2+ uptake and Ca2+-Mg2+-dependent hydrolysis of ATP, were found to be 1.48 +/- 0.06 and 2.08 +/- 0.05 in the euthyroid and hypothyroid groups, respectively. Identical values were obtained with a recently described Ca2+-pulse method (Meltzer, S. and Berman, M.C. (1984) Anal. Biochem. 138, 458-464), i.e., 1.53 +/- 0.06 and 2.01 +/- 0.03 in the euthyroid and hypothyroid groups, respectively. Passive Ca2+ efflux from sarcoplasmic reticulum was the same in both groups (30 nmol/mg per min), as was the fraction of vesicles that did not show net uptake of Ca2+ (less than 10%), which makes it unlikely that these parameters provide an explanation for the differences in the coupling ratio. The energy of activation of the (Ca2+ + Mg2+)-ATPase was increased in hypothyroidism, which may point to changes in the phospholipid environment of the enzyme. Physiological concentrations of T3 and T4 had no effect on the (Ca2+ + Mg2+)-ATPase in vitro, but all observed changes in the hypothyroid state could be reversed within 14 days by administration of T3 to hypothyroid animals. Approximate calculations indicate that the observed changes in the sarcoplasmic reticulum as a result of thyroid-hormone depletion may contribute significantly to the decrease in relaxation rate and the decrease in energy consumption during contraction.

Toxicity of D-galactosamine for rat hepatocytes in monolayer culture

Hepatocellular injury was induced by exposure of primary cultures of rat hepatocytes to 4 mM D-galactosamine. The cell damage was very similar to that seen in vivo and in the isolated perfused rat liver, both in biochemical and in structural terms. The severity of the lesions caused by D-galactosamine was dependent on the age of the culture being treated. Less severe damage was found with older cultures. Since the primary metabolic effects of D-galactosamine were age-independent, the reduction in cell damage seems to be due to progressive cell dedifferentiation. Dexamethasone (1 microM) suppressed the full development of the injury, while 1 microM triiodo-L-thyronine enhanced it. A protection of hepatocytes by alpha 2-macroglobulin against the effects of D-galactosamine could be observed neither in vivo nor in vitro. Direct cytotoxic effects of endotoxin from Salmonella minnesota R 595 could be demonstrated only on hepatocytes in the early phases of primary culture using rather high doses of the purified lipopolysaccharide. It is unlikely that they play a major role in the hepatocellular injury seen following endotoxinemia in vivo. Lowering of extracellular Ca2+ concentration and additions of calcium/calmodulin inhibitors did not prevent cell injury after treatment with D-galactosamine. The results suggest that cell death is not due to an increased influx of Ca2+ into the cells.

Kwashiorkor revisited: the pathogenesis of oedema in kwashiorkor and its significance

Cicely Williams, in her original description of kwashiorkor, implied that deficiency of protein in the baby's food could be a main cause of the syndrome. The hallmark of kwashiorkor is oedema. According to the 'classical' theory, an inadequate intake of protein leads to a low plasma albumin concentration, which in turn causes oedema. This theory has been contested from several points of view: that hypoalbuminaemia is not the major factor determining the presence of oedema, and that there is no real evidence of dietary protein deficiency. The resolution of this question is of some importance from the point of view of public health diagnosis and prevention. A crucial point in the argument is the pathogenesis of oedema, which is discussed in some detail. Although it is clearly multifactorial, with electrolyte disturbances--potassium deficiency and sodium retention--playing an important role, it is contended that the classical theory is essentially correct. On the dietary side, recent experimental work supports the earlier view that the development of oedema depends on a relative deficiency of protein with a relative excess of energy. Comparisons of intakes with requirements are unconvincing in view of uncertainty about the validity of the estimates of children's needs for protein.