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PubMed2002

Adrenalectomy enhances the insulin sensitivity of muscle protein synthesis.

After confirming that adrenalectomy per se does not affect skeletal muscle protein synthesis rates, we examined whether endogenously produced glucocorticoids modulate the effect of physiological insulin concentrations on protein synthesis in overnight-fasted rats 4 days after either a bilateral adrenalectomy (ADX), ADX with dexamethasone treatment (ADX + DEX), or a sham operation (Sham; n = 6 each). Rats received a 3-h euglycemic insulin clamp (3 mU. min(-1). kg(-1)). Rectus muscle protein synthesis was measured at the end of the clamp, and the phosphorylation states of protein kinase B (Akt), eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), and ribosomal protein S6 kinase (p70(S6K)) were quantitated before and after the insulin clamp. The basal phosphorylation states of Akt, 4E-BP1, and p70(S6K) were similar between ADX and Sham rats. Insulin significantly enhanced the phosphorylation of Akt (P < 0.03), 4E-BP1 (P = 0.003), and p70(S6K) (P < 0.002) in ADX but not in Sham rats. Protein synthesis was significantly greater after insulin infusion in ADX than in Sham rats (P = 0.01). Glucocorticoid replacement blunted the effect of insulin on Akt, 4E-BP1, and p70(S6K) phosphorylation and protein synthesis. In conclusion, glucocorticoid deficiency enhances the insulin sensitivity of muscle protein synthesis, which is mediated by increased phosphorylation of translation initiation-regulatory proteins.

Adrenalectomy↗
PubMed1987

Ornithine-alpha-ketoglutarate improves skeletal muscle protein synthesis as assessed by ribosome analysis and nitrogen use after surgery.

Total parenteral nutrition (TPN) was given for 3 days after elective abdominal surgery. The control group (N = 9) received TPN only and one group of patients (N = 6) received TPN supplemented with ornithine-alpha-ketoglutarate (0.35 g/kg bw/day). Protein synthesis in skeletal muscle was assessed from the total ribosome concentration and the percentage of polyribosomes. In the control group the total concentration of ribosomes decreased after surgery by 23% (p less than 0.05) and the percentage of polyribosomes decreased by 21% (p less than 0.01), whereas in the ornithine-alpha-ketoglutarate group both variables remained unaffected. The cumulative urinary urea excretion was significantly larger in the control group than in the ornithine-alpha-ketoglutarate group (p less than 0.05). The calculated nitrogen balance was negative in the control group on each day of the study (p less than 0.05), but that of the ornithine-alpha-ketoglutarate group was not statistically different from zero. The results show that postoperative maintenance of muscle protein synthesis and a more effective nitrogen use was achieved by supplementing TPN with ornithine-alpha-ketoglutarate, 0.35 g/kg bw/day.

Cholecystectomy↗
PubMed2006

Changes in skeletal muscle protein metabolism and myosin heavy chain isoform messenger ribonucleic acid abundance after treatment of hyperthyroidism.

BACKGROUND: Hyperthyroidism causes a hypermetabolic state and skeletal muscle dysfunction, but the underlying mechanism remains incompletely defined. OBJECTIVE: The objective of the study was to determine whether treatment of hyperthyroidism causes changes in amino acid fluxes, synthesis rates of muscle proteins, and expression of muscle myosin heavy chain (MHC) that may impact skeletal muscle function and metabolic rate. METHODS: Eight hyperthyroid patients were studied (TSH 0.008 +/- 0.001 mU/liter) before treatment and at least 9 months after correction of hyperthyroidism (TSH 2.3 +/- 0.4) (P < 0.03). Fluxes of leucine and phenylalanine as well as muscle protein synthesis rates were measured using L[1,2 13C] leucine and L(15N) phenylalanine as tracers. mRNA levels of selected genes were measured in muscle biopsy samples. RESULTS: Treatment decreased resting metabolic rate that paralleled changes in fluxes of leucine and phenylalanine accompanied by improved muscle strength and mass. Synthesis rates of mixed muscle proteins (P = 0.01), sarcoplasmic (P = 0.04), and mitochondrial (P = 0.08) proteins decreased, whereas MHC synthesis was unchanged. Selective increases in mRNA abundance of muscle MHC1 isoform (P = 0.04) and decrease of MHCIIA (P = 0.007) and MHCIIx (P = 024) were observed. Muscle mitochondrial oxidative enzymes and mRNA levels of mitochondrial proteins were unchanged, but uncoupling protein2 and uncoupling protein3 mRNA levels (P = 0.02) decreased. CONCLUSION: Increased amino acid flux, mixed muscle protein synthesis, and synthesis of sarcoplasmic proteins are consistent with the hypermetabolic state in hyperthyroidism. After treatment, MHC synthesis rates were unchanged, but mRNA levels of isoforms of MHC found in slow-twitch and fast-twitch fibers increased and decreased, respectively. These results offer a mechanistic explanation for posttreatment improvement in muscle functions in hyperthyroidism.

Adrenergic beta-Antagonists↗
PubMed1988

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.

Energy Metabolism↗
PubMed2002

[Changes in skeletal muscle protein metabolism in burned rats with sepsis and the role of glucocorticoid in skeletal muscle proteolysis].

OBJECTIVE: To study the effects of glucocorticoid on skeletal muscle protein metabolism in burn sepsis and its possible mechanism. METHODS: The rats were randomly divided into four groups with 15 rats in each group. Group B, 30% TBSA full-thickness burn was produced on the back and endotoxin (6 mg/kg bw) was given intraperitoneally after the injury to simulate burn sepsis. Groups C and D, glucocorticoid receptor antagonist RU38486 (10 mg/kg bw) was given by gavage 2 hours before or 2 hours after burn with endotoxin, respectively. Group A, the rats received only normal saline in same volume as endotoxin. Plasma levels of cortisol were determined with standard procedure. Extensor digitorium longus muscles (EDL) were procured from both legs 12 hours after the injury. After weighing, the proteolytic rate was determined in vitro in an incubation system with oxygen rich environment by high performance liquid chromatography. The gene expressions of ubiquitin, E(2)-14kDa and C2 in the muscles were determined by Northern blot analysis. RESULTS: The weight of EDL was significantly lower in group B than in group A (t = 9.03, P < 0.01). Although the weight of EDL muscles was also lower in groups C and D than in group A, it was significantly higher than in group B (t = 2.26, 6.42, P < 0.05 or P < 0.01). The concentrations of plasma cortisol were markedly higher in groups B, C and D than in group A (t = 9.03 - 22.94, P < 0.01). A 58.8% (210/357) of the total and 335.5% (4.16/1.24) of myofibrillar proteolytic rate in group B was higher than in group A (t = 36.99 and t = 46.19, P < 0.01), respectively. The total and myofibrillar proteolytic rate in group D was 28.3% (161/567) and 49.6% (2.68/5.40) and in group C 18.9% (108/567) and 23.2% (1.25/5.40), which were lower than those in group B (t = 5.34 approximately 34.68, P < 0.01), respectively. Although the expressions of ubiquitin mRNA (2.4 kb), E(2)-14 kDa mRNA (1.2 kb) and C2 mRNA in groups C and D were significantly higher than in group A, all the values were lower than those in group B (t = 3.22, 11.32, P < 0.01), especially in group C. CONCLUSIONS: The proteolytic rate of skeletal muscle, especially the myofibrillar proteolytic rate, was enhanced during burn with sepsis. Hypersecretion of glucocorticoid could upgrade the gene expression of ubiquitin system, resulting in hyperdegradation of skeletal muscle protein during burn with sepsis. Glucocorticoid receptor antagonist RU38486 could decrease the hyperdegradation of skeletal muscle during burn with sepsis.

Animals↗
PubMed2003

Extremity hyperinsulinemia stimulates muscle protein synthesis in severely injured patients.

Insulin has a well-recognized anabolic effect on muscle protein, yet critically ill, severely injured patients are often considered "resistant" to the action of insulin. The purpose of this study was to assess the in vivo effects of hyperinsulinemia on human skeletal muscle in severely injured patients. To accomplish this goal, 14 patients with burns encompassing >40% of their body surface area underwent metabolic evaluation utilizing isotopic dilution of phenylalanine, femoral artery and vein blood sampling, and sequential muscle biopsies of the leg. After baseline metabolic measurements were taken, insulin was infused into the femoral artery at 0.45 mIU.min(-1).100 ml leg volume(-1) to create a local hyperinsulinemia but with minimal systemic perturbations. Insulin administration increased femoral venous concentration of insulin (P < 0.01) but with only a 4% (insignificant) decrease in the arterial glucose concentration and a 7% (insignificant) decrease in the arterial concentration of phenylalanine. Extremity hyperinsulinemia significantly increased leg blood flow (P < 0.05) and the rate of muscle protein synthesis (P < 0.05). Neither the rate of muscle protein breakdown nor the rate of transmembrane transport of phenylalanine was significantly altered with extremity hyperinsulinemia. In conclusion, this study demonstrates that insulin directly stimulates muscle protein synthesis in severely injured patients.

Adult↗
PubMed1998

Evaluation of muscle protein mass in children with solid tumors by muscle thickness measurement with ultrasonography, as compared with anthropometric methods and visceral protein concentrations.

OBJECTIVE: To compare the merits of regional ultrasonography, anthropometry, and measurement of visceral protein concentrations for determining muscle protein mass and nutritional status. DESIGN: Prospective follow-up study. SUBJECTS: Nineteen children (median age 4.7 y, range, 1.4-14.1 y) with malignant solid tumors were examined at diagnosis, monthly during preoperative chemotherapy, and 1-2 months after operation; 19 healthy age- and sex-matched children served as controls. INTERVENTIONS: We measured the thickness of the quadriceps muscle by ultrasonography (muscle index), and body weight, mid-arm circumference, triceps skinfold thickness, and serum concentrations of albumin, prealbumin, and transferrin. RESULTS: At diagnosis, the muscle index was smaller in the patients (mean 5.6 cm2/m2, 95% CI 4.5-6.7 cm2/m2) than in the controls (8.5 cm2/m2, 95% CI 7.7-9.3 cm2/m2) (P = 0.0007), whereas the anthropometric measurements did not differ. The sensitivity, specificity, and predictive value of anthropometry and measurements of visceral proteins were poor in detecting reduced muscle protein reserves when compared with the muscle index as the baseline standard. During the anticancer therapy the mean concentrations of serum albumin and prealbumin increased. CONCLUSIONS: In children with cancer, nutritional status should be assessed with methods more sensitive than anthropometry and visceral protein concentration measurements. We advocate regional ultrasonography for the evaluation of nutrition in these patients.

Adolescent↗
PubMed1996

Role of insulin and IGF-I in activation of muscle protein synthesis after oral feeding.

The aim was to evaluate the role of insulin and insulin-like growth factor I (IGF-I) in activation of muscle protein synthesis after oral feeding. Synthesis rate of globular and myofibrillar proteins in muscle tissue was quantified by a flooding dose of radioactive phenylalanine. Muscle tissue expression of IGF-I mRNA was measured. Normal (C57 Bl) and diabetic mice (type I and type II) were subjected to an overnight fast (18 h) with subsequent refeeding procedures for 3 h with either oral chow intake or provision of insulin, IGF-I, glucose, and amino acids. Anti-insulin and anti-IGF-I were provided intraperitoneally before oral refeeding in some experiments. An overnight fast reduced synthesis of both globular (38 +/- 3%) and myofibrillar proteins (54 +/- 3%) in skeletal muscles, which was reversed by oral refeeding. Muscle protein synthesis, after starvation/ refeeding, was proportional and similar to changes in skeletal muscle IGF-I mRNA expression. Diabetic mice responded quantitatively similarly to starvation/refeeding in muscle protein synthesis compared with normal mice (C57 Bl). Both anti-insulin and anti-IGF-I attenuated significantly the stimulation of muscle protein synthesis in response to oral feeding, whereas exogenous provision of either insulin or IGF-I to overnight-starved and freely fed mice did not clearly stimulate protein synthesis in skeletal muscles. Our results support the suggestion that insulin and IGF-I either induce or facilitate the protein synthesis machinery in skeletal muscles rather than exerting a true stimulation of the biosynthetic process during feeding.

Animal Feed↗
PubMed1994

Effects of dietary corticosterone and trilostane on growth and skeletal muscle protein turnover in broiler cockerels.

1. The effects of dietary corticosterone and trilostane, an inhibitor of glucocorticoid synthesis, on: growth, rates of synthesis and breakdown of skeletal muscle protein, and content of abdominal fat were studied in broiler chickens. 2. Dietary corticosterone (5, 10 or 20 mg/kg) depressed body weight gain and increased abdominal fat content in a dose-dependent manner while dietary trilostane (1.4 or 7.0 mg/kg) had no effect. 3. The rate of protein breakdown in skeletal muscle estimated from N tau-methylhistidine excretion was increased in a dose-dependent manner by dietary corticosterone but it was decreased by trilostane. 4. The rate of skeletal muscle protein synthesis was not affected by corticosterone although it was decreased by trilostane. 5. Plasma corticosterone concentration was increased in a dose-dependent manner by dietary corticosterone and decreased by treatment with 7 mg trilostane/kg diet. 6. The results indicate that higher concentrations of plasma corticosterone increase protein breakdown in skeletal muscle but do not affect muscle protein synthesis while both the rates of synthesis and breakdown are decreased when plasma corticosterone concentration is reduced.

3-Hydroxysteroid Dehydrogenases↗
PubMed1993

Regulation of actomyosin interactions in Limulus muscle proteins.

Contraction of striated muscle from Limulus polyphemus, the horseshoe crab, is regulated by both calcium binding to a troponin-tropomyosin-dependent thin filament array and a myosin light chain kinase-dependent phosphorylation of myosin. We have isolated myosin from Limulus striated muscle and examined how these two regulatory systems affect the sliding velocity of actin filaments over myosin, using an in vitro motility assay. Our results show that in the presence of ATP, Limulus myosin must be phosphorylated in order to move actin filaments. No movement was observed for actin filaments interacting with dephosphorylated Limulus myosin. Calcium was not required for actin movement. In contrast, when both troponin and tropomyosin are bound to actin filaments, calcium is required for the movement of actin filaments over phosphorylated myosin. These results demonstrate that the "off" state of either the thin filament or thick filament regulatory system is dominant and that for the movement to occur, both phosphorylated Limulus myosin and an activated troponin-tropomyosin system are required. Tropomyosin by itself increases the sliding velocity of actin filaments over phosphorylated Limulus myosin about 10-fold in a calcium-independent manner. Tropomyosins from turkey gizzard smooth muscle, bovine cardiac muscle, and Limulus muscle all have a profound effect in increasing the velocity. Troponin alone does not change the velocity. Partial sequences of the tryptic phosphopeptides of Limulus myosin regulatory light chains generated following the phosphorylation by gizzard myosin light chain kinase yield ATS(PO4)NVFAMFEQNQIA for 21 kDa and SGS(PO4)NVFSMFT for 31-kDa light chain.

Actins↗
PubMed2003

Amino acid ingestion improves muscle protein synthesis in the young and elderly.

We recently demonstrated that muscle protein synthesis was stimulated to a similar extent in young and elderly subjects during a 3-h amino acid infusion. We sought to determine if a more practical bolus oral ingestion would also produce a similar response in young (34 +/- 4 yr) and elderly (67 +/- 2 yr) individuals. Arteriovenous blood samples and muscle biopsies were obtained during a primed (2.0 micromol/kg) constant infusion (0.05 micromol.kg(-1).min(-1)) of L-[ring-2H5]phenylalanine. Muscle protein kinetics and mixed muscle fractional synthetic rate (FSR) were calculated before and after the bolus ingestion of 15 g of essential amino acids (EAA) in young (n = 6) and elderly (n = 7) subjects. After EAA ingestion, the rate of increase in femoral artery phenylalanine concentration was slower in elderly subjects but remained elevated for a longer period. EAA ingestion increased FSR in both age groups by approximately 0.04%/h (P < 0.05). However, muscle intracellular (IC) phenylalanine concentration remained significantly higher in elderly subjects at the completion of the study (young: 115.6 +/- 5.4 nmol/ml; elderly: 150.2 +/- 19.4 nmol/ml). Correction for the free phenylalanine retained in the muscle IC pool resulted in similar net phenylalanine uptake values in the young and elderly. EAA ingestion increased plasma insulin levels in young (6.1 +/- 1.2 to 21.3 +/- 3.1 microIU/ml) but not in elderly subjects (3.0 +/- 0.6 to 4.3 +/- 0.4 microIU/ml). Despite differences in the time course of plasma phenylalanine kinetics and a greater residual IC phenylalanine concentration, amino acid supplementation acutely stimulated muscle protein synthesis in both young and elderly individuals.

Administration, Oral↗
PubMed1991

Increased weight gain, nitrogen retention and muscle protein synthesis following treatment of diabetic rats with insulin-like growth factor (IGF)-I and des(1-3)IGF-I.

We have examined the effects of infusing recombinant human growth hormone (hGH), insulin-like growth factor-I (IGF-I), the truncated IGF-I analogue, des(1-3)IGF-I, and insulin over a 7-day period in streptozotocin-induced diabetic rats. IGF-I at a dose of 1.05 or 1.08 mg/kg per day in two experiments increased body weight and nitrogen retention above those of vehicle-infused controls to about 30% of the improvement achieved with 25 or 30 units of insulin/kg per day, but only in the second experiment were the differences statistically significant (P less than 0.05). A 2.5-fold higher IGF-I dose, or des(1-3)IGF-I at 1.08 mg/kg per day, gave effects that were approx. 70% of those obtained with insulin. hGH at 1.38 mg/kg per day was not effective. The IGF peptides, unlike insulin, did not ameliorate the diabetic glucosuria. The improvements in nitrogen balance could be accounted for in part by increases in muscle protein synthesis. Muscle protein breakdown, as assessed by 3-methylhistidine excretion, was inhibited by insulin, but not by the IGF peptides. Carcass fat increased substantially following insulin administration. This did not occur with the IGF peptides, suggesting that IGF predominantly stimulates the growth of lean tissue. IGF-I concentrations and IGF-I-binding proteins in plasma were increased by IGF-I, especially at the higher dose, whereas hGH produced only a transient increase in IGF-I. Des(1-3)IGF-I induced binding proteins, but had only a slight effect on measured IGF-I concentrations. We conclude that IGF peptides stimulate muscle protein synthesis and improve nitrogen balance in diabetes without obviously influencing the abnormal carbohydrate metabolism. Moreover, des(1-3)IGF-I is at least as potent as the full-length IGF-I.

Animals↗
PubMed1998

Availability of eIF4E regulates skeletal muscle protein synthesis during recovery from exercise.

We examined the association of the mRNA cap binding protein eIF4E with the translational inhibitor 4E-BP1 in the acute modulation of skeletal muscle protein synthesis during recovery from exercise. Fasting male rats were run on a treadmill for 2 h at 26 m/min and were realimented immediately after exercise with either saline, a carbohydrate-only meal, or a nutritionally complete meal (54.5% carbohydrate, 14% protein, and 31.5% fat). Exercised animals and nonexercised controls were studied 1 h postexercise. Muscle protein synthesis decreased 26% after exercise and was associated with a fourfold increase in the amount of eIF4E present in the inactive eIF4E.4E-BP1 complex and a concomitant 71% decrease in the association of eIF4E with eIF4G. Refeeding the complete meal, but not the carbohydrate meal, increased muscle protein synthesis equal to controls, despite similar plasma concentrations of insulin. Additionally, eIF4E.4E-BP1 association was inversely related and eIF4E.eIF4G association was positively correlated to muscle protein synthesis. This study demonstrates that recovery of muscle protein synthesis after exercise is related to the availability of eIF4E for 48S ribosomal complex formation, and postexercise meal composition influences recovery via modulation of translation initiation.

Animal Nutritional Physiological Phenomena↗
PubMed1998

Regulation of skeletal muscle protein turnover during sepsis.

Wasting of skeletal muscle protein is a prominent feature of the metabolic response to sepsis. Persistent protein wasting leads to muscle dysfunction and prolongs recovery from the septic insult. Unfortunately, conventional nutritional support alone does not prevent the sepsis-induced weight loss and catabolism of muscle. Hence, mechanisms other than substrate deficiency appear to be involved in the derangements in protein metabolism during sepsis. The catabolism of muscle during sepsis results from a stimulation of proteolysis and an inhibition of protein synthesis. Despite the importance of these pathways in maintaining muscle mass, the regulation of protein synthesis and proteolysis during sepsis remains poorly understood. This review summarizes the mechanisms responsible for alterations in protein synthesis and degradation in muscle during sepsis at the biochemical level. The ability of hormones (insulin, insulin-like growth factor I, glucocorticoids) or cytokines (tumor necrosis factor, interleukin-1) to act as mediators or modulators of protein catabolism is also examined. A picture is emerging suggesting that cytokines may influence skeletal muscle protein metabolism during sepsis both indirectly, through inhibition of the regulatory actions of anabolic hormones on protein turnover, and directly, through modulation of the protein synthesis and degradation enzymatic machinery.

Animals↗
PubMed1984

Immunoperoxidase demonstration of a new muscle protein (Z-protein) in myogenic tumors as a diagnostic aid.

Z-protein, a new muscle protein with a molecular weight of 55,000 daltons recently discovered in Japan, is an excellent marker for muscle differentiation in paraffin sections. Positive staining by a peroxidase-antiperoxidase technique for detection of intracellular Z-protein was observed in 2 of 2 cardiac rhabdomyomas, 5 of 5 leiomyomas, 9 of 10 leiomyosarcomas, and in 15 of 18 rhabdomyosarcomas including 5 of 5 of the pleomorphic type, 7 of 9 of the embryonal type, and 3 of 4 of the alveolar type. Other types of tumors (77 cases) that sometimes mimic myogenic tumors on histologic examination gave negative results. The immunoperoxidase method for detection of intracellular Z-protein is of use in distinguishing myogenic tumors from other types of tumors.

Adolescent↗
PubMed1997

An abundant supply of amino acids enhances the metabolic effect of exercise on muscle protein.

Six normal untrained men were studied during the intravenous infusion of a balanced amino acid mixture (approximately 0.15 g.kg-1.h-1 for 3 h) at rest and after a leg resistance exercise routine to test the influence of exercise on the regulation of muscle protein kinetics by hyperaminoacidemia. Leg muscle protein kinetics and transport of selected amino acids (alanine, phenylalanine, leucine, and lysine) were isotopically determined using a model based on arteriovenous blood samples and muscle biopsy. The intravenous amino acid infusion resulted in comparable increases in arterial amino acid concentrations at rest and after exercise, whereas leg blood flow was 64 +/- 5% greater after exercise than at rest. During hyperaminoacidemia, the increases in amino acid transport above basal were 30-100% greater after exercise than at rest. Increases in muscle protein synthesis were also greater after exercise than at rest (291 +/- 42% vs. 141 +/- 45%). Muscle protein breakdown was not significantly affected by hyperminoacidemia either at rest or after exercise. We conclude that the stimulatory effect of exogenous amino acids on muscle protein synthesis is enhanced by prior exercise, perhaps in part because of enhanced blood flow. Our results imply that protein intake immediately after exercise may be more anabolic than when ingested at some later time.

Adult↗
PubMed1981

Interaction of thyroid status and diet on muscle protein breakdown in the rat, as measured by N tau-methylhistidine excretion.

The effects of thyroid status and of dietary protein and energy content on muscle protein breakdown have been compared in young rats, using urinary N tau-methylhistidine (3-methylhistidine; 3-Mehis) output as a measure of myofibrillar protein breakdown. Recently, thyroidectomized young rats receiving an adequate diet grew slowly and showed a reduction in 3-Mehis output, relative to the weight of their leg muscles. When the thyroidectomized rats were given 2-5 microgram thyroxine (T4) per 100 g body weight, output of 3-Mehis increased, even when growth was prevented by feeding a diet low in protein. This demonstrates that thyroid-status affects muscle protein breakdown through a mechanism other than via a change in growth rate. In confirmation of previous studies, administration of a diet low in protein to young rats prevented growth and reduced output of 3-Mehis relative to leg muscle weight. The reduction in protein breakdown is not likely to be due to reduced thyroid function, since the diet low in protein increased plasma total triiodothyronine (T3) levels. Furthermore, thyroidectomized rats fed the low-protein diet also underwent a decrease in output of 3-Mehis. In contrast to protein deficiency, a moderate dietary energy deficiency allowed some growth to occur, and output of 3-Mehis, relative to muscle mass, was not depressed. Nevertheless, total T3 concentration in the plasma of these animals was considerably reduced, indicating once more that total circulating thyroid hormone levels are not the only determinants of muscle protein breakdown. Finally, reduced plasma total T3 levels occurred in energy-deficient diets even in thyroidectomized rats receiving exogenous T4. This suggests that diet affects metabolism of the hormone independently of changes in secretory output from the thyroid gland.

Animals↗