Muscle Anabolism Research Articles

Characterization of primary human skeletal muscle cells from multiple commercial sources.

There is a significant unmet need for safe, anabolic muscle therapies to treat diseases and conditions associated with severe muscle weakness and frailty. The identification of such therapies requires appropriate cell-based screening assays to select compounds for further development using animal models. Primary human skeletal muscle cells have recently become available from a number of commercial vendors. Such cells may be valuable for studying the mechanisms that direct muscle differentiation, and for identifying and characterizing novel therapeutic approaches for the treatment of age- and injury-induced muscle disorders. However, only limited characterization of these cells has been reported to date. Therefore, we have examined four primary human muscle cell preparations from three different vendors for their capacity to differentiate into multinucleated myotubes. Two of the preparations demonstrated robust myotube formation and expressed characteristic markers of muscle differentiation. Furthermore, these myotubes could be induced to undergo morphological atrophy- and hypertrophy-like responses, and atrophy could be blocked with an inhibitor of myostatin signaling, a pathway that is known to negatively regulate muscle mass. Finally, the myotubes were efficiently infected with recombinant adenovirus, providing a tool for genetic modification. Taken together, our results indicate that primary human muscle cells can be a useful system for studying muscle differentiation, and may also provide tools for studying new therapeutic molecules for the treatment of muscle disease.

Effects of high‐protein diets on fat‐free mass and muscle protein synthesis following weight loss: a randomized controlled trial

The purpose of this work was to determine the effects of varying levels of dietary protein on body composition and muscle protein synthesis during energy deficit (ED). A randomized controlled trial of 39 adults assigned the subjects diets providing protein at 0.8 (recommended dietary allowance; RDA), 1.6 (2×-RDA), and 2.4 (3×-RDA) g kg(-1) d(-1) for 31 d. A 10-d weight-maintenance (WM) period was followed by a 21 d, 40% ED. Body composition and postabsorptive and postprandial muscle protein synthesis were assessed during WM (d 9-10) and ED (d 30-31). Volunteers lost (P<0.05) 3.2 ± 0.2 kg body weight during ED regardless of dietary protein. The proportion of weight loss due to reductions in fat-free mass was lower (P<0.05) and the loss of fat mass was higher (P<0.05) in those receiving 2×-RDA and 3×-RDA compared to RDA. The anabolic muscle response to a protein-rich meal during ED was not different (P>0.05) from WM for 2×-RDA and 3×-RDA, but was lower during ED than WM for those consuming RDA levels of protein (energy × protein interaction, P<0.05). To assess muscle protein metabolic responses to varied protein intakes during ED, RDA served as the study control. In summary, we determined that consuming dietary protein at levels exceeding the RDA may protect fat-free mass during short-term weight loss.

Pattern of protein ingestion to maximise muscle protein synthesis after resistance exercise

Pattern of protein ingestion to maximise muscle protein synthesis after resistance exercise

Role of protein and amino acids in promoting lean mass accretion with resistance exercise and attenuating lean mass loss during energy deficit in humans

Amino acids are major nutrient regulators of muscle protein turnover. After protein ingestion, hyperaminoacidemia stimulates increased rates of skeletal muscle protein synthesis, suppresses muscle protein breakdown, and promotes net muscle protein accretion for several hours. These acute observations form the basis for strategized protein intake to promote lean mass accretion, or prevent lean mass loss over the long term. However, factors such as protein dose, protein source, and timing of intake are important in mediating the anabolic effects of amino acids on skeletal muscle and must be considered within the context of evaluating the reported efficacy of long-term studies investigating protein supplementation as part of a dietary strategy to promote lean mass accretion and/or prevent lean mass loss. Current research suggests that dietary protein supplementation can augment resistance exercise-mediated gains in skeletal muscle mass and strength and can preserve skeletal muscle mass during periods of diet-induced energy restriction. Perhaps less appreciated, protein supplementation can augment resistance training-mediated gains in skeletal muscle mass even in individuals habitually consuming 'adequate' (i.e., >0.8 g kg⁻¹ day⁻¹) protein. Additionally, overfeeding energy with moderate to high-protein intake (15-25 % protein or 1.8-3.0 g kg⁻¹ day⁻¹) is associated with lean, but not fat mass accretion, when compared to overfeeding energy with low protein intake (5 % protein or ~0.68 g kg⁻¹ day⁻¹). Amino acids represent primary nutrient regulators of skeletal muscle anabolism, capable of enhancing lean mass accretion with resistance exercise and attenuating the loss of lean mass during periods of energy deficit, although factors such as protein dose, protein source, and timing of intake are likely important in mediating these effects.

Effects of leucine and its metabolite β‐hydroxy‐β‐methylbutyrate on human skeletal muscle protein metabolism

Maintenance of skeletal muscle mass is contingent upon the dynamic equilibrium (fasted losses–fed gains) in protein turnover. Of all nutrients, the single amino acid leucine (Leu) possesses the most marked anabolic characteristics in acting as a trigger element for the initiation of protein synthesis. While the mechanisms by which Leu is ‘sensed’ have been the subject of great scrutiny, as a branched-chain amino acid, Leu can be catabolized within muscle, thus posing the possibility that metabolites of Leu could be involved in mediating the anabolic effect(s) of Leu. Our objective was to measure muscle protein anabolism in response to Leu and its metabolite HMB. Using [1,2-13C2]Leu and [2H5]phenylalanine tracers, and GC-MS/GC-C-IRMS we studied the effect of HMB or Leu alone on MPS (by tracer incorporation into myofibrils), and for HMB we also measured muscle proteolysis (by arteriovenous (A–V) dilution). Orally consumed 3.42 g free-acid (FA-HMB) HMB (providing 2.42 g of pure HMB) exhibited rapid bioavailability in plasma and muscle and, similarly to 3.42 g Leu, stimulated muscle protein synthesis (MPS; HMB +70%vs. Leu +110%). While HMB and Leu both increased anabolic signalling (mechanistic target of rapamycin; mTOR), this was more pronounced with Leu (i.e. p70S6K1 signalling ≤90 min vs. ≤30 min for HMB). HMB consumption also attenuated muscle protein breakdown (MPB; −57%) in an insulin-independent manner. We conclude that exogenous HMB induces acute muscle anabolism (increased MPS and reduced MPB) albeit perhaps via distinct, and/or additional mechanism(s) to Leu.

Expanding sports drug testing assays: Mass spectrometric characterization of the selective androgen receptor modulator drug candidates RAD140 and ACP‐105

Anabolic agents have been top-ranked for many years among statistics of adverse analytical findings compiled by the World Anti-Doping Agency (WADA). Besides archetypical anabolic-androgenic steroids (AAS), alternative substances with similar effects concerning bone and muscle anabolism have been therapeutically pursued. A prominent emerging class of drugs is the chemically heterogeneous group of selective androgen receptor modulators (SARMs), some of which have been detected in doping control samples between 2009 and 2012 despite missing clinical approval. In order to support the momentum of expanding the preventive and proactive measures among anti-doping laboratories, the analytical characterization of substances with misuse potential is of great importance. In the present study, the SARM drug candidates RAD140 (comprising a 5-phenyloxadiazole nucleus) and ACP-105 (bearing an N-substituted tropanol pharmacophore) were studied regarding their mass spectrometric behavior under ESI-MS(/MS) and EI-MS(/MS) conditions. Reference material was synthesized according to established protocols and dissociation pathways of RAD140 and ACP-105 were elucidated with liquid chromatography/electrospray ionization quadrupole/time-of-flight or iontrap/orbitrap and gas chromatography/electron ionization quadrupole/time-of-flight high resolution/high accuracy mass spectrometry. Fragmentation pathways to diagnostic product ions of RAD140 (e.g. m/z 223 and 205 using ESI-MS/MS and m/z 421 and 349 using EI-MS/MS) and ACP-105 (such as m/z 233 and 193 or 231 and 217 for ESI-MS/MS and EI-MS/MS measurements, respectively) were proposed as substantiated by determined elemental compositions and MS(n) experiments as well as comparison to spectra of a structural analog. Notably, for the formation of the characteristic fragment ion at m/z 421 of RAD140, the comparably seldom intramolecular migration of a trimethylsilyl residue triggered by electron ionization was suggested as corroborated by all of the above-mentioned analytical means. The obtained data will support future sports drug testing methods and facilitate and accelerate the implementation of this analyte and related compounds or metabolites in both GC/MS(/MS)- and LC/MS(/MS)-based routine doping control procedures.

Abstract 1375: Benefits of adding an exercise intervention to conventional immunosuppression in chronic graft versus host disease: insights from a murine model.

Abstract Purpose.- Graft-versus-host-disease (GVHD) is a major complication of allogeneic hematopoietic stem cell transplantation (allo-HSCT), and it is associated with high morbimortality and decreased patients’ physical capacity. We determined the effects of an exercise training program performed after allo-HSCT on survival, clinical evolution and physical capacity, using a murine model of chronic GVHD treated with cyclosporine A. Methods.- BALB/C female mice (aged 8 weeks) were randomly assigned to an exercise (n=11) or control group (n=12). Allo-HSCT was performed with donor bone marrow cells and splenocytes from B10.D2 female mice. Mice of the exercise group performed a moderate-intensity treadmill running program, and both groups received Cyclosporine A (15 mg/kg/day) during ∼11wks after transplant. We analyzed survival and total clinical severity score (which is the sum of five individual scores). We measured mice basal heart rate as an indicator of physical fitness status before and after allo-HSCT. We collected blood and serum samples for immune cell reconstitution at days +21 and +82, and for cytokine analysis (IL2, IL4, IL6, IL10, IL17a, IFNγ and TNFα) at days -10 and +82. On sacrifice day, we removed target tissues (liver, intestine, skin) and spleen, for analyzing the GVHD damage and the immune cell reconstitution respectively, and skeletal muscles evaluating the muscle adaptations. Results.- Exercise training resulted in an increased survival (p=0.011) and lesser deterioration of physical capacity after ∼11 weeks (p=0.03 for the between-group comparison) in the exercise group. Regarding disease clinical course/severity, the exercise intervention induced a significant group x interaction effect for total severity score (p=0.002), reflecting a better disease evolution in the exercise group. We found a beneficial effect in three individual scores: weight (a reliable indicator of the severity of the disease, p=0.02), spontaneous activity (p=0.001) and posture (p=0.002). Exercise training reduced blood IL4 and TNFα (p=0.025 and p=0.031, respectively) while increasing circulating B220 (p=0.008) and CD4 lymphocytes (p=0.043). We found no between-group difference in citrate synthase activity (p=0.322), and a trend towards higher muscle anabolism (as determined by the ratio of phospho-p70 S6 kinase / p70 S6 kinase) in the exercise compared with the control group (p=0.083). Conclusion.- A moderate-intensity exercise program mimicking the widely accepted public health recommendations for physical activity in humans (≥ 30min/day) was well tolerated and had significant positive effects on clinical and biological indicators of chronic GVHD, and resulted in better survival. Citation Format: Carmen Fiuza-Luces, Luisa Soares-Miranda, África González-Murillo, Jesús Martínez Palacio, Isabel Colmenero, Fernando Casco, Gustavo Melén, María Morán, Manuel Ramírez, Alejandro Lucía. Benefits of adding an exercise intervention to conventional immunosuppression in chronic graft versus host disease: insights from a murine model. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1375. doi:10.1158/1538-7445.AM2013-1375

The effects of bolus versus pulse feeding strategies on muscle anabolism in older men

A topical advancement in understanding the control of muscle protein synthesis (MPS) was the discovery of a “Muscle‐Full” phenomenon, whereby feeding stimulates only transient increases in MPS, which return to postabsorptive values despite sustained availability of plasma and muscle essential amino acids (EAA) including leucine. Herein, we have explored this phenomenon in relation to aging by measuring MPS (13C6‐phenylalanine bound muscle) over a 6 h period (2 h fasted ‐4 h fed) using two distinct feeding regimens: 16 older men received either a single 15g mixed EAA feed (N=8, 70±2 y; ‘BOLUS’) or the same dose in 4 × 3.75g fractions at 45 min intervals (N=8, 70±3 y; ‘PULSE’). While MPS was unchanged in either group 0–90 min after EAA ingestion, it was elevated 91–180 min, irrespective of feeding strategy (BOLUS: 0.049 to 0.082 vs. PULSE: 0.046 to 0.071%.h−1, P&lt;0.05 2‐way RM ANOVA). In contrast, while MPS returned to baseline rates in BOLUS between 181–240 min (to 0.052%.h−1) this was not the case in PULSE (0.070%.h−1, P&lt;0.05). Despite these temporal differences equal total MPS responses to feeding regimens were revealed by area under the curve analysis (0.27% vs. 0.24% in 4 h, P&gt;;0.05). While equal total MPS between regimens suggests a dose, not time‐dependent mechanism regulates the muscle‐full state in older men, greater sustainment of the anabolic response over longer durations with PULSE feeding regimens may transpire.

Obesity: A Somatotrope Perspective

With the obesity epidemic increasing steadily in the United States, learning how energy homeostasis is regulated is imperative (1, 2). Leptin has been a molecule of great interest for this reason. The road to leptin discovery began in 1950 at Jackson Laboratories when a spontaneous mutation arose resulting in hyperphagia and severe obesity (3). The mutation was named the obese mutation, and Ob/Ob mice were the focus of decades of research. The gene encoding leptin was finally cloned in 1994 (4). Its receptor was cloned the year after (5). Leptin acts as an indicator of energy stores and regulates appetite through regulation of orexogenic (NPY and AgRP) and anorexogenic ( MSH) signals in the hypothalamus (6). Although mutations in leptin and its receptor are associated with severe obesity in mice, rats, and humans (3–5, 7, 8), these mutations account for a very small fraction of human obesity cases (9–13). Even so, learning more about leptin and its role in energy homeostasis promises to provide researchers and health professionals with important information about obesity. Sensing energy stores is important for growth and metabolism, which need to be closely linked to nutrition. Somatotrope cells in the anterior pituitary gland secrete GH, which regulates growth and body mass through stimulation of long bone growth, muscle anabolism, and lipolysis (14, 15). Thus, GH decreases fat and increases muscle mass. Several nutrition-sensing hormones regulate somatotrope function, including ghrelin, insulin, and leptin (16–19). Although many studies aimed at understanding obesity have focused on the role of ghrelin, a potent secretagogue for GH, very few have ventured downstream to the somatotrope cells (20–23). Ghrelin is released by distinct endocrine cells of the stomach in response to fasting and is important for energy homeostasis (16). Leptin, which is produced by adipose tissue and by the pituitary, acts as a satiety factor in the hypothalamus and can stimulate GH production (14, 17). Over 80% of somatotrope cells express the receptor for leptin, Leprb (24). Ob/Ob and Db/Db mice, which lack leptin signaling, have reduced somatotrope numbers and reduced serum GH levels (15, 17). Whether the effect of leptin on somatotrope function is direct or indirect due to increased adipose tissue, metabolic disease, or hypogonadism has been difficult to ascertain. The paper by Syed et al in this issue of Endocrinology addresses the role of the leptin receptor in somatotrope function. The authors employed a Gh-Cre mouse (25) to delete the long form of the leptin receptor (Leprb) specifically in somatotrope cells (30). Somatotrope-specific Leprb deletion mutants exhibit a reduction in serum GH levels leading to reduced fat burning and changes in adipokines resulting in obesity by 5–6 months of age (26–28). Syed et al find that the number of somatotrope cells in leptin receptor deletion mutants is normal based on in situ hybridization for Gh mRNA but that most these somatotrope cells do not contain enough GH to be detected by immunohistochemistry, suggesting that somatotrope cells are not functioning normally. The authors demonstrate that the failure in somatotrope function is due to reduced sensitivity to GHRH. However, treatment with both GHRH and ghrelin restored GHRH sensitivity, increased the number of GH-immunoreactive cells to normal proportions, and increased serum GH to normal levels (30). These studies suggest that leptin receptor is necessary for maintaining somatotrope sensitivity to GHRH and that ghrelin is key for this process. This is valuable mechanistic information about how leptin signaling regulates somatotrope function to affect energy homeostasis. Syed et al answer several key questions about the interplay between metabolism and somatotrope function and uncover many more questions that need to be ad-

Interactions between exercise and nutrition to prevent muscle waste during ageing.

The underlying cause of sarcopenia and dynapenia (age-related strength loss) are not fully elucidated, but may be the result, or combination, of alterations in lifestyle or inflammatory and endocrine profiles. What is clear is that functional ability is limited and mortality risk is elevated. Mechanistically, muscle atrophy is the result of the prolonged periods of net negative muscle protein balance, brought about by the imbalance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). Contractile loading of skeletal muscle, through resistive-type exercise and amino acid ingestion both act as a strong stimulus for MPS and, when combined, can induce a net positive protein balance and muscle hypertrophy. Given that MPS in older muscles displays a blunted response to anabolic stimuli compared with the young, the combined effect and manipulation of contractile and nutrient interventions to optimize muscle anabolism could be extremely important for counteracting sarcopenia. Specifically, the dose, absorption kinetics, leucine content, but less-so the timing of ingestion, are important determinants of the mRNA translational signalling response regulating MPS. In addition, resistance exercise-induced rates of MPS and hypertrophy appear to be dependent on exercise volume (to achieve maximal muscle fibre recruitment), as opposed to the absolute load that is lifted. A number of recent studies in young adults lend weight to this notion by showing that contraction can be manipulated; allowing low load weight lifting to effectively stimulate rates of MPS to a level comparable with traditional high loads, a finding with important implications for older adults interested in undertaking resistance exercise.

MTOR signaling response to resistance exercise is altered by chronic resistance training and detraining in skeletal muscle

Resistance training-induced muscle anabolism and subsequent hypertrophy occur most rapidly during the early phase of training and become progressively slower over time. Currently, little is known about the intracellular signaling mechanisms underlying changes in the sensitivity of muscles to training stimuli. We investigated the changes in the exercise-induced phosphorylation of hypertrophic signaling proteins during chronic resistance training and subsequent detraining. Male rats were divided into four groups: 1 bout (1B), 12 bouts (12B), 18 bouts (18B), and detraining (DT). In the DT group, rats were subjected to 12 exercise sessions, detrained for 12 days, and then were subjected to 1 exercise session before being killed. Isometric training consisted of maximum isometric contraction, which was produced by percutaneous electrical stimulation of the gastrocnemius muscle every other day. Muscles were removed 24 h after the final exercise session. Levels of total and phosphorylated p70S6K, 4E-BP1, rpS6, and p90RSK levels were measured, and phosphorylation of p70S6K, rpS6, and p90RSK was elevated in the 1B group compared with control muscle (CON) after acute resistance exercise, whereas repeated bouts of exercise suppressed those phosphorylation in both 12B and 18B groups. Interestingly, these phosphorylation levels were restored after 12 days of detraining in the DT group. On the contrary, phosphorylation of 4E-BP1 was not altered with chronic training and detraining, indicating that, with chronic resistance training, anabolic signaling becomes less sensitive to resistance exercise stimuli but is restored after a short detraining period.

Muscle Kinetics and Exercise: Any Advantages of Dietary Proteins

Muscle Kinetics and Exercise: Any Advantages of Dietary Proteins

Post-Meal Responses of Elongation Factor 2 (eEF2) and Adenosine Monophosphate-Activated Protein Kinase (AMPK) to Leucine and Carbohydrate Supplements for Regulating Protein Synthesis Duration and Energy Homeostasis in Rat Skeletal Muscle

Previous research demonstrates that the anabolic response of muscle protein synthesis (MPS) to a meal is regulated at the level of translation initiation with signals derived from leucine (Leu) and insulin to activate mTORC1 signaling. Recent evidence suggests that the duration of the meal response is limited by energy status of the cell and inhibition of translation elongation factor 2 (eEF2). This study evaluates the potential to extend the anabolic meal response with post-meal supplements of Leu or carbohydrates. Adult (~256 g) male Sprague-Dawley rats were food deprived for 12 h, then either euthanized before a standard meal (time 0) or at 90 or 180 min post-meal. At 135 min post-meal, rats received one of five oral supplements: 270 mg leucine (Leu270), 80:40:40 mg leucine, isoleucine, and valine (Leu80), 2.63 g carbohydrates (CHO2.6), 1 g carbohydrates (CHO1.0), or water (Sham control). Following the standard meal, MPS increased at 90 min then declined to pre-meal baseline at 180 min. Rats administered Leu270, Leu80, CHO2.6, or CHO1.0 maintained elevated rates of MPS at 180 min, while Sham controls declined from peak values. Leu80 and CHO1.0 treatments maintained MPS, but with values intermediate between Sham controls and Leu270 and CHO2.6 supplements. Consistent with MPS findings, the supplements maintained elongation activity and cellular energy status by preventing increases in AMP/ATP and phosphorylation of adenosine monophosphate-activated protein kinase (AMPK), acetyl-CoA carboxylase ACC and eEF2. The impact of the supplements on MPS and cellular energy status was in proportion to the energy content within the individual treatments (i.e., Leu270 > Leu80; CHO2.6 > CHO1.0), but the Leu supplements produced a disproportionate anabolic stimulation of MPS, eEF2 and energy status with significantly lower energy content. In summary, the incongruity between MPS and translation initiation at 180 min reflects a block in translation elongation due to reduced cellular energy, and the extent to which Leu or carbohydrate supplements are able to enhance energy status and prolong the period of muscle anabolism are dose and time-dependent.

Effect of tumor burden and subsequent surgical resection on skeletal muscle mass and protein turnover in colorectal cancer patients

Cachexia is a consequence of tumor burden caused by ill-defined catabolic alterations in muscle protein turnover. We aimed to explore the effect of tumor burden and resection on muscle protein turnover in patients with nonmetastatic colorectal cancer (CRC), which is a surgically curable tumor that induces cachexia. We recruited the following 2 groups: patients with CRC [n = 13; mean ± SEM age: 66 ± 3 y; BMI (in kg/m(2)): 27.6 ± 1.1] and matched healthy controls (n = 8; age: 71 ± 2 y; BMI: 26.2 ± 1). Control subjects underwent a single study, whereas CRC patients were studied twice before and ~6 wk after surgical resection to assess muscle protein synthesis (MPS), muscle protein breakdown (MPB), and muscle mass by using dual-energy X-ray absorptiometry. Leg muscle mass was lower in CRC patients than in control subjects (6290 ± 456 compared with 7839 ± 617 g; P < 0.05) and had an additional decline after surgery (5840 ± 456 g; P < 0.001). Although postabsorptive MPS was unaffected, catabolic changes with tumor burden included the complete blunting of postprandial MPS (0.038 ± 0.004%/h in the CRC group compared with 0.065 ± 0.006%/h in the control group; P < 0.01) and a trend toward increased MPB under postabsorptive conditions (P = 0.09). Although surgical resection exacerbated muscle atrophy (-7.2%), catabolic changes in protein metabolism had normalized 6 wk after surgery. The recovery in postprandial MPS after surgery was inversely related to the degree of muscle atrophy (r = 0.65, P < 0.01). CRC patients display reduced postprandial MPS and a trend toward increased MPB, and tumor resection reverses these derangements. With no effective treatment of cancer cachexia, future therapies directed at preserving muscle mass should concentrate on alleviating proteolysis and enhancing anabolic responses to nutrition before surgery while augmenting muscle anabolism after resection.

Technical note: Variation in muscle mass in wild chimpanzees: Application of a modified urinary creatinine method

Individual body size and composition are important variables for a variety of questions about the behavioral ecology and life histories of non-human primates. Standard methodologies for obtaining body mass involve either capture, which poses risks to the subject, or provisioning, which can disrupt the processes being studied. There are no methods currently available to assess body composition from living animals in the wild. Because of its derivation in muscle, the amount of creatinine that an individual excretes in 24 hours is a reliable and frequently used indicator of relative muscle mass in humans and laboratory animals. Although it is not feasible to collect 24-hour urine samples from wild primates, we apply here a simple method to approximate muscle mass variation from collections of spot urine samples. Specific gravity (SG), an alternative method for assessing urinary water content, is both highly correlated to creatinine and free of mass-dependent effects. Individuals with greater muscle mass should excrete more creatinine for a given SG. We examine this relationship in a dataset of 12,598 urine samples from wild chimpanzees in the Kibale National Park, Uganda. As expected from known differences in body composition, the slope of the relationship between SG and creatinine is significantly greater in adult males than adult females and in adults versus immature individuals. Growth curves generated through this method closely approximate published weight curves for wild chimpanzees. Consistent with the role of testosterone in muscle anabolism, urinary testosterone predicted relative creatinine excretion among adult male chimpanzees.

Role and potential mechanisms of anabolic resistance in sarcopenia

There is pressing need to understand the aging process to better cope with its associated physical and societal costs. The age-related muscle wasting known as sarcopenia is a major contributor to the problems faced by the elderly. By hindering mobility and reducing strength, it greatly diminishes independence and quality of life. In studying the factors that contribute to the development of sarcopenia, the focus is shifting to the study of disordered muscle anabolism. The abnormal response of muscle to previously well-established anabolic stimuli is known as anabolic resistance, and may be a key factor in the development and progression of sarcopenia. Factors such as age, obesity, inflammation, and lipotoxicity contribute to anabolic resistance, and have been studied either directly or indirectly in cell systems and whole animals. Understanding the physiologic and mechanistic basis of anabolic resistance could be the key to formulating new and targeted interventions that would ease the burden currently borne by the world’s aged population.

Influence of carbohydrate supplementation during resistance training on concentrations of the hormones cortisol and insulin

This study analysed the influence of maltodextrin ingestion before and during a single bout of resistance exercise on muscle damage and catabolic/anabolic hormone responses. Seven young men previously trained in resistance exercises performed two sessions each comprising ten exercises for the upper limbs, accompanied by ingestion of a solution containing 8% carbohydrate or placebo. Blood samples were obtained before and after training for analysis of insulin, cortisol and creatine kinase, and during the training period (after the second, fourth, sixth, eighth and tenth exercise) for glucose analysis. At all time-points, glucose was found to be significantly higher following ingestion of carbohydrate. Although the training led to an increase in cortisol, no differences were noted between the carbohydrate and placebo sessions. However, the levels of insulin after training were approximately three times higher following ingestion of carbohydrate (28.7 ± 3 μU/ml) than following ingestion of placebo (9.5 ± 2.2 μU/ml, p <0.05. We conclude that carbohydrate ingestion before and during resistance exercises does not affect catabolic activity, but provides an important anabolic muscle stimulus.

Leucine: a nutrient 'trigger' for muscle anabolism, but what more?

Leucine: a nutrient 'trigger' for muscle anabolism, but what more?

MTORC1 and the regulation of skeletal muscle anabolism and mass

The mass and integrity of skeletal muscle is vital to whole-body substrate metabolism and health. Indeed, defects in muscle metabolism and functions underlie or exacerbate diseases like diabetes, rheumatoid arthritis, and cancer. Physical activity and nutrition are the 2 most important environmental factors that can affect muscle health. At the molecular level, the mammalian target of rapamycin complex 1 (mTORC1) is a critical signalling complex that regulates muscle mass. In response to nutrition and resistance exercise, increased muscle mass and activation of mTORC1 occur in parallel. In this review, we summarize recent findings on mTORC1 and its regulation in skeletal muscle in response to resistance exercise, alone or in combination with intake of protein or amino acids. Because increased activity of the complex is implicated in the development of muscle insulin resistance, obesity, and some cancers (e.g., ovarian, breast), drugs that target mTORC1 are being developed or are in clinical trials. However, various cancers are associated with extensive muscle wasting, due in part to tumour burden and malnutrition. This muscle wasting may also be a side effect of anticancer drugs. Because loss of muscle mass is associated not only with metabolic abnormalities but also dose limiting toxicity, we review the possible implications for skeletal muscle of long-term inhibition of mTORC1, especially in muscle wasting conditions.

Effects of twice‐daily ingestion of essential amino acids on amino acid transporter transcript and protein expression in older adults prior to total knee arthroplasty

Following total knee arthroplasty (TKA) surgery, persistent muscle atrophy and weakness are the greatest clinical barriers to functional recovery. Essential amino acid (EAA) ingestion has been shown to be a potent stimulator of muscle anabolism and to acutely stimulate amino acid transporter mRNA and protein levels in muscle. We examined the effect of twice‐daily EAA supplementation on amino acid transporter gene expression and total protein levels. In this placebo controlled study muscle biopsies were obtained after an overnight fast, under anesthesia immediately prior to TKA surgery following seven days of twice‐daily ingestion of 20g EAA (n=9) or placebo (n=7). Biopsies were analyzed for transcript and protein expression of amino acid transporters using quantitative PCR and Western blotting and compared to placebo. Preliminary results indicate significantly higher mRNA levels for SNAT2 (p&lt;0.01), SNAT4 (p&lt;0.05), and LAT3 (p&lt;0.01) and suggest trends toward increased mRNA for LAT1 (p&lt;0.10) and PAT2 (p&lt;0.10) and decreased mRNA for CAT2 (p&lt;0.10). Total protein levels for CD98 and LAT1 were unchanged between groups. Total protein levels for CD98 and LAT1 were unchanged between groups. These preliminary data suggest EAA supplementation may up‐regulate amino acid transporters at the transcriptional level and regulation of these transporters may involve independent pathways. Support: NICHD K01HD057332