Obesity Appears to Be Associated With Altered Muscle Protein Synthetic and Breakdown Responses to Increased Nutrient Delivery in Older Men, but Not Reduced Muscle Mass or Contractile Function

Obesity is increasing, yet despite the necessity of maintaining muscle mass and function with age, the effect of obesity on muscle protein turnover in older adults remains unknown. Eleven obese (BMI 31.9 ± 1.1 kg · m(-2)) and 15 healthy-weight (BMI 23.4 ± 0.3 kg · m(-2)) older men (55-75 years old) participated in a study that determined muscle protein synthesis (MPS) and leg protein breakdown (LPB) under postabsorptive (hypoinsulinemic-euglycemic clamp) and postprandial (hyperinsulinemic hyperaminoacidemic-euglycemic clamp) conditions. Obesity was associated with systemic inflammation, greater leg fat mass, and patterns of mRNA expression consistent with muscle deconditioning, whereas leg lean mass, strength, and work done during maximal exercise were no different. Under postabsorptive conditions, MPS and LPB were equivalent between groups, whereas insulin and amino acid administration increased MPS in only healthy-weight subjects and was associated with lower leg glucose disposal (LGD) (63%) in obese men. Blunting of MPS in the obese men was offset by an apparent decline in LPB, which was absent in healthy-weight subjects. Lower postprandial LGD in obese subjects and blunting of MPS responses to amino acids suggest that obesity in older adults is associated with diminished muscle metabolic quality. This does not, however, appear to be associated with lower leg lean mass or strength.

Lipid-induced insulin resistance is associated with an impaired skeletal muscle protein synthetic response to amino acid ingestion in healthy young men

The ability to maintain skeletal muscle mass appears to be impaired in insulin-resistant conditions, such as type 2 diabetes, that are characterized by muscle lipid accumulation. The current study investigated the effect of acutely increasing lipid availability on muscle protein synthesis. Seven healthy young male volunteers underwent a 7-h intravenous infusion of l-[ring-(2)H5]phenylalanine on two randomized occasions combined with 0.9% saline or 10% Intralipid at 100 mL/h. After a 4-h "basal" period, a 21-g bolus of amino acids was administered and a 3-h hyperinsulinemic-euglycemic clamp was commenced ("fed" period). Muscle biopsy specimens were obtained from the vastus lateralis at 1.5, 4, and 7 h. Lipid infusion reduced fed whole-body glucose disposal by 20%. Furthermore, whereas the mixed muscle fractional synthetic rate increased from the basal to the fed period during saline infusion by 2.2-fold, no change occurred during lipid infusion, despite similar circulating insulin and leucine concentrations. This "anabolic resistance" to insulin and amino acids with lipid infusion was associated with a complete suppression of muscle 4E-BP1 phosphorylation. We propose that increased muscle lipid availability may contribute to anabolic resistance in insulin-resistant conditions by impairing translation initiation.

Mechanisms regulating muscle mass during disuse atrophy and rehabilitation in humans

Muscle mass loss accompanies periods of bedrest and limb immobilization in humans and requires rehabilitation exercise to effectively restore mass and function. Although recent evidence points to an early and transient rise in muscle protein breakdown contributing to this decline in muscle mass, the driving factor seems to be a reduction in muscle protein synthesis, not least in part due to the development of anabolic resistance to amino acid provision. Although the AKT signaling pathway has been identified in small animals as central to the regulation of muscle protein synthesis, several studies in humans have now demonstrated a disassociation between AKT signaling and muscle protein synthesis during feeding, exercise, and immobilization, suggesting that the mechanisms regulating protein synthesis in human skeletal muscle are more complex than initially thought (at least in non-inflammatory states). During rehabilitation, exercise-induced myogenesis may in part be responsible for the recovery of muscle mass. Rapid and sustained exercise-induced suppression of myostatin mRNA expression, that precedes any gain in muscle mass, points to this, along with other myogenic proteins, as being potential regulators of muscle regeneration during exercise rehabilitation in humans.

Temporal changes in the involvement of pyruvate dehydrogenase complex in muscle lactate accumulation during lipopolysaccharide infusion in rats

A characteristic manifestation of sepsis is muscle lactate accumulation. This study examined any putative (causative) association between pyruvate dehydrogenase complex (PDC) inhibition and lactate accumulation in the extensor digitorum longus (EDL) muscle of rats infused with lipopolysaccharide (LPS), and explored the involvement of increased transcription of muscle-specific pyruvate dehydrogenase kinase (PDK) isoenzymes. Conscious, male Sprague-Dawley rats were infused i.v. with saline (0.4 ml h(-1), control) or LPS (150 mug kg(-1) h(-1)) for 2 h, 6 h or 24 h (n = 6-8). Muscle lactate concentration was elevated after 2, 6 and 24 h LPS infusion. Muscle PDC activity was the same at 2 h and 6 h, but was 65% lower after 24 h of LPS infusion (P < 0.01), when there was a 47% decrease in acetylcarnitine concentration (P < 0.05), and a 24-fold increase in PDK4 mRNA expression (P < 0.001). These changes were preceded by marked increases in tumour necrosis factor-alpha and interleukin-6 mRNA expression at 2 h. The findings indicate that the early (2 and 6 h) elevation in muscle lactate concentration during LPS infusion was not attributable to limited muscle oxygen availability or ATP production (evidenced by unchanged ATP and phosphocreatine (PCr) concentrations) or to PDC inhibition, whereas after 24 h, muscle lactate accumulation appears to have resulted from PDC activation status limiting pyruvate flux, most probably due to cytokine-mediated up-regulation of PDK4 transcription.

Therapeutic exercise in cancer cachexia

This review takes into account experimental and clinical evidence to provide an overview of the rationale for the use of exercise in cancer cachexia, its clinical application, and future developments. Studies targeting cachectic patients have demonstrated that even in advanced disease peripheral muscle has the capacity to respond to exercise training. Effects of exercise include enhancing muscle protein synthesis, attenuating the catabolic effects of cachexia, and modulating levels of inflammation. Nonetheless, there are challenges in applying therapeutic exercise, particularly once cachexia is established, and not all patients are able or willing to undertake programs currently being offered. Strategies to make exercise a more accessible therapy are required and could include offering it earlier in the course of the disease, at lower intensities, and in various forms, including novel approaches. In conclusion, the use of therapeutic exercise has a sound rationale, even in patients with advanced disease and cachexia, and has the potential to help maintain or slow the loss of physical function. Because of practical issues with its application, further study is required to examine if the benefits achieved in small studies can be translated to a wider clinical population.

Physiological control of muscle mass in humans during resistance exercise, disuse and rehabilitation

PURPOSE OF REVIEW

The preservation of skeletal muscle mass is central to maintaining mobility and quality of life with aging and also impacts on our capacity to recover from illness. However, our understanding of the processes that regulate muscle mass in humans during exercise, chronic disuse and rehabilitation remains unclear. This brief review aims to highlight some of the more recent and important findings concerning these physiological stimuli.

RECENT FINDINGS

Although several studies have detailed the molecular events that occur following an acute bout of resistance exercise, a paucity of data appears to remain concerning the molecular and signaling events that underpin resistance exercise training. Reports of increased transcripts for inflammatory proteins following eccentric but not concentric exercise could represent the stimulus for the instigation of structural adaptations that occur following intense muscle lengthening contractions. Studies investigating processes underlying disuse-induced muscle atrophy provide initial evidence to support the notion that transient increases in muscle protein degradation occur following the onset of muscle disuse in humans.

SUMMARY

The need for further studies to improve our basic understanding of muscle-associated processes in humans remains, particularly in relation to the temporal changes in muscle processes that occur during resistance training.

Resistance exercise and the mechanisms of muscle mass regulation in humans: acute effects on muscle protein turnover and the gaps in our understanding of chronic resistance exercise training adaptation

Increasing muscle mass is important when attempting to maximize sports performance and achieve physique augmentation. However, the preservation of muscle mass is essential to maintaining mobility and quality of life with aging, and also impacts on our capacity to recover from illness. Nevertheless, our understanding of the processes that regulate muscle mass in humans during resistance exercise training, chronic disuse and rehabilitation training following atrophy remains very unclear. Here, we report on some of the recent developments in the study of those processes thought to be responsible for governing human muscle protein turnover in response to intense physical activity. Specifically, the effects of acute and chronic resistance exercise in healthy volunteers and also in response to rehabilitation resistance exercise training following muscle atrophy will be discussed, with discrepancies and gaps in our understanding highlighted. In particular, ubiquitin-proteasome mediated muscle proteolysis (Muscle Atrophy F-box/Atrogin-1 and Muscle RING Finger 1), translation initiation of muscle protein synthesis (mammalian target of rapamycin signaling), and satellite cell mediated myogenesis are highlighted as pathways of special relevance to muscle protein metabolism in response to acute resistance exercise. Furthermore, research focused on quantifying signaling and molecular events that modulate muscle protein synthesis and protein degradation under conditions of chronic resistance training is highlighted as being urgently needed to improve knowledge gaps. These studies need to include multiple time-point measurements over the course of any training intervention and must include dynamic measurements of muscle protein synthesis and degradation and sensitive measures of muscle mass. This article is part of a Directed Issue entitled Molecular basis of muscle wasting.

Consequences of Late-Stage Non-Small-Cell Lung Cancer Cachexia on Muscle Metabolic Processes

INTRODUCTION

The loss of muscle is common in patients with advanced non-small-cell lung cancer (NSCLC) and contributes to the high morbidity and mortality of this group. The exact mechanisms behind the muscle loss are unclear.

PATIENTS AND METHODS

To investigate this, 4 patients with stage IV NSCLC who met the clinical definitions for sarcopenia and cachexia were recruited, along with 4 age-matched healthy volunteers. After an overnight fast, biopsy specimens were obtained from the vastus lateralis, and the key components associated with inflammation and the control of muscle protein, carbohydrate, and fat metabolism were assessed.

RESULTS

Compared with the healthy volunteers, significant increases in mRNA levels for interleukin-6 and NF-κB signaling and lower intramyocellular lipid content in slow-twitch fibers were observed in NSCLC patients. Although a significant decrease in phosphorylation of the mechanistic target of rapamycin (mTOR) signaling protein 4E-BP1 (Ser⁶⁵) was observed, along with a trend toward reduced p70 S6K (Thr³⁸⁹) phosphorylation (P = .06), no difference was found between groups for the mRNA levels of MAFbx (muscle atrophy F box) and MuRF1 (muscle ring finger protein 1), chymotrypsin-like activity of the proteasome, or protein levels of multiple proteasome subunits. Moreover, despite decreases in intramyocellular lipid content, no robust changes in mRNA levels for key proteins involved in insulin signaling, glycolysis, oxidative metabolism, or fat metabolism were observed.

CONCLUSION

These findings suggest that examining the contribution of suppressed mTOR signaling in the loss of muscle mass in late-stage NSCLC patients is warranted and reinforces our need to understand the potential contribution of impaired fat metabolism and muscle protein synthesis in the etiology of cancer cachexia.

A mycoprotein-based high-protein vegan diet supports equivalent daily myofibrillar protein synthesis rates compared with an isonitrogenous omnivorous diet in older adults: a randomised controlled trial

Animal-derived dietary protein ingestion and physical activity stimulate myofibrillar protein synthesis rates in older adults. We determined whether a non-animal-derived diet can support daily myofibrillar protein synthesis rates to the same extent as an omnivorous diet. Nineteen healthy older adults (aged 66 (sem 1) years; BMI 24 (sem 1) kg/m2; twelve males, seven females) participated in a randomised, parallel-group, controlled trial during which they consumed a 3-d isoenergetic high-protein (1·8 g/kg body mass per d) diet, where the protein was provided from predominantly (71 %) animal (OMNI; n 9; six males, three females) or exclusively vegan (VEG; n 10; six males, four females; mycoprotein providing 57 % of daily protein intake) sources. During the dietary control period, participants conducted a daily bout of unilateral resistance-type leg extension exercise. Before the dietary control period, participants ingested 400 ml of deuterated water, with 50-ml doses consumed daily thereafter. Saliva samples were collected throughout to determine body water 2H enrichments, and muscle samples were collected from rested and exercised muscle to determine daily myofibrillar protein synthesis rates. Deuterated water dosing resulted in body water 2H enrichments of approximately 0·78 (sem 0·03) %. Daily myofibrillar protein synthesis rates were 13 (sem 8) (P = 0·169) and 12 (sem 4) % (P = 0·016) greater in the exercised compared with rested leg (1·59 (sem 0·12) v. 1·77 (sem 0·12) and 1·76 (sem 0·14) v. 1·93 (sem 0·12) %/d) in OMNI and VEG groups, respectively. Daily myofibrillar protein synthesis rates did not differ between OMNI and VEG in either rested or exercised muscle (P > 0·05). Over the course of a 3-d intervention, omnivorous- or vegan-derived dietary protein sources can support equivalent rested and exercised daily myofibrillar protein synthesis rates in healthy older adults consuming a high-protein diet.

Improved recovery from skeletal muscle damage is largely unexplained by myofibrillar protein synthesis or inflammatory and regenerative gene expression pathways

The contribution of myofibrillar protein synthesis (MyoPS) to recovery from skeletal muscle damage in humans is unknown. Recreationally active men and women consumed a daily protein-polyphenol beverage targeted at increasing amino acid availability and reducing inflammation (PPB; n = 9), both known to affect MyoPS, or an isocaloric placebo (PLA; n = 9) during 168 h of recovery from 300 maximal unilateral eccentric contractions (EE). Muscle function was assessed daily. Muscle biopsies were collected for 24, 27, 36, 72, and 168 h for MyoPS measurements using ²H₂O and expression of 224 genes using RT-qPCR and pathway analysis. PPB improved recovery of muscle function, which was impaired for 5 days after EE in PLA (interaction P < 0.05). Acute postprandial MyoPS rates were unaffected by nutritional intervention (24-27 h). EE increased overnight (27-36 h) MyoPS versus the control leg (PLA: 33 ± 19%; PPB: 79 ± 25%; leg P < 0.01), and PPB tended to increase this further (interaction P = 0.06). Daily MyoPS rates were greater with PPB between 72 and 168 h after EE, albeit after function had recovered. Inflammatory and regenerative signaling pathways were dramatically upregulated and clustered after EE but were unaffected by nutritional intervention. These results suggest that accelerated recovery from EE is not explained by elevated MyoPS or suppression of inflammation.NEW & NOTEWORTHY The present study investigated the contribution of myofibrillar protein synthesis (MyoPS) and associated gene signaling to recovery from 300 muscle-damaging, eccentric contractions. Measured with ²H₂O, MyoPS rates were elevated during recovery and observed alongside expression of inflammatory and regenerative signaling pathways. A nutritional intervention accelerated recovery; however, MyoPS and gene signaling were unchanged compared with placebo. These data indicate that MyoPS and associated signaling do not explain accelerated recovery from muscle damage.

The commercial pig as a model of spontaneously-occurring osteoarthritis

BACKGROUND

Preclinical osteoarthritis models where damage occurs spontaneously may better reflect the initiation and development of human osteoarthritis. The aim was to assess the commercial pig as a model of spontaneous osteoarthritis development by examining pain-associated behaviour, joint cartilage integrity, as well as the use of porcine cartilage explants and isolated chondrocytes and osteoblasts for ex vivo and in vitro studies.

METHODS

Female pigs (Large white x Landrace x Duroc) were examined at different ages from 6 weeks to 3-4 years old. Lameness was assessed as a marker of pain-associated behaviour. Femorotibial joint cartilage integrity was determined by chondropathy scoring and histological staining of proteoglycan. IL-6 production and proteoglycan degradation was assessed in cartilage explants and primary porcine chondrocytes by ELISA and DMMB assay. Primary porcine osteoblasts from damaged and non-damaged joints, as determined by chondropathy scoring, were assessed for mineralisation, proliferative and mitochondrial function as a marker of metabolic capacity.

RESULTS

Pigs aged 80 weeks and older exhibited lameness. Osteoarthritic lesions in femoral condyle and tibial plateau cartilage were apparent from 40 weeks and increased in severity with age up to 3-4 years old. Cartilage from damaged joints exhibited proteoglycan loss, which positively correlated with chondropathy score. Stimulation of porcine cartilage explants and primary chondrocytes with either IL-1β or visfatin induced IL-6 production and proteoglycan degradation. Primary porcine osteoblasts from damaged joints exhibited reduced proliferative, mineralisation, and metabolic capacity.

CONCLUSION

In conclusion, the commercial pig represents an alternative model of spontaneous osteoarthritis and an excellent source of tissue for in vitro and ex vivo studies.

Muscle atrophy in immobilization and senescence in humans

PURPOSE OF REVIEW

Recent reports exploring the mechanisms thought to be responsible for the determination of muscle mass during health, ageing and immobilization in humans have presented findings that have wide ranging implications. This brief review highlights some of the more important findings.

RECENT FINDINGS

Contrary to expectations, recent findings suggest an apparent dissociation between muscle signalling pathways and their associated events in humans, particularly in relation to muscle protein synthesis. Although debate concerning the relative importance of muscle protein synthesis and degradation to muscle mass loss during immobilization continues, and the mechanisms responsible for this loss and its restoration during rehabilitation remain unclear, new evidence has emerged showing that anabolic resistance to protein nutrition develops during immobilization. This latter observation is in agreement with earlier evidence pointing to anabolic resistance of muscle to protein nutrition existing in the elderly, which is of clinical importance. Recent observations also suggest that a sex difference exists in the rate of muscle protein synthesis under postabsorptive conditions in the elderly and may explain why women lose muscle mass at a slower rate with age than men.

SUMMARY

These recent findings highlight our current lack of understanding of the mechanisms that regulate muscle mass in humans.

Vegan and Omnivorous High Protein Diets Support Comparable Daily Myofibrillar Protein Synthesis Rates and Skeletal Muscle Hypertrophy in Young Adults

BACKGROUND

It remains unclear whether non-animal-derived dietary protein sources (and therefore vegan diets) can support resistance training-induced skeletal muscle remodeling to the same extent as animal-derived protein sources.

METHODS

In Phase 1, 16 healthy young adults (m = 8, f = 8; age: 23 ± 1 y; BMI: 23 ± 1 kg/m²) completed a 3-d dietary intervention (high protein, 1.8 g·kg bm^-1·d^-1) where protein was derived from omnivorous (OMNI1; n = 8) or exclusively non-animal (VEG1; n = 8) sources, alongside daily unilateral leg resistance exercise. Resting and exercised daily myofibrillar protein synthesis (MyoPS) rates were assessed using deuterium oxide. In Phase 2, 22 healthy young adults (m = 11, f = 11; age: 24 ± 1 y; BMI: 23 ± 0 kg/m²) completed a 10 wk, high-volume (5 d/wk), progressive resistance exercise program while consuming an omnivorous (OMNI2; n = 12) or non-animal-derived (VEG2; n = 10) high-protein diet (∼2 g·kg bm^-1·d^-1). Muscle fiber cross-sectional area (CSA), whole-body lean mass (via DXA), thigh muscle volume (via MRI), muscle strength, and muscle function were determined pre, after 2 and 5 wk, and postintervention.

OBJECTIVES

To investigate whether a high-protein, mycoprotein-rich, non-animal-derived diet can support resistance training-induced skeletal muscle remodeling to the same extent as an isonitrogenous omnivorous diet.

RESULTS

Daily MyoPS rates were ∼12% higher in the exercised than in the rested leg (2.46 ± 0.27%·d^-1 compared with 2.20 ± 0.33%·d^-1 and 2.62 ± 0.56%·d^-1 compared with 2.36 ± 0.53%·d^-1 in OMNI1 and VEG1, respectively; P < 0.001) and not different between groups (P > 0.05). Resistance training increased lean mass in both groups by a similar magnitude (OMNI2 2.6 ± 1.1 kg, VEG2 3.1 ± 2.5 kg; P > 0.05). Likewise, training comparably increased thigh muscle volume (OMNI2 8.3 ± 3.6%, VEG2 8.3 ± 4.1%; P > 0.05), and muscle fiber CSA (OMNI2 33 ± 24%, VEG2 32 ± 48%; P > 0.05). Both groups increased strength (1 repetition maximum) of multiple muscle groups, to comparable degrees.

CONCLUSIONS

Omnivorous and vegan diets can support comparable rested and exercised daily MyoPS rates in healthy young adults consuming a high-protein diet. This translates to similar skeletal muscle adaptive responses during prolonged high-volume resistance training, irrespective of dietary protein provenance. This trial was registered at clinicaltrials.gov as NCT03572127.

Rehabilitative Exercise Training for Burn Injury

Due to improvements in acute burn care over the last few decades, most patients with severe burns (up to 90% of the total body surface) survive. However, the metabolic and cardiovascular complications that accompany a severe burn can persist for up to 3 years post injury. Accordingly, there is now a greater appreciation of the need for strategies that can hasten recovery and reduce long-term morbidity post burn. Rehabilitation exercise training (RET) is a proven effective treatment to restore lean body mass, glucose and protein metabolism, cardiorespiratory fitness, and muscle strength in burn survivors. Despite this, very few hospitals incorporate RET in programs to aid the rehabilitation of patients with severe burns. Given that RET is a safe and efficacious treatment that restores function and reduces post-burn morbidity, we propose that a long-term exercise prescription plan should be considered for all patients with severe burns. In this literature review, we discuss the current understanding of burn trauma on major organ systems, and the positive benefits of incorporating RET as a part of the long-term rehabilitation of severely burned individuals. We also provide burn-specific exercise prescription guidelines for clinical exercise physiologists.

Reducing NF-κB Signaling Nutritionally is Associated with Expedited Recovery of Skeletal Muscle Function After Damage

CONTEXT

The early events regulating the remodeling program following skeletal muscle damage are poorly understood.

OBJECTIVE

The objective of this study was to determine the association between myofibrillar protein synthesis (myoPS) and nuclear factor-kappa B (NF-κB) signaling by nutritionally accelerating the recovery of muscle function following damage.

DESIGN, SETTING, PARTICIPANTS, AND INTERVENTIONS

Healthy males and females consumed daily postexercise and prebed protein-polyphenol (PP; n = 9; 4 females) or isocaloric maltodextrin placebo (PLA; n = 9; 3 females) drinks (parallel design) 6 days before and 3 days after 300 unilateral eccentric contractions of the quadriceps during complete dietary control.

MAIN OUTCOME MEASURES

Muscle function was assessed daily, and skeletal muscle biopsies were taken after 24, 27, and 36 hours for measurements of myoPS rates using deuterated water, and gene ontology and NF-κB signaling analysis using a quantitative reverse transcription PCR (RT-qPCR) gene array.

RESULTS

Eccentric contractions impaired muscle function for 48 hours in PLA intervention, but just for 24 hours in PP intervention (P = 0.047). Eccentric quadricep contractions increased myoPS compared with the control leg during postexercise (24-27 hours; 0.14 ± 0.01 vs 0.11 ± 0.01%·h-1, respectively; P = 0.075) and overnight periods (27-36 hours; 0.10 ± 0.01 vs 0.07 ± 0.01%·h-1, respectively; P = 0.020), but was not further increased by PP drinks (P > 0.05). Protein-polyphenol drinks decreased postexercise and overnight muscle IL1R1 (PLA = 2.8 ± 0.4, PP = 1.1 ± 0.4 and PLA = 1.9 ± 0.4, PP = 0.3 ± 0.4 log2 fold-change, respectively) and IL1RL1 (PLA = 4.9 ± 0.7, PP = 1.6 ± 0.8 and PLA = 3.7 ± 0.6, PP = 0.7 ± 0.7 log2 fold-change, respectively) messenger RNA expression (P < 0.05) and downstream NF-κB signaling compared with PLA.

CONCLUSION

Protein-polyphenol drink ingestion likely accelerates recovery of muscle function by attenuating inflammatory NF-κB transcriptional signaling, possibly to reduce aberrant tissue degradation rather than increase myoPS rates.

Effects of endotoxaemia on protein metabolism in rat fast-twitch skeletal muscle and myocardium

BACKGROUND

It is unclear if the rat myocardium undergoes the same rapid reductions in protein content that are classically observed in fast-twitch skeletal muscle during endotoxaemia.

METHODOLOGY/PRINCIPAL FINDINGS

To investigate this further, and to determine if there is any divergence in the response of skeletal muscle and myocardium in the mechanisms that are thought to be largely responsible for eliciting changes in protein content, Sprague Dawley rats were implanted with vascular catheters and administered lipopolysaccharide (LPS; 150 microg kg(-1) h(-1)) intravenously for 2 h, 6 h or 24 h (saline administered control animals were also included), after which the extensor digitorum longus (EDL) and myocardium were removed under terminal anaesthesia. The protein-to-DNA ratio, a marker of protein content, was significantly reduced in the EDL following 24 h LPS administration (23%; P<0.05), but was no different from controls in the myocardium. At the same time point, a significant increase in MAFbx/atrogin-1 and MuRF1 mRNA (3.7+/-0.7- and 19.5+/-1.9-fold increase vs. controls, respectively; P<0.05), in addition to protein levels of alpha1-3, 5-7 subunits of the 20S proteasome, were observed in EDL but not myocardium. In contrast, elevations in phosphorylation of p70 S6K residues Thr(421)/Ser(424), and 4E-BP1 residues Thr(37)/Thr(46) (P<0.05), consistent with an elevation in translation initiation, were seen exclusively in the myocardium of LPS-treated animals.

CONCLUSIONS/SIGNIFICANCE

In summary, these findings suggest that the myocardium does not undergo the same catabolic response as skeletal muscle during early endotoxaemia, partly due to the absence of transcriptional and signalling events in the myocardium typically associated with increased muscle proteolysis and the suppression of protein synthesis.

Quantifying label enrichment from two mass isotopomers increases proteome coverage for in vivo protein turnover using heavy water metabolic labeling

Heavy water metabolic labeling followed by liquid chromatography coupled with mass spectrometry is a powerful high throughput technique for measuring the turnover rates of individual proteins in vivo. The turnover rate is obtained from the exponential decay modeling of the depletion of the monoisotopic relative isotope abundance. We provide theoretical formulas for the time course dynamics of six mass isotopomers and use the formulas to introduce a method that utilizes partial isotope profiles, only two mass isotopomers, to compute protein turnover rate. The use of partial isotope profiles alleviates the interferences from co-eluting contaminants in complex proteome mixtures and improves the accuracy of the estimation of label enrichment. In five different datasets, the technique consistently doubles the number of peptides with high goodness-of-fit characteristics of the turnover rate model. We also introduce a software tool, d2ome+, which automates the protein turnover estimation from partial isotope profiles.

Short-term disuse does not affect postabsorptive or postprandial muscle protein fractional breakdown rates

BACKGROUND

The decline in postabsorptive and postprandial muscle protein fractional synthesis rates (FSR) does not quantitatively account for muscle atrophy during uncomplicated, short-term disuse, when atrophy rates are the highest. We sought to determine whether 2 days of unilateral knee immobilization affects mixed muscle protein fractional breakdown rates (FBR) during postabsorptive and simulated postprandial conditions.

METHODS

Twenty-three healthy, male participants (age: 22 ± 1 year; height: 179 ± 1 cm; body mass: 73.4 ± 1.5 kg; body mass index 22.8 ± 0.5 kg·m-2 ) took part in this randomized, controlled study. After 48 h of unilateral knee immobilization, primed continuous intravenous l-[15 N]-phenylalanine and l-[ring-2 H5 ]-phenylalanine infusions were used for parallel determinations of FBR and FSR, respectively, in a postabsorptive (saline infusion; FAST) or simulated postprandial state (67.5 mg·kg body mass-1 ·h-1 amino acid infusion; FED). Bilateral m. vastus lateralis biopsies from the control (CON) and immobilized (IMM) legs, and arterialized-venous blood samples, were collected throughout.

RESULTS

Amino acid infusion rapidly increased plasma phenylalanine (59 ± 9%), leucine (76 ± 5%), isoleucine (109 ± 7%) and valine (42 ± 4%) concentrations in FED only (all P < 0.001), which was sustained for the remainder of infusion. Serum insulin concentrations peaked at 21.8 ± 2.2 mU·L-1 at 15 min in FED only (P < 0.001) and were 60% greater in FED than FAST (P < 0.01). Immobilization did not influence FBR in either FAST (CON: 0.150 ± 0.018; IMM: 0.143 ± 0.017%·h-1 ) or FED (CON: 0.134 ± 0.012; IMM: 0.160 ± 0.018%·h-1 ; all effects P > 0.05). However, immobilization decreased FSR (P < 0.05) in both FAST (0.071 ± 0.004 vs. 0.086 ± 0.007%·h-1 ; IMM vs CON, respectively) and FED (0.066 ± 0.016 vs. 0.119 ± 0.016%·h-1 ; IMM vs CON, respectively). Consequently, immobilization decreased net muscle protein balance (P < 0.05) and to a greater extent in FED (CON: -0.012 ± 0.025; IMM: -0.095 ± 0.023%·h-1 ; P < 0.05) than FAST (CON: -0.064 ± 0.020; IMM: -0.072 ± 0.017%·h-1 ).

CONCLUSIONS

We conclude that merely 2 days of leg immobilization does not modulate postabsorptive and simulated postprandial muscle protein breakdown rates. Instead, under these conditions the muscle negative muscle protein balance associated with brief periods of experimental disuse is driven near exclusively by reduced basal muscle protein synthesis rates and anabolic resistance to amino acid administration.

Brown adipose tissue recruitment in a rodent model of severe burns

BACKGROUND

Severe burns results in a prolonged hypermetabolic response. Brown adipose tissue (BAT), abundant in uncoupling protein 1 (UCP1), plays a key role in non-shivering thermogenesis. We set out to determine if BAT is recruited in response to severe burns.

METHODS

Male balb-c mice underwent scald burns on approximately 20-25% of their total body surface. BAT was harvested from the interscapular fat pad of sham and burned mice at 3h, 24h, 4 days, and 10 days after injury. High-resolution respirometry was used to determine mitochondrial respiratory function in BAT. BAT protein concentration, and mitochondrial enzyme activity were also determined.

RESULTS

Respiration increased in BAT of burned mice, peaking at 24h after injury (after injury, P<0.001). While UCP1 independent respiration was not significantly altered by burn, UCP1 dependent respiration increased >2-fold at 24h after injury when compared to the 3h and sham group (P<0.01). Normalized to citrate synthase activity, total uncoupled (P<0.05) and UCP1 dependent (P<0.01) respiration remained elevated at 24h after injury.

CONCLUSIONS

We show a time-dependent recruitment of rodent BAT in response to severe burns. Given recent reports that humans, including patients with severe burns, have functional BAT, these data support a role for BAT in the hypermetabolic response to severe burns.

Ingestion of mycoprotein, pea protein, and their blend support comparable postexercise myofibrillar protein synthesis rates in resistance-trained individuals

Pea protein is an attractive nonanimal-derived protein source to support dietary protein requirements. However, although high in leucine, a low methionine content has been suggested to limit its anabolic potential. Mycoprotein has a complete amino acid profile which, at least in part, may explain its ability to robustly stimulate myofibrillar protein synthesis (MyoPS) rates. We hypothesized that an inferior postexercise MyoPS response would be seen following ingestion of pea protein compared with mycoprotein, which would be (partially) rescued by blending the two sources. Thirty-three healthy, young [age: 21 ± 1 yr, body mass index (BMI): 24 ± 1 kg·m-2] and resistance-trained participants received primed, continuous infusions of l-[ring-2H5]phenylalanine and completed a bout of whole body resistance exercise before ingesting 25 g of protein from mycoprotein (MYC, n = 11), pea protein (PEA, n = 11), or a blend (39% MYC, 61% PEA) of the two (BLEND, n = 11). Blood and muscle samples were taken pre-, 2 h, and 4 h postexercise/protein ingestion to assess postabsorptive and postprandial postexercise myofibrillar protein fractional synthetic rates (FSRs). Protein ingestion increased plasma essential amino acid and leucine concentrations (time effect; P < 0.0001), but more rapidly in BLEND and PEA compared with MYC (time × condition interaction; P < 0.0001). From similar postabsorptive values (MYC, 0.026 ± 0.008%·h-1; PEA, 0.028 ± 0.007%·h-1; BLEND, 0.026 ± 0.006%·h-1), resistance exercise and protein ingestion increased myofibrillar FSRs (time effect; P < 0.0001) over a 4-h postprandial period (MYC, 0.076 ± 0.004%·h-1; PEA, 0.087 ± 0.01%·h-1; BLEND, 0.085 ± 0.01%·h-1), with no differences between groups (all; P > 0.05). These data show that all three nonanimal-derived protein sources have utility in supporting postexercise muscle reconditioning.NEW & NOTEWORTHY This study provides evidence that pea protein (PEA), mycoprotein (MYC), and their blend (BLEND) can support postexercise myofibrillar protein synthesis rates following a bout of whole body resistance exercise. Furthermore, these data suggest that a methionine deficiency in pea may not limit its capacity to stimulate an acute increase in muscle protein synthesis (MPS).

The impact of catecholamines on skeletal muscle following massive burns: Friend or foe?

Profound skeletal muscle wasting in the setting of total body hypermetabolism is a defining characteristic of massive burns, compromising the patient's recovery and necessitating a protracted period of rehabilitation. In recent years, the prolonged use of the non-selective beta-blocker, propranolol, has gained prominence as an effective tool to assist with suppressing epinephrine-dependent burn-induced hypermetabolism and by extension, blunting muscle catabolism. However, synthetic β-adrenergic agonists, such as clenbuterol, are widely associated with the promotion of muscle growth in both animals and humans. Moreover, experimental adrenodemedullation is known to result in muscle catabolism. Therefore, the blunting of muscle β-adrenergic signaling via the use of propranolol would be expected to negatively impair muscle protein homeostasis. This review explores these paradoxical observations and identifies the manner by which propranolol is thought to exert its anti-catabolic effects in burn patients. Moreover, we identify potential avenues by which the use of beta-blocker therapy in the treatment of massive burns could potentially be further refined to promote the recovery of muscle mass in these critically ill patients while continuing to ameliorate total body hypermetabolism.

Mycoprotein ingestion within or without its wholefood matrix results in equivalent stimulation of myofibrillar protein synthesis rates in resting and exercised muscle of young men

Ingestion of mycoprotein stimulates skeletal muscle protein synthesis (MPS) rates to a greater extent than concentrated milk protein when matched for leucine content, potentially attributable to the wholefood nature of mycoprotein. We hypothesised that bolus ingestion of mycoprotein as part of its wholefood matrix would stimulate MPS rates to a greater extent compared with a leucine-matched bolus of protein concentrated from mycoprotein. Twenty-four healthy young (age, 21 ± 2 years; BMI, 24 ± 3 kg.m2) males received primed, continuous infusions of L-[ring-2H5]phenylalanine and completed a bout of unilateral resistance leg exercise before ingesting either 70 g mycoprotein (MYC; 31·4 g protein, 2·5 g leucine; n 12) or 38·2 g of a protein concentrate obtained from mycoprotein (PCM; 28·0 g protein, 2·5 g leucine; n 12). Blood and muscle samples (vastus lateralis) were taken pre- and (4 h) post-exercise/protein ingestion to assess postabsorptive and postprandial myofibrillar protein fractional synthetic rates (FSR) in resting and exercised muscle. Protein ingestion increased plasma essential amino acid and leucine concentrations (P < 0·0001), but more rapidly (both 60 v. 90 min; P < 0·0001) and to greater magnitudes (1367 v. 1346 μmol·l-1 and 298 v. 283 μmol·l-1, respectively; P < 0·0001) in PCM compared with MYC. Protein ingestion increased myofibrillar FSR (P < 0·0001) in both rested (MYC, Δ0·031 ± 0·007 %·h-1 and PCM, Δ0·020 ± 0·008 %·h-1) and exercised (MYC, Δ0·057 ± 0·011 %·h-1 and PCM, Δ0·058 ± 0·012 %·h-1) muscle, with no differences between conditions (P > 0·05). Mycoprotein ingestion results in equivalent postprandial stimulation of resting and post-exercise myofibrillar protein synthesis rates irrespective of whether it is consumed within or without its wholefood matrix.

Cafeteria Diet Impacts the Body Weight and Energy Expenditure of Brown Norway Rats in an Apparent Age Dependent Manner, but Has no Effect on Muscle Anabolic Sensitivity to Nutrition

While obesity blunts the ability of muscle to mount a protein synthetic response to an amino acid infusion in older adults, it is unclear if this insensitivity to nutrition persists long term and in response to complete foods is unknown. To address this, young (2 months old) and old (17–20 months old) Brown Norway rats were randomized to receive either chow or a 12 wk diet of calorie-dense human foods. At wk 10 drinking water was supplemented with 2% heavy water, followed 2 weeks later by a flooding dose of [²H₅]-phenylalanine and an oral leucine bolus, allowing the short and long-term effects of age and diet on muscle protein synthesis rates to be determined. The experimental diet increased energy intake in both young (7.4 ± 0.9%) and old (18.2 ± 1.8%) animals (P < 0.01), but only led to significant increases in body weight in the former (young: 10.2 ± 3.0% (P < 0.05) and old: 3.1 ± 5.1% (NS) vs. age-matched controls). Notably, energy expenditure in response to the cafeteria diet was increased in old animals only (chow: 5.1 ± 0.4; cafe: 8.2 ± 1.6 kcal.kg b.w⁻¹.h⁻¹; P < 0.05). Gastrocnemius protein fractional synthetic rates in response to either an acute leucine bolus or two weeks of feeding were equivalent across groups irrespective of age or diet. Rodents in old age appear capable of preventing weight gain in response to a calorie-dense diet by increasing energy expenditure while maintaining the anabolic sensitivity of muscle to nutrition; the mechanisms of which could have important implications for the aging obese human.