Activation of mTORC1 signaling and protein synthesis in human muscle following blood flow restriction exercise is inhibited by rapamycin

Blood Flow Restriction Training After Achilles Tendon Rupture

Blood flow restriction (BFR) training is a technique shown to be safe and effective at increasing muscular strength and endurance in healthy fitness populations and is under study for its use in postinjury rehabilitation. BFR stimulates muscular strength and hypertrophy gains at much lower loads than traditional methods, allowing patients to begin the rehabilitation process much sooner. We report on 2 patients who incorporated BFR training into their traditional rehabilitation program after Achilles tendon ruptures. Patient 1 was a 29-year-old active duty soldier who sustained a left Achilles tendon rupture while playing competitive football. After operative repair and traditional rehabilitative measures, he was unable to ambulate without assistive devices owing to persistent weakness. The patient subsequently started a 5-week "return to run" program using BFR training. He experienced plantarflexion peak torque improvements of 522% and 108.9% and power gains of 4475% and 211% at 60°/s and 120°/s, respectively. He was able to ambulate without assistive devices at the 5-week follow-up examination. Patient 2 was a 38-year-old male soldier who experienced a complete left Achilles tendon rupture while exercising. After nonoperative treatment with an accelerated rehabilitation program, the patient still experienced significant strength and functional deficits. He was subsequently enrolled in a 6-week course of BFR training. He experienced plantarflexion strength improvements of 55.8% and 47.1% and power gains of 68.8% and 78.7% at 60°/s and 120°/s, respectively. He was able to return to running and sports on completion of 6 weeks of BFR-assisted therapy. Incorporating tourniquet-assisted blood flow restriction with rehabilitation programs can improve strength, endurance, and function after Achilles tendon rupture.

Environmental hypoxia favors myoblast differentiation and fast phenotype but blunts activation of protein synthesis after resistance exercise in human skeletal muscle

We hypothesized that a single session of resistance exercise performed in moderate hypoxic (FiO₂: 14%) environmental conditions would potentiate the anabolic response during the recovery period spent in normoxia. Twenty subjects performed a 1-leg knee extension session in normoxic or hypoxic conditions. Muscle biopsies were taken 15 min and 4 h after exercise in the vastus lateralis of the exercised and the nonexercised legs. Blood and saliva samples were taken at regular intervals before, during, and after the exercise session. The muscle fractional-protein synthetic rate was determined by deuterium incorporation into proteins, and the protein-degradation rate was determined by methylhistidine release from skeletal muscle. We found that: 1) hypoxia blunted the activation of protein synthesis after resistance exercise; 2) hypoxia down-regulated the transcriptional program of autophagy; 3) hypoxia regulated the expression of genes involved in glucose metabolism at rest and the genes involved in myoblast differentiation and fusion and in muscle contraction machinery after exercise; and 4) the hypoxia-inducible factor-1α pathway was not activated at the time points studied. Contrary to our hypothesis, environmental hypoxia did not potentiate the short-term anabolic response after resistance exercise, but it initiated transcriptional regulations that could potentially translate into satellite cell incorporation and higher force production in the long term.-Gnimassou, O., Fernández-Verdejo, R., Brook, M., Naslain, D., Balan, E., Sayda, M., Cegielski, J., Nielens, H., Decottignies, A., Demoulin, J.-B., Smith, K., Atherton, P. J., Fancaux, M., Deldicque, L. Environmental hypoxia favors myoblast differentiation and fast phenotype but blunts activation of protein synthesis after resistance exercise in human skeletal muscle.

Blood Flow Restriction Therapy After Knee Surgery: Indications, Safety Considerations, and Postoperative Protocol

Blood flow restriction (BFR) training involves occluding venous outflow while maintaining arterial inflow by the application of an extremity tourniquet after surgery. BFR ultimately reduces oxygen delivery to muscle cells, similar to an anaerobic environment, and allows patients to exercise with low resistance and stimulates muscle hypertrophy and strength using heavy resistance. Thus orthopaedic surgeons and physical therapists are incorporating this type of training into their postoperative rehabilitation protocols, particularly after injuries or surgical procedures about the knee joint. The purpose of this Technical Note is to describe a BFR clinical application technique and to report on the indications, safety considerations, and postoperative knee surgery rehabilitation protocols for BFR.

The Role of Blood Flow Restriction Therapy Following Knee Surgery: Expert Opinion

Blood flow restriction (BFR) therapy is becoming increasingly popular in musculoskeletal injury rehabilitation. In particular, this form of therapy is being utilized more often in the postoperative setting following knee surgery, including anterior cruciate ligament reconstruction. BFR therapy provides patients and clinicians an alternative treatment option to standard muscle strengthening and hypertrophy guidelines in the setting of postoperative pain, weakness, and postoperative activity restrictions that contribute to muscle atrophy. The ability to complete exercise in a low load environment and achieve similar physiological adaptations as high-intensity strength training makes this modality appealing. With poor patient-related outcomes associated with continued muscle atrophy, pain, and muscle weakness, some researchers have investigated BFR training postoperatively following arthroscopic knee surgery with promising results. However, owing to the current paucity of research studies, inconsistency among reported protocols, and mixed results, it may be some time before a mass adoption of BFR therapy is made into the world of orthopaedic rehabilitation. Although the current data is inconclusive, we choose to utilize BFR in postoperative knee patients, regardless of weight-bearing status, for whom maintenance of existing muscle mass or improvement of decreased postoperative strength levels is important. Therefore, the purpose of this expert opinion is to review the background of BFR, describe the clinical evidence of BFR following knee surgery, and report the authors' current recommendations for application of BFR postoperatively.

Muscle Protein Anabolic Resistance to Essential Amino Acids Does Not Occur in Healthy Older Adults Before or After Resistance Exercise Training

Background

The muscle protein anabolic response to contraction and feeding may be blunted in older adults. Acute bouts of exercise can improve the ability of amino acids to stimulate muscle protein synthesis (MPS) by activating mechanistic target of rapamycin complex 1 (mTORC1) signaling, but it is not known whether exercise training may improve muscle sensitivity to amino acid availability.

Objective

The aim of this study was to determine if muscle protein anabolism is resistant to essential amino acids (EAAs) and whether resistance exercise training (RET) improves muscle sensitivity to EAA in healthy older adults.

Methods

In a longitudinal study, 19 healthy older adults [mean ± SD age: 71 ± 4 y body mass index (kg/m2): 28 ± 3] were trained for 12 wk with a whole-body program of progressive RET (60-75% 1-repetition maximum). Body composition, strength, and metabolic health were measured pre- and posttraining. We also performed stable isotope infusion experiments with muscle biopsies pre- and posttraining to measure MPS and markers of amino acid sensing in the basal state and in response to 6.8 g of EAA ingestion.

Results

RET increased muscle strength by 16%, lean mass by 2%, and muscle cross-sectional area by 27% in healthy older adults (P < 0.05). MPS and mTORC1 signaling (i.e., phosphorylation status of protein kinase B, 4E binding protein 1, 70-kDa S6 protein kinase, and ribosomal protein S6) increased after EAA ingestion (P < 0.05) pre- and posttraining. RET increased basal MPS by 36% (P < 0.05); however, RET did not affect the response of MPS and mTORC1 signaling to EAA ingestion.

Conclusion

RET increases strength and basal MPS, promoting hypertrophy in healthy older adults. In these subjects, a small dose of EAAs stimulates muscle mTORC1 signaling and MPS, and this response to EAAs does not improve after RET. Our data indicate that anabolic resistance to amino acids may not be a problem in healthy older adults. This trial was registered at www.clinicaltrials.gov as NCT02999802.

mTORC1 signaling-associated protein synthesis in porcine mammary glands was regulated by the local available methionine depending on methionine sources

Mechanistic target of rapamycin complex1 (mTORC1) activation and protein synthesis varied with methionine sources; however, the related mechanisms are largely unknown. Porcine mammary epithelial cells (PMEC) and mammary tissue slices (MTS) were used to test whether methionine precursors differ in providing the available methionine and thus differ in mTORC1 signaling-associated protein synthesis. PMEC with methionine deprivation for 8 h and MTS from lactating sows were cultured for 24 and 2 h, respectively, with treatment media without methionine (negative control, NC) or supplemented with 0.6 mM (for PMEC) and 0.1 mM (for MTS) of L-methionine (L-MET), D-methionine (D-MET), DL-2-hydroxy-4-(methylthio) butyric acid (HMTBA), or keto-methyl(thio)butanoic acid (KMB). The measurements included: phosphorylation of mTORC1 signaling, fractional protein synthesis rate (FSR), amino acids (AA) profile, and enzyme activities. Compared with the NC treatment, activated mTORC1 signaling as manifested by higher (P < 0.05) protein abundance of phosphorylated-S6 Kinase 1 (P-S6K1) and phosphorylated-4E-binding Protein 1 (P-4E-BP1) in PMEC and MTS, and increased protein synthesis as indicated by higher (P < 0.05) FSR in MTS occurred in L-MET and HMTBA treatments rather than in D-MET treatment. Compared with the NC treatment, methionine concentration and ratio of methionine to lysine in MTS increased (P < 0.05) in L-MET and HMTBA treatments but not in D-MET treatment, and activities of enzymes responsible for conversion of D-MET and HMTBA to keto-methionine in mammary tissues were about 10 and 50%, respectively, of that in liver. Taken together, mTORC1 signaling-associated protein synthesis in porcine mammary glands was regulated by the local available methionine depending on methionine sources.

Attenuated PGC-1α Isoforms following Endurance Exercise with Blood Flow Restriction

INTRODUCTION

Exercise performed with blood flow restriction simultaneously enhances the acute responses to both myogenic and mitochondrial pathways with roles in training adaptation. We investigated isoform-specific gene expression of the peroxisome proliferator-activated receptor gamma coactivator 1 and selected target genes and proteins regulating skeletal muscle training adaptation.

METHODS

Nine healthy, untrained males participated in a randomized, counterbalanced, crossover design in which each subject completed a bout of low-intensity endurance exercise performed with blood flow restriction (15 min cycling at 40% of V˙O2peak, BFR-EE), endurance exercise (30 min cycling at 70% of V˙O2peak, EE), or resistance exercise (4 × 10 repetitions of leg press at 70% of one-repetition maximum) separated by at least 1 wk of recovery. A single resting muscle biopsy (vastus lateralis) was obtained 2 wk before the first exercise trial (rest) and 3 h after each bout.

RESULTS

Total PGC-1α mRNA abundance, along with all four isoforms, increased above rest with EE only (P < 0.05) being higher than BFR-EE (P < 0.05). PGC-1α1, 2, and 4 were higher after EE compared with resistance exercise (P < 0.05). EE also increased vascular endothelial growth factor, Hif-1α, and MuRF-1 mRNA abundance above rest (P < 0.05), whereas COXIV mRNA expression increased with EE compared with BFR-EE (P < 0.05).

CONCLUSION

The attenuated expression of all four PGC-1α isoforms when EE is performed with blood flow restriction suggests this type of exercise provides an insufficient stimulus to activate the signaling pathways governing mitochondrial and angiogenesis responses observed with moderate- to high-intensity EE.

Do metabolites that are produced during resistance exercise enhance muscle hypertrophy?

Many reviews conclude that metabolites play an important role with respect to muscle hypertrophy during resistance exercise, but their actual physiologic contribution remains unknown. Some have suggested that metabolites may work independently of muscle contraction, while others have suggested that metabolites may play a secondary role in their ability to augment muscle activation via inducing fatigue. Interestingly, the studies used as support for an anabolic role of metabolites use protocols that are not actually designed to test the importance of metabolites independent of muscle contraction. While there is some evidence in vitro that metabolites may induce muscle hypertrophy, the only study attempting to answer this question in humans found no added benefit of pooling metabolites within the muscle post-exercise. As load-induced muscle hypertrophy is thought to work via mechanotransduction (as opposed to being metabolically driven), it seems likely that metabolites simply augment muscle activation and cause the mechanotransduction cascade in a larger proportion of muscle fibers, thereby producing greater muscle growth. A sufficient time under tension also appears necessary, as measurable muscle growth is not observed after repeated maximal testing. Based on current evidence, it is our opinion that metabolites produced during resistance exercise do not have anabolic properties per se, but may be anabolic in their ability to augment muscle activation. Future studies are needed to compare protocols which produce similar levels of muscle activation, but differ in the magnitude of metabolites produced, or duration in which the exercised muscles are exposed to metabolites.

Conservative Management for Stable High Ankle Injuries in Professional Football Players

Context:

High ankle “syndesmosis” injuries are common in American football players relative to the general population. At the professional level, syndesmotic sprains represent a challenging and unique injury lacking a standardized rehabilitation protocol during conservative management.

Evidence Acquisition:

PubMed, Biosis Preview, SPORTDiscus, PEDro, and EMBASE databases were searched using the terms syndesmotic injuries, American football, conservative management, and rehabilitation.

Study Design:

Clinical review.

Level of Evidence:

Level 3.

Results:

When compared with lateral ankle sprains, syndesmosis injuries result in significantly prolonged recovery times and games lost. For stable syndesmotic injuries, conservative management features a brief period of immobilization and protected weightbearing followed by progressive strengthening exercises and running, and athletes can expect to return to competition in 2 to 6 weeks. Further research investigating the efficacy of dry needling and blood flow restriction therapy is necessary to evaluate the benefit of these techniques in the rehabilitation process.

Conclusion:

Successful conservative management of stable syndesmotic injuries in professional American football athletes requires a thorough understanding of the anatomy, injury mechanisms, diagnosis, and rehabilitation strategies utilized in elite athletes.

Blood Flow Restriction Training After Knee Arthroscopy: A Randomized Controlled Pilot Study

INTRODUCTION

Quadriceps strength after arthroscopic knee procedures is frequently diminished several years postoperation. Blood flow restriction (BFR) training uses partial venous occlusion while performing submaximal exercise to induce muscle hypertrophy and strength improvements. The purpose of this study was to evaluate BFR as a postoperative therapeutic intervention after knee arthroscopy.

METHODS

A randomized controlled pilot study comparing physical therapy with and without BFR after knee arthroscopy was conducted. Patients underwent 12 sessions of supervised physical therapy. Subjects followed the same postoperative protocol with the addition of 3 additional BFR exercises. Outcome measures included thigh girth, physical function measures, Knee Osteoarthritis Outcome Score (KOOS), Veterans RAND 12-Item Health Survey (VR12), and strength testing. Bilateral duplex ultrasonography was used to evaluate for deep venous thrombosis preintervention and postintervention.

RESULTS

Seventeen patients completed the study. Significant increases in thigh girth were observed in the BFR group at 6-cm and 16-cm proximal to the patella (P = 0.0111 and 0.0001). All physical outcome measures significantly improved in the BFR group, and the timed stair ascent improvements were greater than conventional therapy (P = 0.0281). The VR-12 and KOOS subscales significantly improved in the BFR group, and greater improvement was seen in VR-12 mental component score (P = 0.0149). The BFR group displayed approximately 2-fold greater improvements in extension and flexion strength compared with conventional therapy (74.59% vs 33.5%, P = 0.034). No adverse events were observed during the study.

CONCLUSIONS

This study suggests that BFR is an effective intervention after knee arthroscopy. Further investigation is warranted to elucidate the benefits of this intervention in populations with greater initial impairment.

Intramuscular Anabolic Signaling and Endocrine Response Following Resistance Exercise: Implications for Muscle Hypertrophy

Maintaining skeletal muscle mass and function is critical for disease prevention, mobility and quality of life, and whole-body metabolism. Resistance exercise is known to be a major regulator for promoting muscle protein synthesis and muscle mass accretion. Manipulation of exercise intensity, volume, and rest elicit specific muscular adaptations that can maximize the magnitude of muscle growth. The stimulus of muscle contraction that occurs during differing intensities of resistance exercise results in varying biochemical responses regulating the rate of protein synthesis, known as mechanotransduction. At the cellular level, skeletal muscle adaptation appears to be the result of the cumulative effects of transient changes in gene expression following acute bouts of exercise. Thus, maximizing the resistance exercise-induced anabolic response produces the greatest potential for hypertrophic adaptation with training. The mechanisms involved in converting mechanical signals into the molecular events that control muscle growth are not completely understood; however, skeletal muscle protein synthesis appears to be regulated by the multi-protein phosphorylation cascade, mTORC1 (mammalian/mechanistic target of rapamycin complex 1). The purpose of this review is to examine the physiological response to resistance exercise, with particular emphasis on the endocrine response and intramuscular anabolic signaling through mTORC1. It appears that resistance exercise protocols that maximize muscle fiber recruitment, time-under-tension, and metabolic stress will contribute to maximizing intramuscular anabolic signaling; however, the resistance exercise parameters for maximizing the anabolic response remain unclear.

Effects of phosphatidic acid supplementation on muscle thickness and strength in resistance-trained men

The purpose of this study was to investigate the effects of phosphatidic acid (PA) supplementation on muscle thickness and strength following an 8 week supervised resistance-training program. Fifteen resistance trained men (22.8 ± 3.5 years; 80.6 ± 8.7 kg; 178.1 ± 5.6 cm; 14.6% ± 8.8% body fat) were randomly assigned to a group that either consumed 750 mg of PA or a placebo (PL). Testing was carried out before (PRE) and after (POST) training/supplementation for muscle thickness and strength. Muscle thickness of the rectus femoris (RF), vastus lateralis (VL), biceps brachii (BB), and triceps brachii (TB) muscles were measured via ultrasonography, along with 1 repetition maximum (1RM) of squat, deadlift, and bench press. Analysis of covariance (ANCOVA), using PRE values as the covariate, did not reveal any group differences for measures of muscle thickness in the RF (PA: 3.6% ± 5.2%; PL: 3.2% ± 4.2%, p = 0.97), VL (PA: 23.4% ± 18.1%, PL: 12.5% ± 15.4%, p = 0.37), BB (PA: 3.7% ± 6.4%, PL: 9.6% ± 12.4%, p = 0.86), or TB (PA: 15.1% ± 17.9%, PL: 10.7% ± 19.3%, p = 0.79). Likewise, no group differences were observed in changes in squat (PA: 8.4% ± 4.1%, PL: 8.1% ± 4.2%, p = 0.79), deadlift (PA: 10.1% ± 10.1%, PL: 8.9% ± 9.5%, p = 0.66), or bench press (PA: 5.7% ± 5.5%, PL: 5.1% ± 3.0%, p = 0.76) exercises. Collectively, however, all participants experienced significant (p < 0.05) improvements in each measure of muscle thickness and strength. Results of this study suggest that PA supplementation, in combination with a 3 days·week−1 resistance-training program for 8 weeks, did not have a differential effect compared with PL on changes in muscle thickness or 1RM strength.

Post-absorptive muscle protein turnover affects resistance training hypertrophy

PURPOSE

Acute bouts of resistance exercise and subsequent training alters protein turnover in skeletal muscle. The mechanisms responsible for the changes in basal post-absorptive protein turnover and its impact on muscle hypertrophy following resistance exercise training are unknown. Our goal was to determine whether post-absorptive muscle protein turnover following 12 weeks of resistance exercise training (RET) plays a role in muscle hypertrophy. In addition, we were interested in determining potential molecular mechanisms responsible for altering post-training muscle protein turnover.

METHODS

Healthy young men (n = 31) participated in supervised whole body progressive RET at 60-80% 1 repetition maximum (1-RM), 3 days/week for 3 months. Pre- and post-training vastus lateralis muscle biopsies and blood samples taken during an infusion of ¹³C₆ and ¹⁵N phenylalanine and were used to assess skeletal muscle protein turnover in the post-absorptive state. Lean body mass (LBM), muscle strength (determined by dynamometry), vastus lateralis muscle thickness (MT), myofiber type-specific cross-sectional area (CSA), and mRNA were assessed pre- and post-RET.

RESULTS

RET increased strength (12-40%), LBM (~5%), MT (~15%) and myofiber CSA (~20%) (p < 0.05). Muscle protein synthesis (MPS) increased 24% while muscle protein breakdown (MPB) decreased 21%, respectively. These changes in protein turnover resulted in an improved net muscle protein balance in the basal state following RET. Further, the change in basal MPS is positively associated (r = 0.555, p = 0.003) with the change in muscle thickness.

CONCLUSION

Post-absorptive muscle protein turnover is associated with muscle hypertrophy during resistance exercise training.

Mechanosensitive Molecular Networks Involved in Transducing Resistance Exercise-Signals into Muscle Protein Accretion

Loss of skeletal muscle myofibrillar protein with disease and/or inactivity can severely deteriorate muscle strength and function. Strategies to counteract wasting of muscle myofibrillar protein are therefore desirable and invite for considerations on the potential superiority of specific modes of resistance exercise and/or the adequacy of low load resistance exercise regimens as well as underlying mechanisms. In this regard, delineation of the potentially mechanosensitive molecular mechanisms underlying muscle protein synthesis (MPS), may contribute to an understanding on how differentiated resistance exercise can transduce a mechanical signal into stimulation of muscle accretion. Recent findings suggest specific upstream exercise-induced mechano-sensitive myocellular signaling pathways to converge on mammalian target of rapamycin complex 1 (mTORC1), to influence MPS. This may e.g. implicate mechanical activation of signaling through a diacylglycerol kinase (DGKζ)-phosphatidic acid (PA) axis or implicate integrin deformation to signal through a Focal adhesion kinase (FAK)-Tuberous Sclerosis Complex 2 (TSC2)-Ras homolog enriched in brain (Rheb) axis. Moreover, since initiation of translation is reliant on mRNA, it is also relevant to consider potentially mechanosensitive signaling pathways involved in muscle myofibrillar gene transcription and whether some of these pathways converge with those affecting mTORC1 activation for MPS. In this regard, recent findings suggest how mechanical stress may implicate integrin deformation and/or actin dynamics to signal through a Ras homolog gene family member A protein (RhoA)-striated muscle activator of Rho signaling (STARS) axis or implicate deformation of Notch to affect Bone Morphogenetic Protein (BMP) signaling through a small mother of decapentaplegic (Smad) axis.

Soy-Dairy Protein Blend or Whey Protein Isolate Ingestion Induces Similar Postexercise Muscle Mechanistic Target of Rapamycin Complex 1 Signaling and Protein Synthesis Responses in Older Men

Background: Previous work demonstrated that a soy-dairy protein blend (PB) prolongs hyperaminoacidemia and muscle protein synthesis in young adults after resistance exercise.

Objective: We investigated the effect of PB in older adults. We hypothesized that PB would prolong hyperaminoacidemia, enhancing mechanistic target of rapamycin complex 1 (mTORC1) signaling and muscle protein anabolism compared with a whey protein isolate (WPI).

Methods: This double-blind, randomized controlled trial studied men 55–75 y of age. Subjects consumed 30 g protein from WPI or PB (25% soy, 25% whey, and 50% casein) 1 h after leg extension exercise (8 sets of 10 repetitions at 70% one-repetition maximum). Blood and muscle amino acid concentrations and basal and postexercise muscle protein turnover were measured by using stable isotopic methods. Muscle mTORC1 signaling was assessed by immunoblotting.

Results: Both groups increased amino acid concentrations (P < 0.05) and mTORC1 signaling after protein ingestion (P < 0.05). Postexercise fractional synthesis rate (FSR; P ≥ 0.05), fractional breakdown rate (FBR; P ≥ 0.05), and net balance (P = 0.08) did not differ between groups. WPI increased FSR by 67% (mean ± SEM: rest: 0.05% ± 0.01%; postexercise: 0.09% ± 0.01%; P < 0.05), decreased FBR by 46% (rest: 0.17% ± 0.01%; postexercise: 0.09% ± 0.03%; P < 0.05), and made net balance less negative (P < 0.05). PB ingestion did not increase FSR (rest: 0.07% ± 0.03%; postexercise: 0.09% ± 0.01%; P ≥ 0.05), tended to decrease FBR by 42% (rest: 0.25% ± 0.08%; postexercise: 0.15% ± 0.08%; P = 0.08), and made net balance less negative (P < 0.05). Within-group percentage of change differences were not different between groups for FSR, FBR, or net balance (P ≥ 0.05).

Conclusions: WPI and PB ingestion after exercise in older men induced similar responses in hyperaminoacidemia, mTORC1 signaling, muscle protein synthesis, and breakdown. These data add new evidence for the use of whey or soy-dairy PBs as targeted nutritional interventions to counteract sarcopenia. This trial was registered at clinicaltrials.gov as NCT01847261.

The impact of postexercise essential amino acid ingestion on the ubiquitin proteasome and autophagosomal-lysosomal systems in skeletal muscle of older men

Essential amino acid (EAA) ingestion enhances postexercise muscle protein synthesis, and, in particular, the anabolic response of older adults appears sensitive to the quantity of ingested leucine. The effect of leucine ingestion on muscle breakdown following resistance exercise (RE) is less understood. The purpose of this study was to identify the impact of postexercise leucine ingestion on the ubiquitin proteasome and autophagosomal-lysosomal systems following acute RE in older men. Subjects (72 ± 2 yr) performed RE and 1 h postexercise ingested 10 g of EAA containing a leucine quantity similar to quality protein (control, 1.8 g leucine, n = 7) or enriched in leucine (leucine, 3.5 g leucine, n = 8). Stable isotope infusion and muscle biopsies (vastus lateralis) obtained at rest and 2, 5, and 24 h postexercise were used to examine protein content (Western blot), mRNA expression (RT-quantitative PCR), and muscle protein fractional breakdown rate (FBR). Muscle-specific RING finger 1 mRNA increased in both groups at 2 and 5 h (P < 0.05). LC3 mRNA increased, and the LC3BII-to-LC3BI ratio decreased at all postexercise time points in control (P < 0.05). Conversely, LC3 mRNA only increased at 2 h, and the LC3BII-to-LC3BI ratio only decreased at 2 and 5 h in leucine (P < 0.05). Tumor necrosis factor receptor-associated factor-6 mRNA increased (P < 0.05) in control at 5 h. FBR was not statistically different between groups or from basal 24 h postexercise (P > 0.05). These data indicate that ingesting a larger quantity of leucine following RE may further reduce postexercise skeletal muscle autophagy in older men; however, it does not appear to influence the acute postexercise elevation in markers of the ubiquitin proteasome system or the breakdown of intact proteins.NEW & NOTEWORTHY The impact of postexercise leucine ingestion on processes of skeletal muscle breakdown in older adults is not well understood. Additional postexercise leucine ingestion appears to further reduce autophagy, but it does not interfere with the increase in ubiquitin proteasome system markers or the breakdown of intact proteins in skeletal muscle of older men. Postexercise leucine ingestion may promote a healthier protein pool and favorable muscle adaptations in older adults through greater accretion of myofibrillar proteins.

Hypermetabolism and hypercatabolism of skeletal muscle accompany mitochondrial stress following severe burn trauma

Burn trauma results in prolonged hypermetabolism and skeletal muscle wasting. How hypermetabolism contributes to muscle wasting in burn patients remains unknown. We hypothesized that oxidative stress, cytosolic protein degradation, and mitochondrial stress as a result of hypermetabolism contribute to muscle cachexia postburn. Patients (n = 14) with burns covering >30% of their total body surface area were studied. Controls (n = 13) were young healthy adults. We found that burn patients were profoundly hypermetabolic at both the skeletal muscle and systemic levels, indicating increased oxygen consumption by mitochondria. In skeletal muscle of burn patients, concurrent activation of mTORC1 signaling and elevation in the fractional synthetic rate paralleled increased levels of proteasomes and elevated fractional breakdown rate. Burn patients had greater levels of oxidative stress markers as well as higher expression of mtUPR-related genes and proteins, suggesting that burns increased mitochondrial stress and protein damage. Indeed, upregulation of cytoprotective genes suggests hypermetabolism-induced oxidative stress postburn. In parallel to mtUPR activation postburn, mitochondrial-specific proteases (LONP1 and CLPP) and mitochondrial translocases (TIM23, TIM17B, and TOM40) were upregulated, suggesting increased mitochondrial protein degradation and transport of preprotein, respectively. Our data demonstrate that proteolysis occurs in both the cytosolic and mitochondrial compartments of skeletal muscle in severely burned patients. Increased mitochondrial protein turnover may be associated with increased protein damage due to hypermetabolism-induced oxidative stress and activation of mtUPR. Our results suggest a novel role for the mitochondria in burn-induced cachexia.

Role of Ingested Amino Acids and Protein in the Promotion of Resistance Exercise-Induced Muscle Protein Anabolism

The goal of this critical review is to comprehensively assess the evidence for the molecular, physiologic, and phenotypic skeletal muscle responses to resistance exercise (RE) combined with the nutritional intervention of protein and/or amino acid (AA) ingestion in young adults. We gathered the literature regarding the translational response in human skeletal muscle to acute exposure to RE and protein/AA supplements and the literature describing the phenotypic skeletal muscle adaptation to RE and nutritional interventions. Supplementation of protein/AAs with RE exhibited clear protein dose-dependent effects on translational regulation (protein synthesis) through mammalian target of rapamycin complex 1 (mTORC1) signaling, which was most apparent through increases in p70 ribosomal protein S6 kinase 1 (S6K1) phosphorylation, compared with postexercise recovery in the fasted or carbohydrate-fed state. These acute findings were critically tested via long-term exposure to RE training (RET) and protein/AA supplementation, and it was determined that a diminishing protein/AA supplement effect occurs over a prolonged exposure stimulus after exercise training. Furthermore, we found that protein/AA supplements, combined with RET, produced a positive, albeit minor, effect on the promotion of lean mass growth (when assessed in >20 participants/treatment); a negligible effect on muscle mass; and a negligible to no additional effect on strength. A potential concern we discovered was that the majority of the exercise training studies were underpowered in their ability to discern effects of protein/AA supplementation. Regardless, even when using optimal methodology and large sample sizes, it is clear that the effect size for protein/AA supplementation is low and likely limited to a subset of individuals because the individual variability is high. With regard to nutritional intakes, total protein intake per day, rather than protein timing or quality, appears to be more of a factor on this effect during long-term exercise interventions. There were no differences in strength or mass/muscle mass on RET outcomes between protein types when a leucine threshold (>2 g/dose) was reached. Future research with larger sample sizes and more homogeneity in design is necessary to understand the underlying adaptations and to better evaluate the individual variability in the muscle-adaptive response to protein/AA supplementation during RET.

Blood flow-restricted strength training displays high functional and biological efficacy in women: a within-subject comparison with high-load strength training

Limited data exist on the efficacy of low-load blood flow-restricted strength training (BFR), as compared directly to heavy-load strength training (HST). Here, we show that 12 wk of twice-a-week unilateral BFR [30% of one repetition maximum (1RM) to exhaustion] and HST (6-10RM) of knee extensors provide similar increases in 1RM knee extension and cross-sectional area of distal parts of musculus quadriceps femoris in nine untrained women (age 22 ± 1 yr). The two protocols resulted in similar acute increases in serum levels of human growth hormone. On the cellular level, 12 wk of BFR and HST resulted in similar shifts in muscle fiber composition in musculus vastus lateralis, evident as increased MyHC2A proportions and decreased MyHC2X proportions. They also resulted in similar changes of the expression of 29 genes involved in skeletal muscle function, measured both in a rested state following 12 wk of training and subsequent to singular training sessions. Training had no effect on myonuclei proportions. Of particular interest, 1) gross adaptations to BFR and HST were greater in individuals with higher proportions of type 2 fibers, 2) both BFR and HST resulted in approximately four-fold increases in the expression of the novel exercise-responsive gene Syndecan-4, and 3) BFR provided lesser hypertrophy than HST in the proximal half of musculus quadriceps femoris and also in CSApeak, potentially being a consequence of pressure from the tourniquet utilized to achieve blood flow restriction. In conclusion, BFR and HST of knee extensors resulted in similar adaptations in functional, physiological, and cell biological parameters in untrained women.

Protein supplementation does not alter intramuscular anabolic signaling or endocrine response after resistance exercise in trained men

The mammalian/mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway appears to be the primary regulator of muscle protein synthesis. A variety of stimuli including resistance exercise, amino acids, and hormonal signals activate mTORC1 signaling. The purpose of this study was to investigate the effect of a protein supplement on mTORC1 signaling following a resistance exercise protocol designed to promote elevations in circulating hormone concentrations. We hypothesized that the protein supplement would augment the intramuscular anabolic signaling response. Ten resistance-trained men (age, 24.7 ± 3.4 years; weight, 90.1 ± 11.3 kg; height, 176.0 ± 4.9 cm) received either a placebo or a supplement containing 20 g protein, 6 g carbohydrates, and 1 g fat after high-volume, short-rest lower-body resistance exercise. Blood samples were obtained at baseline, immediately, 30 minutes, 1 hour, 2 hours, and 5 hours after exercise. Fine-needle muscle biopsies were completed at baseline, 1 hour, and 5 hours after exercise. Myoglobin, lactate dehydrogenase, and lactate concentrations were significantly elevated after resistance exercise (P < .0001); however, no differences were observed between trials. Resistance exercise also elicited a significant insulin, growth hormone, and cortisol response (P < .01); however, no differences were observed between trials for insulin-like growth factor-1, insulin, testosterone, growth hormone, or cortisol. Intramuscular anabolic signaling analysis revealed significant elevations in RPS6 phosphorylation after resistance exercise (P = .001); however, no differences were observed between trials for signaling proteins including Akt, mTOR, p70S6k, and RPS6. The endocrine response and phosphorylation status of signaling proteins within the mTORC1 pathway did not appear to be altered by ingestion of supplement after resistance exercise in resistance-trained men.

Exercise and the Regulation of Skeletal Muscle Hypertrophy

Skeletal muscle is a critical organ serving as the primary site for postprandial glucose disposal and the generation of contractile force. The size of human skeletal muscle mass is dependent upon the temporal relationship between changes in muscle protein synthesis (MPS) and muscle protein breakdown. The aim of this chapter is to review our current understanding of how resistance exercise influences protein turnover with a specific emphasis on the molecular factors regulating MPS. We also will discuss recent data relating to the prescription of resistance exercise to maximize skeletal muscle hypertrophy. Finally, we evaluate the impact of age and periods of disuse on the loss of muscle mass and the controversy surround the etiology of muscle disuse atrophy.

Intramuscular anabolic signaling and endocrine response following high volume and high intensity resistance exercise protocols in trained men

Resistance exercise paradigms are often divided into high volume (HV) or high intensity (HI) protocols, however, it is unknown whether these protocols differentially stimulate mTORC1 signaling. The purpose of this study was to examine mTORC1 signaling in conjunction with circulating hormone concentrations following a typical HV and HI lower-body resistance exercise protocol. Ten resistance-trained men (24.7 ± 3.4 years; 90.1 ± 11.3 kg; 176.0 ± 4.9 cm) performed each resistance exercise protocol in a random, counterbalanced order. Blood samples were obtained at baseline (BL), immediately (IP), 30 min (30P), 1 h (1H), 2 h (2H), and 5 h (5H) postexercise. Fine needle muscle biopsies were completed at BL, 1H, and 5H. Electromyography of the vastus lateralis was also recorded during each protocol. HV and HI produced a similar magnitude of muscle activation across sets. Myoglobin and lactate dehydrogenase concentrations were significantly greater following HI compared to HV (P = 0.01–0.02), whereas the lactate response was significantly higher following HV compared to HI (P = 0.003). The growth hormone, cortisol, and insulin responses were significantly greater following HV compared to HI (P = 0.0001–0.04). No significant differences between protocols were observed for the IGF-1 or testosterone response. Intramuscular anabolic signaling analysis revealed a significantly greater (P = 0.03) phosphorylation of IGF-1 receptor at 1H following HV compared to HI. Phosphorylation status of all other signaling proteins including mTOR, p70S6k, and RPS6 were not significantly different between trials. Despite significant differences in markers of muscle damage and the endocrine response following HV and HI, both protocols appeared to elicit similar mTORC1 activation in resistance-trained men.

Blood flow restriction prevents muscle damage but not protein synthesis signaling following eccentric contractions

There is a growing body of evidence to suggest that resistance training exercise combined with blood flow restriction (BFR) increases muscle size and strength in humans. Eccentric contraction (ECC) frequently induces severe muscle damage. However, it is not known whether and to what extent muscle damage occurs following ECC + BFR due to the difficulty of conducting definitive invasive studies. The purpose of this study was to examine muscle fiber damage following ECC + BFR at the cellular level. High-intensity ECC was purposefully selected to maximize the opportunity for muscle damage and hypertrophic signaling in our novel in vivo animal model. Male Wistar rats were assigned randomly to the following groups: ECC and ECC + BFR at varying levels of occlusion pressure (140, 160, and 200 Torr). In all conditions, electrical stimulation was applied to the dorsiflexor muscles simultaneously with electromotor-induced plantar flexion. We observed severe histochemical muscle fiber damage (area of damaged fibers/total fiber area analyzed) following ECC (26.4 ± 4.0%). Surprisingly, however, muscle damage was negligible following ECC + BFR140 (2.6 ± 1.2%), ECC+BFR160 (3.0 ± 0.5%), and ECC + BFR200 (0.2 ± 0.1%). Ribosomal S6 kinase 1 (S6K1) phosphorylation, a downstream target of rapamycin (mTOR)-phosphorylation kinase, increased following ECC + BFR200 as well as ECC. In contrast, S6K1 phosphorylation was not altered by BFR alone. The present findings suggest that ECC combined with BFR, even at high exercise intensities, may enhance muscle protein synthesis without appreciable muscle fiber damage.

The role of mTOR signaling in the regulation of protein synthesis and muscle mass during immobilization in mice

The maintenance of skeletal muscle mass contributes substantially to health and to issues associated with the quality of life. It has been well recognized that skeletal muscle mass is regulated by mechanically induced changes in protein synthesis, and that signaling by mTOR is necessary for an increase in protein synthesis and the hypertrophy that occurs in response to increased mechanical loading. However, the role of mTOR signaling in the regulation of protein synthesis and muscle mass during decreased mechanical loading remains largely undefined. In order to define the role of mTOR signaling, we employed a mouse model of hindlimb immobilization along with pharmacological, mechanical and genetic means to modulate mTOR signaling. The results first showed that immobilization induced a decrease in the global rates of protein synthesis and muscle mass. Interestingly, immobilization also induced an increase in mTOR signaling, eIF4F complex formation and cap-dependent translation. Blocking mTOR signaling during immobilization with rapamycin not only impaired the increase in eIF4F complex formation, but also augmented the decreases in global protein synthesis and muscle mass. On the other hand, stimulating immobilized muscles with isometric contractions enhanced mTOR signaling and rescued the immobilization-induced decrease in global protein synthesis through a rapamycin-sensitive mechanism that was independent of ribosome biogenesis. Unexpectedly, the effects of isometric contractions were also independent of eIF4F complex formation. Similar to isometric contractions, overexpression of Rheb in immobilized muscles enhanced mTOR signaling, cap-dependent translation and global protein synthesis, and prevented the reduction in fiber size. Therefore, we conclude that the activation of mTOR signaling is both necessary and sufficient to alleviate the decreases in protein synthesis and muscle mass that occur during immobilization. Furthermore, these results indicate that the activation of mTOR signaling is a viable target for therapies that are aimed at preventing muscle atrophy during periods of mechanical unloading.

Summary: The activation of mTOR signaling is both necessary and sufficient to alleviate the decreases in protein synthesis and muscle mass that occur during immobilization.

Modulating exercise-induced hormesis: Does less equal more?

Hormesis encompasses the notion that low levels of stress stimulate or upregulate existing cellular and molecular pathways that improve the capacity of cells and organisms to withstand greater stress. This notion underlies much of what we know about how exercise conditions the body and induces long-term adaptations. During exercise, the body is exposed to various forms of stress, including thermal, metabolic, hypoxic, oxidative, and mechanical stress. These stressors activate biochemical messengers, which in turn activate various signaling pathways that regulate gene expression and adaptive responses. Historically, antioxidant supplements, nonsteroidal anti-inflammatory drugs, and cryotherapy have been favored to attenuate or counteract exercise-induced oxidative stress and inflammation. However, reactive oxygen species and inflammatory mediators are key signaling molecules in muscle, and such strategies may mitigate adaptations to exercise. Conversely, withholding dietary carbohydrate and restricting muscle blood flow during exercise may augment adaptations to exercise. In this review article, we combine, integrate, and apply knowledge about the fundamental mechanisms of exercise adaptation. We also critically evaluate the rationale for using interventions that target these mechanisms under the overarching concept of hormesis. There is currently insufficient evidence to establish whether these treatments exert dose-dependent effects on muscle adaptation. However, there appears to be some dissociation between the biochemical/molecular effects and functional/performance outcomes of some of these treatments. Although several of these treatments influence common kinases, transcription factors, and proteins, it remains to be determined if these interventions complement or negate each other, and whether such effects are strong enough to influence adaptations to exercise.