Resistance exercise increases AMPK activity and reduces 4E-BP1 phosphorylation and protein synthesis in human skeletal muscle

Whey protein intake after resistance exercise activates mTOR signaling in a dose-dependent manner in human skeletal muscle

PURPOSE

Protein ingestion after resistance exercise increases muscle protein synthesis (MPS) in a dose-dependent manner. However, the molecular mechanism(s) for the dose-dependency of MPS remains unclear. This study aimed to determine the dose response of mammalian target of rapamycin (mTOR) signaling in muscle with ingestion of protein after resistance exercise.

METHODS

Fifteen male subjects performed four sets of six unilateral isokinetic concentric knee extensions. Immediately after exercise, eight subjects consumed water only. The other seven subjects, in a randomized-order crossover design, took either a 10 [3.6 g essential amino acids (EAA)] or 20 g (7.1 g EAA) solution of whey protein. Muscle biopsies from the vastus lateralis muscle were taken 30 min before and 1 h after resistance exercise. Phosphorylation of Akt (Ser473), mTOR (Ser2448), 4E-BP1 (Thr37/46), and S6K1 (Thr389) was measured by western blotting.

RESULTS

Concentric knee extension exercise alone did not increase phosphorylation of Akt and mTOR 1 h after exercise, but ingesting protein after exercise significantly increased the phosphorylation of Akt and mTOR in a dose-dependent manner (P < 0.05). 4E-BP1 phosphorylation significantly decreased after resistance exercise (P < 0.05), but subjects who took 10 or 20 g of protein after exercise showed increased 4E-BP1 from post-exercise dephosphorylation (P < 0.05). S6K1 phosphorylation significantly increased after resistance exercise (P < 0.05), and 20 g of protein further increased S6K1 phosphorylation compared with ingestion of 10 g (P < 0.05).

CONCLUSIONS

These findings suggest that whey protein intake after resistance exercise activates mTOR signaling in a dose-dependent manner in untrained men.

Periexercise coingestion of branched-chain amino acids and carbohydrate in men does not preferentially augment resistance exercise-induced increases in phosphatidylinositol 3 kinase/protein kinase B-mammalian target of rapamycin pathway markers indicative of muscle protein synthesis

The effects of a single bout of resistance exercise (RE) in conjunction with periexercise branched-chain amino acid (BCAA) and carbohydrate (CHO) ingestion on skeletal muscle signaling markers indicative of muscle protein synthesis were determined. It was hypothesized that CHO + BCAA would elicit a more profound effect on these signaling markers compared with CHO. Twenty-seven males were randomly assigned to CHO, CHO + BCAA, or placebo (PLC) groups. Four sets of leg presses and leg extensions were performed at 80% 1 repetition maximum. Supplements were ingested 30 minutes and immediately before and after RE. Venous blood and muscle biopsy samples were obtained immediately before supplement ingestion and 0.5, 2, and 6 hours after RE. Serum insulin and glucose and phosphorylated levels of muscle insulin receptor substrate 1 (IRS-1), protein kinase B, mammalian target of rapamycin, phosphorylated 70S6 kinase, and 4E binding protein 1 were assessed. Data were analyzed by 2-way repeated-measures analysis of variance. Significant group × time interactions were observed for glucose and insulin (P < .05) showing that CHO and CHO + BCAA were significantly greater than PLC. Significant time main effects were observed for IRS-1 (P = .001), protein kinase B (P = .031), mammalian target of rapamycin (P = .003), and phosphorylated 70S6 kinase (P = .001). Carbohydrate and CHO + BCAA supplementation significantly increased IRS-1 compared with PLC (P = .002). However, periexercise coingestion of CHO and BCAA did not augment RE-induced increases in skeletal muscle signaling markers indicative of muscle protein synthesis when compared with CHO.

How to explain exercise-induced phenotype from molecular data: rethink and reconstruction based on AMPK and mTOR signaling

During endurance and resistance exercise training, AMPK and mTOR signaling were known as selective pathways implicating the differentiation of exercise-induced phenotype in skeletal muscle. Among the previous studies, however, the differences in exercise protocol, the individuality and the genetic heterogeneity within species make it difficult to reach a consistent conclusion in the roles of AMPK and mTOR signaling. In this review, we aim not to reanalyze the previous articles and present the research progress of AMPK and mTOR signaling in exercise, but to propose an abstract general hypothesis for exercise-induced phenotype. Generally, exercise- induced skeletal muscle phenotype is independent of one and a few genes, proteins and signaling pathways. Convergent adaptation will better summarize the specificity of skeletal muscle phenotype in response to a single mode of exercise. Backward adaptation will open a new concept to illustrate the process of exercise-induced adaptation, such as mitochondrial quality control and muscle mass homeostasis.

Resistance training for diabetes prevention and therapy: experimental findings and molecular mechanisms

Type 2 diabetes mellitus (T2D) is characterized by insulin resistance, impaired glycogen synthesis, lipid accumulation, and impaired mitochondrial function. Exercise training has received increasing recognition as a cornerstone in the prevention and treatment of T2D. Emerging research suggests that resistance training (RT) has the power to combat metabolic dysfunction in patients with T2D and seems to be an effective measure to improve overall metabolic health and reduce metabolic risk factors in diabetic patients. However, there is limited mechanistic insight into how these adaptations occur. This review provides an overview of the intervention data on the impact of RT on glucose metabolism. In addition, the molecular mechanisms that lead to adaptation in skeletal muscle in response to RT and that are associated with possible beneficial metabolic responses are discussed. Some of the beneficial adaptations exerted by RT include increased GLUT4 translocation in skeletal muscle, increased insulin sensitivity and hence restored metabolic flexibility. Increased energy expenditure and excess postexercise oxygen consumption in response to RT may be other beneficial effects. RT is increasingly establishing itself as an effective measure to improve overall metabolic health and reduce metabolic risk factors in diabetic patients.

Acute molecular responses in untrained and trained muscle subjected to aerobic and resistance exercise training versus resistance training alone

AIM

This study assessed and compared acute muscle molecular responses before and after 5-week training, employing either aerobic (AE) and resistance exercise (RE) or RE only.

METHODS

Ten men performed one-legged RE, while the contralateral limb performed AE followed by RE 6 h later (AE+RE). Before (untrained) and after (trained) the intervention, acute bouts of RE were performed with or without preceding AE. Biopsies were obtained from m. vastus lateralis of each leg pre- and 3 h post-RE to determine mRNA levels of VEGF, PGC-1α, MuRF-1, atrogin-1, myostatin and phosphorylation of mTOR, p70S6K, rpS6 and eEF2.

RESULTS

PGC-1α and VEGF expression increased (P < 0.05) after acute RE in the untrained, but not the trained state. These markers showed greater response after AE+RE than RE in either condition. Myostatin was lower after AE+RE than RE, both before and after training. AE+RE showed higher MuRF-1 and atrogin-1 expression than RE in the untrained, not the trained state. Exercise increased (P < 0.05) p70S6K phosphorylation both before and after training, yet this increase tended to be more prominent for AE+RE than RE before training. Phosphorylation of p70S6K was greater in trained muscle. Changes in these markers did not correlate with exercise-induced alterations in strength or muscle size.

CONCLUSION

Concurrent exercise in untrained skeletal muscle prompts global molecular responses consistent with resulting whole muscle adaptations. Yet, training blunts the more robust anabolic response shown after AE+RE compared with RE. This study challenges the concept that single molecular markers could predict training-induced changes in muscle size or strength.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Role of stearoyl-CoA desaturase-1 in skeletal muscle function and metabolism

Stearoyl-CoA desaturase-1 (SCD1) converts saturated fatty acids (SFA) into monounsaturated fatty acids and is necessary for proper liver, adipose tissue, and skeletal muscle lipid metabolism. While there is a wealth of information regarding SCD1 expression in the liver, research on its effect in skeletal muscle is scarce. Furthermore, the majority of information about its role is derived from global knockout mice, which are known to be hypermetabolic and fail to accumulate SCD1's substrate, SFA. We now know that SCD1 expression is important in regulating lipid bilayer fluidity, increasing triglyceride formation, and enabling lipogenesis and may protect against SFA-induced lipotoxicity. Exercise has been shown to increase SCD1 expression, which may contribute to an increase in intramyocellular triglyceride at the expense of free fatty acids and diacylglycerol. This review is intended to define the role of SCD1 in skeletal muscle and discuss the potential benefits of its activity in the context of lipid metabolism, insulin sensitivity, exercise training, and obesity.

Increased net muscle protein balance in response to simultaneous and separate ingestion of carbohydrate and essential amino acids following resistance exercise

Relative to essential amino acids (EAAs), carbohydrate (CHO) ingestion stimulates a delayed response of net muscle protein balance (NBAL). We investigated if staggered ingestion of CHO and EAA would superimpose the response of NBAL following resistance exercise, thus resulting in maximal anabolic stimulation. Eight recreationally trained subjects completed 2 trials: combined (COMB - drink 1, CHO+EAA; drink 2, placebo) and separated (SEP - drink 1, CHO; drink 2, EAA) post-exercise ingestion of CHO and EAA. Drink 1 was administered 1 h following an acute exercise bout and was followed 1 h later by drink 2. A primed, continuous infusion of l-[ring-(13)C6]-phenylalanine was combined with femoral arteriovenous sampling and muscle biopsies for the determination of muscle protein kinetics. Arterial amino acid concentrations increased following ingestion of EAA in both conditions. No difference between conditions was observed for phenylalanine delivery to the leg (COMB: 167 ± 23 μmol·min(-1)·(100 mL leg vol)(-1) × 6 h; SEP: 167 ± 21 μmol·min(-1)·(100 mL leg vol)(-1) × 6 h, P > 0.05). In the first hour following ingestion of the drink containing EAA, phenylalanine uptake was 50% greater for the SEP trial than the COMB trial. However, phenylalanine uptake was similar for COMB (110 ± 19 mg) and SEP (117 ± 24 mg) over the 6 h period. These data suggest that whereas separation of CHO and EAA ingestion following exercise may have a transient physiological impact on NBAL, this response is not reflected over a longer period. Thus, separation of CHO and EAA ingestion is unnecessary to optimize post-exercise muscle protein metabolism.

Induction of amino acid transporters expression by endurance exercise in rat skeletal muscle

We here investigated whether an acute bout of endurance exercise would induce the expression of amino acid transporters that regulate leucine transport across plasma and lysosomal membranes in rat skeletal muscle. Rats ran on a motor-driven treadmill at a speed of 28 m/min for 90 min. Immediately after the exercise, we observed that expression of mRNAs encoding L-type amino acid transporter 1 (LAT1) and CD98 was induced in the gastrocnemius, soleus, and extensor digitorum longus (EDL) muscles. Sodium-coupled neutral amino acid transporter 2 (SNAT2) mRNA was also induced by the exercise in those three muscles. Expression of proton-assisted amino acid transporter 1 (PAT1) mRNA was slightly but not significantly induced by a single bout of exercise in soleus and EDL muscles. Exercise-induced mRNA expression of these amino acid transporters appeared to be attenuated by repeated bouts of the exercise. These results suggested that the expression of amino acid transporters for leucine may be induced in response to an increase in the requirement for this amino acid in the cells of working skeletal muscles.

Absence of leucine in an essential amino acid supplement reduces activation of mTORC1 signalling following resistance exercise in young females

The purpose of the study was to investigate the specific effect of leucine on mTORC1 signalling and amino acid metabolism in connection with resistance exercise. Comparisons were made between ingestion of supplements with and without leucine. Eight young women performed leg press exercise on 2 occasions. In randomized order they received either an aqueous solution of essential amino acids with leucine (EAA) or without leucine (EAA-Leu), given as small boluses throughout the experiment. Muscle biopsies were taken after an overnight fast before exercise and 1 and 3 h postexercise and samples of blood were taken repeatedly during the experiment. Plasma and muscle concentrations of leucine rose 60%-140% (p < 0.05) with EAA and fell 35%-45% (p < 0.05) with the EAA-Leu supplement. In the EAA-trial, plasma and muscle levels of tyrosine (not present in the supplement) and the sum of the EAA were 15%-25% (p < 0.05) lower during recovery. Phosphorylation of mTOR and p70S6k was elevated to a larger extent following 1 h of recovery with leucine in the supplement (120% vs. 49% (p < 0.05) and 59- vs. 8-fold (p < 0.05) for EAA and EAA-Leu, respectively). The levels of MAFbx and MuRF-1 mRNA and of the corresponding proteins were not significantly altered after 3 h recovery from exercise. In conclusion, the presence of leucine in the supplement enhances the stimulatory effect on mTORC1 signalling and reduces the level of tyrosine and the sum of the EAA in muscle and plasma, suggesting a stimulation of protein synthesis and (or) inhibition of breakdown, leading to improvement in net protein balance.

Resistance exercise induced mTORC1 signaling is not impaired by subsequent endurance exercise in human skeletal muscle

The current dogma is that the muscle adaptation to resistance exercise is blunted when combined with endurance exercise. The suggested mechanism (based on rodent experiments) is that activation of adenosine monophosphate-activated protein kinase (AMPK) during endurance exercise impairs muscle growth through inhibition of the mechanistic target of rapamycin complex 1 (mTORC1). The purpose of this study was to investigate potential interference of endurance training on the signaling pathway of resistance training [mTORC1 phosphorylation of ribosomal protein S6 kinase 1 (S6K1)] in human muscle. Ten healthy and moderately trained male subjects performed on two separate occasions either acute high-intensity and high-volume resistance exercise (leg press, R) or R followed by 30 min of cycling (RE). Muscle biopsies were collected before and 1 and 3 h post resistance exercise. Phosphorylation of mTOR (Ser²⁴⁴⁸) increased 2-fold (P < 0.05) and that of S6K1 (Thr³⁸⁹) 14-fold (P < 0.05), with no difference between R and RE. Phosphorylation of eukaryotic elongation factor 2 (eEF2, Thr⁵⁶) was reduced ~70% during recovery in both trials (P < 0.05). An interesting finding was that phosphorylation of AMPK (Thr¹⁷²) and acetyl-CoA carboxylase (ACC, Ser⁷⁹) decreased ~30% and ~50%, respectively, 3 h postexercise (P < 0.05). Proliferator-activated receptor-γ coactivator-1 (PGC-1α) mRNA increased more after RE (6.5-fold) than after R (4-fold) (RE vs. R: P < 0.01) and was the only gene expressed differently between trials. These data show that the signaling of muscle growth through the mTORC1-S6K1 axis after heavy resistance exercise is not inhibited by subsequent endurance exercise. It is also suggested that prior activation of mTORC1 signaling may repress subsequent phosphorylation of AMPK.

Skeletal muscle signaling associated with impaired glucose tolerance in spinal cord-injured men and the effects of contractile activity

The mechanisms underlying poor glucose tolerance in persons with spinal cord injury (SCI), along with its improvement after several weeks of neuromuscular electrical stimulation-induced resistance exercise (NMES-RE) training, remain unclear, but presumably involve the affected skeletal musculature. We, therefore, investigated skeletal muscle signaling pathways associated with glucose transporter 4 (GLUT-4) translocation at rest and shortly after a single bout of NMES-RE in SCI (n = 12) vs. able-bodied (AB, n = 12) men. Subjects completed an oral glucose tolerance test during visit 1 and ≈90 NMES-RE isometric contractions of the quadriceps during visit 2. Muscle biopsies were collected before, and 10 and 60 min after, NMES-RE. We assessed transcript levels of GLUT-4 by quantitative PCR and protein levels of GLUT-4 and phosphorylated- and total AMP-activated protein kinase (AMPK)-α, CaMKII, Akt, and AS160 by immunoblotting. Impaired glucose tolerance in SCI was confirmed by higher (P < 0.05) plasma glucose concentrations than AB at all time points after glucose ingestion, despite equivalent insulin responses to the glucose load. GLUT-4 protein content was lower (P < 0.05) in SCI vs. AB at baseline. Main group effects revealed higher phosphorylation in SCI of AMPK-α, CaMKII, and Akt (P < 0.05), and Akt phosphorylation increased robustly (P < 0.05) following NMES-RE in SCI only. In SCI, low skeletal muscle GLUT-4 protein concentration may, in part, explain poor glucose tolerance, whereas heightened phosphorylation of relevant signaling proteins (AMPK-α, CaMKII) suggests a compensatory effort. Finally, it is encouraging to find (based on Akt) that SCI muscle remains both sensitive and responsive to mechanical loading (NMES-RE) even ≈22 yr after injury.

Safety and Efficacy of Mobility Interventions in Patients with Femoral Catheters in the ICU: A Prospective Observational Study

INTRODUCTION

There are limited data describing mobility interventions provided to patients with femoral catheters. The purpose of this study was to examine the incidence of femoral catheter related adverse effects during physical therapy (PT) sessions in a cardiovascular intensive care unit (ICU).

METHODS

This was a prospective, observational study and included patients with at least one femoral catheter. Data were collected after each PT session.

RESULTS

There were 77 subjects with a total of 92 femoral catheters (50 arterial, 15 central venous, and 27 dialysis) treated. A total of 210 separate PT sessions occurred with 630 mobility activities including sitting on side of bed, standing at the bedside, transfers to stretcher chair or regular chair, and walking. There were no catheter related mechanical or thrombotic complications during any of the PT sessions.

CONCLUSIONS

Physical therapy sessions, including standing and walking were feasible and safe in cardiovascular ICU patients with femoral catheters who met the criteria for mobility interventions. The results from this study support the hypothesis that early mobilization in patients with femoral catheters is important to minimize functional decline and provide evidence that the presence of femoral catheters alone should not be a reason to limit progressive mobility interventions.

Cancer cachexia prevention via physical exercise: molecular mechanisms

Cancer cachexia is a debilitating consequence of disease progression, characterised by the significant weight loss through the catabolism of both skeletal muscle and adipose tissue, leading to a reduced mobility and muscle function, fatigue, impaired quality of life and ultimately death occurring with 25-30 % total body weight loss. Degradation of proteins and decreased protein synthesis contributes to catabolism of skeletal muscle, while the loss of adipose tissue results mainly from enhanced lipolysis. These mechanisms appear to be at least, in part, mediated by systemic inflammation. Exercise, by virtue of its anti-inflammatory effect, is shown to be effective at counteracting the muscle catabolism by increasing protein synthesis and reducing protein degradation, thus successfully improving muscle strength, physical function and quality of life in patients with non-cancer-related cachexia. Therefore, by implementing appropriate exercise interventions upon diagnosis and at various stages of treatment, it may be possible to reverse protein degradation, while increasing protein synthesis and lean body mass, thus counteracting the wasting seen in cachexia.

Mammalian target of rapamycin-independent S6K1 and 4E-BP1 phosphorylation during contraction in rat skeletal muscle

Muscle protein synthesis rates decrease during contraction/exercise, but rapidly increase post-exercise. Previous studies mainly focused on signaling pathways that control protein synthesis during post-exercise recovery, such as mTOR and its downstream targets S6K1 and 4E-BP1. In this study, we investigated the effect of high-frequency electrical stimulation on the phosphorylation state of signaling components controlling protein synthesis in rat skeletal muscle. Electrical stimulation increased S6K1 Thr389 phosphorylation, which was unaffected by Torin1, a selective mTOR inhibitor, suggesting that S6K1 phosphorylation by contraction was mTOR-independent. Phosphorylation of eIF4B Ser422 was also increased during electrical stimulation, which was abrogated by inhibition of MEK/ERK/RSK1 activation. Moreover, although phosphorylation of conventional mTOR sites in 4E-BP1 decreased during contraction, mTOR-independent phosphorylation was also apparent, which was associated with the release of 4E-BP1 from eIF4E. The results indicate mTOR-independent phosphorylation of S6K1 and 4E-BP1 and suggest MEK/ERK/RSK1-dependent phosphorylation of eIF4B during skeletal muscle contraction. These phosphorylation events would keep the translation initiation machinery "primed" in an active state so that protein synthesis could quickly resume post-exercise.

Vitamin C administration attenuates overload-induced skeletal muscle hypertrophy in rats

AIM

This study aimed to investigate the effects of vitamin C administration on skeletal muscle hypertrophy induced by mechanical overload in rats.

METHODS

Male Wistar rats were randomly assigned to three groups: (i) sham-operated group (n = 8), (ii) placebo-administered group (n = 8) and (iii) vitamin C-administered group (n = 8). In the placebo-administered and vitamin C-administered groups, the gastrocnemius and soleus muscles of the right hindlimb were surgically removed to overload the plantaris muscle. Vitamin C (500 mg kg(-1)) was orally administered to the vitamin C-administered group once a day for 14 days.

RESULTS

Synergist muscle ablation caused significant increases in wet weight and protein concentration of the plantaris muscle in both the placebo-administered (P < 0.01) and vitamin C-administered groups (P < 0.01) compared with the sham-operated group (SHA). However, the magnitude of plantaris muscle hypertrophy (expressed as a percentage of the contralateral plantaris muscle) was significantly smaller (P < 0.01) in the vitamin C-administered group (141%) than in the placebo-administered group (PLA) (152%). Compared with the SHA, only the PLA showed higher expressions of phosphorylated p70s6k and Erk1/2 (positive regulators of muscle protein synthesis) and a lower expression of atrogin-1 (a muscle atrophy marker). Concentrations of vitamin C and oxidative stress markers in the overloaded muscle were similar between the placebo-administered and vitamin C-administered groups.

CONCLUSION

Oral vitamin C administration can attenuate overload-induced skeletal muscle hypertrophy, which may have implications for antioxidant supplementation during exercise training.

The acute effects of strength, endurance and concurrent exercises on the Akt/mTOR/p70(S6K1) and AMPK signaling pathway responses in rat skeletal muscle

The activation of competing intracellular pathways has been proposed to explain the reduced training adaptations after concurrent strength and endurance exercises (CE). The present study investigated the acute effects of CE, strength exercises (SE), and endurance exercises (EE) on phosphorylated/total ratios of selected AMPK and Akt/mTOR/p70^S6K1 pathway proteins in rats. Six animals per exercise group were killed immediately (0 h) and 2 h after each exercise mode. In addition, 6 animals in a non-exercised condition (NE) were killed on the same day and under the same conditions. The levels of AMPK, phospho-Thr¹⁷²AMPK (p-AMPK), Akt, phospho-Ser⁴⁷³Akt (p-Akt), p70^S6K1, phospho-Thr³⁸⁹-p70^S6K1 (p-p70^S6K1), mTOR, phospho-Ser²⁴⁴⁸mTOR (p-mTOR), and phospho-Thr¹⁴⁶²-TSC2 (p-TSC2) expression were evaluated by immunoblotting in total plantaris muscle extracts. The only significant difference detected was an increase (i.e., 87%) in Akt phosphorylated/total ratio in the CE group 2 h after exercise compared to the NE group (P = 0.002). There were no changes in AMPK, TSC2, mTOR, or p70^S6K1 ratios when the exercise modes were compared to the NE condition (P ≥ 0.05). In conclusion, our data suggest that low-intensity and low-volume CE might not blunt the training-induced adaptations, since it did not activate competing intracellular pathways in an acute bout of strength and endurance exercises in rat skeletal muscle.

The effect of electrical muscle stimulation on the prevention of disuse muscle atrophy in patients with consciousness disturbance in the intensive care unit

PURPOSE

Disuse atrophy of the lower limbs of patients with consciousness disturbance has often been recognized as "an unavoidable consequence," such that the mechanism was not investigated diligently. In this study, we examined the preventive effects of electrical muscle stimulation (EMS) against disuse atrophy of the lower limbs in patients in coma after stroke or traumatic brain injury in the intensive care unit.

MATERIALS AND METHODS

We evaluated changes in cross-sectional area of lower limb muscles weekly with computed tomography in 6 control group patients and 9 EMS group patients. Electrical muscle stimulation was performed daily from day 7 after admission. We evaluated the anterior thigh muscle compartment, posterior thigh muscle compartment, anterior leg muscle compartment, and posterior leg muscle compartment.

RESULTS

In the control group, the decrease in cross-sectional area progressed in all compartments every week (P < .0001). Cross-sectional areas of all compartments at day 14 were significantly decreased in the control group compared with those in the EMS group at day 7 (P < .001). We were able to limit the rate of muscle atrophy as measured in the cross-sectional areas to within 4% during the period of EMS (days 7-42) in 5 patients. The difference between the control and the EMS groups was statistically significant (P < .001).

CONCLUSION

Electrical muscle stimulation is effective in the prevention of disuse muscle atrophy in patients with consciousness disorder.

Protein blend ingestion following resistance exercise promotes human muscle protein synthesis

High-quality proteins such as soy, whey, and casein are all capable of promoting muscle protein synthesis postexercise by activating the mammalian target of rapamycin (mTORC1) signaling pathway. We hypothesized that a protein blend of soy and dairy proteins would capitalize on the unique properties of each individual protein and allow for optimal delivery of amino acids to prolong the fractional synthetic rate (FSR) following resistance exercise (RE). In this double-blind, randomized, clinical trial, 19 young adults were studied before and after ingestion of ∼19 g of protein blend (PB) or ∼18 g whey protein (WP) consumed 1 h after high-intensity leg RE. We examined mixed-muscle protein FSR by stable isotopic methods and mTORC1 signaling with western blotting. Muscle biopsies from the vastus lateralis were collected at rest (before RE) and at 3 postexercise time points during an early (0-2 h) and late (2-4 h) postingestion period. WP ingestion resulted in higher and earlier amplitude of blood branched-chain amino acid (BCAA) concentrations. PB ingestion created a lower initial rise in blood BCAA but sustained elevated levels of blood amino acids later into recovery (P < 0.05). Postexercise FSR increased equivalently in both groups during the early period (WP, 0.078 ± 0.009%; PB, 0.088 ± 0.007%); however, FSR remained elevated only in the PB group during the late period (WP, 0.074 ± 0.010%; PB, 0.087 ± 0.003%) (P < 0.05). mTORC1 signaling similarly increased between groups, except for no increase in S6K1 phosphorylation in the WP group at 5 h postexercise (P < 0.05). We conclude that a soy-dairy PB ingested following exercise is capable of prolonging blood aminoacidemia, mTORC1 signaling, and protein synthesis in human skeletal muscle and is an effective postexercise nutritional supplement.

Safety and feasibility of femoral catheters during physical rehabilitation in the intensive care unit

OBJECTIVE

Femoral catheters pose a potential barrier to early rehabilitation in the intensive care unit (ICU) due to concerns, such as catheter removal, local trauma, bleeding, and infection. We prospectively evaluated the feasibility and safety of physical therapy (PT) in ICU patients with femoral catheters.

DESIGN, SETTING, AND PATIENTS

We evaluated consecutive medical ICU patients who received PT with a femoral venous, arterial, or hemodialysis catheter(s) in situ.

MEASUREMENTS AND MAIN RESULTS

Of 1074 consecutive patients, 239 (22%) received a femoral catheter (81% venous, 29% arterial, 6% hemodialysis; some patients had >1 catheter). Of those, 101 (42%) received PT interventions, while the catheter was in situ, for a total of 253 sessions over 210 medical ICU (MICU) days. On these 210 MICU days, the highest daily activity level achieved was 49 (23%) standing or walking, 57 (27%) sitting, 25 (12%) supine cycle ergometry, and 79 (38%) in-bed exercises. During 253 PT sessions, there were no catheter-related adverse events giving a 0% event rate (95% upper confidence limit of 2.1% for venous catheters).

CONCLUSIONS

Physical therapy interventions in MICU patients with in situ femoral catheters appear to be feasible and safe. The presence of a femoral catheter should not automatically restrict ICU patients to bed rest.

Influence of amino acids, dietary protein, and physical activity on muscle mass development in humans

Ingestion of protein is crucial for maintenance of a variety of body functions and within the scope of this review we will specifically focus on the regulation of skeletal muscle mass. A quantitative limitation exists as to how much muscle protein the body can synthesize in response to protein intake. Ingestion of excess protein exerts an unwanted load to the body and therefore, it is important to find the least amount of protein that provides the maximal hypertrophic stimulus. Hence, research has focused on revealing the relationship between protein intake (dose) and its resulting stimulation of muscle protein synthesis (response). In addition to the protein amount, the protein digestibility and, hence, the availability of its constituent amino acids is decisive for the response. In this regard, recent studies have provided in-depth knowledge about the time-course of the muscle protein synthetic response dependent on the characteristics of the protein ingested. The effect of protein intake on muscle protein accretion can further be stimulated by prior exercise training. In the ageing population, physical training may counteract the development of “anabolic resistance” and restore the beneficial effect of protein feeding. Presently, our knowledge is based on measures obtained in standardized experimental settings or during long-term intervention periods. However, to improve coherence between these types of data and to further improve our knowledge of the effects of protein ingestion, other investigative approaches than those presently used are requested.

Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis

Quantity and timing of protein ingestion are major factors regulating myofibrillar protein synthesis (MPS). However, the effect of specific ingestion patterns on MPS throughout a 12 h period is unknown. We determined how different distributions of protein feeding during 12 h recovery after resistance exercise affects anabolic responses in skeletal muscle. Twenty-four healthy trained males were assigned to three groups (n = 8/group) and undertook a bout of resistance exercise followed by ingestion of 80 g of whey protein throughout 12 h recovery in one of the following protocols: 8 × 10 g every 1.5 h (PULSE); 4 × 20 g every 3 h (intermediate: INT); or 2 × 40 g every 6 h (BOLUS). Muscle biopsies were obtained at rest and after 1, 4, 6, 7 and 12 h post exercise. Resting and post-exercise MPS (l-[ring-(13)C6] phenylalanine), and muscle mRNA abundance and cell signalling were assessed. All ingestion protocols increased MPS above rest throughout 1-12 h recovery (88-148%, P < 0.02), but INT elicited greater MPS than PULSE and BOLUS (31-48%, P < 0.02). In general signalling showed a BOLUS>INT>PULSE hierarchy in magnitude of phosphorylation. MuRF-1 and SLC38A2 mRNA were differentially expressed with BOLUS. In conclusion, 20 g of whey protein consumed every 3 h was superior to either PULSE or BOLUS feeding patterns for stimulating MPS throughout the day. This study provides novel information on the effect of modulating the distribution of protein intake on anabolic responses in skeletal muscle and has the potential to maximize outcomes of resistance training for attaining peak muscle mass.

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.

Addition of carbohydrate or alanine to an essential amino acid mixture does not enhance human skeletal muscle protein anabolism

In humans, essential amino acids (EAAs) stimulate muscle protein synthesis (MPS) with no effect on muscle protein breakdown (MPB). Insulin can stimulate MPS, and carbohydrates (CHOs) and insulin decrease MPB. Net protein balance (NB; indicator of overall anabolism) is greatest when MPS is maximized and MPB is minimized. To determine whether adding CHO or a gluconeogenic amino acid to EAAs would improve NB compared with EAA alone, young men and women (n = 21) ingested 10 g EAA alone, with 30 g sucrose (EAA+CHO), or with 30 g alanine (EAA+ALA). The fractional synthetic rate and phenylalanine kinetics (MPS, MPB, NB) were assessed by stable isotopic methods on muscle biopsies at baseline and 60 and 180 min following nutrient ingestion. Insulin increased 30 min postingestion in all groups and remained elevated in the EAA+CHO and EAA+ALA groups for 60 and 120 min, respectively. The fractional synthetic rate increased from baseline at 60 min in all groups (P < 0.05; EAA = 0.053 ± 0.018 to 0.090 ± 0.039% · h(-1); EAA+ALA = 0.051 ± 0.005 to 0.087 ± 0.015% · h(-1); EAA+CHO = 0.049 ± 0.006 to 0.115 ± 0.024% · h(-1)). MPS and NB peaked at 30 min in the EAA and EAA+CHO groups but at 60 min in the EAA+ALA group and NB was elevated above baseline longer in the EAA+ALA group than in the EAA group (P < 0.05). Although responses were more robust in the EAA+CHO group and prolonged in the EAA+ALA group, AUCs were similar among all groups for fractional synthetic rate, MPS, MPB, and NB. Because the overall muscle protein anabolic response was not improved in either the EAA+ALA or EAA+CHO group compared with EAA, we conclude that protein nutritional interventions to enhance muscle protein anabolism do not require such additional energy.

Heat stress activates the Akt/mTOR signalling pathway in rat skeletal muscle

AIM

It is well known that various stimuli, such as mechanical stress and nutrients, induce muscle hypertrophy thorough the Akt/mTOR signalling pathway, which is a key mediator of protein synthesis and hypertrophy in skeletal muscle. It was recently reported that heat stress also induces an increase in muscle weight and muscle protein content. In addition, heat stress enhances Akt/mTOR signalling after one bout of resistance exercise. However, it remains unclear whether increased temperature itself stimulates the Akt/mTOR signalling pathway.

METHODS

Forty-two male Wistar rats (279.5 ± 1.2 g) were divided into a control group (CON) or one of five thermal stress groups at 37, 38, 39, 40 or 41 °C (n = 7 each group). After overnight fasting, both legs were immersed in different temperatures of hot water for 30 min under sodium pentobarbital anaesthesia. The soleus and plantaris muscles were immediately removed from both legs after the thermal stress.

RESULTS

The phosphorylation of mTOR or 4E-BP1 and heat shock protein (HSP) expression levels were similar among groups in both the soleus and plantaris muscles. However, Akt and p70S6K phosphorylation significantly increased at 41 °C in the soleus and plantaris muscles. Moreover, we observed a temperature-dependent increase in Akt and p70S6K phosphorylation in both muscles.

CONCLUSION

Our data indicate that the altered temperature increased phosphorylation in a temperature-dependent manner in rat skeletal muscle and may itself be a key stimulator of Akt/mTOR signalling.

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.

Rapamycin does not affect post-absorptive protein metabolism in human skeletal muscle

Administration of the mTORC1 inhibitor, rapamycin, to humans blocks the increase in skeletal muscle protein synthesis in response to resistance exercise or amino acid ingestion.

OBJECTIVE

To determine whether rapamycin administration influences basal post-absorptive protein synthesis or breakdown in human skeletal muscle.

MATERIALS/METHODS

Six young (26±2 years) subjects were studied during two separate trials, in which each trial was divided into two consecutive 2 h basal periods. The trials were identical except during one trial a single oral dose (16 mg) of rapamycin was administered immediately prior to the second basal period. Muscle biopsies were obtained from the vastus lateralis at 0, 2, and 4 h to examine protein synthesis, mTORC1 signaling, and markers of autophagy (LC3B-I and LC3B-II protein) associated with each 2 h basal period.

RESULTS

During the Control trial, muscle protein synthesis, whole body protein breakdown (phenylalanine Ra), mTORC1 signaling, and markers of autophagy were similar between both basal periods (p>0.05). During the Rapamycin trial, these variables were similar to the Control trial (p>0.05) and were unaltered by rapamycin administration (p>0.05). Thus, post-absorptive muscle protein metabolism and mTORC1 signaling were not affected by rapamycin administration.

CONCLUSIONS

Short-term rapamycin administration may only impair protein synthesis in human skeletal muscle when combined with a stimulus such as resistance exercise or increased amino acid availability.

The role of mTORC1 in regulating protein synthesis and skeletal muscle mass in response to various mechanical stimuli

Skeletal muscle plays a fundamental role in mobility, disease prevention, and quality of life. Skeletal muscle mass is, in part, determined by the rates of protein synthesis, and mechanical loading is a major regulator of protein synthesis and skeletal muscle mass. The mammalian/mechanistic target of rapamycin (mTOR), found in the multi-protein complex, mTORC1, is proposed to play an essential role in the regulation of protein synthesis and skeletal muscle mass. The purpose of this review is to examine the function of mTORC1 in relation to protein synthesis and cell growth, the current evidence from rodent and human studies for the activation of mTORC1 signaling by different types of mechanical stimuli, whether mTORC1 signaling is necessary for changes in protein synthesis and skeletal muscle mass that occur in response to different types of mechanical stimuli, and the proposed molecular signaling mechanisms that may be responsible for the mechanical activation of mTORC1 signaling.

Paraplegia increases skeletal muscle autophagy

INTRODUCTION

Paraplegia results in significant skeletal muscle atrophy through increases in skeletal muscle protein breakdown. Recent work has identified a novel SIRT1-p53 pathway that is capable of regulating autophagy and protein breakdown.

METHODS

Soleus muscle was collected from 6 male Sprague-Dawley rats 10 weeks after complete T4-5 spinal cord transection (paraplegia group) and 6 male sham-operated rats (control group). We utilized immunoblotting methods to measure intracellular proteins and quantitative real-time polymerase chain reaction to measure the expression of skeletal muscle microRNAs.

RESULTS

SIRT1 protein expression was 37% lower, and p53 acetylation (LYS379) was increased in the paraplegic rats (P < 0.05). Atg7 and Beclin-1, markers of autophagy induction, were elevated in the paraplegia group compared with controls (P < 0.05).

CONCLUSIONS

Severe muscle atrophy resulting from chronic paraplegia appears to increase skeletal muscle autophagy independent of SIRT1 signaling. We conclude that chronic paraplegia may cause an increase in autophagic cell death and negatively impact skeletal muscle protein balance.

Intense resistance exercise induces early and transient increases in ryanodine receptor 1 phosphorylation in human skeletal muscle

Background

While ryanodine receptor 1 (RyR1) critically contributes to skeletal muscle contraction abilities by mediating Ca²⁺ion oscillation between sarcoplasmatic and myofibrillar compartments, AMP-activated protein kinase (AMPK) senses contraction-induced energetic stress by phosphorylation at Thr¹⁷². Phosphorylation of RyR1 at serine²⁸⁴³ (pRyR1Ser²⁸⁴³) results in leaky RyR1 channels and impaired Ca²⁺homeostasis. Because acute resistance exercise exerts decreased contraction performance in skeletal muscle, preceded by high rates of Ca²⁺-oscillation and energetic stress, intense myofiber contractions may induce increased RyR1 and AMPK phosphorylation. However, no data are available regarding the time-course and magnitude of early RyR1 and AMPK phosphorylation in human myofibers in response to acute resistance exercise.

Purpose

Determine the effects and early time-course of resistance exercise on pRyR1Ser²⁸⁴³ and pAMPKThr¹⁷² in type I and II myofibers.

Methods

7 male subjects (age 23±2 years, height: 185±7 cm, weight: 82±5 kg) performed 3 sets of 8 repetitions of maximum eccentric knee extensions. Muscle biopsies were taken at rest, 15, 30 and 60 min post exercise. pRyR1Ser²⁸⁴³ and pAMPKThr¹⁷² levels were determined by western blot and semi-quantitative immunohistochemistry techniques.

Results

While total RyR1 and total AMPK levels remained unchanged, RyR1 was significantly more abundant in type II than type I myofibers. pRyR1Ser²⁸⁴³ increased 15 min and peaked 30 min (p<0.01) post exercise in both myofiber types. Type I fibers showed relatively higher increases in pRyR1Ser²⁸⁴³ levels than type II myofibers and remained elevated up to 60 min post resistance exercise (p<0.05). pAMPKThr¹⁷² also increased 15 to 30 min post exercise (p<0.01) in type I and II myofibers and in whole skeletal muscle.

Conclusion

Resistance exercise induces acutely increased pRyR1Ser²⁸⁴³ and concomitantly pAMPKThr¹⁷² levels for up to 30 min in resistance exercised myofibers. This provides a time-course by which pRyR1Ser²⁸⁴³ can mechanistically impact Ca²⁺handling properties and consequently induce reduced myofiber contractility beyond immediate fatiguing mechanisms.

AMP kinase activation improves angiogenesis in pulmonary artery endothelial cells with in utero pulmonary hypertension

Pulmonary artery endothelial cells (PAEC) isolated from fetal lambs with in utero pulmonary hypertension (IPH) have phenotypical changes that lead to increased reactive oxygen species (ROS) formation and impaired angiogenesis. AMP-activated protein kinase (AMPK) is known to be activated by ROS, which is expected to help angiogenesis in IPH-PAEC. The objectives of this study were to investigate AMPK responses in IPH and its role in angiogenesis. We observed that, compared with control PAEC, IPH-PAEC have decreased phosphorylation of AMPKα catalytic subunit and AMPK downstream enzymes, indicating a decrease in AMPK activity. In addition, the expression of AMPK kinases is decreased, and protein phosphatase 2 is increased in IPH-PAEC, potentially contributing to the decreased AMPK activation. Metformin, an AMPK activator, improved IPH-PAEC angiogenesis while increasing endothelial NO synthase (eNOS) serine(1179) phosphorylation and decreasing the eNOS-caveolin-1 association. Metformin also increased MnSOD activity and the expression of both eNOS and MnSOD. The increase in angiogenesis by Metformin is abolished by pretreatment with AMPK inhibitor, Compound C. Expression of vascular endothelial growth factor (VEGF) and platelet-derived growth factor β (PDGFβ) are decreased in IPH-PAEC compared with control PAEC and were not altered by Metformin. These data indicate that Metformin improves angiogenesis through mechanisms independent of these angiogenic factors. In conclusion, activation of AMPK restores angiogenesis and increases the bioavailability of nitric oxide in IPH. Whether Metformin is beneficial in the management of pulmonary hypertension requires further investigation.

Skeletal muscle autophagy and protein breakdown following resistance exercise are similar in younger and older adults

BACKGROUND

The loss of skeletal muscle mass and strength during aging, sarcopenia, increases the risk for falls and dependency. Resistance exercise (RE) training is effective at improving muscle mass and strength in older adults; however, aging is associated with reduced training-induced hypertrophy. Recent research has illustrated an impaired muscle protein synthetic response following an acute bout of RE in older adults but much less is known regarding the effect of acute RE on muscle protein breakdown (MPB). We hypothesize that the ubiquitin proteasome system and the autophagosomal-lysosomal system may regulate the overall rate of MPB during postexercise recovery.

METHODS

Muscle biopsies of the vastus lateralis were sampled from 16 older (age = 70±2 years) and 16 younger (age = 27±2 years) participants at baseline and at 3, 6, and 24 hours following an acute bout of RE. In conjunction with stable isotopic techniques to measure MPB, we utilized immunoblotting and RT-PCR to examine protein and mRNA expression for key signaling molecules in both the ubiquitin proteasome system and the autophagosomal-lysosomal system.

RESULTS

MuRF1 mRNA expression increased, whereas GABARAP mRNA decreased after RE in both younger and older adults (p < .05). The LC3B-II/LC3B-I protein ratio decreased in both groups after RE (p < .05), but MPB was not different 24 hour post-RE in either group (p > .05).

CONCLUSIONS

Aging does not influence skeletal MPB, autophagy, or the ubiquitin proteasome system following an acute bout of RE. Therefore, targeting the muscle protein synthesis response to exercise may hold more promise in the prevention of sarcopenia.

Reduced AMPK-ACC and mTOR signaling in muscle from older men, and effect of resistance exercise

AMP-activated protein kinase (AMPK) is a key energy-sensitive enzyme that controls numerous metabolic and cellular processes. Mammalian target of rapamycin (mTOR) is another energy/nutrient-sensitive kinase that controls protein synthesis and cell growth. In this study we determined whether older versus younger men have alterations in the AMPK and mTOR pathways in skeletal muscle, and examined the effect of a long term resistance type exercise training program on these signaling intermediaries. Older men had decreased AMPKα2 activity and lower phosphorylation of AMPK and its downstream signaling substrate acetyl-CoA carboxylase (ACC). mTOR phosphylation also was reduced in muscle from older men. Exercise training increased AMPKα1 activity in older men, however, AMPKα2 activity, and the phosphorylation of AMPK, ACC and mTOR, were not affected. In conclusion, older men have alterations in the AMPK-ACC and mTOR pathways in muscle. In addition, prolonged resistance type exercise training induces an isoform-selective up regulation of AMPK activity.

Targeting anabolic impairment in response to resistance exercise in older adults with mobility impairments: potential mechanisms and rehabilitation approaches

Muscle atrophy is associated with healthy aging (i.e., sarcopenia) and may be compounded by comorbidities, injury, surgery, illness, and physical inactivity. While a bout of resistance exercise increases protein synthesis rates in healthy young skeletal muscle, the effectiveness of resistance exercise to mount a protein synthetic response is less pronounced in older adults. Improving anabolic sensitivity to resistance exercise, thereby enhancing physical function, is most critical in needy older adults with clinical conditions that render them “low responders”. In this paper, we discuss potential mechanisms contributing to anabolic impairment to resistance exercise and highlight the need to improve anabolic responsiveness in low responders. This is followed with evidence suggesting that the recovery period of resistance exercise provides an opportunity to amplify the exercise-induced anabolic response using protein/essential amino acid ingestion. This anabolic strategy, if repeated chronically, may improve lean muscle gains, decrease time to recovery of function during periods of rehabilitation, and overall, maintain/improve physical independence and reduce mortality rates in older adults.

Genetic strain-dependent protein metabolism and muscle hypertrophy under chronic isometric training in rat gastrocnemius muscle

Genetic strain-dependent reactivity to mechanical stimuli in rat skeletal muscle has not been examined. This study aimed to examine whether genetic strain-dependency is associated with reactivity in protein metabolism and the resultant muscle hypertrophy after isometric resistance training (RT). The right triceps of Sprague-Dawley (SD) and Wistar rats underwent 12 sessions of RT. After RT, a transition from the IIb to the IIx myosin heavy-chain isoform was observed in both strains. In SD rats, the lateral gastrocnemius muscle (LG) mass of the trained legs (TRN) was significantly higher than that of the control legs (CON) (7.8 %, P<0.05). Meanwhile, in Wistar rats, the LG mass was unchanged. In SD rats, the levels of 70-kDa ribosomal protein S6 kinase (p70S6k) and forkhead box 3a (FOXO3a) phosphorylation in the TRN were significantly greater than those of the CON (2.2- and 1.9-fold, respectively; P<0.05). The expression of muscle ring finger-1 (MuRF1) and muscle atrophy F-box (MAFbx/atrogin-1) in the TRN were significantly lower than those of the CON (0.6- and 0.7-fold, respectively; P<0.05). However, in Wistar rats, there was no significant difference. These results suggest a genetic strain difference in protein metabolism. This phenomenon may be useful for studying individual differences in response to RT.

Aging differentially affects human skeletal muscle amino acid transporter expression when essential amino acids are ingested after exercise

BACKGROUND & AIMS

Amino acid transporters have been proposed as regulators of protein synthesis. The primary aim of this study was to determine whether amino acid transporter expression is increased in human muscle following resistance exercise (RE) coupled with essential amino acid (EAA) ingestion, and whether a differential response occurs with aging. Secondly, we aimed to compare this response to a previous study examining RE alone.

METHODS

Young (n = 7, 30 ± 2 yr) and older men (n = 6, 70 ± 2 yr) ingested EAA 1 h after RE. Muscle biopsies were obtained at rest and 3 and 6 h post exercise to examine amino acid transporter mRNA and protein expression.

RESULTS

In both age groups, RE + EAA increased mRNA of L-type amino acid transporter 1 (LAT1)/solute linked carrier (SLC)7A5, sodium-coupled neutral amino acid transporter 2 (SNAT2)/SLC38A2, and cationic amino acid transporter 1/SLC7A1 (p < 0.05). SNAT2 protein increased in young at 3 and 6 h (p < 0.05), whereas old maintained higher LAT1 protein (p < 0.05). Compared to RE alone, RE + EAA enhanced amino acid transporter expression only in young (p < 0.05).

CONCLUSIONS

RE increases muscle amino acid transporter expression in young and older adults, however, post exercise EAA ingestion enhances amino acid transporter expression only in young indicating that aging may influence the function of specific amino acid transporters.

MAFbx, MuRF1, and the stress-activated protein kinases are upregulated in muscle cells during total knee arthroplasty

Total knee arthroplasty (TKA) is the most common and a cost-effective surgical remediation for older adults with long-standing osteoarthritis. In parallel with the expanding population of older adults, the number of TKAs performed annually is projected to be 3.48 million by 2030. During this surgery, a tourniquet is used to stop blood flow to the operative leg. However, the molecular pathways that are affected by tourniquet use during TKA continue to be elucidated. We hypothesized that components of the catabolic FoxO3a (i.e., MuRF1, MAFbx, and Bnip3) pathway, as well as the cellular stress pathways [i.e., stress-activated protein kinase (SAPK)/JNK and MAPKs], are upregulated during TKA. The purpose of this study was to measure changes in transcripts and proteins involved in muscle cell catabolic and stress-activated pathways. We obtained muscle biopsies from subjects, 70 ± 1.3 yr, during TKA, from the vastus lateralis at baseline (before tourniquet inflation), during maximal ischemia (just before tourniquet release), and during reperfusion. Total tourniquet time was 43 ± 2 min and reperfusion time was 16 ± 1. Significant increases in FoxO3a downstream targets, MAFbx and MuRF1, were present for mRNA levels during ischemia (MAFbx, P = 0.04; MuRF1, P = 0.04), and protein expression during ischemia (MAFbx, P = 0.002; MuRF1, P = 0.001) and reperfusion (MuRF1, P = 0.002). Additionally, stress-activated JNK gene expression (P = 0.01) and protein were elevated during ischemia (P = 0.001). The results of this study support our hypothesis that protein degradation pathways are stimulated during TKA. Muscle protein catabolism is likely to play a role in the rapid loss of muscle volume measured within 2 wk of this surgery.

Age effect on myocellular remodeling: response to exercise and nutrition in humans

Aging is associated with decline in muscle mass and muscle functions. Muscle strength declines disproportionate to the decline in muscle mass indicating that muscle quality or protein quality also declines with age. Human studies have shown a progressive decline in muscle protein synthesis including proteins in the contractile apparatus and mitochondria with age. However, the decline in muscle protein synthesis is disproportionate to the decline in muscle mass that occurs with age prompting to hypothesize that muscle protein degradation also declines with age. A decline in mitochondrial capacity to synthesize ATP is likely a limiting factor of both synthesis and degradation, which are ATP dependent processes. In support of the above hypothesis, several studies have shown a decline in whole body protein turnover (synthesis and degradation). The timely and efficient degradation of irreversibly damaged or modified proteins is critical to maintain the quality of protein. It is proposed that a failure to degrade the damaged proteins and replacing them with newly synthesized proteins contribute to age related decline in muscle mass and quality of muscle proteins. The underlying molecular mechanism of these age related changes in human muscle needs further investigation.

Concurrent aerobic exercise interferes with the satellite cell response to acute resistance exercise

The addition of aerobic exercise (AE) to a resistance exercise (RE) program (concurrent exercise, CE) can interfere with maximum muscle fiber growth achieved with RE. Further, CE appears to markedly affect the growth of myosin heavy chain (MHC) I, but not MHC IIa fibers. The mechanism responsible for this “interference” is unclear. Satellite cell (SC) responsiveness to exercise appears to influence muscle adaptation but has not yet been examined following acute concurrent exercise. Thus, we assessed the fiber-type-specific SC response to RE, AE, and CE exercise. Eight college-aged males completed the following two exercise trials: the RE trial, which consisted of unilateral leg extensions and presses (4 sets ≥ 10 repetitions: 75% 1 repetition maximum, RM); and the AE/CE trial, which included an identical RE protocol with the opposite leg, immediately followed by subjects cycling for 90 min (60% Wmax). Muscle biopsies were obtained from the vastus lateralis before and 4 days after each session. Samples were cross-sectioned, stained with antibodies against NCAM, Ki-67, and MHC I, counterstained with DAPI, and analyzed for SC density (SC per fiber), SC activation, and fiber type. SC density increased to a greater extent following RE (38 ± 10%), compared with CE (−6 ± 8%). Similarly, MHC I muscle fiber SC density displayed a greater increase following RE (46 ± 14%), compared with AE (−7 ± 17%) and CE (−8 ± 8%). Our data indicate that the SC response to RE is blunted when immediately followed by AE, at least in MHC I muscle fibers, and possibly MHC II fibers. This suggests that the physiological environment evoked by AE might attenuate the eventual addition of myonuclei important for maximum muscle fiber growth and consequent force-producing capacity.

The role of AMP-activated protein kinase in the coordination of skeletal muscle turnover and energy homeostasis

The AMP-activated protein kinase (AMPK) is a serine/threonine protein kinase that acts as a sensor of cellular energy status switch regulating several systems including glucose and lipid metabolism. Recently, AMPK has been implicated in the control of skeletal muscle mass by decreasing mTORC1 activity and increasing protein degradation through regulation of ubiquitin-proteasome and autophagy pathways. In this review, we give an overview of the central role of AMPK in the control of skeletal muscle plasticity. We detail particularly its implication in the control of the hypertrophic and atrophic signaling pathways. In the light of these cumulative and attractive results, AMPK appears as a key player in regulating muscle homeostasis and the modulation of its activity may constitute a therapeutic potential in treating muscle wasting syndromes in humans.

Protein turnover, amino acid requirements and recommendations for athletes and active populations

Skeletal muscle is the major deposit of protein molecules. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. Stable isotopic tracers (¹³C-lysine, ¹⁵N-glycine, ²H₅-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g·kg⁻¹·day⁻¹ compared to 0.8 g·kg⁻¹·day⁻¹ in resting individuals. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.

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.

A moderate acute increase in physical activity enhances nutritive flow and the muscle protein anabolic response to mixed nutrient intake in older adults

BACKGROUND

Nutrient stimulation of muscle protein anabolism is blunted with aging and may contribute to the development and progression of sarcopenia in older adults. This is likely due to insulin resistance of protein metabolism and/or endothelial dysfunction with a reduction in nutritive flow, both of which can be improved by aerobic exercise.

OBJECTIVE

Our objective was to determine whether increasing physical activity can enhance the muscle protein anabolic effect of essential amino acid (EAA) + sucrose intake in older subjects by improving nutritive flow and/or insulin signaling.

DESIGN

Using a randomized crossover design, we measured in older subjects [n = 6, 70 ± 3 y of age, BMI (in kg/m2) of 25 ± 1] the acute effects of increasing physical activity with aerobic exercise, as compared with normal sedentary lifestyle, on the response of blood flow, microvascular perfusion, insulin signaling, and muscle protein kinetics to EAA+sucrose intake.

RESULTS

No differences between treatment groups were found in the basal state. The change from the basal state in blood flow, muscle perfusion, phenylalanine delivery, net balance, and muscle protein synthesis during the consumption of EAA+sucrose was significantly higher after the exercise than after the control treatment (P < 0.05). Insulin signaling increased during EAA+sucrose ingestion in both groups (P < 0.05).

CONCLUSIONS

Our data indicate that a prior bout of aerobic exercise increases the anabolic effect of nutrient intake in older adults. This effect appears to be mediated by an exercise-induced improvement in nutrient-stimulated vasodilation and nutrient delivery to muscle rather than to improved insulin signaling. This trial was registered at clinicaltrials.gov as NCT00690534.

Low muscle glycogen concentration does not suppress the anabolic response to resistance exercise

We determined the effect of muscle glycogen concentration and postexercise nutrition on anabolic signaling and rates of myofibrillar protein synthesis after resistance exercise (REX). Sixteen young, healthy men matched for age, body mass, peak oxygen uptake (Vo(2peak)) and strength (one repetition maximum; 1RM) were randomly assigned to either a nutrient or placebo group. After 48 h diet and exercise control, subjects undertook a glycogen-depletion protocol consisting of one-leg cycling to fatigue (LOW), whereas the other leg rested (NORM). The next morning following an overnight fast, a primed, constant infusion of l-[ring-(13)C(6)] phenylalanine was commenced and subjects completed 8 sets of 5 unilateral leg press repetitions at 80% 1RM. Immediately after REX and 2 h later, subjects consumed a 500 ml bolus of a protein/CHO (20 g whey + 40 g maltodextrin) or placebo beverage. Muscle biopsies from the vastus lateralis of both legs were taken at rest and 1 and 4 h after REX. Muscle glycogen concentration was higher in the NORM than LOW at all time points in both nutrient and placebo groups (P < 0.05). Postexercise Akt-p70S6K-rpS6 phosphorylation increased in both groups with no differences between legs (P < 0.05). mTOR(Ser2448) phosphorylation in placebo increased 1 h after exercise in NORM (P < 0.05), whereas mTOR increased ~4-fold in LOW (P < 0.01) and ~11 fold in NORM with nutrient (P < 0.01; different between legs P < 0.05). Post-exercise rates of MPS were not different between NORM and LOW in nutrient (0.070 ± 0.022 vs. 0.068 ± 0.018 %/h) or placebo (0.045 ± 0.021 vs. 0.049 ± 0.017 %/h). We conclude that commencing high-intensity REX with low muscle glycogen availability does not compromise the anabolic signal and subsequent rates of MPS, at least during the early (4 h) postexercise recovery period.

Bed rest impairs skeletal muscle amino acid transporter expression, mTORC1 signaling, and protein synthesis in response to essential amino acids in older adults

Skeletal muscle atrophy during bed rest is attributed, at least in part, to slower basal muscle protein synthesis (MPS). Essential amino acids (EAA) stimulate mammalian target of rapamycin (mTORC1) signaling, amino acid transporter expression, and MPS and are necessary for muscle mass maintenance, but there are no data on the effect of inactivity on this anabolic mechanism. We hypothesized that bed rest decreases muscle mass in older adults by blunting the EAA stimulation of MPS through reduced mTORC1 signaling and amino acid transporter expression in older adults. Six healthy older adults (67 ± 2 yr) participated in a 7-day bed rest study. We used stable isotope tracers, Western blotting, and real-time qPCR to determine the effect of bed rest on MPS, muscle mTORC1 signaling, and amino acid transporter expression and content in the postabsorptive state and after acute EAA ingestion. Bed rest decreased leg lean mass by ∼4% (P < 0.05) and increased postabsorptive mTOR protein (P < 0.05) levels while postabsorptive MPS was unchanged (P > 0.05). Before bed rest acute EAA ingestion increased MPS, mTOR (Ser(2448)), S6 kinase 1 (Thr(389), Thr(421)/Ser(424)), and ribosomal protein S6 (Ser(240/244)) phosphorylation, activating transcription factor 4, L-type amino acid transporter 1 and sodium-coupled amino acid transporter 2 protein content (P < 0.05). However, bed rest blunted the EAA-induced increase in MPS, mTORC1 signaling, and amino acid transporter protein content. We conclude that bed rest in older adults significantly attenuated the EAA-induced increase in MPS with a mechanism involving reduced mTORC1 signaling and amino acid transporter protein content. Together, our data suggest that a blunted EAA stimulation of MPS may contribute to muscle loss with inactivity in older persons.

Both resistance- and endurance-type exercise reduce the prevalence of hyperglycaemia in individuals with impaired glucose tolerance and in insulin-treated and non-insulin-treated type 2 diabetic patients

AIMS/HYPOTHESIS

The present study compares the impact of endurance- vs resistance-type exercise on subsequent 24 h blood glucose homeostasis in individuals with impaired glucose tolerance (IGT) and type 2 diabetes.

METHODS

Fifteen individuals with IGT, 15 type 2 diabetic patients treated with exogenous insulin (INS), and 15 type 2 diabetic patients treated with oral glucose-lowering medication (OGLM) participated in a randomised crossover experiment. Participants were studied on three occasions for 3 days under strict dietary standardisation, but otherwise free-living conditions. Blood glucose homeostasis was assessed by ambulatory continuous glucose monitoring over the 24 h period following a 45 min session of resistance-type exercise (75% one repetition maximum), endurance-type exercise (50% maximum workload capacity) or no exercise at all.

RESULTS

Average 24 h blood glucose concentrations were reduced from 7.4 ± 0.2, 9.6 ± 0.5 and 9.2 ± 0.7 mmol/l during the control experiment to 6.9 ± 0.2, 8.6 ± 0.4 and 8.1 ± 0.5 mmol/l (resistance-type exercise) and 6.8 ± 0.2, 8.6 ± 0.5 and 8.5 ± 0.5 mmol/l (endurance-type exercise) over the 24 h period following a single bout of exercise in the IGT, OGLM and INS groups, respectively (p < 0.001 for both treatments). The prevalence of hyperglycaemia (blood glucose >10 mmol/l) was reduced by 35 ± 7 and 33 ± 11% over the 24 h period following a single session of resistance- and endurance-type exercise, respectively (p < 0.001 for both treatments).

CONCLUSIONS/INTERPRETATION

A single session of resistance- or endurance-type exercise substantially reduces the prevalence of hyperglycaemia during the subsequent 24 h period in individuals with IGT, and in insulin-treated and non-insulin-treated type 2 diabetic patients. Both resistance- and endurance-type exercise can be integrated in exercise intervention programmes designed to improve glycaemic control.

TRIAL REGISTRATION

Clinicaltrials.gov NCT00945165.

FUNDING

The Netherlands Organization for Health Research and Development (ZonMw, the Netherlands).

Exercise intensity and muscle hypertrophy in blood flow-restricted limbs and non-restricted muscles: a brief review

Although evidence for high-intensity resistance training-induced muscle hypertrophy has accumulated over the last several decades, the basic concept of the training can be traced back to ancient Greece: Milo of Croton lifted a bull-calf daily until it was fully grown, which would be known today as progressive overload. Now, in the 21st century, different types of training are being tested and studied, such as low-intensity exercise combined with arterial as well as venous blood flow restriction (BFR) to/from the working muscles. Because BFR training requires the use of a cuff that is placed at the proximal ends of the arms and/or legs, the BFR is only applicable to limb muscles. Consequently, most previous BFR training studies have focused on the physiological adaptations of BFR limb muscles. Muscle adaptations in non-BFR muscles of the hip and trunk are lesser known. Recent studies that have reported both limb and trunk muscle adaptations following BFR exercise training suggest that low-intensity (20-30% of 1RM) resistance training combined with BFR elicits muscle hypertrophy in both BFR limb and non-BFR muscles. However, the combination of leg muscle BFR with walk training elicits muscle hypertrophy only in the BFR leg muscles. In contrast to resistance exercise with BFR, the exercise intensity may be too low during BFR walk training to cause muscle hypertrophy in the non-BFR gluteus maximus and other trunk muscles. Other mechanisms including hypoxia, local and systemic growth factors and muscle cell swelling may also potentially affect the hypertrophic response of non-BFR muscles to BFR resistance exercise.