Loss of Muscle Mass and Strength After Hip Fracture: an Intervention Target for Nutrition Supplementation.

PURPOSEOF REVIEW: To summarize what is known about the deleterious effect of hip fracture on muscle mass and strength as well as the scientific evidence for post-surgical nutrition supplementation to maintain muscle and improve function. RECENT FINDINGS: This review provides a discussion of the relationship between muscle mass, strength, and physical function following hip fracture, briefly describes the approaches to measuring lean mass, discusses prevalence of sarcopenia and malnutrition among older men and women with hip fracture, and reviews the effects of essential amino acids on muscle. Loss of muscle mass and strength following hip fracture is substantial with consequences for recovery of functional independence. EAA-based nutrition supplementation, which directly effects muscle, has potential to improve outcomes.

Adaptive remodeling of skeletal muscle energy metabolism in high-altitude hypoxia: Lessons from AltitudeOmics.

Metabolic responses to hypoxia play important roles in cell survival strategies and disease pathogenesis in humans. However, the homeostatic adjustments that balance changes in energy supply and demand to maintain organismal function under chronic low oxygen conditions remain incompletely understood, making it difficult to distinguish adaptive from maladaptive responses in hypoxia-related pathologies. We integrated metabolomic and proteomic profiling with mitochondrial respirometry and blood gas analyses to comprehensively define the physiological responses of skeletal muscle energy metabolism to 16 days of high-altitude hypoxia (5260 m) in healthy volunteers from the AltitudeOmics project. In contrast to the view that hypoxia down-regulates aerobic metabolism, results show that mitochondria play a central role in muscle hypoxia adaptation by supporting higher resting phosphorylation potential and enhancing the efficiency of long-chain acylcarnitine oxidation. This directs increases in muscle glucose toward pentose phosphate and one-carbon metabolism pathways that support cytosolic redox balance and help mitigate the effects of increased protein and purine nucleotide catabolism in hypoxia. Muscle accumulation of free amino acids favor these adjustments by coordinating cytosolic and mitochondrial pathways to rid the cell of excess nitrogen, but might ultimately limit muscle oxidative capacity in vivo Collectively, these studies illustrate how an integration of aerobic and anaerobic metabolism is required for physiological hypoxia adaptation in skeletal muscle, and highlight protein catabolism and allosteric regulation as unexpected orchestrators of metabolic remodeling in this context. These findings have important implications for the management of hypoxia-related diseases and other conditions associated with chronic catabolic stress.

Evidence of a broad histamine footprint on the human exercise transcriptome.

KEY POINTS: Histamine is a primordial signalling molecule, capable of activating cells in an autocrine or paracrine fashion via specific cell surface receptors, in a variety of pathways that probably predate its more recent role in innate and adaptive immunity. Although histamine is normally associated with pathological conditions or allergic and anaphylactic reactions, it may contribute beneficially to the normal changes that occur within skeletal muscle during the recovery from exercise. We show that the human response to exercise includes an altered expression of thousands of protein-coding genes, and much of this response appears to be driven by histamine. Histamine may be an important molecular transducer contributing to many of the adaptations that accompany chronic exercise training. ABSTRACT: Histamine is a primordial signalling molecule, capable of activating cells in an autocrine or paracrine fashion via specific cell surface receptors. In humans, aerobic exercise is followed by a post-exercise activation of histamine H1 and H2 receptors localized to the previously exercised muscle. This could trigger a broad range of cellular adaptations in response to exercise. Thus, we exploited RNA sequencing to explore the effects of H1 and H2 receptor blockade on the exercise transcriptome in human skeletal muscle tissue harvested from the vastus lateralis. We found that exercise exerts a profound influence on the human transcriptome, causing the differential expression of more than 3000 protein-coding genes. The influence of histamine blockade post-exercise was notable for 795 genes that were differentially expressed between the control and blockade condition, which represents >25% of the number responding to exercise. The broad histamine footprint on the human exercise transcriptome crosses many cellular functions, including inflammation, vascular function, metabolism, and cellular maintenance.

Tourniquet Use During Knee Replacement Surgery May Contribute to Muscle Atrophy in Older Adults.

Muscle atrophy after total knee arthroplasty (TKA) occurs at a rate of 1% per day for the first 2 wk. Our hypothesis is that tourniquet-induced ischemia-reperfusion injury occurring during TKA influences metabolism and may contribute to atrophy. Identifying pathways that are upregulated during this critical "14-d window" after surgery may help us delineate therapeutic approaches to avoid muscle loss.

AICAR administration ameliorates hypertension and angiogenic imbalance in a model of preeclampsia in the rat.

Previous studies suggest restoration of angiogenic balance can lower blood pressure and improve vascular endothelium function in models of preeclampsia. Our laboratory has recently reported exercise training mitigates hypertension in an animal model of preeclampsia, but the mechanisms are unknown. AMP-activated protein kinase (AMPK) is stimulated during exercise and has been shown to increase expression of VEGF. Therefore, the purpose of this study was to determine whether AICAR (5-aminoimidazole-4-carboxamide-3-ribonucleoside), a potent AMPK stimulator, would increase circulating VEGF, improve angiogenic potential, decrease oxidative stress, and abrogate placental ischemia-induced hypertension. In rats, reduced uteroplacental perfusion pressure (RUPP) was induced on day 14 of gestation by introducing silver clips on the inferior abdominal aorta and ovarian arteries. AICAR was administered intraperitoneally (50 mg/kg b.i.d.) days 14-18, and blood pressure and tissues were collected on day 19. RUPP-induced hypertension was ameliorated (P < 0.05) with AICAR versus RUPP. AICAR increased (P < 0.05) plasma VEGF and decreased (P < 0.05) plasma soluble VEGF receptor-1 in the RUPP + AICAR versus RUPP. Antioxidant capacity was restored (P < 0.05) by AICAR in RUPP placenta. Renal and placental catalase activity was decreased (P < 0.05) in RUPP + AICAR versus RUPP. Angiogenic potential was increased (P < 0.05) in RUPP + AICAR versus RUPP. Fetal and placental weights were unaffected by AICAR. Placental AMPK phosphorylation was increased (P < 0.05) in RUPP + AICAR versus normal pregnant and RUPP. These findings suggest AICAR may be useful to mitigate angiogenic imbalance, renal, and placental oxidative stress and increase in blood pressure associated with RUPP hypertension. Furthermore, placental AMPK phosphorylation was observed only in the setting of ischemia.

Placental and vascular adaptations to exercise training before and during pregnancy in the rat.

Although exercise during pregnancy is generally recommended and thought to be beneficial to mother and fetus, the nature of the adaptations to exercise during pregnancy and how they may be beneficial remain poorly understood. Recent studies suggest that exercise may stimulate expression of several cytoprotective and pro-angiogenic molecules such as heat shock proteins (HSP) and vascular endothelial growth factors (VEGF). We hypothesized that exercise training during pregnancy improves angiogenic balance, increases HSP expression, and improves endothelial function. Female rats were given access to an exercise wheel for 6 wk before and during pregnancy. On day 19 of pregnancy tissues were collected and snap frozen for later analysis. Western blots were performed in skeletal muscle and placenta. HSP 27 (3.7 ± 0.36 vs. 2.2 ± 0.38; P < 0.05), HSP 60 (2.2 ± 0.73 vs. 0.49 ± 0.08; P < 0.05), and HSP 90 (0.33 ± 0.09 vs. 0.11 ± 0.02; P < 0.05) were increased in the placentas of exercise-trained rats compared with sedentary controls. In addition, exercise training increased (P < 0.05) plasma free VEGF and augmented (P < 0.05) endothelium-dependent vascular relaxation compared with nonexercise control rats. The present data indicates chronic exercise training stimulates HSP expression in the placenta and that regular exercise training increases circulating VEGF in pregnant but not in nonpregnant rats. Although the present findings suggest that exercise before and during pregnancy may promote the expression of molecules that could attenuate placental and vascular dysfunction in complicated pregnancies, further studies are needed to determine the safety and effectiveness of exercise training as a therapeutic modality in pregnancy.

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.

Proteins regulating cap-dependent translation are downregulated during total knee arthroplasty.

Total knee arthroplasty (TKA) utilizes a tourniquet to reduce blood loss, maintain a clear surgical "bloodless" field, and to ensure proper bone-implant cementing. In 2007, over 600,000 TKAs were performed in the United States, and this number is projected to increase to 3.48 million procedures performed annually by 2030. The acute effects of tourniquet-induced ischemia-reperfusion (I/R) on human skeletal muscle cells are poorly understood and require critical investigation, as muscle atrophy following this surgery is rapid and represents the most significant clinical barrier to long-term normalization of physical function. To determine the acute effects of I/R on skeletal muscle cells, biopsies were obtained at baseline, maximal ischemia (prior to tourniquet release), and reperfusion (following tourniquet release). Quadriceps volume was determined before and 2 wk post-TKA by MRI. We measured a 36% decrease in phosphorylation of Akt Ser(473) during ischemia and 37% during reperfusion (P < 0.05). 4E-BP1 Thr(37/46) phosphorylation decreased 29% during ischemia and 22% during reperfusion (P < 0.05). eEF2 Thr(56) phosphorylation increased 25% during ischemia and 43% during reperfusion (P < 0.05). Quadriceps volume decreased 12% in the TKA leg (P < 0.05) and tended to decrease (6%) in the contralateral leg (P = 0.1). These data suggest cap-dependent translation initiation, and elongation may be inhibited during and after TKA surgery. We propose that cap-dependent translational events occurring during surgery may precipitate postoperative changes in muscle cells that contribute to the etiology of muscle atrophy following TKA.

Muscle protein breakdown has a minor role in the protein anabolic response to essential amino acid and carbohydrate intake following resistance exercise.

Muscle protein breakdown (MPB) is increased following resistance exercise, but ingestion of carbohydrate during postexercise recovery can decrease MPB with no effect on muscle protein synthesis (MPS). We sought to determine whether a combination of essential amino acids (EAA) with low carbohydrate or high carbohydrate could effectively reduce MPB following resistance exercise and improve muscle protein net balance (NB). We hypothesized that higher levels of carbohydrate and resulting increases in circulating insulin would inhibit MPB and associated signaling, resulting in augmented NB. Thirteen male subjects were assigned to one of two groups receiving equivalent amounts of EAA (approximately 20 g) but differing carbohydrate levels (low = 30, high = 90 g). Groups ingested nutrients 1 h after an acute bout of leg resistance exercise. Leg phenylalanine kinetics (e.g., MPB, MPS, NB), signaling proteins, and mRNA expression were assessed on successive muscle biopsies using stable isotopic techniques, immunoblotting, and real-time quantitative PCR, respectively. MPB tended to decrease (P < 0.1) and MPS increased (P < 0.05) similarly in both groups following nutrient ingestion. No group differences were observed, but muscle ring finger 1 (MuRF1) protein content and MuRF1 mRNA expression increased following resistance exercise and remained elevated following nutrient ingestion, while autophagy marker (light-chain 3B-II) decreased after nutrient ingestion (P < 0.05). Forkhead box-O3a phosphorylation, total muscle atrophy F-box (MAFbx) protein, and MAFbx and caspase-3 mRNA expression were unchanged. We conclude that the enhanced muscle protein anabolic response detected when EAA+carbohydrate are ingested postresistance exercise is primarily due to an increase in MPS with minor changes in MPB, regardless of carbohydrate dose or circulating insulin level.

Essential amino acid supplementation alters the p53 transcriptional response and cytokine gene expression following total knee arthroplasty.

Reducing muscle atrophy following orthopedic surgery is critical during the postoperative period. Our previous work in patients who underwent total knee arthroplasty (TKA) showed that the vast majority of atrophy occurs within 2 wk following surgery and that essential amino acid (EAA) supplementation attenuates this atrophy. We used RNA-sequencing (RNA-seq) to identify genes associated with atrophy after TKA with and without EAAs. Analysis of overrepresented gene-ontology terms revealed that p53 signaling and the cytokine-cytokine receptor pathways were highly upregulated after TKA. Relative to the placebo group, the EAA group had altered expression of p53 regulators such as MDM2. This altered expression may account for differences between groups in timing of upregulation of some p53 targets such as apoptosis genes, and may account for the reduction in muscle loss in the subjects receiving EAAs. Furthermore, we observed altered expression of a large number of cytokine-signaling genes including TNFRSF12A, which plays a critical role in muscle atrophy, myogenesis, fibrosis, and the noncanonical NF-κB pathway.NEW & NOTEWORTHY Total knee arthroplasty is the most frequently performed inpatient surgical procedure for those over 45 yr in the United States. Following surgery, patients lose a large amount of muscle, which impacts functional mobility. Previously, our laboratory found that supplementing patients' diets with essential amino acids (EAAs) reduces postsurgical muscle loss. Here, our goal was to characterize the transcriptional changes associated with surgery with and without EAA supplementation to uncover the underlying mechanisms by which EAAs attenuate this muscle loss.

Rapamycin administration in humans blocks the contraction-induced increase in skeletal muscle protein synthesis.

Muscle protein synthesis and mTORC1 signalling are concurrently stimulated following muscle contraction in humans. In an effort to determine whether mTORC1 signalling is essential for regulating muscle protein synthesis in humans, we treated subjects with a potent mTORC1 inhibitor (rapamycin) prior to performing a series of high-intensity muscle contractions. Here we show that rapamycin treatment blocks the early (1-2 h) acute contraction-induced increase ( approximately 40%) in human muscle protein synthesis. In addition, several downstream components of the mTORC1 signalling pathway were also blunted or blocked by rapamycin. For instance, S6K1 phosphorylation (Thr421/Ser424) was increased post-exercise 6-fold in the control group while being unchanged with rapamycin treatment. Furthermore, eEF2 phosphorylation (Thr56) was reduced by approximately 25% post-exercise in the control group but phosphorylation following rapamycin treatment was unaltered, indicating that translation elongation was inhibited. Rapamycin administration prior to exercise also reduced the ability of raptor to associate with mTORC1 during post-exercise recovery. Surprisingly, rapamycin treatment prior to resistance exercise completely blocked the contraction-induced increase in the phosphorylation of ERK1/2 (Thr202/Tyr204) and blunted the increase in MNK1 (Thr197/202) phosphorylation. However, the phosphorylation of a known target of MNK1, eIF4E (Ser208), was similar in both groups (P > 0.05) which is consistent with the notion that rapamycin does not directly inhibit MAPK signalling. We conclude that mTORC1 signalling is, in part, playing a key role in regulating the contraction-induced stimulation of muscle protein synthesis in humans, while dual activation of mTORC1 and ERK1/2 stimulation may be required for full stimulation of human skeletal muscle protein synthesis.

Essential amino acid and carbohydrate ingestion before resistance exercise does not enhance postexercise muscle protein synthesis.

Ingestion of an essential amino acid-carbohydrate (EAA + CHO) solution following resistance exercise enhances muscle protein synthesis during postexercise recovery. It is unclear whether EAA + CHO ingestion before resistance exercise can improve direct measures of postexercise muscle protein synthesis (fractional synthetic rate; FSR). We hypothesized that EAA + CHO ingestion before a bout of resistance exercise would prevent the exercise-induced decrease in muscle FSR and would result in an enhanced rate of muscle FSR during postexercise recovery. We studied 22 young healthy subjects before, during, and for 2 h following a bout of high-intensity leg resistance exercise. The fasting control group (n = 11) did not ingest nutrients, and the EAA + CHO group (n = 11) ingested a solution of EAA + CHO 1 h before beginning the exercise bout. Stable isotopic methods were used in combination with muscle biopsies to determine FSR. Immunoblotting procedures were utilized to assess cell signaling proteins associated with the regulation of FSR. We found that muscle FSR increased in the EAA + CHO group immediately following EAA + CHO ingestion (P < 0.05), returned to basal values during exercise, and remained unchanged at 1 h postexercise. Muscle FSR decreased in the fasting group during exercise and increased at 1 h postexercise (P < 0.05). However, the 2 h postexercise FSR increased by approximately 50% in both groups with no differences between groups (P > 0.05). Eukaryotic elongation factor 2 phosphorylation was reduced in both groups at 2 h postexercise (EAA + CHO: 39 +/- 7%; fasting: 47 +/- 9%; P < 0.05). We conclude that EAA + CHO ingestion before resistance exercise does not enhance postexercise FSR compared with exercise without nutrients.

Expression of growth-related genes in young and older human skeletal muscle following an acute stimulation of protein synthesis.

Muscle growth is associated with an activation of the mTOR signaling pathway and satellite cell regulators. The purpose of this study was to determine whether 17 selected genes associated with mTOR/muscle protein synthesis and the satellite cells/myogenic program are differentially expressed in young and older human skeletal muscle at rest and in response to a potent anabolic stimulus [resistance exercise + essential amino acid ingestion (RE+EAA)]. Twelve male subjects (6 young, 6 old) completed a bout of heavy resistance exercise. Muscle biopsies were obtained before and at 3 and 6 h post RE+EAA. Subjects ingested leucine-enriched essential amino acids at 1 h postexercise. mRNA expression was determined using qRT-PCR. At rest, hVps34 mRNA was elevated in the older subjects (P < 0.05) while there was a tendency for levels of myoD, myogenin, and TSC2 mRNA to be higher than young. The anabolic stimulus (RE+EAA) altered mRNAs associated with mTOR regulation. Notably, REDD2 decreased in both age groups (P < 0.05) but the expression of Rheb mRNA increased only in the young. Finally, cMyc mRNA was elevated (P < 0.05) in both young and old at 6 h post RE+EAA. Furthermore, RE+EAA also increased expression of several mRNAs associated with satellite function in the young (P < 0.05), while expression of these mRNAs did not change in the old. We conclude that several anabolic genes in muscle are more responsive in young men post RE+EAA. Our data provide new insights into the regulation of genes important for transcription and translation in young and old human skeletal muscle post RE+EAA.

Nutritional and contractile regulation of human skeletal muscle protein synthesis and mTORC1 signaling.

In this review we discuss current findings in the human skeletal muscle literature describing the acute influence of nutrients (leucine-enriched essential amino acids in particular) and resistance exercise on muscle protein synthesis and mammalian target of rapamycin complex 1 (mTORC1) signaling. We show that essential amino acids and an acute bout of resistance exercise independently stimulate human skeletal muscle protein synthesis. It also appears that ingestion of essential amino acids following resistance exercise leads to an even larger increase in the rate of muscle protein synthesis compared with the independent effects of nutrients or muscle contraction. Until recently the cellular mechanisms responsible for controlling the rate of muscle protein synthesis in humans were unknown. In this review, we highlight new studies in humans that have clearly shown the mTORC1 signaling pathway is playing an important regulatory role in controlling muscle protein synthesis in response to nutrients and/or muscle contraction. We propose that essential amino acid ingestion shortly following a bout of resistance exercise is beneficial in promoting skeletal muscle growth and may be useful in counteracting muscle wasting in a variety of conditions such as aging, cancer cachexia, physical inactivity, and perhaps during rehabilitation following trauma or surgery.

Cellular and morphological changes with EAA supplementation before and after total knee arthroplasty.

Investigate the underlying cellular basis of muscle atrophy (Placebo) and atrophy reduction (essential amino acid supplementation, EAAs) in total knee arthroplasty (TKA) patients by examining satellite cells and other key histological markers of inflammation, recovery, and fibrosis. Forty-one subjects (53-76 yr) scheduled for TKA were randomized into two groups, ingesting 20 g of EAAs or placebo, twice-daily, for 7 days before TKA and for 6 wk after surgery. A first set of muscle biopsies was obtained from both legs before surgery in the operating room, and patients were randomly assigned and equally allocated to have two additional biopsies at either 1 or 2 wk after surgery. Biopsies were processed for gene expression and immunohistochemistry. Satellite cells were significantly higher in patients ingesting 20 g of essential amino acids twice daily for the 7 days leading up to surgery compared with Placebo (operative leg P = 0.03 for satellite cells/fiber and P = 0.05 for satellite cell proportions for Type I-associated cells and P = 0.05 for satellite cells/fiber for Type II-associated cells.) Myogenic regulatory factor gene expression was different between groups, with the Placebo Group having elevated MyoD expression at 1 wk and EAAs having elevated myogenin expression at 1 wk. M1 macrophages were more prevalent in Placebo than the EAAs Group. IL-6 and TNF-α transcripts were elevated postsurgery in both groups; however, TNF-α declined by 2 wk in the EAAs Group. EAAs starting 7 days before surgery increased satellite cells on the day of surgery and promoted a more favorable inflammatory environment postsurgery.NEW & NOTEWORTHY Clinical studies by our group indicate that the majority of muscle atrophy after total knee arthroplasty (TKA) in older adults occurs rapidly, within the first 2 wks. We have also shown that essential amino acid supplementation (EAAs) before and after TKA mitigates muscle atrophy; however, the mechanisms are unknown. These results suggest that satellite cell numbers are elevated with EAA ingestion before surgery, and after surgery, EAA ingestion positively influences markers of inflammation. Combined, these data may help inform further studies designed to address the accelerated sarcopenia that occurs in older adults after major surgery.

Resistance exercise increases human skeletal muscle AS160/TBC1D4 phosphorylation in association with enhanced leg glucose uptake during postexercise recovery.

Akt substrate of 160 kDa (AS160/TBC1D4) is associated with insulin and contraction-mediated glucose uptake. Human skeletal muscle AS160 phosphorylation is increased during aerobic exercise but not immediately following resistance exercise. It is not known whether AS160 phosphorylation is altered during recovery from resistance exercise. Therefore, we hypothesized that muscle AS160/TBC1D4 phosphorylation and glucose uptake across the leg would be increased during recovery following resistance exercise. We studied 9 male subjects before, during, and for 2 h of postexercise recovery. We utilized femoral catheterizations and muscle biopsies in combination with indirect calorimetry and immunoblotting to determine whole body glucose and fat oxidation, leg glucose uptake, muscle AMPKalpha2 activity, and the phosphorylation of muscle Akt and AS160/TBC1D4. Glucose oxidation was reduced while fat oxidation increased ( approximately 35%) during postexercise recovery (P

Chronic paraplegia-induced muscle atrophy downregulates the mTOR/S6K1 signaling pathway.

Ribosomal S6 kinase 1 (S6K1) is a downstream component of the mammalian target of rapamycin (mTOR) signaling pathway and plays a regulatory role in translation initiation, protein synthesis, and muscle hypertrophy. AMP-activated protein kinase (AMPK) is a cellular energy sensor, a negative regulator of mTOR, and an inhibitor of protein synthesis. The purpose of this study was to determine whether the hypertrophy/cell growth-associated mTOR pathway was downregulated during muscle atrophy associated with chronic paraplegia. Soleus muscle was collected from male Sprague-Dawley rats 10 wk following complete T(4)-T(5) spinal cord transection (paraplegic) and from sham-operated (control) rats. We utilized immunoprecipitation and Western blotting techniques to measure upstream [AMPK, Akt/protein kinase B (PKB)] and downstream components of the mTOR signaling pathway [mTOR, S6K1, SKAR, 4E-binding protein 1 (4E-BP1), and eukaryotic initiation factor (eIF) 4G and 2alpha]. Paraplegia was associated with significant soleus muscle atrophy (174 +/- 8 vs. 240 +/- 13 mg; P < 0.05). There was a reduction in phosphorylation of mTOR, S6K1, and eIF4G (P < 0.05) with no change in Akt/PKB or 4E-BP1 (P > 0.05). Total protein abundance of mTOR, S6K1, eIF2alpha, and Akt/PKB was decreased, and increased for SKAR (P < 0.05), whereas 4E-BP1 and eIF4G did not change (P > 0.05). S6K1 activity was significantly reduced in the paraplegic group (P < 0.05); however, AMPKalpha2 activity was not altered (3.5 +/- 0.4 vs. 3.7 +/- 0.5 pmol x mg(-1) x min(-1), control vs. paraplegic rats). We conclude that paraplegia-induced muscle atrophy in rats is associated with a general downregulation of the mTOR signaling pathway. Therefore, in addition to upregulation of atrophy signaling during muscle wasting, downregulation of muscle cell growth/hypertrophy-associated signaling appears to be an important component of long-term muscle loss.

Skeletal muscle protein anabolic response to resistance exercise and essential amino acids is delayed with aging.

Skeletal muscle loss during aging leads to an increased risk of falls, fractures, and eventually loss of independence. Resistance exercise is a useful intervention to prevent sarcopenia; however, the muscle protein synthesis (MPS) response to resistance exercise is less in elderly compared with young subjects. On the other hand, essential amino acids (EAA) increase MPS equally in both young and old subjects when sufficient EAA is ingested. We hypothesized that EAA ingestion following a bout of resistance exercise would stimulate anabolic signaling and MPS similarly between young and old men. Each subject ingested 20 g of EAA 1 h following leg resistance exercise. Muscle biopsies were obtained before and 1, 3, and 6 h after exercise to measure the rate of MPS and signaling pathways that regulate translation initiation. MPS increased early in young (1-3 h postexercise) and later in old (3-6 h postexercise). At 1 h postexercise, ERK1/2 MNK1 phosphorylation increased and eIF2alpha phosphorylation decreased only in the young. mTOR signaling (mTOR, S6K1, 4E-BP1, eEF2) was similar between groups at all time points, but MNK1 phosphorylation was lower at 3 h and AMP-activated protein kinase-alpha (AMPKalpha) phosphorylation was higher in old 1-3 h postexercise. We conclude that the acute MPS response after resistance exercise and EAA ingestion is similar between young and old men; however, the response is delayed with aging. Unresponsive ERK1/2 signaling and AMPK activation in old muscle may be playing a role in the delayed activation of MPS. Notwithstanding, the combination of resistance exercise and EAA ingestion should be a useful strategy to combat sarcopenia.

Human muscle gene expression following resistance exercise and blood flow restriction.

INTRODUCTION: Blood flow restriction in combination with low-intensity resistance exercise (REFR) increases skeletal muscle size to a similar extent as compared with traditional high-intensity resistance exercise training. However, there are limited data describing the molecular adaptations that occur after REFR. PURPOSE: To determine whether hypoxia inducible factor-1 alpha (HIF-1alpha) and REDD1 mRNA are expressed differently in REFR compared with low-intensity resistance exercise with no blood flow restriction (CONTROL). Secondly, to determine whether low-intensity resistance exercise is able to induce changes in mRNA expression of several anabolic and catabolic genes as typically seen with high-intensity resistance exercise. METHODS: Six subjects were studied at baseline and 3 h after a bout of leg resistance exercise (20% 1RM) in REFR and CONTROL subjects. Each subject participated in both groups, with 3 wk separating each visit. Muscle biopsy samples were analyzed for mRNA expression, using qRT-PCR. RESULT: Our primary finding was that there were no differences between CONTROL and REFR for any of the selected genes at 3 h after exercise (P > 0.05). However, low-intensity resistance exercise increased HIF-1alpha, p21, MyoD, and muscle RING finger 1 (MuRF1) mRNA expression and decreased REDD1 and myostatin mRNA expression in both groups (P < 0.05). CONCLUSION: Low-intensity resistance exercise can alter skeletal muscle mRNA expression of several genes associated with muscle growth and remodeling, such as REDD1, HIF-1alpha, MyoD, MuRF1, and myostatin. Further, the results from REFR and CONTROL were similar, indicating that the changes in early postexercise gene expression were attributable to the low-intensity resistance exercise bout, and not blood flow restriction.