Human Skeletal Muscle In Response Research Articles

Exercise intensity-dependent regulation of peroxisome proliferator-activated receptor γ coactivator-1α mRNA abundance is associated with differential activation of upstream signalling kinases in human skeletal muscle

Skeletal muscle contraction increases intracellular ATP turnover, calcium flux, and mechanical stress, initiating signal transduction pathways that modulate peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha)-dependent transcriptional programmes. The purpose of this study was to determine if the intensity of exercise regulates PGC-1alpha expression in human skeletal muscle, coincident with activation of signalling cascades known to regulate PGC-1alpha transcription. Eight sedentary males expended 400 kcal (1674 kj) during a single bout of cycle ergometer exercise on two separate occasions at either 40% (LO) or 80% (HI) of . Skeletal muscle biopsies from the m. vastus lateralis were taken at rest and at +0, +3 and +19 h after exercise. Energy expenditure during exercise was similar between trials, but the high intensity bout was shorter in duration (LO, 69.9 +/- 4.0 min; HI, 36.0 +/- 2.2 min, P < 0.05) and had a higher rate of glycogen utilization (P < 0.05). PGC-1alpha mRNA abundance increased in an intensity-dependent manner +3 h after exercise (LO, 3.8-fold; HI, 10.2-fold, P < 0.05). AMP-activated protein kinase (AMPK) (2.8-fold, P < 0.05) and calcium/calmodulin-dependent protein kinase II (CaMKII) phosphorylation (84%, P < 0.05) increased immediately after HI but not LO. p38 mitogen-activated protein kinase (MAPK) phosphorylation increased after both trials (2.0-fold, P < 0.05), but phosphorylation of the downstream transcription factor, activating transcription factor-2 (ATF-2), increased only after HI (2.4-fold, P < 0.05). Cyclic-AMP response element binding protein (CREB) phosphorylation was elevated at +3 h after both trials (80%, P < 0.05) and class IIa histone deacetylase (HDAC) phosphorylation increased only after HI (2.0-fold, P < 0.05). In conclusion, exercise intensity regulates PGC-1alpha mRNA abundance in human skeletal muscle in response to a single bout of exercise. This effect is mediated by differential activation of multiple signalling pathways, with ATF-2 and HDAC phosphorylation proposed as key intensity-dependent mediators.

A Method To Study In Vivo Protein Synthesis In Slow- And Fast-twitch Human Muscle Fibers

PURPOSE: To develop an approach to directly assess protein fractional synthesis rate (FSR) in isolated human muscle fibers and to examine potential differences in FSR between slow (MHC I) and fast twitch (MHC IIa) fibers. METHODS: Individual muscle fibers were isolated from biopsies of the vastus lateralis (VL) and soleus (SOL) obtained from eight young men during a primed, continuous infusion of 2H3 leucine performed under basal conditions. To determine mixed protein FSR, a portion of each fiber was used to identify fiber type, fibers of the same fiber type were pooled, and the 2H3 leucine enrichment determined via GC-MS. RESULTS: In the VL, MHC I fibers exhibited a 33% faster (p<0.05) mixed protein FSR compared to MHC IIa fibers (0.0900.005 vs. 0.0680.002 %/hr). Similar (p>0.05) mixed protein FSR values were observed between MHC I fibers from the SOL (0.0820.006 %/hr) and MHC I fibers from the VL. Feasibility of processing isolated fibers for myofibrillar (MF) protein FSR was also determined in non-fiber typed pooled fibers from the VL, resulting in a MF protein FSR of 0.0730.004 %/hr. CONCLUSION: These methods can be applied to the study of fiber type specific responses in human skeletal muscle, and the need for this level of investigation is underscored by the different basal protein synthesis rates in MHC I and MHC IIa fibers originating from the same muscle. Supported by NIH R01 AG025032; NASA NNJ06HF59G

Anabolic and energetic signaling in human skeletal muscle in response to amino acids and endurance exercise

Resistance exercise potentiates anabolic signaling in response to amino acids. However, endurance exercise may interfere with amino acid‐mediated anabolic signaling through activation of AMPK. Thus, we examined nutrient and energy sensing signaling pathways in response to amino acids and endurance exercise (AA+EE) in young (n=9) and older (n=8) men. Muscle biopsies were obtained at rest and again at 10 and 180 min of recovery from 45 min of walking at 40% VO2PEAK. AMPKα and ACC were unchanged in response to AA+EE. Although Akt, mTOR, and 4E‐BP1 appear unaltered by AA+EE, phosphorylation of the downstream effector p70S6K was significantly increased. Our data show, for the first time in humans, that AA+EE increases the phosphorylation of p70S6K, which is a key anabolic signaling protein.

A new method to study in vivo protein synthesis in slow- and fast-twitch muscle fibers and initial measurements in humans

The aim of this study was to develop an approach to directly assess protein fractional synthesis rate (FSR) in isolated human muscle fibers in a fiber type-specific fashion. Individual muscle fibers were isolated from biopsies of the vastus lateralis (VL) and soleus (SOL) obtained from eight young men during a primed, continuous infusion of [5,5,5-(2)H3]leucine performed under basal conditions. To determine mixed protein FSR, a portion of each fiber was used to identify fiber type, fibers of the same type were pooled, and the [5,5,5-(2)H3]leucine enrichment was determined via GC-MS. Processing isolated slow-twitch [myosin heavy chain (MHC) I] and fast-twitch (MHC IIa) fibers for mixed protein bound [5,5,5-(2)H3]leucine enrichment yielded mass ion chromatographic peaks that were similar in shape, abundance, and measurement reliability as tissue homogenates. In the VL, MHC I fibers exhibited a 33% faster (P<0.05) mixed protein FSR compared with MHC IIa fibers (0.068+/-0.006 vs. 0.051+/-0.003%/h). MHC I fibers from the SOL (0.060+/-0.005%/h) and MHC I fibers from the VL displayed similar (P>0.05) mixed protein FSR. Feasibility of processing isolated human muscle fibers for analysis of myofibrillar protein [5,5,5-(2)H3]leucine enrichment was also confirmed in non-fiber-typed pooled fibers from the VL. These methods can be applied to the study of fiber type-specific responses in human skeletal muscle. The need for this level of investigation is underscored by the different contributions of each fiber type to whole muscle function and the numerous distinct adaptive functional and metabolic changes in MHC I and MHC II fibers originating from the same muscle.

Exercise‐induced histone modifications in human skeletal muscle

Skeletal muscle adaptations to exercise confer many of the health benefits of physical activity and occur partly through alterations in skeletal muscle gene expression. The exact mechanisms mediating altered skeletal muscle gene expression in response to exercise are unknown. However, in recent years, chromatin remodelling through epigenetic histone modifications has emerged as a key regulatory mechanism controlling gene expression in general. The purpose of this study was to examine the effect of exercise on global histone modifications that mediate chromatin remodelling and transcriptional activation in human skeletal muscle in response to exercise. In addition, we sought to examine the signalling mechanisms regulating these processes. Following 60 min of cycling, global histone 3 acetylation at lysine 9 and 14, a modification associated with transcriptional initiation, was unchanged from basal levels, but was increased at lysine 36, a site associated with transcriptional elongation. We examined the regulation of the class IIa histone deacetylases (HDACs), which are enzymes that suppress histone acetylation and have been implicated in the adaptations to exercise. While we found no evidence of proteasomal degradation of the class IIa HDACs, we found that HDAC4 and 5 were exported from the nucleus during exercise, thereby removing their transcriptional repressive function. We also observed activation of the AMP-activated protein kinase (AMPK) and the calcium-calmodulin-dependent protein kinase II (CaMKII) in response to exercise, which are two kinases that induce phosphorylation-dependent class IIa HDAC nuclear export. These data delineate a signalling pathway that might mediate skeletal muscle adaptations in response to exercise.

The response of human skeletal muscle tissue to hypoxia.

Hypoxia refers to environmental or clinical settings that potentially threaten tissue oxygen homeostasis. One unique aspect of skeletal muscle is that, in addition to hypoxia, oxygen balance in this tissue may be further compromised when exercise is superimposed on hypoxia. This review focuses on the cellular and molecular responses of human skeletal muscle to acute and chronic hypoxia, with emphasis on physical exercise and training. Based on published work, it is suggested that hypoxia does not appear to promote angiogenesis or to greatly alter oxidative enzymes in skeletal muscle at rest. Although the HIF-1 pathway in skeletal muscle is still poorly documented, emerging evidence suggests that muscle HIF-1 signaling is only activated to a minor degree by hypoxia. On the other hand, combining hypoxia with exercise appears to improve some aspects of muscle O(2) transport and/or metabolism.

Fibroblast Growth Factor-21 Is Induced in Human Skeletal Muscles by Hyperinsulinemia

OBJECTIVEFibroblast growth factor-21 (FGF-21) is a potent metabolic regulator, which in animal models has been shown to improve glucose metabolism and insulin sensitivity. Recently, FGF-21 was shown to be expressed and secreted from murine muscle cells in response to insulin stimulation.RESEARCH DESIGN AND METHODSWe studied muscular FGF-21 expression and plasma FGF-21 after acute insulin stimulation in young healthy men during a hyperinsulinemic- euglycemic clamp. Furthermore, we investigated systemic levels and muscle FGF-21 expression in humans with or without insulin resistance and chronic elevated insulin.RESULTSFGF-21 was barely detectable in young healthy men before insulin infusion. After 3 or 4 h of insulin infusion during a hyperinsulinemic-euglycemic clamp, muscular FGF-21 expression increased significantly. Plasma FGF-21 followed the same pattern. In individuals with chronic elevated insulin, muscular FGF-21 expression was associated with hyperinsulinemia in men but not in women. In plasma, hyperinsulinemia and fasting glucose were positively associated with plasma FGF-21 while plasma FGF-21 correlated negatively with HDL cholesterol. No associations between muscle and plasma FGF-21 were found in the individuals with chronic hyperinsulinemia.CONCLUSIONSFGF-21 is expressed in human skeletal muscle in response to insulin stimulation, suggesting that FGF-21 is an insulin-regulated myokine. In support, we found an association between chronic hyperinsulinemia and levels of FGF-21.

Effect of consecutive repeated sprint and resistance exercise bouts on acute adaptive responses in human skeletal muscle

We examined acute molecular responses in skeletal muscle to repeated sprint and resistance exercise bouts. Six men [age, 24.7 +/- 6.3 yr; body mass, 81.6 +/- 7.3 kg; peak oxygen uptake, 47 +/- 9.9 mlxkg(-1)xmin(-1); one repetition maximum (1-RM) leg extension 92.2 +/- 12.5 kg; means +/- SD] were randomly assigned to trials consisting of either resistance exercise (8 x 5 leg extension, 80% 1-RM) followed by repeated sprints (10 x 6 s, 0.75 Nxm torquexkg(-1)) or vice-versa. Muscle biopsies from vastus lateralis were obtained at rest, 15 min after each exercise bout, and following 3-h recovery to determine early signaling and mRNA responses. There was divergent exercise order-dependent phosphorylation of p70 S6K (S6K). Specifically, initial resistance exercise increased S6K phosphorylation ( approximately 75% P < 0.05), but there was no effect when resistance exercise was undertaken after sprints. Exercise decreased IGF-I mRNA following 3-h recovery ( approximately 50%, P = 0.06) independent of order, while muscle RING finger mRNA was elevated with a moderate exercise order effect (P < 0.01). When resistance exercise was followed by repeated sprints PGC-1alpha mRNA was increased (REX1-SPR2; P = 0.02) with a modest distinction between exercise orders. Repeated sprints may promote acute interference on resistance exercise responses by attenuating translation initiation signaling and exacerbating ubiquitin ligase expression. Indeed, repeated sprints appear to generate the overriding acute exercise-induced response when undertaking concurrent repeated sprint and resistance exercise. Accordingly, we suggest that sprint-activities are isolated from resistance training and that adequate recovery time is considered within periodized training plans that incorporate these divergent exercise modes.

Potential role of TBC1D4 in enhanced post-exercise insulin action in human skeletal muscle

Aims/hypothesisTBC1 domain family, member 4 (TBC1D4; also known as AS160) is a cellular signalling intermediate to glucose transport regulated by insulin-dependent and -independent mechanisms. Skeletal muscle insulin sensitivity is increased after acute exercise by an unknown mechanism that does not involve modulation at proximal insulin signalling intermediates. We hypothesised that signalling through TBC1D4 is involved in this effect of exercise as it is a common signalling element for insulin and exercise.MethodsInsulin-regulated glucose metabolism was evaluated in 12 healthy moderately trained young men 4 h after one-legged exercise at basal and during a euglycaemic–hyperinsulinaemic clamp. Vastus lateralis biopsies were taken before and immediately after the clamp.ResultsInsulin stimulation increased glucose uptake in both legs, with greater effects (~80%, p < 0.01) in the previously exercised leg. TBC1D4 phosphorylation, assessed using the phospho–AKT (protein kinase B)substrate antibody and phospho- and site-specific antibodies targeting six phosphorylation sites on TBC1D4, increased at similar degrees to insulin stimulation in the previously exercised and rested legs (p < 0.01). However, TBC1D4 phosphorylation on Ser-318, Ser-341, Ser-588 and Ser-751 was higher in the previously exercised leg, both in the absence and in the presence of insulin (p < 0.01; Ser-588, p = 0.09; observed power = 0.39). 14–3–3 binding capacity for TBC1D4 increased equally (p < 0.01) in both legs during insulin stimulation.Conclusion/interpretationWe provide evidence for site-specific phosphorylation of TBC1D4 in human skeletal muscle in response to physiological hyperinsulinaemia. The data support the idea that TBC1D4 is a nexus for insulin- and exercise-responsive signals that may mediate increased insulin action after exercise.

Kinetics of GLUT4 Trafficking in Rat and Human Skeletal Muscle

OBJECTIVEIn skeletal muscle, insulin stimulates glucose transport activity three- to fourfold, and a large part of this stimulation is associated with a net translocation of GLUT4 from an intracellular compartment to the cell surface. We examined the extent to which insulin or the AMP-activated protein kinase activator AICAR can lead to a stimulation of the exocytosis limb of the GLUT4 translocation pathway and thereby account for the net increase in glucose transport activity.RESEARCH DESIGN AND METHODSUsing a biotinylated photoaffinity label, we tagged endogenous GLUT4 and studied the kinetics of exocytosis of the tagged protein in rat and human skeletal muscle in response to insulin or AICAR. Isolated epitrochlearis muscles were obtained from male Wistar rats. Vastus lateralis skeletal muscle strips were prepared from open muscle biopsies obtained from six healthy men (age 39 ± 11 years and BMI 25.8 ± 0.8 kg/m2).RESULTSIn rat epitrochlearis muscle, insulin exposure leads to a sixfold stimulation of the GLUT4 exocytosis rate (with basal and insulin-stimulated rate constants of 0.010 and 0.067 min−1, respectively). In human vastus lateralis muscle, insulin stimulates GLUT4 translocation by a similar sixfold increase in the exocytosis rate constant (with basal and insulin-stimulated rate constants of 0.011 and 0.075 min−1, respectively). In contrast, AICAR treatment does not markedly increase exocytosis in either rat or human muscle.CONCLUSIONSInsulin stimulation of the GLUT4 exocytosis rate constant is sufficient to account for most of the observed increase in glucose transport activity in rat and human muscle.

Consecutive bouts of diverse contractile activity alter acute responses in human skeletal muscle

We examined acute molecular responses in skeletal muscle to divergent exercise stimuli by combining consecutive bouts of resistance and endurance exercise. Eight men [22.9 +/- 6.3 yr, body mass of 73.2 +/- 4.5 kg, peak O(2) uptake (Vo(2peak)) of 54.0 +/- 5.7 ml.kg(-1) x min(-1)] were randomly assigned to complete trials consisting of either resistance exercise (8 x 5 leg extension, 80% 1 repetition maximum) followed by a bout of endurance exercise (30 min cycling, 70% Vo(2peak)) or vice versa. Muscle biopsies were obtained from the vastus lateralis at rest, 15 min after each exercise bout, and after 3 h of passive recovery to determine early signaling and mRNA responses. Phosphorylation of Akt and Akt1(Ser473) were elevated 15 min after resistance exercise compared with cycling, with the greatest increase observed when resistance exercise followed cycling ( approximately 55%; P < 0.01). TSC2-mTOR-S6 kinase phosphorylation 15 min after each bout of exercise was similar regardless of the exercise mode. The cumulative effect of combined exercise resulted in disparate mRNA responses. IGF-I mRNA content was reduced when cycling preceded resistance exercise (-42%), whereas muscle ring finger mRNA was elevated when cycling was undertaken after resistance exercise ( approximately 52%; P < 0.05). The hexokinase II mRNA level was higher after resistance cycling ( approximately 45%; P < 0.05) than after cycling-resistance exercise, whereas modest increases in peroxisome proliferator-activated receptor gamma coactivator-1alpha mRNA did not reveal an order effect. We conclude that acute responses to diverse bouts of contractile activity are modified by the exercise order. Moreover, undertaking divergent exercise in close proximity influences the acute molecular profile and likely exacerbates acute "interference."

Gene expression profiling of human skeletal muscle in response to stabilized weight loss

BackgroundDiet-induced weight reduction promotes a decrease in resting energy expenditure that could partly explain the difficulty in maintaining reduced body mass. Whether this reduction remains after stabilized weight loss is still controversial, and the molecular mechanisms are unknown. ObjectiveThe objective was to investigate the effect of a stabilized 10% weight loss on body composition, metabolic profile, and skeletal muscle gene expression profiling. DesignObese women were assigned to a 4-wk very-low-calorie diet, a 3–6-wk low-calorie diet, and a 4-wk weight-maintenance program to achieve a 10% weight loss. Resting energy expenditure, body composition, plasma variables, and skeletal muscle transcriptome were compared before weight loss and during stabilized weight reduction. ResultsEnergy restriction caused an 11% weight loss. Stabilization to the new weight was accompanied by an 11% decrease in the resting metabolic rate normalized to the body cellular mass. A large number of genes were regulated with a narrow range of regulation. The main regulated genes were slow/oxidative fiber markers, which were overexpressed, and the gene encoding the glucose metabolism inhibitor PDK4, which tended to be down-regulated. The knowledge-based approach gene set enrichment analysis showed that a set of genes related to long-term calorie restriction was up-regulated, whereas sets of genes related to insulin, interleukin 6, and ubiquitin-mediated proteolysis were down regulated. ConclusionsWeight loss–induced decreases in resting metabolic rate persist after weight stabilization. Changes in skeletal muscle gene expression indicate a shift toward oxidative metabolism.

Phosphorylation of the JAK2-STAT5 Pathway in Response to Acute Aerobic Exercise

Growth hormone (GH) is a powerful stimulator of the Janus kinase 2 (JAK2)-signal transducer and activator of transcription 5 (STAT5) pathway. Acute exercise is a known stimulus for GH secretion. The purpose of this study was to determine the phosphorylation of the JAK2-STAT5 pathway in human skeletal muscle in response to acute aerobic exercise. Eleven young (22.5 +/- 0.6, mean +/- SE), healthy, aerobically trained males performed 30 min of cycling at 70% V O2max. Blood samples were collected at 10- to 15-min intervals and analyzed for human GH, immunofunctional (IF) GH, GH binding protein, and insulin-like growth factor I (IGF-I). Muscle biopsies were taken from the vastus lateralis before exercise, immediately after exercise, as well as, 30 and 60 min postexercise. Muscle samples were analyzed for changes in JAK2 and STAT5 tyrosine phosphorylation, as well as changes in JAK2 and STAT5 protein content. Multivariate ANOVA with post hoc comparisons demonstrated that GH and IF GH were significantly elevated immediately after exercise compared with preexercise (P < 0.001). Exercise significantly increased the phosphorylation of JAK2 immediately after exercise (P = 0.004). A trend toward increasing levels of STAT5 phosphorylation was observed immediately after exercise (P = 0.08) and was significantly elevated 30 min after exercise (P = 0.002), compared with preexercise levels. Muscle JAK2 and STAT5 protein content did not change. The results demonstrate that the JAK2-STAT5 pathway is activated in response to acute aerobic exercise in human skeletal muscle and suggests that the exercise-induced release of GH may play a role in the activation of this pathway.

Rebuttal from Prof. Le Gallais

Rebuttal from Prof. Le Gallais

Human Skeletal Muscle Responses to Prolonged Space Flight: Whole Muscle Size and Strength

Magnetic resonance imaging (MRI) and isokinetic dynamometry was used to assess lower leg skeletal muscle characteristics from crewmembers (n=9; 45±2 y) that averaged 181-d aboard the International Space Station (ISS). While on the ISS, crewmembers had access to a running treadmill, cycle ergometer and resistance exercise device. The exercise regimen varied among the individuals with groups identified as high treadmill users (HT; n=4) and low treadmill users (LT; n=5). On average, all crewmembers lost 13±2% (p<0.05) of their lower leg muscle mass. HT lost less (p<0.05) muscle mass (8±2%) compared to LT (15±2%). Maximal isometric strength was reduced similarly for all crewmembers (14±2%; p<0.05). Isokinetic strength at six speeds was reduced 20–29% (p<0.05) with no differences between HT and LT. These data show that lower leg muscle volume and strength are significantly reduced after 6 months in space. These data also indicate that the level of treadmill use in space may aid in preserving lower limb muscle mass. Future long duration space missions should consider modifications to the current ISS exercise prescription and/or hardware that would preserve human skeletal muscle mass and function. Supported by NASA grant NCC9-116.

Human skeletal muscle responses to prolonged spaceflight: functional capacity of single slow and fast fibers

Soleus (Sol) and gastrocnemius (Gast) biopsies were obtained preflight and on landing day from 9 crewmembers (45±2 y), averaging 181d aboard the Space Station. Subjects self-distributed into those performing high (>200 min/wk) treadmill exercise (HT) and low treadmill exercise (<100 min/wk) (LT). For 540 preflight Sol type I fibers, the mean values ± SE were diameter 98 ± 1 (μm), peak force (0.86 ± 0.01 mN), maximal velocity (0.86 ± 0.02 V0 in fiber lengths (FL)/s), and peak power (14.01 ± 0.47 μN·FL·s−1). Following flight, for 458 fibers these parameters declined by 19, 35, 19, and 47 %, respectively. The decline in peak force was due to fiber atrophy as mean relative force (kN/m2) pre (115 ± 1) vs post (114 ± 1) was unaltered. Peak power fell from reduced velocity (19%) and force (33%). Sol type II and Gast type I fibers showed less dramatic declines in size, force and power. HT exercise partially protected type I fibers. The declines in the HT group were fiber diameter (9%), force (23%), and power (34%) compared to 26, 43, and 52 % for these variables in the LT group. The 19% decline in V0 was the same in both HT and LT crewmembers and contrasts to a 30% increase in V0 observed following 17 d spaceflight. These data indicate that while the level of treadmill use in space may aid in preserving lower limb muscle mass, modifications to the current exercise prescription are needed to better protect fiber power. Supported by NASA Grant NCC9-116.

Human skeletal muscle responses to prolonged spaceflight: morphological and fiber type changes

Human skeletal muscle responses to prolonged spaceflight: morphological and fiber type changes

Human skeletal muscle responses to prolonged spaceflight: enzyme and substrate adaptations

Soleus and gastrocnemius biopsies from 5 astronauts and 5 cosmonauts were obtained preflight and on landing day after 6 months on the International Space Station. Subjects were segregated into those performing more than 200 (HT) or less than 100 (LT) min/wk of treadmill exercise. Post-flight, the HT group retained histochemical cytochrome oxidase activity and oil red O-positive lipids in soleus type I fibers. The LT group had type I fibers with less cytochrome oxidase activity and larger reductions in lipids. PAS staining showed increased glycogen content in type I fibers, more pronounced in LT. Citrate synthase (CS), carnitine palmitoyl transferase 1b and phosphofructokinase (PFK) were unchanged immunohistochemically. Single fiber biochemistry revealed no changes in PFK activity. CS activity (mole·kg−1dry fiber·hr−1) was unchanged in the LT group but increased significantly in soleus type I (4.14 ± 0.34 to 6.84 ± 0.23) and gastrocnemius type II (3.26 ± 0.17 to 4.10 ± 0.24) fibers of the HT group. In gastrocnemius type II fibers, lactate dehydrogenase decreased (43.85 ± 2.48 to 33.06 ± 1.87) in HT and increased (22.84 ± 0.90 to 31.00 ± 1.34) in LT. Prolonged spaceflight depleted type I fiber lipids and increased glycogen with greater effect in LT than the HT groups. While metabolic enzymes were relatively unaltered, oxidative marker enzymes were better protected by higher exercise. Supported by NASA Grant NCC9-116.

Time course and differential responses of the major heat shock protein families in human skeletal muscle following acute nondamaging treadmill exercise

The exercise-induced expression of heat shock proteins (HSPs) in rodent models is relatively well defined. In contrast, comparable data from human studies are limited and the exercise-induced stress response of human skeletal muscle is far from understood. This study has characterized the time course and magnitude of the HSP response in the skeletal muscles of a healthy active, but untrained, young male population following a running exercise protocol. Eight subjects performed 45 min of treadmill running at a speed corresponding to their lactate threshold (11.7 +/- 0.5 km/h; 69.8 +/- 4.8% maximum O2 uptake). Muscle biopsies were obtained from the vastus lateralis muscle immediately before and at 24 h, 48 h, 72 h, and 7 days postexercise. Exercise induced a significant (P < 0.05) but variable increase in HSP70, heat shock cognate (HSC) 70, and HSP60 expression with peak increases (typically occurring at 48 h postexercise) to 210, 170, and 139% of preexercise levels, respectively. In contrast, exercise did not induce a significant increase in either HSP27, alphaB-crystallin, SOD 2 (MnSOD) protein content, or the activity of SOD and catalase. When examining baseline protein levels, HSC70, HSP27, and alphaB-crystallin appeared consistently expressed between subjects, whereas HSP70 and MnSOD displayed marked individual variation of up to 3- and 1.5-fold, respectively. These data are the first to define the time course and extent of HSP production in human skeletal muscle following a moderately demanding and nondamaging running exercise protocol. Data demonstrate a differential effect of aerobic exercise on specific HSPs.

Growth Hormone Signaling in Response to Acute Exercise

Growth hormone (GH) is a powerful stimulator of the Janus Kinase-2/Signal Transducer and Activator of Transcription 5 (JAK2/STAT5) pathway in animal and cellular research. Acute exercise is a known stimulus for GH secretion. The purpose of this study was to determine the phosphorylation of the JAK2/STAT5 pathway in human skeletal muscle in response to acute aerobic exercise. Twelve young (22.1 ± 0.7, Mean ± SE), healthy, aerobically trained males performed 30 min of cycling at 70% VO2max. Blood samples were collected at 10 to 15 min intervals during rest, exercise (E) and recovery and analyzed for human GH and immunofunctional (IF) GH. Muscle biopsies from the vastus lateralis were taken pre-E, post-E, 30 min and 60 min into recovery. Muscle samples were analyzed for tyrosine phosphorylation changes in JAK2 and STAT5, as well as, changes in JAK2 and STAT5 protein content. Multivariate ANOVA with post hoc comparisons demonstrated that GH and IF GH were significantly elevated immediately after E compared to pre-E (P<0.003). Exercise significantly increased the phosphorylation of JAK2 immediately after E (P=0.001) and an increasing trend in the phosphorylation of STAT5 after E was observed (P=0.057). Muscle JAK2, and STAT5 protein content did not change. Peak IF GH was positively related to the fold-change in STAT5 phosphorylation (r=0.66, P=0.037). Results demonstrate that the JAK2/STAT5 pathway is activated in response to acute aerobic exercise in human skeletal muscle and suggests that the exercise-induced release of GH may play a pivotal role in the activation of this pathway. Funding: Kate R Barrett and Susan Stout awards.