Time course evaluation of protein synthesis and glucose uptake after acute resistance exercise in rats

Network model of skeletal muscle cell signalling predicts differential responses to endurance and resistance exercise training

Exercise-induced muscle adaptations vary based on exercise modality and intensity. We constructed a signalling network model from 87 published studies of human or rodent skeletal muscle cell responses to endurance or resistance exercise in vivo or simulated exercise in vitro. The network comprises 259 signalling interactions between 120 nodes, representing eight membrane receptors and eight canonical signalling pathways regulating 14 transcriptional regulators, 28 target genes and 12 exercise-induced phenotypes. Using this network, we formulated a logic-based ordinary differential equation model predicting time-dependent molecular and phenotypic alterations following acute endurance and resistance exercises. Compared with nine independent studies, the model accurately predicted 18/21 (85%) acute responses to resistance exercise and 12/16 (75%) acute responses to endurance exercise. Detailed sensitivity analysis of differential phenotypic responses to resistance and endurance training showed that, in the model, exercise regulates cell growth and protein synthesis primarily by signalling via mechanistic target of rapamycin, which is activated by Akt and inhibited in endurance exercise by AMP-activated protein kinase. Endurance exercise preferentially activates inflammation via reactive oxygen species and nuclear factor κB signalling. Furthermore, the expected preferential activation of mitochondrial biogenesis by endurance exercise was counterbalanced in the model by protein kinase C in response to resistance training. This model provides a new tool for investigating cross-talk between skeletal muscle signalling pathways activated by endurance and resistance exercise, and the mechanisms of interactions such as the interference effects of endurance training on resistance exercise outcomes.

A Receptor Story: Insulin Resistance Pathophysiology and Physiologic Insulin Resensitization's Role as a Treatment Modality

Physiologic insulin secretion consists of an oscillating pattern of secretion followed by distinct trough periods that stimulate ligand and receptor activation. Apart from the large postprandial bolus release of insulin, β cells also secrete small amounts of insulin every 4-8 min independent of a meal. Insulin resistance is associated with a disruption in the normal cyclical pattern of insulin secretion. In the case of type-2 diabetes, β-cell mass is reduced due to apoptosis and β cells secrete insulin asynchronously. When ligand/receptors are constantly exposed to insulin, a negative feedback loop down regulates insulin receptor availability to insulin, creating a relative hyperinsulinemia. The relative excess of insulin leads to insulin resistance (IR) due to decreased receptor availability. Over time, progressive insulin resistance compromises carbohydrate metabolism, and may progress to type-2 diabetes (T2D). In this review, we discuss insulin resistance pathophysiology and the use of dynamic exogenous insulin administration in a manner consistent with more normal insulin secretion periodicity to reverse insulin resistance. Administration of insulin in such a physiologic manner appears to improve insulin sensitivity, lower HgbA1c, and, in some instances, has been associated with the reversal of end-organ damage that leads to complications of diabetes. This review outlines the rationale for how the physiologic secretion of insulin orchestrates glucose metabolism, and how mimicking this secretion profile may serve to improve glycemic control, reduce cellular inflammation, and potentially improve outcomes in patients with diabetes.

MicroRNAs associated with signaling pathways and exercise adaptation in sarcopenia

Considering the expansion of human life-span over the past few decades; sarcopenia, a physiological consequence of aging process characterized with a diminution in mass and strength of skeletal muscle, has become more frequent. Thus, there is a growing need for expanding our knowledge on the molecular mechanisms of muscle atrophy in sarcopenia which are complex and involve many signaling pathways associated with protein degradation and synthesis. MicroRNAs (miRNAs) as evolutionary conserved small RNAs, could complementarily bind to their target mRNAs and post-transcriptionally inhibit their translation. Aberrant expression of miRNAs contributes to the development of sarcopenia by regulating the expression of critical genes involved in age-related skeletal muscle mass loss. Here we have a review on the signaling pathways along with the miRNAs controlling their components expression and subsequently we provide a brief overview on the effects of exercise on expression pattern of miRNAs in sarcopenia.

Acute aggressive behavior perturbates the oxidative status of a wild bird independently of testosterone and progesterone

Aerobically demanding activities like aggression can lead to an elevated oxidative metabolism affecting the concentration of pro-oxidant and antioxidant compounds and can result in an overall perturbation of the oxidative status. Aggression may also alter the oxidative status indirectly through an increase in testosterone and progesterone concentrations. Given that changes in the oxidative status could represent a physiological cost of aggression, we tested the hypothesis that acute conspecific aggression impairs the oxidative status and evaluated the role of testosterone and progesterone as potential mediators. To achieve this, we experimentally manipulated the aggressive behavior of wild female and male birds and measured the concentrations of pro-oxidants, enzymatic- and non-enzymatic antioxidants, testosterone and progesterone in blood. After 20 min of conspecific aggressive behavior, both sexes had lower concentrations of non-enzymatic antioxidants than control individuals. This effect was independent of testosterone and progesterone concentrations, and much stronger in females than in males. Further, only in females (but not in males) being more aggressive came at the expense of lower antioxidant concentration. We provide the first experimental evidence that acute aggressive behavior perturbates the oxidative state of a wild vertebrate independently of testosterone and progesterone, with potential ecological and evolutionary implications given the role of the redox system in shaping life-history traits.

Glucose promotes cell growth by suppressing branched-chain amino acid degradation

Glucose and branched-chain amino acids (BCAAs) are essential nutrients and key determinants of cell growth and stress responses. High BCAA level inhibits glucose metabolism but reciprocal regulation of BCAA metabolism by glucose has not been demonstrated. Here we show that glucose suppresses BCAA catabolism in cardiomyocytes to promote hypertrophic response. High glucose inhibits CREB stimulated KLF15 transcription resulting in downregulation of enzymes in the BCAA catabolism pathway. Accumulation of BCAA through the glucose-KLF15-BCAA degradation axis is required for the activation of mTOR signaling during the hypertrophic growth of cardiomyocytes. Restoration of KLF15 prevents cardiac hypertrophy in response to pressure overload in wildtype mice but not in mutant mice deficient of BCAA degradation gene. Thus, regulation of KLF15 transcription by glucose is critical for the glucose-BCAA circuit which controls a cascade of obligatory metabolic responses previously unrecognized for cell growth.

Effect of endurance training and branched-chain amino acids on the signaling for muscle protein synthesis in CKD model rats fed a low-protein diet

A low-protein diet (LPD) protects against the progression of renal injury in patients with chronic kidney disease (CKD). However, LPD may accelerate muscle wasting in these patients. Both exercise and branched-chain amino acids (BCAA) are known to increase muscle protein synthesis by activating the mammalian target of rapamycin (mTOR) pathway. The aim of this study was to investigate whether endurance exercise and BCAA play a role for increasing muscle protein synthesis in LPD-fed CKD (5/6 nephrectomized) rats. Both CKD and sham rats were pair-fed on LPD or LPD fortified with a BCAA diet (BD), and approximately one-half of the animals in each group was subjected to treadmill exercise (15 m/min, 1 h/day, 5 days/wk). After 7 wk, renal function was measured, and soleus muscles were collected to evaluate muscle protein synthesis. Renal function did not differ between LPD- and BD-fed CKD rats, and the treadmill exercise did not accelerate renal damage in either group. The treadmill exercise slightly increased the phosphorylation of p70s6 kinase, a marker of mTOR activity, in the soleus muscle of LPD-fed CKD rats compared with the sham group. Furthermore, BCAA supplementation of the LPD-fed, exercise-trained CKD rats restored the phosphorylation of p70s6 kinase to the same level observed in the sham group; however, the corresponding induced increase in muscle protein synthesis and muscle mass was marginal. These results indicate that the combination of treadmill exercise and BCAA stimulates cell signaling to promote muscle protein synthesis; however, the implications of this effect for muscle growth remain to be clarified.

Co-ingestion of carbohydrate and whey protein increases fasted rates of muscle protein synthesis immediately after resistance exercise in rats

The objective of the study was to investigate whether co-ingestion of carbohydrate and protein as compared with protein alone augments muscle protein synthesis (MPS) during early exercise recovery. Two months old rats performed 10 repetitions of ladder climbing with 75% of body weight attached to their tails. Placebo (PLA), whey protein (WP), or whey protein plus carbohydrate (CP) was then given to rats by gavage. An additional group of sedentary rats (SED) was used as controls. Blood samples were collected immediately and at either 1 or 2 h after exercise. The flexor hallucis longus muscle was excised at 1 or 2 h post exercise for analysis of MPS and related signaling proteins. MPS was significantly increased by CP compared with PLA (p<0.05), and approached significance compared with WP at 1 h post exercise (p = 0.08). CP yielded a greater phosphorylation of mTOR compared with SED and PLA at 1 h post exercise and SED and WP at 2 h post exercise. CP also increased phosphorylation of p70S6K compared with SED at 1 and 2 h post exercise. 4E-BP1 phosphorylation was inhibited by PLA at 1 h but elevated by WP and CP at 2 h post exercise relative to SED. The phosphorylation of AMPK was elevated by exercise at 1 h post exercise, and this elevated level was sustained only in the WP group at 2 h. The phosphorylation of Akt, GSK3, and eIF2Bε were unchanged by treatments. Plasma insulin was transiently increased by CP at 1 h post exercise. In conclusion, post-exercise CP supplementation increases MPS post exercise relative to PLA and possibly WP, which may have been mediated by greater activation of the mTOR signaling pathway.

Animal models of resistance exercise and their application to neuroscience research

BACKGROUND

Numerous studies have demonstrated that participation in regular resistance exercise (e.g., strength training) is associated with improvements in mental health, memory, and cognition. However, less is known about the neurobiological mechanisms mediating these effects. The goal of this mini-review is to describe and evaluate the available animal models of resistance exercise that may prove useful for examining CNS activity.

NEW METHOD

Various models have been developed to examine resistance exercise in laboratory animals.

COMPARISON WITH EXISTING METHODS

Resistance exercise models vary in how the resistance manipulation is applied, either through direct stimulation of the muscle (e.g., in situ models) or through behavior maintained by operant contingencies (e.g., whole organism models). Each model presents distinct advantages and disadvantages for examining central nervous system (CNS) activity, and consideration of these attributes is essential for the future investigation of underlying neurobiological substrates.

RESULTS

Potential neurobiological mechanisms mediating the effects of resistance exercise on pain, anxiety, memory, and drug use have been efficiently and effectively investigated using resistance exercise models that minimize stress and maximize the relative contribution of resistance over aerobic factors.

CONCLUSIONS

Whole organism resistance exercise models that (1) limit the use of potentially stressful stimuli and (2) minimize the contribution of aerobic factors will be critical for examining resistance exercise and CNS function.

Greater Amino Acid Intake Is Required to Maximize Whole-Body Protein Synthesis Immediately after Endurance Exercise Than at Rest in Endurance-Trained Rats, as Determined by an Indicator Amino Acid Oxidation Method

BACKGROUND

The indicator amino acid oxidation (IAAO) method has contributed to establishing protein and amino acid (AA) requirements by determining the optimal protein and AA intake that maximizes whole-body protein synthesis. However, it has not been used with endurance-trained subjects.

OBJECTIVE

This study aimed to determine the optimal AA intake immediately after endurance exercise and at rest in endurance-trained rats by using the IAAO method.

METHODS

Four-week-old male Fischer rats were divided into a sedentary (SED) group and a trained (TR) group, which underwent treadmill training 5 d/wk for 6 wk at 26 m/min for 60 min/d. On the metabolic trial day, half of the TR group was provided with test diets after daily treadmill running (TR-PostEx). The other half of the TR group (TR-Rest) and all of the SED group were provided with test diets while at rest. The test diets contained different amounts of AAs (3.3-37.3 g ⋅ kg(-1) ⋅ d(-1)). Phenylalanine in the test diet was replaced with L-[1-(13)C]phenylalanine. The phenylalanine oxidation rate (PheOx) was determined with (13)CO2 enrichment in breath, CO2 excretion rate, and enrichment of phenylalanine in blood during the feeding period. The optimal AA intake was determined with biphasic mixed linear regression crossover analysis for PheOx, which identified a breakpoint at the minimal PheOx in response to graded amounts of AA intake.

RESULTS

The optimal AA intake in the TR-PostEx group (26.8 g ⋅ kg(-1) ⋅ d(-1); 95% CI: 21.5, 32.1 g ⋅ kg(-1) ⋅ d(-1)) was significantly higher than in the SED (15.1 g ⋅ kg(-1) ⋅ d(-1); 95% CI: 11.1, 19.1 g ⋅ kg(-1) ⋅ d(-1)) and TR-Rest (13.3 g ⋅ kg(-1) ⋅ d(-1); 95% CI: 10.9, 15.7 g ⋅ kg(-1) ⋅ d(-1)) groups, which did not differ.

CONCLUSIONS

Greater AA intake is required to maximize whole-body protein synthesis immediately after endurance exercise than at rest, but not at rest in endurance-trained rats.

The effects of resistance exercise on cocaine self-administration, muscle hypertrophy, and BDNF expression in the nucleus accumbens

BACKGROUND

Exercise is associated with positive outcomes in drug abusing populations and reduces drug self-administration in laboratory animals. To date, most research has focused on aerobic exercise, and other types of exercise have not been examined. This study examined the effects of resistance exercise (strength training) on cocaine self-administration and BDNF expression, a marker of neuronal activation regulated by aerobic exercise.

METHODS

Female rats were assigned to either exercising or sedentary conditions. Exercising rats climbed a ladder wearing a weighted vest and trained six days/week. Training consisted of a three-set "pyramid" in which the number of repetitions and resistance varied across three sets: eight climbs carrying 70% body weight (BW), six climbs carrying 85% BW, and four climbs carrying 100% BW. Rats were implanted with intravenous catheters and cocaine self-administration was examined. Behavioral economic measures of demand intensity and demand elasticity were derived from the behavioral data. BDNF mRNA expression was measured via qRT-PCR in the nucleus accumbens following behavioral testing.

RESULTS

Exercising rats self-administered significantly less cocaine than sedentary rats. A behavioral economic analysis revealed that exercise increased demand elasticity for cocaine, reducing consumption at higher unit prices. Exercising rats had lower BDNF expression in the nucleus accumbens core than sedentary rats.

CONCLUSIONS

These data indicate that resistance exercise decreases cocaine self-administration and reduces BDNF expression in the nucleus accumbens after a history of cocaine exposure. Collectively, these findings suggest that strength training reduces the positive reinforcing effects of cocaine and may decrease cocaine use in human populations.

Cumulative Muscle Protein Synthesis and Protein Intake Requirements

Muscle protein synthesis (MPS) fluctuates widely over the course of a day and is influenced by many factors. The time course of MPS responses to exercise and the influence of training and nutrition can only be pieced together from several different investigations and methods, many of which create unnatural experimental conditions. Measurements of cumulative MPS, the sum synthesis over an extended period, using deuterium oxide have been shown to accurately reflect muscle responses and may allow investigations of the response to exercise, total protein intake requirements, and interaction with protein timing in free-living experimental conditions; these factors have yet to be carefully integrated. Such studies could include clinical and athletic populations to integrate nutritional and exercise recommendations and help guide their revisions to optimize the skeletal muscle function that is so important to overall health.

Herbal supplement Kamishimotsuto augments resistance exercise-induced mTORC1 signaling in rat skeletal muscle

OBJECTIVES

Kamishimotsuto (KST) is a supplement containing 13 different herbs including Phellodendron bark, Anemarrhena rhizome and ginseng that have been shown to activate mammalian target of rapamycin complex 1 (mTORC1) and thereby increase muscle protein synthesis in vitro. However, the combined effect of KST and resistance exercise on muscle protein anabolism has not been investigated in vivo. Therefore, the purpose of this study was to investigate the effect of KST supplementation, resistance exercise on (mTORC1) signaling and subsequent muscle protein synthesis.

METHODS

Male Sprague-Dawley rats were divided into two groups: one group received KST (500 mg/kg/d in water) and the other group received placebo (PLA) for 7 d. After 12 h of fasting, the right gastrocnemius muscle was isometrically exercised via percutaneous electrical stimulation. Muscle samples were analyzed for muscle protein synthesis (MPS) and by western blotting analysis to assess the phosphorylation of p70S6K (Thr389), rpS6 (Ser240/244), and Akt (Ser473 and Thr308).

RESULTS

KST supplementation for 7 d significantly increased basal p-Akt (Ser473) levels compared with PLA, phosphorylation of the signaling proteins and MPS at baseline were otherwise unaffected. p-p70S6K and p-rpS6 levels significantly increased 1 h and 3 h after exercise in the PLA group, and these elevations were augmented in the KST group (P < 0.05). Furthermore, MPS at 6 h after resistance exercise was greater in the KST group than in the PLA group (P < 0.05).

CONCLUSIONS

While resistance exercise alone was able to increase p70S6K and rpS6 phosphorylation, Kamishimotsuto supplementation further augmented resistance exercise-induced muscle protein synthesis through mTORC1 signaling.

Training transfer: scientific background and insights for practical application

Training transfer as an enduring, multilateral, and practically important problem encompasses a large body of research findings and experience, which characterize the process by which improving performance in certain exercises/tasks can affect the performance in alternative exercises or motor tasks. This problem is of paramount importance for the theory of training and for all aspects of its application in practice. Ultimately, training transfer determines how useful or useless each given exercise is for the targeted athletic performance. The methodological background of training transfer encompasses basic concepts related to transfer modality, i.e., positive, neutral, and negative; the generalization of training responses and their persistence over time; factors affecting training transfer such as personality, motivation, social environment, etc. Training transfer in sport is clearly differentiated with regard to the enhancement of motor skills and the development of motor abilities. The studies of bilateral skill transfer have shown cross-transfer effects following one-limb training associated with neural adaptations at cortical, subcortical, spinal, and segmental levels. Implementation of advanced sport technologies such as motor imagery, biofeedback, and exercising in artificial environments can facilitate and reinforce training transfer from appropriate motor tasks to targeted athletic performance. Training transfer of motor abilities has been studied with regard to contralateral effects following one limb training, cross-transfer induced by arm or leg training, the impact of strength/power training on the preparedness of endurance athletes, and the impact of endurance workloads on strength/power performance. The extensive research findings characterizing the interactions of these workloads have shown positive transfer, or its absence, depending on whether the combinations conform to sport-specific demands and physiological adaptations. Finally, cross-training as a form of concurrent exercising in different athletic disciplines has been examined in reference to the enhancement of general fitness, the preparation of recreational athletes, and the preparation of athletes for multi-sport activities such as triathlon, duathlon, etc.

Abnormal protein turnover and anabolic resistance to exercise in sarcopenic obesity

Obesity may impair protein synthesis rates and cause anabolic resistance to growth factors, hormones, and exercise, ultimately affecting skeletal muscle mass and function. To better understand muscle wasting and anabolic resistance with obesity, we assessed protein 24-h fractional synthesis rates (24-h FSRs) in selected hind-limb muscles of sedentary and resistance-exercised lean and obese Zucker rats. Despite atrophied hind-limb muscles (-28% vs. lean rats), 24-h FSRs of mixed proteins were significantly higher in quadriceps (+18%) and red or white gastrocnemius (+22 or +38%, respectively) of obese animals when compared to lean littermates. Basal synthesis rates of myofibrillar (+8%) and mitochondrial proteins (-1%) in quadriceps were not different between phenotypes, while manufacture of cytosolic proteins (+12%) was moderately elevated in obese cohorts. Western blot analyses revealed a robust activation of p70S6k (+178%) and a lower expression of the endogenous mTOR inhibitor DEPTOR (-28%) in obese rats, collectively suggesting that there is an obesity-induced increase in net protein turnover favoring degradation. Lastly, the protein synthetic response to exercise of mixed (-7%), myofibrillar (+6%), and cytosolic (+7%) quadriceps subfractions was blunted compared to the lean phenotype (+34, +40, and +17%, respectively), indicating a muscle- and subfraction-specific desensitization to the anabolic stimulus of exercise in obese animals.

Acute heat stress prior to downhill running may enhance skeletal muscle remodeling

Heat shock proteins (HSPs) are chaperones that are known to have important roles in facilitating protein synthesis, protein assembly and cellular protection. While HSPs are known to be induced by damaging exercise, little is known about how HSPs actually mediate skeletal muscle adaption to exercise. The purpose of this study was to determine the effects of a heat shock pretreatment and the ensuing increase in HSP expression on early remodeling and signaling (2 and 48 h) events of the soleus (Sol) muscle following a bout of downhill running. Male Wistar rats (10 weeks old) were randomly assigned to control, eccentric exercise (EE; downhill running) or heat shock + eccentric exercise (HS; 41°C for 20 min, 48 h prior to exercise) groups. Markers of muscle damage, muscle regeneration and intracellular signaling were assessed. The phosphorylation (p) of HSP25, Akt, p70s6k, ERK1/2 and JNK proteins was also performed. As expected, following exercise the EE group had increased creatine kinase (CK; 2 h) and mononuclear cell infiltration (48 h) compared to controls. The EE group had an increase in p-HSP25, but there was no change in HSP72 expression, total protein concentration, or neonatal MHC content. Additionally, the EE group had increased p-p70s6k, p-ERK1/2, and p-JNK (2 h) compared to controls; however no changes in p-Akt were seen. In contrast, the HS group had reduced CK (2 h) and mononuclear cell infiltration (48 h) compared to EE. Moreover, the HS group had increased HSP72 content (2 and 48 h), total protein concentration (48 h), neonatal MHC content (2 and 48 h), p-HSP25 and p-p70s6k (2 h). Lastly, the HS group had reduced p-Akt (48 h) and p-ERK1/2 (2 h). These data suggest that heat shock pretreatment and/or the ensuing HSP72 response may protect against muscle damage, and enhance increases in total protein and neonatal MHC content following exercise. These changes appear to be independent of Akt and MAPK signaling pathways.

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.

Acute high-fat feeding does not prevent the improvement in glucose tolerance after resistance exercise in lean individuals

Our first aim was to investigate whether the ingestion of a single high-fat meal impairs glucose tolerance. Our second aim was to investigate whether improvements in glucose tolerance that are seen after resistance exercise remain when exercise is performed after ingestion of a high-fat meal. Eight young males consumed either a high fat (HF) or an isocaloric control (CON) meal in the morning and underwent an oral glucose tolerance test (OGTT) 6 h later. On two other occasions, a single 1 h bout of resistance exercise was completed 2 h after consumption of each meal (HFE and CONE). There were no significant differences in plasma glucose and plasma insulin areas under the curve (AUC) or estimates of insulin sensitivity between the HF and CON trials (P > 0.05). The HFE and CONE trials elicited a ~20% lower plasma glucose AUC (P < 0.05) compared to their respective control trials. The HFE also elicited a ~25% lower plasma insulin AUC (P < 0.05) in comparison to the HF trial. The HFE trial also significantly improved estimates of insulin sensitivity in comparison to the HF condition (P < 0.05). In conclusion, this study demonstrates that consumption of a single HF meal does not impair glucose tolerance in the resting state in lean individuals and that an acute bout of resistance exercise remains effective in enhancing glucose tolerance following the ingestion of a single high-fat meal.

Cumulative responses of muscle protein synthesis are augmented with chronic resistance exercise training

AIM

The purpose of this study was to determine the anabolic response of a single bout of high intensity resistance exercise (RE) following 5 weeks of RE training.

METHODS

To complete these studies, Sprague-Dawley rats were assigned by body mass to RE, exercise control (EC), or sedentary cage control (CC) groups and studied over 36 h after 5 weeks of RE (squat-like) training. Cumulative (final 36 h) fractional rates of muscle protein synthesis (FSR) were determined by ²H₂O, and acute (16 h post-RE) rates of muscle protein synthesis (RPS) were determined by flooding with l-[2,3,4,5,6-³H]phenylalanine. Regulators of peptide-chain initiation, 4E-BP1, eIF4E and the association of the two were determined by Western blotting and immunoprecipitation respectively.

RESULTS

No differences were observed with acute measures of RPS obtained 16 h following the final exercise bout in the plantaris or soleus muscles (P > 0.05). Consistent with this observation, 4E-BP1 was similarly phosphorylated and bound to eIF4E among all groups. However, upon determination of the cumulative response, FSR was significantly increased in the plantaris of RE vs. EC and CC (0.929±0.094, 0.384±0.039, 0.300±0.022% h(-1) respectively; P<0.001), but not the soleus.

CONCLUSION

With the advantage of determining cumulative FSR, the present study demonstrates that anabolic responses to RE are still evident after chronic RE training, primarily in muscle composed of fast-twitch fibres.

Resistance training associated with the administration of anabolic-androgenic steroids improves insulin sensitivity in ovariectomized rats

The aim of the present study was to investigate the effects of anabolic-androgenic steroids and resistance training (RT) on insulin sensitivity in ovariectomized rats. Adult female Wistar rats were divided into ten experimental groups (n = 5 animals per group): (1) sedentary (Sed-Intact); (2) sedentary ovariectomized (Sed-Ovx); (3) sedentary nandrolone (Sed-Intact-ND); (4) sedentary ovariectomized plus nandrolone (Sed-Ovx-ND); (5) trained (TR-Intact); (6) trained nandrolone (TR-Intact-ND); (7) trained ovariectomized (TR-Ovx); (8) trained ovariectomized plus nandrolone; (9) trained sham; and (10) trained ovariectomized plus sham. Four sessions of RT were used, during which the animals climbed a 1.1 m vertical ladder with weights attached to their tails. The sessions were performed once every 3 days, with between four and nine climbs and with eight to twelve dynamic movements per climb. To test the sensitivity of insulin in the pancreas, glucose and insulin tolerance tests were performed. For insulin sensitivity, there was a statistically significant interaction for the TR-Ovx group, which presented higher sensitivity than the Sed-Intact, Sed-Ovx, and TR-Intact groups. Sed-Intact-ND and TR-Intact-ND groups exhibited higher values of insulin sensitivity than the Sed-Intact group. Except for the TR-Intact group, sensitivity was greater in trained groups than in the Sed-Intact group. There was higher insulin sensitivity in the TR-Intact-ND group than in the Sed-Intact and Sed-Intact-ND groups (P < 0.05). In conclusion, ovariectomy and short-term RT alone induced no change on insulin action. Administration of nandrolone decanoate improved insulin action, mainly when it was associated with RT.

Insulin resistance syndrome blunts the mitochondrial anabolic response following resistance exercise

Metabolic risk factors associated with insulin resistance syndrome may attenuate augmentations in skeletal muscle protein anabolism following contractile activity. The purpose of this study was to investigate whether or not the anabolic response, as defined by an increase in cumulative fractional protein synthesis rates (24-h FSR) following resistance exercise (RE), is blunted in skeletal muscle of a well-established rodent model of insulin resistance syndrome. Four-month-old lean (Fa/?) and obese (fa/fa) Zucker rats engaged in four lower body RE sessions over 8 days, with the last bout occurring 16 h prior to muscle harvest. A priming dose of deuterium oxide ((2)H(2)O) and (2)H(2)O-enriched drinking water were administered 24 h prior to euthanization for assessment of cumulative FSR. Fractional synthesis rates of mixed (-5%), mitochondrial (-1%), and cytosolic (+15%), but not myofibrillar, proteins (-16%, P = 0.012) were normal or elevated in gastrocnemius muscle of unexercised obese rats. No statistical differences were found in the anabolic response of cytosolic and myofibrillar subfractions between phenotypes, but obese rats were not able to augment 24-h FSR of mitochondria to the same extent as lean rats following RE (+14% vs. +28%, respectively). We conclude that the mature obese Zucker rat exhibits a mild, myofibrillar-specific suppression in basal FSR and a blunted mitochondrial response to contractile activity in mixed gastrocnemius muscle. These findings underscore the importance of assessing synthesis rates of specific myocellular subfractions to fully elucidate perturbations in basal protein turnover rates and differential adaptations to exercise stimuli in metabolic disease.

Impaired overload-induced hypertrophy in obese Zucker rat slow-twitch skeletal muscle

The effect of insulin resistance (IR) on the adaptation of skeletal muscle loading is not well understood. Here we examine whether the soleus muscles of the lean Zucker (LZ) and insulin-resistant obese Zucker (OZ) rat exhibit differences in their ability to undergo muscle hypertrophy following 8 wk of mechanical overload. Four-week-old male LZ ( n = 5) and OZ ( n = 5) rats underwent unilateral surgical ablation of the gastrocnemius muscle while the contralateral hindlimb was used as an internal control. Mechanical overload increased soleus muscle wet weight (LZ 57% and OZ 33%, respectively; P < 0 .05) and average type 1 fiber cross-sectional area (LZ 32% and OZ 5%, respectively; P < 0.05) in LZ and OZ rats, while the magnitude of these increases was greater in the LZ animals ( P < 0 .05). The reduced degree of muscle hypertrophy observed in the OZ animals was associated with decreases in the ability of the OZ soleus muscle to phosphorylate p70s6kThr 389 and mTOR, while phosphorylation of p70s6kThr 389 was increased in the LZ overloaded soleus by 83% ( P < 0 .05). The amount of Tuberin/TSC2 phosphorylation, an inhibitor of mTOR, was unchanged in the LZ soleus after overload while it was increased (68.3%, P < 0.05) in OZ animals. Conversely, AMPK phosphorylation was decreased in the LZ (−22.77%, P < 0 .05) but increased (57%, P < 0 .05) in the OZ soleus with overload. Taken together, these data suggest that IR or other related comorbidities may impair the ability of the soleus to activate mTOR signaling and undergo load-induced muscle hypertrophy.

Protective effects of exercise preconditioning on hindlimb unloading-induced atrophy of rat soleus muscle

AIM

A chronic decrease in the activation and loading levels of skeletal muscles as occurs with hindlimb unloading (HU) results in a number of detrimental changes. Several proteolytic pathways are involved with an increase in myofibrillar protein degradation associated with HU. Exercise can be used to counter this increase in proteolytic activity and, thus, may be able to protect against some of the detrimental changes associated with chronic decreased use. The purpose of the present study was to determine the potential of a single bout of preconditioning endurance exercise in attenuating the effects of 2 weeks of HU on the mass, phenotype and force-related properties of the soleus muscle in adult rats.

METHODS

Male Wistar rats were subjected to HU for 2 weeks. One half of the rats performed a single bout of treadmill exercise for 25 min immediately prior to the 2 weeks of HU.

RESULTS

Soleus mass, maximum tetanic tension, myofibrillar protein content, fatigue resistance and percentage of type I (slow) myosin heavy chain were decreased in HU rats. In addition, markers for the cathepsin, calpain, caspase and ATP-ubiquitin-proteasome proteolytic pathways were increased. The preconditioning endurance exercise bout attenuated all of the detrimental changes associated with HU, and increased HSP72 mRNA expression and protein levels.

CONCLUSION

These findings indicate that exercise preconditioning may be an effective countermeasure to the detrimental effects of chronic decreases in activation and loading levels on skeletal muscles and that an elevation in HSP72 may be one of the mechanisms associated with these responses.

Stimulation of muscle anabolism by resistance exercise and ingestion of leucine plus protein

Leucine is known to stimulate muscle protein synthesis and anabolism. However, evidence for the efficacy of additional leucine to enhance the response of muscle anabolism to resistance exercise and protein ingestion is unclear. Thus, we investigated the response of net muscle protein balance to ingestion of additional leucine with protein in association with resistance exercise. Two groups of untrained subjects performed an intense bout of leg resistance exercise following ingestion of 1 of 2 drinks: flavored water (PL); or 16.6 g of whey protein + 3.4 g of leucine (W+L). Arteriovenous amino acid balance across the leg was measured to assess the anabolic response of muscle in each group. Arterial amino acid concentrations increased in response to ingestion of W+L. Amino acid concentrations peaked between 60 and 120 min after ingestion, and then declined to baseline values. Valine concentration decreased to levels significantly lower than baseline. Net balance of leucine, threonine, and phenylalanine did not change following PL ingestion, but increased and remained elevated above baseline for 90–120 min following W+L ingestion. Leucine (138 ± 37 and –23 ± 23 mg), phenylalanine (58 ± 28 and –38 ± 14 mg), and threonine (138 ± 37 and –23 ± 23 mg) uptake was greater for W+L than for PL over the 5.5 h following drink ingestion. Our results indicate that the whey protein plus leucine in healthy young volunteers results in an anabolic response in muscle that is not greater than the previously reported response to whey protein alone.

Activation of the mammalian target of rapamycin complex 1 is both necessary and sufficient to stimulate eukaryotic initiation factor 2Bvarepsilon mRNA translation and protein synthesis

In a previous study we demonstrated a requirement for activation of mTORC1 in the stimulation of eIF2Bepsilon mRNA translation in skeletal muscle in response to resistance exercise. Although that study established the necessity of mTORC1 activation, the experimental model used did not lend itself readily to address the question of whether or not mTORC1 activation was sufficient to produce the response. Therefore, the present study was designed to address the sufficiency of mTORC1 activation, using cultures of Rat2 fibroblasts in which mTORC1 signaling was repressed by serum/leucine-depletion and stimulated by repletion of leucine and/or IGF-1. Repletion with leucine and IGF-1 caused a shift of eIF2Bepsilon mRNA into actively translating polysomes and a stimulation of new eIF2Bepsilon protein synthesis, but had no effect on mRNAs encoding the other four eIF2B subunits. Stimulation of eIF2Bepsilon translation was reversed by pre-treatment with the mTORC1 inhibitor rapamycin. Exogenous overexpression of FLAG-Rheb, a proximal activator of mTORC1, also caused a re-distribution of eIF2Bepsilon mRNA into polysomes and a stimulation of eIF2Bepsilon protein synthesis. The stimulation of eIF2Bepsilon mRNA translation occurred in the absence of any effect on eIF2Bepsilon mRNA abundance. RNAi-mediated knockdown of eIF2Bepsilon resulted in reduced cellular proliferation, a result that phenocopied the known cytostatic effect of mTORC1 repression. Overall the results demonstrate that activation of mTORC1 is both necessary and sufficient to stimulate eIF2Bepsilon mRNA translation and that this response may represent a novel mechanism through which mTORC1 can affect mRNA translation initiation, rates of protein synthesis, and cellular growth/proliferation.

Mechanical stimuli of skeletal muscle: implications on mTOR/p70s6k and protein synthesis

The skeletal muscle is a tissue with adaptive properties which are essential to the survival of many species. When mechanically stimulated it is liable to undergo remodeling, namely, changes in its mass/volume resulting mainly from myofibrillar protein accumulation. The mTOR pathway (mammalian target of rapamycin) via its effector p70s6k (ribosomal protein kinase S6) has been reported to be of importance to the control of skeletal muscle mass, particularly under mechanical stimulation. However, not all mechanical stimuli are capable of activating this pathway, and among those who are, there are differences in the activation magnitude. Likewise, not all skeletal muscle fibers respond to the same extent to mechanical stimulation. Such evidences suggest specific mechanical stimuli through appropriate cellular signaling to be responsible for the final physiological response, namely, the accumulation of myofibrillar protein. Lately, after the mTOR signaling pathway has been acknowledged as of importance for remodeling, the interest for the mechanical/chemical mediators capable of activating it has increased. Apart from the already known MGF (mechano growth factor), some other mediators such as phosphatidic acid (PA) have been identified. This review article comprises and discusses relevant information on the mechano-chemical transduction of the pathway mTOR, with special emphasis on the muscle protein synthesis.

Effect of acute exercise on glucose tolerance following post-exercise feeding

It is well documented that a single bout of endurance exercise (EE) can improve insulin sensitivity, whereas relatively little is known about the acute effects of resistance exercise (RE) in humans. The objective of this study is to investigate the insulin and glucose responses to an oral glucose tolerance test (OGTT) following a high intensity bout of either EE or RE followed by post-exercise carbohydrate-protein hydrolysate ingestion. Eighteen participants were divided into two groups: a group in which nine participants completed 1 h of EE (cycle ergometry at 75% W (max)) and a RE group in which nine participants completed a RE circuit (3 sets of 10 repetitions). Participants ingested 1.5 l of a carbohydrate (200 g)-protein hydrolysate (50 g) beverage within 1 h of exercise completion. An OGTT was performed 6 h post-exercise. On the control day the endurance and resistance groups performed the above protocol without the prior exercise (CEE or CRE). The control and exercise days were counterbalanced. RE reduced plasma glucose AUC (822 +/- 68 vs. 694 +/- 23 mmol l(-1).120 min; CRE vs. RE, respectively; P < 0.05) but EE did not lead to a change (784 +/- 40 vs. 835 +/- 59 mmol l(-1).120 min; CEE vs. EE, respectively). Plasma insulin AUC remained unchanged compared to the control in both the RE and EE groups. The results suggest that the benefit of RE on glucose tolerance following CHO intake remains for 6 h even when a carbohydrate-protein hydrolysate beverage was ingested within 1 h after exercise, while the well documented benefit of EE was not observed.

Lengthening contractions differentially affect p70s6k phosphorylation compared to isometric contractions in rat skeletal muscle

The purpose of this investigation was to determine if p70(s6k) phosphorylation is dependent on the mode of resistance exercise (e.g. isometric vs. lengthening). Two groups (n = 5 each) of Female Sprague Dawley rats, approximately 12 weeks old, were tested. Rats were anesthetized and indwelling electrodes used to stimulate the right hind limb muscles via the sciatic nerve. The tibialis anterior (TA) muscle of Group 1 rats were exposed to three sets of ten isometric resistance contractions while the TA of Group 2 rats were exposed to three sets of ten resistance contractions that involved lengthening. Contralateral TA muscles served as non-exercised controls. The TA muscle was harvested 6 h post exercise and then the rat was euthanized. Muscle samples were processed to compare p70(s6k) phosphorylation between groups. A single bout of TA contractions that involved muscle lengthening resulted in significantly (p < 0.05) higher levels of phospho-p70(s6k) six hours post exercise compared to controls and isometric contractions. The differences in total p70(s6k) six hours post exercise were not significantly different between groups. Results suggest that signal transduction pathways activated by isometric exercise may differ (i.e., a non-p70(s6k) activation pathway) from that activated by lengthening exercise.

Regulation of protein synthesis associated with skeletal muscle hypertrophy by insulin-, amino acid- and exercise-induced signalling

Although insulin, amino acids and exercise individually activate multiple signal transduction pathways in skeletal muscle, one pathway, the phosphatidylinositol 3-kinase (PI3K)–mammalian target of rapamycin (mTOR) signalling pathway, is a target of all three. Activation of the PI3K–mTOR signal transduction pathway results in both acute (i.e. occurring in minutes to hours) and long-term (i.e. occurring in hours to days) up-regulation of protein synthesis through modulation of multiple steps involved in mediating the initiation of mRNA translation and ribosome biogenesis respectively. In addition, changes in gene expression through altered patterns of mRNA translation promote cell growth, which in turn promotes muscle hypertrophy. The focus of the present discussion is to review current knowledge concerning the mechanism(s) through which insulin, amino acids and resistance exercise act to activate the PI3K–mTOR signal transduction pathway and thereby enhance the rate of protein synthesis in muscle.

How nutrition and exercise maintain the human musculoskeletal mass

In this article we review some of our recent work concerning the effects of nutrition and exercise on protein synthesis and signal transduction in human musculoskeletal tissues. A great deal of new information is being generated by the application of recently refined techniques for measuring protein turnover. The field remains one that is largely descriptive but increasingly we are beginning to discern mechanisms underlying lean tissue maintenance, growth and wasting especially as multidisciplinary tools are applied to its study. Several types of exercise and nutrition are potent stimuli for protein synthesis in skeletal muscle. By contrast, collagen in the extracellular matrix in muscle and tendon appears to be mechanically but not nutritionally sensitive. The rates of collagen turnover in a variety of tissues are sufficiently high to account for a sizeable proportion of whole body protein turnover. One of the most recent surprises is the high turnover rate of human bone collagen and its anabolic response to feeding. As our understanding of the normal physiology of these processes advances, we become better able to construct testable hypotheses concerning the effects of ageing and disease on the musculoskeletal mass. Current evidence suggests that one of the major problems with loss of muscle during ageing is an inability of the tissue to respond adequately to increased availability of nutrients.

Insulin-facilitated increase of muscle protein synthesis after resistance exercise involves a MAP kinase pathway

Recent studies have implicated the mTOR-signaling pathway as a primary component for muscle growth in mammals. The purpose of this investigation was to examine signaling pathways for muscle protein synthesis after resistance exercise. Sprague-Dawley rats (male, 6 mo old) were assigned to either resistance exercise or control groups. Resistance exercise was accomplished in operantly conditioned animals using a specially designed flywheel apparatus. Rats performed two sessions of resistance exercise, separated by 48 h, each consisting of 2 sets of 25 repetitions. Sixteen hours after the second session, animals were killed, and soleus muscles were examined for rates of protein synthesis with and without insulin and/or rapamycin (mTOR inhibitor) and/or PD-098059 (PD; MEK kinase inhibitor). Results of this study demonstrated that rates of synthesis were higher (P < 0.05) with insulin after exercise compared with without insulin, or to control muscles, regardless of insulin. Rapamycin lowered (P < 0.05) rates of synthesis in controls, with or without insulin, and after exercise without insulin. However, insulin was able to overcome the inhibition of rapamycin after exercise (P < 0.05). PD had no effect on protein synthesis in control rats, but the addition of PD to exercised muscle resulted in lower (P < 0.05) rates of synthesis, and this inhibition was not rescued by insulin. Western blot analyses demonstrated that the inhibitors used in the present study were selective and effective for preventing activation of specific signaling proteins. Together, these results suggest that the insulin-facilitated increase of muscle protein synthesis after resistance exercise requires multiple signaling pathways.

Impaired overload-induced muscle growth is associated with diminished translational signalling in aged rat fast-twitch skeletal muscle

Impaired overload-induced protein synthesis and growth in aged fast-twitch skeletal muscle may result from diminished responsiveness of signalling intermediates controlling protein translation. Yet, potential age-related signalling decrements have never been examined in direct parallel with impaired overload-induced muscle growth in any model. To this end, we used Western blotting to examine the contents and phosphorylation states of mammalian target of rapamycin (mTOR) and its downstream translational signalling intermediates, 70 kDa ribosomal protein S6 kinase (S6k), ribosomal protein S6 (rpS6), eukaryotic elongation factor 2 (eEF2), and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), in conjunction with impaired growth in 1 week overloaded fast-twitch plantaris muscles (via unilateral gastrocnemius ablation) of old (O; 30 months) versus young adult (YA; 8 months) male Fischer344 x Brown Norway rats. The significantly (P <or= 0.05) diminished growth (assessed by total muscle protein content) in overloaded O muscles (5.6 +/- 1.7 versus 19.3 +/- 2.9% in YA) was accompanied by significant impairments in the phosphorylation states of mTOR (Ser2448), S6k (impaired at the mTOR-specific Thr389 residue but not at Thr421/Ser424), rpS6 (Ser235/236) and 4E-BP1 (gel shift), as well as deficits in total eEF2 accretion. Moreover, in overloaded muscles across both age groups, phospho-S6k at Thr389 (but not at Thr421/Ser424), 4E-BP1 phosphorylation status, and total eEF2 accretion were all positively correlated with percentage muscle hypertrophy, and negatively correlated with the phosphorylation (Thr172) of 5'-AMP-activated protein kinase (AMPK; which inhibits translational signalling and protein synthesis in young muscle at rest). As previously published by ourselves, AMPK was hyperphosphorylated in O versus YA muscles used in the current investigation. The present results provide solid evidence that impaired overload-induced growth in aged fast-twitch muscle may partly result from multiple-level decrements in signalling pathway(s) controlling protein translation, and also provide an initial indication that AMPK hyperactivation with age may potentially lie upstream of these decrements.

Anabolic signaling and protein synthesis in human skeletal muscle after dynamic shortening or lengthening exercise

We hypothesized a differential activation of the anabolic signaling proteins protein kinase B (PKB) and p70 S6 kinase (p70(S6K)) and subsequent differential stimulation of human muscle protein synthesis (MPS) after dynamic shortening or lengthening exercise. Eight healthy men [25 +/- 5 yr, BMI 26 +/- 3 kg/m(-2) (means +/- SD)] were studied before and after 12 min of repeated stepping up to knee height, and down again, while carrying 25% of their body weight, i.e., shortening exercise with the "up" leg and lengthening exercise with contralateral "down" leg. Quadriceps biopsies were taken before and 3, 6, and 24 h after exercise. After exercise, over 2 h before the biopsies, the subjects ingested 500 ml of water containing 45 g of essential amino acids and 135 g of sucrose. Rates of muscle protein synthesis were determined via incorporation over time of [1-(13)C]leucine (<or=6 h after exercise) or [1-(13)C]valine (21-24 h after exercise) and phosphorylation of signaling proteins by Western analysis. PKB and p70(S6K) phosphorylation increased approximately 3-fold after 3 h and remained elevated at 6 and 24 h. After exercise, rates of myofibrillar and sarcoplasmic protein synthesis were unchanged over the period including exercise and 3 h of recovery but had increased significantly at 6 (approximately 3.0- and 2.4-fold, respectively) and 24 h (approximately 3.2- and 2.0-fold, respectively), independently of the mode of exercise. Short-term dynamic exercise in either shortening or lengthening mode increases MPS at least as much as resistance exercise and is associated with long-term activation of PKB and p70(S6K).

Molecular determinants of skeletal muscle mass: getting the “AKT” together

Skeletal muscle is the most abundant tissue in the human body and its normal physiology plays a fundamental role in health and disease. During many disease states, a dramatic loss of skeletal muscle mass (atrophy) is observed. In contrast, physical exercise is capable of producing significant increases in muscle mass (hypertrophy). Maintenance of skeletal muscle mass is often viewed as the net result of the balance between two separate processes, namely protein synthesis and protein degradation. However, these two biochemical processes are not occurring independent of each other but they rather appear to be finely coordinated by a web of intricate signaling networks. Such signaling networks are in charge of executing environmental and cellular cues that will ultimate determine whether muscle proteins are synthesized or degraded. In this review, recent findings are discussed demonstrating that the AKT1/FOXOs/Atrogin-1(MAFbx)/MuRF1 signaling network plays an important role in the progression of skeletal muscle atrophy. These novel findings highlight an important mechanism that coordinates the activation of the protein synthesis machinery with the activation of a genetic program responsible for the degradation of muscle proteins during skeletal muscle atrophy.

Selective activation of AMPK-PGC-1alpha or PKB-TSC2-mTOR signaling can explain specific adaptive responses to endurance or resistance training-like electrical muscle stimulation

Endurance training induces a partial fast-to-slow muscle phenotype transformation and mitochondrial biogenesis but no growth. In contrast, resistance training mainly stimulates muscle protein synthesis resulting in hypertrophy. The aim of this study was to identify signaling events that may mediate the specific adaptations to these types of exercise. Isolated rat muscles were electrically stimulated with either high frequency (HFS; 6x10 repetitions of 3 s-bursts at 100 Hz to mimic resistance training) or low frequency (LFS; 3 h at 10 Hz to mimic endurance training). HFS significantly increased myofibrillar and sarcoplasmic protein synthesis 3 h after stimulation 5.3- and 2.7-fold, respectively. LFS had no significant effect on protein synthesis 3 h after stimulation but increased UCP3 mRNA 11.7-fold, whereas HFS had no significant effect on UCP3 mRNA. Only LFS increased AMPK phosphorylation significantly at Thr172 by approximately 2-fold and increased PGC-1alpha protein to 1.3 times of control. LFS had no effect on PKB phosphorylation but reduced TSC2 phosphorylation at Thr1462 and deactivated translational regulators. In contrast, HFS acutely increased phosphorylation of PKB at Ser473 5.3-fold and the phosphorylation of TSC2, mTOR, GSK-3beta at PKB-sensitive sites. HFS also caused a prolonged activation of the translational regulators p70 S6k, 4E-BP1, eIF-2B, and eEF2. These data suggest that a specific signaling response to LFS is a specific activation of the AMPK-PGC-1alpha signaling pathway which may explain some endurance training adaptations. HFS selectively activates the PKB-TSC2-mTOR cascade causing a prolonged activation of translational regulators, which is consistent with increased protein synthesis and muscle growth. We term this behavior the "AMPK-PKB switch." We hypothesize that the AMPK-PKB switch is a mechanism that partially mediates specific adaptations to endurance and resistance training, respectively.

Regulation of mTOR by amino acids and resistance exercise in skeletal muscle

Resistance exercise disturbs skeletal muscle homeostasis leading to activation of catabolic and anabolic processes within the muscle cell. A current challenge of exercise biology is to describe the molecular mechanisms of regulation by which contractile activity stimulates net protein breakdown during exercise and net protein synthesis during recovery. Muscle growth is optimized by combining exercise and appropriate nutritional strategies, such as amino acid (AA) and carbohydrate ingestion. The effects are integrated at the level of one central regulatory protein, mTOR (mammalian target of rapamycin). mTOR is a complex protein integrating signals of the energetic status of the cell and environmental stimuli to control protein synthesis, protein breakdown and therefore cell growth. mTOR is known to be activated by insulin, and the mechanisms involved are well documented. The ways by which exercise and AA lead to mTOR activation remain partially unclear. Exercise and AA use different signalling pathways upstream of mTOR. Exercise seems to recruit partially the same pathway as insulin, whereas AA could act more directly on mTOR. During resistance exercise, the activity of mTOR could be acutely blunted by AMP-activated protein kinase (AMPK), thus inhibiting protein synthesis and enhancing AA availability for energy metabolism. During recovery, the inhibition of mTOR by AMPK is suppressed, and its activation is maximized by the presence of AA. There appears to be a requirement for a minimal concentration of plasma insulin to stimulate muscle protein synthesis in response to resistance exercise and AA ingestion.

Resistance exercise increases muscle protein synthesis and translation of eukaryotic initiation factor 2Bepsilon mRNA in a mammalian target of rapamycin-dependent manner

The contribution of mammalian target of rapamycin (mTOR) signaling to the resistance exercise-induced stimulation of skeletal muscle protein synthesis was assessed by administering rapamycin to Sprague-Dawley rats 2 h prior to a bout of resistance exercise. Animals were sacrificed 16 h postexercise, and gastrocnemius protein synthesis, mTOR signaling, and biomarkers of translation initiation were assessed. Exercise stimulated the rate of protein synthesis; however, this effect was prevented by pretreatment with rapamycin. The stimulation of protein synthesis was mediated by an increase in translation initiation, since exercise caused an increase in polysome aggregation that was abrogated by rapamycin administration. Taken together, the data suggest that the effect of rapamycin was not mediated by reduced phosphorylation of eukaryotic initiation factor 4E (eIF4E) binding protein 1 (BP1), because exercise did not cause a significant change in 4E-BP1(Thr-70) phosphorylation, 4E-BP1-eIF4E association, or eIF4F complex assembly concomitant with increased protein synthetic rates. Alternatively, there was a rapamycin-sensitive decrease in relative eIF2Bepsilon(Ser-535) phosphorylation that was explained by a significant increase in the expression of eIF2Bepsilon protein. The proportion of eIF2Bepsilon mRNA in polysomes was increased following exercise, an effect that was prevented by rapamycin treatment, suggesting that the increase in eIF2Bepsilon protein expression was mediated by an mTOR-dependent increase in translation of the mRNA encoding the protein. The increase in eIF2Bepsilon mRNA translation and protein abundance occurred independent of similar changes in other eIF2B subunits. These data suggest a novel link between mTOR signaling and eIF2Bepsilon mRNA translation that could contribute to the stimulation of protein synthesis following acute resistance exercise.

Insulin facilitation of muscle protein synthesis following resistance exercise in hindlimb-suspended rats is independent of a rapamycin-sensitive pathway

Hindlimb suspension (HS) results in rapid losses of muscle mass, which may in part be explained by attenuated rates of protein synthesis. Mammalian target of rapamycin (mTOR) regulates protein synthesis and has been implicated as a potential mediator of the muscle mass decrement with HS. This study examined the effect of resistance exercise, a muscle hypertrophy stimulant, on rates of protein synthesis after 4 days of HS in mature male Sprague-Dawley rats. Flywheel resistance exercise (2 sets x 25 repetitions) was conducted on days 2 and 4 of HS (HSRE). Sixteen hours after the last exercise bout, soleus muscles were assessed for in vitro rates of protein synthesis, with and without insulin (signaling agonist) and/or rapamycin (mTOR inhibitor). Results demonstrated that soleus mass was reduced (P < 0.05) with HS, but this loss of mass was not observed (P > 0.05) with HSRE. Muscle protein synthesis was diminished (P < 0.05) with HS, with or without insulin. HSRE also had reduced rates of synthesis without insulin; however, insulin administration yielded higher (P < 0.05) rates in HSRE compared with HS or control. Rapamycin diminished protein synthesis in all groups (P < 0.05), but insulin rescued synthesis rates in HS and HSRE to levels similar to insulin alone for each group, suggesting that alternate signaling pathways develop to increase protein synthesis with HS. These results demonstrate that the capacity for an augmented anabolic response to resistance exercise is maintained after 4 days of HS and is independent of a rapamycin-sensitive pathway.

Human soleus muscle protein synthesis following resistance exercise

AIM

It is generally believed the calf muscles in humans are relatively unresponsive to resistance training when compared with other muscles of the body. The purpose of this investigation was to determine the muscle protein synthesis response of the soleus muscle following a standard high intensity bout of resistance exercise.

METHODS

Eight recreationally active males (27 +/- 4 years) completed three unilateral calf muscle exercises: standing calf press/heel raise, bent-knee calf press/heel raise, and seated calf press/heel raise. Each exercise consisted of four sets of 15 repetitions (approximately 15 repetition maximum, RM, or approximately 70% 1RM). Fractional rate of muscle protein synthesis (FSR) was determined with a primed constant infusion of [2H5]phenylalanine coupled with muscle biopsies immediately and 3 h following the exercise in both the exercise and non-exercise (resting control) leg.

RESULTS

FSR was elevated (P < 0.05) in the exercise (0.069 +/- 0.010) vs. the control (0.051 +/- 0.012) leg. Muscle glycogen concentration was lower (P < 0.05) in the exercise compared with the control leg (Decrease from control; immediate post-exercise: 54 +/- 5; 3 h post-exercise: 36 +/- 4 mmol kg(-1) wet wt.). This relatively high amount of glycogen use is comparable with previous studies of resistance exercise of the thigh (i.e. vastus lateralis; approximately 41-49 mmol kg(-1) wet wt.). However, the exercise-induced increase in FSR that has been consistently reported for the vastus lateralis (approximately 0.045-0.060% h(-1)) is on average approximately 200% higher than reported here for the soleus (0.019 +/- 0.003% h(-1)).

CONCLUSIONS

These results suggest the relatively poor response of soleus muscle protein synthesis to an acute bout of resistance exercise may be the basis for the relative inability of the calf muscles to respond to resistance training programs.

Deletion of ribosomal S6 kinases does not attenuate pathological, physiological, or insulin-like growth factor 1 receptor-phosphoinositide 3-kinase-induced cardiac hypertrophy

Ribosomal S6 kinases (S6Ks) have been depicted as critical effectors downstream of growth factor pathways, which play an important role in the regulation of protein synthesis by phosphorylating the ribosomal protein, S6. The goal of this study was to determine whether S6Ks regulate heart size, are critical for the induction of cardiac hypertrophy in response to a pathological or physiological stimulus, and whether S6Ks are critical downstream effectors of the insulin-like growth factor 1 (IGF1)-phosphoinositide 3-kinase (PI3K) pathway. For this purpose, we generated and characterized cardiac-specific S6K1 and S6K2 transgenic mice and subjected S6K1(-/-), S6K2(-/-), and S6K1(-/-) S6K2(-/-) mice to a pathological stress (aortic banding) or a physiological stress (exercise training). To determine the genetic relationship between S6Ks and the IGF1-PI3K pathway, S6K transgenic and knockout mice were crossed with cardiac-specific transgenic mice overexpressing the IGF1 receptor (IGF1R) or PI3K mutants. Here we show that overexpression of S6K1 induced a modest degree of hypertrophy, whereas overexpression of S6K2 resulted in no obvious cardiac phenotype. Unexpectedly, deletion of S6K1 and S6K2 had no impact on the development of pathological, physiological, or IGF1R-PI3K-induced cardiac hypertrophy. These studies suggest that S6Ks alone are not essential for the development of cardiac hypertrophy.

Postprandial metabolism in resistance-trained versus sedentary males

INTRODUCTION

This investigation examined if postprandial metabolism differed between resistance-trained [(RT), N = 12] and sedentary [(SED), N = 12] males. A secondary objective was to determine whether different resistance-training programs [bodybuilding (BB), N = 8 and power/weight-lifting (PL), N = 8] resulted in disparate effects on postprandial energy metabolism.

METHODS

Moderate fat [(MF), 37% carbohydrate, 18% protein, and 45% fat] and high carbohydrate [(HC), 79% carbohydrate, 20% protein, and 1% fat] meals were randomly administered, and postprandial metabolism was measured for 240 min. Carbohydrate oxidation, fat oxidation, diet-induced thermogenesis (DIT), and glucose and insulin areas under the curve (AUC) were calculated.

RESULTS

Fat oxidization/lean body mass (LBM) was significantly greater in SED after the HC (RT, 0.27 +/- 0.02 g vs SED, 0.33 +/- 0.02 g, P = 0.017) and MF (RT, 0.34 +/- 0.02 g vs SED, 0.39 +/- 0.02 g, P = 0.036) meals. Carbohydrate oxidation/LBM was significantly greater in RT after the HC meal (RT, 0.87 +/- 0.03 g vs SED, 0.74 +/- 0.04 g, P = 0.017) only. DIT and DIT/LBM were significantly greater in RT compared with SED after the HC meal (DIT: RT, 351 +/- 21 kJ vs SED, 231 +/- 23 kJ, P = 0.001; DIT/LBM: RT, 5.25 +/- 0.028 kJ vs SED, 3.92 +/- 0.37 kJ, P = 0.009). The AUC for both glucose and insulin were significantly greater in SED compared with RT in response to the HC meal but not the MF meal. There were no differences in the BB and PL groups for any measured variables in response to either the HC or MF meals.

CONCLUSION

These data indicate that postprandial metabolism is different between resistance-trained and sedentary males but that no such differences exist with different resistance training styles.

Branched-chain amino acids increase p70S6k phosphorylation in human skeletal muscle after resistance exercise

The aim of the study was to investigate the effect of resistance exercise alone or in combination with oral intake of branched-chain amino acids (BCAA) on phosphorylation of the 70-kDa S6 protein kinase (p70(S6k)) and mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK1/2), and p38 MAPK in skeletal muscle. Seven male subjects performed one session of quadriceps muscle resistance training (4 x 10 repetitions at 80% of one repetition maximum) on two occasions. In a randomized order, double-blind, crossover test, subjects ingested a solution of BCAA or placebo during and after exercise. Ingestion of BCAA increased plasma concentrations of isoleucine, leucine, and valine during exercise and throughout recovery after exercise (2 h postexercise), whereas no change was noted after the placebo trial. Resistance exercise led to a robust increase in p70(S6k) phosphorylation at Ser(424) and/or Thr(421), which persisted 1 and 2 h after exercise. BCAA ingestion further enhanced p70(S6k) phosphorylation 3.5-fold during recovery. p70(S6k) phosphorylation at Thr(389) was unaltered directly after resistance exercise. However, during recovery, Thr(389) phosphorylation was profoundly increased, but only during the BCAA trial. Furthermore, phosphorylation of the ribosomal protein S6 was also increased in the recovery period only during the BCAA trial. Exercise led to a marked increase in ERK1/2 and p38 MAPK phosphorylation, which was completely suppressed upon recovery and unaltered by BCAA. In conclusion, BCAA, ingested during and after resistance exercise, mediate signal transduction through p70(S6k) in skeletal muscle.

Control of the Size of the Human Muscle Mass

This review is divided into two parts, the first dealing with the cell and molecular biology of muscle in terms of growth and wasting and the second being an account of current knowledge of physiological mechanisms involved in the alteration of size of the human muscle mass. Wherever possible, attempts have been made to interrelate the information in each part and to provide the most likely explanation for phenomena that are currently only partially understood. The review should be of interest to cell and molecular biologists who know little of human muscle physiology and to physicians, physiotherapists, and kinesiologists who may be familiar with the gross behavior of human muscle but wish to understand more about the underlying mechanisms of change.

Alterations in the expression of mRNAs and proteins that code for species relevant to eIF2B activity after an acute bout of resistance exercise

The focus of the study described herein was to examine the relative expression levels of mRNAs and proteins relevant to the regulation of translational initation, and hence protein synthesis, in the time course after an acute bout of resistance exercise in male Sprague-Dawley rats. Significant increases in the relative abundance of the mRNAs coding for the epsilon (33%) and gamma (26%) subunits of eukaryotic initiation factor (eIF) 2B were observed 48 h after the exercise bout. Furthermore, the mRNA coding for the delta subunit of eIF2B was also significantly increased, both 24 h (46%) and 48 h (44%) postexercise. There was a relative decrease in three eIF2Bϵ kinase mRNAs, namely sequences coding for glycogen synthase kinase 3β (49%), casein kinase I (48%), and casein kinase II (42%) 48 h into the recovery period. Additionally, there was a significant decrease in expression of the mRNAs coding for eIF2α (28% 24 h postexercise) and one of its regulatory kinases, double-stranded RNA-activated protein kinase (33% 48 h postexercise). Finally, an increase in eIF2B total protein (124%) was observed within 3 h postexercise. These results suggest that there may be rapid translational regulation of mRNAs coding for species relevant to translational initiation after an acute bout of resistance exercise. Furthermore, transcription of these mRNAs is altered further into the recovery period, and this might play a role in protein synthetic capacity on subsequent bouts of resistance exercise.

Immediate response of mammalian target of rapamycin (mTOR)-mediated signalling following acute resistance exercise in rat skeletal muscle

The purpose of the present investigation was to determine whether mammalian target of rapamycin (mTOR)-mediated signalling and some key regulatory proteins of translation initiation are altered in skeletal muscle during the immediate phase of recovery following acute resistance exercise. Rats were operantly conditioned to reach an illuminated bar located high on a Plexiglass cage, such that the animals completed concentric and eccentric contractions involving the hindlimb musculature. Gastrocnemius muscle was extracted immediately after acute exercise and 5, 10, 15, 30 and 60 min of recovery. Phosphorylation of protein kinase B (PKB) on Ser-473 peaked at 10 min of recovery (282% of control, P < 0.05) with no significant changes noted for mTOR phosphorylation on Ser-2448. Eukaryotic initiation factor (eIF) 4E-binding protein-1 (4E-BP1) and S6 kinase-1 (S6K1), both downstream effectors of mTOR, were altered during recovery as well. 4E-BP1 phosphorylation was significantly elevated at 10 min (292%, P < 0.01) of recovery. S6K1 phosphorylation on Thr-389 demonstrated a trend for peak activation at 10 min following exercise (336%, P = 0.06) with ribosomal protein S6 phosphorylation being maximally activated at 15 min of recovery (647%, P < 0.05). Components of the eIF4F complex were enhanced during recovery as eIF4E association with eIF4G peaked at 10 min (292%, P < 0.05). Events regulating the binding of initiator methionyl-tRNA to the 40S ribosomal subunit were assessed through eIF2B activity and eIF2 alpha phosphorylation on Ser-51. No differences were noted with either eIF2B or eIF2 alpha. Collectively, these results provide strong evidence that mTOR-mediating signalling is transiently upregulated during the immediate period following resistance exercise and this response may constitute the most proximal growth response of the cell.

Molecular basis of skeletal muscle plasticity--from gene to form and function

Skeletal muscle shows an enormous plasticity to adapt to stimuli such as contractile activity (endurance exercise, electrical stimulation, denervation), loading conditions (resistance training, microgravity), substrate supply (nutritional interventions) or environmental factors (hypoxia). The presented data show that adaptive structural events occur in both muscle fibres (myofibrils, mitochondria) and associated structures (motoneurons and capillaries). Functional adaptations appear to involve alterations in regulatory mechanisms (neuronal, endocrine and intracellular signalling), contractile properties and metabolic capacities. With the appropriate molecular techniques it has been demonstrated over the past 10 years that rapid changes in skeletal muscle mRNA expression occur with exercise in human and rodent species. Recently, gene expression profiling analysis has demonstrated that transcriptional adaptations in skeletal muscle due to changes in loading involve a broad range of genes and that mRNA changes often run parallel for genes in the same functional categories. These changes can be matched to the structural/functional adaptations known to occur with corresponding stimuli. Several signalling pathways involving cytoplasmic protein kinases and nuclear-encoded transcription factors are recognized as potential master regulators that transduce physiological stress into transcriptional adaptations of batteries of metabolic and contractile genes. Nuclear reprogramming is recognized as an important event in muscle plasticity and may be related to the adaptations in the myosin type, protein turnover, and the cytoplasma-to-myonucleus ratio. The accessibility of muscle tissue to biopsies in conjunction with the advent of high-throughput gene expression analysis technology points to skeletal muscle plasticity as a particularly useful paradigm for studying gene regulatory phenomena in humans.

Mitogen-activated protein kinase (MAPK) pathway activation: effects of age and acute exercise on human skeletal muscle

The purpose of this investigation was to examine the activation (phosphorylation) and total protein content of MAPK signalling cascade proteins (ERK 1/2, p90RSK, Mnk 1, eIF4E, p38 MAPK, JNK/SAPK, MKP 1) at rest and following exercise, in sedentary young and old men. Eight young (22 +/- 1 years; YM) and eight old (79 +/- 3 years; OM) men underwent a resting muscle biopsy of the vastus lateralis; they then performed a knee extensor resistance exercise session (29 contractions at approximately 70 % of max), followed by a post-exercise biopsy. Western immunoblot analysis demonstrated that the OM had higher resting phosphorylation of ERK 1/2, p90RSK, Mnk 1, p38 MAPK and JNK/SAPK proteins versus YM (P < 0.05). The resistance exercise bout caused an increase in phosphorylation of the ERK 1/2, p90RSK and Mnk 1 proteins (P < 0.05) in the YM. Conversely, the OM had a decrease in ERK 1/2, p90RSK, Mnk 1, p38 MAPK and JNK/SAPK phosphorylation (P < 0.05) after the exercise bout. Neither group showed a change in eIF4E phosphorylation. The total amount of protein expression of the MAPK signalling proteins was not different between the YM and OM, except that there was a higher (P < 0.05) MKP 1 protein content in the OM. This investigation is the first to provide evidence that MAPK proteins are differentially activated at rest and in response to a bout of resistance exercise in skeletal muscle of young and old men. These findings may have implications for other processes (e.g. transcription and translation) involved in skeletal muscle type and growth, when examining the changes occurring with ageing muscle before and after resistance exercise/training.

Invited Review: Role of insulin in translational control of protein synthesis in skeletal muscle by amino acids or exercise

Protein synthesis in skeletal muscle is modulated in response to a variety of stimuli. Two stimuli receiving a great deal of recent attention are increased amino acid availability and exercise. Both of these effectors stimulate protein synthesis in part through activation of translation initiation. However, the full response of translation initiation and protein synthesis to either effector is not observed in the absence of a minimal concentration of insulin. The combination of insulin and either increased amino acid availability or endurance exercise stimulates translation initiation and protein synthesis in part through activation of the ribosomal protein S6 protein kinase S6K1 as well as through enhanced association of eukaryotic initiation factor eIF4G with eIF4E, an event that promotes binding of mRNA to the ribosome. In contrast, insulin in combination with resistance exercise stimulates translation initiation and protein synthesis through enhanced activity of a guanine nucleotide exchange protein referred to as eIF2B. In both cases, the amount of insulin required for the effects is low, and a concentration of the hormone that approximates that observed in fasting animals is sufficient for maximal stimulation. This review summarizes the results of a number of recent studies that have helped to establish our present understanding of the interactions of insulin, amino acids, and exercise in the regulation of protein synthesis in skeletal muscle.