Protein synthesis versus energy state in contracting muscles of perfused rat hindlimb

Protein Biomarkers of Bovine Defective Meats at a Glance: Gel-Free Hybrid Quadrupole-Orbitrap Analysis for Rapid Screening

An understanding of biological mechanisms that could be involved in the stress response of animal cattle prior to slaughter is critical to create effective strategies aiming at the production of high-quality meat. The sarcoplasmic proteome of directly extracted samples from normal and high ultimate pH (pHu) meat groups was studied through a straightforward gel-free strategy supported by liquid chromatography hybrid quadrupole-Orbitrap high-resolution mass spectrometry (LC-HRMS) analysis. A stepped proteomic pipeline combining rapid biomarker hunting supported by qualitative protein Mascot scores followed by targeted label-free peptide quantification revealed 26 descriptors that characterized meat groups assayed. The functional study of the proposed biomarkers suggested their relevant role in metabolic, chaperone/stress-related, muscle contractility/fiber organization, and transport activities. The efficiency, flexibility, rapidity, and easiness of the methodology proposed can positively contribute to the creation of innovative proteomic alternatives addressing meat quality assessment.

Endurance exercise decreases protein synthesis and ER-mitochondria contacts in mouse skeletal muscle

Exercise is important to maintain skeletal muscle mass through stimulation of protein synthesis, which is a major ATP-consuming process for cells. However, muscle cells have to face high energy demand during contraction. The present study aimed to investigate protein synthesis regulation during aerobic exercise in mouse hindlimb muscles. Male C57Bl/6J mice ran at 12 m/min for 45 min or at 12 m/min for the first 25 min followed by a progressive increase in velocity up to 20 m/min for the last 20 min. Animals were injected intraperitoneally with 40 nmol/g of body weight of puromycin and euthanized by cervical dislocation immediately after exercise cessation. Analysis of gastrocnemius, plantaris, quadriceps, soleus, and tibialis anterior muscles revealed a decrease in protein translation assessed by puromycin incorporation, without significant differences among muscles or running intensities. The reduction of protein synthesis was associated with a marked inhibition of mammalian target of rapamycin complex 1 (mTORC1)-dependent phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1, a mechanism consistent with reduced translation initiation. A slight activation of AMP-activated protein kinase consecutive to the running session was measured but did not correlate with mTORC1 inhibition. More importantly, exercise resulted in a strong upregulation of regulated in development and DNA damage 1 (REDD1) protein and gene expressions, whereas transcriptional regulation of other recognized exercise-induced genes (IL-6, kruppel-like factor 15, and regulator of calcineurin 1) did not change. Consistently with the recently discovered role of REDD1 on mitochondria-associated membranes, we observed a decrease in mitochondria-endoplasmic reticulum interaction following exercise. Collectively, these data raise questions concerning the role of mitochondria-associated endoplasmic reticulum membrane disruption in the regulation of muscle proteostasis during exercise and, more generally, in cell adaptation to metabolic stress.NEW & NOTEWORTHY How muscles regulate protein synthesis to cope with the energy demand during contraction is poorly documented. Moreover, it is unknown whether protein translation is differentially affected among mouse hindlimb muscles under different physiological exercise modalities. We showed here that 45 min of running decreases puromycin incorporation similarly in 5 different mouse muscles. This decrease was associated with a strong increase in regulated in development and DNA damage 1 protein expression and a significant disruption of the mitochondria and sarcoplasmic reticulum interaction.

Divergent Anabolic Signalling responses of Murine Soleus and Tibialis Anterior Muscles to Chronic 2G Hypergravity

The purpose of the study was to assess the rate of protein synthesis (PS) and elucidate signalling pathways regulating PS in mouse soleus (Sol) and tibialis anterior (TA) muscles following chronic hypergravity (30-day centrifugation at 2G). The content of the key signalling proteins of the various anabolic signalling pathways was determined by Western-blotting. The rate of PS was assessed using in-vivo SUnSET technique. An exposure to 2G centrifugation did not induce any significant changes in the rate of PS as well as phosphorylation status of the key anabolic markers (AKT, p70s6k, 4E-BP1, GSK-3beta, eEF2) in Sol. On the contrary, a significant 55% increase in PS (p < 0.05) was found in TA. The cause of such a rise in PS could be associated with an increase in AKT (+72%, p < 0.05), GSK-3beta (+60%, p < 0.05) and p70s6k (+40%, p < 0.05) phosphorylation, as well as a decrease in eEF2 phosphorylation (−46%, p < 0.05) as compared to control values. Thus, the results of our study indicate that 30-day 2G centrifugation induces a distinct anabolic response in mouse Sol and TA muscles. The activation of the PS rate in TA could be linked to an up-regulation of both mTORC1-dependent and mTORC1-independent signalling pathways.

Endurance exercise induces REDD1 expression and transiently decreases mTORC1 signaling in rat skeletal muscle

Working muscle conserves adenosine triphosphate (ATP) for muscle contraction by attenuating protein synthesis through several different pathways. Regulated in development and DNA damage response 1 (REDD1) is one candidate protein that can itself attenuate muscle protein synthesis during muscle contraction. In this study, we investigated whether endurance exercise induces REDD1 expression in association with decreased mammalian target of rapamycin (mTOR) complex I (mTORC1) signaling and global protein synthesis in rat skeletal muscle. After overnight fasting, rats ran on a treadmill at a speed of 28 m/min for 60 min, and were killed before and immediately, 1, 3, 6, 12, and 24 h after exercise. REDD1 mRNA and corresponding protein levels increased rapidly immediately after exercise, and gradually decreased back to the basal level over a period of 6 h in the gastrocnemius muscle. Phosphorylation of mTOR Ser2448 and S6K1 Thr389 increased with the exercise, but diminished in 1–3 h into the recovery period after cessation of exercise. The rate of protein synthesis, as determined by the surface sensing of translation (SUnSET) method, was not altered by exercise in fasted muscle. These results suggest that REDD1 attenuates exercise‐induced mTORC1 signaling. This may be one mechanism responsible for blunting muscle protein synthesis during exercise and in the early postexercise recovery period.

We show that REDD1 expression is rapidly induced by an acute bout of endurance exercise in association with a decrease in mTORC1 signaling in rat muscle. The rate of mixed‐muscle protein synthesis was, however, not altered by exercise in fasted state. These results suggest that REDD1‐induced suppression of mTORC1 signaling may be one mechanism to blunt muscle protein synthesis during exercise and postexercise early recovery period.

Alcohol impairs skeletal muscle protein synthesis and mTOR signaling in a time-dependent manner following electrically stimulated muscle contraction

Alcohol (EtOH) decreases protein synthesis and mammalian target of rapamycin (mTOR)-mediated signaling and blunts the anabolic response to growth factors in skeletal muscle. The purpose of the current investigation was to determine whether acute EtOH intoxication antagonizes the contraction-induced increase in protein synthesis and mTOR signaling in skeletal muscle. Fasted male mice were injected intraperitoneally with 3 g/kg EtOH or saline (control), and the right hindlimb was electrically stimulated (10 sets of 6 contractions). The gastrocnemius muscle complex was collected 30 min, 4 h, or 12 h after stimulation. EtOH decreased in vivo basal protein synthesis (PS) in the nonstimulated muscle compared with time-matched Controls at 30 min, 4 h, and 12 h. In Control, but not EtOH, PS was decreased 15% after 30 min. In contrast, PS was increased in Control 4 h poststimulation but remained unchanged in EtOH. Last, stimulation increased PS 10% in Control and EtOH at 12 h, even though the absolute rate remained reduced by EtOH. The stimulation-induced increase in the phosphorylation of S6K1 Thr(421)/Ser(424) (20-52%), S6K1 Thr(389) (45-57%), and its substrate rpS6 Ser(240/244) (37-72%) was blunted by EtOH at 30 min, 4 h, and 12 h. Phosphorylation of 4E-BP1 Ser(65) was also attenuated by EtOH (61%) at 4 h. Conversely, phosphorylation of extracellular signal-regulated kinase Thr(202)/Tyr(204) was increased by stimulation in Control and EtOH mice at 30 min but only in Control at 4 h. Our data indicate that acute EtOH intoxication suppresses muscle protein synthesis for at least 12 h and greatly impairs contraction-induced changes in synthesis and mTOR signaling.

Autophagy is essential to support skeletal muscle plasticity in response to endurance exercise

Physical exercise is a stress that can substantially modulate cellular signaling mechanisms to promote morphological and metabolic adaptations. Skeletal muscle protein and organelle turnover is dependent on two major cellular pathways: Forkhead box class O proteins (FOXO) transcription factors that regulate two main proteolytic systems, the ubiquitin-proteasome, and the autophagy-lysosome systems, including mitochondrial autophagy, and the MTORC1 signaling associated with protein translation and autophagy inhibition. In recent years, it has been well documented that both acute and chronic endurance exercise can affect the autophagy pathway. Importantly, substantial efforts have been made to better understand discrepancies in the literature on its modulation during exercise. A single bout of endurance exercise increases autophagic flux when the duration is long enough, and this response is dependent on nutritional status, since autophagic flux markers and mRNA coding for actors involved in mitophagy are more abundant in the fasted state. In contrast, strength and resistance exercises preferentially raise ubiquitin-proteasome system activity and involve several protein synthesis factors, such as the recently characterized DAGK for mechanistic target of rapamycin activation. In this review, we discuss recent progress on the impact of acute and chronic exercise on cell component turnover systems, with particular focus on autophagy, which until now has been relatively overlooked in skeletal muscle. We especially highlight the most recent studies on the factors that can impact its modulation, including the mode of exercise and the nutritional status, and also discuss the current limitations in the literature to encourage further works on this topic.

Cytokine mRNA expression responses to resistance, aerobic, and concurrent exercise in sedentary middle-aged men

Concurrent resistance and aerobic exercise (CE) is recommended to ageing populations, though is postulated to induce diminished acute molecular responses. Given that contraction-induced cytokine mRNA expression reportedly mediates remunerative postexercise molecular responses, it is necessary to determine whether cytokine mRNA expression may be diminished after CE. Eight middle-aged men (age, 53.3 ±1.8 years; body mass index, 29.4 ± 1.4 kg·m−2) randomly completed (balanced for completion order) 8 × 8 leg extensions at 70% maximal strength (RE), 40 min of cycling at 55% of peak aerobic workload (AE), or (workload-matched) 50% RE and 50% AE (CE). Muscle (vastus lateralis) was obtained pre-exercise, and at 1 h and 4 h postexercise, and analyzed for changes of glycogen concentration, tumor necrosis factor (TNF)α, TNF receptor-1 and -2 (TNF-R1 and TNF-R2, respectively), interleukin (IL)-6, IL-6R, IL-1β, and IL-1 receptor-antagonist (IL-1ra). All exercise modes upregulated cytokine mRNA expression at 1 h postexercise comparably (TNFα, TNF-R1, TNF-R2, IL-1β, IL-6) (p < 0.05). Expression remained elevated at 4 h after RE and AE (p < 0.05), though returned to pre-exercise levels after CE (p > 0.05). Moreover, AE and RE upregulated IL-1β and IL-1ra expression, whereas CE upregulated IL-1β expression only (p < 0.05). Only AE reduced muscle glycogen concentration (p < 0.05), whilst upregulating receptor expression the greatest; though, IL-6R expression remained unchanged after all modes (p > 0.05). In conclusion, in middle-aged men, all modes induced commensurate cytokine mRNA expression at 1 h postexercise; however, only CE resulted in ameliorated expression at 4 h postexercise. Whether the RE or AE components of CE are independently or cumulatively sufficient to upregulate cytokine responses, or whether they collectively inhibit cytokine mRNA expression, remains to be determined.

Integrated MALDI-MS imaging and LC-MS techniques for visualizing spatiotemporal metabolomic dynamics in a rat stroke model

Spatiotemporal information about biomolecules is indispensable for precise pathological analysis, but it remains largely unclear. Here we show a novel analytical platform combing mass spectrometry imaging (MSI) with its complementary technique, liquid chromatography-mass spectrometry (LC-MS), to elucidate more comprehensive metabolic behaviors, with spatiotemporal information, in tissues. Analysis of a rat transient middle cerebral artery occlusion (MCAO) brain tissue after ischemia-reperfusion was performed to characterize the detailed metabolomic response to pathological alterations. To compare the spatially resolved metabolic state between ischemic and contralateral hemispheres of the MCAO brain, coronally sliced tissues were subjected to MSI. We also measured the metabolites extracted from three different cerebral regions, including whole cortex (CTX), hippocampus (HI) and corpus striatum (CPu), by LC-MS. In the ischemic hemisphere, significant metabolic changes at the CTX and CPu were observed after reperfusion, while not at the HI. A region-specific metabolic behavior was observed in amino acid and nucleotide metabolism, as well as in the TCA cycle. Correlation between MSI and LC-MS data was relatively high in the CTX and CPu. Combination of both MS platforms visualized the diverse spatiotemporal metabolic dynamics during pathological progress. Thus, our proposed strategy will contribute to the understanding of the complex pathogenesis of ischemia-reperfusion.

Changes in body composition in triathletes during an Ironman race

PURPOSE

Triathletes lose body mass during an Ironman triathlon. However, the associated body composition changes remain enigmatic. Thus, the purpose of this study was to investigate Ironman-induced changes in segmental body composition, using for the first time dual-energy X-ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT).

METHODS

Before and after an Ironman triathlon, segmental body composition and lower leg tissue mass, areas and densities were assessed using DXA and pQCT, respectively, in eight non-professional male triathletes. In addition, blood and urine samples were collected for the determination of hydration status.

RESULTS

Body mass decreased by 1.9 ± 0.8 kg. This loss was due to 0.4 ± 0.3 and 1.4 ± 0.8 kg decrease in fat and lean mass, respectively (P < 0.01). Calf muscle density was reduced by 1.93 ± 1.04 % (P < 0.01). Hemoglobin, hematocrit, and plasma [K(+)] remained unchanged, while plasma [Na(+)] (P < 0.05), urine specific gravity and plasma and urine osmolality increased (P < 0.01).

CONCLUSIONS

The loss in lean mass was explained by a decrease in muscle density, as an indicator of glycogen loss, and increases in several indicators for dehydration. The measurement of body composition with DXA and pQCT before and after an Ironman triathlon provided exact values for the loss in fat and lean mass. Consequently, these results yielded more detailed insights into tissue catabolism during ultra-endurance exercise.

Protein intake does not increase vastus lateralis muscle protein synthesis during cycling

PURPOSE

This study aimed to investigate the effect of protein ingestion on leg protein turnover and vastus lateralis muscle protein synthesis during bicycle exercise and recovery.

METHODS

Eight healthy males participated in two experiments in which they ingested either a carbohydrate solution (CHO) providing 0.49 g·kg(-1)·h(-1), or a carbohydrate and protein solution (CHO + P) providing 0.49 and 0.16 g·kg(-1)·h(-1), during 3 h of bicycle exercise and 3 h of recovery. Leg protein turnover was determined from stable isotope infusion (l-[ring-C6]phenylalanine), femoral-arterial venous blood sampling, and blood flow measurements. Muscle protein synthesis was calculated from the incorporation of l-[ring-C6]phenylalanine into protein.

RESULTS

Consuming protein during exercise increased leg protein synthesis and decreased net leg protein breakdown; however, protein ingestion did not increase protein synthesis within the highly active vastus lateralis muscle (0.029%·h(-1), ± 0.004%·h(-1), and 0.030%·h(-1), ± 0.003%·h(-1), in CHO and CHO + P, respectively; P = 0.88). In contrast, consuming protein, during exercise and recovery, increased postexercise vastus lateralis muscle protein synthesis by 51% ± 22% (0.070%·h(-1), ± 0.003%·h(-1), and 0.105%·h(-1), ± 0.013%·h(-1), in CHO and CHO+P, respectively; P < 0.01). Furthermore, leg protein net balance was negative during recovery with CHO intake, whereas positive leg protein net balance was achieved with CHO+P intake.

CONCLUSIONS

We conclude that consuming protein during prolonged bicycle exercise does not increase protein synthesis within highly active leg muscles. However, protein intake may have stimulated protein synthesis within less active leg muscles and/or other nonmuscle leg tissue. Finally, protein supplementation, during exercise and recovery, enhanced postexercise muscle protein synthesis and resulted in positive leg protein net balance.

Human muscle protein synthesis and breakdown during and after exercise

Skeletal muscle demonstrates extraordinary mutability in its responses to exercise of different modes, intensity, and duration, which must involve alterations of muscle protein turnover, both acutely and chronically. Here, we bring together information on the alterations in the rates of synthesis and degradation of human muscle protein by different types of exercise and the influences of nutrition, age, and sexual dimorphism. Where possible, we summarize the likely changes in activity of signaling proteins associated with control of protein turnover. Exercise of both the resistance and nonresistance types appears to depress muscle protein synthesis (MPS), whereas muscle protein breakdown (MPB) probably remains unchanged during exercise. However, both MPS and MPB are elevated after exercise in the fasted state, when net muscle protein balance remains negative. Positive net balance is achieved only when amino acid availability is increased, thereby raising MPS markedly. However, postexercise-increased amino acid availability is less important for inhibiting MPB than insulin, the secretion of which is stimulated most by glucose availability, without itself stimulating MPS. Exercise training appears to increase basal muscle protein turnover, with differential responses of the myofibrillar and mitochondrial protein fractions to acute exercise in the trained state. Aging reduces the responses of myofibrillar protein and anabolic signaling to resistance exercise. There appear to be few, if any, differences in the response of young women and young men to acute exercise, although there are indications that, in older women, the responses may be blunted more than in older men.

Cyclic stretch reduces myofibrillar protein synthesis despite increases in FAK and anabolic signalling in L6 cells

Muscle protein synthesis is increased after exercise, but evidence is now accruing that during muscular activity it is suppressed. In life, muscles are subjected to shortening forces due to contraction, but may also be subject to stretching forces during lengthening. It would be biologically inefficient if contraction and stretch have different effects on muscle protein turnover, but little is known about the metabolic effects of stretch. To investigate this, we assessed myofibrillar and sarcoplasmic protein synthesis (MPS, SPS, respectively) by incorporation of [1-13C]proline (using gas chromatography-mass spectrometry) and anabolic signalling (by phospho-immunoblotting and kinase assays) in cultured L6 skeletal muscle cells during 30 min of cyclic stretch and over 30 min intervals for up to 120 min afterwards. SPS was unaffected, whereas MPS was suppressed by 40 +/- 0.03% during stretch, before returning to basal rates by 90-20 min afterwards. Paradoxically, stretch stimulated anabolic signalling with peak values after 2-30 min: e.g. focal adhesion kinase (FAK Tyr576/577; +28 +/- 6%), protein kinase B activity (Akt; +113 +/- 31%), p70S6K1 (ribosomal S6 kinase Thr389; 25 +/- 5%), 4E binding protein 1 (4EBP1 Thr37/46; 14 +/- 3%), eukaryotic elongation factor 2 (eEF2 Thr56; -47 +/- 4%), extracellular regulated protein kinase 1/2 (ERK1/2 Tyr202/204; +65% +/- 9%), eukaryotic initiation factor 2alpha (eIF2alpha Ser51; -20 +/- 5%, P < 0.05) and eukaryotic initiation factor 4E (eIF4E Ser209; +33 +/- 10%, P < 0.05). After stretch, except for Akt activity, stimulatory phosphorylations were sustained: e.g. FAK (+26 +/- 11%) for > or =30 min, eEF2 for > or =60 min (peak -45 +/- 4%), 4EBP1 for > or =90 min (+33 +/- 5%), and p70S6K1 remained elevated throughout (peak +64 +/- 7%). Adenosine monophosphate-activated protein kinase (AMPK) phosphorylation was unchanged throughout. We report for the first time that acute cyclic stretch specifically suppresses MPS, despite increases in activity/phosphorylation of elements thought to increase anabolism.

Regulatory mechanisms of skeletal muscle protein turnover during exercise

Skeletal muscle protein turnover is a relatively slow metabolic process that is altered by various physiological stimuli such as feeding, fasting, and exercise. During exercise, catabolism of amino acids contributes very little to ATP turnover in working muscle. With regard to protein turnover, there are now consistent data from tracer studies in rodents and humans showing that global protein synthesis is blunted in working skeletal muscle. Whether there is altered skeletal muscle protein breakdown during exercise remains unclear. The blunting of protein synthesis is believed to be mediated by suppressed mRNA translation initiation and elongation steps involving, but not limited to, changes in eukaryotic initiation factor 4E binding protein 1 and eukaryotic elongation factor 2 phosphorylation (eEF2), respectively. Evidence is provided that upstream signaling to translation factors is mediated by signaling downstream of changes in intracellular Ca2+ and energy turnover. In particular, a signaling cascade involving Ca2+/calmodulin-eEF2 kinase-eEF2 is implicated. The possible functional significance of altered protein turnover in working skeletal muscle during exercise is discussed. Further work with available and new techniques will undoubtedly reveal the functional significance and signaling mechanisms behind changes in skeletal muscle protein turnover during exercise.

A Ca(2+)-calmodulin-eEF2K-eEF2 signalling cascade, but not AMPK, contributes to the suppression of skeletal muscle protein synthesis during contractions

Skeletal muscle protein synthesis rate decreases during contractions but the underlying regulatory mechanisms are poorly understood. It was hypothesized that there would be a coordinated regulation of eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E-binding protein 1 (4EBP1) phosphorylation by signalling cascades downstream of rises in intracellular [Ca(2+)] and decreased energy charge via AMP-activated protein kinase (AMPK) in contracting skeletal muscle. When fast-twitch skeletal muscles were contracted ex vivo using different protocols, the suppression of protein synthesis correlated more closely with changes in eEF2 than 4EBP1 phosphorylation. Using a combination of Ca(2+) release agents and ATPase inhibitors it was shown that the 60-70% suppression of fast-twitch skeletal muscle protein synthesis during contraction was equally distributed between Ca(2+) and energy turnover-related mechanisms. Furthermore, eEF2 kinase (eEF2K) inhibition completely blunted increases in eEF2 phosphorylation and partially blunted (i.e. 30-40%) the suppression of protein synthesis during contractions. The 3- to 5-fold increase in skeletal muscle eEF2 phosphorylation during contractions in situ was rapid and sustained and restricted to working muscle. The increase in eEF2 phosphorylation and eEF2K activation were downstream of Ca(2+)-calmodulin (CaM) but not other putative activating factors such as a fall in intracellular pH or phosphorylation by protein kinases. Furthermore, blunted protein synthesis and 4EBP1 dephosphorylation were unrelated to AMPK activity during contractions, which was exemplified by normal blunting of protein synthesis during contractions in muscles overexpressing kinase-dead AMPK. In summary, in fast-twitch skeletal muscle, the inhibition of eEF2 activity by phosphorylation downstream of Ca(2+)-CaM-eEF2K signalling partially contributes to the suppression of protein synthesis during exercise/contractions.

AMPK and the biochemistry of exercise: implications for human health and disease

AMPK (AMP-activated protein kinase) is a phylogenetically conserved fuel-sensing enzyme that is present in all mammalian cells. During exercise, it is activated in skeletal muscle in humans, and at least in rodents, also in adipose tissue, liver and perhaps other organs by events that increase the AMP/ATP ratio. When activated, AMPK stimulates energy-generating processes such as glucose uptake and fatty acid oxidation and decreases energy-consuming processes such as protein and lipid synthesis. Exercise is perhaps the most powerful physiological activator of AMPK and a unique model for studying its many physiological roles. In addition, it improves the metabolic status of rodents with a metabolic syndrome phenotype, as does treatment with AMPK-activating agents; it is therefore tempting to attribute the therapeutic benefits of regular physical activity to activation of AMPK. Here we review the acute and chronic effects of exercise on AMPK activity in skeletal muscle and other tissues. We also discuss the potential role of AMPK activation in mediating the prevention and treatment by exercise of specific disorders associated with the metabolic syndrome, including Type 2 diabetes and Alzheimer's disease.

Skeletal muscle eEF2 and 4EBP1 phosphorylation during endurance exercise is dependent on intensity and muscle fiber type

Protein synthesis in skeletal muscle is known to decrease during exercise, and it has been suggested that this may depend on the magnitude of the relative metabolic stress within the contracting muscle. To examine the mechanisms behind this, the effect of exercise intensity on skeletal muscle eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E binding protein 1 (4EBP1) phosphorylation, key components in the mRNA translation machinery, were examined together with AMP-activated protein kinase (AMPK) in healthy young men. Skeletal muscle eEF2 phosphorylation at Thr56 increased during exercise but was not influenced by exercise intensity, and was lower than rest 30 min after exercise. On the other hand, 4EBP1 phosphorylation at Thr37/46 decreased during exercise, and this decrease was greater at higher exercise intensities and was similar to rest 30 min after exercise. AMPK activity, as indexed by AMPK alpha-subunit phosphorylation at Thr172 and phosphorylation of the AMPK substrate ACCbeta at Ser221, was higher with higher exercise intensities, and these indices were higher than rest after high-intensity exercise only. Using immunohistochemistry, it was shown that the increase in skeletal muscle eEF2 Thr56 phosphorylation was restricted to type I myofibers. Taken together, these data suggest that the depression of skeletal muscle protein synthesis with endurance-type exercise may be regulated at both initiation (i.e., 4EBP1) and elongation (i.e., eEF2) steps, with eEF2 phosphorylation contributing at all exercise intensities but 4EBP1 dephosphorylation contributing to a greater extent at high vs. low exercise intensities.

Timing of creatine or protein supplementation and resistance training in the elderly

Muscle loss with age has a negative effect on strength and functional independence. Age-related loss of muscle is the result of decreased muscle fiber number and size, which are functions of altered hormonal status, physical inactivity, and variations in nutritional intake. Resistance training has a positive effect on muscle mass and strength in the elderly. Studies of protein or creatine supplementation for increasing muscle mass and strength in older individuals are equivocal. The timing of nutritional supplementation may be more important than the absolute daily intake of supplements. Protein or creatine ingestion proximate to resistance-training sessions may be more beneficial for increasing muscle mass and strength than ingestion of protein or creatine at other times of the day, possibly because of increased blood flow and therefore increased transport of amino acids and creatine to skeletal 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.

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

Resistance exercise is a potent stimulator of muscle protein synthesis and muscle cell growth, with the increase in protein synthesis being detected within 2-3 h post-exercise and remaining elevated for up to 48 h. However, during exercise, muscle protein synthesis is inhibited. An increase in AMP-activated protein kinase (AMPK) activity has recently been shown to decrease mammalian target of rapamycin (mTOR) signalling to key regulators of translation initiation. We hypothesized that the cellular mechanism for the inhibition of muscle protein synthesis during an acute bout of resistance exercise in humans would be associated with an activation of AMPK and an inhibition of downstream components of the mTOR pathway (4E-BP1 and S6K1). We studied 11 subjects (seven men, four women) before, during, and for 2 h following a bout of resistance exercise. Muscle biopsy specimens were collected at each time point from the vastus lateralis. We utilized immunoprecipitation and immunoblotting methods to measure muscle AMPKalpha2 activity, and mTOR-associated upstream and downstream signalling proteins, and stable isotope techniques to measure muscle fractional protein synthetic rate (FSR). AMPKalpha2 activity (pmol min(-1) (mg protein)(-1)) at baseline was 1.7 +/- 0.3, increased immediately post-exercise (3.0 +/- 0.6), and remained elevated at 1 h post-exercise (P < 0.05). Muscle FSR decreased during exercise and was significantly increased at 1 and 2 h post-exercise (P < 0.05). Phosphorylation of 4E-BP1 at Thr37/46 was significantly reduced immediately post-exercise (P < 0.05). We conclude that AMPK activation and a reduced phosphorylation of 4E-BP1 may contribute to the inhibition of muscle protein synthesis during resistance exercise. However, by 1-2 h post-exercise, muscle protein synthesis increased in association with an activation of protein kinase B, mTOR, S6K1 and eEF2.

Milk ingestion stimulates net muscle protein synthesis following resistance exercise

PURPOSE

Previous studies have examined the response of muscle protein to resistance exercise and nutrient ingestion. Net muscle protein synthesis results from the combination of resistance exercise and amino acid intake. No study has examined the response of muscle protein to ingestion of protein in the context of a food. This study was designed to determine the response of net muscle protein balance following resistance exercise to ingestion of nutrients as components of milk.

METHOD

Three groups of volunteers ingested one of three milk drinks each: 237 g of fat-free milk (FM), 237 g of whole milk (WM), and 393 g of fat-free milk isocaloric with the WM (IM). Milk was ingested 1 h following a leg resistance exercise routine. Net muscle protein balance was determined by measuring amino acid balance across the leg.

RESULTS

Arterial concentrations of representative amino acids increased in response to milk ingestion. Threonine balance and phenylalanine balance were both > 0 following milk ingestion. Net amino acid uptake for threonine was 2.8-fold greater (P < 0.05) for WM than for FM. Mean uptake of phenylalanine was 80 and 85% greater for WM and IM, respectively, than for FM, but not statistically different. Threonine uptake relative to ingested was significantly (P < 0.05) higher for WM (21 +/- 6%) than FM (11 +/- 5%), but not IM (12 +/- 3%). Mean phenylalanine uptake/ingested also was greatest for WM, but not significantly.

CONCLUSIONS

Ingestion of milk following resistance exercise results in phenylalanine and threonine uptake, representative of net muscle protein synthesis. These results suggest that whole milk may have increased utilization of available amino acids for protein synthesis.

Role of AMPK in skeletal muscle metabolic regulation and adaptation in relation to exercise

The 5'-AMP-activated protein kinase (AMPK) is a potent regulator of skeletal muscle metabolism and gene expression. AMPK is activated both in response to in vivo exercise and ex vivo contraction. AMPK is therefore believed to be an important signalling molecule in regulating muscle metabolism during exercise as well as in adaptation of skeletal muscle to exercise training. The first part of this review is focused on different mechanisms regulating AMPK activity during muscle work such as alterations in nucleotide concentrations, availability of energy substrates and upstream AMPK kinases. We furthermore discuss the possible role of AMPK as a master switch in skeletal muscle metabolism with the main focus on AMPK in metabolic regulation during muscle work. Finally, AMPK has a well established role in regulating expression of genes encoding various enzymes in muscle, and this issue is discussed in relation to adaptation of skeletal muscle to exercise training.

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).

Exercise rapidly increases eukaryotic elongation factor 2 phosphorylation in skeletal muscle of men

Protein synthesis in skeletal muscle is known to decrease during contractions but the underlying regulatory mechanisms are unknown. Here, the effect of exercise on skeletal muscle eukaryotic elongation factor 2 (eEF2) phosphorylation, a key component in protein translation machinery, was examined. Eight healthy men exercised on a cycle ergometer at a workload eliciting approximately 67% peak pulmonary oxygen consumption (VO2 peak) with skeletal muscle biopsies taken from the vastus lateralis muscle at rest as well as after 1, 10, 30, 60 and 90 min of exercise. In response to exercise, there was a rapid (i.e. < 1 min) 5- to 7-fold increase in eEF2 phosphorylation at Thr56 that was sustained for 90 min of continuous exercise. The in vitro activity of skeletal muscle eEF2 kinase was not altered by exercise indicating that the increased activity of eEF2 kinase to eEF2 is not mediated by covalent mechanisms. In support of this, the increase in AMPK activity was temporally unrelated to eEF2 phosphorylation. However, skeletal muscle eEF2 kinase was potently activated by Ca(2)(+)-calmodulin in vitro, suggesting that the higher eEF2 phosphorylation in working skeletal muscle is mediated by allosteric activation of eEF2 kinase by Ca(2)(+) signalling via calmodulin. Given that eEF2 phosphorylation inhibits eEF2 activity and mRNA translation, these findings suggest that the inhibition of protein synthesis in contracting skeletal muscle is due to the Ca(2)(+)-induced stimulation of eEF2 kinase.

IGF-I stimulates protein synthesis in skeletal muscle through multiple signaling pathways during sepsis

Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius not observed in rats with a sterile abscess. Inhibition is associated with an impaired mRNA translation initiation that can be ameliorated by elevating IGF-I but not insulin. The present study investigated the ability of IGF-I signaling to stimulate protein synthesis in gastrocnemius by accelerating mRNA translation initiation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Protein synthesis in gastrocnemius from septic rats was accelerated twofold by the addition of IGF-I (10 nM) to perfusate. IGF-I increased the phosphorylation of translation repressor 4E-binding protein-1 (4E-BP1). Hyperphosphorylation of 4E-BP1 in response to IGF-I resulted in its dissociation from the inactive eukaryotic initiation factor (eIF) 4E.4E-BP1 complex. Assembly of the active eIF4F complex (as assessed by the association eIF4G with eIF4E) was increased twofold by IGF-I in the perfusate. In addition, phosphorylation of eIF4G and ribosomal protein S6 kinase-1 (S6K1) was also enhanced by IGF-I. Activation of mammalian target of rapamycin, an upstream kinase implicated in phosphorylating both 4E-BP1 and S6K1, was also observed. Thus the ability of IGF-I to accelerate protein synthesis during sepsis may be related to a stimulation of signaling to multiple steps in translation initiation with an ensuing increased phosphorylation of eIF4G, eIF4E availability, and S6K1 phosphorylation.

Myofibrillar and collagen protein synthesis in human skeletal muscle in young men after maximal shortening and lengthening contractions

We aimed to determine whether there were differences in the extent and time course of skeletal muscle myofibrillar protein synthesis (MPS) and muscle collagen protein synthesis (CPS) in human skeletal muscle in an 8.5-h period after bouts of maximal muscle shortening (SC; average peak torque = 225 +/- 7 N.m, means +/- SE) or lengthening contractions (LC; average peak torque = 299 +/- 18 N.m) with equivalent work performed in each mode. Eight healthy young men (21.9 +/- 0.6 yr, body mass index 24.9 +/- 1.3 kg/m2) performed 6 sets of 10 maximal unilateral LC of the knee extensors on an isokinetic dynamometer. With the contralateral leg, they then performed 6 sets of maximal unilateral SC with work matched to the total work performed during LC (10.9 +/- 0.7 vs. 10.9 +/- 0.8 kJ, P = 0.83). After exercise, the participants consumed small intermittent meals to provide 0.1 g.kg(-1).h(-1) of protein and carbohydrate. Prior exercise elevated MPS above rest in both conditions, but there was a more rapid rise after LC (P < 0.01). The increases (P < 0.001) in CPS above rest were identical for both SC and LC and likely represent a remodeling of the myofibrillar basement membrane. Therefore, a more rapid rise in MPS after maximal LC could translate into greater protein accretion and muscle hypertrophy during chronic resistance training utilizing maximal LC.

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.

No effect of creatine supplementation on human myofibrillar and sarcoplasmic protein synthesis after resistance exercise

Muscle hypertrophy during resistance training is reportedly increased by creatine supplementation. Having previously failed to find an anabolic effect on muscle protein turnover at rest, either fed or fasted, we have now examined the possibility of a stimulatory effect of creatine in conjunction with acute resistance exercise. Seven healthy men (body mass index, 23 +/- 2 kg/m2, 21 +/- 1 yr, means +/- SE) performed 20 x 10 repetitions of leg extension-flexion at 75% one-repetition maximum in one leg, on two occasions, 4 wk apart, before and after ingesting 21 g/day creatine for 5 days. The subjects ate approximately 21 g maltodextrin + 6 g protein/h for 3 h postexercise. We measured incorporation of [1-13C]leucine into quadriceps muscle proteins in the rested and exercised legs. Leg protein breakdown (as dilution of [2H5]phenylalanine) was also assessed in the exercised and rested leg postexercise. Creatine supplementation increased muscle total creatine by approximately 21% (P < 0.01). Exercise increased the synthetic rates of myofibrillar and sarcoplasmic proteins by two- to threefold (P < 0.05), and leg phenylalanine balance became more positive, but creatine was without any anabolic effect.

Beta -oxidation of free fatty acids is required to maintain translational control of protein synthesis in heart

The study described herein investigated the role of free fatty acids (FFAs) in the maintenance of protein synthesis in vivo in rat cardiac and skeletal muscle. Suppression of FFA beta-oxidation by methyl palmoxirate caused a marked reduction in protein synthesis in the heart. The effect on protein synthesis was mediated in part by changes in the function of eukaryotic initiation factors (eIFs) involved in the initiation of mRNA translation. The guanine nucleotide exchange activity of eIF2B was repressed, phosphorylation of the alpha-subunit of eIF2 was enhanced, and phosphorylation of eIF4E-binding protein-1 and ribosomal protein S6 kinase was reduced. Similar changes in protein synthesis and translation initiation were not observed in the gastrocnemius following treatment with methyl palmoxirate. In heart, repressed beta-oxidation of FFA correlated, as demarcated by changes in the ATP/AMP ratio and phosphorylation of AMP-activated kinase, with alterations in the energy status of the tissue. Therefore, the activation state of signal transduction pathways that are responsive to cellular energy stress represents one mechanism whereby translation initiation may be regulated in cardiac muscle.

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.

Insulin fails to stimulate muscle protein synthesis in sepsis despite unimpaired signaling to 4E-BP1 and S6K1

Induction of sepsis in rats causes an inhibition of protein synthesis in skeletal muscle that is resistant to the stimulatory actions of insulin. To gain a better understanding of the underlying reason for this lack of response, the present study was undertaken to investigate sepsis-induced alterations in insulin signaling to regulatory components of mRNA translation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Sepsis resulted in a 50% reduction in protein synthesis in the gastrocnemius. Protein synthesis in muscles from septic rats, but not controls, was unresponsive to stimulation by insulin. The insulin-induced hyperphosphorylation response of the translation repressor protein 4E-binding protein 1 (4E-BP1) and of the 70-kDa S6 kinase (S6K1) (1), two targets of insulin action on mRNA translation, was unimpaired in gastrocnemius of septic rats. Hyperphosphorylation of 4E-BP1 in response to insulin resulted in its dissociation from the inactive eukaryotic initiation factor (eIF)4E. 4E-BP1 complex in both control and septic rats. However, assembly of the active eIF4F complex as assessed by the association of eIF4E with eIF4G did not follow the pattern predicted by the increased availability of eIF4E resulting from changes in the phosphorylation of 4E-BP1. Indeed, sepsis caused a dramatic reduction in the amount of eIF4G associated with eIF4E in the presence or absence of insulin. Thus the inability of insulin to stimulate protein synthesis during sepsis may be related to a defect in signaling to a step in translation initiation involved in assembly of an active eIF4F complex.

Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise

The present study was designed to determine whether consumption of an oral essential amino acid-carbohydrate supplement (EAC) before exercise results in a greater anabolic response than supplementation after resistance exercise. Six healthy human subjects participated in two trials in random order, PRE (EAC consumed immediately before exercise), and POST (EAC consumed immediately after exercise). A primed, continuous infusion of L-[ring-(2)H(5)]phenylalanine, femoral arteriovenous catheterization, and muscle biopsies from the vastus lateralis were used to determine phenylalanine concentrations, enrichments, and net uptake across the leg. Blood and muscle phenylalanine concentrations were increased by approximately 130% after drink consumption in both trials. Amino acid delivery to the leg was increased during exercise and remained elevated for the 2 h after exercise in both trials. Delivery of amino acids (amino acid concentration times blood flow) was significantly greater in PRE than in POST during the exercise bout and in the 1st h after exercise (P < 0.05). Total net phenylalanine uptake across the leg was greater (P = 0.0002) during PRE (209 +/- 42 mg) than during POST (81 +/- 19). Phenylalanine disappearance rate, an indicator of muscle protein synthesis from blood amino acids, increased after EAC consumption in both trials. These results indicate that the response of net muscle protein synthesis to consumption of an EAC solution immediately before resistance exercise is greater than that when the solution is consumed after exercise, primarily because of an increase in muscle protein synthesis as a result of increased delivery of amino acids to the leg.

IGF-I/IGFBP-3 binary complex modulates sepsis-induced inhibition of protein synthesis in skeletal muscle

The present study evaluated the ability of insulin-like growth factor I (IGF-I) complexed with IGF binding protein-3 (IGFBP-3) to modulate the sepsis-induced inhibition of protein synthesis in gastrocnemius. Beginning 16 h after the induction of sepsis, either the binary complex or saline was injected twice daily via a tail vein, with measurements made 3 and 5 days later. By day 3, sepsis had reduced plasma IGF-I concentrations approximately 50% in saline-treated rats. Administration of the binary complex provided exogenous IGF-I to compensate for the sepsis-induced diminished plasma IGF-I. Sepsis decreased rates of protein synthesis in gastrocnemius relative to controls by limiting translational efficiency. Treatment of septic rats with the binary complex for 5 days attenuated the sepsis-induced inhibition of protein synthesis and restored translational efficiency to control values. Assessment of potential mechanisms regulating translational efficiency showed that neither the sepsis-induced change in gastrocnemius content of eukaryotic initiation factor 2B (eIF2B), the amount of eIF4E associated with 4E binding protein-1 (4E-BP1), nor the phosphorylation state of 4E-BP1 or eIF4E were altered by the binary complex. Overall, the results are consistent with the hypothesis that decreases in plasma IGF-I are partially responsible for enhanced muscle catabolism during sepsis.

Eukaryotic initiation factors and protein synthesis after resistance exercise in rats

Translational control of protein synthesis depends on numerous eukaryotic initiation factors (eIFs) and we have previously shown (Am. J. Physiol. Endocrinol. Metab. 276: E721-E727, 1999) that increases in one factor, eIF2B, are associated with increases in rates of protein synthesis after resistance exercise in rats. In the present study we investigated whether the eIF4E family of initiation factors is also involved with an anabolic response to exercise. Male Sprague-Dawley rats either remained sedentary (n = 6) or performed acute resistance exercise (n = 6), and rates of protein synthesis were assessed in vivo 16 h after the last session of resistance exercise. eIF4E complexed to eIF4G (eIF4E x eIF4G), eIF4E binding protein 1 (4E-BP1) complexed to eIF4E, and phosphorylation state of eIF4E and 4E-BP1 (gamma-form) were assessed in gastrocnemius. Rates of protein synthesis were higher in exercised rats compared with sedentary rats [205 +/- 8 (SE) vs. 164 +/- 5.5 nmol phenylalanine incorporated x g muscle(-1) x h(-1), respectively; P < 0.05]. Arterial plasma insulin concentrations were not different between the two groups. A trend (P = 0.09) for an increase in eIF4E x eIF4G with exercise was noted; however, no statistically significant differences were observed in any of the components of the eIF4E family in response to resistance exercise. These new data, along with our previous report on eIF2B, suggest that the regulation of peptide chain initiation after exercise is more dependent on eIF2B than on the eIF4E system.

Role of eIF4E in stimulation of protein synthesis by IGF-I in perfused rat skeletal muscle

Insulin-like growth factor I (IGF-I) promotes anabolism by stimulating protein synthesis in skeletal muscle. In the present study, we have examined mechanisms by which IGF-I stimulates protein synthesis in skeletal muscle with a perfused rat hindlimb preparation. IGF-I (10 nM) stimulated protein synthesis over 2.7-fold. Total RNA content was unaffected, but translational efficiency was increased by IGF-I. We next examined the effect of IGF-I on eukaryotic initiation factor (eIF) 4E as a mechanism regulating translation initiation. IGF-I did not alter either the amount of eIF4E associated with the eIF4E binding protein 4E-BP1 or the phosphorylation state of 4E-BP1. Likewise, the phosphorylation state of eIF4E was unaltered by IGF-I. In contrast, the amount of eIF4E bound to eIF4G was increased threefold by IGF-I. We conclude that IGF-I regulates protein synthesis in skeletal muscle by enhancing formation of the active eIF4E x eIF4G complex.

Reduced 40S initiation complex formation in skeletal muscle during sepsis

Severe muscle wasting is a characteristic feature of sepsis. We have previously established that the rate of protein synthesis in muscles composed of fast-twitch fibers is severely diminished in response to sepsis. The present studies investigate the biochemical reactions responsible for the decreased rate of protein synthesis using gastrocnemius from control and septic rats perfused in situ. Analysis of free ribosomal subunits indicated peptide-chain initiation was impaired by infection. To characterize biochemical reactions in the pathway of peptide-chain initiation affected, the effect of sepsis on the incorporation of initiator [35S]methionyl-tRNA (met-tRNA(imet)) into the 40S initiation complex was examined. Sepsis caused a 65% decrease in the binding of radiolabelled met-tRNA(imet) to the 40S initiation complex compared with controls. The binding of met-tRNA(met) to the 40S ribosome is regulated by eukaryotic initiation factor eIF-2B, whose activity can be modulated in part by the redox state of pyridine dinucleotides. The mean cytoplasmic NADH/NAD+ ratio was increased 2 fold in sepsis, while the NADPH/NADP+ ratio was unchanged. These findings identify the formation of the 40S initiation complex as a defect in the protein synthesis machinery during sepsis. The decreased formation of the 40S initiation complex in muscle could not be explained by changes in the cytoplasmic redox state.

Age-dependent decrease in the amount of eukaryotic initiation factor 2 in various rat tissues

Recent studies have suggested that the decline in protein synthesis that occurs in rat liver and brain during development and aging is associated with a decrease in the activity of eukaryotic initiation factor 2 (eIF-2). One way in which eIF-2 activity could be decreased in tissue extracts would be through a decrease in the activity of the GDP exchange factor, eIF-2B. In the present study, the activity of eIF-2B was measured in tissue extracts and was found to be less in older than in younger rats. Thus a decrease in eIF-2B activity could account for part of the decrease in protein synthesis that occurs during aging. Another way in which eIF-2 activity could be decreased would be through a decrease in amount of the protein. Therefore the amount of eIF-2 in various tissues was quantified by protein immunoblot analysis. We found that the amount of eIF-2 relative to total protein tended to fall with increasing age. Furthermore, eIF-2 content was directly proportional to the rate of protein synthesis in the tissues examined. Finally, slot-blot analysis of polyadenylated RNA revealed no significant change in the relative abundance of eIF-2 alpha mRNA with age. The last-mentioned experiments suggest that the synthesis of eIF-2 may be regulated through changes in the deficiency of translation of eIF-2 alpha mRNA rather than through changes in gene transcription.

Acute effects of phorbol esters on the protein-synthetic rate and carbohydrate metabolism of normal and mdx mouse muscles

1. mdx mice do not express dystrophin, the product of the gene which is defective in Duchenne and Becker muscular dystrophy. We have previously shown that protein-synthetic rates (ks) are increased in mdx mouse muscles [MacLennan & Edwards (1990) Biochem. J. 268, 795-797]. 2. The tumour-promoting stereoisomer of phorbol 12,13-didecanoate (4 beta-PDD) acutely increased the ks of muscles from mdx and wild-type (C57BL/10) mice incubated in vitro in the absence of insulin. The effects of 4 beta-PDD are presumably mediated by activation of protein kinase C (PKC). 3. The muscle glycogen concentrations of mdx mice were higher than those of C57BL/10 mice. Studies performed in vivo and in vitro suggested that the effect might be at least partially due to increased rate of glycogen synthesis in mdx muscle. 4. 4 beta-PDD increased the glycogen-synthetic rates rates of C57BL/10, but not mdx, muscles incubated in vitro in the absence of insulin. 5. In muscles from both species incubated in the absence of insulin, treatment with 4 beta-PDD also induced increased rates of glucose uptake and lactate production. Kinetic studies of C57BL/10 and mdx muscles suggested that 4 beta-PDD raised the Vmax. of glucose uptake, but did not alter the Km for the process. 6. The possible role of PKC in controlling the protein and carbohydrate metabolism of normal and mdx mouse muscles is discussed.

Acute alpha 1-adrenergic stimulation of cardiac protein synthesis may involve increased intracellular pH and protein kinase activity

In the presence of 5 microM-DL-propranolol and in HCO3(-)-containing buffers, 1 microM-adrenaline acutely stimulated protein synthesis by about 25% in the anterogradely perfused rat heart. This stimulation was opposed by low (1-10 nM) concentrations of prazosin, but not by similar concentrations of yohimbine, suggesting involvement of the alpha 1-adrenoceptor. Under the same conditions, adrenaline raised intracellular pH (pHi) by about 0.1 unit. The increase in pHi induced by adrenaline was prevented by 5 nM-prazosin, but not by 5 nM-yohimbine, again suggesting involvement of the alpha 1-adrenoceptor. Since an increase in pHi stimulates protein synthesis in the heart [Sugden & Fuller (1991) Biochem. J. 273, 339-346], the increase in pHi induced by adrenaline may be involved in its stimulation of protein synthesis. Adrenaline also increased phosphocreatine concentrations. As discussed, the increase in pHi induced by adrenaline may be responsible for this effect. Using second-order polynomial regression analysis, we showed that rates of protein synthesis were significantly correlated (P less than 0.0001) with phosphocreatine concentrations. We discuss two possible reasons for this correlation: (i) increases in pHi stimulate protein synthesis and separately raise phosphocreatine concentrations, or (ii) the increase in protein synthesis rates is a consequence of the raised phosphocreatine concentrations induced by the increase in pHi. Rates of protein synthesis were not significantly correlated with ATP/ADP concentration ratios, nor with any of the following: ATP, ADP, AMP or total adenine nucleotide concentrations. In freshly isolated adult rat cardiomyocytes, the protein kinase inhibitor staurosporine (1 microM) prevented stimulation of protein synthesis by 0.3 microM-adrenaline (and by 1 microM-phorbol 12-myristate 13-acetate or 1 m-unit of insulin/ml). The results are discussed within a mechanistic framework initiated by stimulation of the hydrolysis of membrane phospholipids by alpha 1-adrenergic agonists.

Effects of ischaemia, blood loss and reperfusion on rat muscle protein synthesis, metabolite concentrations and polyribosome profiles in vivo

In adult rat gastrocnemius muscles, on reperfusion after 45 min of tourniquet ischaemia, protein synthetic rates were depressed by over half for 1 h compared to normal (12%/day), and were at least one-third below normal for up to 5 h afterwards. Ischaemia caused muscle concentrations of phosphocreatine to be depressed by 70%, and those of lactate to be elevated by 350%; the proportion of ribosomes as polyribosomes was decreased by half. Unlike the rates of protein synthesis, all of these variables returned to normal after 35 min of reperfusion. When 25% of the blood volume was removed (for 10-45 min), there were falls in the rate of gastrocnemius protein synthesis and in phosphocreatine concentration, and an increase in lactate concentration. On blood replacement, protein synthesis and metabolite concentrations returned to normal within 15 min. Polyribosome profiles were unaffected by blood loss or replacement. There were highly significant correlations between the rate of gastrocnemius protein synthesis and both phosphocreatine concentration and 1/(lactate concentration), during blood loss and replacement, i.e. during both the fall and rise in muscle energy status. We conclude that the effects of ischaemia and blood loss on protein synthesis are not equivalent.

Resistance of protein and glucose metabolism to insulin in denervated rat muscle

Denervated (1-10 days) rat epitrochlearis muscles were isolated, and basal and insulin-stimulated protein and glucose metabolism were studied. Although basal rates of glycolysis and glucose transport were increased in 1-10-day-denervated muscles, basal glycogen-synthesis rates were unaltered and glycogen concentrations were decreased. Basal rates of protein degradation and synthesis were increased in 1-10-day-denervated muscles. The increase in degradation was greater than that in synthesis, resulting in muscle atrophy. Increased rates of proteolysis and glycolysis were accompanied by elevated release rates of leucine, alanine, glutamate, pyruvate and lactate from 3-10-day-denervated muscles. ATP and phosphocreatine were decreased in 3-10-day-denervated muscles. Insulin resistance of glycogen synthesis occurred in 1-10-day denervated muscles. Insulin-stimulated glycolysis and glucose transport were inhibited by day 3 of denervation, and recovered by day 10. Inhibition of insulin-stimulated protein synthesis was observed only in 3-day-denervated muscles, whereas regulation by insulin of net proteolysis was unaffected in 1-10-day-denervated muscles. Thus the results demonstrate enhanced glycolysis, proteolysis and protein synthesis, and decreased energy stores, in denervated muscle. They further suggest a defect in insulin's action on protein synthesis in denervated muscles as well as on glucose metabolism. However, the lack of concurrent changes in all insulin-sensitive pathways and the absence of insulin-resistance for proteolysis suggest multiple and specific cellular defects in insulin's action in denervated muscle.

Acute alterations in sodium flux in vitro lead to decreased myofibrillar protein breakdown in rat skeletal muscle

Myofibrillar protein breakdown was evaluated by measuring the release of N tau-methylhistidine by isolated rat skeletal muscles or perfused rat muscles in the presence of a variety of agents known to affect Na+ flux. Total cell proteolysis was evaluated simultaneously by measuring tyrosine release by muscles after the inhibition of protein synthesis with cycloheximide. Treatment of muscles with the Na+ ionophore monensin or inhibitors of Na+-K+ ATPase (ouabain, digoxin or vanadate) decreased N tau-methylhistidine release by muscles by 21-35%. A phorbol ester (phorbol 12-myristate 13-acetate) as well as a synthetic diacylglycerol known to activate protein kinase C and a Na+/H+ antiport also decreased N tau-methylhistidine release by muscles. Removal of extracellular Na+ blocked the ability of these agents to attenuate N tau-methylhistidine release by muscles, suggesting that their effectiveness required a change in Na+ flux. In contrast with N tau-methylhistidine release by muscles, these agents, except for monensin, did not effect the release of tyrosine, suggesting that they attenuate specifically the breakdown of myofibrillar proteins. Overall these results indicate a link between Na+ and the regulation of protein breakdown in rat skeletal muscle, whereby an influx of Na+ can result in a decrease in myofibrillar proteolysis. Left unresolved is whether phospholipid hydrolysis is involved in this scheme.

Differential effects of acute changes in cell Ca2+ concentration on myofibrillar and non-myofibrillar protein breakdown in the rat extensor digitorum longus muscle in vitro. Assessment by production of tyrosine and N tau-methylhistidine

The influence of Ca2+ on myofibrillar proteolysis was evaluated in the isolated extensor digitorum longus muscle incubated in vitro with agents previously shown to increase the intracellular concentration of Ca2+. Myofibrillar proteolysis was evaluated by measuring the release of N tau-methylhistidine, and total proteolysis was evaluated by measuring tyrosine release by incubated muscles after the inhibition of protein synthesis with cycloheximide. Incubated muscles released measurable quantities of N tau-methylhistidine, and muscle contents of the amino acids remained stable over 2 h of incubation. The release of N tau-methylhistidine by incubated muscles was similar to its release by perfused rat muscle in response to brief starvation, indicating the integrity of the incubated muscles. Ca2+ ionophore A23187, dibucaine, procaine, caffeine and elevated K+ concentration increased lactate release by incubated muscles and decreased tissue contents of ATP and phosphocreatine to varying degrees, indicating the metabolic effectiveness of the agents tested. Only A23187 and dibucaine increased total cell Ca2+, and they increased tyrosine release. Caffeine and elevated [K+] increased neither cell Ca2+ nor tyrosine release; however, only A23187 and dibucaine increased tyrosine release significantly. On the other hand, these agents were without effect on myofibrillar proteolysis as assessed by N tau-methylhistidine release by incubated muscles and changes in tissue contents of the amino acid. In fact, some of the agents tested tended to decrease myofibrillar proteolysis slightly. These results indicate that acute elevation of intracellular Ca2+ is associated with increased breakdown of non-myofibrillar but not myofibrillar proteins. Because of this, the role of elevated Ca2+ in muscle atrophy in certain pathological states is questioned. The data also indicate that the breakdown of myofibrillar and non-myofibrillar proteins in muscle is regulated independently and by different pathways, a conclusion reached in previous studies with perfused rat muscle.

Response of muscle protein turnover to insulin after acute exercise and training

To determine whether the enhanced insulin-sensitivity of glucose metabolism in muscle after acute exercise also extends to protein metabolism, untrained and exercise-trained rats were subjected to an acute bout of exercise, and the responses of protein synthesis and degradation to insulin were measured in epitrochlearis muscles in vitro. Acute exercise of both untrained and trained rats decreased protein synthesis in muscle in the absence or presence of insulin, but protein degradation was not altered. Exercise training alone had no effect on protein synthesis or degradation in muscle in the absence or presence of insulin. Acute exercise or training alone enhanced the sensitivities of both protein synthesis and degradation to insulin, but the enhanced insulin-sensitivities from training alone were not additive to those after acute exercise. These results indicate that: a decrease in protein synthesis is the primary change in muscle protein turnover after acute exercise and is not altered by prior exercise training, and the enhanced insulin-sensitivities of metabolism of both glucose and protein after either acute exercise or training suggest post-binding receptor events.

Contrasting response of protein degradation to starvation and insulin as measured by release of N tau-methylhistidine or phenylalanine from the perfused rat heart

An isotope-dilution method is described for the measurement of N tau-methylhistidine release from the perfused rat heart. We argue that release of N tau-methylhistidine is indicative of cardiac actin degradation. N tau-Methylhistidine release is compared with phenylalanine release in the presence of cycloheximide (phenylalanine release being a measure of degradation of mixed proteins). In hearts perfused with glucose plus acetate, the rate of actin degradation was increased by starvation and was not inhibited by insulin. In contrast, the rate of mixed-protein degradation was decreased by starvation and was inhibited by insulin. The fractional rate of degradation of mixed proteins in hearts from fed or starved rats was greater than that for actin. It is suggested that there are at least two pools of intracellular protein, the degradation rates of which differ in terms of their response to insulin and starvation.