Guillaume Py

The concept of maximal lactate steady state: A bridge between biochemistry, physiology and sport science

The maximal lactate steady state (MLSS) is defined as the highest blood lactate concentration (MLSSc) and work load (MLSSw) that can be maintained over time without a continual blood lactate accumulation. A close relationship between endurance sport performance and MLSSw has been reported and the average velocity over a marathon is just below MLSSw. This work rate delineates the low-to high-intensity exercises at which carbohydrates contribute more than 50% of the total energy need and at which the fuel mix switches (crosses over) from predominantly fat to predominantly carbohydrate. The rate of metabolic adenosine triphosphate (ATP) turnover increases as a direct function of metabolic power output and the blood lactate at MLSS represents the highest point in the equilibrium between lactate appearance and disappearance both being equal to the lactate turnover. However, MLSSc has been reported to demonstrate a great variability between individuals (from 2–8 mmol/L) in capillary blood and not to be related to MLSSw. The fate of enhanced lactate clearance in trained individuals has been attributed primarily to oxidation in active muscle and gluconeogenesis in liver. The transport of lactate into and out of the cells is facilitated by monocarboxylate transporters (MCTs) which are transmembrane proteins and which are significantly improved by training. Endurance training increases the expression of MCT1 with intervariable effects on MCT4. The relationship between the concentration of the two MCTs and the performance parameters (i.e. the maximal distance run in 20 minutes) in elite athletes has not yet been reported. However, lactate exchange and removal indirectly estimated with velocity constants of the individual blood lactate recovery has been reported to be related to time to exhaustion at maximal oxygen uptake.

Early Activation of Rat Skeletal Muscle IL-6/STAT1/STAT3 Dependent Gene Expression in Resistance Exercise Linked to Hypertrophy

Cytokine interleukin-6 (IL-6) is an essential regulator of satellite cell-mediated hypertrophic muscle growth through the transcription factor signal transducer and activator of transcription 3 (STAT3). The importance of this pathway linked to the modulation of myogenic regulatory factors expression in rat skeletal muscle undergoing hypertrophy following resistance exercise, has not been investigated. In this study, the phosphorylation and nuclear localization of STAT3, together with IL-6/STAT3-responsive gene expression, were measured after both a single bout of resistance exercise and 10 weeks of training. Flexor Digitorum Profundus muscle samples from Wistar rats were obtained 2 and 6 hours after a single bout of resistance exercise and 72 h after the last bout of either 2, 4, or 10 weeks of resistance training. We observed an increase in IL-6 and SOCS3 mRNAs concomitant with phosphorylation of STAT1 and STAT3 after 2 and 6 hours of a single bout of exercise (p<0.05). STAT3-dependent early responsive genes such as CyclinD1 and cMyc were also upregulated whereas MyoD and Myf5 mRNAs were downregulated (p<0.05). BrdU-positive satellite cells increased at 2 and 6 hours after exercise (p<0.05). Muscle fiber hypertrophy reached up to 100% after 10 weeks of training and the mRNA expression of Myf5, c-Myc and Cyclin-D1 decreased, whereas IL-6 mRNA remained upregulated. We conclude that the IL-6/STAT1/STAT3 signaling pathway and its responsive genes after a single bout of resistance exercise are an important event regulating the SC pool and behavior involved in muscle hypertrophy after ten weeks of training in rat skeletal muscle.

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.

Autophagy and protein turnover signaling in slow-twitch muscle during exercise.

PURPOSE The aim of this study was to characterize skeletal muscle protein breakdown and mitochondrial dynamics markers at different points of endurance exercise. METHODS Mice run at 10 m·min(-1) during 1 h, and running speed was increased by 0.5 m·min(-1) every minute during 40 min and then by 1 m·min(-1) until exhaustion. Animals were killed by cervical dislocation at 30, 60, 90, and 120 min; at time to exhaustion (Te); and at 3 and 24 h during recovery. The soleus and the deep red regions of the quadriceps muscles were pooled. RESULTS AMPK phosphorylation (Thr172) increased from 30 min to Te, and FoxO3a phosphorylation (Thr32 and Ser253) decreased from 120 min to 3 h after exercise. FoxO3a-dependent E3 ligases Mul1 and MuRF1 proteins increased from 30 min to Te and at Te and 3 h after exercise, respectively, whereas MAFbx/atrogin-1 protein expression did not change significantly. The autophagic markers LC3B-II increased at 120 min and Te, and p62 significantly decreased at Te. The AMPK-dependent phosphorylation of Ulk1 at Ser317 and Ser555 increased from 60 min to Te and at 30 and 60 min, respectively. Akt (Ser473), MTOR (Ser2448), and 4E-BP1 (Thr37/46) phosphorylation decreased from 90 min to Te, and the MTOR-dependent phosphorylation of Ulk1 (Ser757) decreased from 120 min to Te. Ser616 phosphorylation of the mitochondrial fission marker DRP1 increased from 60 min to Te, but protein expression of the fusion markers mitofusin-2, a substrate of Mul1, and OPA1 did not significantly change. CONCLUSIONS These results fit with a regulation of protein breakdown triggered by FoxO3a and Ulk1 pathways after AMPK activation and Akt/MTOR inhibition. Furthermore, our data suggest that mitochondrial fission is quickly increased, and mitochondrial fusion is unchanged during exercise.

Antioxidants and mitochondrial respiration in lung, diaphragm, and locomotor muscles : effect of exercise

Previous studies have shown that exhaustive exercise may increase reactive oxygen species (ROS) generation in oxidative muscles that may in turn impair mitochondrial respiration. Locomotor muscles have been extensively examined, but there is few report about diaphragm or lung. The later is a privileged site for oxygen transit. To compare the antioxidant defense system and mitochondrial function in lung, diaphragm and locomotor muscles after exercise, 24 young adult male rats were randomly assigned to a control (C) or exercise (E) group. E group rats performed an exhaustive running test on a motorized treadmill at 80-85% VO2max Mean exercise duration was 66+/-2.7 min. Lung, costal diaphragm, mixed gastrocnemius, and oxidative muscles (red gastrocnemius and soleus: RG/SOL homogenate) were sampled. Mitochondrial respiration was assessed in tissue homogenates by respiratory control index (RCI: rate of uncoupled respiration/rate of basal respiration) measurement. Lipid peroxidation was evaluated by malondialdehyde concentration (MDA) and we determined the activity of two antioxidant enzymes: superoxide dismutase (SOD) and glutathione peroxidase (GPX). We found elevated basal (C group data) SOD and GPX activities in both lung and diaphragm compared to locomotor muscles (p<.001). Exercise led to a rise in GPX activity in red locomotor muscles homogenate (GR/SOL; C = 10.3+/-0.29 and E = 14.4+/-1.51 micromol x min(-1) x gww(-1); p<.05), whereas there was no significant change in lung and diaphragm. MDA concentration and mitochondrial RCI values were not significantly changed after exercise. We conclude that lung and diaphragm had higher antioxidant protection than locomotor muscles. The exercise test did not lead to significant oxidative stress or alteration in mitochondrial respiration, suggesting that antioxidant function was adequate in both lung and diaphragm in the experimental condition.

N-acetylcysteine protects against bupivacaine-induced myotoxicity caused by oxidative and sarcoplasmic reticulum stress in human skeletal myotubes.

Background:Local anesthetics offer the benefits of extended analgesia with greater patient satisfaction and faster rehabilitation compared with intravenous morphine. These benefits, however, can be offset by adverse iatrogenic muscle pain. Here, the authors investigate the mechanisms of local anesthetic-induced myotoxicity and assess the protective effect of N-acetylcysteine. Methods:The authors used primary cell cultures of human skeletal muscle myoblasts to study local anesthetic adverse effects. Production of reactive oxygen species was investigated in human skeletal myotubes by fluorescence microscopy. Expression of sarcoplasmic/endoplasmic reticulum stress markers and induction of apoptosis were followed by immunofluorescence and Western blot analysis. Finally, the effect of N-acetylcysteine on bupivacaine-induced myotoxicity was investigated in vitro. Results:Bupivacaine sequentially induced reactive oxygen species production, oxidative stress, sarcoplasmic/endoplasmic reticulum stress, and activation of caspases 9 and 7 in human differentiated myoblasts. These iatrogenic effects were prevented by N-acetylcysteine. Conclusions:The authors demonstrated a protective effect of N-acetylcysteine against bupivacaine-induced sarcoplasmic/endoplasmic reticulum stress and apoptosis in primary human skeletal muscle cell.

Polyphenols decreased liver NADPH oxidase activity, increased muscle mitochondrial biogenesis and decreased gastrocnemius age-dependent autophagy in aged rats

Abstract This study explored major systems of reactive oxygen species (ROS) production and their consequences on oxidative stress, mitochondriogenesis and muscle metabolism in aged rats, and evaluated the efficiency of 30-day oral supplementation with a moderate dose of a red grape polyphenol extract (RGPE) on these parameters. In the liver of aged rats, NADPH oxidase activity was increased and mitochondrial respiratory chain complex activities were altered, while xanthine oxidase activity remained unchanged. In muscles, only mitochondrial activity was modified with aging. The oral intake of RGPE decreased liver NADPH oxidase activity in the aged rats without affecting global oxidative stress, suggesting that NADPH oxidase was probably not the dominant detrimental source of production of O2·− in the liver. Interestingly, RGPE supplementation increased mitochondrial biogenesis and improved antioxidant status in the gastrocnemius of aged rats, while it had no significant effect in soleus. RGPE supplementation also decreased age-dependent autophagy in gastrocnemius of aged rats. These results extended existing findings on the beneficial effects of RGPE on mitochondriogenesis and muscle metabolism in aged rats.

Combined effects of hypoxia and endurance training on lipid metabolism in rat skeletal muscle

Aim:  To determine whether endurance training can counterbalance the negative effects of hypoxia on mitochondrial phosphorylation and expression of the long chain mitochondrial fatty acid transporter muscle carnitine palmitoyl transferase 1 (mCPT‐1).

Muscle wasting and aging: Experimental models, fatty infiltrations, and prevention.

Identification of cost-effective interventions to maintain muscle mass, muscle strength, and physical performance during muscle wasting and aging is an important public health challenge. It requires understanding of the cellular and molecular mechanisms involved. Muscle-deconditioning processes have been deciphered by means of several experimental models, bringing together the opportunities to devise comprehensive analysis of muscle wasting. Studies have increasingly recognized the importance of fatty infiltrations or intermuscular adipose tissue for the age-mediated loss of skeletal-muscle function and emphasized that this new important factor is closely linked to inactivity. The present review aims to address three main points. We first mainly focus on available experimental models involving cell, animal, or human experiments on muscle wasting. We next point out the role of intermuscular adipose tissue in muscle wasting and aging and try to highlight new findings concerning aging and muscle-resident mesenchymal stem cells called fibro/adipogenic progenitors by linking some cellular players implicated in both FAP fate modulation and advancing age. In the last part, we review the main data on the efficiency and molecular and cellular mechanisms by which exercise, replacement hormone therapies, and β-hydroxy-β-methylbutyrate prevent muscle wasting and sarcopenia. Finally, we will discuss a potential therapeutic target of sarcopenia: glucose 6-phosphate dehydrogenase.

Effects of Chronic Administration of Clenbuterol on Contractile Properties and Calcium Homeostasis in Rat Extensor Digitorum Longus Muscle

Clenbuterol, a β2-agonist, induces skeletal muscle hypertrophy and a shift from slow-oxidative to fast-glycolytic muscle fiber type profile. However, the cellular mechanisms of the effects of chronic clenbuterol administration on skeletal muscle are not completely understood. As the intracellular Ca2+ concentration must be finely regulated in many cellular processes, the aim of this study was to investigate the effects of chronic clenbuterol treatment on force, fatigue, intracellular calcium (Ca2+) homeostasis and Ca2+-dependent proteolysis in fast-twitch skeletal muscles (the extensor digitorum longus, EDL, muscle), as they are more sensitive to clenbuterol-induced hypertrophy. Male Wistar rats were chronically treated with 4 mg.kg−1 clenbuterol or saline vehicle (controls) for 21 days. Confocal microscopy was used to evaluate sarcoplasmic reticulum Ca2+ load, Ca2+ -transient amplitude and Ca2+ spark properties. EDL muscles from clenbuterol-treated animals displayed hypertrophy, a shift from slow to fast fiber type profile and increased absolute force, while the relative force remained unchanged and resistance to fatigue decreased compared to control muscles from rats treated with saline vehicle. Compared to control animals, clenbuterol treatment decreased Ca2+-transient amplitude, Ca2+ spark amplitude and frequency and the sarcoplasmic reticulum Ca2+ load was markedly reduced. Conversely, calpain activity was increased by clenbuterol chronic treatment. These results indicate that chronic treatment with clenbuterol impairs Ca2+ homeostasis and this could contribute to the remodeling and functional impairment of fast-twitch skeletal muscle.