Acute exercise activates the AHR in peripheral blood mononuclear cells in an intensity-dependent manner.

The kynurenine pathway of tryptophan degradation generates several metabolites such as kynurenine or kynurenic acid that serve as endogenous ligands of the aryl hydrocarbon receptor (AHR). Due to its distinct biological roles particularly modulating the immune system, the AHR is a current therapeutic target across different inflammation-related diseases. Here, we show an acute exercise-induced increase in AHR ligand availability on a systemic level and a kynurenine pathway activation in peripheral blood mononuclear cells (PBMCs). Concurrently, the AHR is activated in PBMCs following acute exercise. Exercise effects on both, kynurenic acid and AHR activation in PBMCs were greater in response to high-intensity interval exercise (50 min., six three-minute intervals á 90% V̇O2peak, and three-minute intervals at 50% V̇O2peak in between) compared to workload-matched moderate intensity continuous exercise (50 min.). In conclusion, these data indicate a novel mechanistic link how exercise modulates the immune system through the kynurenine pathway-AHR axis, potentially underlying exercise-induced benefits in various chronic diseases.

Resistance training does not increase myocellular garbage dumps: A pilot study on lipofuscin in skeletal muscle fibers of resistance trained young men.

Lipofuscin (LF) is an intracellular aggregate associated with proteostatic impairments, especially prevalent in nondividing skeletal muscle fibers. Reactive oxygen species (ROS) drive LF-formation. Resistance training (RT) improves muscle performance but also increases ROS production, potentially promoting LF-formation. Thus, we aimed to investigate if RT of a mesocycle duration increases LF-formation in type-I and II muscle fibers and whether RT increases the antioxidant capacity (AOC) in terms of SOD1 and SOD2 content. An intervention group (IG) performed 14 eccentrically accented RT-sessions within 7 weeks. Vastus lateralis muscle biopsies were collected before and after the intervention from IG as well as from a control group (CG) which refrained from RT for the same duration. LF was predominantly found near nuclei, followed by membrane-near and a minor amount in the fiber core, with corresponding spot sizes. Overall, LF-content was higher in type-I than type-II fibers (p < 0.05). There was no increase in LF-content in type-I or IIA fibers, neither for the IG following RT nor for the CG. The same is valid for SOD1/2. We conclude that, in healthy subjects, RT can be safely performed, without adverse effects on increased LF-formation.

Enhanced capacity for CaMKII signaling mitigates calcium release related contractile fatigue with high intensity exercise.

BACKGROUND: We tested whether enhancing the capacity for calcium/calmodulin-dependent protein kinase type II (CaMKII) signaling would delay fatigue of excitation-induced calcium release and improve contractile characteristics of skeletal muscle during fatiguing exercise. METHODS: Fast and slow type muscle, gastrocnemius medialis (GM) and soleus (SOL), of rats and mouse interosseus (IO) muscle fibers, were transfected with pcDNA3-based plasmids for rat α and ß CaMKII or empty controls. Levels of CaMKII, its T287-phosphorylation (pT287-CaMKII), and phosphorylation of components of calcium release and re-uptake, ryanodine receptor 1 (pS2843-RyR1) and phospholamban (pT17-PLN), were quantified biochemically. Sarcoplasmic calcium in transfected muscle fibers was monitored microscopically during trains of electrical excitation based on Fluo-4 FF fluorescence (n = 5-7). Effects of low- (n = 6) and high- (n = 8) intensity exercise on pT287-CaMKII and contractile characteristics were studied in situ. RESULTS: Co-transfection with αCaMKII-pcDNA3/ßCaMKII-pcDNA3 increased α and ßCaMKII levels in SOL (+45.8 %, +250.5 %) and GM (+40.4 %, +89.9 %) muscle fibers compared to control transfection. High-intensity exercise increased pT287-ßCaMKII and pS2843-RyR1 levels in SOL (+269 %, +151 %) and GM (+354 %, +119 %), but decreased pT287-αCaMKII and p17-PLN levels in GM compared to SOL (-76 % vs. +166 %; 0 % vs. +128 %). α/ß CaMKII overexpression attenuated the decline of calcium release in muscle fibers with repeated excitation, and mitigated exercise-induced deterioration of rates in force production, and passive force, in a muscle-dependent manner, in correlation with pS2843-RyR1 and pT17-PLN levels (|r| > 0.7). CONCLUSION: Enhanced capacity for α/ß CaMKII signaling improves fatigue-resistance of active and passive contractile muscle properties in association with RyR1- and PLN-related improvements in sarcoplasmic calcium release.

Resistance exercise: a mighty tool that adapts, destroys, rebuilds and modulates the molecular and structural environment of skeletal muscle.

PURPOSE: Skeletal muscle regulates health and performance by maintaining or increasing strength and muscle mass. Although the molecular mechanisms in response to resistance exercise (RE) significantly target the activation of protein synthesis, a plethora of other mechanisms and structures must be involved in orchestrating the communication, repair, and restoration of homeostasis after RE stimulation. In practice, RE can be modulated by variations in intensity, continuity and volume, which affect molecular responses and skeletal muscle adaptation. Knowledge of these aspects is important with respect to planning of training programs and assessing the impact of RE training on skeletal muscle. METHODS: In this narrative review, we introduce general aspects of skeletal muscle substructures that adapt in response to RE. We further highlighted the molecular mechanisms that control human skeletal muscle anabolism, degradation, repair and memory in response to acute and repeated RE and linked these aspects to major training variables. RESULTS: Although RE is a key stimulus for the activation of skeletal muscle anabolism, it also induces myofibrillar damage. Nevertheless, to increase muscle mass accompanied by a corresponding adaptation of the essential substructures of the sarcomeric environment, RE must be continuously repeated. This requires the permanent engagement of molecular mechanisms that re-establish skeletal muscle integrity after each RE-induced muscle damage. CONCLUSION: Various molecular regulators coordinately control the adaptation of skeletal muscle after acute and repeated RE and expand their actions far beyond muscle growth. Variations of key resistance training variables likely affect these mechanisms without affecting muscle growth.

Repeated and Interrupted Resistance Exercise Induces the Desensitization and Re-Sensitization of mTOR-Related Signaling in Human Skeletal Muscle Fibers.

The acute resistance exercise (RE)-induced phosphorylation of mTOR-related signaling proteins in skeletal muscle can be blunted after repeated RE. The time frame in which the phosphorylation (p) of mTORS2448, p70S6kT421/S424, and rpS6S235/236 will be reduced during an RE training period in humans and whether progressive (PR) loading can counteract such a decline has not been described. (1) To enclose the time frame in which pmTORS2448, prpS6S235/236, and pp70S6kT421/S424 are acutely reduced after RE occurs during repeated RE. (2) To test whether PR will prevent that reduction compared to constant loading (CO) and (3) whether 10 days without RE may re-increase blunted signaling. Fourteen healthy males (24 ± 2.8 yrs.; 1.83 ± 0.1 cm; 79.3 ± 8.5 kg) were subjected to RE with either PR (n = 8) or CO (n = 6) loading. Subjects performed RE thrice per week, conducting three sets with 10−12 repetitions on a leg press and leg extension machine. Muscle biopsies were collected at rest (T0), 45 min after the first (T1), seventh (T7), 13th (T13), and 14th (X-T14) RE session. No differences were found between PR and CO for any parameter. Thus, the groups were combined, and the results show the merged values. prpS6S235/236 and pp70s6kT421/S424 were increased at T1, but were already reduced at T7 and up to T13 compared to T1. Ten days without RE re-increased prpS6S235/236 and pp70S6kT421/S424 at X-T14 to a level comparable to that of T1. pmTORS2448 was increased from T1 to X-T14 and did not decline over the training period. Single-fiber immunohistochemistry revealed a reduction in prpS6S235/236 in type I fibers from T1 to T13 and a re-increase at X-T14, which was more augmented in type II fibers at T13 (p < 0.05). The entity of myofibers revealed a high heterogeneity in the level of prpS6S235/236, possibly reflecting individual contraction-induced stress during RE. The type I and II myofiber diameter increased from T0 and T1 to T13 and X-T14 (p < 0.05) prpS6S235/236 and pp70s6kT421/S424 reflect RE-induced states of desensitization and re-sensitization in dependency on frequent loading by RE, but also by its cessation.

Effects of Acute and Chronic Resistance Exercise on the Skeletal Muscle Metabolome.

Resistance training promotes metabolic health and stimulates muscle hypertrophy, but the precise routes by which resistance exercise (RE) conveys these health benefits are largely unknown. AIM: To investigate how acute RE affects human skeletal muscle metabolism. METHODS: We collected vastus lateralis biopsies from six healthy male untrained volunteers at rest, before the first of 13 RE training sessions, and 45 min after the first and last bouts of RE. Biopsies were analysed using untargeted mass spectrometry-based metabolomics. RESULTS: We measured 617 metabolites covering a broad range of metabolic pathways. In the untrained state RE altered 33 metabolites, including increased 3-methylhistidine and N-lactoylvaline, suggesting increased protein breakdown, as well as metabolites linked to ATP (xanthosine) and NAD (N1-methyl-2-pyridone-5-carboxamide) metabolism; the bile acid chenodeoxycholate also increased in response to RE in muscle opposing previous findings in blood. Resistance training led to muscle hypertrophy, with slow type I and fast/intermediate type II muscle fibre diameter increasing by 10.7% and 10.4%, respectively. Comparison of post-exercise metabolite levels between trained and untrained state revealed alterations of 46 metabolites, including decreased N-acetylated ketogenic amino acids and increased beta-citrylglutamate which might support growth. Only five of the metabolites that changed after acute exercise in the untrained state were altered after chronic training, indicating that training induces multiple metabolic changes not directly related to the acute exercise response. CONCLUSION: The human skeletal muscle metabolome is sensitive towards acute RE in the trained and untrained states and reflects a broad range of adaptive processes in response to repeated stimulation.

PKM2 Determines Myofiber Hypertrophy In Vitro and Increases in Response to Resistance Exercise in Human Skeletal Muscle.

Nearly 100 years ago, Otto Warburg investigated the metabolism of growing tissues and discovered that tumors reprogram their metabolism. It is poorly understood whether and how hypertrophying muscle, another growing tissue, reprograms its metabolism too. Here, we studied pyruvate kinase muscle (PKM), which can be spliced into two isoforms (PKM1, PKM2). This is of interest, because PKM2 redirects glycolytic flux towards biosynthetic pathways, which might contribute to muscle hypertrophy too. We first investigated whether resistance exercise changes PKM isoform expression in growing human skeletal muscle and found that PKM2 abundance increases after six weeks of resistance training, whereas PKM1 decreases. Second, we determined that Pkm2 expression is higher in fast compared to slow fiber types in rat skeletal muscle. Third, by inducing hypertrophy in differentiated C2C12 cells and by selectively silencing Pkm1 and/or Pkm2 with siRNA, we found that PKM2 limits myotube growth. We conclude that PKM2 contributes to hypertrophy in C2C12 myotubes and indicates a changed metabolic environment within hypertrophying human skeletal muscle fibers. PKM2 is preferentially expressed in fast muscle fibers and may partly contribute to the increased potential for hypertrophy in fast fibers.

Coordinated alpha-crystallin B phosphorylation and desmin expression indicate adaptation and deadaptation to resistance exercise-induced loading in human skeletal muscle.

Skeletal muscle is a target of contraction-induced loading (CiL), leading to protein unfolding or cellular perturbations, respectively. While cytoskeletal desmin is responsible for ongoing structural stabilization, in the immediate response to CiL, alpha-crystallin B (CRYAB) is phosphorylated at serine 59 (pCRYABS59) by P38, acutely protecting the cytoskeleton. To reveal adaptation and deadaptation of these myofibrillar subsystems to CiL, we examined CRYAB, P38, and desmin regulation following resistance exercise at diverse time points of a chronic training period. Mechanosensitive JNK phosphorylation (pJNKT183/Y185) was determined to indicate the presence of mechanical components in CiL. Within 6 wk, subjects performed 13 resistance exercise bouts at the 8-12 repetition maximum, followed by 10 days detraining and a final 14th bout. Biopsies were taken at baseline and after the 1st, 3rd, 7th, 10th, 13th, and 14th bout. To assess whether potential desensitization to CiL can be mitigated, one group trained with progressive and a second with constant loading. As no group differences were found, all subjects were combined for statistics. Total and phosphorylated P38 was not regulated over the time course. pCRYABS59 and pJNKT183/Y185 strongly increased following the unaccustomed first bout. This exercise-induced pCRYABS59/pJNKT183/Y185 increase disappeared with the 10th until 13th bout. As response to the detraining period, the 14th bout led to a renewed increase in pCRYABS59. Desmin content followed pCRYABS59 inversely, i.e., was up- when pCRYABS59 was downregulated and vice versa. In conclusion, the pCRYABS59 response indicates increase and decrease in resistance to CiL, in which a reinforced desmin network could play an essential role by structurally stabilizing the cells.

Phosphorylation of αB-crystallin and its cytoskeleton association differs in skeletal myofiber types depending on resistance exercise intensity and volume.

αB-crystallin (CRYAB) is an important actor in the immediate cell stabilizing response following mechanical stress in skeletal muscle. Yet, only little is known regarding myofiber type-specific stress responses of CRYAB. We investigated whether the phosphorylation of CRYAB at serine 59 (pCRYABSer59) and its cytoskeleton association are influenced by varying load-intensity and -volume in a fiber type-specific manner. Male subjects were assigned to 1, 5, and 10 sets of different acute resistance exercise protocols: hypertrophy (HYP), maximum strength (MAX), strength endurance (SE), low intensity (LI), and three sets of maximum eccentric resistance exercise (ECC). Skeletal muscle biopsies were taken at baseline and 30 min after exercise. Western blot revealed an increase in pCRYABSer59 only following 5 and 10 sets in groups HYP, MAX, SE, and LI as well as following 3 sets in the ECC group. In type I fibers, immunohistochemistry determined increased pCRYABSer59 in all groups. In type II fibers, pCRYABSer59 only increased in MAX and ECC groups, with the increase in type II fibers exceeding that of type I fibers in ECC. Association of CRYAB and pCRYABSer59 with the cytoskeleton reflected the fiber type-specific phosphorylation pattern. Phosphorylation of CRYAB and its association with the cytoskeleton in type I and II myofibers is highly specific in terms of loading intensity and volume. Most likely, this is based on specific recruitment patterns of the different myofiber entities due to the different resistance exercise loadings. We conclude that pCRYABSer59 indicates contraction-induced mechanical stress exposure of single myofibers in consequence of resistance exercise. NEW & NOTEWORTHY We determined that the phosphorylation of αB-crystallin at serine 59 (pCRYABSer59) after resistance exercise differs between myofiber types in a load- and intensity-dependent manner. The determination of pCRYABSer59 could serve as a marker indirectly indicating contractile involvement and applied mechanical stress on individual fibers. By that, it is possible to retrospectively assess the impact of resistance exercise loading on skeletal muscle fiber entities.

Resistance exercise-induced muscle fatigue is not accompanied by increased phosphorylation of ryanodine receptor 1 at serine 2843.

Skeletal muscle fatigue has been shown to be associated with hyperphosphorylation of the ryanodine receptor 1 at serine 2843 (pRyR1Ser2843), due to chronic overloading exercise. We investigated whether pRyR1Ser2843, is a mechanism relevant for muscle fatigue also under acute, in contrast to chronic, muscle loading. 24 male subjects (age: 24,8±3,8; height: 182,8±7,2 cm; weight: 82,5±9,9 kg) were evenly (n = 6) assigned to the following four different resistance exercise (RE) groups: hypertrophy- (HYP), strength endurance- (SE), maximum power- (MAX) at the subjects' 10, 25 and 3 repetition maximum, respectively, and low intensity (LI) RE with 70% of the 10 repetition maximum. Each group completed three different RE volumes (1 set, 5, and 10 sets). Muscle biopsies from the vastus lateralis were taken before and after exercise, analyzed for pRyR1Ser2843 and examined for association with RE-induced muscle fatigue which was determined as reduction in maximum isometric force (isoFmax) in the quadriceps femoris muscle also before and after exercise.The degree of RE-induced muscle fatigue was specific in terms of set volume as well as of RE mode. isoFmax was not reduced in any group after one set of RE. Five sets led to a significant reduction of isoFmax in HYP and SE but not in LI and MAX (p<0,05). Ten sets of RE, as compared to five sets, exclusively induced further muscle fatigue in LI. In terms of RE mode differences, isoFmax reduction was generally higher in HYP and SE than in MAX and Li after five and ten sets of RE (p<0,05). However, pRyR1Ser2843 did not show any significant regulation, regardless of exercise condition. We conclude that despite its relevance in reducing muscle contractility in chronic overloading, pRyR1Ser2843 does not reflect the degree of muscle fatigue exerted by acute hypertrophy-, strength endurance-, maximum power and low intensity-oriented exercise.

Acute alterations in the hematological and hemorheological profile induced by resistance training and possible implication for microvascular functionality.

Depending on the exercise variables and training design, resistance exercise can be applied to gain muscle mass, prevent diseases like osteoporosis and sarcopenia or generally increase strength capacity. But the influence on blood flow parameters and possible consequences in health and disease are less understood. To examine the possible impact of resistance exercise of different duration on hemorheology, oxidative stress and microvascular function, participants (n = 6) performed lower-limb resistance exercise of the quadriceps femoris. Loading consisted of 1 (S1), 5 (S5) and 10 (S10) sets, on separated days, at the individual 10 repetition maximum. Blood samples were taken before (Pre) and after (Post0) each set as well after a 25-min recovery period (Post25). Hemograms were measured to analyze hematocrit, white blood cell (WBC) count and red blood cell (RBC) count. RBC deformability and aggregation were measured by ektacytometry and syllectometry to determine hemorheological responses. Plasma and RBC nitrate were measured by chemiluminescence detection to determine nitric oxide production. Formation of N-tyrosine and plasma malondialdehyde to determine oxidative stress and lipid peroxidation were measured by immunostaining and ELISA, respectively. Hematocrit, RBC, WBC count and aggregation increased Post0 in each protocol with subsequently decreased values Post25 below Pre values. High effect size was observed regarding deformability during the different sets. RBC nitrite analysis revealed effect size alterations between the trainings, whereas plasma nitrite was not affected. Effects size was evident in lipid peroxidation, whereas N-tyrosine concentration was not altered. Lower-limb resistance exercise induced acute changes in hematological and hemorheological parameters, whereby intermittent hemodilution and plasma shifts seemed the major contributor. The acute adaptations of RBC function seen during short duration resistance exercise might contribute to beneficial effects on microvascular circulation with a low oxidative stress response.

p38 MAPK activation and H3K4 trimethylation is decreased by lactate in vitro and high intensity resistance training in human skeletal muscle.

Exercise induces adaptation of skeletal muscle by acutely modulating intracellular signaling, gene expression, protein turnover and myogenic activation of skeletal muscle stem cells (Satellite cells, SCs). Lactate (La)-induced metabolic stimulation alone has been shown to modify SC proliferation and differentiation. Although the mechanistic basis remains elusive, it was demonstrated that La affects signaling via p38 mitogen activated protein kinase (p38 MAPK) which might contribute to trimethylation of histone 3 lysine 4 (H3K4me3) known to regulate satellite cell proliferation and differentiation. We investigated the effects of La on p38 MAPK and H3K4me3 in a model of activated SCs. Differentiating C2C12 myoblasts were treated with La (20 mM) and samples analysed using qRT-PCR, immunofluorescence, and western blotting. We determined a reduction of p38 MAPK phosphorylation, decreased H3K4me3 and reduced expression of Myf5, myogenin, and myosin heavy chain (MHC) leading to decreased differentiation of La-treated C2C12 cells after 5 days of repeated La treatment. We further investigated whether this regulatory pathway would be affected in human skeletal muscle by the application of two different resistance exercise regimes (RE) associated with distinct metabolic demands and blood La accumulation. Muscle biopsies were obtained 15, 30 min, 1, 4, and 24 h post exercise after moderate intensity RE (STD) vs. high intensity RE (HIT). Consistent with in vitro results, reduced p38 phosphorylation and blunted H3K4me3 were also observed upon metabolically demanding HIT RE in human skeletal muscle. Our data provide evidence that La-accumulation acutely affects p38 MAPK signaling, gene expression and thereby cell differentiation and adaptation in vitro, and likely in vivo.

Different Training-Induced Skeletal Muscle Adaptations in COPD Patients with and without Alpha-1 Antitrypsin Deficiency.

BACKGROUND: Pulmonary rehabilitation (PR) improves oxidative capacity of peripheral muscles in patients with chronic obstructive pulmonary disease (COPD). The exercise-induced oxidative skeletal muscle adaptation in COPD patients with inherited alpha-1 antitrypsin deficiency (A1ATD) has not been studied. OBJECTIVES: To compare PR effects on skeletal muscle adaptation in COPD patients with and without A1ATD. METHODS: Nine COPD patients with A1ATD (genotype PiZZ, 6 receiving A1AT augmentation therapy), and 10 'usual' COPD patients (genotype PiMM) performed an incremental cycling test and underwent musculus vastus lateralis biopsies before and after a 3-week PR program including exercise training. RESULTS: PiZZ and PiMM patients improved peak work rate following PR (+9 ± 11 W, p < 0.05, and +18 ± 9 W, p < 0.001, between-group difference p < 0.05). PiMM patients increased fibre type I (+8.1%), reduced fibre type IIA (-2.1%) and hybrid fibre type IIA/IIX proportion (-3.9%). Following PR, PiMM patients also raised mitochondrial signalling proteins PGC-1α (4.5-fold), and TFAM (6.4-fold). PiZZ patients had no change in fibre type I but showed a shift of type IIA/IIX (-8.8%) towards fibre type IIA distribution (+8.9%). The capillary to fibre ratio increased by 28% (p < 0.05) in PiZZ, whereas no change was observed in PiMM patients. Linear regression analysis revealed that diffusion capacity and A1AT therapy are predictor variables for myofibre type I response to PR (r2 = 0.684, p < 0.01). CONCLUSIONS: Following a 3-week PR with comparable training modalities, PiMM but not PiZZ patients increased the oxidative myofibre type I proportion. This skeletal muscle adaptation pattern suggests better improvement of exercise capacity in PiMM than in PiZZ patients with COPD.

High force development augments skeletal muscle signalling in resistance exercise modes equalized for time under tension.

How force development and time under tension (TUT) during resistance exercise (RE) influence anabolic signalling of skeletal muscle is incompletely understood. We hypothesized that high force development during RE is more important for post-exercise-induced signalling than submaximal and fatiguing RE with lower force development but similar TUT. Twenty-two male subjects (24 ± 6 years, 181 ± 9 cm, 79 ± 2 kg) performed three distinct RE modes in the fed state with equal TUT but distinct force output: (i) maximal eccentric RE (ECC, n = 7) three sets, eight reps, 100% eccentric dynamic force; (ii) standard RE (STD, n = 7), three sets, 10 reps, 75% dynamic force; and (iii) high fatiguing single-set RE (HIT, n = 8), 20 reps, 100% eccentric-concentric force; vastus lateralis biopsies were collected at baseline, 15, 30, 60, 240 min and 24 h after RE, and the signalling of mechanosensitive and mammalian target of rapamycin (mTOR)-related proteins was determined. The phosphorylation levels of pFAK(Tyr397), pJNK(Thr183/Tyr185), pAKT(Thr308/Ser473), pmTOR(Ser2448), p4E-BP1(Thr37/46), p70s6k(Thr389)/(Ser421/Thr424) and pS6(Ser235/236) were significantly higher in ECC than those in STD and HIT at several time points (P < 0.01). pJNK(Thr183/Tyr185) and pS6(Ser235/236) levels were significantly higher in type II myofibres in ECC compared with STD and HIT. HIT exerted throughout the weakest signalling response. We conclude that high force development during acute RE is superior for anabolic skeletal muscle signalling than fatiguing RE with lower force output but similar TUT. Our results suggest that this response is substantially driven by the higher activation of type II myofibres during RE.