Stimulated myotube contractions regulate membrane-bound and soluble TLR4 to prevent LPS-induced signaling and myotube atrophy in skeletal muscle cells.

Toll-like receptor 4 (TLR4) activation by lipopolysaccharides (LPS) increases proinflammatory cytokine production and upregulation of muscle atrophy signaling pathways. Muscle contractions can suppress LPS/TLR4 axis activation by reducing the protein expression of TLR4 on immune cells. However, the mechanism by which muscle contractions decrease TLR4 remains undefined. Moreover, it is not clear whether muscle contractions affect TLR4 expressed on skeletal muscle cells. The purpose of this study was to uncover the nature and mechanisms by which stimulated myotube contractions using electrical pulse stimulation (EPS) as an in vitro model of skeletal muscle contractions affect TLR4 expression and intracellular signaling to combat LPS-induced muscle atrophy. C2C12 myotubes were stimulated to contract via EPS with and without subsequent LPS exposure. We then examined the isolated effects of conditioned media (CM) collected following EPS and soluble TLR4 (sTLR4) alone on LPS-induced myotube atrophy. Exposure to LPS decreased membrane-bound and sTLR4, increased TLR4 signaling (decreased inhibitor of κBα), and induced myotube atrophy. However, EPS decreased membrane-bound TLR4, increased sTLR4, and prevented LPS-induced signaling and myotube atrophy. CM, which contained elevated levels of sTLR4, prevented LPS-induced upregulation of atrophy-related gene transcripts muscle ring finger 1 (MuRF1) and atrogin-1 and reduced myotube atrophy. Recombinant sTLR4 added to media prevented LPS-induced myotube atrophy. In summary, our study provides the first evidence that sTLR4 has anticatabolic effects by reducing TLR4-mediated signaling and atrophy. In addition, the study reveals a novel finding, by demonstrating that stimulated myotube contractions decrease membrane-bound TLR4 and increase the secretion of sTLR4 by myotubes.NEW & NOTEWORTHY Excessive Toll-like receptor 4 (TLR4) activation causes muscle atrophy. Muscle contractions can limit TLR4 activation on immune cells, but its impact on TLR4 expressed on skeletal muscle cells remains unclear. Here, we demonstrate in C2C12 myotubes for the first time that stimulated myotube contractions reduce membrane-bound TLR4 and increase soluble TLR4, preventing TLR4-mediated signaling and myotube atrophy. Further analyses revealed soluble TLR4 independently prevents myotube atrophy, supporting a potential therapeutic role in combating TLR4-mediated atrophy.

Effect of heat stress on heat shock protein expression and hypertrophy-related signaling in the skeletal muscle of trained individuals.

Muscle mass is balanced between hypertrophy and atrophy by cellular processes, including activation of the protein kinase B-mechanistic target of rapamycin (Akt-mTOR) signaling cascade. Stressors apart from exercise and nutrition, such as heat stress, can stimulate the heat shock protein A (HSPA) and C (HSPC) families alongside hypertrophic signaling factors and muscle growth. The effects of heat stress on HSP expression and Akt-mTOR activation in human skeletal muscle and their magnitude of activation compared with known hypertrophic stimuli are unclear. Here, we show a single session of whole body heat stress following resistance exercise increases the expression of HSPA and activation of the Akt-mTOR cascade in skeletal muscle compared with resistance exercise in a healthy, resistance-trained population. Heat stress alone may also exert similar effects, though the responses are notably variable and require further investigation. In addition, acute heat stress in C2C12 muscle cells enhanced myotube growth and myogenic fusion, albeit to a lesser degree than growth factor-mediated hypertrophy. Though the mechanisms by which heat stress stimulates hypertrophy-related signaling and the potential mechanistic role of HSPs remain unclear, these findings provide additional evidence implicating heat stress as a novel growth stimulus when combined with resistance exercise in human skeletal muscle and alone in isolated murine muscle cells. We believe these findings will help drive further applied and mechanistic investigation into how heat stress influences muscular hypertrophy and atrophy.NEW & NOTEWORTHY We show that acute resistance exercise followed by whole body heat stress increases the expression of HSPA and increases activation of the Akt-mTOR cascade in a physically active and resistance-trained population.

Exercise in hypobaric hypoxia increases markers of intestinal injury and symptoms of gastrointestinal distress.

NEW FINDINGS: What is the central question of this study? What is the effect of hypobaric hypoxia on markers of exercise-induced intestinal injury and symptoms of gastrointestinal (GI) distress? What is the main finding and its importance? Exercise performed at 4300 m of simulated altitude increased intestinal fatty acid binding protein (I-FABP), claudin-3 (CLDN-3) and lipopolysaccharide binding protein (LBP), which together suggest that exercise-induced intestinal injury may be aggravated by concurrent hypoxic exposure. Increases in I-FABP, LBP and CLDN-3 were correlated to exercise-induced GI symptoms, providing some evidence of a link between intestinal barrier injury and symptoms of GI distress. ABSTRACT: We sought to determine the effect of exercise in hypobaric hypoxia on markers of intestinal injury and gastrointestinal (GI) symptoms. Using a randomized and counterbalanced design, nine males completed two experimental trials: one at local altitude of 1585 m (NORM) and one at 4300 m of simulated hypobaric hypoxia (HYP). Participants performed 60 min of cycling at a workload that elicited 65% of their NORM V̇O2max${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{max}}}}$ . GI symptoms were assessed before and every 15 min during exercise. Pre- and post-exercise blood samples were assessed for intestinal fatty acid binding protein (I-FABP), claudin-3 (CLDN-3) and lipopolysaccharide binding protein (LBP). All participants reported at least one GI symptom in HYP compared to just one participant in NORM. I-FABP significantly increased from pre- to post-exercise in HYP (708 ± 191 to 1215 ± 518 pg ml-1 ; P = 0.011, d = 1.10) but not NORM (759 ± 224 to 828 ± 288 pg ml-1 ; P > 0.99, d = 0.27). CLDN-3 significantly increased from pre- to post-exercise in HYP (13.8 ± 0.9 to 15.3 ± 1.2 ng ml-1 ; P = 0.003, d = 1.19) but not NORM (13.7 ± 1.8 to 14.2 ± 1.6 ng ml-1 ; P = 0.435, d = 0.45). LBP significantly increased from pre- to post-exercise in HYP (10.8 ± 1.2 to 13.9 ± 2.8 µg ml-1 ; P = 0.006, d = 1.12) but not NORM (11.3 ± 1.1 to 11.7 ± 0.9 µg ml-1 ; P > 0.99, d = 0.32). I-FABP (d = 0.85), CLDN-3 (d = 0.95) and LBP (d = 0.69) were all significantly higher post-exercise in HYP compared to NORM (P ≤ 0.05). Overall GI discomfort was significantly correlated to ΔI-FABP (r = 0.71), ΔCLDN-3 (r = 0.70) and ΔLBP (r = 0.86). These data indicate that cycling exercise performed in hypobaric hypoxia can cause intestinal injury, which might cause some commonly reported GI symptoms.

Effect of Cardiorespiratory Fitness on Verifying VO2max in Middle-aged and Older Adults.

We investigated the effect of cardiorespiratory fitness (CRF) on the probability of achieving the verification criterion to confirm that VO2max was obtained in a sample of middle-aged to older adults. Data from twelve men and nine women (60.7±8.5 years, VO2max: 34.8±9.4 mL/kg/min) were used for analysis. Participants had their VO2max measured via a maximal graded exercise test and confirmed using a verification bout on a cycle ergometer. Logistic regression was used to evaluate the effect of CRF (VO2max) on the probability a participant would successfully achieve the verification criterion. Odds ratios are reported to quantify the effect size. No statistically significant relationship was observed between CRF and achieving the verification criterion (ß=.081, SE=.0619, Wald=1.420, p=.156). Estimated odds ratio for the effect of CRF on the verification criterion indicated an increase of 8% [Exp(ß)=1.08, 95% CI (0.96, 1.22)] in the probability of achieving the verification criterion given a one unit increase in VO2max. Each 1 mL/kg/min increase in VO2max results in an 8% increase in the chance that an individual achieves the verification criterion confirming that VO2max was obtained. Therefore, CRF is likely of practical significance and should be considered when deciding to use a verification trial.

Heat acclimation during low-intensity exercise increases V̇O2max and Hsp72, but not markers of mitochondrial biogenesis and oxidative phosphorylation, in skeletal tissue.

NEW FINDINGS: What is the central question of this study? Heat acclimation increases tolerance to exercise performed in the heat and may improve maximal oxygen uptake (VO2 max) and performance in temperate environments. However, it is unknown if HA affects the expression of proteins related to mitochondrial biogenesis and oxidative capacity in skeletal muscle. What is the main finding and its importance? We showed that heat acclimation increased VO2 max in a temperate environment but did not change markers of mitochondrial biogenesis and oxidative phosphorylation in the skeletal muscle. ABSTRACT: Heat acclimation (HA) increases tolerance to exercise performed in the heat and may improve maximal oxygen uptake ( V̇O2max ) in temperate environments. However, it is unknown if HA affects the expression of proteins related to mitochondrial biogenesis and oxidative capacity in skeletal muscle. The purpose of this study was to investigate the effect of HA on skeletal muscle markers of mitochondrial biogenesis and oxidative phosphorylation in recreationally trained adults. Thirteen (7 males and 6 females) individuals underwent 10 days of HA. Participants performed two 45 min bouts of exercise (walking at 30-40% maximal velocity at 3% grade) with 10 min rest per session in a hot environment (∼42°C and 30-50% relative humidity). V̇O2max , ventilatory thresholds (VT), and protein expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), mitochondrial transcription factor A (TFAM), calcium/calmodulin-dependent protein kinase (CaMK), electron transport chain (ETC) complexes I-IV, and heat shock protein 72 (Hsp72) in skeletal muscle were measured pre- and post-HA. Comparing day 1 to day 10, HA was confirmed by lower resting core temperature (Tcore ) (P = 0.026), final Tcore (P < 0.0001), mean heart rate (HR) (P = 0.002), final HR (P = 0.003), mean ratings of perceived exertion (RPE) (P = 0.026) and final RPE (P = 0.028). Pre- to post-HA V̇O2max (P = 0.045) increased but VT1 (P = 0.263) and VT2 (P = 0.239) were unchanged. Hsp72 (P = 0.007) increased, but skeletal muscle protein expression (PGC-1α, P = 0.119; TFAM, P = 0.763; CaMK, P = 0.19; ETC I, P = 0.629; ETC II, P = 0.724; ETC III, P = 0.206; ETC IV, P = 0.496) were not affected with HA. HA during low-intensity exercise increased V̇O2max in a temperate environment and Hsp72 but it did not affect markers of mitochondrial biogenesis and oxidative phosphorylation in the skeletal muscle.

Metabolic effects of two high-intensity circuit training protocols: Does sequence matter?

BACKGROUND/OBJECTIVE: The integration of high-intensity interval training (HIIT) and circuit weight training (CWT) is seamless and practical for meeting recommended exercise guidelines. The purpose of this study was to determine the ideal combination of HIIT and CWT to elicit desired acute cardiorespiratory and metabolic responses in variables such as energy expenditure (EE), oxygen consumption (VO2), heart rate (HR), blood lactate (BLa-), excess post-exercise oxygen consumption (EPOC), rating of perceived exertion (RPE), and enjoyment. METHODS: Fourteen trained males (25.7 ±â€¯4.4 yr) completed two exercise protocols matched for volume and recovery periods. On one day, participants performed six HIIT bouts prior to three rounds of a nine exercise CWT protocol (HIC). The second day (separated by ≥ 72 h) consisted of three rounds of three mini-circuits (three exercises per circuit) integrated with three HIIT bouts between the first and second and second and third mini-circuits (TRI). VO2, HR, and EE were monitored throughout both protocols. EPOC for a 20-min duration, [BLa-] (five time points), RPE, and enjoyment were measured post-exercise. RESULTS: Energy expenditure was significantly higher during the HIC compared to the TRI protocol (p = .012), as well as EPOC (p = .034). [BLa-] was significantly greater immediate-, 5min-, 10min- and 20min-post-exercise following HIC as compared to TRI. Mean values for HIC and TRI were similar (p > .05) for HR and RPE. CONCLUSION: Performing HIIT prior to CWT elicits a higher metabolic perturbation compared to the TRI protocol. Although a significant EE difference was detected between the two trials, the practical difference (∼20 kcal) between protocols indicates both protocols are similarly effective for caloric expenditure, metabolic and cardiorespiratory response.

Autophagy in peripheral blood mononuclear cells is associated with body fat percentage.

CONTEXT: Numerous chronic conditions including obesity exhibit autophagic dysfunction. Association of immune cell autophagic marker regulation by body fat percentage (%BF) is unknown. OBJECTIVE: Investigate autophagy activity in peripheral blood mononuclear cells (PBMCs) of adults with distinct %BFs and obesity-related circulating inflammatory markers. MATERIALS AND METHODS: Sixteen individuals (eight males) with %BF above (n = 8, 36.9 ± 3.6 years, 27.1 ± 8.1%BF) and below (n = 8, 37.1 ± 3.7 years, 13.3 ± 3.7%BF) their age- and sex-specific 50th percentile value based on the American College Sports Medicine guidelines participated. Body fat percentage was calculated from hydrostatic weighing. PBMCs were isolated from venous blood, and PBMC autophagic flux markers (LC3-I, LC3-II, and p62) were measured via Western blot. CRP, resistin, leptin, and adiponectin were measured via ELISA. RESULTS: LC3-II/LC3-I ratio correlated with %BF (r=-0.56, p=.023). Insulin (p=.05) and CRP (p=.018) were higher in high %BF participants. DISCUSSION AND CONCLUSIONS: Autophagic activity markers in PBMCs correlate with %BF, but are not different between %BF groups.

A comprehensive evaluation of heat stress and heat strain in a sample of sugarcane cutters in Brazil.

Sugarcane cutters are vulnerable to extreme heat and are at risk for heat-related illness and chronic kidney disease, potentially due to high heat strain. We performed a comprehensive assessment of the physiological demands of sugarcane cutters via measurements of metabolic, thermal, and cardiovascular responses. In addition, we assessed cross-shift changes in markers of kidney function. Nine male sugarcane cutters were monitored while working during the spring harvest season in Brazil. Core temperature (Tcore) and heart rate (HR) were continuously recorded, and oxygen consumption was measured during the work shift. Urine and blood samples were collected pre- and postwork shifts. Total sweat loss was calculated using body weight changes and adjusting for water ingestion and urine output. A wet-bulb globe temperature (WBGT) station was used to monitor environmental heat stress. WBGT was ≥30°C on 7 of the 8 study days. Mean and peak Tcore during the work shift were 37.96 ± 0.47°C and 38.60 ± 0.41°C, respectively, with all participants surpassing a Tcore of 38°C. Mean and peak HR during the work shift were 137 ± 14 and 164 ± 11 beats/min, respectively. Percent of maximal oxygen consumption was, on average, 53 ± 11%. Workers had a total sweat loss of 7.63 ± 2.31 L and ingested 6.04 ± 1.95 L of fluid. Kidney function (estimated glomerular filtration rate) was reduced from pre- to postwork shift (Δ -20 ± 18 mL·min·1.73 m2). We demonstrated that sugarcane cutters performing prolonged work during a period of high environmental heat stress display high levels of heat strain, high water turnover, and reduced kidney function.NEW & NOTEWORTHY We demonstrate that a shift of sugarcane cutting performed outdoors during the spring harvest season results in a high level of heat strain. In fact, all the studied workers sustained core temperatures above 38°C and heart rates above 75% of the measured maximum heart rate. Additionally, workers displayed a high water turnover with sweat loss close to 10% of their body weight. Finally, we report elevated muscle damage and reductions in kidney function following the work shift.

Canagliflozin Prevents Hyperglycemia-Associated Muscle Extracellular Matrix Accumulation and Improves the Adaptive Response to Aerobic Exercise.

Chronic hyperglycemia is associated with low response to aerobic exercise training in rodent models and humans, including reduced aerobic exercise capacity and impaired oxidative remodeling in skeletal muscle. Here, we investigated whether glucose lowering with the sodium-glucose cotransporter 2 inhibitor (SGLT2i), canagliflozin (Cana; 30 mg/kg/day), could restore exercise training response in a model of hyperglycemia (low-dose streptozotocin [STZ]). Cana effectively prevented increased blood glucose in STZ-treated mice. After 6 weeks of voluntary wheel running, Cana-treated mice displayed improvements in aerobic exercise capacity, higher capillary density in striated muscle, and a more oxidative fiber-type in skeletal muscle. In contrast, these responses were blunted or absent in STZ-treated mice. Recent work implicates glucose-induced accumulation of skeletal muscle extracellular matrix (ECM) and hyperactivation of c-Jun N-terminal kinase (JNK)/SMAD2 mechanical signaling as potential mechanisms underlying poor exercise response. In line with this, muscle ECM accretion was prevented by Cana in STZ-treated mice. JNK/SMAD2 signaling with acute exercise was twofold higher in STZ compared with control but was normalized by Cana. In human participants, ECM accumulation was associated with increased JNK signaling, low VO2peak, and impaired metabolic health (oral glucose tolerance test-derived insulin sensitivity). These data demonstrate that hyperglycemia-associated impairments in exercise adaptation can be ameliorated by cotherapy with SGLT2i.

Body fat percentage is independently and inversely associated with serum antibody responses to SARS-CoV-2 mRNA vaccines.

Vaccination is widely considered the most effective preventative strategy to protect against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection. An individual's exercise habits, and physical fitness have been shown to impact the immune response following vaccination using traditional vaccine platforms, but their effects are not well characterized following administration of newer vaccination technology (mRNA vaccines). We investigated these effects on the magnitude of antibody responses following SARS-CoV-2 mRNA vaccination while accounting for known covariates (age, sex, time since vaccination, and the type of vaccine administered). Adults of varying fitness levels (18-65 years; N = 50) who had received either the Moderna or Pfizer SARS-CoV-2 mRNA vaccine between 2 weeks and 6 months prior, completed health history and physical activity questionnaires, had their blood drawn, body composition, cardiorespiratory fitness, and strength assessed. Multiple linear regressions assessed the effect of percent body fat, hand grip strength, cardiorespiratory fitness, and physical activity levels on the magnitude of receptor binding domain protein (RBD) and spike protein subunit 1 (S1) and 2 (S2) while accounting for known covariates. Body fat percentage was inversely associated with the magnitude of S1 (p = 0.006, ß = - 366.56), RBD (p = 0.003, ß = - 249.30), and S2 (p = 0.106, ß = - 190.08) antibodies present in the serum following SARS-CoV-2 mRNA vaccination. Given the increasing number of infections, variants, and the known waning effects of vaccination, future mRNA vaccinations such as boosters are encouraged to sustain immunity; reducing excess body fat may improve the efficacy of these vaccinations.

The Effect of Acute Glutamine Supplementation on Markers of Inflammation and Fatigue During Consecutive Days of Simulated Wildland Firefighting.

OBJECTIVE: To examine the effect of oral glutamine supplementation on inflammation and fatigue during and after simulated wildland firefighting (WLFF) tasks in hot conditions over 2 consecutive days. METHODS: Eleven men and women ingested a glutamine supplement or a placebo before and after simulated wildland firefighting in an environmental chamber (38 °C, 35% relative humidity). Subjective fatigue, markers of inflammation, and cellular stress were measured pre, post and 4 hours post-exercise on both days. RESULTS: Gastrointestinal damage, subjective fatigue, and ratings of perceived exertion were lower after glutamine supplementation compared with placebo. Heat shock protein 70 (HSP70) and nuclear factor kappa-inhibitor alpha (IκBα) levels were higher on both days of the glutamine trial compared with placebo. CONCLUSIONS: Glutamine supplementation may improve recovery after fire suppression in WLFFs. This may result from the upregulation of HSP70 which inhibits inflammation and protects against gastrointestinal (GI) barrier damage.