Physiological assessment of a 16 day, 4385 km ultra-endurance mountain bike race: A case study.

The Tour Divide (TD) is a 4385 km ultra-endurance bicycle race that follows the continental divide from Canada to Mexico. In this case study, we performed a comprehensive molecular and physiological profile before and after the completion of the TD. Assessments were performed 35 days before the start (Pre-TD) and ∼36 h after the finish (Post-TD). Total energy expenditure was assessed during the first 9 days by doubly labelled water (2 H2 18 O), abdominal and leg tissue volumes via MRI, and graded exercise tests to quantify fitness and substrate preference. Vastus lateralis muscle biopsies were taken to measure mitochondrial function via respirometry, and vascular function was assessed using Doppler ultrasound. The 47-year-old male subject took 16 days 7 h 45 min to complete the route. He rode an average of 16.8 h/day. Neither maximal O2 uptake nor maximal power output changed pre- to post-TD. Measurement of total energy expenditure and dietary recall records suggested maintenance of energy balance, which was supported by the lack of change in body weight. The subject lost both appendicular and trunk fat mass and gained leg lean mass pre- to post-TD. Skeletal muscle mitochondrial and vascular endothelial function decreased pre- to post-TD. Overall, exercise performance was maintained despite reductions in muscle mitochondrial and vascular endothelial function post-TD, suggesting a metabolic reserve in our highly trained athlete.

Passive heat stress induces mitochondrial adaptations in skeletal muscle.

The mitochondria are central to skeletal muscle metabolic health. Impaired mitochondrial function is associated with various muscle pathologies, including insulin resistance and muscle atrophy. As a result, continuous efforts are made to find ways to improve mitochondrial health in the context of disuse and disease. While exercise is known to cause robust improvements in mitochondrial health, not all individuals are able to exercise. This creates a need for alternate interventions which elicit some of the same benefits as exercise. Passive heating (i.e., application of heat in the absence of muscle contractions) is one potential intervention which has been shown to increase mitochondrial enzyme content and activity, and to improve mitochondrial respiration. Associated with increases in mitochondrial content and/or function, passive heating can also improve insulin sensitivity in the context of type II diabetes and preserve muscle mass in the face of limb disuse. This area of research remains in its infancy, with many questions yet to be answered about how to maximize the benefits of passive heating and elucidate the mechanisms by which heat stress affects muscle mitochondria.

Body shape perception in men and women without obesity during caloric restriction: a secondary analysis from the CALERIE study.

OBJECTIVE: To examine body shape perception in 218 adults without obesity or history of eating disorders during caloric restriction (CR). METHODS: Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE) is a 2-year, randomized clinical trial using a 2:1 assignment (CR, 25% reduction in calories; Control, typical diet). For this secondary analysis, we examined perceived body shape using the Body Shape Questionnaire (BSQ). Analyses of BSQ scores are reported by group, over time, by sex, and by BMI. Data for body fat percentage, symptoms of depression, food cravings, maximal oxygen consumption, and stress were analyzed for their association with BSQ scores. RESULTS: Compared to control, CR reduced BSQ scores. Women tended to have greater concern with body shape than men across all measurement times. There was no difference in change in BSQ scores at 12 or 24 months between those with a BMI < 25 kg/m2 or ≥ 25 kg/m2. Change in body fat percentage was most correlated with change in BSQ score from 0 to 12 (r = 0.39) and 0-24 months (r = 0.38). For change in BSQ score, Akaike/ Bayesian information criterion (AIC/BIC) found that the model of best fit included the following three change predictors: change in body fat percentage, depression symptoms, and food cravings. For 0-12 months, AIC/BIC = 1482.0/1505.6 and for 0-24 months AIC/BIC = 1364.8/1386.5. CONCLUSIONS: CR is associated with reduced concern for body shape in men and women without obesity and with no history of eating disorders. Body shape perception among this sample was complex and influenced by multiple factors. LEVEL OF EVIDENCE: Level I, randomized controlled trial.

Localized Heat Therapy Improves Mitochondrial Respiratory Capacity but Not Fatty Acid Oxidation.

AIM: Mild heat stress can improve mitochondrial respiratory capacity in skeletal muscle. However, long-term heat interventions are scarce, and the effects of heat therapy need to be understood in the context of the adaptations which follow the more complex combination of stimuli from exercise training. The purpose of this work was to compare the effects of 6 weeks of localized heat therapy on human skeletal muscle mitochondria to single-leg interval training. METHODS: Thirty-five subjects were assigned to receive sham therapy, short-wave diathermy heat therapy, or single-leg interval exercise training, localized to the quadriceps muscles of the right leg. All interventions took place 3 times per week. Muscle biopsies were performed at baseline, and after 3 and 6 weeks of intervention. Mitochondrial respiratory capacity was assessed on permeabilized muscle fibers via high-resolution respirometry. RESULTS: The primary finding of this work was that heat therapy and exercise training significantly improved mitochondrial respiratory capacity by 24.8 ± 6.2% and 27.9 ± 8.7%, respectively (p < 0.05). Fatty acid oxidation and citrate synthase activity were also increased following exercise training by 29.5 ± 6.8% and 19.0 ± 7.4%, respectively (p < 0.05). However, contrary to our hypothesis, heat therapy did not increase fatty acid oxidation or citrate synthase activity. CONCLUSION: Six weeks of muscle-localized heat therapy significantly improves mitochondrial respiratory capacity, comparable to exercise training. However, unlike exercise, heat does not improve fatty acid oxidation capacity.

Exercise, but Not Metformin Prevents Loss of Muscle Function Due to Doxorubicin in Mice Using an In Situ Method.

Though effective in treating various types of cancer, the chemotherapeutic doxorubicin (DOX) is associated with skeletal muscle wasting and fatigue. The purpose of this study was to assess muscle function in situ following DOX administration in mice. Furthermore, pre-treatments with exercise (EX) or metformin (MET) were used in an attempt to preserve muscle function following DOX. Mice were assigned to the following groups: control, DOX, DOX + EX, or DOX + MET, and were given a single injection of DOX (15 mg/kg) or saline 3 days prior to sacrifice. Preceding the DOX injection, DOX + EX mice performed 60 min/day of running for 5 days, while DOX + MET mice received 5 daily oral doses of 500 mg/kg MET. Gastrocnemius-plantaris-soleus complex function was assessed in situ via direct stimulation of the sciatic nerve. DOX treatment increased time to half-relaxation following contractions, indicating impaired recovery (p < 0.05). Interestingly, EX prevented any increase in half-relaxation time, while MET did not. An impaired relaxation rate was associated with a reduction in SERCA1 protein content (p = 0.07) and AMPK phosphorylation (p < 0.05). There were no differences between groups in force production or mitochondrial respiration. These results suggest that EX, but not MET may be an effective strategy for the prevention of muscle fatigue following DOX administration in mice.

Multitissue analysis of exercise and metformin on doxorubicin-induced iron dysregulation.

Doxorubicin (DOX) is an effective chemotherapeutic treatment with lasting side effects in heart and skeletal muscle. DOX is known to bind with iron, contributing to oxidative damage resulting in cardiac and skeletal muscle toxicity. However, major cellular changes to iron regulation in response to DOX are poorly understood in liver, heart, and skeletal muscle. Additionally, two cotreatments, exercise (EX) and metformin (MET), were studied for their effectiveness in reducing DOX toxicity by ameliorating iron dysregulation and preventing oxidative stress. The purposes of this study were to 1) characterize the DOX-induced changes of the major iron regulation pathway in liver, heart, and skeletal muscle and 2) to determine whether EX and MET exert their benefits by minimizing DOX-induced iron dysregulation. Mice were assigned to receive saline or DOX (15 mg/kg) treatments, paired with either EX (5 days) or MET (500 mg/kg), and were euthanized 3 days after DOX treatment. Results suggest that the cellular response to DOX is protective against oxidative stress by reducing iron availability. DOX increased iron storage capacity through elevated ferritin levels in liver, heart, and skeletal muscle. DOX reduced iron transport capacity through reduced transferrin receptor levels in heart and skeletal muscle. EX and MET cotreatments had protective effects in the liver through reduced transferrin receptor levels. At 3 days after DOX, oxidative stress was mild, as shown by normal glutathione and lipid peroxidation levels. Together these results suggest that the cellular response to reduce iron availability in response to DOX treatment is sufficient to match oxidative stress.

AMPK and PPARß positive feedback loop regulates endurance exercise training-mediated GLUT4 expression in skeletal muscle.

The objective of this study is to determine whether AMP-activated protein kinase (AMPK), peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α), or peroxisome proliferator-activated receptor ß (PPARß) can independently mediate the increase of glucose transporter type 4 (GLUT4) expression that occurs in response to exercise training. We found that PPARß can regulate GLUT4 expression without PGC-1α. We also found AMPK and PPARß are important for maintaining normal physiological levels of GLUT4 protein in the sedentary condition as well following exercise training. However, AMPK and PPARß are not essential for the increase in GLUT4 protein expression that occurs in response to exercise training. We discovered that AMPK activation increases PPARß via myocyte enhancer factor 2A (MEF2A), which acted as a transcription factor for PPARß. Furthermore, exercise training increases the cooperation of AMPK and PPARß to regulate glucose uptake. In conclusion, cooperation between AMPK and PPARß via NRF-1/MEF2A pathway enhances the exercise training mediated adaptive increase in GLUT4 expression and subsequent glucose uptake in skeletal muscle.

Effects of curcumin and ursolic acid on the mitochondrial coupling efficiency and hydrogen peroxide emission of intact skeletal myoblasts.

Curcumin may improve blood glucose management, but the mechanism is not fully established. We demonstrated that curcumin (40 µM) reduced the mitochondrial coupling efficiency (percentage of oxygen consumption coupled to ATP synthesis) of intact skeletal muscle cells. A 30-minute pretreatment with curcumin reduced mitochondrial coupling efficiency by 17.0 ± 0.4% relative to vehicle (p < 0.008). Curcumin pretreatment also decreased the rate of hydrogen peroxide emission by 43 ± 13% compared to vehicle (p < 0.05). Analysis of cell respiration in the presence of curcumin revealed a 40 ± 4% increase in the rate of oxygen consumption upon curcumin administration (p < 0.05 compared to vehicle). No difference in mitochondrial coupling efficiency was observed between vehicle- and curcumin-pretreated cells after permeabilization of cell membranes (p > 0.7). The interaction between curcumin and ursolic acid, another natural compound that may improve blood glucose management, was also examined. Pretreatment with ursolic acid (0.12 µM) increased the mitochondrial coupling efficiency of intact cells by 4.1 ± 1.1% relative to vehicle (p < 0.008) and attenuated the effect of curcumin when the two compounds were used in combination. The observed changes to mitochondrial coupling efficiency and hydrogen peroxide emission were consistent with the established effects of curcumin on blood glucose control. Our findings also show that changes to mitochondrial coupling efficiency after curcumin pretreatment may go undetected unless cells are assessed in the intact condition.

ß-Cell deletion of Nr4a1 and Nr4a3 nuclear receptors impedes mitochondrial respiration and insulin secretion.

ß-Cell insulin secretion is dependent on proper mitochondrial function. Various studies have clearly shown that the Nr4a family of orphan nuclear receptors is essential for fuel utilization and mitochondrial function in liver, muscle, and adipose. Previously, we have demonstrated that overexpression of Nr4a1 or Nr4a3 is sufficient to induce proliferation of pancreatic ß-cells. In this study, we examined whether Nr4a expression impacts pancreatic ß-cell mitochondrial function. Here, we show that ß-cell mitochondrial respiration is dependent on the nuclear receptors Nr4a1 and Nr4a3. Mitochondrial respiration in permeabilized cells was significantly decreased in ß-cells lacking Nr4a1 or Nr4a3. Furthermore, respiration rates of intact cells deficient for Nr4a1 or Nr4a3 in the presence of 16 mM glucose resulted in decreased glucose mediated oxygen consumption. Consistent with this reduction in respiration, a significant decrease in glucose-stimulated insulin secretion rates is observed with deletion of Nr4a1 or Nr4a3. Interestingly, the changes in respiration and insulin secretion occur without a reduction in mitochondrial content, suggesting decreased mitochondrial function. We establish that knockdown of Nr4a1 and Nr4a3 results in decreased expression of the mitochondrial dehydrogenase subunits Idh3g and Sdhb. We demonstrate that loss of Nr4a1 and Nr4a3 impedes production of ATP and ultimately inhibits glucose-stimulated insulin secretion. These data demonstrate for the first time that the orphan nuclear receptors Nr4a1 and Nr4a3 are critical for ß-cell mitochondrial function and insulin secretion.

Does calorie restriction induce mitochondrial biogenesis? A reevaluation.

It has been reported that 30% calorie restriction (CR) for 3 mo results in large increases in mitochondrial biogenesis in heart, brain, liver, and adipose tissue, with concomitant increases in respiration and ATP synthesis. We found these results surprising, and performed this study to determine whether 30% CR does induce an increase in mitochondria in heart, brain, liver, adipose tissue, and/or skeletal muscle. To this end, we measured the levels of a range of mitochondrial proteins, and mRNAs. With the exception of long-chain acyl-CoA dehydrogenase protein level, which was increased ∼60% in adipose tissue, none of the mitochondrial proteins or mRNAs that we measured were increased in rats subjected to 30% CR for 14 wk. There was also no increase in citrate synthase activity. Because it is not possible to have an increase in mitochondria without any increase in key mitochondrial proteins, we conclude that 30% CR does not induce an increase in mitochondria in heart, brain, liver, adipose tissue, or skeletal muscle in laboratory rodents.

Soy content of basal diets determines the effects of supplemental selenium in male mice.

The effects of supplemental Se in rodent models may depend upon composition of the basal diet to which it is added. Wild-type male littermates of Transgenic Adenocarcinoma of Mouse Prostate mice were fed until 18 wk of age 1 of 2 Se-adequate stock diets high in soy (HS) or low in phytoestrogens (LP) or the same diets supplemented with 3.0 mg Se/kg diet as seleno-methylselenocysteine. Body and abdominal fat pad weights were lower (P < 0.01) in mice fed the HS diet. Supplemental Se reduced fat pad weights in mice receiving the LP diet but increased body and fat pad weights in mice consuming the HS formulation (P-interaction < 0.005). Serum free triiodothyronine concentrations were unaffected by supplemental Se in mice fed the LP diet but were decreased by Se supplementation of mice given the HS feed (P-interaction < 0.02). Free thyroxine concentrations were higher in mice consuming the HS diet regardless of Se intake (P < 0.001). Hepatic mRNA for iodothyronine deiodinase I was lower (P < 0.001) in mice fed the HS diet. Supplementation of Se increased this mRNA (P < 0.001) in both diet groups. Results from this study show a significant interaction between the composition of basal diets and the effects of supplemental Se with respect to body composition. These findings have important implications for future studies in rodent models of the effects of supplemental Se on heart disease, cancer, diabetes, and other conditions related to body weight and composition.

High-fat diets cause insulin resistance despite an increase in muscle mitochondria.

It has been hypothesized that insulin resistance is mediated by a deficiency of mitochondria in skeletal muscle. In keeping with this hypothesis, high-fat diets that cause insulin resistance have been reported to result in a decrease in muscle mitochondria. In contrast, we found that feeding rats high-fat diets that cause muscle insulin resistance results in a concomitant gradual increase in muscle mitochondria. This adaptation appears to be mediated by activation of peroxisome proliferator-activated receptor (PPAR)delta by fatty acids, which results in a gradual, posttranscriptionally regulated increase in PPAR gamma coactivator 1alpha (PGC-1alpha) protein expression. Similarly, overexpression of PPARdelta results in a large increase in PGC-1alpha protein in the absence of any increase in PGC-1alpha mRNA. We interpret our findings as evidence that raising free fatty acids results in an increase in mitochondria by activating PPARdelta, which mediates a posttranscriptional increase in PGC-1alpha. Our findings argue against the concept that insulin resistance is mediated by a deficiency of muscle mitochondria.

Skeletal Muscle Mitochondrial Function after a 100-km Ultramarathon: A Case Study in Monozygotic Twins.

PURPOSE: Very little research has investigated the effects of ultraendurance exercise on the bioenergetic status of muscle. The primary objective of this case study was to characterize the changes that occur in skeletal muscle mitochondria in response to a 100-km ultramarathon in monozygotic twins. A second objective was to determine whether mitochondrial function is altered by consuming a periodized low-carbohydrate, high-fat diet during training compared with a high-carbohydrate diet. METHODS: One pair of male monozygotic twins ran 100 km on treadmills after 4 wk of training on either a high-carbohydrate or periodized low-carbohydrate, high-fat diet. Muscle biopsies were collected 4 wk before the run, as well as 4 and 52 h postrun. Blood draws were also performed immediately before as well as 4 and 52 h after the run. RESULTS: Four hours postrun, respiratory capacity, citrate synthase activity, and mitochondrial complex protein content were decreased. Two days later, both twins showed signs of rapid recovery in several of these measures. Furthermore, blood levels of creatine phosphokinase, C-reactive protein, and aspartate transaminase were elevated 4 h after the run but partially recovered 2 d later. CONCLUSION: Although there were some differences between the twins, the primary finding is that there is significant mitochondrial impairment induced by running 100 km, which rapidly recovers within 2 d. These results provide ample rationale for future investigations of the effects of ultraendurance activity on mitochondrial function.

Valproic acid promotes SOD2 acetylation: a potential mechanism of valproic acid-induced oxidative stress in developing systems.

Valproic acid (VPA) is an antiepileptic, bipolar, and migraine medication, which is associated with embryonic dysmorphology, more specifically neural tube defects (NTDs), if taken while pregnant. One mechanism by which VPA may cause NTDs is through oxidative stress that cause disruption of cell signaling. However, mechanisms of VPA-induced oxidative stress are not fully understood. Since VPA is a deacetylase inhibitor, we propose that VPA promotes mitochondrial superoxide dismutase-2 (SOD2) acetylation, decreasing SOD2 activity and increasing oxidant levels. Using the pluripotent embryonal carcinoma cell line, P19, VPA effects were evaluated in undifferentiated and neurodifferentiated cells. VPA treatments increased oxidant levels, oxidized the glutathione (GSH)/glutathione disulfide (GSSG) redox couple, and decreased total SOD and SOD2 activity in undifferentiated P19 cells but not in differentiated P19 cells. VPA caused a specific increase in mitochondrial oxidants in undifferentiated P19 cells, VPA did not alter respirometry measurements. Immunoblot analyses demonstrated that VPA increased acetylation of SOD2 at lysine68 (AcK68 SOD2) in undifferentiated P19 cells but not in differentiated P19 cells. Pretreatments with the Nrf2 inducer, dithiol-3-thione (D3T), in undifferentiated P19 cells prevented increased oxidant levels, GSH/GSSG redox oxidation and restored total SOD and SOD2 activity, correlating with a decrease in AcK68 SOD2 levels. In embryos, VPA decreased total SOD and SOD2 activity and increased levels of AcK68 SOD2, and D3T pretreatments prevented VPA effects, increasing total SOD and SOD2 activity and lowering levels of AcK68 SOD2. These data demonstrate a potential, contributing oxidizing mechanism by which VPA incites teratogenesis in developing systems. Moreover, these data also suggest that Nrf2 interventions may serve as a means to protect developmental signaling and inhibit VPA-induced malformations.

Accumulation of Skeletal Muscle T Cells and the Repeated Bout Effect in Rats.

PURPOSE: The purpose of this investigation was to characterize skeletal muscle T-cell accumulation after contraction-induced muscle damage and test the hypothesis that T cells contribute to postdamage muscle protection (i.e., the repeated bout effect) in a way reminiscent of their role in adaptive immunity. METHODS: In vivo lengthening contractions were used to model the repeated bout effect and contralateral repeated bout effect in rats. Intramuscular T-cell subsets were characterized by flow cytometry after single and repeated bouts of lengthening contractions, and an adoptive T-cell transfer experiment was done to test whether T cells from muscle damage-experienced rats can confer protection from injury to damage-naive rats. RESULTS: Electrically stimulated lengthening contractions elicited the repeated bout effect, but not the contralateral repeated bout effect. Although leukocytes (CD45+) were scarce in undamaged muscle (2.1% of all cells), substantially more (63% of all cells) were observed after a single bout of lengthening contractions. Within the leukocyte population were several subsets of T cells, including conventional CD4+, CD8+, memory, and regulatory T cells. In contrast, a minimal increase in T cells was observed after a second bout of lengthening contractions. Conventional CD4+ T cells (FoxP3-) were the most abundant subset in muscle after lengthening contractions. Adoptive T-cell transfer from damage-experienced rats did not confer protection to damage-naive recipient rats. CONCLUSIONS: The robust T-cell accumulation, particularly the CD4 subset, after contraction-induced damage suggests a role for these cells in muscle repair and adaptation to muscle damaging contractions. Moreover, T cells are unlikely to mediate the protective adaptations of the repeated bout effect in a manner similar to their role in adaptive immunity.

Daily heat treatment maintains mitochondrial function and attenuates atrophy in human skeletal muscle subjected to immobilization.

Skeletal muscle immobilization leads to atrophy, decreased metabolic health, and substantial losses in function. Animal models suggest that heat stress can provide protection against atrophy in skeletal muscle. This study investigated the effects of daily heat therapy on human skeletal muscle subjected to 10 days of immobilization. Muscle biopsies were collected, and MRIs were analyzed from the vastus lateralis of 23 healthy volunteers (11 women, 12 men) before and after either 10 days of immobilization with a daily sham treatment (Imm) or with a targeted, daily 2-h heat treatment using pulsed shortwave diathermy (Imm + H). Diathermy increased intramuscular temperature 4.2 ± 0.29°C (P < 0.0001), with no change during sham treatment. As a result, heat shock protein (HSP)70 and HSP90 increased (P < 0.05) following Imm + H (25 ± 6.6 and 20 ± 7.4%, respectively) but were unaltered with Imm only. Heat treatment prevented the immobilization-induced loss of coupled (-27 ± 5.2% vs. -8 ± 6.0%, P = 0.0041) and uncoupled (-25 ± 7.0% vs. -10 ± 3.9%, P = 0.0302) myofiber respiratory capacity. Likewise, heat treatment prevented the immobilization-induced loss of proteins associated with all five mitochondrial respiratory complexes (P < 0.05). Furthermore, decreases in muscle cross-sectional area following Imm were greater than Imm + H at both the level of the whole muscle (-7.6 ± 0.96% vs. -4.5 ± 1.09%, P = 0.0374) and myofiber (-10.8 ± 1.52% vs. -5.8 ± 1.49%, P = 0.0322). Our findings demonstrate that daily heat treatments, applied during 10 days of immobilization, prevent the loss of mitochondrial function and attenuate atrophy in human skeletal muscle. NEW & NOTEWORTHY Limb immobilization results in substantial decreases in skeletal muscle size, function, and metabolic capacity. To date, there are few, if any, interventions to prevent the deleterious effects of limb immobilization on skeletal muscle health. Heat stress has been shown to elicit a stress response, resulting in increased heat shock protein expression and improved mitochondrial function. We show that during 10 days of lower-limb immobilization in humans, daily exposure to heat stress maintains mitochondrial respiratory capacity and attenuates atrophy in skeletal muscle. Our findings suggest that heat stress may serve as an effective therapeutic strategy to attenuate the decreases of muscle mass and metabolic function that accompany periods of disuse.

High-resolution Respirometry to Measure Mitochondrial Function of Intact Beta Cells in the Presence of Natural Compounds.

High-resolution respirometry allows for the measurement of oxygen consumption of isolated mitochondria, cells and tissues. Beta cells play a critical role in the body by controlling blood glucose levels through insulin secretion in response to elevated glucose concentrations. Insulin secretion is controlled by glucose metabolism and mitochondrial respiration. Therefore, measuring intact beta cell respiration is essential to be able to improve beta cell function as a treatment for diabetes. Using intact 832/13 INS-1 derived beta cells we can measure the effect of increasing glucose concentration on cellular respiration. This protocol allows us to measure beta cell respiration in the presence or absence of various compounds, allowing one to determine the effect of given compounds on intact cell respiration. Here we demonstrate the effect of two naturally occurring compounds, monomeric epicatechin and curcumin, on beta cell respiration under the presence of low (2.5 mM) or high glucose (16.7 mM) conditions. This technique can be used to determine the effect of various compounds on intact beta cell respiration in the presence of differing glucose concentrations.

Repeated exposure to heat stress induces mitochondrial adaptation in human skeletal muscle.

The heat stress response is associated with several beneficial adaptations that promote cell health and survival. Specifically, in vitro and animal investigations suggest that repeated exposures to a mild heat stress (~40°C) elicit positive mitochondrial adaptations in skeletal muscle comparable to those observed with exercise. To assess whether such adaptations translate to human skeletal muscle, we produced local, deep tissue heating of the vastus lateralis via pulsed shortwave diathermy in 20 men and women ( n = 10 men; n = 10 women). Diathermy increased muscle temperature by 3.9°C within 30 min of application. Immediately following a single 2-h heating session, we observed increased phosphorylation of AMP-activated protein kinase and ERK1/2 but not of p38 MAPK or JNK. Following repeated heat exposures (2 h daily for 6 consecutive days), we observed a significant cellular heat stress response, as heat shock protein 70 and 90 increased 45% and 38%, respectively. In addition, peroxisome proliferator-activated receptor gamma, coactivator-1 alpha and mitochondrial electron transport protein complexes I and V expression were increased after heating. These increases were accompanied by augmentation of maximal coupled and uncoupled respiratory capacity, measured via high-resolution respirometry. Our data provide the first evidence that mitochondrial adaptation can be elicited in human skeletal muscle in response to repeated exposures to mild heat stress. NEW & NOTEWORTHY Heat stress has been shown to elicit mitochondrial adaptations in cell culture and animal research. We used pulsed shortwave diathermy to produce deep tissue heating and explore whether beneficial mitochondrial adaptations would translate to human skeletal muscle in vivo. We report, for the first time, positive mitochondrial adaptations in human skeletal muscle following recurrent heat stress. The results of this study have clinical implications for many conditions characterized by diminished skeletal muscle mitochondrial function.

CXCL10 increases in human skeletal muscle following damage but is not necessary for muscle regeneration.

CXCL10 is a chemokine for activated and memory T cells with many important immunological functions. We recently found that CXCL10 is upregulated in human muscle following contraction-induced damage. No information is available on the role of CXCL10 in the context of muscle damage or repair. In this study, we confirm that CXCL10 is elevated in human muscle at 2 and 3 days following damage and perform cell culture and animal studies to examine the role of CXCL10 in muscle repair. CXCL10 did not impact proliferation rates of human primary myoblasts but it did promote myogenic differentiation in vitro, suggesting a possible direct impact on muscle regeneration. To test if CXCL10 was dispensable for effective muscle regeneration in vivo, we measured functional and histological markers of muscle repair out to 14 days postmuscle injury caused by a myotoxin in wild-type (WT) mice and CXCL10 knockout (KO) mice. Between genotypes, no significant differences were found in loss or restoration of in situ muscle force, cross-sectional area of newly formed myofibers, or the number of embryonic myosin heavy chain-positive myofibers. In addition, KO animals were not deficient in T-cell accumulation in the damaged muscle following injury. Gene expression of the other two ligands (CXCL9 and 11) that bind to the same receptor as CXCL10 were also elevated in the damaged muscle of KO mice. Thus, other ligands may have compensated for the lack of CXCL10 in the KO mice. We conclude that CXCL10 is not necessary for effective muscle regeneration.

PPARß Is Essential for Maintaining Normal Levels of PGC-1α and Mitochondria and for the Increase in Muscle Mitochondria Induced by Exercise.

The objective of this study was to evaluate the specific mechanism(s) by which PPARß regulates mitochondrial content in skeletal muscle. We discovered that PPARß increases PGC-1α by protecting it from degradation by binding to PGC-1α and limiting ubiquitination. PPARß also induces an increase in nuclear respiratory factor 1 (NRF-1) expression, resulting in increases in mitochondrial respiratory chain proteins and MEF2A, for which NRF-1 is a transcription factor. There was also an increase in AMP kinase phosphorylation mediated by an NRF-1-induced increase in CAM kinase kinase-ß (CaMKKß). Knockdown of PPARß resulted in large decreases in the levels of PGC-1α and mitochondrial proteins and a marked attenuation of the exercise-induced increase in mitochondrial biogenesis. In conclusion, PPARß induces an increase in PGC-1α protein, and PPARß is a transcription factor for NRF-1. Thus, PPARß plays essential roles in the maintenance and adaptive increase in mitochondrial enzymes in skeletal muscle by exercise.