Effects of acute and chronic endurance exercise on mitochondrial uncoupling in human skeletal muscle

Mitochondrial plasticity in obesity and diabetes mellitus

SIGNIFICANCE

Insulin resistance and its related diseases, obesity and type 2 diabetes mellitus (T2DM), have been linked to changes in aerobic metabolism, pointing to a possible role of mitochondria in the development of insulin resistance.

RECENT ADVANCES

Refined methodology of ex vivo high-resolution respirometry and in vivo magnetic resonance spectroscopy now allows describing several features of mitochondria in humans. In addition to measuring mitochondrial function at baseline and after exercise-induced submaximal energy depletion, the response of mitochondria to endocrine and metabolic challenges, termed mitochondrial plasticity, can be assessed using hyperinsulinemic clamp tests. While insulin resistant states do not uniformly relate to baseline and post-exercise mitochondrial function, mitochondrial plasticity is typically impaired in insulin resistant relatives of T2DM, in overt T2DM and even in type 1 diabetes mellitus (T1DM).

CRITICAL ISSUES

The variability of baseline mitochondrial function in the main target tissue of insulin action, skeletal muscle and liver, may be attributed to inherited and acquired changes in either mitochondrial quantity or quality. In addition to certain gene polymorphisms and aging, circulating glucose and lipid concentrations correlate with both mitochondrial function and plasticity.

FUTURE DIRECTIONS

Despite the associations between features of mitochondrial function and insulin sensitivity, the question of a causal relationship between compromised mitochondrial plasticity and insulin resistance in the development of obesity and T2DM remains to be resolved.

Exercise-induced heart mitochondrial cholesterol depletion influences the inhibition of mitochondrial swelling

The significance of the reduction of the cholesterol pool in heart mitochondria after exercise is still unknown. Recently, published data have suggested that cholesterol may influence the components of mitochondrial contact site and affect mitochondrial swelling. Therefore, the aim of this study was to determine whether the decreased cholesterol content in heart mitochondria caused by prolonged swimming may provoke changes in their bioenergetics and result in an increased resistance to calcium chloride-induced mitochondrial swelling. Male Wistar rats were divided into a sedentary control group and an exercise group. The rats exercised for 3 h, burdened with an additional 3% of their body weight. Their hearts were removed immediately after completing the exercise. The left ventricle was divided and used for experiments. Mitochondrial cholesterol content, membrane fluidity and mitochondrial bioenergetics were measured in the control and exercised rat heart mitochondria. To assess whether mitochondrial modifications are linked to disruption of lipid microdomains, methyl-β-cyclodextrin, a well-known lipid microdomain-disrupting agent and cholesterol chelator, was applied to the mitochondria of the control group. Cholesterol depletion, increased membrane fluidity and increased resistance to calcium chloride-induced swelling were observed in postexercise heart crude mitochondrial fraction. Similar results were achieved in control mitochondria treated with 2% methyl-β-cyclodextrin. All of the mitochondrial bioenergetics parameters were similar between the groups. Therefore, the disruption of raft-like microdomains appears to be an adaptive change in the rat heart following exercise.

Eccentric exercise transiently affects mice skeletal muscle mitochondrial function

Eccentric exercise (EE) is known to induce damage and dysfunction in skeletal muscle. However, the possible role of mitochondrial (dys)function, including the vulnerability to mitochondrial permeability transition pore (MPTP) opening, is unclear. Therefore, this study aimed to analyze the impact of a single acute bout of downhill running on skeletal muscle mitochondrial function. Thirty 12-week-old Charles River CD1 male mice were randomly assigned into control (C) or exercised groups. EE consisted of 120 min of downhill treadmill running at a –16° gradient. Exercised animals were sacrificed immediately (Ecc0h) and 48 h (Ecc48h) after the end of the running bout. Plasma and skeletal muscles were then obtained. Muscle mitochondrial function, including oxygen consumption prior to and after anoxia and reoxygenation, membrane potential, and MPTP opening, were evaluated. Respiratory chain complexI, II, and V activities were determined. EE significantly increased plasma creatine kinase activity (119.4 ± 5.6 vs. 1061.3 ± 46.3 vs. 256.8 ± 15.3 U·L–1, C, Ecc0h and Ecc48h, respectively) and myoglobin and interleukin-6 content. Impaired state 3 and respiratory control ratio (8.4 ± 0.4 vs. 5.6 ± 0.9 vs. 8.4 ± 0.5, C, Ecc0h and Ecc48h, respectively), as well as increased susceptibility to MPTP opening, seen by cyclosporin A-sensitive high swelling amplitude, lower time to maximal swelling velocity (313.8 ± 17.7 vs. 244.5 ± 19.4 vs. 298.5 ± 8.7 s, C, Ecc0h and Ecc48h, respectively), and calcium release immediately after the end of exercise (C vs. Ecc0h) were observed. EE induced a transient impairment in the activity of complex V (C vs. Ecc0h). No significant changes from the C group were observed 48 h after the end of EE (C vs. Ecc48h) in any analyzed parameters. In conclusion, prolonged EE transiently impaired mice skeletal muscle mitochondrial function and increased susceptibility to calcium-induced MPTP opening.

Effect of eccentric training on mitochondrial function and oxidative stress in the skeletal muscle of rats

The objective of the present study was to investigate the effects of eccentric training on the activity of mitochondrial respiratory chain enzymes, oxidative stress, muscle damage, and inflammation of skeletal muscle. Eighteen male mice (CF1) weighing 30-35 g were randomly divided into 3 groups (N = 6): untrained, trained eccentric running (16°; TER), and trained running (0°) (TR), and were submitted to an 8-week training program. TER increased muscle oxidative capacity (succinate dehydrogenase and complexes I and II) in a manner similar to TR, and TER did not decrease oxidative damage (xylenol and creatine phosphate) but increased antioxidant enzyme activity (superoxide dismutase and catalase) similar to TR. Muscle damage (creatine kinase) and inflammation (myeloperoxidase) were not reduced by TER. In conclusion, we suggest that TER improves mitochondrial function but does not reduce oxidative stress, muscle damage, or inflammation induced by eccentric contractions.

Higher mitochondrial respiration and uncoupling with reduced electron transport chain content in vivo in muscle of sedentary versus active subjects

OBJECTIVE

This study investigated the disparity between muscle metabolic rate and mitochondrial metabolism in human muscle of sedentary vs. active individuals.

RESEARCH DESIGN AND METHODS

Chronic activity level was characterized by a physical activity questionnaire and a triaxial accelerometer as well as a maximal oxygen uptake test. The ATP and O(2) fluxes and mitochondrial coupling (ATP/O(2) or P/O) in resting muscle as well as mitochondrial capacity (ATP(max)) were determined in vivo in human vastus lateralis muscle using magnetic resonance and optical spectroscopy on 24 sedentary and seven active subjects. Muscle biopsies were analyzed for electron transport chain content (using complex III as a representative marker) and mitochondrial proteins associated with antioxidant protection.

RESULTS

Sedentary muscle had lower electron transport chain complex content (65% of the active group) in proportion to the reduction in ATP(max) (0.69 ± 0.07 vs. 1.07 ± 0.06 mM sec(-1)) as compared with active subjects. This lower ATP(max) paired with an unchanged O(2) flux in resting muscle between groups resulted in a doubling of O(2) flux per ATP(max) (3.3 ± 0.3 vs. 1.7 ± 0.2 μM O(2) per mM ATP) that reflected mitochondrial uncoupling (P/O = 1.41 ± 0.1 vs. 2.1 ± 0.3) and greater UCP3/complex III (6.0 ± 0.7 vs. 3.8 ± 0.3) in sedentary vs. active subjects.

CONCLUSION

A smaller mitochondrial pool serving the same O(2) flux resulted in elevated mitochondrial respiration in sedentary muscle. In addition, uncoupling contributed to this higher mitochondrial respiration. This finding resolves the paradox of stable muscle metabolism but greater mitochondrial respiration in muscle of inactive vs. active subjects.

High-intensity interval training increases in vivo oxidative capacity with no effect on P(i)→ATP rate in resting human muscle

Mitochondrial ATP production is vital for meeting cellular energy demand at rest and during periods of high ATP turnover. We hypothesized that high-intensity interval training (HIT) would increase ATP flux in resting muscle (VPi→ATP) in response to a single bout of exercise, whereas changes in the capacity for oxidative ATP production (Vmax) would require repeated bouts. Eight untrained men (27 ± 4 yr; peak oxygen uptake = 36 ± 4 ml·kg(-1)·min(-1)) performed six sessions of HIT (4-6 × 30-s bouts of all-out cycling with 4-min recovery). After standardized meals and a 10-h fast, VPi→ATP and Vmax of the vastus lateralis muscle were measured using phosphorus magnetic resonance spectroscopy at 4 Tesla. Measurements were obtained at baseline, 15 h after the first training session, and 15 h after completion of the sixth session. VPi→ATP was determined from the unidirectional flux between Pi and ATP, using the saturation transfer technique. The rate of phosphocreatine recovery (kPCr) following a maximal contraction was used to calculate Vmax. While kPCr and Vmax were unchanged after a single session of HIT, completion of six training sessions resulted in a ∼14% increase in muscle oxidative capacity (P ≤ 0.004). In contrast, neither a single nor six training sessions altered VPi→ATP (P = 0.74). This novel analysis of resting and maximal high-energy phosphate kinetics in vivo in response to HIT provides evidence that distinct aspects of human skeletal muscle metabolism respond differently to this type of training.

Elevated energy coupling and aerobic capacity improves exercise performance in endurance-trained elderly subjects

Increased maximal oxygen uptake (V(O(2)max)), mitochondrial capacity and energy coupling efficiency are reported after endurance training (ET) in adult subjects. Here we test whether leg exercise performance (power output of the legs, P(max), at V(O(2)max)) reflects these improvements with ET in the elderly. Fifteen male and female subjects were endurance trained for a 6 month programme, with 13 subjects (69.5 ± 1.2 years old, range 65-80 years old; n = 7 males; n = 6 females) completing the study. This training significantly improved P(max) (Δ17%; P = 0.003), V(O(2)max) (Δ5.4%; P = 0.021) and the increment in oxygen uptake (V(O(2))) above resting (ΔV(O(2)m-r) = V(O(2)max) - V(O(2)rest; Δ9%; P < 0.02). In addition, evidence of improved energy coupling came from elevated leg power output per unit V(O(2))at the aerobic capacity [Δ(P(max)/ΔV(O(2)m-r)); P = 0.02] and during submaximal exercise in the ramp test as measured by delta efficiency (ΔP(ex)/ΔV(O(2)); P = 0.04). No change was found in blood lactate, muscle glycolysis or fibre type. The rise in P(max) paralleled the improvement in muscle oxidative phosphorylation capacity (ATP(max)) in these subjects. In addition, the greater exercise energy coupling [Δ(P(max)/ΔV(O(2)m-r)) and delta efficiency] was accompanied by increased mitochondrial energy coupling as measured by elevated ATP production per unit mitochondrial content in these subjects. These results suggest that leg exercise performance benefits from elevations in energy coupling and oxidative phosphorylation capacity at both the whole-body and muscle levels that accompany endurance training in the elderly.

Training-induced acceleration of O(2) uptake on-kinetics precedes muscle mitochondrial biogenesis in humans

The effects of 5 weeks of moderate-intensity endurance training on pulmonary oxygen uptake kinetics (V(O(2)) on-kinetics) were studied in 15 healthy men (mean ± SD: age 22.7 ± 1.8 years, body weight 76.4 ± 8.9 kg and maximal V(O(2)) 46.0 ± 3.7 ml kg(-1) min(-1)). Training caused a significant acceleration (P = 0.003) of V(O(2)) on-kinetics during moderate-intensity cycling (time constant of the 'primary' component 30.0 ± 6.6 versus 22.8 ± 5.6 s before and after training, respectively) and a significant decrease (P = 0.04) in the amplitude of the primary component (837 ± 351 versus 801 ± 330 ml min(-1)). No changes in myosin heavy chain distribution, muscle fibre capillarization, level of peroxisome proliferator-activated receptor γ coactivator 1α and other markers of mitochondrial biogenesis (mitochondrial DNA copy number, cytochrome c and cytochrome oxidase subunit I contents) in the vastus lateralis were found after training. A significant downregulation in the content of the sarcoplasmic reticulum ATPase 2 (SERCA2; P = 0.03) and a tendency towards a decrease in SERCA1 (P = 0.055) was found after training. The decrease in SERCA1 was positively correlated (P = 0.05) with the training-induced decrease in the gain of the V(O(2)) on-kinetics (ΔV(O(2)) at steady state/Δpower output). In the early stage of training, the acceleration in V(O(2)) on-kinetics during moderate-intensity cycling can occur without enhanced mitochondrial biogenesis or changes in muscle myosin heavy chain distribution and in muscle fibre capillarization. The training-induced decrease of the O(2) cost of cycling could be caused by the downregulation of SERCA pumps.

Exercise efficiency is reduced by mitochondrial uncoupling in the elderly

A reduction in exercise efficiency accompanies ageing in humans. Here we evaluated the impact of changes in the contractile-coupling and mitochondrial-coupling efficiencies on the reduction in exercise efficiency in the elderly. Nine adult (mean, 38.8 years old) and 40 elderly subjects (mean, 68.8 years old) performed a cycle ergometer test to measure O2 uptake and leg power output up to the aerobic limit ( ). Reduced leg power output per unit O2 uptake was reflected in a drop in delta efficiency (εD) from 0.27 ± 0.01 (mean ± SEM) in adults to 0.22 ± 0.01 in the elderly group. Similar declines with age were apparent for both the leg power output at and the ATP generation capacity (ATPmax) determined in vivo using (31)P magnetic resonance spectroscopy. These similar declines resulted in unchanged contractile-coupling efficiency values (εC) in the adult (0.50 ± 0.05) versus the elderly group (0.58 ± 0.04) and agreed with independent measures of muscle contractile-coupling efficiency in human quadriceps (0.5). The mitochondrial-coupling efficiency calculated from the ratio of delta to contractile-coupling efficiencies in the adults (εD/εC = 0.58 ± 0.08) corresponded to values for well-coupled mitochondria (0.6); however, εD/εC was significantly lower in the elderly subjects (0.44 ± 0.03). Conversion of ATPmax per mitochondrial volume (ATPmax/Vv[mt,f]) reported in these groups into thermodynamic units confirmed this drop in mitochondrial-coupling efficiency from 0.57 ± 0.08 in adults to 0.41 ± 0.03 in elderly subjects. Thus, two independent methods revealed that reduced mitochondrial-coupling efficiency was a key part of the drop in exercise efficiency in these elderly subjects and may be an important part of the loss of exercise performance with age.

Biomarkers of mitochondrial content in skeletal muscle of healthy young human subjects

Skeletal muscle mitochondrial content varies extensively between human subjects. Biochemical measures of mitochondrial proteins, enzyme activities and lipids are often used as markers of mitochondrial content and muscle oxidative capacity (OXPHOS). The purpose of this study was to determine how closely associated these commonly used biochemical measures are to muscle mitochondrial content and OXPHOS. Sixteen young healthy male subjects were recruited for this study. Subjects completed a graded exercise test to determine maximal oxygen uptake (VO2peak) and muscle biopsies were obtained from the vastus lateralis. Mitochondrial content was determined using transmission electron microscopy imaging and OXPHOS was determined as the maximal coupled respiration in permeabilized fibres. Biomarkers of interest were citrate synthase (CS) activity, cardiolipin content, mitochondrial DNA content (mtDNA), complex I–V protein content, and complex I–IV activity. Spearman correlation coefficient tests and Lin's concordance tests were applied to assess the absolute and relative association between the markers and mitochondrial content or OXPHOS. Subjects had a large range of VO2peak (range 29.9–71.6ml min−1 kg−1) and mitochondrial content (4–15% of cell volume).Cardiolipin content showed the strongest association with mitochondrial content followed by CS and complex I activities. mtDNA was not related to mitochondrial content. Complex IV activity showed the strongest association with muscle oxidative capacity followed by complex II activity.We conclude that cardiolipin content, and CS and complex I activities are the biomarkers that exhibit the strongest association with mitochondrial content, while complex IV activity is strongly associated with OXPHOS capacity in human skeletal muscle.

Erythropoietin treatment enhances muscle mitochondrial capacity in humans

Erythropoietin (Epo) treatment has been shown to induce mitochondrial biogenesis in cardiac muscle along with enhanced mitochondrial capacity in mice. We hypothesized that recombinant human Epo (rhEpo) treatment enhances skeletal muscle mitochondrial oxidative phosphorylation (OXPHOS) capacity in humans. In six healthy volunteers rhEpo was administered by sub-cutaneous injection over 8 weeks with oral iron (100 mg) supplementation taken daily. Mitochondrial OXPHOS was quantified by high-resolution respirometry in saponin-permeabilized muscle fibers obtained from biopsies of the vastus lateralis before and after rhEpo treatment. OXPHOS was determined with the mitochondrial complex I substrates malate, glutamate, pyruvate, and complex II substrate succinate in the presence of saturating ADP concentrations, while maximal electron transport capacity (ETS) was assessed by addition of an uncoupler. rhEpo treatment increased OXPHOS (from 92 ± 5 to 113 ± 7 pmol·s⁻¹·mg⁻¹) and ETS (107 ± 4 to 143 ± 14 pmol·s⁻¹·mg⁻¹, p < 0.05), demonstrating that Epo treatment induces an upregulation of OXPHOS and ETS in human skeletal muscle.

Variation in the uncoupling protein 2 and 3 genes and human performance

Uncoupling proteins 2 and 3 (UCP2 and UCP3) may negatively regulate mitochondrial ATP synthesis and, through this, influence human physical performance. However, human data relating to both these issues remain sparse. Examining the association of common variants in the UCP3/2 locus with performance phenotypes offers one means of investigation. The efficiency of skeletal muscle contraction, delta efficiency (DE), was assessed by cycle ergometry in 85 young, healthy, sedentary adults both before and after a period of endurance training. Of these, 58 were successfully genotyped for the UCP3-55C>T (rs1800849) and 61 for the UCP2-866G>A (rs659366) variant. At baseline, UCP genotype was unrelated to any physical characteristic, including DE. However, the UCP2-866G>A variant was independently and strongly associated with the DE response to physical training, with UCP2-866A allele carriers exhibiting a greater increase in DE with training (absolute change in DE of -0.2 ± 3.6% vs. 1.7 ± 2.8% vs. 2.3 ± 3.7% for GG vs. GA vs. AA, respectively; P = 0.02 for A allele carriers vs. GG homozygotes). In multivariate analysis, there was a significant interaction between UCP2-866G>A and UCP3-55C>T genotypes in determining changes in DE (adjusted R(2) = 0.137; P value for interaction = 0.003), which was independent of the effect of either single polymorphism or baseline characteristics. In conclusion, common genetic variation at the UCP3/2 gene locus is associated with training-related improvements in DE, an index of skeletal muscle performance. Such effects may be mediated through differences in the coupling of mitochondrial energy transduction in human skeletal muscle, but further mechanistic studies are required to delineate this potential role.

The higher susceptibility of congenital analbuminemic rats to Ca2+-induced mitochondrial permeability transition is associated with the increased expression of cyclophilin D and nitrosothiol depletion

Congenital analbuminemia is a rare autosomal recessive disorder characterized by a trace level of albumin in blood plasma and mild clinical symptoms. Analbuminemic patients generally present associated abnormalities, among which dyslipidemia is a hallmark. In this study, we show that mitochondria isolated from different tissues (liver, heart and brain) from 3-month-old analbuminemic rats (NAR) present a higher susceptibility to Ca(2+)-induced mitochondrial permeability transition (MPT), as assessed by either Ca(2+)-induced mitochondrial swelling, dissipation of membrane potential or mitochondrial Ca(2+) release. The Ca(2+) retention capacity of the liver mitochondria isolated from 3-month-old NAR was about 50% that of the control. Interestingly, the assessment of this variable in 21-day-old NAR indicated that the mitochondrial Ca(2+) retention capacity was preserved at this age, as compared to age-matched controls, which indicates that a reduced capacity for mitochondrial Ca(2+) retention is not a constitutive feature. The search for putative mediators of MPT sensitization in NAR revealed a 20% decrease in mitochondrial nitrosothiol content and a 30% increase in cyclophilin D expression. However, the evaluation of other variables related to mitochondrial redox status showed similar results between the controls and NAR, i.e., namely the contents of reduced mitochondrial membrane protein thiol groups and total glutathione, H(2)O(2) release rate, and NAD(P)H reduced state. We conclude that the higher expression of cyclophilin D, a major component of the MPT pore, and decreased nitrosothiol content in NAR mitochondria may underlie MPT sensitization in these animals.

Similar qualitative and quantitative changes of mitochondrial respiration following strength and endurance training in normoxia and hypoxia in sedentary humans

Endurance and strength training are established as distinct exercise modalities, increasing either mitochondrial density or myofibrillar units. Recent research, however, suggests that mitochondrial biogenesis is stimulated by both training modalities. To test the training “specificity” hypothesis, mitochondrial respiration was studied in permeabilized muscle fibers from 25 sedentary adults after endurance (ET) or strength training (ST) in normoxia or hypoxia [fraction of inspired oxygen (FiO2) = 21% or 13.5%]. Biopsies were taken from the musculus vastus lateralis, and cycle-ergometric incremental maximum oxygen uptake (V̇o2max) exercise tests were performed under normoxia, before and after the 10-wk training program. The main finding was a significant increase ( P < 0.05) of fatty acid oxidation capacity per muscle mass, after endurance and strength training under normoxia [2.6- and 2.4-fold for endurance training normoxia group (ETN) and strength training normoxia group (STN); n = 8 and 3] and hypoxia [2.0-fold for the endurance training hypoxia group (ETH) and strength training hypoxia group (STH); n = 7 and 7], and higher coupling control of oxidative phosphorylation. The enhanced lipid oxidative phosphorylation (OXPHOS) capacity was mainly (87%) due to qualitative mitochondrial changes increasing the relative capacity for fatty acid oxidation ( P < 0.01). Mitochondrial tissue-density contributed to a smaller extent (13%), reflected by the gain in muscle mass-specific respiratory capacity with a physiological substrate cocktail (glutamate, malate, succinate, and octanoylcarnitine). No significant increase was observed in mitochondrial DNA (mtDNA) content. Physiological OXPHOS capacity increased significantly in ETN ( P < 0.01), with the same trend in ETH and STH ( P < 0.1). The limitation of flux by the phosphorylation system was diminished after training. Importantly, key mitochondrial adaptations were similar after endurance and strength training, regardless of normoxic or hypoxic exercise. The transition from a sedentary to an active lifestyle induced muscular changes of mitochondrial quality representative of mitochondrial health.

Aerobic exercise training adaptations are increased by postexercise carbohydrate-protein supplementation

Carbohydrate-protein supplementation has been found to increase the rate of training adaptation when provided postresistance exercise. The present study compared the effects of a carbohydrate and protein supplement in the form of chocolate milk (CM), isocaloric carbohydrate (CHO), and placebo on training adaptations occurring over 4.5 weeks of aerobic exercise training. Thirty-two untrained subjects cycled 60 min/d, 5 d/wk for 4.5 wks at 75-80% of maximal oxygen consumption (VO(2) max). Supplements were ingested immediately and 1 h after each exercise session. VO(2) max and body composition were assessed before the start and end of training. VO(2) max improvements were significantly greater in CM than CHO and placebo. Greater improvements in body composition, represented by a calculated lean and fat mass differential for whole body and trunk, were found in the CM group compared to CHO. We conclude supplementing with CM postexercise improves aerobic power and body composition more effectively than CHO alone.

Dietary inorganic nitrate improves mitochondrial efficiency in humans

Nitrate, an inorganic anion abundant in vegetables, is converted in vivo to bioactive nitrogen oxides including NO. We recently demonstrated that dietary nitrate reduces oxygen cost during physical exercise, but the mechanism remains unknown. In a double-blind crossover trial we studied the effects of a dietary intervention with inorganic nitrate on basal mitochondrial function and whole-body oxygen consumption in healthy volunteers. Skeletal muscle mitochondria harvested after nitrate supplementation displayed an improvement in oxidative phosphorylation efficiency (P/O ratio) and a decrease in state 4 respiration with and without atractyloside and respiration without adenylates. The improved mitochondrial P/O ratio correlated to the reduction in oxygen cost during exercise. Mechanistically, nitrate reduced the expression of ATP/ADP translocase, a protein involved in proton conductance. We conclude that dietary nitrate has profound effects on basal mitochondrial function. These findings may have implications for exercise physiology- and lifestyle-related disorders that involve dysfunctional mitochondria.

Role and treatment of mitochondrial DNA-related mitochondrial dysfunction in sporadic neurodegenerative diseases

Several sporadic neurodegenerative diseases display phenomena that directly or indirectly relate to mitochondrial function. Data suggesting altered mitochondrial function in these diseases could arise from mitochondrial DNA (mtDNA) are reviewed. Approaches for manipulating mitochondrial function and minimizing the downstream consequences of mitochondrial dysfunction are discussed.

Reduced respiratory capacity in muscle mitochondria of obese subjects

BACKGROUND/AIMS

The extent of weight gain varies among individuals despite equal calorie overconsumption. Furthermore, weight gain is often less than expected from energy excess. This suggests differences in metabolic efficiency and basal metabolism. Since mitochondrial uncoupling accounts for a substantial portion of the basal metabolic rate, we compared skeletal muscle mitochondrial respiration in obese subjects to normal-weight reference groups with various degrees of physical activity.

METHODS

Muscle biopsies were taken from the vastus lateralis muscle of 9 healthy obese subjects (BMI 40 ± 3). Mitochondria were isolated and analyzed for coupled (state 3) and uncoupled (state 4) respirations as well as mitochondrial efficiency (P/O ratio) using pyruvate as a substrate. Respiratory data were compared to reference groups A, normal-weight untrained (BMI 24 ± 0.7), and B, normal-weight trained (BMI 24 ± 0.6).

RESULTS

Obese subjects had a decreased respiratory capacity per mitochondrial volume compared to the reference groups: this was evident in state 4 (65% and 35% of reference group A and B, respectively) and state 3 (53% and 29% of A and B, respectively) (p < 0.05).

CONCLUSION

Obese subjects had a low capacity for fuel oxidation, which may play a role in the predisposition of obesity. However, whether lower mitochondrial capacity is a cause or a consequence of obesity requires further research.

Exercise changes regional vascular control by commissural NTS in spontaneously hypertensive rats

Inhibition of the commissural nucleus of the solitary tract (commNTS) induces a fall in sympathetic nerve activity and blood pressure in spontaneously hypertensive rats (SHR), which suggests that this subnucleus of the NTS is a source of sympathoexcitation. Exercise training reduces sympathetic activity and arterial pressure. The purpose of the present study was to investigate whether the swimming exercise can modify the regional vascular responses evoked by inhibition of the commNTS neurons in SHR and normotensive Wistar-Kyoto (WKY) rats. Exercise consisted of swimming, 1 h/day, 5 days/wk for 6 wks, with a load of 2% of the body weight. The day after the last exercise session, the rats were anesthetized with intravenous α-chloralose, tracheostomized, and artificially ventilated. The femoral artery was cannulated for mean arterial pressure (MAP) and heart rate recordings, and Doppler flow probes were placed around the lower abdominal aorta and superior mesenteric artery. Microinjection of 50 mM GABA into the commNTS caused similar reductions in MAP in swimming and sedentary SHR (−25 ± 6 and −30 ± 5 mmHg, respectively), but hindlimb vascular conductance increased twofold in exercised vs. sedentary SHR (54 ± 8 vs. 24 ± 5%). GABA into the commNTS caused smaller reductions in MAP in swimming and sedentary WKY rats (−20 ± 4 and −16 ± 2 mmHg). Hindlimb conductance increased fourfold in exercised vs. sedentary WKY rats (75 ± 2% vs. 19 ± 3%). Therefore, our data suggest that the swimming exercise induced changes in commNTS neurons, as shown by a greater enhancement of hindlimb vasodilatation in WKY vs. SHR rats in response to GABAergic inhibition of these neurons.

Mitochondrial Medicine and the Neurodegenerative Mitochondriopathies

Neurodegenerative diseases are a common late-life scourge for which disease-modifying treatments are sorely needed. Mitochondrial perturbation is commonly observed in these diseases, so pursuing treatment development strategies that target mitochondria or processes affected by mitochondria seems reasonable. This review discusses the rationale underlying past and current efforts to treat neurodegenerative diseases using mitochondrial medicine, and tries to predict how future efforts might proceed.

Assessment of physiological capacities of elite athletes & respiratory limitations to exercise performance

Physiological assessment of athletes is an important process for the characterization of the athlete, monitoring progress and the trained state or 'level of preparedness' of an athlete, as well as aiding the process of training program design. Interestingly, the majority of physiological assessments performed on athletes can also be performed on children with disease, and therefore clinicians can learn a great deal about physiology and assessment of patient populations through the examination of the physiological responses of elite athletes. This review describes typical physiological responses of elite athletes to tests of aerobic and anaerobic metabolism and provides a specific focus upon respiratory limitations to exercise performance. Typical responses of elite athletes are described to provide the scientist and clinician with a perspective of the upper range of physiological capacities of elite athletes.

Steady exercise removes VO(2max) difference between mitochondrial genomic variants

It has been clearly established that mitochondrial variants, among other potential factors, influence on VO(2max). With this study we sought to determine whether this genetic predisposition could be modified by steady exercise. Mitochondrial genetic variants were determined in 70 healthy controls (CON) and in 77 athletes who trained regularly (50 cyclists, aerobic training (AER), and 27 runners of 400m, anaerobic training (NoAER)). All of them were male Spanish Caucasian individuals. A maximum graded exercise test (GXT) in cycle-ergometer was performed to determine VO(2max) (mL kg(-1)min(-1)). Our results confirmed that, in CON, VO(2max) (P=0.007) was higher in Non-J than J individuals. Furthermore, we found that AER and NoAER showed, as it could be expected, higher VO(2max) than CON, but not differences between mitochondrial variants have been found. According with these findings, the influence of mitochondrial DNA (mtDNA) variants on VO(2max) has been confirmed, and a new conclusion has arisen: the steady exercise is able to remove this influence. The interest of these promising findings in muscular performance should be further explored, in particular, the understanding of potential applications in sport training and in muscle pathological syndromes.

Recent advances in adaptive thermogenesis: potential implications for the treatment of obesity

Large inter-individual differences in cold-induced (non-shivering) and diet-induced adaptive thermogenesis exist in animals and humans. These differences in energy expenditure can have a large impact on long-term energy balance and thus body weight (when other factors remain stable). Therefore, the level of adaptive thermogenesis might relate to the susceptibility to obesity; efforts to increase adaptive thermogenesis might be used to treat obesity. In small mammals, the main process involved is mitochondrial uncoupling in brown adipose tissue (BAT), which is regulated by the sympathetic nervous system. For a long time, it was assumed that mitochondrial uncoupling is not a major physiological contributor to adaptive thermogenesis in adult humans. However, several studies conducted in recent years suggest that mitochondrial uncoupling in BAT and skeletal muscle tissue in adult humans can be physiologically significant. Other mechanisms besides mitochondrial uncoupling that might be involved are futile calcium cycling, protein turnover and substrate cycling. In conjunction with recent advances on signal transduction studies, this knowledge makes manipulation of adaptive thermogenesis a more realistic option and thus a pharmacologically interesting target to treat obesity.

Beneficial effects of exercise on muscle mitochondrial function in diabetes mellitus

The physiopathology of diabetes mellitus has been closely associated with a variety of alterations in mitochondrial histology, biochemistry and function. Generally, the alterations comprise increased mitochondrial reactive oxygen and nitrogen species (RONS) generation, resulting in oxidative stress and damage; decreased capacity to metabolize lipids, leading to intramyocyte lipid accumulation; and diminished mitochondrial density and reduced levels of uncoupling proteins (UCPs), with consequent impairment in mitochondrial function. Chronic physical exercise is a physiological stimulus able to induce mitochondrial adaptations that can counteract the adverse effects of diabetes on muscle mitochondria. However, the mechanisms responsible for mitochondrial adaptations in the muscles of diabetic patients are still unclear. The main mechanisms by which exercise may be considered an important non-pharmacological strategy for preventing and/or attenuating diabetes-induced mitochondrial impairments may involve (i) increased mitochondrial biogenesis, which is dependent on the increased expression of some important proteins, such as the 'master switch' peroxisome proliferator-activated receptor (PPAR)-gamma-coactivator-1alpha (PGC-1alpha) and heat shock proteins (HSPs), both of which are severely downregulated in the muscles of diabetic patients; and (ii) the restoration or attenuation of the low UCP3 expression in skeletal muscle mitochondria of diabetic patients, which is suggested to play a pivotal role in mitochondrial dysfunction.There is evidence that chronic exercise and lifestyle interventions reverse impairments in mitochondrial density and size, in the activity of respiratory chain complexes and in cardiolipin content; however, the mechanisms by which chronic exercise alters mitochondrial respiratory parameters, mitochondrial antioxidant systems and other specific proteins involved in mitochondrial metabolism in the muscles of diabetic patients remain to be elucidated.

Increased substrate oxidation and mitochondrial uncoupling in skeletal muscle of endurance-trained individuals

Endurance exercise training is accompanied by physiological changes that improve muscle function and performance. Several studies have demonstrated that markers of mitochondrial capacity are elevated, however, these studies tend to be performed ex vivo under conditions that yield maximal enzyme activities or in vivo but monitoring the response to exercise. Therefore, it is unclear whether basal mitochondrial metabolism is affected by exercise training. To explore whether resting muscle metabolism was altered in trained individuals in vivo, two independent parameters of metabolic function-tricarboxylic acid (TCA) cycle flux (V(TCA)), and ATP synthesis (V(ATP))-were assessed noninvasively by using magnetic resonance spectroscopy in a cohort of young endurance trained subjects (n = 7) and a group of matched sedentary subjects (n = 8). V(TCA) was 54% higher in the muscle of endurance trained compared with sedentary subjects (91.7 +/- 7.6 vs. 59.6 +/- 4.9 nmol/g/min, P < 0.01); however, V(ATP) was not different between the trained and sedentary subjects (5.98 +/- 0.43 vs. 6.35 +/- 0.70 mumol/g/min, P = 0.67). The ratio V(ATP)/V(TCA) (an estimate of mitochondrial coupling) was also significantly reduced in trained subjects (P < 0.04). These data demonstrate that basal mitochondrial substrate oxidation is increased in the muscle of endurance trained individuals yet energy production is unaltered, leading to an uncoupling of oxidative phosphorylation at rest. Increased mitochondrial uncoupling may represent another mechanism by which exercise training enhances muscle insulin sensitivity via increased fatty acid oxidation in the resting state.

Four weeks of speed endurance training reduces energy expenditure during exercise and maintains muscle oxidative capacity despite a reduction in training volume

We studied the effect of an alteration from regular endurance to speed endurance training on muscle oxidative capacity, capillarization, as well as energy expenditure during submaximal exercise and its relationship to mitochondrial uncoupling protein 3 (UCP3) in humans. Seventeen endurance-trained runners were assigned to either a speed endurance training (SET; n = 9) or a control (Con; n = 8) group. For a 4-wk intervention (IT) period, SET replaced the ordinary training ( approximately 45 km/wk) with frequent high-intensity sessions each consisting of 8-12 30-s sprint runs separated by 3 min of rest (5.7 +/- 0.1 km/wk) with additional 9.9 +/- 0.3 km/wk at low running speed, whereas Con continued the endurance training. After the IT period, oxygen uptake was 6.6, 7.6, 5.7, and 6.4% lower (P < 0.05) at running speeds of 11, 13, 14.5, and 16 km/h, respectively, in SET, whereas remained the same in Con. No changes in blood lactate during submaximal running were observed. After the IT period, the protein expression of skeletal muscle UCP3 tended to be higher in SET (34 +/- 6 vs. 47 +/- 7 arbitrary units; P = 0.06). Activity of muscle citrate synthase and 3-hydroxyacyl-CoA dehydrogenase, as well as maximal oxygen uptake and 10-km performance time, remained unaltered in both groups. In SET, the capillary-to-fiber ratio was the same before and after the IT period. The present study showed that speed endurance training reduces energy expenditure during submaximal exercise, which is not mediated by lowered mitochondrial UCP3 expression. Furthermore, speed endurance training can maintain muscle oxidative capacity, capillarization, and endurance performance in already trained individuals despite significant reduction in the amount of training.

Acute responses in muscle mitochondrial and cytosolic enzyme activities during heavy intermittent exercise

To examine the effects of repetitive bouts of heavy exercise on the maximal activities of enzymes representative of the major metabolic pathways and segments, 13 untrained volunteers [peak aerobic power (V̇o2 peak) = 44.3 ± 2.3 ml·kg−1·min−1] cycled at ∼91% V̇o2 peak for 6 min once per hour for 16 h. Maximal enzyme activities ( Vmax, mol·kg−1·protein·h−1) were measured in homogenates from tissue extracted from the vastus lateralis before and after exercise at repetitions 1 (R1), 2 (R2), 9 (R9), and 16 (R16). For the mitochondrial enzymes, exercise resulted in reductions ( P < 0.05) in cytochrome- c oxidase (COX, 14.6%), near significant reductions in malate dehydrogenase (4.06%; P = 0.06) and succinic dehydrogenase (4.82%; P = 0.09), near significant increases in β-hydroxyacyl-CoA dehydrogenase (4.94%; P = 0.08), and no change in citrate synthase (CS, 2.88%; P = 0.37). For the cytosolic enzymes, exercise reduced ( P < 0.05) Vmax in hexokinase (Hex, 4.4%), creatine phosphokinase (9.0%), total phosphorylase (13.5%), phosphofructokinase (16.6%), pyruvate kinase (PK, 14.1%) and lactate dehydrogenase (10.7%). Repetition-dependent reductions ( P < 0.05) in Vmax were observed for CS (R1, R2 > R16), COX (R1, R2 > R16), Hex (1R, 2R > R16), and PK (R9 > R16). It is concluded that heavy exercise results in transient reductions in a wide range of enzymes involved in different metabolic functions and that in the case of selected enzymes, multiple repetitions of the exercise reduce average Vmax.

Effect of a single session of electrical stimulation on activity and expression of citrate synthase and antioxidant enzymes in rat soleus muscle

The aim of our study was to investigate the effect of a single high intensity session of muscle contractions on the activity and expression of citrate synthase (CS) and of the following major antioxidant enzymes: Mn-superoxide dismutase (Mn-SOD), Cu,Zn-superoxide dismutase (Cu,Zn-SOD), catalase (CAT), and glutathione peroxidase (GPX). To accomplish this, soleus muscles of male Wistar rats were subjected to contractions using a intense electrical stimulation (ES) protocol. Soleus muscles were isolated either immediately or 1 h after the contractions and utilized for enzyme activity determination, and for analysis of gene expression by quantitative PCR. A significant increase in maximal activity (63%) and expression (80%) of CS was observed in stimulated soleus muscles, isolated 1 h after ES as compared to controls. However, this effect was not observed in muscles isolated immediately after ES. By using macroarray and Real Time RT-PCR analysis, an increase in expression of Mn-SOD, Cu,Zn-SOD, CAT, and GPX was also found. Interestingly, of these enzymes, only CAT activity was significantly increased (44%) 1 h after ES in soleus muscle. These results indicate that acute ES up-regulates activity and expression of CS and CAT in soleus muscles. This increase in expression of CAT may play an important role in counteracting the potential deleterious effects of elevated oxidative stress induced by a high oxidative demand in skeletal muscles subjected to exercise training.

Skeletal muscle uncoupling protein-3 restores upon intervention in the prediabetic and diabetic state: implications for diabetes pathogenesis?

AIM

Skeletal muscle uncoupling protein-3 (UCP3) is reduced in type 2 diabetes, and in the pre-diabetic condition of impaired glucose tolerance (IGT). Here we examined whether intervention programs known to improve insulin sensitivity are paralleled by an increase in skeletal muscle UCP3 protein levels.

METHODS

Skeletal muscle UCP3 protein content was measured before and after one year of an exercise intervention in muscle biopsies of eight diabetic subjects. In addition, UCP3 was measured in IGT subjects before and after 1 year of following a lifestyle-intervention program or serving as control.

RESULTS

In the diabetic patients a significant increase of approximately 75% in UCP3 protein was found after 1 year of exercise training (P < 0.05). In IGT subjects UCP3 protein increased in the intervention group (P = 0.02), while UCP3 remained unaltered in the control group (P = 0.64).

CONCLUSION

Both, exercise training and a lifestyle-intervention program increase UCP3 protein content in skeletal muscle of subjects with reduced glycaemic control, indicating a restoration towards normal UCP3 levels. These data support the idea that UCP3 has a role in the aetiology of type 2 diabetes mellitus.

Role of mitochondria in exercise-induced oxidative stress in skeletal muscle from hyperthyroid rats

Previous study showed that exercise induces higher oxidative damage and respiratory capacity reduction in hyperthyroid than in euthyroid skeletal muscle. Because impaired cell function can result from mitochondrial dysfunction, we evaluated the changes induced by exercise in oxygen consumption of skeletal muscle mitochondria from euthyroid and hyperthyroid rats. The mitochondrial function was related with indices of oxidative damage and nitric oxide production, scavenger levels and mitochondrial ROS production rates. Our results show that exercise increased state 4 and decreased state 3 respiration, and the highest changes happened in hyperthyroid preparations. This was consistent with the observation that oxidative damage and NO(*) derivative content were increased by T(3) administration and exercise, reaching the highest levels in hyperthyroid exercised rats. Our results also indicate that the high mitochondrial oxidative damage induced by T(3) and exercise is due to enhanced ROS production, which is dependent on increases in mitochondrial content and reduction degree, respectively, of autoxidizable electron carriers.

Mitochondrial oxidative function and type 2 diabetes

The cause of insulin resistance and type 2 diabetes is unknown. The major part of insulin-mediated glucose disposal takes place in the skeletal muscle, and increased amounts of intramyocellular lipid has been associated with insulin resistance and linked to decreased activity of mitochondrial oxidative phosphorylation. This review will cover the present knowledge and literature on the topics of the activity of oxidative enzymes and the electron transport chain (ETC) in skeletal muscle of patients with type 2 diabetes. Different methods of studying mitochondrial function are described, including biochemical measurements of oxidative enzyme and electron transport activity, isolation of mitochondria for measurements of respiration, and ATP production and indirect measurements of ATP production using nuclear magnetic resonance (NMR) - spectroscopy. Biochemical markers of mitochondrial content are also discussed. Several studies show reduced activity of oxidative enzymes in skeletal muscle of type 2 diabetics. The reductions are independent of muscle fiber type, and are accompanied by visual evidence of damaged mitochondria. In most studies, the reduced oxidative enzyme activity is explained by decreases in mitochondrial content; thus, evidence of a functional impairment in mitochondria in type 2 diabetes is not convincing. These impairments in oxidative function and mitochondrial morphology could reflect the sedentary lifestyle of the diabetic subjects, and the influence of physical activity on oxidative activity and mitochondrial function is discussed. The studies on insulin-resistant offspring of type 2 diabetic parents have provided important insights in the earliest metabolic defects in type 2 diabetes. These defects include reductions in basal ATP production and an attenuated response to insulin stimulation. The decreased basal ATP production does not affect overall lipid or glucose oxidation, and no studies linking changes in oxidative activity and insulin sensitivity in type 2 diabetes have been published. It is concluded that evidence of a functional impairment in mitochondria in type 2 diabetes is not convincing, and that intervention studies describing the correlation between changes in insulin resistance and mitochondrial function in type 2 diabetes are lacking. Specific effects of regular physical training and muscular work on mitochondrial function and plasticity in type 2 diabetes remain an important area of research.

Mitochondrial respiration is decreased in skeletal muscle of patients with type 2 diabetes

We tested the hypothesis of a lower respiratory capacity per mitochondrion in skeletal muscle of type 2 diabetic patients compared with obese subjects. Muscle biopsies obtained from 10 obese type 2 diabetic and 8 obese nondiabetic male subjects were used for assessment of 3-hydroxy-Acyl-CoA-dehydrogenase (HAD) and citrate synthase activity, uncoupling protein (UCP)3 content, oxidative stress measured as 4-hydroxy-2-nonenal (HNE), fiber type distribution, and respiration in isolated mitochondria. Respiration was normalized to citrate synthase activity (mitochondrial content) in isolated mitochondria. Maximal ADP-stimulated respiration (state 3) with pyruvate plus malate and respiration through the electron transport chain (ETC) were reduced in type 2 diabetic patients, and the proportion of type 2X fibers were higher in type 2 diabetic patients compared with obese subjects (all P < 0.05). There were no differences in respiration with palmitoyl-l-carnitine plus malate, citrate synthase activity, HAD activity, UCP3 content, or oxidative stress measured as HNE between the groups. In the whole group, state 3 respiration with pyruvate plus malate and respiration through ETC were negatively associated with A1C, and the proportion of type 2X fibers correlated with markers of insulin resistance (P < 0.05). In conclusion, we provide evidence for a functional impairment in mitochondrial respiration and increased amount of type 2X fibers in muscle of type 2 diabetic patients. These alterations may contribute to the development of type 2 diabetes in humans with obesity.

Reduced efficiency, but increased fat oxidation, in mitochondria from human skeletal muscle after 24-h ultraendurance exercise

The hypothesis that ultraendurance exercise influences muscle mitochondrial function has been investigated. Athletes in ultraendurance performance performed running, kayaking, and cycling at 60% of their peak O(2) consumption for 24 h. Muscle biopsies were taken preexercise (Pre-Ex), postexercise (Post-Ex), and after 28 h of recovery (Rec). Respiration was analyzed in isolated mitochondria during state 3 (coupled to ATP synthesis) and state 4 (noncoupled respiration), with fatty acids alone [palmitoyl carnitine (PC)] or together with pyruvate (Pyr). Electron transport chain activity was measured with NADH in permeabilized mitochondria. State 3 respiration with PC increased Post-Ex by 39 and 41% (P < 0.05) when related to mitochondrial protein and to electron transport chain activity, respectively. State 3 respiration with Pyr was not changed (P > 0.05). State 4 respiration with PC increased Post-Ex but was lower than Pre-Ex at Rec (P < 0.05 vs. Pre-Ex). Mitochondrial efficiency [amount of added ADP divided by oxygen consumed during state 3 (P/O ratio)] decreased Post-Ex by 9 and 6% (P < 0.05) with PC and PC + Pyr, respectively. P/O ratio remained reduced at Rec. Muscle uncoupling protein 3, measured with Western blotting, was not changed Post-Ex but tended to decrease at Rec (P = 0.07 vs. Pre-Ex). In conclusion, extreme endurance exercise decreases mitochondrial efficiency. This will increase oxygen demand and may partly explain the observed elevation in whole body oxygen consumption during standardized exercise (+13%). The increased mitochondrial capacity for PC oxidation indicates plasticity in substrate oxidation at the mitochondrial level, which may be of advantage during prolonged exercise.

Mild mitochondrial uncoupling impacts cellular aging in human muscles in vivo

Faster aging is predicted in more active tissues and animals because of greater reactive oxygen species generation. Yet age-related cell loss is greater in less active cell types, such as type II muscle fibers. Mitochondrial uncoupling has been proposed as a mechanism that reduces reactive oxygen species production and could account for this paradox between longevity and activity. We distinguished these hypotheses by using innovative optical and magnetic resonance spectroscopic methods applied to noninvasively measured ATP synthesis and O(2) uptake in vivo in human muscle. Here we show that mitochondrial function is unchanged with age in mildly uncoupled tibialis anterior muscle (75% type I) despite a high respiratory rate in adults. In contrast, substantial uncoupling and loss of cellular [ATP] indicative of mitochondrial dysfunction with age was found in the lower respiring and well coupled first dorsal interosseus (43-50% type II) of the same subjects. These results reject respiration rate as the sole factor impacting the tempo of cellular aging. Instead, they support mild uncoupling as a mechanism protecting mitochondrial function and contributing to the paradoxical longevity of the most active muscle fibers.

Mitochondrial function, fibre types and ageing: new insights from human muscle in vivo

Mitochondrial changes are at the centre of a wide range of maladies, including diabetes, neurodegeneration and ageing-related dysfunctions. Here we describe innovative optical and magnetic resonance spectroscopic methods that non-invasively measure key mitochondrial fluxes, ATP synthesis and O(2) uptake, to permit the determination of mitochondrial coupling efficiency in vivo (P/O: half the ratio of ATP flux to O(2) uptake). Three new insights result. First, mitochondrial coupling can be measured in vivo with the rigor of a biochemical determination and provides a gold standard to define well-coupled mitochondria (P/O approximately 2.5). Second, mitochondrial coupling differs substantially among muscles in healthy adults, from values reflective of well-coupled oxidative phosphorylation in a hand muscle (P/O = 2.7) to mild uncoupling in a leg muscle (P/O = 2.0). Third, these coupling differences have an important impact on cell ageing. We found substantial uncoupling and loss of cellular [ATP] in a hand muscle indicative of mitochondrial dysfunction with age. In contrast, stable mitochondrial function was found in a leg muscle, which supports the notion that mild uncoupling is protective against mitochondrial damage with age. Thus, greater mitochondrial dysfunction is evident in muscles with higher type II muscle fibre content, which may be at the root of the preferential loss of type II fibres found in the elderly. Our results demonstrate that mitochondrial function and the tempo of ageing varies among human muscles in the same individual. These technical advances, in combination with the range of mitochondrial properties available in human muscles, provide an ideal system for studying mitochondrial function in normal tissue and the link between mitochondrial defects and cell pathology in disease.

Repeated static contractions increase mitochondrial vulnerability toward oxidative stress in human skeletal muscle

Repeated static contractions (RSC) induce large fluctuations in tissue oxygen tension and increase the generation of reactive oxygen species (ROS). This study investigated the effect of RSC on muscle contractility, mitochondrial respiratory function, and in vitro sarcoplasmic reticulum (SR) Ca(2+) kinetics in human muscle. Ten male subjects performed five bouts of static knee extension with 10-min rest in between. Each bout of RSC (target torque 66% of maximal voluntary contraction torque) was maintained to fatigue. Muscle biopsies were taken preexercise and 0.3 and 24 h postexercise from vastus lateralis. Mitochondria were isolated and respiratory function measured after incubation with H(2)O(2) (HPX) or control medium (Con). Mitochondrial function was not affected by RSC during Con. However, RSC exacerbated mitochondrial dysfunction during HPX, resulting in decreased respiratory control index, decreased mitochondrial efficiency (phosphorylated ADP-to-oxygen consumed ratio), and increased noncoupled respiration (HPX/Con post- vs. preexercise). SR Ca(2+) uptake rate was lower 0.3 vs. 24 h postexercise, whereas SR Ca(2+) release rate was unchanged. RSC resulted in long-lasting changes in muscle contractility, including reduced maximal torque, low-frequency fatigue, and faster torque relaxation. It is concluded that RSC increases mitochondrial vulnerability toward ROS, reduces SR Ca(2+) uptake rate, and causes low-frequency fatigue. Although conclusive evidence is lacking, we suggest that these changes are related to increased formation of ROS during RSC.

Cycling efficiency in humans is related to low UCP3 content and to type I fibres but not to mitochondrial efficiency

The purpose of this study was to investigate the hypothesis that cycling efficiency in vivo is related to mitochondrial efficiency measured in vitro and to investigate the effect of training status on these parameters. Nine endurance trained and nine untrained male subjects (V(O2peak) = 60.4 +/- 1.4 and 37.0 +/- 2.0 ml kg(-1) min(-1), respectively) completed an incremental submaximal efficiency test for determination of cycling efficiency (gross efficiency, work efficiency (WE) and delta efficiency). Muscle biopsies were taken from m. vastus lateralis and analysed for mitochondrial respiration, mitochondrial efficiency (MEff; i.e. P/O ratio), UCP3 protein content and fibre type composition (% MHC I). MEff was determined in isolated mitochondria during maximal (state 3) and submaximal (constant rate of ADP infusion) rates of respiration with pyruvate. The rates of mitochondrial respiration and oxidative phosphorylation per muscle mass were about 40% higher in trained subjects but were not different when expressed per unit citrate synthase (CS) activity (a marker of mitochondrial density). Training status had no influence on WE (trained 28.0 +/- 0.5, untrained 27.7 +/- 0.8%, N.S.). Muscle UCP3 was 52% higher in untrained subjects, when expressed per muscle mass (P < 0.05 versus trained). WE was inversely correlated to UCP3 (r = -0.57, P < 0.05) and positively correlated to percentage MHC I (r = 0.58, P < 0.05). MEff was lower (P < 0.05) at submaximal respiration rates (2.39 +/- 0.01 at 50% V(O2max)) than at state 3 (2.48 +/- 0.01) but was neither influenced by training status nor correlated to cycling efficiency. In conclusion cycling efficiency was not influenced by training status and not correlated to MEff, but was related to type I fibres and inversely related to UCP3. The inverse correlation between WE and UCP3 indicates that extrinsic factors may influence UCP3 activity and thus MEff in vivo.

Polyethylene glycol-superoxide dismutase prevents endotoxin-induced cardiac dysfunction

RATIONALE

Sepsis produces significant mitochondrial and contractile dysfunction in the heart, but the role of superoxide-derived free radicals in the genesis of these abnormalities is not completely understood.

OBJECTIVES

The study was designed to test the hypothesis that superoxide scavenger administration prevents endotoxin-induced cardiac mitochondrial and contractile dysfunction.

METHODS

Four groups of rats were studied, and animals were injected with either saline, endotoxin, endotoxin plus polyethylene glycol-adsorbed-superoxide dismutase (PEG-SOD; a free-radical scavenger), or PEG-SOD alone. Animals were killed 48 h after injections. We then measured cardiac mitochondrial generation of reactive oxygen species (ROS), formation of free-radical reaction products (protein carbonyls, lipid aldehydes, nitrotyrosine), mitochondrial function, and cardiac contractile function.

MEASUREMENTS AND MAIN RESULTS

Endotoxin elicited increases in cardiac mitochondrial ROS formation (p < 0.001), increases in cardiac levels of free-radical reaction products, reductions in mitochondrial ATP generation (p < 0.001), and decrements in cardiac pressure-generating capacity (p < 0.01). Administration of PEG-SOD blocked formation of free-radical reaction products, prevented mitochondrial dysfunction, and preserved cardiac contractility. For example, mitochondrial ATP generation was 923 +/- 50, 392 +/- 32, 753 +/- 25, and 763 +/- 36 nmol/min/mg, respectively, for control, endotoxin, endotoxin + PEG-SOD, and PEG-SOD groups (p < 0.001). In addition, cardiac systolic pressure generation at a diastolic pressure of 15 mm Hg averaged 110 +/- 11, 66 +/- 7, 129 +/- 10 and 124 +/- 5 mm Hg, respectively, for control, endotoxin, endotoxin + PEG-SOD, and PEG-SOD groups (p < 0.01).

CONCLUSION

These data indicate that superoxide-derived oxidants play a critical role in the development of cardiac mitochondrial and contractile dysfunction in endotoxin-induced sepsis.

Mitochondrial H2O2 production is reduced with acute and chronic eccentric exercise in rat skeletal muscle

Oxidative stress with acute/chronic exercise has been so far examined using exercise involving a combination of concentric and eccentric contractions, but skeletal muscles are likely to be injured to a greater extent by pliometric contractions. In the present study, the effects of acute and chronic bouts of downhill running exercise on mitochondrial hydrogen peroxide (H2O2) generation (fluorimetric detection of a dimer with homovanillic acid in presence of horseradish peroxidase) and oxygen consumption in conjunction with antioxidant enzymes activity were examined. The results show that acute eccentric exercise was accompanied by a significantly reduced mitochondrial H2O2 production that is likely due to a decrease in complex I of the electron transport chain (ETC). On the other hand, eccentric training leads to positive adaptations, reflected by a higher citrate synthase activity and decreased mitochondrial H2O2 production. The decrease in mitochondrial H2O2 cannot be attributed to alterations in antioxidant capacities but rather to changes in mitochondrial membrane composition characterized by an increased polyunsaturated to saturated fatty acids ratio, and decreased contents in arachidonic acid and plasmalogens. These results suggest that changes in mitochondrial membrane properties with eccentric training can affect H2O2 production by muscle mitochondria. It is hypothesized that these changes resulted in a mild uncoupling sufficient to reduce electron back flow through complex I of the ETC, the major generator of reactive oxygen species by skeletal muscle mitochondria.

Reduced insulin-mediated citrate synthase activity in cultured skeletal muscle cells from patients with type 2 diabetes: Evidence for an intrinsic oxidative enzyme defect

In myotubes established from patients with type 2 diabetes (T2D), lipid oxidation and insulin-mediated glucose oxidation are reduced, whereas in myotubes from obese non-diabetic subjects, exposure to palmitate impairs insulin-mediated glucose oxidation. To determine the underlying mechanisms of these metabolic malfunctions, we studied mitochondrial respiration, uncoupled respiration and oxidative enzyme activities (citrate synthase (CS), 3-hydroxy-acyl-CoA-dehydrogenase activity (HAD)) before and after acute exposure to insulin and/or palmitate in myotubes established from healthy lean and obese subjects and T2D patients. Basal CS activity was lower (14%) in diabetic myotubes compared with myotubes from lean controls (P=0.03). Incubation with insulin (1 microM) for 4 h increased the CS activity (26-33%) in myotubes from both lean (P=0.02) and obese controls (P<0.001), but not from diabetic subjects. Co-incubation with palmitate (0.6 mM) for 4 h abolished the stimulatory effect of insulin on CS activity in non-diabetic myotubes. No differences were detected in mitochondrial respiration and HAD activity between myotubes from non-diabetic subjects and T2D patients, and none of these measures responded to high levels of insulin and/or palmitate. These results provide evidence for an intrinsic defect in CS activity, which may play a role in the pathogenesis of T2D. Moreover, the data suggest that insulin resistance at the CS level can be induced by exposure to high free fatty acid levels.

Effect of 2 weeks of endurance training on uncoupling protein 3 content in untrained human subjects

AIM

The mitochondrial uncoupling protein-3 (UCP3) is able to lower the proton gradient across the inner mitochondrial membrane, thereby uncoupling substrate oxidation from ATP production and dissipating energy as heat. What the effect of endurance training on UCP3 is, is still controversial. Endurance-trained athletes are characterized by lower levels of UCP3, but longitudinal studies in rodents reported no effect of endurance training on muscular UCP3 levels. Here, we examined the effect of a 2-week training programme on skeletal muscle UCP3 protein content in untrained human subjects, and hypothesized that UCP3 will be reduced after the training programme.

METHODS

Nine untrained men [age: 23.3 +/- 3.2 years; BMI: 22.6 +/- 2.6 kg m(-2); maximal power output (W(max)): 3.8 +/- 0.6 W kg(-1) body weight] trained for 2 weeks. Before and at least 72 h after the training period, muscle biopsies were taken for determination of UCP3 protein content.

RESULTS

UCP3 protein content tended to be lower after the training programme [95 +/- 10 vs. 109 +/- 12 arbitrary units (AU), P = 0.08]. Cytochrome c content tended to increase with 33% in response to endurance training (52 +/- 6 vs. 39 +/- 6 AU, P = 0.08). The ratio UCP3 relative to cytochrome c tended to decrease significantly upon endurance training (2.0 +/- 0.4 vs. 3.2 +/- 0.6 AU, P = 0.01).

CONCLUSION

A short-term (2-week) endurance training programme decreased UCP3 protein levels and significantly reduced the ratio of UCP3 to cytochrome c.

Postexercise insulin sensitivity is not impaired after an overnight lipid infusion

High plasma fatty acid availability and a positive energy balance in sedentary individuals reduce insulin sensitivity. This study's purpose was to determine whether high plasma fatty acid availability and systemic caloric excess after exercise also impair insulin sensitivity. On two separate occasions, seven nonobese women performed 90 min of exercise at approximately 65% peak oxygen uptake. In one trial, a lipid + heparin emulsion (Lipid) was infused overnight to increase plasma fatty acid availability. In the other trial, saline was infused as control. The next morning, a muscle biopsy was taken to measure muscle glycogen and intramuscular triglyceride (IMTG) concentrations. Three hours after the overnight infusion was stopped, insulin sensitivity was assessed with an intravenous glucose tolerance test, using minimal model analysis (Si). During the overnight infusions, plasma fatty acid concentration was approximately fourfold higher [means (SD): 0.84 (0.36) vs. 0.22 (0.09) mmol/l; P = 0.003], and the next morning IMTG concentration was approximately 30% greater [49.2 (6.6) vs. 38.3 (7.7) mmol/kg dry wt; P = 0.036] in Lipid compared with saline. However, muscle glycogen concentration was not different between trials (P = 0.82). Lipid caused a 24-h surplus of approximately 1100 kcal above energy balance (P = 0.00001), whereas energy balance was maintained in saline. Despite these differences in fatty acid and energy availability, Si the morning after exercise was not different between trials (P = 0.72). Thus insulin sensitivity the morning after a single exercise session was not reduced despite overnight exposure to a fourfold increase in plasma fatty acid concentration, elevated IMTG concentration, and systemic delivery of approximately 1,100-kcal excess.

Rapid upregulation of pyruvate dehydrogenase kinase activity in human skeletal muscle during prolonged exercise

Prolonged moderate-intensity exercise is characterized by a progressive reduction in carbohydrate oxidation and concomitant increase in fat oxidation. Pyruvate dehydrogenase (PDH) controls the entry of pyruvate into oxidative pathways and is a rate-limiting enzyme for carbohydrate metabolism. PDH is controlled by the activities of a kinase (PDK, inhibitory) and phosphatase (stimulatory). To test the hypothesis that increased PDK activity was associated with decreased PDH activity and carbohydrate oxidation during an acute exercise bout, seven recreationally active men completed 4 h of cycle exercise at 55% peak oxygen consumption. Muscle samples were obtained before and at 10 min and 4 h of exercise for the measurement of PDH activity and the extraction of intact mitochondria for the measurements of PDK activity and PDK-2 and PDK-4 protein expression. Carbohydrate oxidation was reduced ( P < 0.05) with exercise duration. Muscle glycogen content was lower ( P ≤ 0.05) at 4 h compared with rest and there was no change in muscle pyruvate content from 10 to 240 min during exercise (10 min: 0.28 ± 0.05; 240 min: 0.35 ± 0.09 mmol/kg dry muscle). PDH activity increased ( P < 0.05) above resting values at 10 min (2.86 ± 0.26 mmol·min−1·kg wet muscle−1), but was lower than 10 min after 4 h (2.23 ± 0.24 mmol·min−1·kg wet muscle−1) of exercise. PDK-2 and PDK-4 protein expression was not different from rest at 10 min and 4 h of exercise. PDK activity at rest averaged 0.081 ± 0.016 min−1, was similar at 10 min, and increased ( P < 0.05) to 0.189 ± 0.013 min−1 at 4 h. Although reduced glycolytic flux may have played a role in decreasing carbohydrate oxidation, the results suggest that increased PDK activity contributed to the reduction in PDH activity and carbohydrate oxidation late in prolonged exercise. The increased PDK activity was independent of changes in intra-mitochondrial effectors, and PDK-2 and PDK-4 protein content, suggesting that it was caused by a change in the specific activity of the existing kinases.