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

Mitochondrial inefficiencies and anoxic ATP hydrolysis capacities in diabetic rat heart

As ~80% of diabetic patients die from heart failure, an understanding of diabetic cardiomyopathy is crucial. Mitochondria occupy 35-40% of the mammalian cardiomyocyte volume and supply 95% of the heart's ATP, and diabetic heart mitochondria show impaired structure, arrangement, and function. We predict that bioenergetic inefficiencies are present in diabetic heart mitochondria; therefore, we explored mitochondrial proton and electron handling by linking oxygen flux to steady-state ATP synthesis, reactive oxygen species (ROS) production, and mitochondrial membrane potential (ΔΨ) within rat heart tissues. Sprague-Dawley rats were injected with streptozotocin (STZ, 55 mg/kg) to induce type 1 diabetes or an equivalent volume of saline (control, n = 12) and fed standard rat chow for 8 wk. By coupling high-resolution respirometers with purpose-built fluorometers, we followed Magnesium Green (ATP synthesis), Amplex UltraRed (ROS production), and safranin-O (ΔΨ). Relative to control rats, the mass-specific respiration of STZ-diabetic hearts was depressed in oxidative phosphorylation (OXPHOS) states. Steady-state ATP synthesis capacity was almost one-third lower in STZ-diabetic heart, which, relative to oxygen flux, equates to an estimated 12% depression in OXPHOS efficiency. However, with anoxic transition, STZ-diabetic and control heart tissues showed similar ATP hydrolysis capacities through reversal of the F1F0-ATP synthase. STZ-diabetic cardiac mitochondria also produced more net ROS relative to oxygen flux (ROS/O) in OXPHOS. While ΔΨ did not differ between groups, the time to develop ΔΨ with the onset of OXPHOS was protracted in STZ-diabetic mitochondria. ROS/O is higher in lifelike OXPHOS states, and potential delays in the time to develop ΔΨ may delay ATP synthesis with interbeat fluctuations in ADP concentrations. Whereas diabetic cardiac mitochondria produce less ATP in normoxia, they consume as much ATP in anoxic infarct-like states.

Cardiac, skeletal, and smooth muscle mitochondrial respiration: are all mitochondria created equal?

Unlike cardiac and skeletal muscle, little is known about vascular smooth muscle mitochondrial respiration. Therefore, the present study examined mitochondrial respiratory rates in smooth muscle of healthy human feed arteries and compared with that of healthy cardiac and skeletal muscles. Cardiac, skeletal, and smooth muscles were harvested from a total of 22 subjects (53 ± 6 yr), and mitochondrial respiration was assessed in permeabilized fibers. Complex I + II, state 3 respiration, an index of oxidative phosphorylation capacity, fell progressively from cardiac to skeletal to smooth muscles (54 ± 1, 39 ± 4, and 15 ± 1 pmol·s(-1)·mg(-1), P < 0.05, respectively). Citrate synthase (CS) activity, an index of mitochondrial density, also fell progressively from cardiac to skeletal to smooth muscles (222 ± 13, 115 ± 2, and 48 ± 2 μmol·g(-1)·min(-1), P < 0.05, respectively). Thus, when respiration rates were normalized by CS (respiration per mitochondrial content), oxidative phosphorylation capacity was no longer different between the three muscle types. Interestingly, complex I state 2 normalized for CS activity, an index of nonphosphorylating respiration per mitochondrial content, increased progressively from cardiac to skeletal to smooth muscles, such that the respiratory control ratio, state 3/state 2 respiration, fell progressively from cardiac to skeletal to smooth muscles (5.3 ± 0.7, 3.2 ± 0.4, and 1.6 ± 0.3 pmol·s(-1)·mg(-1), P < 0.05, respectively). Thus, although oxidative phosphorylation capacity per mitochondrial content in cardiac, skeletal, and smooth muscles suggest all mitochondria are created equal, the contrasting respiratory control ratio and nonphosphorylating respiration highlight the existence of intrinsic functional differences between these muscle mitochondria. This likely influences the efficiency of oxidative phosphorylation and could potentially alter ROS production.

Fibre-specific responses to endurance and low volume high intensity interval training: striking similarities in acute and chronic adaptation

The current study involved the completion of two distinct experiments. Experiment 1 compared fibre specific and whole muscle responses to acute bouts of either low-volume high-intensity interval training (LV-HIT) or moderate-intensity continuous endurance exercise (END) in a randomized crossover design. Experiment 2 examined the impact of a six-week training intervention (END or LV-HIT; 4 days/week), on whole body and skeletal muscle fibre specific markers of aerobic and anaerobic capacity. Six recreationally active men (Age: 20.7±3.8 yrs; VO₂peak: 51.9±5.1 mL/kg/min) reported to the lab on two separate occasions for experiment 1. Following a muscle biopsy taken in a fasted state, participants completed an acute bout of each exercise protocol (LV-HIT: 8, 20-second intervals at ∼170% of VO₂peak separated by 10 seconds of rest; END: 30 minutes at ∼65% of VO₂peak), immediately followed by a muscle biopsy. Glycogen content of type I and IIA fibres was significantly (p<0.05) reduced, while p-ACC was significantly increased (p<0.05) following both protocols. Nineteen recreationally active males (n = 16) and females (n = 3) were VO₂peak-matched and assigned to either the LV-HIT (n = 10; 21±2 yrs) or END (n = 9; 20.7±3.8 yrs) group for experiment 2. After 6 weeks, both training protocols induced comparable increases in aerobic capacity (END: Pre: 48.3±6.0, Mid: 51.8±6.0, Post: 55.0±6.3 mL/kg/min LV-HIT: Pre: 47.9±8.1, Mid: 50.4±7.4, Post: 54.7±7.6 mL/kg/min), fibre-type specific oxidative and glycolytic capacity, glycogen and IMTG stores, and whole-muscle capillary density. Interestingly, only LV-HIT induced greater improvements in anaerobic performance and estimated whole-muscle glycolytic capacity. These results suggest that 30 minutes of END exercise at ∼65% VO₂peak or 4 minutes of LV-HIT at ∼170% VO₂peak induce comparable changes in the intra-myocellular environment (glycogen content and signaling activation); correspondingly, training-induced adaptations resulting for these protocols, and other HIT and END protocols are strikingly similar.

Skeletal muscle mitochondria in the elderly: effects of physical fitness and exercise training

CONTEXT

Sarcopenia is thought to be associated with mitochondrial (Mito) loss. It is unclear whether the decrease in Mito content is consequent to aging per se or to decreased physical activity.

OBJECTIVES

The objective of the study was to examine the influence of fitness on Mito content and function and to assess whether exercise could improve Mito function in older adults.

DESIGN AND SUBJECTS

Three distinct studies were conducted: 1) a cross-sectional observation comparing Mito content and fitness in a large heterogeneous cohort of older adults; 2) a case-control study comparing chronically endurance-trained older adults and sedentary (S) subjects matched for age and gender; and 3) a 4-month exercise intervention in S.

SETTING

The study was conducted at a university-based clinical research center.

OUTCOMES

Mito volume density (MitoVd) was assessed by electron microscopy from vastus lateralis biopsies, electron transport chain proteins by Western blotting, mRNAs for transcription factors involved in M biogenesis by quantitative RT-PCR, and in vivo oxidative capacity (ATPmax) by (31)P-magnetice resonance spectroscopy. Peak oxygen uptake was measured by graded exercise test.

RESULTS

Peak oxygen uptake was strongly correlated with MitoVd in 80 60- to 80-year-old adults. Comparison of chronically endurance-trained older adults vs S revealed differences in MitoVd, ATPmax, and some electron transport chain protein complexes. Finally, exercise intervention confirmed that S subjects are able to recover MitoVd, ATPmax, and specific transcription factors.

CONCLUSIONS

These data suggest the following: 1) aging per se is not the primary culprit leading to Mito dysfunction; 2) an aerobic exercise program, even at an older age, can ameliorate the loss in skeletal muscle Mito content and may prevent aging muscle comorbidities; and 3) the improvement of Mito function is all about content.

Metabolic adaptations in skeletal muscle, adipose tissue, and whole-body oxidative capacity in response to resistance training

PURPOSE

The effects of resistance training on mitochondrial biogenesis and oxidative capacity in skeletal muscle are not fully characterized, and even less is known about alterations in adipose tissue. We aimed to investigate adaptations in oxidative metabolism in skeletal muscle and adipose tissue after 8 weeks of heavy resistance training in apparently healthy young men.

METHODS

Expression of genes linked to oxidative metabolism in the skeletal muscle and adipose tissue was assessed before and after the training program. Body composition, peak oxygen uptake (VO2 peak), fat oxidation, activity of mitochondrial enzyme in muscle, and serum adiponectin levels were also determined before and after resistance training.

RESULTS

In muscle, the expression of the genes AdipoR1 and COX4 increased after resistance training (9 and 13 %, respectively), whereas the expression levels of the genes PGC-1α, SIRT1, TFAM, CPT1b, and FNDC5 did not change. In adipose tissue, the expression of the genes SIRT1 and CPT1b decreased after training (20 and 23 %, respectively). There was an increase in lean mass (from 59.7 ± 6.1 to 61.9 ± 6.2 kg), VO2 peak (from 49.7 ± 5.5 to 56.3 ± 5.0 ml/kg/min), and fat oxidation (from 6.8 ± 2.1 to 9.1 ± 2.7 mg/kg fat-free mass/min) after training, whereas serum adiponectin levels decreased significantly and enzyme activity of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase did not change.

CONCLUSION

Despite significant increases in VO2 peak, fat oxidation, and lean mass following resistance training, the total effect on gene expression and enzyme activity linked to oxidative metabolism was moderate.

Physiological underpinnings associated with differences in pace of life and metabolic rate in north temperate and neotropical birds

Animal life-history traits fall within limited ecological space with animals that have high reproductive rates having short lives, a continuum referred to as a "slow-fast" life-history axis. Animals of the same body mass at the slow end of the life-history continuum are characterized by low annual reproductive output and low mortality rate, such as is found in many tropical birds, whereas at the fast end, rates of reproduction and mortality are high, as in temperate birds. These differences in life-history traits are thought to result from trade-offs between investment in reproduction or self-maintenance as mediated by the biotic and abiotic environment. Thus, tropical and temperate birds provide a unique system to examine physiological consequences of life-history trade-offs at opposing ends of the "pace of life" spectrum. We have explored the implications of these trade-offs at several levels of physiological organization including whole-animal, organ systems, and cells. Tropical birds tend to have higher survival, slower growth, lower rates of whole-animal basal metabolic rate and peak metabolic rate, and smaller metabolically active organs compared with temperate birds. At the cellular level, primary dermal fibroblasts from tropical birds tend to have lower cellular metabolic rates and appear to be more resistant to oxidative cell stress than those of temperate birds. However, at the subcellular level, lipid peroxidation rates, a measure of the ability of lipid molecules within the cell membranes to thwart the propagation of oxidative damage, appear not to be different between tropical and temperate species. Nevertheless, lipids in mitochondrial membranes of tropical birds tend to have increased concentrations of plasmalogens (phospholipids with antioxidant properties), and decreased concentrations of cardiolipin (a complex phospholipid in the electron transport chain) compared with temperate birds.

The ratio of macronutrients, not caloric intake, dictates cardiometabolic health, aging, and longevity in ad libitum-fed mice

The fundamental questions of what represents a macronutritionally balanced diet and how this maintains health and longevity remain unanswered. Here, the Geometric Framework, a state-space nutritional modeling method, was used to measure interactive effects of dietary energy, protein, fat, and carbohydrate on food intake, cardiometabolic phenotype, and longevity in mice fed one of 25 diets ad libitum. Food intake was regulated primarily by protein and carbohydrate content. Longevity and health were optimized when protein was replaced with carbohydrate to limit compensatory feeding for protein and suppress protein intake. These consequences are associated with hepatic mammalian target of rapamycin (mTOR) activation and mitochondrial function and, in turn, related to circulating branched-chain amino acids and glucose. Calorie restriction achieved by high-protein diets or dietary dilution had no beneficial effects on lifespan. The results suggest that longevity can be extended in ad libitum-fed animals by manipulating the ratio of macronutrients to inhibit mTOR activation.

Early mitochondrial dysfunction in glycolytic muscle, but not oxidative muscle, of the fructose-fed insulin-resistant rat

Although evidence that type 2 diabetes mellitus (T2DM) is accompanied by mitochondrial dysfunction in skeletal muscle has been accumulating, a causal link between mitochondrial dysfunction and the pathogenesis of the disease remains unclear. Our study focuses on an early stage of the disease to determine whether mitochondrial dysfunction contributes to the development of T2DM. The fructose-fed (FF) rat was used as an animal model of early T2DM. Mitochondrial respiration and acylcarnitine species were measured in oxidative (soleus) and glycolytic [extensor digitorum longus (EDL)] muscle. Although FF rats displayed characteristic signs of T2DM, including hyperglycemia, hyperinsulinemia, and hypertriglyceridemia, mitochondrial content was preserved in both muscles from FF rats. The EDL muscle had reduced complex I and complex I and II respiration in the presence of pyruvate but not glutamate. The decrease in pyruvate-supported respiration was due to a decrease in pyruvate dehydrogenase activity. Accumulation of C14:1 and C14:2 acylcarnitine species and a decrease in respiration supported by long-chain acylcarnitines but not acetylcarnitine indicated dysfunctional β-oxidation in the EDL muscle. In contrast, the soleus muscle showed preserved mitochondrial respiration, pyruvate dehydrogenase activity, and increased fatty acid oxidation, as evidenced by overall reduced acylcarnitine levels. Aconitase activity, a sensitive index of reactive oxygen species production in mitochondria, was reduced exclusively in EDL muscle, which showed lower levels of the antioxidant enzymes thioredoxin reductase and glutathione peroxidase. Here, we show that the glycolytic EDL muscle is more prone to an imbalance between energy supply and oxidation caused by insulin resistance than the oxidative soleus muscle.

Intermittent and continuous high-intensity exercise training induce similar acute but different chronic muscle adaptations

High-intensity interval training (HIIT) performed in an 'all-out' manner (e.g. repeated Wingate tests) is a time-efficient strategy to induce skeletal muscle remodelling towards a more oxidative phenotype. A fundamental question that remains unclear, however, is whether the intermittent or 'pulsed' nature of the stimulus is critical to the adaptive response. In study 1, we examined whether the activation of signalling cascades linked to mitochondrial biogenesis was dependent on the manner in which an acute high-intensity exercise stimulus was applied. Subjects performed either four 30 s Wingate tests interspersed with 4 min of rest (INT) or a bout of continuous exercise (CONT) that was matched for total work (67 ± 7 kJ) and which required ∼4 min to complete as fast as possible. Both protocols elicited similar increases in markers of adenosine monophosphate-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase activation, as well as Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) mRNA expression (main effects for time, P ≤ 0.05). In study 2, we determined whether 6 weeks of the CONT protocol (3 days per week) would increase skeletal muscle mitochondrial content to a similar extent to what we have previously reported after 6 weeks of INT. Despite similar acute signalling responses to the CONT and INT protocols, training with CONT did not increase the maximal activity or protein content of a range of mitochondrial markers. However, peak oxygen uptake was higher after CONT training (from 45.7 ± 5.4 to 48.3 ± 6.5 ml kg(-1) min(-1); P < 0.05) and 250 kJ time trial performance was improved (from 26:32 ± 4:48 to 23:55 ± 4:16 min:s; P < 0.001) in our recreationally active participants. We conclude that the intermittent nature of the stimulus is important for maximizing skeletal muscle adaptations to low-volume, all-out HIIT. Despite the lack of skeletal muscle mitochondrial adaptations, our data show that a training programme based on a brief bout of high-intensity exercise, which lasted <10 min per session including warm-up, and performed three times per week for 6 weeks, improved peak oxygen uptake in young healthy subjects.

Vitamin C and E supplementation hampers cellular adaptation to endurance training in humans: a double-blind, randomised, controlled trial

In this double-blind, randomised, controlled trial, we investigated the effects of vitamin C and E supplementation on endurance training adaptations in humans. Fifty-four young men and women were randomly allocated to receive either 1000 mg of vitamin C and 235 mg of vitamin E or a placebo daily for 11 weeks. During supplementation, the participants completed an endurance training programme consisting of three to four sessions per week (primarily of running), divided into high-intensity interval sessions [4-6 × 4-6 min; >90% of maximal heart rate (HRmax)] and steady state continuous sessions (30-60 min; 70-90% of HRmax). Maximal oxygen uptake (VO2 max ), submaximal running and a 20 m shuttle run test were assessed and blood samples and muscle biopsies were collected, before and after the intervention. Participants in the vitamin C and E group increased their VO2 max (mean ± s.d.: 8 ± 5%) and performance in the 20 m shuttle test (10 ± 11%) to the same degree as those in the placebo group (mean ± s.d.: 8 ± 5% and 14 ± 17%, respectively). However, the mitochondrial marker cytochrome c oxidase subunit IV (COX4) and cytosolic peroxisome proliferator-activated receptor-γ coactivator 1 α (PGC-1α) increased in the m. vastus lateralis in the placebo group by 59 ± 97% and 19 ± 51%, respectively, but not in the vitamin C and E group (COX4: -13 ± 54%; PGC-1α: -13 ± 29%; P ≤ 0.03, between groups). Furthermore, mRNA levels of CDC42 and mitogen-activated protein kinase 1 (MAPK1) in the trained muscle were lower in the vitamin C and E group than in the placebo group (P ≤ 0.05). Daily vitamin C and E supplementation attenuated increases in markers of mitochondrial biogenesis following endurance training. However, no clear interactions were detected for improvements in VO2 max and running performance. Consequently, vitamin C and E supplementation hampered cellular adaptations in the exercised muscles, and although this did not translate to the performance tests applied in this study, we advocate caution when considering antioxidant supplementation combined with endurance exercise.

Substrate-specific changes in mitochondrial respiration in skeletal and cardiac muscle of hibernating thirteen-lined ground squirrels

During torpor, the metabolic rate (MR) of thirteen-lined ground squirrels (Ictidomys tridecemlineatus) is considerably lower relative to euthermia, resulting in part from temperature-independent mitochondrial metabolic suppression in liver and skeletal muscle, which together account for ~40% of basal MR. Although heart accounts for very little (<0.5%) of basal MR, in the present study, we showed that respiration rates were decreased up to 60% during torpor in both subsarcolemmal (SS) and intermyofibrillar (IM) mitochondria from cardiac muscle. We further demonstrated pronounced seasonal (summer vs. winter [i.e., interbout] euthermia) changes in respiration rates in both mitochondrial subpopulations in this tissue, consistent with a shift in fuel use away from carbohydrates and proteins and towards fatty acids and ketones. By contrast, these seasonal changes in respiration rates were not observed in either SS or IM mitochondria isolated from hind limb skeletal muscle. Both populations of skeletal muscle mitochondria, however, did exhibit metabolic suppression during torpor, and this suppression was 2- to 3-fold greater in IM mitochondria, which provide ATP for Ca(2+)- and myosin ATPases, the activities of which are likely quite low in skeletal muscle during torpor because animals are immobile. Finally, these changes in mitochondrial respiration rates were still evident when standardized to citrate synthase activity rather than to total mitochondrial protein.

Mitochondrial respiratory capacity and content are normal in young insulin-resistant obese humans

Considerable debate exists about whether alterations in mitochondrial respiratory capacity and/or content play a causal role in the development of insulin resistance during obesity. The current study was undertaken to determine whether such alterations are present during the initial stages of insulin resistance in humans. Young (∼23 years) insulin-sensitive lean and insulin-resistant obese men and women were studied. Insulin resistance was confirmed through an intravenous glucose tolerance test. Measures of mitochondrial respiratory capacity and content as well as H(2)O(2) emitting potential and the cellular redox environment were performed in permeabilized myofibers and primary myotubes prepared from vastus lateralis muscle biopsy specimens. No differences in mitochondrial respiratory function or content were observed between lean and obese subjects, despite elevations in H(2)O(2) emission rates and reductions in cellular glutathione. These findings were apparent in permeabilized myofibers as well as in primary myotubes. The results suggest that reductions in mitochondrial respiratory capacity and content are not required for the initial manifestation of peripheral insulin resistance.

Loss of STAT3 in mouse embryonic fibroblasts reveals its Janus-like actions on mitochondrial function and cell viability

STAT3 has been implicated in mitochondrial function; however, the physiological relevance of this action is not established. Here we studied the importance of STAT3 to the cellular response to stimuli, TNFα and serum deprivation, which increase mitochondrial reactive oxygen species (ROS) formation. Experiments were performed using wild type (WT) and STAT3 knockout (KO) mouse embryonic fibroblasts (MEF). Both WT and STAT3 KO MEF expressed similar levels of tumor necrosis factor receptor 1 (TNFR1) and exhibited comparable IκBα degradation with TNFα. However, in the absence of STAT3 nuclear accumulation of NFκB p65 with TNFα was attenuated and induction of the survival protein c-FLIPL was eliminated. Nonetheless, WT MEF were more sensitive to TNFα-induced death which was attributed to necrosis. Deletion of STAT3 decreased ROS formation induced by TNFα and serum deprivation. STAT3 deletion was associated with lower levels of complex I and rates of respiration. Relative to WT cells, mitochondria of STAT3 KO cells released significantly more cytochrome c in response to oxidative stress and had greater caspase 3 cleavage due to serum deprivation. Our findings are consistent with STAT3 being important for mitochondrial function and cell viability by ensuring mitochondrial integrity and the expression of pro-survival genes.

Effects of hypoxia-cadmium interactions on rainbow trout (Oncorhynchus mykiss) mitochondrial bioenergetics: attenuation of hypoxia-induced proton leak by low doses of cadmium

The goal of the present study was to elucidate the modulatory effects of cadmium (Cd) on hypoxia/reoxygenation-induced mitochondrial dysfunction in light of the limited understanding of the mechanisms of multiple stressor interactions in aquatic organisms. Rainbow trout (Oncorhynchus mykiss) liver mitochondria were isolated and energized with complex I substrates (malate-glutamate), and exposed to hypoxia (0>PO2<2 Torr) for 0-60 min followed by reoxygenation and measurement of coupled and uncoupled respiration and complex I enzyme activity. Thereafter, 5 min hypoxia was used to probe interactions with Cd (0-20 μmol l(-1)) and to test the hypothesis that deleterious effects of hypoxia/reoxygenation on mitochondria were mediated by reactive oxygen species (ROS). Hypoxia/reoxygenation inhibited state 3 and uncoupler-stimulated (state 3u) respiration while concomitantly stimulating states 4 and 4ol (proton leak) respiration, thus reducing phosphorylation and coupling efficiencies. Low doses of Cd (≤5 μmol l(-1)) reduced, while higher doses enhanced, hypoxia-stimulated proton leak. This was in contrast to the monotonic enhancement by Cd of hypoxia/reoxygenation-induced reductions of state 3 respiration, phosphorylation efficiency and coupling. Mitochondrial complex I activity was inhibited by hypoxia/reoxygenation, hence confirming the impairment of at least one component of the electron transport chain (ETC) in rainbow trout mitochondria. Similar to the effect on state 4 and proton leak, low doses of Cd partially reversed the hypoxia/reoxygenation-induced complex I activity inhibition. The ROS scavenger and sulfhydryl group donor N-acetylcysteine, administrated immediately prior to hypoxia exposure, reduced hypoxia/reoxygenation-stimulated proton leak without rescuing the inhibited state 3 respiration, suggesting that hypoxia/reoxygenation influences distinct aspects of mitochondria via different mechanisms. Our results indicate that hypoxia/reoxygenation impairs the ETC and sensitizes mitochondria to Cd via mechanisms that involve, at least in part, ROS. Moreover, we provide, for the first time in fish, evidence for a hormetic effect of Cd on mitochondrial bioenergetics--the attenuation of hypoxia/reoxygenation-stimulated proton leak and partial rescue of complex I inhibition by low Cd doses.

Calorie restriction influences key metabolic enzyme activities and markers of oxidative damage in distinct mouse liver mitochondrial sub-populations

AIMS

The purpose of the study was to establish if enzyme activities from key metabolic pathways and levels of markers of oxidative damage to proteins and lipids differed between distinct liver mitochondrial sub-populations, and which specific sub-populations contributed to these differences.

MAIN METHODS

Male C57BL/6J mice were fed non-purified diet for one month then separated into two groups, control and calorie-restricted (CR). The two groups were fed semi-purified diet (AIN93G), with the CR group receiving 40% less calories than controls. After two months, enzyme activities and markers of oxidative damage in mitochondria were determined.

KEY FINDINGS

In all mitochondrial sub-populations, enzyme activities and markers of oxidative damage, from control and CR groups, showed a pattern of M1>M3>M10. Higher acyl-CoA dehydrogenase (β-oxidation) and β-hydroxybutyrate dehydrogenase (ketogenesis) activities and lower carbonyl and TBARS levels were observed in M1 and M3 fractions from CR mice. ETC enzyme activities did not show a consistent pattern. In the Krebs cycle, citrate synthase and aconitase activities decreased while succinate dehydrogenase and malate dehydrogenase activities increased in the M1 mitochondria from the CR versus control mice.

SIGNIFICANCE

CR does not produce uniform changes in enzyme activities or markers of oxidative damage in mitochondrial sub-populations, with changes occurring primarily in the heavy mitochondrial populations. Centrifugation at 10,000 g to isolate mitochondria likely dilutes the mitochondrial populations which show the greatest response to CR. Use of lower centrifugal force (3000 g or lower) may be beneficial for some studies.

The role of weight loss and exercise in correcting skeletal muscle mitochondrial abnormalities in obesity, diabetes and aging

Mitochondria within skeletal muscle have been implicated in insulin resistance of obesity and type 2 diabetes mellitus as well as impaired muscle function with normal aging. Evaluating the potential of interventions to improve mitochondria is clearly relevant to the prevention or treatment of metabolic diseases and age-related dysfunction. This review provides an overview and critical evaluation of the effects of weight loss and exercise interventions on skeletal muscle mitochondria, along with implications for insulin resistance, obesity, type 2 diabetes and aging. The available literature strongly suggests that the lower mitochondrial capacity associated with obesity, type 2 diabetes and aging is not an irreversible lesion. However, weight loss does not appear to affect this response, even when the weight loss is extreme. In contrast, increasing physical activity improves mitochondrial content and perhaps the function of individual mitochondrion. Despite the consistent effect of exercise to improve mitochondrial capacity, studies mechanistically linking mitochondria to insulin resistance, reductions in intramyocellular lipid or improvement in muscle function remain inconclusive. In summary, studies of diet and exercise training have advanced our understanding of the link between mitochondrial oxidative capacity and insulin resistance in obesity, type 2 diabetes and aging. Nevertheless, additional inquiry is necessary to establish the significance and clinical relevance of those perturbations, which could lead to targeted therapies for a myriad of conditions and diseases involving mitochondria.

Can we optimise the exercise training prescription to maximise improvements in mitochondria function and content?

BACKGROUND

While there is agreement that exercise is a powerful stimulus to increase both mitochondrial function and content, we do not know the optimal training stimulus to maximise improvements in mitochondrial biogenesis.

SCOPE OF REVIEW

This review will focus predominantly on the effects of exercise on mitochondrial function and content, as there is a greater volume of published research on these adaptations and stronger conclusions can be made.

MAJOR CONCLUSIONS

The results of cross-sectional studies, as well as training studies involving rats and humans, suggest that training intensity may be an important determinant of improvements in mitochondrial function (as determined by mitochondrial respiration), but not mitochondrial content (as assessed by citrate synthase activity). In contrast, it appears that training volume, rather than training intensity, may be an important determinant of exercise-induced improvements in mitochondrial content. Exercise-induced mitochondrial adaptations are quickly reversed following a reduction or cessation of physical activity, highlighting that skeletal muscle is a remarkably plastic tissue. Due to the small number of studies, more research is required to verify the trends highlighted in this review, and further studies are required to investigate the effects of different types of training on the mitochondrial sub-populations and also mitochondrial adaptations in different fibre types. Further research is also required to better understand how genetic variants influence the large individual variability for exercise-induced changes in mitochondrial biogenesis.

GENERAL SIGNIFICANCE

The importance of mitochondria for both athletic performance and health underlines the importance of better understanding the factors that regulate exercise-induced changes in mitochondrial biogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Improvements in exercise performance with high-intensity interval training coincide with an increase in skeletal muscle mitochondrial content and function

Six sessions of high-intensity interval training (HIT) are sufficient to improve exercise capacity. The mechanisms explaining such improvements are unclear. Accordingly, the aim of this study was to perform a comprehensive evaluation of physiologically relevant adaptations occurring after six sessions of HIT to determine the mechanisms explaining improvements in exercise performance. Sixteen untrained (43 ± 6 ml·kg−1·min−1) subjects completed six sessions of repeated ( 8 – 12 ) 60 s intervals of high-intensity cycling (100% peak power output elicited during incremental maximal exercise test) intermixed with 75 s of recovery cycling at a low intensity (30 W) over a 2-wk period. Potential training-induced alterations in skeletal muscle respiratory capacity, mitochondrial content, skeletal muscle oxygenation, cardiac capacity, blood volumes, and peripheral fatigue resistance were all assessed prior to and again following training. Maximal measures of oxygen uptake (V̇o2peak; ∼8%; P = 0.026) and cycling time to complete a set amount of work (∼5%; P = 0.008) improved. Skeletal muscle respiratory capacities increased, most likely as a result of an expansion of skeletal muscle mitochondria (∼20%, P = 0.026), as assessed by cytochrome c oxidase activity. Skeletal muscle deoxygenation also increased while maximal cardiac output, total hemoglobin, plasma volume, total blood volume, and relative measures of peripheral fatigue resistance were all unaltered with training. These results suggest that increases in mitochondrial content following six HIT sessions may facilitate improvements in respiratory capacity and oxygen extraction, and ultimately are responsible for the improvements in maximal whole body exercise capacity and endurance performance in previously untrained individuals.

Time course analysis reveals gene-specific transcript and protein kinetics of adaptation to short-term aerobic exercise training in human skeletal muscle

Repeated bouts of episodic myofibrillar contraction associated with exercise training are potent stimuli for physiological adaptation. However, the time course of adaptation and the continuity between alterations in mRNA expression and protein content are not well described in human skeletal muscle. Eight healthy, sedentary males cycled for 60 min at 80% of peak oxygen consumption (VO_2peak) each day for fourteen consecutive days, resulting in an increase in VO_2peak of 17.5±3.8%. Skeletal muscle biopsies were taken at baseline, and on the morning following (+16 h after exercise) the first, third, seventh, tenth and fourteenth training sessions. Markers of mitochondrial adaptation (Cyt c and COXIV expression, and citrate synthase activity) were increased within the first week of training, but the mtDNA/nDNA ratio was unchanged by two weeks of training. Accumulation of PGC-1α and ERRα protein during training suggests a regulatory role for these factors in adaptations of mitochondrial and metabolic gene expression. A subset of genes were transiently increased after one training session, but returned to baseline levels thereafter, which is supportive of the concept of transcriptional capacity being particularly sensitive to the onset of a new level of contractile activity. Thus, gene-specific temporal patterns of induction of mRNA expression and protein content are described. Our results illustrate the phenomenology of skeletal muscle plasticity and support the notion that transcript level adjustments, coupled to accumulation of encoded protein, underlie the modulation of skeletal muscle metabolism and phenotype by regular exercise.

Silencing of maternal heme-binding protein causes embryonic mitochondrial dysfunction and impairs embryogenesis in the blood sucking insect Rhodnius prolixus

The heme molecule is the prosthetic group of many hemeproteins involved in essential physiological processes, such as electron transfer, transport of gases, signal transduction, and gene expression modulation. However, heme is a pro-oxidant molecule capable of propagating reactions leading to the generation of reactive oxygen species. The blood-feeding insect Rhodnius prolixus releases enormous amounts of heme during host blood digestion in the midgut lumen when it is exposed to a physiological oxidative challenge. Additionally, this organism produces a hemolymphatic heme-binding protein (RHBP) that transports heme to pericardial cells for detoxification and to growing oocytes for yolk granules and as a source of heme for embryo development. Here, we show that silencing of RHBP expression in female fat bodies reduced total RHBP circulating in the hemolymph, promoting oxidative damage to hemolymphatic proteins. Moreover, RHBP knockdown did not cause reduction in oviposition but led to the production of heme-depleted eggs (white eggs). A lack of RHBP did not alter oocyte fecundation. However, produced white eggs were nonviable. Embryo development cellularization and vitellin yolk protein degradation, processes that normally occur in early stages of embryogenesis, were compromised in white eggs. Total cytochrome c content, cytochrome c oxidase activity, citrate synthase activity, and oxygen consumption, parameters that indicate mitochondrial function, were significantly reduced in white eggs compared with normal dark red eggs. Our results showed that reduction of heme transport from females to growing oocytes by RHBP leads to embryonic mitochondrial dysfunction and impaired embryogenesis.

Autophagic flux and oxidative capacity of skeletal muscles during acute starvation

Autophagy is an important proteolytic pathway in skeletal muscles. The roles of muscle fiber type composition and oxidative capacity remain unknown in relation to autophagy. The diaphragm (DIA) is a fast-twitch muscle fiber with high oxidative capacity, the tibialis anterior (TA) muscle is a fast-twitch muscle fiber with low oxidative capacity, and the soleus muscle (SOL) is a slow-twitch muscle with high oxidative capacity. We hypothesized that oxidative capacity is a major determinant of autophagy in skeletal muscles. Following acute (24 h) starvation of adult C57/Bl6 mice, each muscle was assessed for autophagy and compared with controls. Autophagy was measured by monitoring autophagic flux following leupeptin (20 mg/kg) or colchicine (0.4 mg/kg/day) injection. Oxidative capacity was measured by monitoring citrate synthase activity. In control mice, autophagic flux values were significantly greater in the TA than in the DIA and SOL. In acutely starved mice, autophagic flux increased, most markedly in the TA, and several key autophagy-related genes were significantly induced. In both control and starved mice, there was a negative linear correlation of autophagic flux with citrate synthase activity. Starvation significantly induced AMPK phosphorylation and inhibited AKT and RPS6KB1 phosphorylation, again most markedly in the TA. Starvation induced Foxo1, Foxo3, and Foxo4 expression and attenuated the phosphorylation of their gene products. We conclude that both basal and starvation-induced autophagic flux are greater in skeletal muscles with low oxidative capacity as compared with those with high oxidative capacity and that this difference is mediated through selective activation of the AMPK pathway and inhibition of the AKT-MTOR pathways.

Low hydrogen peroxide production in mitochondria of the long-lived Arctica islandica: underlying mechanisms for slow aging

The observation of an inverse relationship between lifespan and mitochondrial H₂O₂ production rate would represent strong evidence for the disputed oxidative stress theory of aging. Studies on this subject using invertebrates are surprisingly lacking, despite their significance in both taxonomic richness and biomass. Bivalve mollusks represent an interesting taxonomic group to challenge this relationship. They are exposed to environmental constraints such as microbial H₂S, anoxia/reoxygenation, and temperature variations known to elicit oxidative stress. Their mitochondrial electron transport system is also connected to an alternative oxidase that might improve their ability to modulate reactive oxygen species (ROS) yield. Here, we compared H₂O₂ production rates in isolated mantle mitochondria between the longest-living metazoan--the bivalve Arctica islandica--and two taxonomically related species of comparable size. In an attempt to test mechanisms previously proposed to account for a reduction of ROS production in long-lived species, we compared oxygen consumption of isolated mitochondria and enzymatic activity of different complexes of the electron transport system in the two species with the greatest difference in longevity. We found that A. islandica mitochondria produced significantly less H₂O₂ than those of the two short-lived species in nearly all conditions of mitochondrial respiration tested, including forward, reverse, and convergent electron flow. Alternative oxidase activity does not seem to explain these differences. However, our data suggest that reduced complex I and III activity can contribute to the lower ROS production of A. islandica mitochondria, in accordance with previous studies. We further propose that a lower complex II activity could also be involved.

Rice bran extract protects from mitochondrial dysfunction in guinea pig brains

Mitochondrial dysfunction plays a major role in the development of age-related neurodegenerative diseases and recent evidence suggests that food ingredients can improve mitochondrial function. In the current study we investigated the effects of feeding a stabilized rice bran extract (RBE) on mitochondrial function in the brain of guinea pigs. Key components of the rice bran are oryzanols, tocopherols and tocotrienols, which are supposed to have beneficial effects on mitochondrial function. Concentrations of α-tocotrienol and γ-carboxyethyl hydroxychroman (CEHC) but not γ-tocotrienol were significantly elevated in brains of RBE fed animals and thus may have provided protective properties. Overall respiration and mitochondrial coupling were significantly enhanced in isolated mitochondria, which suggests improved mitochondrial function in brains of RBE fed animals. Cells isolated from brains of RBE fed animals showed significantly higher mitochondrial membrane potential and ATP levels after sodium nitroprusside (SNP) challenge indicating resistance against mitochondrial dysfunction. Experimental evidence indicated increased mitochondrial mass in guinea pig brains, e.g. enhanced citrate synthase activity, increased cardiolipin as well as respiratory chain complex I and II and TIMM levels. In addition levels of Drp1 and fis1 were also increased in brains of guinea pigs fed RBE, indicating enhanced fission events. Thus, RBE represents a potential nutraceutical for the prevention of mitochondrial dysfunction and oxidative stress in brain aging and neurodegenerative diseases.

Gastrocnemius mitochondrial respiration: are there any differences between men and women?

INTRODUCTION

Work on human and mouse skeletal muscle by our group and others has demonstrated that aging and age-related degenerative diseases are associated with mitochondrial dysfunction, which may be more prevalent in males. There have been, however, no studies that specifically examine the influence of male or female sex on human skeletal muscle mitochondrial respiration. The purpose of this study was to compare mitochondrial respiration in the gastrocnemius of adult men and women.

METHODS

Gastrocnemius muscle was obtained from male (n = 19) and female (n = 11) human subjects with healthy lower-extremity musculoskeletal and arterial systems and normal ambulatory function. All patients were undergoing operations for the treatment of varicose veins in their legs. Mitochondrial respiration was determined with a Clark electrode in an oxygraph cell containing saponin-skinned muscle bundles. Complex I-, II-, III-, and IV-dependent respiration was measured individually and normalized to muscle weight, total protein content, and citrate synthase (CS, index of mitochondrial content).

RESULTS

Male and female patients had no evidence of musculoskeletal or arterial disease and did not differ with regard to age, race, body mass index, or other clinical characteristics. Complex I-, II-, III-, and IV-dependent respiration normalized to muscle weight, total protein content, and CS did not statistically differ for males compared with females.

CONCLUSIONS

Our study evaluates, for the first time, gastrocnemius mitochondrial respiration of adult men and women who have healthy musculoskeletal and arterial systems and normal ambulatory function. Our data demonstrate there are no differences in the respiration of gastrocnemius mitochondria between men and women.

Diet influences the intake target and mitochondrial functions of Drosophila melanogaster males

In this study, we examine the dietary protein to carbohydrate ratio (P:C) on the mitochondrial functions of two Drosophila melanogaster mtDNA haplotypes. We investigated multiple physiological parameters on flies fed with either 1:12 P:C or 1:3 P:C diets. Our results provide experimental evidence that a specific haplotype has a reduction of complex I activity when the flies are fed with the 1:12 P:C diet. This study is of particular importance to understand the influence of diet on mitochondrial evolution in invasive and broadly distributed species including humans.

The impact of severe burns on skeletal muscle mitochondrial function

Severe burns induce a pathophysiological response that affects almost every physiological system within the body. Inflammation, hypermetabolism, muscle wasting, and insulin resistance are all hallmarks of the pathophysiological response to severe burns, with perturbations in metabolism known to persist for several years post injury. Skeletal muscle is the principal depot of lean tissue within the body and as the primary site of peripheral glucose disposal, plays an important role in metabolic regulation. Following a large burn, skeletal muscle functions as and endogenous amino acid store, providing substrates for more pressing functions, such as the synthesis of acute phase proteins and the deposition of new skin. Subsequently, burn patients become cachectic, which is associated with poor outcomes in terms of metabolic health and functional capacity. While a loss of skeletal muscle contractile proteins per se will no doubt negatively impact functional capacity, detriments in skeletal muscle quality, i.e. a loss in mitochondrial number and/or function may be quantitatively just as important. The goal of this review article is to summarise the current understanding of the impact of thermal trauma on skeletal muscle mitochondrial content and function, to offer direction for future research concerning skeletal muscle mitochondrial function in patients with severe burns, and to renew interest in the role of these organelles in metabolic dysfunction following severe burns.

Physiological adaptations to reproduction. II. Mitochondrial adjustments in livers of lactating mice

Reproduction imposes significant costs and is characterized by an increased energy demand. As a consequence, individuals adjust their cellular structure and function in response to this physiological constraint. Because mitochondria are central to energy production, changes in their functional properties are likely to occur during reproduction. Such changes could cause adjustments in reactive oxygen species (ROS) production and consequently in oxidative stress levels. In this study, we investigated several mechanisms involved in energy production, including mitochondrial respiration at different steps of the electron transport system (ETS) and related the results to citrate synthase activity in the liver of non-reproductive and reproductive (two and eight pups) female house mice at peak lactation. Whereas we did not find differences between females having different litter sizes, liver mitochondria of reproductive females showed lower ETS activity and an increase in mitochondrial density when compared with the non-reproductive females. Although it is possible that these changes were due to combined processes involved in reproduction and not to the relative investment in lactation, we propose that the mitochondrial adjustment in liver might help to spare substrates and therefore energy for milk production in the mammary gland. Moreover, our results suggest that these changes lead to an increase in ROS production that subsequently upregulates antioxidant defence activity and decreases oxidative stress.

Xanthine oxidase inhibition attenuates skeletal muscle signaling following acute exercise but does not impair mitochondrial adaptations to endurance training

The aim of this research was to examine the impact of the xanthine oxidase (XO) inhibitor allopurinol on the skeletal muscle activation of cell signaling kinases' and adaptations to mitochondrial proteins and antioxidant enzymes following acute endurance exercise and endurance training. Male Sprague-Dawley rats performed either acute exercise (60 min of treadmill running, 27 m/min, 5% incline) or 6 wk of endurance training (5 days/wk) while receiving allopurinol or vehicle. Allopurinol treatment reduced XO activity to 5% of the basal levels (P < 0.05), with skeletal muscle uric acid levels being almost undetectable. Following acute exercise, skeletal muscle oxidized glutathione (GSSG) significantly increased in allopurinol- and vehicle-treated groups despite XO activity and uric acid levels being unaltered by acute exercise (P < 0.05). This suggests that the source of ROS was not from XO. Surprisingly, muscle GSSG levels were significantly increased following allopurinol treatment. Following acute exercise, allopurinol treatment prevented the increase in p38 MAPK and ERK phosphorylation and attenuated the increase in mitochondrial transcription factor A (mtTFA) mRNA (P < 0.05) but had no effect on the increase in peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), nuclear respiratory factor-2, GLUT4, or superoxide dismutase mRNA. Allopurinol also had no impact on the endurance training-induced increases in PGC-1α, mtTFA, and mitochondrial proteins including cytochrome c, citrate synthase, and β-hydroxyacyl-CoA dehydrogenase. In conclusion, although allopurinol inhibits cell signaling pathways in response to acute exercise, the inhibitory effects of allopurinol appear unrelated to exercise-induced ROS production by XO. Allopurinol also has little effect on increases in mitochondrial proteins following endurance training.

Simvastatin impairs exercise training adaptations

OBJECTIVES

This study sought to determine if simvastatin impairs exercise training adaptations.

BACKGROUND

Statins are commonly prescribed in combination with therapeutic lifestyle changes, including exercise, to reduce cardiovascular disease risk in patients with metabolic syndrome. Statin use has been linked to skeletal muscle myopathy and impaired mitochondrial function, but it is unclear whether statin use alters adaptations to exercise training.

METHODS

This study examined the effects of simvastatin on changes in cardiorespiratory fitness and skeletal muscle mitochondrial content in response to aerobic exercise training. Sedentary overweight or obese adults with at least 2 metabolic syndrome risk factors (defined according to National Cholesterol Education Panel Adult Treatment Panel III criteria) were randomized to 12 weeks of aerobic exercise training or to exercise in combination with simvastatin (40 mg/day). The primary outcomes were cardiorespiratory fitness and skeletal muscle (vastus lateralis) mitochondrial content (citrate synthase enzyme activity).

RESULTS

Thirty-seven participants (exercise plus statins: n = 18; exercise only: n = 19) completed the study. Cardiorespiratory fitness increased by 10% (p < 0.05) in response to exercise training alone, but was blunted by the addition of simvastatin resulting in only a 1.5% increase (p < 0.005 for group by time interaction). Similarly, skeletal muscle citrate synthase activity increased by 13% in the exercise-only group (p < 0.05), but decreased by 4.5% in the simvastatin-plus-exercise group (p < 0.05 for group-by-time interaction).

CONCLUSIONS

Simvastatin attenuates increases in cardiorespiratory fitness and skeletal muscle mitochondrial content when combined with exercise training in overweight or obese patients at risk of the metabolic syndrome. (Exercise, Statins, and the Metabolic Syndrome; NCT01700530).

Methods for assessing mitochondrial function in diabetes

A growing body of research is investigating the potential contribution of mitochondrial function to the etiology of type 2 diabetes. Numerous in vitro, in situ, and in vivo methodologies are available to examine various aspects of mitochondrial function, each requiring an understanding of their principles, advantages, and limitations. This review provides investigators with a critical overview of the strengths, limitations and critical experimental parameters to consider when selecting and conducting studies on mitochondrial function. In vitro (isolated mitochondria) and in situ (permeabilized cells/tissue) approaches provide direct access to the mitochondria, allowing for study of mitochondrial bioenergetics and redox function under defined substrate conditions. Several experimental parameters must be tightly controlled, including assay media, temperature, oxygen concentration, and in the case of permeabilized skeletal muscle, the contractile state of the fibers. Recently developed technology now offers the opportunity to measure oxygen consumption in intact cultured cells. Magnetic resonance spectroscopy provides the most direct way of assessing mitochondrial function in vivo with interpretations based on specific modeling approaches. The continuing rapid evolution of these technologies offers new and exciting opportunities for deciphering the potential role of mitochondrial function in the etiology and treatment of diabetes.

Impaired mitochondrial function in chronically ischemic human heart

Chronic ischemic heart disease is associated with myocardial hypoperfusion. The resulting hypoxia potentially inflicts damage upon the mitochondria, leading to a compromised energetic state. Furthermore, ischemic damage may cause excessive production of reactive oxygen species (ROS), producing mitochondrial damage, hereby reinforcing a vicious circle. Ischemic preconditioning has been proven protective in acute ischemia, but the subject of chronic ischemic preconditioning has not been explored in humans. We hypothesized that mitochondrial respiratory capacity would be diminished in chronic ischemic regions of human myocardium but that these mitochondria would be more resistant to ex vivo ischemia and, second, that ROS generation would be higher in ischemic myocardium. The aim of this study was to test mitochondrial respiratory capacity during hyperoxia and hypoxia, to investigate ROS production, and finally to assess myocardial antioxidant levels. Mitochondrial respiration in biopsies from ischemic and nonischemic regions from the left ventricle of the same heart was compared in nine human subjects. Maximal oxidative phosphorylation capacity in fresh muscle fibers was lower in ischemic compared with nonischemic myocardium (P < 0.05), but the degree of coupling (respiratory control ratio) did not differ (P > 0.05). The presence of ex vivo hypoxia did not reveal any chronic ischemic preconditioning of the ischemic myocardial regions (P > 0.05). ROS production was higher in ischemic myocardium (P < 0.05), and the levels of antioxidant protein expression was lower. Diminished mitochondrial respiration capacity and excessive ROS production demonstrate an impaired mitochondrial function in ischemic human heart muscle. No chronic ischemic preconditioning effect was found.

MicroRNA-133 controls brown adipose determination in skeletal muscle satellite cells by targeting Prdm16

Brown adipose tissue (BAT) is an energy-dispensing thermogenic tissue that plays an important role in balancing energy metabolism. Lineage-tracing experiments indicate that brown adipocytes are derived from myogenic progenitors during embryonic development. However, adult skeletal muscle stem cells (satellite cells) have long been considered uniformly determined toward the myogenic lineage. Here, we report that adult satellite cells give rise to brown adipocytes and that microRNA-133 regulates the choice between myogenic and brown adipose determination by targeting the 3'UTR of Prdm16. Antagonism of microRNA-133 during muscle regeneration increases uncoupled respiration, glucose uptake, and thermogenesis in local treated muscle and augments whole-body energy expenditure, improves glucose tolerance, and impedes the development of diet-induced obesity. Finally, we demonstrate that miR-133 levels are downregulated in mice exposed to cold, resulting in de novo generation of satellite cell-derived brown adipocytes. Therefore, microRNA-133 represents an important therapeutic target for the treatment of obesity.

Short-term intensified cycle training alters acute and chronic responses of PGC1α and Cytochrome C oxidase IV to exercise in human skeletal muscle

Reduced activation of exercise responsive signalling pathways have been reported in response to acute exercise after training; however little is known about the adaptive responses of the mitochondria. Accordingly, we investigated changes in mitochondrial gene expression and protein abundance in response to the same acute exercise before and after 10-d of intensive cycle training. Nine untrained, healthy participants (mean±SD; VO_2peak 44.1±17.6 ml/kg/min) performed a 60 min bout of cycling exercise at 164±18 W (72% of pre-training VO_2peak). Muscle biopsies were obtained from the vastus lateralis muscle at rest, immediately and 3 h after exercise. The participants then underwent 10-d of cycle training which included four high-intensity interval training sessions (6×5 min; 90–100% VO_2peak) and six prolonged moderate-intensity sessions (45–90 min; 75% VO_2peak). Participants repeated the pre-training exercise trial at the same absolute work load (64% of pre-training VO_2peak). Muscle PGC1-α mRNA expression was attenuated as it increased by 11- and 4- fold (P<0.001) after exercise pre- and post-training, respectively. PGC1-α protein expression increased 1.5 fold (P<0.05) in response to exercise pre-training with no further increases after the post-training exercise bout. RIP140 protein abundance was responsive to acute exercise only (P<0.01). COXIV mRNA (1.6 fold; P<0.01) and COXIV protein expression (1.5 fold; P<0.05) were increased by training but COXIV protein expression was decreased (20%; P<0.01) by acute exercise pre- and post-training. These findings demonstrate that short-term intensified training promotes increased mitochondrial gene expression and protein abundance. Furthermore, acute indicators of exercise-induced mitochondrial adaptation appear to be blunted in response to exercise at the same absolute intensity following short-term training.

Mitochondria express enhanced quality as well as quantity in association with aerobic fitness across recreationally active individuals up to elite athletes

Changes in skeletal muscle respiratory capacity parallel that of aerobic fitness. It is unknown whether mitochondrial content, alone, can fully account for these differences in skeletal muscle respiratory capacity. The aim of the present study was to examine quantitative and qualitative mitochondrial characteristics across four different groups (n = 6 each), separated by cardiorespiratory fitness. High-resolution respirometry was performed on muscle samples to compare respiratory capacity and efficiency in active, well-trained, highly trained, and elite individuals. Maximal exercise capacity (ml O(2)·min(-1)·kg(-1)) differed across all groups, with mean ± SD values of 51 ± 4, 64 ± 5, 71 ± 2, and 77 ± 3, respectively. Mitochondrial content assessed by citrate synthase activity was higher in elite trained compared with active and well-trained (29 ± 7 vs. 16 ± 4 and 19 ± 4 nmol·min(-1)·mg wet wt(-1), respectively). When normalizing respiration to mitochondrial content, the respiratory capacities during maximal fatty acid oxidation (P = 0.003), maximal state 3 respiration (P = 0.021), and total electron transport system capacity (P = 0.008) improved with respect to maximal exercise capacity. The coupling efficiency of β-oxidation, however, expressed no difference across groups. These data demonstrate the quantitative and qualitative differences that exist in skeletal muscle mitochondrial respiratory capacity and efficiency across individuals that differ in aerobic capacity. Mitochondrial-specific respiration capacities during β-oxidation, maximal oxidative phosphorylation, and electron transport system capacity all correspondingly improve with aerobic capacity, independent of mitochondrial content in human skeletal muscle.

The C57Bl/6 mouse serves as a suitable model of human skeletal muscle mitochondrial function

It is debatable whether differences in mitochondrial function exist across skeletal muscle types and whether mouse skeletal muscle mitochondrial function can serve as a valid model for human skeletal muscle mitochondrial function. The aims of this study were to compare and contrast three different mouse skeletal muscles and to identify the mouse muscle that most closely resembles human skeletal muscle respiratory capacity and control. Mouse quadriceps (QUAD(M)), soleus (SOL(M)) and gastrocnemius (GAST(M)) skeletal muscles were obtained from 8- to 10-week-old healthy mice (n = 8), representing mixed, oxidative and glycolytic muscle, respectively. Skeletal muscle samples were also collected from young, active, healthy human subjects (n = 8) from the vastis lateralis (QUAD(H)). High-resolution respirometry was used to examine mitochondrial function in all skeletal muscle samples, and mitochondrial content was quantified with citrate synthase activity. Mass-specific respiration was higher across all respiratory states in SOL(M) versus both GAST(M) and QUAD(H) (P < 0.01). When controlling for mitochondrial content, however, SOL(M) respiration was lower than GAST(M) and QUAD(H) (P < 0.05 and P < 0.01, respectively). When comparing respiratory capacity between mouse and human muscle, QUAD(M) exhibited only one different respiratory state when compared with QUAD(H). These results demonstrate that qualitative differences in mitochondrial function exist between different mouse skeletal muscles types when respiratory capacity is normalized to mitochondrial content, and that skeletal muscle respiratory capacity in young, healthy QUAD(M) does correspond well with that of young, healthy QUAD(H).

PGC-1α overexpression results in increased hepatic fatty acid oxidation with reduced triacylglycerol accumulation and secretion

Studies have shown that decreased mitochondrial content and function are associated with hepatic steatosis. We examined whether peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) overexpression and a subsequent increase in mitochondrial content and function in rat primary hepatocytes (in vitro) and Sprague-Dawley rats (in vivo) would comprehensively alter mitochondrial lipid metabolism, including complete (CO(2)) and incomplete (acid-soluble metabolites) fatty acid oxidation (FAO), tricarboxylic acid cycle flux, and triacylglycerol (TAG) storage and export. PGC-1α overexpression in primary hepatocytes produced an increase in markers of mitochondrial content and function (citrate synthase, mitochondrial DNA, and electron transport system complex proteins) and an increase in FAO, which was accompanied by reduced TAG storage and TAG secretion compared with control. Also, the PGC-1α-overexpressing hepatocytes were protected from excess TAG accumulation following overnight lipid treatment. PGC-1α overexpression in hepatocytes lowered expression of genes critical to VLDL assembly and secretion (apolipoprotein B and microsomal triglyceride transfer protein). Adenoviral transduction of rats with PGC-1α resulted in a liver-specific increase in PGC-1α expression and produced an in vivo liver phenotype of increased FAO via increased mitochondrial function that also resulted in reduced hepatic TAG storage and decreased plasma TAG levels. In conclusion, overexpression of hepatic PGC-1α and subsequent increases in FAO through elevated mitochondrial content and/or function result in reduced TAG storage and secretion in the in vitro and in vivo milieu.

Insulin resistance and mitochondrial function in skeletal muscle

The mechanism(s) behind the decreased ability of insulin to facilitate glucose uptake in insulin sensitive tissues as seen in type 2 diabetes is not resolved. With the rapidly increasing prevalence of this disease world-wide, and the many complications that follow the disease, large resources are used in the attempt to resolve the mechanisms of insulin resistance. In this context, a dysfunction of mitochondria in the skeletal muscle has been suggested to play a pivotal role. It has been postulated that a decrease in the content of mitochondria in the skeletal muscle can explain the insulin resistance. Complementary to this also specific defects of components in the respiratory chain in the mitochondria have been suggested to play a role in insulin resistance. A key element in these mechanistic suggestions is inability to handle substrate fluxes and subsequently an accumulation of ectopic intramyocellular lipids, interfering with insulin signaling. In this review we will present the prevailing view-points and argue for the unlikelihood of this scenario being instrumental in human insulin resistance. This article is part of a Directed Issue entitled: Bioenergetic dysfunction.

Twenty-eight days at 3454-m altitude diminishes respiratory capacity but enhances efficiency in human skeletal muscle mitochondria

Modifications of skeletal muscle mitochondria following exposure to high altitude (HA) are generally studied by morphological examinations and biochemical analysis of expression. The aim of this study was to examine tangible measures of mitochondrial function following a prolonged exposure to HA. For this purpose, skeletal muscle biopsies were obtained from 8 lowland natives at sea level (SL) prior to exposure and again after 28 d of exposure to HA at 3454 m. High-resolution respirometry was performed on the muscle samples comparing respiratory capacity and efficiency. Exercise capacity was assessed at SL and HA. Respirometric analysis revealed that mitochondrial respiratory capacity diminished in complex I- and complex II-specific respiration in addition to a loss of maximal state-3 oxidative phosphorylation capacity from SL to HA, all independent from alterations in mitochondrial content. Leak control coupling, respiratory control ratio, and oligomycin-induced leak respiration, all measures of mitochondrial efficiency, improved in response to HA exposure. SL respiratory capacities correlated with measures of exercise capacity near SL, whereas mitochondrial efficiency correlated best with exercise capacity following HA. This data demonstrate that 1 mo of exposure to HA reduces respiratory capacity in human skeletal muscle; however, the efficiency of electron transport improves.