Increased mitochondrial substrate sensitivity in skeletal muscle of patients with type 2 diabetes

A practical perspective on how to develop, implement, execute, and reproduce high-resolution respirometry experiments: The physiologist's guide to an Oroboros O2k

The development of high-resolution respirometry (HRR) has greatly expanded the analytical scope to study mitochondrial respiratory control relative to specific tissue/cell types across various metabolic states. Specifically, the Oroboros Oxygraph 2000 (O2k) is a common tool for measuring rates of mitochondrial respiration and is the focus of this perspective. The O2k platform is amenable for answering numerous bioenergetic questions. However, inherent variability with HRR-derived data, both within and amongst users, can impede progress in bioenergetics research. Therefore, we advocate for several vital considerations when planning and conducting O2k experiments to ultimately enhance transparency and reproducibility across laboratories. In this perspective, we offer guidance for best practices of mitochondrial preparation, protocol selection, and measures to increase reproducibility. The goal of this perspective is to propagate the use of the O2k, enhance reliability and validity for both new and experienced O2k users, and provide a reference for peer reviewers.

Comprehensive interrogation of human skeletal muscle reveals a dissociation between insulin resistance and mitochondrial capacity

Insulin resistance and blunted mitochondrial capacity in skeletal muscle are often synonymous, however, this association remains controversial. The aim of this study was to perform an in-depth multifactorial comparison of skeletal muscle mitochondrial capacity between individuals who were lean and active (Active, n = 9), individuals with obesity (Obese, n = 9), and individuals with obesity, insulin resistance, and type 2 diabetes (T2D, n = 22). Mitochondrial capacity was assessed by ex vivo mitochondrial respiration with fatty-acid and glycolytic-supported protocols adjusted for mitochondrial content (mtDNA and citrate synthase activity). Supercomplex assembly was measured by Blue Native (BN)-PAGE and immunoblot. Tricarboxylic (TCA) cycle intermediates were assessed with targeted metabolomics. Exploratory transcriptomics and DNA methylation analyses were performed to uncover molecular differences affecting mitochondrial function among the three groups. We reveal no discernable differences in skeletal muscle mitochondrial content, mitochondrial capacity, supercomplex assembly, TCA cycle intermediates, and mitochondrial molecular profiles between obese individuals with and without T2D that had comparable levels of confounding factors (body mass index, age, and aerobic capacity). We highlight that lean, active individuals have greater mitochondrial content, mitochondrial capacity, supercomplex assembly, and TCA cycle intermediates. These phenotypical changes are reflected at the level of DNA methylation and gene transcription. The collective observation of comparable muscle mitochondrial capacity in individuals with obesity and T2D (vs. individuals without T2D) underscores a dissociation from skeletal muscle insulin resistance. Clinical trial number: NCT01911104.NEW & NOTEWORTHY Whether impaired mitochondrial capacity contributes to skeletal muscle insulin resistance is debated. Our multifactorial analysis shows no differences in skeletal muscle mitochondrial content, mitochondrial capacity, and mitochondrial molecular profiles between obese individuals with and without T2D that had comparable levels of confounding factors (BMI, age, aerobic capacity). We highlight that lean, active individuals have enhanced skeletal muscle mitochondrial capacity that is also reflected at the level of DNA methylation and gene transcription.

Metabolic effects of alternate-day fasting in males with obesity with or without type 2 diabetes

Alternate-day fasting induces oscillations in energy stores. We hypothesized that repeated oscillations increases insulin secretion and sensitivity, and improve metabolic health in patients with obesity with or without type 2 diabetes (T2DM). Twenty-three male patients fasted every other day for 30 h for 6 weeks. Experiments included resting energy expenditure, continuous glucose monitoring, intravenous glucose tolerance test, euglycemic hyperinsulinemic clamp, body composition, hepatic triglyceride content, muscle biopsies which were performed at baseline, during 3 weeks without allowed weight loss, and after additional 3 weeks with weight loss. Bodyweight decreased ∼1% and further ∼3% during weeks one to three and four to six, respectively (p < 0.05). Only minor changes in fat mass occurred in weeks 1-3. With weight loss, visceral fat content decreased by 13 ± 3% and 12 ± 2% from baseline in patients with and without T2DM, respectively (p < 0.05). Hepatic triglyceride content decreased by 17 ± 9% and 36 ± 9% (with diabetes) and 27 ± 8% and 40 ± 8% (without diabetes) from baseline to week 3 and week 6, respectively (all p < 0.05). Muscle lipid and glycogen content oscillated with the intervention. Glucose homeostasis, insulin secretion and sensitivity was impaired in patients with T2DM and did not change without weight loss, but improved (p < 0.05) when alternate day fasting was combined with weight loss. In conclusion, alternate-day fasting is feasible in patients with obesity and T2DM, and decreases visceral fat and liver fat deposits. Energy store oscillations by alternate-day fasting do not improve insulin secretion or sensitivity per se. Clinical Trial registration: (ClinicalTrials.gov), (ID NCT02420054).

Normalization of maternal adiponectin in obese pregnant mice prevents programming of impaired glucose metabolism in adult offspring

Infants born to obese mothers have a greater risk for childhood obesity and insulin resistance. However, the underlying biological mechanism remains elusive, which constitutes a significant roadblock for developing specific prevention strategies. Maternal adiponectin levels are lower in obese pregnant women, which is linked with increased placental nutrient transport and fetal overgrowth. We have previously reported that adiponectin supplementation to obese dams during the last four days of pregnancy prevented the development of obesity, glucose intolerance, muscle insulin resistance, and fatty liver in three months old offspring. In the present study, we tested the hypothesis that 6-9-month-old offspring of obese dams show glucose intolerance associated with muscle insulin resistance and mitochondrial dysfunction and that normalization of maternal adiponectin in obese pregnant mice prevents the development of this phenotype in the offspring. Male and female offspring of obese mice exhibited in vivo glucose intolerance and insulin resistance at 6 and 9 months of age. In gastrocnemius muscles ex vivo, male and female offspring of obese dams showed reduced phosphorylation of insulin receptor substrate 1^Tyr-608 , Akt^Thr-308 , and decreased Glut4 plasma membrane translocation upon insulin stimulation. These metabolic abnormalities in offspring born to obese mice were largely prevented by normalization of maternal adiponectin levels in late pregnancy. We provide evidence that low circulating maternal adiponectin is a critical mechanistic link between maternal obesity and the development of metabolic disease in offspring. Strategies aimed at improving maternal adiponectin levels may prevent long-term metabolic dysfunction in offspring of obese mothers.

Leptin receptor defect with diabetes causes skeletal muscle atrophy in female obese Zucker rats where peculiar depots networked with mitochondrial damages

Tibialis anterior muscles of 45-week-old female obese Zucker rats with defective leptin receptor and non-insulin dependent diabetes mellitus (NIDDM) showed a significative atrophy compared to lean muscles, based on histochemical-stained section's measurements in the sequence: oxidative slow twitch (SO, type I) < oxidative fast twitch (FOG, type IIa) < fast glycolytic (FG, type IIb). Both oxidative fiber's outskirts resembled 'ragged' fibers and, in these zones, ultrastructure revealed small clusters of endoplasm-like reticulum filled with unidentified electron contrasted compounds, contiguous and continuous with adjacent mitochondria envelope. The linings appeared crenated stabbed by circular patterns resembling those found of ceramides. The same fibers contained scattered degraded mitochondria that tethered electron contrasted droplets favoring larger depots while mitoptosis were widespread in FG fibers. Based on other interdisciplinary investigations on the lipid depots of diabetes 2 muscles made us to propose these accumulated contrasted contents to be made of peculiar lipids, including acyl-ceramides, as those were only found while diabetes 2 progresses in aging obese rats. These could interfere in NIDDM with mitochondrial oxidative energetic demands and muscle functions.

The Response of Mitochondrial Respiration and Quantity in Skeletal Muscle and Adipose Tissue to Exercise in Humans with Prediabetes

Background: Mitochondrial dysfunction has been implicated in the pathogenesis of type 2 diabetes, but its contribution to the early stages of dysglycemia remains poorly understood. By collecting a high-resolution stage-based spectrum of dysglycemia, our study fills this gap by evaluating derangement in both the function and quantity of mitochondria. We sampled mitochondria in skeletal muscle and subcutaneous adipose tissues of subjects with progressive advancement of dysglycemia under a three-month exercise intervention. Methods: We measured clinical metabolic parameters and gathered skeletal muscle and adipose tissue biopsies before and after the three-month exercise intervention. We then assayed the number of mitochondria via citrate synthase (CS) activity and functional parameters with high-resolution respirometry. Results: In muscle, there were no differences in mitochondrial quantity or function at baseline between normoglycemics and prediabetics. However, the intervention caused improvement in CS activity, implying an increase in mitochondrial quantity. By contrast in adipose tissue, baseline differences in CS activity were present, with the lowest CS activity coincident with impaired fasting glucose and impaired glucose tolerance (IFG + IGT). Finally, CS activity, but few of the functional metrics, improved under the intervention. Conclusions: We show that in prediabetes, no differences in the function or amount of mitochondria (measured by CS activity) in skeletal muscle are apparent, but in adipose tissue of subjects with IFG + IGT, a significantly reduced activity of CS was observed. Finally, metabolic improvements under the exercise correlate to improvements in the amount, rather than function, of mitochondria in both tissues.

Normal to enhanced intrinsic mitochondrial respiration in skeletal muscle of middle- to older-aged women and men with uncomplicated type 1 diabetes

AIMS/HYPOTHESIS

This study interrogated mitochondrial respiratory function and content in skeletal muscle biopsies of healthy adults between 30 and 72 years old with and without uncomplicated type 1 diabetes.

METHODS

Participants (12 women/nine men) with type 1 diabetes (48 ± 11 years of age), without overt complications, were matched for age, sex, BMI and level of physical activity to participants without diabetes (control participants) (49 ± 12 years of age). Participants underwent a Bergström biopsy of the vastus lateralis to assess mitochondrial respiratory function using high-resolution respirometry and citrate synthase activity. Electron microscopy was used to quantify mitochondrial content and cristae (pixel) density.

RESULTS

Mean mitochondrial area density was 27% lower (p = 0.006) in participants with type 1 diabetes compared with control participants. This was largely due to smaller mitochondrial fragments in women with type 1 diabetes (-18%, p = 0.057), as opposed to a decrease in the total number of mitochondrial fragments in men with diabetes (-28%, p = 0.130). Mitochondrial respiratory measures, whether estimated per milligram of tissue (i.e. mass-specific) or normalised to area density (i.e. intrinsic mitochondrial function), differed between cohorts, and demonstrated sexual dimorphism. Mass-specific mitochondrial oxidative phosphorylation (OXPHOS) capacity with the substrates for complex I and complex II (C_I + II) was significantly lower (-24%, p = 0.033) in women with type 1 diabetes compared with control participants, whereas mass-specific OXPHOS capacities with substrates for complex I only (pyruvate [C_{I pyr}] or glutamate [C_{I glu}]) or complex II only (succinate [C_{II succ}]) were not different (p > 0.404). No statistical differences (p > 0.397) were found in mass-specific OXPHOS capacity in men with type 1 diabetes compared with control participants despite a 42% non-significant increase in C_{I glu} OXPHOS capacity (p = 0.218). In contrast, intrinsic C_I + II OXPHOS capacity was not different in women with type 1 diabetes (+5%, p = 0.378), whereas in men with type 1 diabetes it was 25% higher (p = 0.163) compared with control participants. Men with type 1 diabetes also demonstrated higher intrinsic OXPHOS capacity for C_{I pyr} (+50%, p = 0.159), C_{I glu} (+88%, p = 0.033) and C_{II succ} (+28%, p = 0.123), as well as higher intrinsic respiratory rates with low (more physiological) concentrations of either ADP, pyruvate, glutamate or succinate (p < 0.012). Women with type 1 diabetes had higher (p < 0.003) intrinsic respiratory rates with low concentrations of succinate only. Calculated aerobic fitness (Physical Working Capacity Test [PWC₁₃₀]) showed a strong relationship with mitochondrial respiratory function and content in the type 1 diabetes cohort.

CONCLUSIONS/INTERPRETATION

In middle- to older-aged adults with uncomplicated type 1 diabetes, we conclude that skeletal muscle mitochondria differentially adapt to type 1 diabetes and demonstrate sexual dimorphism. Importantly, these cellular alterations were significantly associated with our metric of aerobic fitness (PWC₁₃₀) and preceded notable impairments in skeletal mass and strength.

High metabolic substrate load induces mitochondrial dysfunction in rat skeletal muscle microvascular endothelial cells

The influence of glucose and palmitic acid (PA) on mitochondrial respiration and emission of hydrogen peroxide (H2 O2 ) was determined in skeletal muscle-derived microvascular endothelial cells. Measurements were assessed in intact and permeabilized (cells treated with 0.025% saponin) low passage endothelial cells with acute-or prolonged (3 days) incubation with regular (1.7 mM) or elevated (2.2 mM) PA concentrations and regular (5 mM) or elevated (11 mM) glucose concentrations. In intact cells, acute incubation with 1.7 mM PA alone or with 1.7 mM PA + 5 mM glucose (p < .001) led to a lower mitochondrial respiration (p < 0.01) and markedly higher H2 O2 /O2 emission (p < 0.05) than with 5 mM glucose alone. Prolonged incubation of intact cells with 1.7 mM PA +5 mM glucose led to 34% (p < 0.05) lower respiration and 2.5-fold higher H2 O2 /O2 emission (p < 0.01) than incubation with 5 mM glucose alone. Prolonged incubation of intact cells with elevated glucose led to 60% lower (p < 0.05) mitochondrial respiration and 4.6-fold higher H2 O2 /O2 production than incubation with 5 mM glucose in intact cells (p < 0.001). All effects observed in intact cells were present also in permeabilized cells (State 2). In conclusion, our results show that acute and prolonged lipid availability, as well as prolonged hyperglycemia, induces mitochondrial dysfunction as evidenced by lower mitochondrial respiration and enhanced H2 O2/ O2 emission. Elevated plasma substrate availability may lead to microvascular dysfunction in skeletal muscle by impairing endothelial mitochondrial function.

Targeting Mitochondria in Diabetes

Type 2 diabetes (T2D), one of the most prevalent noncommunicable diseases, is often preceded by insulin resistance (IR), which underlies the inability of tissues to respond to insulin and leads to disturbed metabolic homeostasis. Mitochondria, as a central player in the cellular energy metabolism, are involved in the mechanisms of IR and T2D. Mitochondrial function is affected by insulin resistance in different tissues, among which skeletal muscle and liver have the highest impact on whole-body glucose homeostasis. This review focuses on human studies that assess mitochondrial function in liver, muscle and blood cells in the context of T2D. Furthermore, different interventions targeting mitochondria in IR and T2D are listed, with a selection of studies using respirometry as a measure of mitochondrial function, for better data comparison. Altogether, mitochondrial respiratory capacity appears to be a metabolic indicator since it decreases as the disease progresses but increases after lifestyle (exercise) and pharmacological interventions, together with the improvement in metabolic health. Finally, novel therapeutics developed to target mitochondria have potential for a more integrative therapeutic approach, treating both causative and secondary defects of diabetes.

Skeletal muscle metabolic responses to physical activity are muscle type specific in a rat model of chronic kidney disease

Chronic kidney disease (CKD) leads to musculoskeletal impairments that are impacted by muscle metabolism. We tested the hypothesis that 10-weeks of voluntary wheel running can improve skeletal muscle mitochondria activity and function in a rat model of CKD. Groups included (n = 12-14/group): (1) normal littermates (NL); (2) CKD, and; (3) CKD-10 weeks of voluntary wheel running (CKD-W). At 35-weeks old the following assays were performed in the soleus and extensor digitorum longus (EDL): targeted metabolomics, mitochondrial respiration, and protein expression. Amino acid-related compounds were reduced in CKD muscle and not restored by physical activity. Mitochondrial respiration in the CKD soleus was increased compared to NL, but not impacted by physical activity. The EDL respiration was not different between NL and CKD, but increased in CKD-wheel rats compared to CKD and NL groups. Our results demonstrate that the soleus may be more susceptible to CKD-induced changes of mitochondrial complex content and respiration, while in the EDL, these alterations were in response the physiological load induced by mild physical activity. Future studies should focus on therapies to improve mitochondrial function in both types of muscle to determine if such treatments can improve the ability to adapt to physical activity in CKD.

PGC-1α-mediated regulation of mitochondrial function and physiological implications

The majority of human energy metabolism occurs in skeletal muscle mitochondria emphasizing the importance of understanding the regulation of myocellular mitochondrial function. The transcriptional co-activator peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α) has been characterized as a major factor in the transcriptional control of several mitochondrial components. Thus, PGC-1α is often described as a master regulator of mitochondrial biogenesis as well as a central player in regulating the antioxidant defense. However, accumulating evidence suggests that PGC-1α is also involved in the complex regulation of mitochondrial quality beyond biogenesis, which includes mitochondrial network dynamics and autophagic removal of damaged mitochondria. In addition, mitochondrial reactive oxygen species production has been suggested to regulate skeletal muscle insulin sensitivity, which may also be influenced by PGC-1α. This review aims to highlight the current evidence for PGC-1α-mediated regulation of skeletal muscle mitochondrial function beyond the effects on mitochondrial biogenesis as well as the potential PGC-1α-related impact on insulin-stimulated glucose uptake in skeletal muscle. Novelty PGC-1α regulates mitochondrial biogenesis but also has effects on mitochondrial functions beyond biogenesis. Mitochondrial quality control mechanisms, including fission, fusion, and mitophagy, are regulated by PGC-1α. PGC-1α-mediated regulation of mitochondrial quality may affect age-related mitochondrial dysfunction and insulin sensitivity.

Type 2 diabetes-induced hyposalivation of the submandibular gland through PINK1/Parkin-mediated mitophagy

Diabetes is often accompanied by dysfunction of salivary glands. However, the molecular mechanism remains unclear. The mechanisms that underlie diabetic hyposalivation were studied by db/db mice and SMG‐C6 cells. We found morphological changes and decreased stimulated salivary flow rates of the submandibular gland (SMG) in diabetic mice. We observed structural changes and dysfunction of mitochondria. More mitophagosomes and higher expression of autophagy‐related proteins were detected. Increased levels of proteins PINK1 and Parkin indicate that PINK1/Parkin‐mediated mitophagy was activated in diabetic SMG. Consistently, high glucose (HG) induced mitochondrial dysfunction and PINK1/Parkin‐mediated mitophagy in cultivated SMG‐C6 cells. HG also increased reactive oxygen species (ROS) and lessened activation of antioxidants in SMG‐C6 cells. In addition, HG lowered ERK1/2 phosphorylation and HG‐induced mitophagy was decreased after ERK1/2 was activated by LM22B‐10. Altogether, these data suggest that ROS played a crucial role in diabetes‐induced mitochondrial dysfunction and PINK1/Parkin‐mediated mitophagy and ERK1/2 was required in HG‐induced mitophagy in SMG.

Influence of exercise amount and intensity on long-term weight loss maintenance and skeletal muscle mitochondrial ROS production in humans

Sustaining a weight loss after a lifestyle intervention is challenging. The objective of the present study was to investigate if mitochondrial function is associated with the ability to maintain a weight loss. Sixty-eight former participants in an 11-12-week lifestyle intervention were recruited into 2 groups; weight loss maintenance (WLM; body mass index (BMI): 32 ± 1 kg/m²) and weight regain (WR; BMI: 43 ± 2 kg/m²) based on weight loss measured at a follow-up visit (WLM: 4.8 ± 0.4; WR: 7.6 ± 0.8 years after lifestyle intervention). Maximal oxygen consumption rate, physical activity level, and blood and muscle samples were obtained at the follow-up experiment. Mitochondrial respiratory capacity and reactive oxygen species (ROS) production were measured. Fasting blood samples were used to calculate glucose homeostasis index. WR had impaired glucose homeostasis and decreased maximal oxygen uptake and physical activity level compared with WLM. The decreased physical activity in WR was due to a lower activity level at vigorous and moderate intensities. Mitochondrial respiratory capacity and citrate synthase (CS) activity was higher in WLM, but intrinsic mitochondrial respiratory capacity (mitochondrial respiratory capacity corrected for mitochondrial content (CS activity)) was similar. ROS production was higher in WR compared with WLM, which was accompanied by a decreased content of antioxidant proteins in WR. Intrinsic mitochondrial respiratory capacity in skeletal muscle is not associated with the ability to maintain a long-term weight loss. WLM had a higher maximal oxygen uptake, physical activity level, mitochondrial respiratory capacity and CS activity compared with WR. The reduced glucose tolerance was concurrent with increased ROS production per mitochondria in WR, and could also be associated with the lower physical activity level in this group.

High-intensity interval training changes mitochondrial respiratory capacity differently in adipose tissue and skeletal muscle

The effect of high-intensity training (HIT) on mitochondrial ADP sensitivity and respiratory capacity was investigated in human skeletal muscle and subcutaneous adipose tissue (SAT). Twelve men and women underwent 6 weeks of HIT (7 × 1 min at app. 100% of maximal oxygen uptake (VO2max )). Mitochondrial respiration was measured in permeabilized muscle fibers and in abdominal SAT. Mitochondrial ADP sensitivity was determined using Michaelis Menten enzyme kinetics. VO2max , body composition and citrate synthase (CS) activity (skeletal muscle) and mtDNA (SAT) were measured before and after training. VO2max increased from 2.6 ± 0.2 to 2.8 ± 0.2 L O2 /min (P = 0.011) accompanied by a decreased mitochondrial ADP sensitivity in skeletal muscle (Km : 0.14 ± 0.02 to 0.29 ± 0.03 mmol/L ADP (P = 0.002)), with no changes in SAT (Km : 0.12 ± 0.02 to 0.16 ± 0.05 mmol/L ADP; P = 0.186), following training. Mitochondrial respiratory capacity increased in skeletal muscle from 57 ± 4 to 67 ± 4 pmol O2 ·mg-1 ·sec-1 (P < 0.001), but decreased with training in SAT from 1.3 ± 0.1 to 1.0 ± 0.1 pmol O2 ·mg-1 ·sec-1 (P < 0.001). CS activity increased (P = 0.027) and mtDNA was unchanged following training. Intrinsic mitochondrial respiratory capacity was unchanged in skeletal muscle, but increased in SAT after HIT. In summary, our results demonstrate that mitochondrial adaptations to HIT in skeletal muscle are comparable to adaptations to endurance training, with an increased mitochondrial respiratory capacity and CS activity. However, mitochondria in SAT adapts differently compared to skeletal muscle mitochondria, where mitochondrial respiratory capacity decreased and mtDNA remained unchanged after HIT.

Four days of bed rest increases intrinsic mitochondrial respiratory capacity in young healthy males

Bed rest leads to impaired glucose tolerance. Whether this is linked to maladaptation's in skeletal muscle mitochondrial function and in particular to the level of reactive oxygen species (ROS) is at present unknown. The aim of this longitudinal study was to quantify skeletal muscle mitochondrial function (respiratory capacity and ROS production) together with glucose tolerance after 4 days of strict bed rest in healthy young male subjects (n = 14). Mitochondrial function was determined in permeabilized muscle fibers using high-resolution respirometry and fluorometry, mitochondrial content (citrate synthase [CS] activity) and antioxidant protein expression levels were assessed in parallel to this. Glucose tolerance was determined by means of oral glucose tolerance tests. Intrinsic mitochondrial respiratory capacity was augmented after the bed rest period (CI + IIP : 0.43 ± 0.12 vs. 0.55 ± 0.14 [pmol/sec/mg]/CS activity), due to a decreased CS activity (158 ± 39 vs. 129 ± 25 mU/mg dw.). No differences were observed in ROS production (per mg of tissue or when normalized to CS activity). Furthermore, the protein content for catalase was increased while superoxide dismutase and glutathione peroxidase remained unaffected. These findings were accompanied by an impaired glucose tolerance after the bed rest period (Matsuda index: 12 ± 6 vs. 9 ± 5). The change in intrinsic mitochondrial respiratory capacity could be an early indication in the development of impaired glucose tolerance. The increased catalase protein content might explain that no change was seen in ROS production after 4 days of bed rest. Whether these findings can be extrapolated to lifestyle-dependent decrements in physical activity and the development of type-2-diabetes remains unknown.

Supplemental Oxygen Improves In Vivo Mitochondrial Oxidative Phosphorylation Flux in Sedentary Obese Adults With Type 2 Diabetes

Type 2 diabetes is associated with impaired exercise capacity. Alterations in both muscle perfusion and mitochondrial function can contribute to exercise impairment. We hypothesized that impaired muscle mitochondrial function in type 2 diabetes is mediated, in part, by decreased tissue oxygen delivery and would improve with oxygen supplementation. Ex vivo muscle mitochondrial content and respiration assessed from biopsy samples demonstrated expected differences in obese individuals with (n = 18) and without (n = 17) diabetes. Similarly, in vivo mitochondrial oxidative phosphorylation capacity measured in the gastrocnemius muscle via ³¹P-MRS indicated an impairment in the rate of ADP depletion with rest (27 ± 6 s [diabetes], 21 ± 7 s [control subjects]; P = 0.008) and oxidative phosphorylation (P = 0.046) in type 2 diabetes after isometric calf exercise compared with control subjects. Importantly, the in vivo impairment in oxidative capacity resolved with oxygen supplementation in adults with diabetes (ADP depletion rate 5.0 s faster, P = 0.012; oxidative phosphorylation 0.046 ± 0.079 mmol/L/s faster, P = 0.027). Multiple in vivo mitochondrial measures related to HbA_1c These data suggest that oxygen availability is rate limiting for in vivo mitochondrial oxidative exercise recovery measured with ³¹P-MRS in individuals with uncomplicated diabetes. Targeting muscle oxygenation could improve exercise function in type 2 diabetes.

Mitochondrial respiratory capacity remains stable despite a comprehensive and sustained increase in insulin sensitivity in obese patients undergoing gastric bypass surgery

AIM

It has been proposed, but not yet demonstrated by convincing evidence in published articles, that insulin resistance and mitochondrial respiratory function are causally related physiological phenomena. Here, we tested the prediction that weight loss-induced increase in insulin sensitivity will correlate with a corresponding change in mitochondrial respiratory capacity over the same time period.

METHODS

Insulin sensitivity was evaluated using the hyperinsulinaemic-euglycaemic clamp technique, and skeletal muscle mitochondrial respiratory capacity was evaluated by high-resolution respirometry in 26 patients with obesity. Each experiment was performed ~2 months and 1-2 weeks before, and ~4 and ~19 months after Roux-en-Y gastric bypass (RYGB) surgery.

RESULTS

A substantial weight loss was observed in all patients, and insulin sensitivity increased in all patients over the 21-months time period of the study. In contrast, skeletal muscle mitochondrial respiratory capacity, intrinsic mitochondrial respiratory capacity and mitochondrial content remained unchanged over the same time period.

CONCLUSION

Among obese patients with and without type 2 diabetes undergoing RYGB surgery, intrinsic mitochondrial respiratory capacity in skeletal muscle is not correlated with insulin sensitivity before or after the surgical intervention. Mitochondrial respiratory function may not be germane to the pathophysiology and/or aetiology of obesity and/or type 2 diabetes.

Defects in skeletal muscle subsarcolemmal mitochondria in a non-obese model of type 2 diabetes mellitus

Skeletal muscle resistance to insulin is related to accumulation of lipid-derived products, but it is not clear whether this accumulation is caused by skeletal muscle mitochondrial dysfunction. Diabetes and obesity are reported to have a selective effect on the function of subsarcolemmal and interfibrillar mitochondria in insulin-resistant skeletal muscle. The current study investigated the role of the subpopulations of mitochondria in the pathogenesis of insulin resistance in the absence of obesity. A non-obese spontaneous rat model of type 2 diabetes mellitus, (Goto-Kakizaki), was used to evaluate function and biochemical properties in both populations of skeletal muscle mitochondria. In subsarcolemmal mitochondria, minor defects are observed whereas in interfibrillar mitochondria function is preserved. Subsarcolemmal mitochondria defects characterized by a mild decline of oxidative phosphorylation efficiency are related to ATP synthase and structural alterations of inner mitochondria membrane but are considered unimportant because of the absence of defects upstream as shown with polarographic and spectrophometric assays. Fatty acid transport and oxidation is preserved in both population of mitochondria, whereas palmitoyl-CoA increased 25% in interfibrillar mitochondria of diabetic rats. Contrary to popular belief, these data provide compelling evidence that mitochondrial function is unaffected in insulin-resistant skeletal muscle from T2DM non-obese rats.

The effects of 2 weeks of statin treatment on mitochondrial respiratory capacity in middle-aged males: the LIFESTAT study

BACKGROUND

Statins are used to lower cholesterol in plasma and are one of the most used drugs in the world. Many statin users experience muscle pain, but the mechanisms are unknown at the moment. Many studies have hypothesized that mitochondrial function could be involved in these side effects.

AIM

The aim of the study was to investigate mitochondrial function after 2 weeks of treatment with simvastatin (S; n = 10) or pravastatin (P; n = 10) in healthy middle-aged participants.

METHODS

Mitochondrial respiratory capacity and substrate sensitivity were measured in permeabilized muscle fibers by high-resolution respirometry. Mitochondrial content (citrate synthase (CS) activity), antioxidant content, as well as coenzyme Q₁₀ concentration (Q₁₀) were determined. Fasting plasma glucose and insulin concentrations were measured, and whole body maximal oxygen uptake (VO_2max) was determined.

RESULTS

No differences were seen in mitochondrial respiratory capacity although a tendency was observed for a reduction when complex IV respiration was analyzed in both S (229 (169; 289 (95% confidence interval)) vs. 179 (146; 211) pmol/s/mg, respectively; P = 0.062) and P (214 (143; 285) vs. 162 (104; 220) pmol/s/mg, respectively; P = 0.053) after treatment. A tendency (1.64 (1.28; 2.00) vs. 1.28 (0.99; 1.58) mM, respectively; P = 0.092) for an increased mitochondrial substrate sensitivity (complex I-linked substrate; glutamate) was seen only in S after treatment. No differences were seen in Q₁₀, CS activity, or antioxidant content after treatment. Fasting glucose and insulin as well as VO_2max were not changed after treatment.

CONCLUSION

Two weeks of statin (S or P) treatment have no major effect on mitochondrial function. The tendency for an increased mitochondrial substrate sensitivity after simvastatin treatment could be an early indication of the negative effects linked to statin treatment.

Dietary Interventions for Type 2 Diabetes: How Millet Comes to Help

Diabetes has become a highly problematic and increasingly prevalent disease world-wide. It has contributed toward 1.5 million deaths in 2012. Management techniques for diabetes prevention in high-risk as well as in affected individuals, beside medication, are mainly through changes in lifestyle and dietary regulation. Particularly, diet can have a great influence on life quality for those that suffer from, as well as those at risk of, diabetes. As such, considerations on nutritional aspects are required to be made to include in dietary intervention. This review aims to give an overview on the general consensus of current dietary and nutritional recommendation for diabetics. In light of such recommendation, the use of plant breeding, conventional as well as more recently developed molecular marker-based breeding and biofortification, are discussed in designing crops with desired characteristics. While there are various recommendations available, dietary choices are restricted by availability due to geo-, political-, or economical- considerations. This particularly holds true for countries such as India, where 65 million people (up from 50 million in 2010) are currently diabetic and their numbers are rising at an alarming rate. Millets are one of the most abundant crops grown in India as well as in Africa, providing a staple food source for many poorest of the poor communities in these countries. The potentials of millets as a dietary component to combat the increasing prevalence of global diabetes are highlighted in this review.

Actovegin, a non-prohibited drug increases oxidative capacity in human skeletal muscle

Actovegin, a deproteinized haemodialysate of calf blood, is suggested to have ergogenic properties, but this potential effect has never been investigated in human skeletal muscle. To investigate this purported ergogenic effect, we measured the mitochondrial respiratory capacity in permeabilized human skeletal muscle fibres acutely exposed to Actovegin in a low and in a high dose. We found that Actovegin, in the presence of complex I-linked substrates increased the oxidative phosphorylation (OXPHOS) capacity significantly in a concentration-dependent manner (19 ± 3, 31 ± 4 and 45 ± 4 pmol/mg/s). Maximal OXPHOS capacity with complex I and II-linked substrate was increased when the fibres were exposed to the high dose of Actovegin (62 ± 6 and 77 ± 6 pmol/mg/s) (p < .05). The respiratory capacity of the electron transfer system as well as Vmax and Km were also increased in a concentration-dependent manner after Actovegin exposure (70 ± 6, 79 ± 6 and 88 ± 7 pmol/mg/s; 13 ± 2, 25 ± 3 and 37 ± 4 pmol/mg/s; 0.08 ± 0.02, 0.21 ± 0.03 and 0.36 ± 0.03 mM, respectively) (p < .05). In summary, we report for the first time that Actovegin has a marked effect on mitochondrial oxidative function in human skeletal muscle. Mitochondrial adaptations like this are also seen after a training program in human subjects. Whether this improvement translates into an ergogenic effect in athletes and thus reiterates the need to include Actovegin on the World Anti-Doping Agency's active list remains to be investigated.

Mitochondrial DNA content and function, childhood obesity, and insulin resistance

OBJECTIVES

The objectives of our study were to compare the mitochondrial enzyme activity between obese and non-obese children and to assess the association between mitochondrial DNA content and function and markers of metabolic syndrome.

METHODS

Clinical and anthropometric data of obese and normal-weight children ages 2-18 years were collected. We collected buccal swabs for mitochondrial respiratory enzymes (complex I, IV, and Citrate Synthase). In obese children only, serum levels of metabolic parameters and mitochondrial DNA from mononuclear cells were quantitated.

RESULTS

We recruited 75 obese and 65 normal-weight children. There was no difference in respiratory complex enzyme activity levels between obese and normal-weight subjects. In obese subjects, mitochondrial to nuclear DNA (mt/nDNA) ratio was significantly correlated with BMI Z-score and BMI percentile (p < 0.05, and p < 0.01, respectively), and the strength of this correlation was proportionate to the degree of obesity. We did not find any association between mt/nDNA ratio and metabolic parameters. We observed a significant positive association between complex IV activity and fasting insulin level (p < 0.05). Finally, fasting insulin explained 45% of the variation in the complex IV activity level (p < 0.05).

CONCLUSION

Our findings indicate that mitochondrial DNA content is directly related to obesity, but not to the markers of metabolic syndrome/insulin resistance in children. Longitudinal studies involving larger samples are needed to confirm our findings and help elucidate the relationship between mitochondrial function, adiposity, and insulin resistance.

Mitochondrial Quality Control as a Therapeutic Target

In addition to oxidative phosphorylation (OXPHOS), mitochondria perform other functions such as heme biosynthesis and oxygen sensing and mediate calcium homeostasis, cell growth, and cell death. They participate in cell communication and regulation of inflammation and are important considerations in aging, drug toxicity, and pathogenesis. The cell's capacity to maintain its mitochondria involves intramitochondrial processes, such as heme and protein turnover, and those involving entire organelles, such as fusion, fission, selective mitochondrial macroautophagy (mitophagy), and mitochondrial biogenesis. The integration of these processes exemplifies mitochondrial quality control (QC), which is also important in cellular disorders ranging from primary mitochondrial genetic diseases to those that involve mitochondria secondarily, such as neurodegenerative, cardiovascular, inflammatory, and metabolic syndromes. Consequently, mitochondrial biology represents a potentially useful, but relatively unexploited area of therapeutic innovation. In patients with genetic OXPHOS disorders, the largest group of inborn errors of metabolism, effective therapies, apart from symptomatic and nutritional measures, are largely lacking. Moreover, the genetic and biochemical heterogeneity of these states is remarkably similar to those of certain acquired diseases characterized by metabolic and oxidative stress and displaying wide variability. This biologic variability reflects cell-specific and repair processes that complicate rational pharmacological approaches to both primary and secondary mitochondrial disorders. However, emerging concepts of mitochondrial turnover and dynamics along with new mitochondrial disease models are providing opportunities to develop and evaluate mitochondrial QC-based therapies. The goals of such therapies extend beyond amelioration of energy insufficiency and tissue loss and entail cell repair, cell replacement, and the prevention of fibrosis. This review summarizes current concepts of mitochondria as disease elements and outlines novel strategies to address mitochondrial dysfunction through the stimulation of mitochondrial biogenesis and quality control.

Effects of metformin on mitochondrial function of leukocytes from polycystic ovary syndrome patients with insulin resistance

OBJECTIVE

Oxidative stress and mitochondrial dysfunction are implicated in polycystic ovary syndrome (PCOS). The present study assesses the effect of metformin treatment on mitochondrial function in polymorphonuclear cells from PCOS subjects. Additionally, we evaluate endocrine parameters and levels of interleukin 6 (IL6) and tumour necrosis factor alpha (TNFα).

DESIGN AND METHODS

Our study population was comprised of 35 women of reproductive age diagnosed with PCOS and treated with metformin for 12 weeks, and their corresponding controls (n=41), adjusted by age and BMI. We evaluated the alteration of endocrinological and anthropometrical parameters and androgen levels. Mitochondrial O2 consumption (using a Clark-type O2 electrode), membrane potential, mitochondrial mass, and levels of reactive oxygen species (ROS) and glutathione (GSH) (by means of fluorescence microscopy) were assessed in poymorphonuclear cells. H2O2 was evaluated with the Amplex Red(R) H2O2/Peroxidase Assay kit. IL6 and TNFα were measured using the Luminex 200 flow analyser system.

RESULTS

Metformin had beneficial effects on patients by increasing mitochondrial O2 consumption, membrane potential, mitochondrial mass and glutathione levels, and by decreasing levels of reactive oxygen species and H2O2. In addition, metformin reduced glucose, follicle-stimulating hormone, IL6 and TNFα levels and increased dehydroepiandrosterone sulfate levels. HOMA-IR and mitochondrial function biomarkers positively correlated with ROS production (r=0.486, P=0.025), GSH content (r=0.710, P=0.049) and H2O2 (r=0.837, P=0.010), and negatively correlated with membrane potential (r=-0.829, P=0.011) at baseline. These differences disappeared after metformin treatment.

CONCLUSIONS

Our results demonstrate the beneficial effects of metformin treatment on mitochondrial function in leukocytes of PCOS patients.

Comprehensive assessment of physical functioning in bariatric surgery candidates compared with subjects without obesity

BACKGROUND

Obesity appears to be associated with reduced physical performance, but comprehensive assessments of physical functioning are lacking in subjects with severe obesity, in particular in comparison with subjects without obesity. This precludes an objective assessment of the degree of impairment.

OBJECTIVE

To compare motor skills and cardiopulmonary fitness between subjects with severe obesity (OB) (i.e., candidates for bariatric surgery) and control subjects without obesity (non-OB).

SETTING

Cantonal Hospital, Switzerland.

METHODS

Flexibility, movement speed, balance, maximal isometric strength, and cardiopulmonary fitness were tested in 45 OB (body mass index: 42.6±.9 kg/m(2); age: 35±1.7 years; 33 women) and 32 non-OB (body mass index: 23±.4 kg/m(2); age: 38.5±2.1 years; 25 women) subjects.

RESULTS

In comparison with the non-OB group, the OB group showed reduced shoulder flexibility (P<.001) but comparable hamstrings flexibility (P = .3). Speed-related tasks (i.e., timed up-and-go test and timed lying-to-standing test) indicated that the OB group was slower than the non-OB group (all P<.007). Strength-related tasks indicated a greater absolute back muscle and knee-extensor strength (all P<.002) in the OB group with no difference in knee-flexor strength (both P>.8). However, when related to weight, the OB group showed reduced maximal strength (all P<.002). Bicycle spiroergometry indicated that absolute oxygen consumption at peak exercise and at the anaerobic threshold did not differ between groups (both P>.06). Related to weight, however, values were lower in the OB than in the non-OB group (both P< .001).

CONCLUSION

Data indicate a differential pattern of functional impairment in bariatric surgery candidates compared with subjects without obesity. These findings might help to establish tailored intervention protocols to improve physical performance in such subjects.

Blood-cell bioenergetics are associated with physical function and inflammation in overweight/obese older adults

BACKGROUND

Physical function and strength decline with age and lead to limited mobility and independence in older adults. Alterations in mitochondrial function are thought to underlie numerous age-related changes, including declining physical ability. Recent studies suggest that systemic changes in bioenergetic capacity may be reported by analyzing mitochondrial function in circulating cells. The objective of this study was to determine whether the bioenergetic capacity of peripheral blood mononuclear cells (PBMCs) is related to differences in physical function among older, overweight/obese, adults. To address this, we tested the hypothesis that greater PBMC respirometric capacity would be associated with better physical function, muscular strength, leg lean mass, and muscle quality. Furthermore, we tested whether the respirometric capacity of PBMCs is related to cellular composition and inflammatory status reported by interleukin-6 (IL-6).

METHODS

Fasted PBMC respiration (pmol/min/500,000 cells), expanded short physical performance battery (Ex-SPPB), peak knee extensor (KE) strength (Nm), grip strength (kg), leg lean mass (kg, via dual energy X-ray absorptiometry [DXA]), muscle quality (Nm/kg), and plasma IL-6 (pg/mL) were analyzed in 15 well-functioning, community-dwelling, sedentary overweight/obese older men (n=9) and women (n=6) aged 65 to 78 (mean 68.3 ± 3.5 years). Pearson and partial correlations were calculated to determine associations between PBMC respiration and these variables.

RESULTS

Higher maximal respiration of PBMCs was associated with better Ex-SPPB (r=0.58, p=0.02), greater KE strength (r=0.60, p=0.02), greater grip strength (r=0.52, p=0.05) and lower IL-6 (r=-0.58, p=0.04). Higher spare respiratory capacity was associated with better Ex-SPPB (r=0.59, p=0.02), greater KE strength (r=0.60, p=0.02), greater grip strength (r=0.54, p=0.04), greater leg muscle quality (r=0.56, p=0.04), and lower IL-6 (r=-0.55, p=0.05). Monocyte and lymphocyte counts were not related to PBMC respiratory capacity.

CONCLUSIONS

Our results indicate that respirometric profiles of readily obtainable blood cells are associated with physical function and strength. Future studies should be undertaken in order to determine whether blood-based bioenergetic profiling can provide an objective index of systemic mitochondrial health.

Reduced efficiency of sarcolipin-dependent respiration in myocytes from humans with severe obesity

OBJECTIVE

Sarcolipin (SLN) regulates muscle energy expenditure through its action on sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump. It is unknown whether SLN-dependent respiration has relevance to human obesity, but whole-transcriptome gene expression profiling revealed that SLN was more highly expressed in myocytes from individuals with severe obesity (OB) than in lean controls (LN). The purpose of this study was to examine SLN-dependent cellular respiratory rates in LN and OB human muscles.

METHODS

Primary myocytes were isolated from muscle biopsy from seven LN and OB Caucasian females. Cellular respiration was assessed with and without lentivirus-mediated SLN knockdown in LN and OB myocytes.

RESULTS

SLN mRNA and protein abundance was greater in OB compared to LN cells. Despite elevated SLN levels in wild-type OB cells, respiratory rates among SLN-deficient cells were higher in OB compared to LN. Obesity-induced reduction in efficiency of SLN-dependent respiration was associated with altered sarcoplasmic reticulum phospholipidome.

CONCLUSIONS

SLN-dependent respiration is reduced in muscles from humans with severe obesity compared to lean controls. Identification of the molecular mechanism that affects SLN efficiency might lead to interventions that promote an increase in skeletal muscle energy expenditure.

High Intensity Interval Training (HIIT) Induces Specific Changes in Respiration and Electron Leakage in the Mitochondria of Different Rat Skeletal Muscles

High intensity interval training (HIIT) is characterized by vigorous exercise with short rest intervals. Hydrogen peroxide (H₂O₂) plays a key role in muscle adaptation. This study aimed to evaluate whether HIIT promotes similar H₂O₂ formation via O₂ consumption (electron leakage) in three skeletal muscles with different twitch characteristics. Rats were assigned to two groups: sedentary (n=10) and HIIT (n=10, swimming training). We collected the tibialis anterior (TA-fast), gastrocnemius (GAST-fast/slow) and soleus (SOL-slow) muscles. The fibers were analyzed for mitochondrial respiration, H₂O₂ production and citrate synthase (CS) activity. A multi-substrate (glycerol phosphate (G3P), pyruvate, malate, glutamate and succinate) approach was used to analyze the mitochondria in permeabilized fibers. Compared to the control group, oxygen flow coupled to ATP synthesis, complex I and complex II was higher in the TA of the HIIT group by 1.5-, 3.0- and 2.7-fold, respectively. In contrast, oxygen consumed by mitochondrial glycerol phosphate dehydrogenase (mGPdH) was 30% lower. Surprisingly, the oxygen flow coupled to ATP synthesis was 42% lower after HIIT in the SOL. Moreover, oxygen flow coupled to ATP synthesis and complex II was higher by 1.4- and 2.7-fold in the GAST of the HIIT group. After HIIT, CS activity increased 1.3-fold in the TA, and H₂O₂ production was 1.3-fold higher in the TA at sites containing mGPdH. No significant differences in H₂O₂ production were detected in the SOL. Surprisingly, HIIT increased H₂O₂ production in the GAST via complex II, phosphorylation, oligomycin and antimycin by 1.6-, 1.8-, 2.2-, and 2.2-fold, respectively. Electron leakage was 3.3-fold higher in the TA with G3P and 1.8-fold higher in the GAST with multiple substrates. Unexpectedly, the HIIT protocol induced different respiration and electron leakage responses in different types of muscle.

Defining meal requirements for protein to optimize metabolic roles of amino acids

Dietary protein provides essential amino acids (EAAs) for the synthesis of new proteins plus an array of other metabolic functions; many of these functions are sensitive to postprandial plasma and intracellular amino acid concentrations. Recent research has focused on amino acids as metabolic signals that influence the rate of protein synthesis, inflammation responses, mitochondrial activity, and satiety, exerting their influence through signaling systems including mammalian/mechanistic target of rapamycin complex 1 (mTORC1), general control nonrepressed 2 (GCN2), glucagon-like peptide 1 (GLP-1), peptide YY (PYY), serotonin, and insulin. These signals represent meal-based responses to dietary protein. The best characterized of these signals is the leucine-induced activation of mTORC1, which leads to the stimulation of skeletal muscle protein synthesis after ingestion of a meal that contains protein. The response of this metabolic pathway to dietary protein (i.e., meal threshold) declines with advancing age or reduced physical activity. Current dietary recommendations for protein are focused on total daily intake of 0.8 g/kg body weight, but new research suggests daily needs for older adults of ≥1.0 g/kg and identifies anabolic and metabolic benefits to consuming at least 20-30 g protein at a given meal. Resistance exercise appears to increase the efficiency of EAA use for muscle anabolism and to lower the meal threshold for stimulation of protein synthesis. Applying this information to a typical 3-meal-a-day dietary plan results in protein intakes that are well within the guidelines of the Dietary Reference Intakes for acceptable macronutrient intakes. The meal threshold concept for dietary protein emphasizes a need for redistribution of dietary protein for optimum metabolic health.

Three-dimensional reconstruction of the human skeletal muscle mitochondrial network as a tool to assess mitochondrial content and structural organization

AIM

Mitochondria undergo continuous changes in shape as result of complex fusion and fission processes. The physiological relevance of mitochondrial dynamics is still unclear. In the field of mitochondria bioenergetics, there is a need of tools to assess cell mitochondrial content. To develop a method to visualize mitochondrial networks in high resolution and assess mitochondrial volume.

METHODS

Confocal fluorescence microscopy imaging of mitochondrial network stains in human vastus lateralis single muscle fibres and focused ion beam/ scanning electron microscopy (FIB/SEM) imaging, combined with 3D reconstruction was used as a tool to analyse mitochondrial morphology and measure mitochondrial fractional volume.

RESULTS

Most type I and type II muscle fibres have tubular highly interconnected profusion mitochondria, which are thicker and more structured in type I muscle fibres (Fig. 1). In some muscle fibres, profission-isolated ellipsoid-shaped mitochondria were observed. Mitochondrial volume was significantly higher in type I muscle fibres and showed no correlation with any of the investigated molecular and biochemical mitochondrial measurements (Fig. 2). Three-dimensional reconstruction of FIB/SEM data sets shows that some subsarcolemmal mitochondria are physically interconnected with some intermyofibrillar mitochondria (Fig. 3).

CONCLUSION

Two microscopy methods to visualize skeletal muscle mitochondrial networks in 3D are described and can be used as tools to investigate mitochondrial dynamics in response to life-style interventions and/or in certain pathologies. Our results question the classification of mitochondria into subsarcolemmal and intermyofibrillar pools, as they are physically interconnected.

Heterozygous SOD2 deletion impairs glucose-stimulated insulin secretion, but not insulin action, in high-fat-fed mice

Elevated reactive oxygen species (ROS) are linked to insulin resistance and islet dysfunction. Manganese superoxide dismutase (SOD2) is a primary defense against mitochondrial oxidative stress. To test the hypothesis that heterozygous SOD2 deletion impairs glucose-stimulated insulin secretion (GSIS) and insulin action, wild-type (sod2(+/+)) and heterozygous knockout mice (sod2(+/-)) were fed a chow or high-fat (HF) diet, which accelerates ROS production. Hyperglycemic (HG) and hyperinsulinemic-euglycemic (HI) clamps were performed to assess GSIS and insulin action in vivo. GSIS during HG clamps was equal in chow-fed sod2(+/-) and sod2(+/+) but was markedly decreased in HF-fed sod2(+/-). Remarkably, this impairment was not paralleled by reduced HG glucose infusion rate (GIR). Decreased GSIS in HF-fed sod2(+/-) was associated with increased ROS, such as superoxide ion. Surprisingly, insulin action determined by HI clamps did not differ between sod2(+/-) and sod2(+/+) of either diet. Since insulin action was unaffected, we hypothesized that the unchanged HG GIR in HF-fed sod2(+/-) was due to increased glucose effectiveness. Increased GLUT-1, hexokinase II, and phospho-AMPK protein in muscle of HF-fed sod2(+/-) support this hypothesis. We conclude that heterozygous SOD2 deletion in mice, a model that mimics SOD2 changes observed in diabetic humans, impairs GSIS in HF-fed mice without affecting insulin action.

Effects of exercise training on mitochondrial function in patients with type 2 diabetes

Type 2 diabetes is characterized by a decreased ability of insulin to facilitate glucose uptake into insulin sensitive tissue, i.e., skeletal muscle. The mechanism behind this is at the moment unresolved. It has been suggested that increased amount of lipids inside the skeletal muscle (intramuscular triglyceride, diacylglycerol and ceramides) will impair insulin action in skeletal muscle, but data are not consistent in the human literature. It has also been hypothesized that the impaired insulin sensitivity is due to a dysfunction in the mitochondria resulting in an impaired ability to oxidize lipids, but the majority of the literature is not supporting this hypothesis. Recently it has been suggested that the production of reactive oxygen species play an essential role in skeletal muscle insulin sensitivity. It is well accepted that physical activity (endurance, strength and high intensity training) improves insulin sensitivity in healthy humans and in patients with type 2 diabetes. Whether patients with type 2 diabetes have the same beneficial effects (same improvement) as control subjects, when it comes to regular physical activity in regard to mitochondrial function, is not established in the literature. This review will focus only on the effect of physical activity on skeletal muscle (mitochondrial function) in patients with type 2 diabetes.

Both linoleic and α-linolenic acid prevent insulin resistance but have divergent impacts on skeletal muscle mitochondrial bioenergetics in obese Zucker rats

The therapeutic use of polyunsaturated fatty acids (PUFA) in preserving insulin sensitivity has gained interest in recent decades; however, the roles of linoleic acid (LA) and α-linolenic acid (ALA) remain poorly understood. We investigated the efficacy of diets enriched with either LA or ALA on attenuating the development of insulin resistance (IR) in obesity. Following a 12-wk intervention, LA and ALA both prevented the shift toward an IR phenotype and maintained muscle-specific insulin sensitivity otherwise lost in obese control animals. The beneficial effects of ALA were independent of changes in skeletal muscle mitochondrial content and oxidative capacity, as obese control and ALA-treated rats showed similar increases in these parameters. However, ALA increased the propensity for mitochondrial H2O2 emission and catalase content within whole muscle and reduced markers of oxidative stress (4-HNE and protein carbonylation). In contrast, LA prevented changes in markers of mitochondrial content, respiratory function, H2O2 emission, and oxidative stress in obese animals, thereby resembling levels seen in lean animals. Together, our data suggest that LA and ALA are efficacious in preventing IR but have divergent impacts on skeletal muscle mitochondrial content and function. Moreover, we propose that LA has value in preserving insulin sensitivity in the development of obesity, thereby challenging the classical view that n-6 PUFAs are detrimental.

Time-course effects of aerobic interval training and detraining in patients with metabolic syndrome

BACKGROUND AND AIMS

Exercise training can improve health of patients with metabolic syndrome (MetS). However, which MetS factors are most responsive to exercise training remains unclear. We studied the time-course of changes in MetS factors in response to training and detraining.

METHODS AND RESULTS

Forty eight MetS patients (52 ± 8.8 yrs old; 33 ± 4 BMI) underwent 4 months (3 days/week) of supervised aerobic interval training (AIT) program. After 1 month of training, there were progressive increases in high density lipoprotein cholesterol (HDL-c) and reductions in waist circumference and blood pressure (12 ± 3, -3.9 ± 0.4, and -12 ± 1%, respectively after 4 months; all P < 0.05). However, fasting plasma concentration of triglycerides and glucose were not reduced by training. Insulin sensitivity (HOMA), cardiorespiratory fitness (VO2peak) and exercise maximal fat oxidation (FOMAx) also progressively improved with training (-17 ± 5; 21 ± 2 and 31 ± 8%, respectively, after 4 months; all P < 0.05). Vastus lateralis samples from seven subjects revealed that mitochondrial O2 flux was markedly increased with training (71 ± 11%) due to increased mitochondrial content. After 1 month of detraining, the training-induced improvements in waist circumference and blood pressure were maintained. HDL-c and VO2peak returned to the values found after 1-2 months of training while HOMA and FOMAx returned to pre-training values.

CONCLUSIONS

The health related variables most responsive to aerobic interval training in MetS patients are waist circumference, blood pressure and the muscle and systemic adaptations to consume oxygen and fat. However, the latter reverse with detraining while blood pressure and waist circumference are persistent to one month of detraining.

Skeletal muscle reactive oxygen species: a target of good cop/bad cop for exercise and disease

Metabolic stresses associated with disease, ageing, and exercise increase the levels of reactive oxygen species (ROS) in skeletal muscle. These ROS have been linked mechanistically to adaptations in skeletal muscle that can be favourable (i.e. in response to exercise) or detrimental (i.e. in response to disease). The magnitude, duration (acute versus chronic), and cellular origin of the ROS are important underlying factors in determining the metabolic perturbations associated with the ROS produced in skeletal muscle. In particular, insulin resistance has been linked to excess ROS production in skeletal muscle mitochondria. A chronic excess of mitochondrial ROS can impair normal insulin signalling pathways and glucose disposal in skeletal muscle. In contrast, ROS produced in skeletal muscle in response to exercise has been linked to beneficial metabolic adaptations including mitochondrial biogenesis and muscle hypertrophy. Moreover, unlike insulin resistance, exercise-induced ROS appears to be primarily of non-mitochondrial origin. The present review summarizes the diverse ROS-targeted metabolic outcomes associated with insulin resistance versus exercise in skeletal muscle, thus, presenting two contrasting perspectives of pathologically harmful versus physiologically beneficial ROS. Here, we discuss the key sites of ROS production during exercise and the effect of ROS in skeletal muscle of people with type 2 diabetes.

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.

Elevated mitochondrial oxidative stress impairs metabolic adaptations to exercise in skeletal muscle

Mitochondrial oxidative stress is a complex phenomenon that is inherently tied to energy provision and is implicated in many metabolic disorders. Exercise training increases mitochondrial oxidative capacity in skeletal muscle yet it remains unclear if oxidative stress plays a role in regulating these adaptations. We demonstrate that the chronic elevation in mitochondrial oxidative stress present in Sod2 (+/-) mice impairs the functional and biochemical mitochondrial adaptations to exercise. Following exercise training Sod2 (+/-) mice fail to increase maximal work capacity, mitochondrial enzyme activity and mtDNA copy number, despite a normal augmentation of mitochondrial proteins. Additionally, exercised Sod2 (+/-) mice cannot compensate for their higher amount of basal mitochondrial oxidative damage and exhibit poor electron transport chain complex assembly that accounts for their compromised adaptation. Overall, these results demonstrate that chronic skeletal muscle mitochondrial oxidative stress does not impact exercise induced mitochondrial biogenesis, but impairs the resulting mitochondrial protein function and can limit metabolic plasticity.

The best approach: homogenization or manual permeabilization of human skeletal muscle fibers for respirometry?

The number of studies on mitochondrial function is growing as a result of the recognition of the pivotal role of an intact mitochondrial function in numerous diseases. Measurements of oxygen consumption by the mitochondria in human skeletal muscle are used in many studies. There are several advantages of studying mitochondrial respiration in permeabilized fibers (Pfi), but the method requires a manual procedure of mechanical separation of the fiber bundles in the biopsy and chemical permeabilization of the cell membrane. This is time-consuming and subject to interpersonal variability. An alternative is to use a semiautomatic tool for preparation of a homogenate of the muscle biopsy. We investigated whether the PBI shredder is useful in preparing a muscle homogenate for measurements of mitochondrial respiratory capacity. The homogenate is compared with the Pfi preparation. Maximal respiratory capacity was significantly reduced in the homogenate compared with the Pfi from human skeletal muscle. A marked cytochrome c response was observed in the homogenate, which was not the case with the Pfi, indicating that the outer mitochondrial membrane was not intact. The mitochondria in the homogenate were more uncoupled compared with the Pfi. Manual permeabilization is an advantageous technique for preparing human skeletal muscle biopsies for respirometry.

Differential changes in exercise performance after massive weight loss induced by bariatric surgery

BACKGROUND

Exercise performance and pulmonary function are often impaired in severely obese subjects. Bariatric surgery represents the most effective therapy for severe obesity, but data on changes in exercise performance after massive weight loss induced by bariatric surgery have rarely been assessed so far.

METHODS

Exercise performance was obtained by bicycle spiroergometry in 18 severely obese patients before and at least 1 year after bariatric surgery. Additionally, pulmonary function was assessed by spirometry.

RESULTS

BMI was reduced from 46.3 ± 1.6 to 33.5 ± 1.4 kg/m(2) after surgery. Pulmonary function (forced expiratory volume within 1 s; inspiratory vital capacity) improved after weight loss (both p ≤ 0.01). At peak exercise, heart rate (HR) peak, absolute oxygen uptake (VO(2)) peak, and load peak did not differ between both assessments (all p > 0.25). However, relative (related to actual body weight) VO(2) peak and workload peak were higher after than before surgery (both p ≤ 0.005), while gross efficiency peak and ventilatory equivalent peak remained unchanged (both p > 0.30). At anaerobic threshold (AT), patients showed lower HR AT and absolute VO(2) AT after than before surgery (both p < 0.05), while absolute workload AT did not differ (p = 0.58). In turn, relative VO(2) AT did not change (p = 0.30), whereas relative workload AT was higher after surgery (p = 0.04). Also, ventilatory efficiency AT and gross efficiency AT tended to be improved (both p = 0.08). Before surgery, the patients performed 27.0 % of VO(2) peak above their AT, while this fraction increased to 35.3 % (p = 0.006).

CONCLUSIONS

Results indicated differential changes in exercise performance, with the relative but not the absolute peak performance being improved after massive weight loss. Interestingly, anaerobic exercise tolerance was markedly improved after surgery.

Physically active vs. inactive lifestyle, muscle properties, and glucose homeostasis in middle-aged and older twins

Exercise-induced positive changes in skeletal muscle properties and metabolism decrease the risk for disability, cardiometabolic diseases and mortality. Here, we studied muscle properties and glucose homeostasis in a non-exercise stage in twin pairs with co-twins discordant for physical activity habits for at least 32 years of their adult lives. Isometric knee extension force, MR imaging of midthigh tissue composition and muscle volume, and fasting blood samples were acquired from 16 same-sex (seven monozygotic, nine dizygotic) middle-aged and older twin pairs. The consistently active twins had 20 % higher knee extension forces than their inactive co-twins (p = 0.006) although the active twins had only 4 % higher midthigh muscle cross-sectional areas (p = 0.072). These results were similar in intrapair analysis in which only the seven identical twin pairs were included. The ratio between the area of midthigh fat and muscle tissues was significantly lower among the active twins (0.65 vs. 0.48, p = 0.006). The active twins had also lower fasting plasma glucose levels (5.1 vs 5.6 mmol/l, p = 0.041). The area of midthigh intramuscular (extramyocellular) fat was associated with the markers of glucose homeostasis, especially with glycated hemoglobin, and these associations were emphasized by the diabetic and inactive twins. Regular exercise throughout the adult life retains muscle strength and quality but not necessarily mass. The regular use of muscles also prevents from the accumulation of intramuscular fat which might be related to maintained glucose metabolism and, thus, prevention of metabolic disorders.

Submaximal ADP-stimulated respiration is impaired in ZDF rats and recovered by resveratrol

Mitochondrial dysfunction and reactive oxygen species (ROS) have been implicated in the aetiology of skeletal muscle insulin resistance, although there is considerable controversy regarding these concepts. Mitochondrial function has been traditionally assessed in the presence of saturating ADP, but ATP turnover and the resultant ADP is thought to limit respiration in vivo. Therefore, we investigated the potential link between submaximal ADP-stimulated respiration rates, ROS generation and skeletal muscle insulin sensitivity in a model of type 2 diabetes mellitus, the ZDF rat. Utilizing permeabilized muscle fibres we observed that submaximal ADP-stimulated respiration rates (250-2000 μm ADP) were lower in ZDF rats than in lean controls, which coincided with decreased adenine nucleotide translocase 2 (ANT2) protein content. This decrease in submaximal ADP-stimulated respiration occurred in the absence of a decrease in electron transport chain function. Treating ZDF rats with resveratrol improved skeletal muscle insulin resistance and this was associated with elevated submaximal ADP-stimulated respiration rates as well as an increase in ANT2 protein content. These results coincided with a greater ability of ADP to attenuate mitochondrial ROS emission and an improvement in cellular redox balance. Together, these data suggest that mitochondrial dysfunction is present in skeletal muscle insulin resistance when assessed at submaximal ADP concentrations and that ADP dynamics may influence skeletal muscle insulin sensitivity through alterations in the propensity for mitochondrial ROS emission.

Increased skeletal muscle mitochondrial efficiency in rats with fructose-induced alteration in glucose tolerance

In the present study, the effect of long-term fructose feeding on skeletal muscle mitochondrial energetics was investigated. Measurements in isolated tissue were coupled with the determination of whole-body energy expenditure and insulin sensitivity. A significant increase in plasma NEFA, as well as in skeletal muscle TAG and ceramide, was found in fructose-fed rats compared with the controls, together with a significantly higher plasma insulin response to a glucose load, while no significant variation in plasma glucose levels was found. Significantly lower RMR values were found in fructose-fed rats starting from week 4 of the dietary treatment. Skeletal muscle mitochondrial mass and degree of coupling were found to be significantly higher in fructose-fed rats compared with the controls. Significantly higher lipid peroxidation was found in fructose-fed rats, together with a significant decrease in superoxide dismutase activity. Phosphorylated Akt levels normalised to plasma insulin levels were significantly lower in fructose-fed rats compared with the controls. In conclusion, a fructose-rich diet has a deep impact on a metabolically relevant tissue such as skeletal muscle. In this tissue, the consequences of high fructose feeding are altered glucose tolerance, elevated mitochondrial biogenesis and increased mitochondrial coupling. This latter modification could have a detrimental metabolic effect by causing oxidative stress and energy sparing that contribute to the high metabolic efficiency of fructose-fed rats.

Does obstructive sleep apnea impair the cardiopulmonary response to exercise?

STUDY OBJECTIVES

The aim of this study was to evaluate cardiopulmonary exercise performance in lean and obese patients with obstructive sleep apnea (OSA) compared with controls.

DESIGN

Case-control study.

SETTING

The study was carried out in Sao Paulo Sleep Institute, Sao Paulo, Brazil.

PATIENTS AND PARTICIPANTS

INDIVIDUALS WITH SIMILAR AGES WERE ALLOCATED INTO GROUPS: 22 to the lean OSA group, 36 to the lean control group, 31 to the obese OSA group, and 26 to the obese control group.

INTERVENTIONS

The participants underwent a clinical evaluation, polysomnography, a maximum limited symptom cardiopulmonary exercise test, two-dimensional transthoracic echocardiography, and spirometry.

MEASUREMENTS AND RESULTS

The apnea-hypopnea index, arousal index, lowest arterial oxygen saturation (SaO2) and time of SaO2 < 90% were different among the groups. There were differences in functional capacity based on the following variables: maximal oxygen uptake (VO2max), P < 0.01 and maximal carbon dioxide production (VCO2max), P < 0.01. The obese patients with OSA and obese controls presented significantly lower VO2max and VCO2max values. However, the respiratory exchange ratio (RER) and anaerobic threshold (AT) did not differ between groups. Peak diastolic blood pressure (BP) was higher among the obese patients with OSA but was not accompanied by changes in peak systolic BP and heart rate (HR). When multiple regression was performed, body mass index (P < 0.001) and male sex in conjunction with diabetes (P < 0.001) independently predicted VO2max (mL/kg/min).

CONCLUSIONS

The results of this study suggest that obesity alone and sex, when associated with diabetes but not OSA, influenced exercise cardiorespiratory function.