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Aim: The purpose of this study was to 1. investigate if glucose tolerance is affected after one acute bout of different types of exercise; 2. assess if potential differences between two exercise paradigms are related to changes in mitochondrial function; and 3. determine if endurance athletes differ from nonendurance‐trained controls in their metabolic responses to the exercise paradigms. Methods: Nine endurance athletes (END) and eight healthy nonendurance‐trained controls (CON) were studied. Oral glucose tolerance tests (OGTT) and mitochondrial function were assessed on three occasions: in the morning, 14 h after an overnight fast without prior exercise (RE), as well as after 3 h of prolonged continuous exercise at 65% of VO2max (PE) or 5 × 4 min at ~95% of VO2max (HIIT) on a cycle ergometer. Results: Glucose tolerance was markedly reduced in END after PE compared with RE. END also exhibited elevated fasting serum FFA and ketones levels, reduced insulin sensitivity and glucose oxidation, and increased fat oxidation during the OGTT. CON showed insignificant changes in glucose tolerance and the aforementioned measurements compared with RE. HIIT did not alter glucose tolerance in either group. Neither PE nor HIIT affected mitochondrial function in either group. END also exhibited increased activity of 3‐hydroxyacyl‐CoA dehydrogenase activity in muscle extracts vs. CON. Conclusion: Prolonged exercise reduces glucose tolerance and increases insulin resistance in endurance athletes the following day. These findings are associated with an increased lipid load, a high capacity to oxidize lipids, and increased fat oxidation. [ABSTRACT FROM AUTHOR]

Fibre type‐specific analyses are required for broader understanding of muscle physiology, but such analyses are difficult to conduct due to the extreme time requirements of dissecting and fibre typing individual fibres. Investigations are often confined to a small number of fibres from few participants with low representativeness of the entire fibre population and the participant population. To increase the feasibility of conducting large‐scale fibre type‐specific studies, a valid and rapid method for high‐throughput fibre typing of individually dissected fibres was developed and named THRIFTY (for high‐THRoughput Immunofluorescence Fibre TYping). Employing THRIFTY, 400 fibre segments were fixed onto microscope slides with a pre‐printed coordinated grid system, probed with antibodies against myosin heavy chain (MyHC)‐I and MyHC‐II and classified using a fluorescence microscope. The validity and speed of THRIFTY was compared to a previously validated protocol (dot blot) on a fibre‐to‐fibre basis. Fibre pool purity was evaluated using 'gold standard' SDS‐PAGE and silver staining. A modified THRIFTY‐protocol using fluorescence western blot equipment was also validated. THRIFTY displayed excellent agreement with the dot blot protocol, κ = 0.955 (95% CI: 0.928, 0.982), P < 0.001. Both the original and modified THRIFTY protocols generated type I and type II fibre pools of absolute purity. Using THRIFTY, 400 fibres were typed just under 11 h, which was approximately 3 times faster than dot blot. THRIFTY is a novel and valid method with high versatility for very rapid fibre typing of individual fibres. THRIFTY can therefore facilitate the generation of large fibre pools for more extensive mechanistic studies into skeletal muscle physiology. Key points: Skeletal muscle is composed of different fibre types, each with distinct physiological properties.To fully understand how skeletal muscle adapts to external cues such as exercise, nutrition and ageing, fibre type‐specific investigations are required.Such investigations are very difficult to conduct due to the extreme time requirements related to classifying individually isolated muscle fibres.To bypass this issue, we have developed a rapid and reliable method named THRIFTY which is cheap as well as versatile and which can easily be implemented in most laboratories.THRIFTY increases the feasibility of conducting larger fibre type‐specific studies and enables time‐sensitive assays where measurements need to be carried out in close connection with tissue sampling.By using THRIFTY, new insights into fibre type‐specific muscle physiology can be gained which may have broad implications in health and disease. [ABSTRACT FROM AUTHOR]

The repair, remodeling, and regeneration of myofibers are dependent on satellite cells (SCs), although, the distribution of SCs in different fiber types of human muscle remains inconclusive. There is also a paucity of research comparing muscle fiber characteristics in a sex‐specific manner. Therefore, the aim of this study was to investigate fiber type‐specific SC content in men and women. Muscle biopsies from vastus lateralis were collected from 64 young (mean age 27 ± 5), moderately trained men (n = 34) and women (n = 30). SCs were identified by Pax7‐staining together with immunofluorescent analyses of fiber type composition, fiber size, and myonuclei content. In a mixed population, comparable number of SCs was associated to type I and type II fibers (0.07 ± 0.02 vs 0.07 ± 0.02 SCs per fiber, respectively). However, unlike men, women displayed a fiber type‐specific distribution, with SC content being lower in type II than type I fibers (P =.041). Sex‐based differences were found specifically for type II fibers, where women displayed lower SC content compared to men (P <.001). In addition, positive correlations (r‐values between 0.36‐0.56) were found between SC content and type I and type II fiber size in men (P =.03 and P <.01, respectively), whereas similar relationships could not be detected in women. Sex‐based differences were also noted for fiber type composition and fiber size, but not for myonuclei content. We hereby provide evidence for sex‐based differences present at the myocellular level, which may have important implications when studying exercise‐ and training‐induced myogenic responses in skeletal muscle. [ABSTRACT FROM AUTHOR]

Aims: The body responds to exercise training by profound adaptations throughout the cardiorespiratory and muscular systems, which may result in improvements in maximal oxygen consumption (VO2peak) and mitochondrial capacity. By convenience, mitochondrial respiration is often measured at supra‐physiological oxygen levels, an approach that ignores any potential regulatory role of mitochondrial affinity for oxygen (p50mito) at physiological oxygen levels. Methods: In this study, we examined the p50mito of mitochondria isolated from the Vastus lateralis and Triceps brachii in 12 healthy volunteers before and after a training intervention with seven sessions of sprint interval training using both leg cycling and arm cranking. The changes in p50mito were compared to changes in whole‐body VO2peak. Results: We here show that p50mito is similar in isolated mitochondria from the Vastus (40 ± 3.8 Pa) compared to Triceps (39 ± 3.3) but decreases (mitochondrial oxygen affinity increases) after seven sessions of sprint interval training (to 26 ± 2.2 Pa in Vastus and 22 ± 2.7 Pa in Triceps, both P <.01). The change in VO2peak modelled from changes in p50mito was correlated to actual measured changes in VO2peak (R2 =.41, P =.002). Conclusion: Together with mitochondrial respiratory capacity, p50mito is a critical factor when measuring mitochondrial function, it can decrease with sprint interval training and should be considered in the integrative analysis of the oxygen cascade from lung to mitochondria. [ABSTRACT FROM AUTHOR]

This study investigated the relationship between aerobic efficiency during cycling exercise and the increase in physical performance with acute hyperoxic exposure (FiO2 ~ 31%) (HOX) and also tested the hypothesis that fat oxidation could be increased by acute hyperoxia. Fourteen males and four females were recruited for two sessions, where they exercised for 2 × 10 min at 100 W to determine efficiency. HOX and normoxia (NOX) were administered randomly on both occasions to account for differences in nitrogen exchange. Thereafter, a progressive ramp test was performed to determine VO2max and maximal power output (Wmax). After 30 min rest, workload was set to 80% of maximal power output (Wmax) for a time to exhaustion test (TTE). At 100W gross efficiency was reduced from 19.4% during NOX to 18.9% during HOX (P ≤ 0.0001). HOX increased fat oxidation at 100 W by 52% from 3.41 kcal min-1 to 5.17 kcal min-1 (P ≤ 0.0001) with a corresponding reduction in carbohydrate oxidation. Wmax increased by 2.4% from 388.8 (±82.1) during NOX to 397.8 (±83.5) during HOX (P ≤ 0.0001). SaO2 was higher in HOX both at the end of the maximal exercise test and TTE. Subjects with a high level of efficiency in NOX had a larger improvement in Wmax with HOX, in agreement with the hypothesis that an optimum level of efficiency exists that maximizes power production. No association between mitochondrial excess capacity and endurance performance was found; increases in oxygen supply seemed to increase maximal aerobic power production and maintain/increase endurance capacity at the same relative workload. [ABSTRACT FROM AUTHOR]

Intense exercise training is a powerful stimulus that activates mitochondrial biogenesis pathways and thus increases mitochondrial density and oxidative capacity. Moderate levels of reactive oxygen species (ROS) during exercise are considered vital in the adaptive response, but high ROS production is a serious threat to cellular homeostasis. Although biochemical markers of the transition from adaptive to maladaptive ROS stress are lacking, it is likely mediated by redox sensitive enzymes involved in oxidative metabolism. One potential enzyme mediating such redox sensitivity is the citric acid cycle enzyme aconitase. In this study, we examined biopsy specimens of vastus lateralis and triceps brachii in healthy volunteers, together with primary human myotubes. An intense exercise regimen inactivated aconitase by 55-72%, resulting in inhibition of mitochondrial respiration by 50-65%. In the vastus, the mitochondrial dysfunction was compensated for by a 15-72% increase in mitochondrial proteins, whereas H2O2 emission was unchanged. In parallel with the inactivation of aconitase, the intermediary metabolite citrate accumulated and played an integral part in cellular protection against oxidative stress. In contrast, the triceps failed to increase mitochondrial density, and citrate did not accumulate. Instead, mitochondrial H2O2 emission was decreased to 40% of the pretraining levels, together with a 6-fold increase in protein abundance of catalase. In this study, a novel mitochondrial stress response was highlighted where accumulation of citrate acted to preserve the redox status of the cell during periods of intense exercise.--Larsen, F. J., Schiffer, T. A., Ørtenblad, N., Zinner, C., Morales-Alamo, D., Willis, S. J., Calbet, J. A., Holmberg, H.-C., Boushel, R. High-intensity sprint training inhibits mitochondrial respiration through aconitase inactivation. [ABSTRACT FROM AUTHOR]

The maximum power principle dictates that open biological systems tend to self-organize to a level of efficiency that allows maximal power production. Applying this principle to cellular energetics and whole-body physiology would suggest that for every metabolic challenge, an optimal efficiency exists that maximizes power production. On exposure to hypoxia, it would be favorable if metabolic efficiency would rapidly adjust so as to better preserve work performance. We tested this idea in humans by measuring metabolic efficiency and exercise tolerance under nor-moxic (FiO2=20.9%) and hypoxic (FiO2=16%) conditions, where FiO2 is fraction of inhaled oxygen. The results were compared with respirometric analyses of skeletal muscle mitochondria from the same individuals. We found that among healthy trained subjects (n = 14) with a wide range of metabolic efficiency (ME), those with a high ME during normoxic exercise were able to better maintain exercise capacity (Wmax) in hypoxia. On hypoxic exposure, these subjects acutely decreased their efficiency from 19.2 to 17.4%, thereby likely shifting it closer to a degree of efficiency where maximal power production is achieved. In addition, mitochondria from these subjects had a lower intrinsic respiration compared to subjects that showed a large drop in Wmax in hypoxia An acute shift in efficiency was also demonstrated in isolated mitochondria exposed to physiological levels of hypoxia as P/O ratio increased from 0.9 to 1.3 with hypoxic exposure. These findings suggest the existence of a physiological adaptive response by which metabolic efficiency is dynamically optimized to maximize power production. [ABSTRACT FROM AUTHOR]

The basal metabolic rate (BMR) is referred to as the minimal rate of metabolism required to support basic body functions. It is well known that individual BMR varies greatly, even when correcting for body weight, fat content, and thyroid hormone levels, but the mechanistic determinants of this phenomenon remain unknown. Here, we show in humans that mass-related BMR correlates strongly to the mitochondrial oxygen affinity (p50mito; R²=0.66, P=0.0004) measured in isolated skeletal muscle mitochondria. A similar relationship was found for oxygen affinity and efficiency during constant-load submaximal exercise (R²=0.46, P=0.007). In contrast, BMR did not correlate to overall mitochondrial density or to proton leak. Mechanistically, part of the p50mito seems to be controlled by the excess of cytochrome c oxidase (COX) protein and activity relative to other mitochondrial proteins. This is illustrated by the 5-fold increase in p50mito after partial cyanide inhibition of COX at doses that do not affect maximal mitochondrial electron flux through the ETS. These data suggest that the interindividual variation in BMR in humans is primarily explained by differences in mitochondrial oxygen affinity. The implications of these findings are discussed in terms of a trade-off between aerobic efficiency and power. [ABSTRACT FROM AUTHOR]