Martin Gram

Skeletal muscle mitochondrial H2O2 emission increases with immobilization and decreases after aerobic training in young and older men

Currently, it is not known whether impaired mitochondrial function contributes to human ageing or whether potential impairments in mitochondrial function with age are secondary to physical inactivity. The present study investigated mitochondrial respiratory function and reactive oxygen species emission at a predefined membrane potential in young and older men subjected to 2 weeks of one‐leg immobilization followed by 6 weeks of aerobic cycle training. Immobilization increased reactive oxygen species emission and decreased ATP generating respiration. Subsequent aerobic training reversed these effects. By contrast, age had no effect on the measured variables. The results of the present study support the notion that increased mitochondrial reactive oxygen species production mediates the detrimental effects seen after physical inactivity and that ageing per se does not cause mitochondrial dysfunction.

Two weeks of one-leg immobilization decreases skeletal muscle respiratory capacity equally in young and elderly men.

Physical inactivity affects human skeletal muscle mitochondrial oxidative capacity but the influence of aging combined with physical inactivity is not known. This study investigates the effect of two weeks of immobilization followed by six weeks of supervised cycle training on muscle oxidative capacity in 17 young (23±1years) and 15 elderly (68±1years) healthy men. We applied high-resolution respirometry in permeabilized fibers from muscle biopsies at inclusion after immobilization and training. Furthermore, protein content of mitochondrial complexes I-V, mitochondrial heat shock protein 70 (mtHSP70) and voltage dependent anion channel (VDAC) were measured in skeletal muscle by Western blotting. The elderly men had lower content of complexes I-V and mtHSP70 but similar respiratory capacity and content of VDAC compared to the young. In both groups the respiratory capacity and protein content of VDAC, mtHSP70 and complexes I, II, IV and V decreased with immobilization and increased with retraining. Moreover, there was no overall difference in the response between the groups. When the intrinsic mitochondrial capacity was evaluated by normalizing respiration to citrate synthase activity, the respiratory differences with immobilization and training disappeared. In conclusion, aging is not associated with a decrease in muscle respiratory capacity in spite of lower complexes I-V and mtHSP70 protein content. Furthermore, immobilization decreased and aerobic training increased the respiratory capacity and protein contents of complexes I-V, mtHSP70 and VDAC similarly in the two groups. This suggests that inactivity and training alter mitochondrial biogenesis equally in young and elderly men.

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.

The effect of age and unilateral leg immobilization for 2 weeks on substrate utilization during moderate-intensity exercise in human skeletal muscle.

This study aimed to provide molecular insight into the differential effects of age and physical inactivity on the regulation of substrate metabolism during moderate‐intensity exercise. Using the arteriovenous balance technique, we studied the effect of immobilization of one leg for 2 weeks on leg substrate utilization in young and older men during two‐legged dynamic knee‐extensor moderate‐intensity exercise, as well as changes in key proteins in muscle metabolism before and after exercise. Age and immobilization did not affect relative carbohydrate and fat utilization during exercise, but the older men had higher uptake of exogenous fatty acids, whereas the young men relied more on endogenous fatty acids during exercise. Using a combined whole‐leg and molecular approach, we provide evidence that both age and physical inactivity result in intramuscular lipid accumulation, but this occurs only in part through the same mechanisms.

Immobilization increases interleukin‐6, but not tumour necrosis factor‐α, release from the leg during exercise in humans

•  What is the central question of this study? Does physical inactivity influence the exercise‐induced release of tumour necrosis factor‐α and interleukin‐6 in healthy humans? In young, healthy subjects, we immobilized one leg for 2 weeks, followed by 45 min two‐legged exercise where one leg served as the control and the other was the previously inactive leg. •  What is the main finding and its importance? We found that prior physical inactivity enhances interleukin‐6 release during exercise, and it is released in the blood from the legs during exercise much faster than previously known. However, tumour necrosis factor‐α is not released in the blood with exercise, even from a previously inactive leg.

The Effect of Reduced Physical Activity and Retraining on Blood Lipids and Body Composition in Young and Older Adult Men.

We studied the effect of physical inactivity and subsequent retraining on cardiovascular risk factors in 17 young (Y; 23.4 ± 0.5 years) and 15 older adult (O; 68.1 ± 1.1 years) men who underwent 14 days of one leg immobilization followed by six weeks of training. Body weight remained unchanged. Daily physical activity decreased by 31 ± 9% (Y) and 37 ± 9% (O) (p < .001). Maximal oxygen uptake decreased with inactivity (Y) and always increased with training. Visceral fat mass decreased (p < .05) with training. Concentrations of lipids in blood were always highest in the older adults. FFA and glycerol increased with reduced activity (p < .05), but reverted with training. Training resulted in increases in HDL-C (p < .05) and a decrease in LDL-C and TC:HDL-C ratio (p < .05). A minor reduction in daily physical activity for two weeks increased blood lipids in both young and older men. Six weeks of training improved blood lipids along with loss of visceral fat.

A novel method for determining human ex vivo submaximal skeletal muscle mitochondrial function.

The present study utilized a novel method aiming to investigate mitochondrial function in human skeletal muscle at submaximal levels and at a predefined membrane potential. The effect of age and training status was investigated using a cross‐sectional design. Ageing was found to be related to decreased leak regardless of training status. Increased training status was associated with increased mitochondrial hydrogen peroxide emission.

Physical inactivity and muscle oxidative capacity in humans

Abstract Physical inactivity is associated with a high prevalence of type 2 diabetes and is an independent predictor of mortality. It is possible that the detrimental effects of physical inactivity are mediated through a lack of adequate muscle oxidative capacity. This short review will cover the present literature on the effects of different models of inactivity on muscle oxidative capacity in humans. Effects of physical inactivity include decreased mitochondrial content, decreased activity of oxidative enzymes, changes in markers of oxidative stress and a decreased expression of genes and contents of proteins related to oxidative phosphorylation. With such a substantial down-regulation, it is likely that a range of adenosine triphosphate (ATP)-dependent pathways such as calcium signalling, respiratory capacity and apoptosis are affected by physical inactivity. However, this has not been investigated in humans, and further studies are required to substantiate this hypothesis, which could expand our knowledge of the potential link between lifestyle-related diseases and muscle oxidative capacity. Furthermore, even though a large body of literature reports the effect of physical training on muscle oxidative capacity, the adaptations that occur with physical inactivity may not always be opposite to that of physical training. Thus, it is concluded that studies on the effect of physical inactivity per se on muscle oxidative capacity in functional human skeletal muscle are warranted.

Exercise promotes IL-6 release from legs in older men with minor response to unilateral immobilization

Abstract Physical inactivity is a major contributor to low-grade systemic inflammation. Most of the studies characterizing interleukin-6 (IL-6) and tumour necrosis factor-α (TNF-α) release from exercising legs have been done in young, healthy men, but studies on inactivity in older people are lacking. The impact of 14 days of one-leg immobilization (IM) on IL-6 and TNF-α release during exercise in comparison to the contralateral control (CON) leg was investigated. Fifteen healthy men (age 68.1 ± 1.1 year (mean ± SEM); BMI 27.0 ± 0.4 kg·m2; VO2max 33.3 ± 1.6 ml·kg‒1·min‒1) performed 45 min of two-leg dynamic knee extensor exercise at 19.5 ± 0.9 W. Arterial and femoral venous blood samples from the CON and the IM legs were collected every 15 min during exercise, and thigh blood flow was measured with ultrasound Doppler. Arterial plasma IL-6 concentration increased with exercise (rest vs. 45 min, main effect p < .05). IL-6 release increased with exercise (rest vs. 30 min, main effect p < .05). Furthermore, IL-6 release was borderline (main effect, p = .085, effect size 0.28) higher in the IM leg compared to the CON leg (288 (95% CI: 213–373) vs. 220 (95% CI: 152–299) pg·min‒1, respectively). There was no release of TNF-α in either leg and arterial concentrations remained unchanged during exercise (p > .05). In conclusion, exercise induces more pronounced IL-6 secretion in healthy older men. Two weeks of unilateral immobilization on the other hand had only a minor influence on IL-6 release. Neither immobilization nor exercise had an effect on TNF-α release across the working legs in older men.