Adaptation of motor unit contractile properties in rat medial gastrocnemius to treadmill endurance training: Relationship to muscle mitochondrial biogenesis

Exercise Training Decreases Nitrite Concentration in the Heart and Locomotory Muscles of Rats Without Changing the Muscle Nitrate Content

BACKGROUND

Skeletal muscles are postulated to be a potent regulator of systemic nitric oxide homeostasis. In this study, we aimed to evaluate the impact of physical training on the heart and skeletal muscle nitric oxide bioavailability (judged on the basis of intramuscular nitrite and nitrate) in rats.

METHODS AND RESULTS

Rats were trained on a treadmill for 8 weeks, performing mainly endurance running sessions with some sprinting runs. Muscle nitrite (NO2-) and nitrate (NO3-) concentrations were measured using a high-performance liquid chromatography-based method, while amino acids, pyruvate, lactate, and reduced and oxidized glutathione were determined using a liquid chromatography coupled with tandem mass spectrometry technique. The content of muscle nitrite reductases (electron transport chain proteins, myoglobin, and xanthine oxidase) was assessed by western immunoblotting. We found that 8 weeks of endurance training decreased basal NO2- in the locomotory muscles and in the heart, without changes in the basal NO3-. In the slow-twitch oxidative soleus muscle, the decrease in NO2- was already present after the first week of training, and the content of nitrite reductases remained unchanged throughout the entire period of training, except for the electron transport chain protein content, which increased no sooner than after 8 weeks of training.

CONCLUSIONS

Muscle NO2- level, opposed to NO3-, decreases in the time course of training. This effect is rapid and already visible in the slow-oxidative soleus after the first week of training. The underlying mechanisms of training-induced muscle NO2- decrease may involve an increase in the oxidative stress, as well as metabolite changes related to an increased muscle anaerobic glycolytic activity contributing to (1) direct chemical reduction of NO2- or (2) activation of muscle nitrite reductases.

Mitochondria as a target for exercise-mitigated type 2 diabetes

Type 2 diabetes mellitus (T2DM) is one of most common metabolic diseases and continues to be a leading cause of death worldwide. Although great efforts have been made to elucidate the pathogenesis of diabetes, the underlying mechanism still remains unclear. Notably, overwhelming evidence has demonstrated that mitochondria are tightly correlated with the development of T2DM, and the defects of mitochondrial function in peripheral insulin-responsive tissues, such as skeletal muscle, liver and adipose tissue, are crucial drivers of T2DM. Furthermore, exercise training is considered as an effective stimulus for improving insulin sensitivity and hence is regarded as the best strategy to prevent and treat T2DM. Although the precise mechanisms by which exercise alleviates T2DM are not fully understood, mitochondria may be critical for the beneficial effects of exercise.

Moderate intensity continuous versus high intensity interval training: Metabolic responses of slow and fast skeletal muscles in rat

The healthy benefits of regular physical exercise are mainly mediated by the stimulation of oxidative and antioxidant capacities in skeletal muscle. Our understanding of the cellular and molecular responses involved in these processes remain often uncomplete particularly regarding muscle typology. The main aim of the present study was to compare the effects of two types of exercise training protocol: a moderate-intensity continuous training (MICT) and a high-intensity interval training (HIIT) on metabolic processes in two muscles with different typologies: soleus and extensor digitorum longus (EDL). Training effects in male Wistar rats were studied from whole organism level (maximal aerobic speed, morphometric and systemic parameters) to muscle level (transcripts, protein contents and enzymatic activities involved in antioxidant defences, aerobic and anaerobic metabolisms). Wistar rats were randomly divided into three groups: untrained (UNTR), n = 7; MICT, n = 8; and HIIT, n = 8. Rats of the MICT and HIIT groups ran five times a week for six weeks at moderate and high intensity, respectively. HIIT improved more than MICT the endurance performance (a trend to increased maximal aerobic speed, p = 0.07) and oxidative capacities in both muscles, as determined through protein and transcript assays (AMPK-PGC-1α signalling pathway, antioxidant defences, mitochondrial functioning and dynamics). Whatever the training protocol, the genes involved in these processes were largely more significantly upregulated in soleus (slow-twitch fibres) than in EDL (fast-twitch fibres). Solely on the basis of the transcript changes, we conclude that the training protocols tested here lead to specific muscular responses.

Tissue Distribution of mtDNA Copy Number And Expression Pattern of An mtDNA-Related Gene in Three Teleost Fish Species

Teleosts are the most speciose vertebrates and have diverse swimming performance. Based on swimming duration and speed, teleosts are broadly divided into sustained, prolonged, and burst swimming fish. Teleosts with different swimming performance have different energy requirements. In addition, energy requirement also varies among different tissues. As mitochondrial DNA (mtDNA) copy number is correlated with ATP production, we speculated that mtDNA copy number varies among fish with different swimming performance, as well as among different tissues. In other species, mtDNA copy number is regulated by tfam (mitochondrial transcription factor A) through mtDNA compaction and mito-genome replication initiation. In order to clarify the tissue distribution of mtDNA copy number and expression pattern of tfam in teleosts with disparate swimming performance, we selected representative fish with sustained swimming (Pseudocaranx dentex), prolonged swimming (Takifugu rubripes), and burst swimming (Paralichthys olivaceus). We measured mtDNA copy number and tfam gene expression in 10 tissues of these three fish. The results showed the mtDNA content pattern of various tissues was broadly consistent among three fish, and high-energy demanding tissues contain higher mtDNA copy number. Slow-twitch muscles with higher oxidative metabolism possess a greater content of mtDNA than fast-twitch muscles. In addition, relatively higher mtDNA content in fast-twitch muscle of P. olivaceus compared to the other two fish could be an adaptation to their frequent burst swimming demands. And the higher mtDNA copy number in heart of P. dentex could meet their oxygen transport demands of long-distance swimming. However, tfam expression was not significantly correlated with mtDNA copy number in these teleosts, suggesting tfam may be not the only factor regulating mtDNA content among various tissues. This study can lay a foundation for studying the role of mtDNA in the adaptive evolution of various swimming ability in teleost fish.

Emerging methods for and novel insights gained by absolute quantification of mitochondrial DNA copy number and its clinical applications

The past thirty years have seen a surge in interest in pathophysiological roles of mitochondria, and the accurate quantification of mitochondrial DNA copy number (mCN) in cells and tissue samples is a fundamental aspect of assessing changes in mitochondrial health and biogenesis. Quantification of mCN between studies is surprisingly variable due to a combination of physiological variability and diverse protocols being used to measure this endpoint. The advent of novel methods to quantify nucleic acids like digital polymerase chain reaction (dPCR) and high throughput sequencing offer the ability to measure absolute values of mCN. We conducted an in-depth survey of articles published between 1969 -- 2020 to create an overview of mCN values, to assess consensus values of tissue-specific mCN, and to evaluate consistency between methods of assessing mCN. We identify best practices for methods used to assess mCN, and we address the impact of using specific loci on the mitochondrial genome to determine mCN. Current data suggest that clinical measurement of mCN can provide diagnostic and prognostic value in a range of diseases and health conditions, with emphasis on cancer and cardiovascular disease, and the advent of means to measure absolute mCN should improve future clinical applications of mCN measurements.

Effect of synchronization of firings of different motor unit types on the force variability in a model of the rat medial gastrocnemius muscle

The synchronized firings of active motor units (MUs) increase the oscillations of muscle force, observed as physiological tremor. This study aimed to investigate the effects of synchronizing the firings within three types of MUs (slow—S, fast resistant to fatigue–FR, and fast fatigable–FF) on the muscle force production using a mathematical model of the rat medial gastrocnemius muscle. The model was designed based on the actual proportion and physiological properties of MUs and motoneurons innervating the muscle. The isometric muscle and MU forces were simulated by a model predicting non-synchronized firing of a pool of 57 MUs (including 8 S, 23 FR, and 26 FF) to ascertain a maximum excitatory signal when all MUs were recruited into the contraction. The mean firing frequency of each MU depended upon the twitch contraction time, whereas the recruitment order was determined according to increasing forces (the size principle). The synchronization of firings of individual MUs was simulated using four different modes and inducing the synchronization of firings within three time windows (± 2, ± 4, and ± 6 ms) for four different combinations of MUs. The synchronization was estimated using two parameters, the correlation coefficient and the cross-interval synchronization index. The four scenarios of synchronization increased the values of the root-mean-square, range, and maximum force in correlation with the increase of the time window. Greater synchronization index values resulted in higher root-mean-square, range, and maximum of force outcomes for all MU types as well as for the whole muscle output; however, the mean spectral frequency of the forces decreased, whereas the mean force remained nearly unchanged. The range of variability and the root-mean-square of forces were higher for fast MUs than for slow MUs; meanwhile, the relative values of these parameters were highest for slow MUs, indicating their important contribution to muscle tremor, especially during weak contractions.

The synchronization of firings of motor units (MUs), the smallest functional elements of skeletal muscle increases fluctuations in muscle force, known as physiological tremor, which can disturb high-precision movements. In this study, we adopted a recently proposed muscle model consisting of MUs of three different types (fast fatigable, fast resistant to fatigue, and slow) to study four different scenarios of MU synchronization during a steady level of excitatory input to motoneurons. The discharge patterns were synchronized between pairs of MUs by shifting in time individual pulses, which occurred within a short time interval, and a degree of synchronization was then estimated. The increased synchronization index resulted in increased force variability for all MU types as well as for the whole muscle output; however, the mean force levels remained nearly unchanged, whereas the frequencies of the force oscillations were decreased. The absolute range of force variability was higher for fast than for slow MUs, indicating their dominant influence on muscle tremor at strong contractions, but the highest relative increase in force variability was observed for synchronized slow MUs, indicating their significant contribution to tremor during weak contractions, in which only slow MUs are active.

PCR-Based Determination of Mitochondrial DNA Copy Number in Multiple Species

Mitochondrial DNA (mtDNA) copy number is a critical component of overall mitochondrial health. In this chapter, we describe methods for simultaneous isolation of mtDNA and nuclear DNA (nucDNA), and measurement of their respective copy numbers using quantitative PCR. Methods differ depending on the species and cell type of the starting material, and availability of specific PCR reagents. We also briefly describe factors that affect mtDNA copy number and discuss caveats to its use as a biomarker.

Lung mitochondria adaptation to endurance training in rats

We elucidated the impact of eight weeks of endurance training on the oxidative metabolism of rat lungs. Adult 3.5-month-old male rats were randomly allocated to a treadmill training group or a sedentary group as control. In the lungs, endurance training raised the expression level of the oxygen sensors hypoxia inducible factor 1α (HIF1α) and lysine-specific demethylase 6A (KDM6A) as well as stimulated mitochondrial oxidative capacity and mitochondrial biogenesis, while lactate dehydrogenase activity was reduced. Endurance training enhanced antioxidant systems (the coenzyme Q content and superoxide dismutase) in lung tissue but decreased them (and uncoupling protein 2) in lung mitochondria. In the lung mitochondria of trained rats, the decreased Q content and Complex I (CI) activity and the enhanced cytochrome pathway activity (CIII + CIV) may account for the diminished Q reduction level, resulting in a general decrease in H₂O₂ formation by mitochondria. Endurance training enhanced oxidation of glutamate and fatty acids and caused opposite effects in functional mitochondrial properties during malate and succinate oxidation, which were related to reduced activity of CI and increased activity of CII, respectively. In addition, endurance training downregulated CI in supercomplexes and upregulated CIII in the CIII₂+CIV supercomplex in the oxidative phosphorylation system. We concluded that the adaptive lung responses observed could be due to hypoxia and oxidative stress induced by strenuous endurance training.

Ketogenic diet feeding improves aerobic metabolism property in extensor digitorum longus muscle of sedentary male rats

Recent studies of the ketogenic diet, an extremely high-fat diet with extremely low carbohydrates, suggest that it changes the energy metabolism properties of skeletal muscle. However, ketogenic diet effects on muscle metabolic characteristics are diverse and sometimes countervailing. Furthermore, ketogenic diet effects on skeletal muscle performance are unknown. After male Wistar rats (8 weeks of age) were assigned randomly to a control group (CON) and a ketogenic diet group (KD), they were fed for 4 weeks respectively with a control diet (10% fat, 10% protein, 80% carbohydrate) and a ketogenic diet (90% fat, 10% protein, 0% carbohydrate). After the 4-week feeding period, the extensor digitorum longus (EDL) muscle was evaluated ex vivo for twitch force, tetanic force, and fatigue. We also analyzed the myosin heavy chain composition, protein expression of metabolic enzymes and regulatory factors, and citrate synthase activity. No significant difference was found between CON and KD in twitch or tetanic forces or muscle fatigue. However, the KD citrate synthase activity and the protein expression of Sema3A, citrate synthase, succinate dehydrogenase, cytochrome c oxidase subunit 4, and 3-hydroxyacyl-CoA dehydrogenase were significantly higher than those of CON. Moreover, a myosin heavy chain shift occurred from type IIb to IIx in KD. These results demonstrated that the 4-week ketogenic diet improves skeletal muscle aerobic capacity without obstructing muscle contractile function in sedentary male rats and suggest involvement of Sema3A in the myosin heavy chain shift of EDL muscle.

Regional Differences in Mitochondrial Capacity in the Finger Flexors of Piano Players

BACKGROUND

Near-infrared spectroscopy (NIRS) has been used to measure oxidative capacity, but regional differences have not been identified. Piano players are also a novel group of subjects for this lab.

METHODS

Controls (n = 13) and piano players (n = 8) were tested in a seated position on the right forearm. A fatigue test was performed for three minutes at 2, 4 and 6 Hz using electrical stimulation, which created an endurance index (EI) as the forearm fatigued. A six-cuff oxidative capacity test was performed using manual exercise to activate the muscle and allow for regional specificity. A rate constant (Rc) was generated from the mitochondrial capacity data.

RESULTS

Overall, piano players (Rc = 1.76 ± 0.6) and controls (Rc = 1.17 ± 0.3) have significant differences for the last two fingers (p = 0.01). While controls have significant differences between the index (Rc = 1.86 ± 0.5) and last two fingers (Rc = 1.17 ± 0.3) (p = 0.001), this difference was not observed in piano players. Overall, piano players (EI = 75.7 ± 12.3) and controls (EI = 73.0 ± 17.3) had no differences in endurance index values (p = 0.71).

CONCLUSIONS

Piano players have significant differences in the mitochondrial capacity of the finger flexors that control the last two fingers compared to controls. The lack of difference between groups in the index fingers and overall endurance test suggests playing the piano produces training adaptations to the finger flexor muscles of the last two digits, which are rarely used by control subjects.

Adult skeletal muscle deletion of Mitofusin 1 and 2 impedes exercise performance and training capacity

Endurance exercise has been shown to be a positive regulator of skeletal muscle metabolic function. Changes in mitochondrial dynamics (fusion and fission) have been shown to influence mitochondrial oxidative capacity. We therefore tested whether genetic disruption of mitofusins (Mfns) affected exercise performance in adult skeletal muscle. We generated adult-inducible skeletal muscle-specific Mfn1 (iMS-Mfn1KO), Mfn2 (iMS-Mfn2KO), and Mfn1/2 (iMS-MfnDKO) knockout mice. We assessed exercise capacity by performing a treadmill time to exhaustion stress test before deletion and up to 8 wk after deletion. Analysis of either the iMS-Mfn1KO or the iMS-Mfn2KO did not reveal an effect on exercise capacity. However, analysis of iMS-MfnDKO animals revealed a progressive reduction in exercise performance. We measured individual electron transport chain (ETC) complex activity and observed a reduction in ETC activity in both the subsarcolemmal and intermyofibrillar mitochondrial fractions specifically for NADH dehydrogenase (complex I) and cytochrome- c oxidase (complex IV), which was associated with a decrease in ETC subunit expression for these complexes. We also tested whether voluntary exercise training would prevent the decrease in exercise capacity observed in iMS-MfnDKO animals ( n = 10/group). However, after 8 wk of training we did not observe any improvement in exercise capacity or ETC subunit parameters in iMS-MfnDKO animals. These data suggest that the decrease in exercise capacity observed in the iMS-MfnDKO animals is in part the result of impaired ETC subunit expression and ETC complex activity. Taken together, these results provide strong evidence that mitochondrial fusion in adult skeletal muscle is important for exercise performance. NEW & NOTEWORTHY This study is the first to utilize an adult-inducible skeletal muscle-specific knockout model for Mitofusin (Mfn)1 and Mfn2 to assess exercise capacity. Our findings reveal a progressive decrease in exercise performance with Mfn1 and Mfn2 deletion. The decrease in exercise capacity was accompanied by impaired oxidative phosphorylation specifically for complex I and complex IV. Furthermore, voluntary exercise training was unable to rescue the impairment, suggesting that normal fusion is essential for exercise-induced mitochondrial adaptations.