Activity attenuates skeletal muscle fiber damage after ischemia and reperfusion

Introduction: In this investigation we aimed to determine whether: (1) physical activity protects rat skeletal muscle from ischemia/reperfusion (I/R) injury; and (2) continued activity after I/R improves the rate of healing. Methods: Rats were divided into sedentary or active (voluntary wheel running) groups. Active rats ran for 4 weeks before I/R or 4 weeks before plus 4 weeks after I/R. Results: Activity before I/R resulted in 73.2% less muscle damage (Evans blue dye inclusion). Sedentary and active rats had a similar decline in neural‐evoked (～99%) and directly stimulated (～70%) in vivo muscle torque, and a similar reduction in junctophilin 1. Active rats produced 19% and 15% greater neural‐evoked torque compared with sedentary rats at 14 and 28 days postinjury, respectively, although the rate of recovery appeared similar. Conclusions: Activity protects against long‐term muscle damage, but not short‐term neural injury or excitation‐contraction uncoupling. Continued activity neither accelerates nor hinders the rate of functional recovery. Muscle Nerve 52: 640–648, 2015     

Mitochondrial function following downhill and/or uphill exercise training in rats

Introduction: The goal of this study was to compare the effects of downhill (DH), uphill (UH), and UH‐DH exercise training, at the same metabolic rate, on exercise capacity and skeletal muscle mitochondrial function. Methods: Thirty‐two Wistar rats were separated into a control and 3 trained groups. The trained groups exercised for 4 weeks, 5 times per week at the same metabolic rate, either in UH, DH, or combined UH‐DH. Twenty‐four hours after the last training session, the soleus, gastrocnemius, and vastus intermedius muscles were removed for assessment of mitochondrial respiration. Results: Exercise training, at the same metabolic rate, improved maximal running speed without specificity for exercise modalities. Maximal fiber respiration was enhanced in soleus and vastus intermedius in the UH group only. Conclusions: Exercise training, performed at the same metabolic rate, improved exercise capacity, but only UH‐trained rats enhanced mitochondrial function in both soleus and vastus intermedius skeletal muscle. Muscle Nerve 54 : 925–935, 2016     

Effects of moderate‐ and high‐intensity chronic exercise on brain‐derived neurotrophic factor expression in fast and slow muscles

Introduction: Brain‐derived neurotrophic factor (BDNF) protein expression is sensitive to cellular activity. In the sedentary state, BDNF expression is affected by the muscle phenotype. Methods: Eighteen Wistar rats were divided into the following 3 groups: sedentary (S); moderate‐intensity training (MIT); and high‐intensity training (HIT). The training protocol lasted 8 weeks. Forty‐eight hours after training, total RNA and protein levels in the soleus and plantaris muscles were obtained. Results: In the plantaris, the BDNF protein level was lower in the HIT than in the S group (P < 0.05). A similar effect was found in the soleus (without significant difference). In the soleus, higher Bdnf mRNA levels were found in the HIT group (P < 0.001 vs. S and MIT groups). In the plantaris muscle, similar Bdnf mRNA levels were found in all groups. Conclusions: These results indicate that high‐intensity chronic exercise reduces BDNF protein level in fast muscles and increases Bdnf mRNA levels in slow muscles. Muscle Nerve 53: 446–451, 2016     

Tongue muscle plasticity following hypoglossal nerve stimulation in aged rats

Introduction: Age‐related decreases in tongue muscle mass and strength have been reported. It may be possible to prevent age‐related tongue muscle changes using neuromuscular electrical stimulation (NMES). Our hypothesis was that alterations in muscle contractile properties and myosin heavy chain composition would be found after NMES. Methods: Fifty‐four young, middle‐aged, and old 344/Brown Norway rats were included in this study. Twenty‐four rats underwent bilateral electrical stimulation of the hypoglossal nerves for 8 weeks and were compared with control or sham rats. Muscle contractile properties and myosin heavy chain (MHC) in the genioglossus (GG), styloglossus (SG), and hyoglossus (HG) muscles were examined. Results: Compared with unstimulated control rats, we found reduced muscle fatigue, increased contraction and half‐decay times, and increased twitch and tetanic tension. Increased type I MHC was found, except for in GG in old and middle‐aged rats. Conclusion: Transitions in tongue muscle contractile properties and phenotype were found after NMES. Muscle Nerve, 2013     

Proteomic analysis of mouse soleus muscles affected by hindlimb unloading and reloading

Introduction: Disuse muscle atrophy, induced by prolonged space flight, bed rest, or denervation, is a common process with obvious changes in slow‐twitch soleus muscles. Methods: Proteomic analysis was performed on mouse soleus subjected to hindlimb unloading (HU) and hindlimb reloading (HR) to identify new dysregulated proteins. Results: Following HU, the mass and cross‐sectional area of muscle fibers decreased, but they recovered after HR. Proteomic analyses revealed 9 down‐regulated and 7 up‐regulated proteins in HU, and 2 down‐regulated and 5 up‐regulated proteins in HR. The dysregulated proteins were mainly involved in energy metabolism, protein degradation, and cytoskeleton stability. Among the dysregulated proteins were fatty acid binding protein 3, α‐B crystalline, and transthyretin. Conclusions: These results indicate that muscle atrophy induced by unloading is related to activation of proteolysis, metabolic alterations toward glycolysis, destruction of myofibrillar integrity, and dysregulation of heat shock proteins (HSPs). The dysregulated proteins may play a role in muscle atrophy and the recovery process. Muscle Nerve 52 : 803–811, 2015     

Low‐intensity running exercise enhances the capillary volume and pro‐angiogenic factors in the soleus muscle of type 2 diabetic rats

Introduction: We determined the effects of low‐intensity exercise on the three‐dimensional capillary structure and associated angiogenic factors in the soleus muscle of Goto‐Kakizaki (GK) diabetic rats. Methods: Four groups of male rats were studied: sedentary nondiabetic (Con), exercised nondiabetic control (Ex), sedentary GK, and exercised GK (GK+Ex). Rats in the Ex and GK+Ex groups were subjected to chronic low‐intensity running on a treadmill (15 m/min, 60 min/session, 5 sessions/week for 3 weeks). Results: Although mean capillary volume and diameter were lower in the GK compared with all other groups, low‐intensity exercise increased both of these measures in GK rats. Mitochondrial markers, i.e., SDH activity and PGC‐1α expression, and the levels of angiogenic factors were higher in the GK+Ex than all other groups. Exercise increased vascular endothelial growth factor (VEGF) protein levels and the VEGF‐to‐TSP‐1 ratio, an indicator of angiogenesis, in GK rats. Conclusions: Combined, the results indicate that low‐intensity exercise reduces some of the microcirculatory complications in type 2 diabetic muscles. Muscle Nerve 51: 391–399, 2015     

Exercise increases utrophin protein expression in the mdx mouse model of Duchenne muscular dystrophy

Introduction: Duchenne muscular dystrophy (DMD) is a lethal genetic disease caused by mutations in the dystrophin gene resulting in chronic muscle damage, muscle wasting, and premature death. Utrophin is a dystrophin protein homologue that increases dystrophic muscle function and reduces pathology. Currently, no treatments that increase utrophin protein expression exist. However, exercise increases utrophin mRNA expression in healthy humans. Therefore, the purpose was to determine whether exercise increases utrophin protein expression in dystrophic muscle. Methods: Utrophin protein was measured in the quadriceps and soleus muscles of mdx mice after 12 weeks of voluntary wheel running exercise or sedentary controls. Muscle pathology was measured in the quadriceps. Results: Exercise increased utrophin protein expression 334 ± 63% in the quadriceps relative to sedentary controls. Exercise increased central nuclei 4 ± 1% but not other measures of pathology. Conclusions: Exercise may be an intervention that increases utrophin expression in patients with DMD. Muscle Nerve 49 : 915–918, 2014     

Specific impulse patterns regulate acetylcholinesterase activity in skeletal muscles of rats and rabbits

In rats, acetylcholinesterase (AChE) activity in the fast muscles is several times higher than in the slow soleus muscle. The hypothesis that specific neural impulse patterns in fast or slow muscles are responsible for different AChE activities was tested by altering the neural activation pattern in the fast extensor digitorum longus (EDL) muscle by chronic low-frequency stimulation of its nerve. In addition, the soleus muscle was examined after hind limb immobilization, which changed its neural activation pattern from tonic to phasic. Myosin heavy-chain (MHC) isoforms were analyzed by gel electrophoresis. Activity of the molecular forms of AChE was determined by velocity sedimentation. Low-frequency stimulation of the rat EDL for 35 days shifted the profile of MHC II isoforms toward a slower MHCIIa isoform. Activity of the globular G₁ and G₄ molecular forms of AChE decreased by a factor of 4 and 10, respectively, and became comparable with those in the soleus muscle. After hind limb immobilization, the fast MHCIId isoform, which is not normally present, appeared in the soleus muscle. Activity of the globular G₁ form of AChE increased approximately three times and approached the levels in the fast EDL muscle. In the rabbit, on the contrary to the rat, activity of the globular forms of AChE in a fast muscle increased after low-frequency stimulation. The results demonstrate that specific neural activation patterns regulate AChE activity in muscles. Great differences, however, exist among different mammalian species in regard to muscle AChE regulation. J. Neurosci. Res. 47:49–57, 1997. © 1997 Wiley-Liss, Inc.     

Effects of exercise and muscle type on BDNF,NT‐4/5, and TrKB expression in skeletal muscle

Muscle‐derived neurotrophins are thought to contribute to the adaptation of skeletal muscle to exercise, but the effects of brief exercise interventions on BDNF, NT‐4/5, and trkB are not understood. RNA was extracted for RT‐PCR from soleus and medial gastrocnemius of Sprague‐Dawley rats exercised on a treadmill at speeds up to 20 m/min at 5% incline for 5 or 10 days. BDNF expression was elevated in soleus following 5 days (184%, P < 0.001) but not 10 days of exercise. NT‐4/5 and trkB were not affected at either time‐point. BDNF mRNA was significantly higher in soleus at rest when compared with medial gastrocnemius (193%, P < 0.05). No significant effects of muscle type were detected for NT‐4/5 and trkB. Our results indicate differential control of BDNF expression between soleus and medial gastrocnemius following 5 days of exercise. BDNF may be a protein with an uncharacterized contribution to the acute adaptation of skeletal muscle to exercise, whereas NT‐4/5 shows no response. Muscle Nerve, 2009     

Voluntary exercise increases neurotrophin-3 and its receptor TrkC in the spinal cord

We have evaluated changes in the expression of neurotrophin-3 (NT-3) and its tyrosine kinase C (TrkC) receptor in the neuromuscular system as a result of voluntary physical activity. We assessed changes in the mRNAs and proteins for NT-3 and TrkC in the lumbar spinal cord and associated soleus muscle following 3 and 7 days of voluntary wheel running. We used quantitative Taqman RT-PCR to measure mRNA and ELISA to assess protein levels. NT-3 mRNA and protein levels increased in the spinal cord to reach statistical significance after 7 days of exercise compared to sedentary control rats. Immunohistochemical analyses localized the elevated NT-3 to the substantia gelatinosa (SG) and nucleus of the dorsal horn. TrkC mRNA levels were significantly elevated in the spinal cord after 3 and 7 days of running. In the soleus muscle, NT-3 mRNA levels and its receptor TrkC were elevated after 3 days, while NT-3 protein levels remained unaffected. The results demonstrate that voluntary exercise has a differential effect on NT-3 as well as its receptor TrkC in the neural and muscular components of the neuromuscular system, and emphasize the role of voluntary activity on the spinal cord and muscle.     

Effect of eccentric versus concentric exercise training on mitochondrial function

Introduction: The effect of eccentric (ECC) versus concentric (CON) training on metabolic properties in skeletal muscle is understood poorly. We determined the responses in oxidative capacity and mitochondrial H₂O₂ production after eccentric (ECC) versus concentric (CON) training performed at similar mechanical power. Methods: Forty‐eight rats performed 5‐ or 20‐day eccentric (ECC) or concentric (CON) training programs. Mitochondrial respiration, H₂O₂ production, citrate synthase activity (CS), and skeletal muscle damage were assessed in gastrocnemius (GAS), soleus (SOL) and vastus intermedius (VI) muscles. Results: Maximal mitochondrial respiration improved only after 20 days of concentric (CON) training in GAS and SOL. H₂O₂ production increased specifically after 20 days of eccentric ECC training in VI. Skeletal muscle damage occurred transiently in VI after 5 days of ECC training. Conclusions: Twenty days of ECC versus CON training performed at similar mechanical power output do not increase skeletal muscle oxidative capacities, but it elevates mitochondrial H₂O₂ production in VI, presumably linked to transient muscle damage. Muscle Nerve 50 : 803–811, 2014     

Electrical stimulation influences satellite cell differentiation after sciatic nerve crush injury in rats

Introduction: Electrical stimulation is often used to prevent muscle atrophy and preserve contractile function, but its effects on the satellite cell population after nerve injury are not well understood. In this study we aimed to determine whether satellite cell differentiation is affected by electrical stimulation after nerve crush. Methods: The sciatic nerves of Sprague‐Dawley (SD) rats were crushed. Half of the injured rats received daily electrical stimulation of the gastrocnemius muscle, and the others did not. Tests for detecting paired box protein 7 (Pax7), myogenic differentiation antigen (MyoD), embryonic myosin heavy chain (eMyHC), and force production were performed 2, 4, and 6 weeks after injury. Results: More Pax7⁺/MyoD⁺ nuclei in stimulated muscles were observed than in non‐stimulated muscles. eMyHC expression was elevated in stimulated muscles and correlated positively with enhanced force production. Conclusions: Increased satellite cell differentiation is correlated with preserved muscle function in response to electrical stimulation after nerve injury. Muscle Nerve 51: 400–411, 2015     

The effects of strength,aerobic, and concurrent exercise on skeletal muscle damage in rats

Introduction: In this study we examined oxidative stress and skeletal muscle damage resulting from acute strength, aerobic, or concurrent exercise in rats. Methods: The animals were divided into control (C), strength (SE), aerobic (AE), and combined (CE) exercise groups. They were euthanized at 3 different time‐points (6, 24, and 48 h) after acute exercise. Results: SE exercise rats had increased dichlorofluorescein oxidation at 6 h post‐exercise and decreased superoxide dismutase activity at all time‐points. Glutathione peroxidase activity and sulfhydryl levels were increased in the AE group at 48 h post‐exercise. Serum lactate dehydrogenase activity was increased in the SE and CE groups at 24 h and in the AE group at 48 h. Echo intensity was elevated at 24 h for all groups. Conclusions: Forty‐eight hours was sufficient for complete recovery from oxidative stress and muscle damage in the SE and CE groups, but not in the AE group. Muscle Nerve 50 : 79–86, 2014     

Recovery of strength is dependent on mTORC1 signaling after eccentric muscle injury

Introduction: Eccentric contractions may cause immediate and long‐term reductions in muscle strength that can be recovered through increased protein synthesis rates. The purpose of this study was to determine whether the mechanistic target‐of‐rapamycin complex 1 (mTORC1), a vital controller of protein synthesis rates, is required for return of muscle strength after injury. Methods: Isometric muscle strength was assessed before, immediately after, and then 3, 7, and 14 days after a single bout of 150 eccentric contractions in mice that received daily injections of saline or rapamycin. Results: The bout of eccentric contractions increased the phosphorylation of mTORC1 (1.8‐fold) and p70s6k1 (13.8‐fold), mTORC1's downstream effector, 3 days post‐injury. Rapamycin blocked mTORC1 and p70s6k1 phosphorylation and attenuated recovery of muscle strength (～20%) at 7 and 14 days. Conclusion: mTORC1 signaling is instrumental in the return of muscle strength after a single bout of eccentric contractions in mice. Muscle Nerve 54 : 914–924, 2016     

Exercise-induced gene expression in soleus muscle is dependent on time after spinal cord injury in rats

Cycling exercise attenuates atrophy in hindlimb muscles and causes changes in spinal cord properties after spinal cord injury in rats. We hypothesized that exercising soleus muscle expresses genes that are potentially beneficial to the injured spinal cord. Rats underwent spinal cord injury at T10 and were exercised on a motor-driven bicycle. Soleus muscle and lumbar spinal cord tissue were used for messenger RNA (mRNA) analysis. Gene expression of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) was elevated 11- and 14-fold, respectively, in soleus muscle after one bout of exercise performed 5 days after spinal cord transection. Also, c-fos and heat shock protein-27 (HSP27) mRNA abundance were increased 11- and 7-fold, respectively. When exercise was started 2 days after the injury, the changes in gene expression were not observed. By contrast, at 2 but not at 5 days after transection, expression of the HSP27 gene was elevated sixfold in the lumbar spinal cord, independent of exercise. Electromyographic activity in soleus muscles was also decreased at 2 days, indicating that the spinal cord was less permissive to exercise at this early time. Long-term exercise for 4 weeks attenuated muscle atrophy equally well in rats started at 2 days or 5 days after injury. We conclude that BDNF and GDNF released from exercising muscle may be involved in exercise-induced plasticity of the spinal cord. Furthermore, the data suggest that the lumbar spinal cord undergoes time-dependent changes that temporarily impede the ability of the muscle to respond to exercise.     

Functional effects of uridine triphosphate on the atrophied soleus muscle of rat after unloading

The purposes of the study were to determine the effects of a pyrimidine nucleotide, the uridine triphosphate (UTP), on the contractile and histochemical properties of the soleus (SOL) muscle following disuse atrophy due to hindlimb unloading (HU) hypokinesia. UTP was injected either during the HU period (2 weeks) or later during the recovery period. In this latter condition, contractile and histochemical properties were studied after 5, 8, 11, and 15 days of spontaneous recovery. HU induced decreases in the SOL weight, force output (twitch and tetanic tensions), time to peak tension during the twitch, and the percentage of type I fibers. The injection of UTP during the HU period did not counteract the modification in speed-related properties, but the decrease in force output was partly counteracted and the proportion of type II C fibers was increased. When UTP was injected during the recovery periods, force-related properties recovered more rapidly. These results suggest that UTP may reduce the loss of force induced by atrophy. © 1997 John Wiley & Sons, Inc. Muscle Nerve, 20, 172–178, 1997.     

Permanent changes in muscle and motoneurones induced by nerve injury during a critical period of development of the rat

The sciatic nerve was crushed in rats at different times during the first two weeks after birth. Following reinnervation the recovery of the fast and slow muscles and their motoneurones was compared. The main factor affecting recovery of muscle weight and tension was the age at which the nerve was crushed; the earlier the injury the greater the impairment. However, recovery also depended upon muscle type. The fast muscles, tibialis anterior and extensor digitorum longus, always recovered less well than the slow soleus muscle. The greatest difference in recovery was seen when the nerve was crushed between 3 and 6 days of age. The fatigue resistance of fast muscles was markedly increased after nerve injury at any time during the first two postnatal weeks and was greatest when the nerve crush was done soon after birth. However, this change was not just related to muscle weakness as the increase in fatigue resistance after nerve crush at 5 and 12 days was similar regardless of the difference in recovery of the muscles. Retrograde labelling of motoneurones with HRP demonstrated that about 60-70% of motoneurones innervating fast or slow muscles were lost following sciatic nerve crush at birth. It is concluded that motoneurone loss probably accounts for most of the impairment of soleus after postnatal nerve crush but only partly explains the poor recovery of fast muscles.     

Effect of locomotor training on muscle performance in the context of nerve-muscle communication dysfunction

Introduction: The effects of locomotor training (LT) on skeletal muscle after peripheral nerve injury and acetylcholinesterase deficiency are not well documented. Methods: We determined the effects of LT on mouse soleus muscle performance after sciatic nerve transection with excision (full and permanent denervation), nerve transection (partial functional reinnervation), nerve crush (full denervation with full functional reinnervation), and acetylcholinesterase deficiency (alteration in neuromuscular junction functioning). Results: We found no significant effect of LT on the recovery of soleus muscle weight, maximal force in response to muscle stimulation, and fatigue resistance after nerve transection with or without excision. However, LT significantly increased soleus muscle fatigue resistance after nerve crush and acetylcholinesterase deficiency. Moreover, hindlimb immobilization significantly aggravated the deficit in soleus muscle maximal force production and atrophy after nerve crush. Conclusions: LT is beneficial, and reduced muscle use is detrimental for intrinsic muscle performance in the context of disturbed nerve–muscle communication. Muscle Nerve, 2012     

Effects of unilateral and bilateral labyrinthectomy on rat postural muscle properties: the soleus

The aim of our study was to determine whether the suppression of the vestibular inputs could have effects on the soleus muscle properties similar to the modifications observed after an episode of microgravity. The inner ear lesion was performed by surgical labyrinthectomy. Twenty-nine male Wistar rats were used for this study and were divided into three experimental groups: control (CONT, n=7), unilateral labyrinthectomized (UL, n=14) and bilateral labyrinthectomized (BL, n=8). Mechanical, histochemical and electrophoretic parameters were determined 17 days after the operation. Furthermore, electromyographic (EMG) activity of the soleus muscle was examined at 1 h, 1 day and 17 days. Our results showed that UL and BL groups did not present any sign of muscle atrophy when compared to CONT group. However, the contractile and phenotypical characteristics of UL and BL soleus muscles revealed that the muscle evolved from slow toward a slower type. This transition was correlated with a more tonic EMG activity pattern. To conclude, our data demonstrated that soleus muscle transformations observed after microgravity (muscle atrophy, slow-to-fast transition, phasic EMG activity) were not directly the consequence of a vestibular silence.