Early changes in muscle fiber size and gene expression in response to spinal cord transection and exercise

Muscles ofspinal cord-transected rats exhibit severe atrophy and a shift toward afaster phenotype. Exercise can partially prevent these changes. Thegoal of this study was to investigate early events involved inregulating the muscle response to spinal transection and passivehindlimb exercise. Adult female Sprague-Dawley rats were anesthetized,and a complete spinal cord transection lesion(T₁₀) was created in all ratsexcept controls. Rats were killed 5 or 10 days after transection orthey were exercised daily on motor-driven bicycles starting at 5 daysafter transection and were killed 0.5, 1, or 5 days after the firstbout of exercise. Structural and biochemical features of soleus andextensor digitorum longus (EDL) muscles were studied. Atrophy wasdecreased in all fiber types of soleus and in type 2a and type 2xfibers of EDL after 5 days of exercise. However, exercise did notappear to affect fiber type that was altered within 5 days of spinalcord transection: fibers expressing myosin heavy chain 2xincreased in soleus and EDL, and extensive coexpression of myosin heavy chain in soleus was apparent. Activation of satellite cells was observed in both muscles of transected rats regardless of exercise status, evidenced by increased accumulation of MyoD and myogenin. Increased expression was transient, except for MyoD, which remained elevated in soleus. MyoD and myogenin were detected both in myofiber and in satellite cell nuclei in both muscles, but in soleus, MyoD waspreferentially expressed in satellite cell nuclei, and in EDL, MyoD wasmore readily detectable in myofiber nuclei, suggesting that MyoD andmyogenin have different functions in different muscles. Exercise didnot affect the level or localization of MyoD and myogenin expression.Similarly, Id-1 expression was transiently increased in soleus and EDLupon spinal cord transection, and no effect of exercise was observed.These results indicate that passive exercise can ameliorate muscleatrophy after spinal cord transection and that satellite cellactivation may play a role in muscle plasticity in response to spinalcord transection and exercise. Finally, the mechanisms underlyingmaintenance of muscle mass are likely distinct from those controllingmyosin heavy chain expression.

     

The COX-2 pathway regulates growth of atrophied muscle via multiple mechanisms

Loss of muscle mass occurs with disease, injury, aging, and inactivity. Restoration of normal muscle mass depends on myofiber growth, the regulation of which is incompletely understood. Cyclooxygenase (COX)-2 is one of two isoforms of COX that catalyzes the synthesis of prostaglandins, paracrine hormones that regulate diverse physiological and pathophysiological processes. Previously, we demonstrated that the COX-2 pathway regulates early stages of myofiber growth during muscle regeneration. However, whether the COX-2 pathway plays a common role in adult myofiber growth or functions specifically during muscle regeneration is unknown. Therefore, we examined the role of COX-2 during myofiber growth following atrophy in mice. Muscle atrophy was induced by hindlimb suspension (HS) for 2 wk, followed by a reloading period, during which mice were treated with either the COX-2-selective inhibitor SC-236 (6 mg·kg^–1·day^–1) or vehicle. COX-2 protein was expressed and SC-236 attenuated myofiber growth during reloading in both soleus and plantaris muscles. Attenuated myofiber growth in the soleus was associated with both decreased myonuclear addition and decreased inflammation, whereas neither of these processes mediated the effects of SC-236 on plantaris growth. In addition, COX-2^–/– satellite cells exhibited impaired activation/proliferation in vitro, suggesting direct regulation of muscle cell activity by COX-2. Together, these data suggest that the COX-2 pathway plays a common regulatory role during various types of muscle growth via multiple mechanisms. cyclooxygenase-2; prostaglandins; myonuclear number; satellite cells; inflammation     

Muscle regeneration during hindlimb unloading results in a reduction in muscle size after reloading.

The hindlimb-unloading model was used to study the ability of muscle injured in a weightless environment to recover after reloading. Satellite cell mitotic activity and DNA unit size were determined in injured and intact soleus muscles from hindlimb-unloaded and age-matched weight-bearing rats at the conclusion of 28 days of hindlimb unloading, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-unloaded rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb unloading, but they were the same (P > 0.05) as those of weight-bearing rats 2 and 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, number of nuclei per millimeter, and DNA unit size were significantly (P < 0.05) smaller for the injured soleus muscles from hindlimb-unloaded rats than for the soleus muscles from weight-bearing rats at each recovery time. Satellite cell mitotic activity was significantly (P < 0.05) higher in the injured soleus muscles from hindlimb-unloaded rats than from weight-bearing rats 2 wk after reloading, but it was the same (P > 0.05) as in the injured soleus muscles from weight-bearing rats 9 wk after reloading. The injured soleus muscles from hindlimb-unloaded rats failed to achieve weight-bearing muscle size 9 wk after reloading, because incomplete compensation for the decrease in myonuclear accretion and DNA unit size expansion occurred during the unloading period.     

Free mobilization and low- to high-intensity exercise in immobilization-induced muscle atrophy

After 3 wk of immobilization, the effects offree cage activity and low- and high-intensity treadmill running (8 wk)on the morphology and histochemistry of the soleus and gastrocnemius muscles in male Sprague-Dawley rats were investigated. In both muscles,immobilization produced a significant(P < 0.001) increase in the meanpercent area of intramuscular connective tissue (soleus: 18.9% inimmobilized left hindlimb vs. 3.6% in nonimmobilized right hindlimb)and in the relative number of muscle fibers with pathologicalalterations (soleus: 66% in immobilized hindlimb vs. 6% in control),with a simultaneous significant (P < 0.001) decrease in the intramuscular capillary density (soleus: mean capillary density in the immobilized hindlimb only 63% of that in thenonimmobilized hindlimb) and muscle fiber size (soleus type I fibers:mean fiber size in the immobilized hindlimb only 69% of that in thenonimmobilized hindlimb). Many of these changes could not be correctedby free remobilization, whereas low- and high-intensity treadmillrunning clearly restored the changes toward control levels, the effectbeing most complete in the high-intensity running group. Collectively,these findings indicate that immobilization-induced pathologicalstructural and histochemical alterations in rat calf muscles are, to agreat extent, reversible phenomena if remobilization is intensified byphysical training. In this respect, high-intensity exercise seems morebeneficial than low-intensity exercise.

     

Activated satellite cells fail to restore myonuclear number in spinal cord transected and exercised rats

In this study, possible mechanisms underlying soleus muscleatrophy after spinal cord transection and attenuation of atrophy withcycling exercise were studied. Adult female Sprague-Dawley rats weredivided into three groups; in two groups the spinal cord was transectedby a lesion at T₁₀. One group wastransected and killed 10 days later, and another group was transectedand exercised for 5 days starting 5 days after transection. The third group served as an uninjured control. All animals received acontinuous-release 5'-bromo-2'-deoxyuridine pellet 10 daysbefore they were killed. Transection alone and transection withexercise lead to activation of satellite cells, but only the exercisegroup showed a trend toward an increase in the number of proliferatingsatellite cells. In all cases the number of activated satellite cellswas significantly higher than the number that divided. Although thenumber of cells undergoing proliferation increased with exercise, noincrease in fusion of satellite cells into muscle fibers was apparent. Spinal cord transection resulted in a 25% decrease in myonuclear number, and exercise was not associated with a restoration of myonuclear number. The number of apoptotic nuclei was increased aftertransection, and exercise attenuated this increase. However, thedecrease in apoptotic nuclei with exercise did not significantly affectmyonuclear number. We conclude that apoptotic nuclear loss likelycontributes to loss of nuclei during muscle atrophy associated withspinal cord transection and that exercise can maintain muscle mass, atleast in the short term, without restoration of myonuclear number.

     

Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading.

The role of satellite cells and DNA unit size in determining muscle size was examined by inhibiting postnatal skeletal muscle development by using hindlimb suspension. Satellite cell mitotic activity and DNA unit size were determined in the soleus muscles from hindlimb-suspended and age-matched weight-bearing rats before the initiation of hindlimb suspension, at the conclusion of a 28-day hindlimb-suspension period, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-suspended rats were significantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb suspension, but they were the same (P > 0. 05) as those of weight-bearing rats 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, nuclei per millimeter, and DNA unit size for the hindlimb-suspended rats were significantly (P < 0.05) smaller than for the weight-bearing rats at all recovery times. Satellite cell mitotic activity was significantly (P < 0.05) higher in the soleus muscles from hindlimb-suspended rats 2 wk after reloading, but it was the same (P > 0.05) as in weight-bearing rats 9 wk after reloading. Juvenile soleus muscles failed to achieve normal muscle size 9 wk after reloading because there was incomplete compensation for the hindlimb-suspension-induced interruptions in myonuclear accretion and DNA unit size expansion.     

Growth hormone/IGF-I and/or resistive exercise maintains myonuclear number in hindlimb unweighted muscles

Allen, David L., Jon K. Linderman, Roland R. Roy, Richard E. Grindeland, Venkat Mukku, and V. Reggie Edgerton. Growth hormone/IGF-I and/or resistive exercise maintains myonuclearnumber in hindlimb unweighted muscles. J. Appl.Physiol. 83(5): 1857-1861, 1997.In the presentstudy of rats, we examined the role, during 2 wk ofhindlimb suspension, of growth hormone/insulin-like growth factor I(GH/IGF-I) administration and/or brief bouts of resistance exercise in ameliorating the loss of myonuclei in fibers of the soleusmuscle that express type I myosin heavy chain. Hindlimb suspensionresulted in a significant decrease in mean soleus wet weight that wasattenuated either by exercise alone or by exercise plus GH/IGF-Itreatment but was not attenuated by hormonal treatment alone. Both meanmyonuclear number and mean fiber cross-sectional area (CSA) of fibersexpressing type I myosin heavy chain decreased after 2 wk of suspensioncompared with control (134 vs. 162 myonuclei/mm and 917 vs. 2,076 µm², respectively). NeitherGH/IGF-I treatment nor exercise alone affected myonuclear number orfiber CSA, but the combination of exercise and growth-factor treatmentattenuated the decrease in both variables. A significant correlationwas found between mean myonuclear number and mean CSA across allgroups. Thus GH/IGF-I administration and brief bouts of muscle loadinghad an interactive effect in attenuating the loss of myonuclei inducedby chronic unloading.

     

A Threshold Dose of Heavy Ion Radiation that Decreases the Oxidative Enzyme Activity of Spinal Motoneurons in Rats

The effect of heavy ion radiation exposure of the spinal cord on the properties of the motoneurons innervating the slow soleus and fast plantaris muscles was investigated. A 15-, 20-, 40-, 50-, or 70-Gy dose of carbon ions (5 Gy/min) was applied to the 2nd to the 6th lumbar segments of the spinal cord in rats. After a 1-month recovery period, the number and cell body size of the irradiated motoneurons innervating the soleus and plantaris muscles did not differ from that of the non-irradiated controls, irrespective of the dose received. However, the oxidative enzyme activity of these motoneurons was decreased by heavy ion radiation at doses of 40, 50, and 70 Gy compared to that of the non-irradiated controls. This decrease in oxidative enzyme activity levels in the motoneurons returned to that of the non-irradiated controls after a 6-month recovery period. We conclude that heavy ion radiation at doses of 40–70 Gy reversibly decreases the oxidative enzyme activity of motoneurons in the spinal cord of rats.     

Exercise training reduces skeletal muscle membrane arachidonate in the obese (fa/fa) Zucker rat

Compared with the lean(Fa/) genotype, obese(fa/fa) Zucker rats have arelative deficiency of muscle phospholipid arachidonate, and skeletalmuscle arachidonate in humans is positively correlated with insulinsensitivity. To assess the hypothesis that the positive effects ofexercise training on insulin sensitivity are mediated by increasedmuscle arachidonate, we randomized 20 lean and 20 obese weanling maleZucker rats to sedentary or treadmill exercise groups. After 9 wk,fasting serum, three skeletal muscles (white gastrocnemius, soleus, andextensor digitorum longus), and heart were obtained. Fasting insulinwas halved by exercise training in the obese rat. In whitegastrocnemius and extensor digitorum longus (fast-twitch muscles), butnot in soleus (a slow-twitch muscle) or heart, phospholipidarachidonate was lower in obese than in lean rats(P < 0.001). In all muscles,exercise in the obese rats reduced arachidonate(P < 0.03, by ANOVA contrast). Weconclude that improved insulin sensitivity with exercise in the obesegenotype is not mediated by increased muscle arachidonate and thatreduced muscle arachidonate in obese Zucker rats is unique tofast-twitch muscles.

     

β-Hydroxy-β-methylbutyrate reduces myonuclear apoptosis during recovery from hind limb suspension-induced muscle fiber atrophy in aged rats

β-Hydroxy-β-methylbutyrate (HMB) is a leucine metabolite shown to reduce protein catabolism in disease states and promote skeletal muscle hypertrophy in response to loading exercise. In this study, we evaluated the efficacy of HMB to reduce muscle wasting and promote muscle recovery following disuse in aged animals. Fisher 344×Brown Norway rats, 34 mo of age, were randomly assigned to receive either Ca-HMB (340 mg/kg body wt) or the water vehicle by gavage (n = 32/group). The animals received either 14 days of hindlimb suspension (HS, n = 8/diet group) or 14 days of unloading followed by 14 days of reloading (R; n = 8/diet group). Nonsuspended control animals were compared with suspended animals after 14 days of HS (n = 8) or after R (n = 8). HMB treatment prevented the decline in maximal in vivo isometric force output after 2 wk of recovery from hindlimb unloading. The HMB-treated animals had significantly greater plantaris and soleus fiber cross-sectional area compared with the vehicle-treated animals. HMB decreased the amount of TUNEL-positive nuclei in reloaded plantaris muscles (5.1% vs. 1.6%, P < 0.05) and soleus muscles (3.9% vs. 1.8%, P < 0.05). Although HMB did not significantly alter Bcl-2 protein abundance compared with vehicle treatment, HMB decreased Bax protein abundance following R, by 40% and 14% (P < 0.05) in plantaris and soleus muscles, respectively. Cleaved caspase-3 was reduced by 12% and 9% (P < 0.05) in HMB-treated reloaded plantaris and soleus muscles, compared with vehicle-treated animals. HMB reduced cleaved caspase-9 by 14% and 30% (P < 0.05) in reloaded plantaris and soleus muscles, respectively, compared with vehicle-treated animals. Although, HMB was unable to prevent unloading-induced atrophy, it attenuated the decrease in fiber area in fast and slow muscles after HS and R. HMB's ability to protect against muscle loss may be due in part to putative inhibition of myonuclear apoptosis via regulation of mitochondrial-associated caspase signaling.     

Cytoplasm-to-myonucleus ratios and succinate dehydrogenase activities in adult rat slow and fast muscle fibers

The relationship between myonuclear number, cellular size, succinate dehydrogenase activity, and myosin type was examined in single fiber segments (n = 54; 9 ± 3 mm long) mechanically dissected from soleus and plantaris muscles of adult rats. One end of each fiber segment was stained for DNA before quantitative photometric analysis of succinate dehydrogenase activity; the other end was double immunolabelled with fast and slow myosin heavy chain monoclonal antibodies. Mean ± S.D. cytoplasmic volume/myonucleus ratio was higher in fast and slow plantaris fibers (112 ± 69 vs. 34 ± 21 x 10 ³µm ³) than fast and slow soleus fibers (40 ± 20 vs. 30 ± 14 x 10 ³µm ³), respectively. Slow fibers always had small volumes/myonucleus, regardless of fiber diameter, succinate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 µm) fast soleus and plantaris fibers with high succinate dehydrogenase activity appeared to have low volumes/myonucleus while larger diameter (>70 µm) fast fibers with low succinate dehydrogenase activity always had large volume/myonucleus. Slow soleus fibers had significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116 ± 51 vs. 55 ± 22 and 44 ± 23), respectively. These data suggest that the myonuclear domain is more limited in slow than fast fibers and in the fibers with a high, compared to a low, oxidative metabolic capability.     

Repair of spinal cord transection and its effects on muscle mass and myosin heavy chain isoform phenotype.

A number of significant advances have been developed for treating spinal cord injury during the past two decades. The combination of peripheral nerve grafts and acidic fibroblast growth factor (hereafter referred to as PNG) has been shown to partially restore hindlimb function. However, very little is known about the effects of such treatments in restoring normal muscle phenotype. The primary goal of the current study was to test the hypothesis that PNG would completely or partially restore 1) muscle mass and muscle fiber cross-sectional area and 2) the slow myosin heavy chain phenotype of the soleus muscle. To test this hypothesis, we assigned female Sprague-Dawley rats to three groups: 1) sham control, 2) spinal cord transection (Tx), and 3) spinal cord transection plus PNG (Tx+PNG). Six months following spinal cord transection, the open-field test was performed to assess locomotor function, and then the soleus muscles were harvested and analyzed. SDS-PAGE for single muscle fiber was used to evaluate the myosin heavy chain (MHC) isoform expression pattern following the injury and treatment. Immunohistochemistry was used to identify serotonin (5-HT) fibers in the spinal cord. Compared with the Tx group, the Tx+PNG group showed 1) significantly improved Basso, Beattie, and Bresnahan scores (hindlimb locomotion test), 2) less muscle atrophy, 3) a higher percentage of slow type I fibers, and 4) 5-HT fibers distal to the lesion site. We conclude that the combined treatment of PNG is partially effective in restoring the muscle mass and slow phenotype of the soleus muscle in a T-8 spinal cord-transected rat model.     

Exercise training alleviates MCT1 and MCT4 reductions in heart and skeletal muscles of STZ-induced diabetic rats.

We compared the changes in monocarboxylate transporter 1 (MCT1) and 4 (MCT4) proteins in heart and skeletal muscles in sedentary control and streptozotocin (STZ)-induced diabetic rats (3 wk) and in trained (3 wk) control and STZ-induced diabetic animals. In nondiabetic animals, training increased MCT1 in the plantaris (+51%; P < 0.01) but not in the soleus (+9%) or the heart (+14%). MCT4 was increased in the plantaris (+48%; P < 0.01) but not in the soleus muscles of trained nondiabetic animals. In sedentary diabetic animals, MCT1 was reduced in the heart (-30%), and in the plantaris (-31%; P < 0.01) and soleus (-26%) muscles. MCT4 content was also reduced in sedentary diabetic animals in the plantaris (-52%; P < 0.01) and soleus (-25%) muscles. In contrast, in trained diabetic animals, MCT1 and MCT4 in heart and/or muscle were similar to those of sedentary, nondiabetic animals (P > 0.05) but were markedly greater than in the sedentary diabetic animals [MCT1: plantaris +63%, soleus +51%, heart +51% (P > 0.05); MCT4: plantaris +107%, soleus +17% (P > 0.05)]. These studies have shown that 1) with STZ-induced diabetes, MCT1 and MCT4 are reduced in skeletal muscle and/or the heart and 2) exercise training alleviated these diabetes-induced reductions.     

Resident stem cells are not required for exercise-induced fiber-type switching and angiogenesis but are necessary for activity-dependent muscle growth

Skeletal muscle undergoes active remodeling in response to endurance exercise training, and the underlying mechanisms of this remodeling remain to be defined fully. We have recently obtained evidence that voluntary running induces cell cycle gene expression and cell proliferation in mouse plantaris muscles that undergo fast-to-slow fiber-type switching and angiogenesis after long-term exercise. To ascertain the functional role of cell proliferation in skeletal muscle adaptation, we performed in vivo 5-bromo-2'-deoxyuridine (BrdU) pulse labeling (a single intraperitoneal injection), which demonstrated a phasic increase (5- to 10-fold) in BrdU-positive cells in plantaris muscle between days 3 and 14 during 4 wk of voluntary running. Daily intraperitoneal injection of BrdU for 4 wk labeled 2.0% and 15.4% of the nuclei in plantaris muscle in sedentary and trained mice, respectively, and revealed the myogenic and angiogenic fates of the majority of proliferative cells. Ablation of resident stem cell activity by X-ray irradiation did not prevent voluntary running-induced increases of type IIa myofibers and CD31-positive endothelial cells but completely blocked the increase in muscle mass. These findings suggest that resident stem cell proliferation is not required for exercise-induced type IIb-to-IIa fiber-type switching and angiogenesis but is required for activity-dependent muscle growth. The origin of the angiogenic cells in this physiological exercise model remains to be determined. endurance exercise; adaptation     

Angiotensin-converting enzyme inhibition attenuates myonuclear addition in overloaded slow-twitch skeletal muscle

Because optimal overload-induced skeletal muscle hypertrophy requires ANG II, we aimed to determine the effects of blocking ANG II production [via angiotensin-converting enzyme (ACE) inhibition] on potential mediators of hypertrophy in overloaded skeletal muscle, namely, myonuclear addition and fibroblast content. In a 2 x 2 design, adult (200-225 g) female Sprague-Dawley rats were placed into one of four groups (n = 8/group): 7-day skeletal muscle overload, sham operation, 7-day skeletal muscle overload with ACE inhibition, or sham operation with ACE inhibition. Functional overloads of the plantaris and soleus muscles were produced via bilateral surgical ablation of the synergistic gastrocnemius muscle, and ACE inhibition was accomplished by the addition of the ACE inhibitor enalapril maleate to the animals' daily drinking water (0.3 mg/ml). Myonuclear addition and extrasarcolemmal nuclear proliferation, as measured by in vivo 5-bromo-2'-deoxyuridine labeling, were significantly (P < or = 0.05) increased by overload in both the slow-twitch soleus and fast-twitch plantaris muscles. Furthermore, ACE inhibition attenuated these overload-induced increases in the soleus muscle but not in the plantaris muscle. However, the effect of ACE inhibition on soleus extrasarcolemmal nuclei was not likely due to differences in fibroblast content because overload elicited significant increases in vimentin-positive areas in soleus and plantaris muscles, and these areas were unaffected by ACE inhibition in either muscle. There was no effect of ACE inhibition on any measure in sham-operated muscles. Collectively, these data indicate that ANG II may mediate the satellite cell response to overload in slow-twitch soleus but not in fast-twitch plantaris muscles and that this effect may occur independently of changes in fibroblast content.     

Calcineurin differentially regulates maintenance and growth of phenotypically distinct muscles

Adequate muscle mass is critical for human health. The molecular pathways regulating maintenance and growth of adult skeletal muscle are little understood. Calcineurin (CN) is implicated as a key signaling molecule in hypertrophy. Whether CN is involved in all forms of muscle growth or in different muscles is unknown. Here, we examine the role of CN in regulating maintenance of muscle size and growth of atrophied muscle in the soleus (slow) and plantaris (fast). The CN inhibitor cyclosporin A (CsA) differentially affects muscle growth and maintenance depending on muscle phenotype. The plantaris is more severely affected by CsA than the soleus in both growth conditions. One-week vs. 2-wk CsA treatment suggests that both CN-dependent and CN-independent growth occur in the atrophied soleus, whereas plantaris growth appears to be totally CN dependent. Our results suggest that CN regulates multiple types of muscle growth, depending both on muscle phenotype and stage of myofiber growth. Differential expression of components of the CN pathway occurs and may contribute to the differences between muscles.     

Contractile function and low-intensity exercise effects of old dystrophic (mdx) mice

Oldmdx mice display a severe myopathyalmost identical to Duchenne's muscular dystrophy. This study examinedthe contractile properties of old mdxmuscles and investigated any effects of low-intensity exercise.Isometric contractile properties of the extensor digitorum longus (EDL)and soleus muscles were tested in adult (8-10 mo) and old (24 mo,split into sedentary and exercised groups)mdx mice. The EDL and soleus from oldmdx mice exhibited decreased absolutetwitch and tetanic forces, and the soleus exhibited a >50% decreasein relative forces (13.4 ± 0.4 vs. 6.0 ± 0.9 N/cm²) compared with adult mice.Old mdx muscles also showed longer contraction times and a higher percentage of type I fibers. Normal andmdx mice completed 10 wk of swimming,but mdx mice spent significantly lesstime swimming than normal animals (7.8 ± 0.4 vs. 15.8 ± 1.1 min, respectively). However, despite their severe dystrophy,mdx muscles responded positively tothe low-intensity exercise. Relative tetanic tensions were increased(~25% and ~45% for the EDL and soleus, respectively) after theswimming, although absolute forces were unaffected. Thus these resultsindicate that, even with a dystrophin-deficient myopathy,mdx muscles can still respond to low-intensity exercise. This study shows that the contractile functionof muscles of old mdx mice displaysmany similarities to that of human dystrophic patients and providesfurther evidence that the use of non-weight-bearing, low-intensityexercises, such as swimming, has no detrimental effect on dystrophicmuscle and could be a useful therapeutic aid for sufferers of musculardystrophy.

     

Effects of 4wk of hindlimb suspension on skeletal muscle mitochondrial respiration in rats

Yajid, Fatima, Jacques G. Mercier, Béatrice M. Mercier, Hervé Dubouchaud, and Christian Préfaut.Effects of 4 wk of hindlimb suspension on skeletal musclemitochondrial respiration in rats. J. Appl.Physiol. 84(2): 479-485, 1998.We investigated inrats the effect of 4 wk of hypodynamia on the respiration of mitochondria isolated from four distinct muscles [soleus,extensor digitorum longus, tibial anterior, and gastrocnemius(Gas)] and from subsarcolemmal (SS) and intermyofibrillar (IMF)regions of mixed hindlimb muscles that mainly contained the four citedmuscles. With pyruvate plus malate as respiratory substrate, 4 wk ofhindlimb suspension produced an 18% decrease in state3 respiration for IMF mitochondria compared with thosein the control group (P < 0.05). TheSS mitochondria state 3 were notsignificantly changed. Concerning the four single muscles, themitochondrial respiration was significantly decreased in the Gasmuscle, which showed a 59% decrease in state3 with pyruvate + malate(P < 0.05). The other musclespresented no significant decrease in respiratory rate in comparisonwith the control group. With succinate + rotenone, there was nosignificant difference in the respiratory rate compared with therespective control group, whatever the mitochondrial origin (SS, orIMF, or from single muscle). We conclude that 4 wk of hindlimbsuspension alters the respiration of IMF mitochondria in hindlimbskeletal muscles and seems to act negatively on complex I of theelectron-transport chain or prior sites. The muscle mitochondria mostaffected are those isolated from the Gas muscle.

     

Satellite cell regulation of muscle mass is altered at old age.

Muscle mass is decreased with advancing age, likely due to altered regulation of muscle fiber size. This study was designed to investigate cellular mechanisms contributing to this process. Analysis of male Fischer 344 X Brown Norway rats at 6, 20, and 32 mo of age demonstrated that, even though significant atrophy had occurred in soleus muscle by old age, myofiber nuclear number did not change, resulting in a decreased myonuclear domain. Also, the number of centrally located nuclei was significantly elevated in soleus muscle of 32-mo-old rats, correlating with an increase in gene expression of MyoD and myogenin. Whereas total 5'-bromo-2'deoxyuridine (BrdU)-positive nuclei were decreased at older ages, BrdU-positive myofiber nuclei were increased. These results suggest that, with age, loss of muscle mass is accompanied by increased myofiber nuclear density that involves fusion of proliferative satellite cells, resembling ongoing regeneration. Interestingly, centrally located myofiber nuclei were not BrdU labeled. Rats were subjected to hindlimb suspension (HS) for 7 or 14 days and intermittent reloading during HS for 1 h each day (IR) to investigate how aging affects the response of soleus muscle to disuse and an atrophy-reducing intervention. After 14 days of HS, soleus muscle size was decreased to a similar extent at all three ages. However, myofiber nuclear number and the total number of BrdU-positive nuclei decreased with HS only in the young rats. IR was associated with an attenuation of atrophy in soleus muscles of 6- and 20- but not 32-mo-old rats. Furthermore, IR was associated with an increase in BrdU-positive myofiber nuclei only in young rats. These data indicate that altered satellite cell function with age contributes to the impaired response of soleus muscle to an intervention that attenuates muscle atrophy in young animals during imposed disuse.     

Stretching prior to resistance training promotes adaptations on the postsynaptic region in different myofiber types

The morphology of the neuromuscular junction adapts according to changes in its pattern of use, especially at the postsynaptic region according to the myofibrillar type and physical exercise. This investigation revealed the morphological adaptations of the postsynaptic region after static stretching, resistance training, and their association in adult male Wistar rats. We processed the soleus and plantaris muscles for histochemical (muscle fibers) and postsynaptic region imaging techniques. We observed muscle hypertrophy in both groups submitted to resistance training, even though the cross-section area is larger when there is no previous static stretching. The soleus postsynaptic region revealed higher compactness and fragmentation index in the combined exercise. The resistance training promoted higher adaptations in the postsynaptic area of plantaris; moreover, the previous static stretching decreased this area. In conclusion, the neuromuscular system’s components responded according to the myofiber type even though it is the same physical exercise. Besides, static stretching (isolated or combined) plays a crucial role in neuromuscular adaptations.Key words: Neuromuscular junction, motor endplate, muscle hypertrophy, static stretching, resistance training