Eccentric contraction-induced injury of mouse soleus muscle: effect of varying [Ca2+]o

Nucleotide- and Protein-Dependent Functions of Actg1

Cytoplasmic β- and γ-actin proteins are 99% identical but support unique organismal functions. The cytoplasmic actin nucleotide sequences Actb and Actg1, respectively, are more divergent but still 89% similar. Actb-/- mice are embryonic lethal and Actb-/- cells fail to proliferate, but editing the Actb gene to express γ-actin (Actbc-g) resulted in none of the overt phenotypes of the knockout revealing protein-independent functions for Actb. To determine if Actg1 has a protein-independent function, we crossed Actbc-g and Actg1-/- mice to generate the bG/0 line, where the only cytoplasmic actin expressed is γ-actin from Actbc-g. The bG/0 mice were viable but showed a survival defect despite expressing γ-actin protein at levels no different from bG/gG with normal survival. A unique myopathy phenotype was also observed in bG/0 mice. We conclude that impaired survival and myopathy in bG/0 mice are due to loss of Actg1 nucleotide-dependent function(s). On the other hand, the bG/0 genotype rescued functions impaired by Actg1-/-, including cell proliferation and auditory function, suggesting a role for γ-actin protein in both fibroblasts and hearing. Together, these results identify nucleotide-dependent functions for Actg1 while implicating γ-actin protein in more cell-/tissue-specific functions.

Absence of the Z-disc protein α-actinin-3 impairs the mechanical stability of Actn3KO mouse fast-twitch muscle fibres without altering their contractile properties or twitch kinetics

BACKGROUND

A common polymorphism (R577X) in the ACTN3 gene results in the complete absence of the Z-disc protein α-actinin-3 from fast-twitch muscle fibres in ~ 16% of the world's population. This single gene polymorphism has been subject to strong positive selection pressure during recent human evolution. Previously, using an Actn3KO mouse model, we have shown in fast-twitch muscles, eccentric contractions at L₀ + 20% stretch did not cause eccentric damage. In contrast, L₀ + 30% stretch produced a significant ~ 40% deficit in maximum force; here, we use isolated single fast-twitch skeletal muscle fibres from the Actn3KO mouse to investigate the mechanism underlying this.

METHODS

Single fast-twitch fibres are separated from the intact muscle by a collagenase digest procedure. We use label-free second harmonic generation (SHG) imaging, ultra-fast video microscopy and skinned fibre measurements from our MyoRobot automated biomechatronics system to study the morphology, visco-elasticity, force production and mechanical strength of single fibres from the Actn3KO mouse. Data are presented as means ± SD and tested for significance using ANOVA.

RESULTS

We show that the absence of α-actinin-3 does not affect the visco-elastic properties or myofibrillar force production. Eccentric contractions demonstrated that chemically skinned Actn3KO fibres are mechanically weaker being prone to breakage when eccentrically stretched. Furthermore, SHG images reveal disruptions in the myofibrillar alignment of Actn3KO fast-twitch fibres with an increase in Y-shaped myofibrillar branching.

CONCLUSIONS

The absence of α-actinin-3 from the Z-disc in fast-twitch fibres disrupts the organisation of the myofibrillar proteins, leading to structural weakness. This provides a mechanistic explanation for our earlier findings that in vitro intact Actn3KO fast-twitch muscles are significantly damaged by L₀ + 30%, but not L₀ + 20%, eccentric contraction strains. Our study also provides a possible mechanistic explanation as to why α-actinin-3-deficient humans have been reported to have a faster decline in muscle function with increasing age, that is, as sarcopenia reduces muscle mass and force output, the eccentric stress on the remaining functional α-actinin-3 deficient fibres will be increased, resulting in fibre breakages.

pQCT- and Ultrasound-based Muscle and Fat Estimate Errors after Resistance Exercise

PURPOSE

Resistance exercise increases blood flow, induces osmotic and hydrostatic fluid shifts during and immediately after exercise, and may trigger inflammatory responses for several days in the working muscle. The resultant muscle swelling can subsequently affect muscle size and quality assessments. However, the effects of muscle swelling on x-ray attenuation of adipose estimate errors are unknown.

METHODS

Peripheral quantitative computed tomography (pQCT) and ultrasonography were used to assess muscle and adipose tissue properties of both upper arms before, 24, 48, and 72 h after unilateral resistance exercise. Recreationally active participants (n = 17) completed the exercise while their contralateral limb served as a control.

RESULTS

Resistance exercise resulted in a significant increase in pQCT-derived muscle cross-sectional area (includes intermuscular adipose tissue [IMAT] area), muscle area (excludes IMAT area) and IMAT area, and ultrasound-derived muscle thickness at 24, 48, and 72 h. A significant decrease in pQCT-derived muscle density was also detected as well as an increase in ultrasound-derived echo intensity at 48 and 72 h. The changes in muscle area, IMAT area, and muscle density were significantly correlated with changes in echo intensity, whereas the changes in muscle cross-sectional area and IMAT area were significantly correlated with changes in muscle thickness.

CONCLUSION

Unaccustomed resistance exercise can cause errors in pQCT- and ultrasound-based muscle and adipose estimates for at least 72 h. These errors are the result of muscle swelling likely caused by muscle blood flow and inflammation-dependent fluid shifts in muscle. These findings may have implications for measurements in other inflammatory conditions.

Neopterin/7,8-dihydroneopterin is elevated in Duchenne muscular dystrophy patients and protects mdx skeletal muscle function

NEW FINDINGS

What is the central question of this study? We examined whether the macrophage-synthesized antioxidant 7,8-dihydroneopterin was elevated in Duchenne muscular dystrophy (DMD) patients. We then examined whether 7,8-dihydroneopterin could protect dystrophic skeletal mouse muscle from eccentric contraction-induced force loss and improve recovery. What is the main finding and its importance? Urinary neopterin/creatinine and 7,8-dihydroneopterin/creatinine were elevated in DMD patients. 7,8-Dihydroneopterin attenuated eccentric contraction-induced force loss of dystrophic skeletal mouse muscle and accelerated recovery of force. These results suggest that eccentric contraction-induced force loss is mediated, in part, by an oxidative component and provides a potential protective role for 7,8-dihydroneopterin in DMD.

ABSTRACT

Macrophage infiltration is a hallmark of dystrophin-deficient muscle. We tested the hypothesis that Duchenne muscular dystrophy (DMD) patients would have elevated levels of the macrophage-synthesized pterins, neopterin and 7,8-dihydroneopterin, compared with unaffected age-matched control subjects. Urinary neopterin/creatinine and 7,8-dihydroneopterin/creatinine were elevated in DMD patients, and 7,8-dihydroneopterin/creatinine was associated with patient age and ambulation. Urinary 7,8-dihydroneopterin corrected for specific gravity was also elevated in DMD patients. Given that 7,8-dihydroneopterin is an antioxidant, we then identified a potential role for 7,8-dihydroneopterin in disease pathology. We assessed whether 7,8-dihydroneopterin could: (i) protect against isometric force loss in wild-type skeletal muscle exposed to various pro-oxidants; and (ii) protect wild-type and mdx muscle from eccentric contraction-induced force loss, which has an oxidative component. Force loss was elicited in isolated extensor digitorum longus (EDL) muscles by 10 eccentric contractions, and recovery of force after the contractions was measured in the presence of exogenous 7,8-dihydroneopterin. 7,8-Dihydroneopterin attenuated isometric force loss by wild-type EDL muscles when challenged by H₂ O₂ and HOCl, but exacerbated force loss when challenged by SIN-1 (NO^• , O₂^• , ONOO^- ). 7,8-Dihydroneopterin attenuated eccentric contraction-induced force loss in mdx muscle. Isometric force production by EDL muscles of mdx mice also recovered to a greater degree after eccentric contractions in the presence of 7,8-dihydroneopterin. The results corroborate macrophage activation in DMD patients, provide a potential protective role for 7,8-dihydroneopterin in the susceptibility of dystrophic muscle to eccentric contractions and indicate that oxidative stress contributes to eccentric contraction-induced force loss in mdx skeletal muscle.

Variable rescue of microtubule and physiological phenotypes in mdx muscle expressing different miniaturized dystrophins

Delivery of miniaturized dystrophin genes via adeno-associated viral vectors is one leading approach in development to treat Duchenne muscular dystrophy. Here we directly compared the functionality of five mini- and micro-dystrophins via skeletal muscle-specific transgenic expression in dystrophin-deficient mdx mice. We evaluated their ability to rescue defects in the microtubule network, passive stiffness and contractility of skeletal muscle. Transgenic mdx mice expressing the short dystrophin isoform Dp116 served as a negative control. All mini- and micro-dystrophins restored elevated detyrosinated α-tubulin and microtubule density of mdx muscle to values not different from C57BL/10, however, only mini-dystrophins restored the transverse component of the microtubule lattice back to C57BL/10. Passive stiffness values in mdx muscles expressing mini- or micro-dystrophins were not different from C57BL/10. While all mini- and micro-dystrophins conferred significant protection from eccentric contraction-induced force loss in vivo and ex vivo compared to mdx, removal of repeats two and three resulted in less protection from force drop caused by eccentric contraction ex vivo. Our data reveal subtle yet significant differences in the relative functionalities for different therapeutic constructs of miniaturized dystrophin in terms of protection from ex vivo eccentric contraction-induced force loss and restoration of an organized microtubule lattice.

Isometric resistance training increases strength and alters histopathology of dystrophin-deficient mouse skeletal muscle

Mutation to the dystrophin gene causes skeletal muscle weakness in patients with Duchenne muscular dystrophy (DMD) or Becker muscular dystrophy (BMD). Deliberation continues regarding implications of prescribing exercise for these patients. The purpose of this study was to determine whether isometric resistance exercise (~10 tetanic contractions/session) improves skeletal muscle strength and histopathology in the mdx mouse model of DMD. Three isometric training sessions increased in vivo isometric torque (22%) and contractility rates (54%) of anterior crural muscles of mdx mice. Mice expressing a BMD-causing missense mutated dystrophin on the mdx background showed comparable increases in torque (22%), while wild-type mice showed less change (11%). Increases in muscle function occurred within 1 h and peaked 3 days posttraining; however, the adaptation was lost after 7 days unless retrained. Six isometric training sessions over 4 wk caused increased isometric torque (28%) and contractility rates (22-28%), reduced fibrosis, as well as greater uniformity of fiber cross-sectional areas, fewer embryonic myosin heavy-chain-positive fibers, and more satellite cells in tibialis anterior muscle compared with the contralateral untrained muscle. Ex vivo functional analysis of isolated extensor digitorum longus (EDL) muscle from the trained hindlimb revealed greater absolute isometric force, lower passive stiffness, and a lower susceptibility to eccentric contraction-induced force loss compared with untrained EDL muscle. Overall, these data support the concept that exercise training in the form of isometric tetanic contractions can improve contractile function of dystrophin-deficient muscle, indicating a potential role for enhancing muscle strength in patients with DMD and BMD. NEW & NOTEWORTHY We focused on adaptive responses of dystrophin-deficient mouse skeletal muscle to isometric contraction training and report that in the absence of dystrophin (or in the presence of a mutated dystrophin), strength and muscle histopathology are improved. Results suggest that the strength gains are associated with fiber hypertrophy, reduced fibrosis, increased number of satellite cells, and blunted eccentric contraction-induced force loss in vitro. Importantly, there was no indication that the isometric exercise training was deleterious to dystrophin-deficient muscle.

Xanthine oxidase is hyper-active in Duchenne muscular dystrophy

Generation of superoxide by xanthine oxidase can be stimulated under ischemic and aberrant calcium homeostasis. Because patients and mice with Duchenne muscular dystrophy (DMD) suffer from ischemia and excessive calcium influx, we tested the hypothesis that xanthine oxidase activity is elevated and contributes to disease pathology. Xanthine oxidase activity was measured by urinary isoxanthopterin in DMD patients at rest and in response to exercise. Urinary isoxanthopterin/creatinine was elevated compared to age-matched controls and Becker muscular dystrophy (BMD) patients. Concentrations were also increased after a six minute walk test in ambulatory patients. We also measured urinary isoxanthopterin in wildtype mice and a number of dystrophic mouse models; the DMD mouse model (mdx), mdx mice overexpressing a variety of transgenic miniaturized and chimeric skeletal muscle-specific dystrophins and utrophin and the β-sarcoglycan deficient (Scgb^-/-) mouse which represents type 2E human limb-girdle muscular dystrophy. Mdx and Scgb^-/-mice had greater urinary isoxanthopterin/creatinine than wildtype mice while mdx mice expressing dystrophin or utrophin linking the extracellular matrix to the actin cytoskeleton were not different than wildtype. We also measured higher levels of urinary ortho-tyrosine in humans and mice deficient for dystrophin to confirm elevated oxidative stress. Surprisingly, mdx had lower xanthine oxidase protein levels and higher mRNA in gastrocnemius muscle compared to wildtype mice, however, the enzymatic activity of skeletal muscle xanthine oxidase was elevated above wildtype and a transgenic rescued mdx mouse (Dys^ΔMTB-mdx). Downhill treadmill running also caused significant increases in mdx urinary isoxanthopterin that was prevented with the xanthine oxidase inhibitor allopurinol. Similarly, in vitro eccentric contraction-induced force drop of mdx muscle was attenuated by the allopurinol metabolite, oxypurinol. Together, our data suggests hyper-activity of xanthine oxidase in DMD, identifies xanthine oxidase activity as a contributing factor in eccentric contraction-induced force drop of dystrophin-deficient skeletal muscle and highlights the potential of isoxanthopterin as a noninvasive biomarker in DMD.

Loss of peroxiredoxin-2 exacerbates eccentric contraction-induced force loss in dystrophin-deficient muscle

Force loss in skeletal muscle exposed to eccentric contraction is often attributed to injury. We show that EDL muscles from dystrophin-deficient mdx mice recover 65% of lost force within 120 min of eccentric contraction and exhibit minimal force loss when the interval between contractions is increased from 3 to 30 min. A proteomic screen of mdx muscle identified an 80% reduction in the antioxidant peroxiredoxin-2, likely due to proteolytic degradation following hyperoxidation by NADPH Oxidase 2. Eccentric contraction-induced force loss in mdx muscle was exacerbated by peroxiredoxin-2 ablation, and improved by peroxiredoxin-2 overexpression or myoglobin knockout. Finally, overexpression of γcyto- or βcyto-actin protects mdx muscle from eccentric contraction-induced force loss by blocking NADPH Oxidase 2 through a mechanism dependent on cysteine 272 unique to cytoplasmic actins. Our data suggest that eccentric contraction-induced force loss may function as an adaptive circuit breaker that protects mdx muscle from injurious contractions.

Role of calpain in eccentric contraction-induced proteolysis of Ca2+-regulatory proteins and force depression in rat fast-twitch skeletal muscle

The aim of this study was to examine the in vivo effects of eccentric contraction (ECC) on calpain-dependent proteolysis of Ca2+-regulatory proteins and force production in fast-twitch skeletal muscles. Rat extensor digitorum longus muscles were exposed to 200 repeated ECC in situ and excised immediately [recovery 0 (REC0)] or 3 days [recovery 3 (REC3)] after cessation of ECC. Calpain inhibitor (CI)-treated rats were intraperitoneally injected with MDL-28170 before ECC and during REC3. Tetanic force was markedly reduced at REC0 and remained reduced at REC3. CI treatment ameliorated the ECC-induced force decline but only at REC3. No evidence was found for proteolysis of dihydropyridine receptor (DHPR), junctophilin (JP)1, JP2, ryanodine receptor (RyR), sarcoplasmic reticulum Ca2+-ATPase (SERCA)1a, or junctional face protein-45 at REC0. At REC3, ECC resulted in decreases in DHPR, JP1, JP2, RyR, and SERCA1a. CI treatment prevented the decreases in DHPR, JP1, and JP2, whereas it had little effect on RyR and SERCA1a. These findings suggest that DHPR, JP1, and JP2, but not RyR and SERCA1a, undergo calpain-dependent proteolysis in in vivo muscles subjected to ECC and that impaired function of DHPR and/or JP might cause prolonged force deficits with ECC.

NEW & NOTEWORTHY Calpain-dependent proteolysis is one of the contributing factors to muscle damage that occurs with eccentric contraction (ECC). It is unclear, however, whether calpains account for proteolysis of Ca2+-regulatory proteins in in vivo muscles subjected to ECC. Here, we provide evidence that dihydropyridine receptor and junctophilin, but not ryanodine receptor and sarcoplasmic reticulum Ca2+-ATPase, undergo calpain-dependent proteolysis.

Eccentric contractions disrupt FKBP12 content in mouse skeletal muscle

Strength deficits associated with eccentric contraction‐induced muscle injury stem, in part, from impaired voltage‐gated sarcoplasmic reticulum (SR) Ca²⁺ release. FKBP12 is a 12‐kD immunophilin known to bind to the SR Ca²⁺ release channel (ryanodine receptor, RyR1) and plays an important role in excitation‐contraction coupling. To assess the effects of eccentric contractions on FKBP12 content, we measured anterior crural muscle (tibialis anterior [TA], extensor digitorum longus [EDL], extensor hallucis longus muscles) strength and FKBP12 content in pellet and supernatant fractions after centrifugation via immunoblotting from mice before and after a single bout of either 150 eccentric or concentric contractions. There were no changes in peak isometric torque or FKBP12 content in TA muscles after concentric contractions. However, FKBP12 content was reduced in the pelleted fraction immediately after eccentric contractions, and increased in the soluble protein fraction 3 day after injury induction. FKBP12 content was correlated (P = 0.025; R²= 0.38) to strength deficits immediately after injury induction. In summary, eccentric contraction‐induced muscle injury is associated with significant alterations in FKBP12 content after injury, and is correlated with changes in peak isometric torque.

Eccentric contraction‐induced muscle injury is associated with immediate and prolonged strength deficits that stem in part from impaired sarcoplasmic reticulum (SR) calcium release. The content of FKBP12, a 12‐kD immunophilin known to bind to the SR calcium release channel and influence SR calcium release, is reduced in mouse skeletal muscle immediately after injury induction and is significantly associated with strength deficits.

Mechanisms of muscle injury, repair, and regeneration

Skeletal muscle continuously adapts to changes in its mechanical environment through modifications in gene expression and protein stability that affect its physiological function and mass. However, mechanical stresses commonly exceed the parameters that induce adaptations, producing instead acute injury. Furthermore, the relatively superficial location of many muscles in the body leaves them further vulnerable to acute injuries by exposure to extreme temperatures, contusions, lacerations or toxins. In this article, the molecular, cellular, and mechanical factors that underlie muscle injury and the capacity of muscle to repair and regenerate are presented. Evidence shows that muscle injuries that are caused by eccentric contractions result from direct mechanical damage to myofibrils. However, muscle pathology following other acute injuries is largely attributable to damage to the muscle cell membrane. Many feaures in the injury-repair-regeneration cascade relate to the unregulated influx of calcium through membrane lesions, including: (i) activation of proteases and hydrolases that contribute muscle damage, (ii) activation of enzymes that drive the production of mitogens and motogens for muscle and immune cells involved in injury and repair, and (iii) enabling protein-protein interactions that promote membrane repair. Evidence is also presented to show that the myogenic program that is activated by acute muscle injury and the inflammatory process that follows are highly coordinated, with myeloid cells playing a central role in modulating repair and regeneration. The early-invading, proinflammatory M1 macrophages remove debris caused by injury and express Th1 cytokines that play key roles in regulating the proliferation, migration, and differentiation of satellite cells. The subsequent invasion by anti-inflammatory, M2 macrophages promotes tissue repair and attenuates inflammation. Although this system provides an effective mechanism for muscle repair and regeneration following acute injury, it is dysregulated in chronic injuries. In this article, the process of muscle injury, repair and regeneration that occurs in muscular dystrophy is used as an example of chronic muscle injury, to highlight similarities and differences between the injury and repair processes that occur in acutely and chronically injured muscle.

Immediate force loss after eccentric contractions is increased with L-NAME administration, a nitric oxide synthase inhibitor

INTRODUCTION

Nitric oxide (NO) signaling regulates many biological processes in skeletal muscle, wherein aberrant signaling contributes to myopathic conditions (e.g., Duchenne muscular dystrophy). NO has been shown to play a role in muscle regeneration after injury. However, less is known about its role during injury. In this study we aimed to determine whether NO synthase (NOS) inhibition exacerbates functional deficits immediately after the performance of eccentric contractions.

METHODS

Wild-type mouse extensor digitorum longus (EDL) muscles underwent in vitro functional testing in the presence or absence of a non-specific NOS inhibitor (L-NAME, 10 mM) before and after performance of 10 eccentric contractions.

RESULTS

After eccentric contractions, P(o) was reduced by ∽25% for muscle in regular physiological solution but by ∽50% with the addition of L-NAME (P = 0.009).

CONCLUSIONS

Non-specific blockade of NOS exacerbates functional deficits immediately after eccentric contractions, suggesting that NO signaling protects skeletal muscle from excessive injury in healthy muscle.

The spectral changes in EMG during a second bout eccentric contraction could be due to adaptation in muscle fibres themselves: a simulation study

The mechanism of marked reduction in damage symptoms after repeated bout of similar eccentric contractions is still unknown. The neuronal adaptation leading to reduction of muscle fibre propagation velocity (MFPV) due to increased activation of slow-twitch motor units (MUs), decrease in activation of fast-twitch MUs, and/or increase in MU synchronization was suggested as a cause for lower EMG frequency characteristics. However, the repeated bout effect could occur also after electrically stimulated exercise. Prolonged elevation of cytoplasmic Ca(2+) due to the increased membrane permeability after eccentric contractions was reported. Elevated Ca(2+) induced peripheral changes that included alteration of intracellular action potential and MFPV reduction. We simulated and compared changes in EMG frequency characteristics related to effects of central nervous system (CNS) or to peripheral changes. The simulations were performed for different electrode arrangements and positions. The results showed that the peripheral effects could be similar or even stronger than the effects related to CNS. We hypothesised that the repeated bout effect was a consequence of the adaptation in muscle fibres necessary for avoiding Ca(2+)-induced protein and lipid degradation due to Ca(2+) overload resulting from the increased membrane permeability after eccentric contraction. The possibilities for noninvasive testing of this hypothesis were discussed.

TAT-μUtrophin mitigates the pathophysiology of dystrophin and utrophin double-knockout mice

Previously, we demonstrated functional substitution of dystrophin by TAT-μUtrophin (TAT-μUtr) in dystrophin-deficient mdx mice. Herein, we addressed whether TAT-μUtr could improve the phenotype of dystrophin and utrophin double-knockout (mdx:utr(-/-)) mice. Specifically, we quantitatively compared survival and quality of life assessments in mdx:utr(-/-) mice receiving TAT-μUtr protein administration against placebo-treated mdx:utr(-/-) mice (PBS). Additionally, skeletal muscles from TAT-μUtr and PBS mice were tested in vivo and ex vivo for strength and susceptibility to eccentric contraction-induced injury. We found the TAT-μUtr treatment extended life span 45% compared with mice administered PBS. This was attributed to significantly increased food consumption (3.1 vs. 1.8 g/24 h) due to improved ability to search for food as daily cage activities were greater in TAT-μUtr mice (e.g., 364 vs. 201 m ambulation/24 h). The extensor digitorum longus muscles of TAT-μUtr-treated double-knockout mice also displayed increased force-generating capacity ex vivo (8.3 vs. 6.4 N/cm(2)) and decreased susceptibility to injury ex vivo and in vivo. These data indicate that the functional benefits of TAT-μUtr replacement treatment extend to the mdx:utr(-/-) double-knockout mouse and support its development as a therapy to mitigate muscle weakness in patients with Duchenne muscular dystrophy.

Junctophilin damage contributes to early strength deficits and EC coupling failure after eccentric contractions

Junctophilins (JP1 and JP2) are expressed in skeletal muscle and are the primary proteins involved in transverse (T)-tubule and sarcoplasmic reticulum (SR) membrane apposition. During the performance of eccentric contractions, the apposition of T-tubule and SR membranes may be disrupted, resulting in excitation-contraction (EC) coupling failure and thus reduced force-producing capacity. In this study, we made three primary observations: 1) through the first 3 days after the performance of 50 eccentric contractions in vivo by the left hindlimb anterior crural muscles of female mice, both JP1 and JP2 were significantly reduced by approximately 50% and 35%, respectively, while no reductions were observed after the performance of nonfatiguing concentric contractions; 2) following the performance of a repeated bout of 50 eccentric contractions in vivo, only JP1 was immediately reduced ( approximately 30%) but recovered by 3-day postinjury in tandem with the recovery of strength and EC coupling; and 3) following the performance of 10 eccentric contractions at either 15 degrees or 35 degrees C by isolated mouse extensor digitorum longus (EDL) muscle, isometric force, EC coupling, and JP1 and JP2 were only reduced after the eccentric contractions performed at 35 degrees C. Regression analysis of JP1 and JP2 content in tibialis anterior and EDL muscles from each set of experiments indicated that JP damage is significantly associated with early (0-3 days) strength deficits after performance of eccentric contractions (R = 0.49; P < 0.001). As a whole, the results of this study indicate that JP damage plays a role in early force deficits due to EC coupling failure following the performance of eccentric contractions.

Effects of prednisolone on skeletal muscle contractility in mdx mice

Current treatment for Duchenne muscular dystrophy (DMD) is chronic administration of the glucocorticoid prednisolone. Prednisolone improves muscle strength in boys with DMD, but the mechanism is unknown. The purpose of this study was to determine how prednisolone improves muscle strength by examining muscle contractility in dystrophic mice over time and in conjunction with eccentric injury. Mdx mice began receiving prednisolone (n = 23) or placebo (n = 16) at 5 weeks of age. Eight weeks of prednisolone increased specific force of the extensor digitorum longus muscle 26%, but other parameters of contractility were not affected. Prednisolone also improved the histological appearance of muscle by decreasing the number of centrally nucleated fibers. Prednisolone treatment did not affect force loss during eccentric contractions or recovery of force following injury. These data are of clinical relevance, because the increase in muscle strength in boys with DMD taking prednisolone does not appear to occur via the same mechanism in dystrophic mice.

Estradiol replacement reverses ovariectomy-induced muscle contractile and myosin dysfunction in mature female mice

Skeletal muscle contractility and myosin function decline following ovariectomy in mature female mice. In the present study we tested the hypothesis that estradiol replacement can reverse those declines. Four-month-old female C57BL/6 mice (n = 69) were ovariectomized (OVX) or sham operated. Some mice were treated immediately with placebo or 17beta-estradiol (OVX + E(2)) while other mice were treated 30 days postsurgery. Thirty or sixty days postsurgery, soleus muscles were assessed in vitro for contractile function and susceptibility to eccentric contraction-induced injury. Myosin structural dynamics was analyzed in extensor digitorum longus (EDL) muscles by electron paramagnetic resonance spectroscopy. Maximal isometric tetanic force was affected by estradiol status (P < 0.001) being approximately 10% less in soleus muscles from OVX compared with sham-operated mice [168 mN (SD 16.7) vs. 180 mN (SD 14.4)] and was restored in OVX + E(2) mice [187 mN (SD 17.6)]. The fraction of strong-binding myosin during contraction was also affected (P = 0.045) and was approximately 15% lower in EDL muscles from OVX compared with OVX + E(2) mice [0.263 (SD 0.034) vs. 0.311 (SD 0.022)]. Plasma estradiol levels were correlated with maximal isometric tetanic force (r = 0.458; P < 0.001) and active stiffness (r = 0.329; P = 0.044), indicating that circulating estradiol influenced muscle and myosin function. Estradiol was not effective in protecting muscle against an acute eccentric contraction-induced injury (P >or= 0.401) but did restore ovariectomy-induced increases in muscle wet mass caused by fluid accumulation. Collectively, estradiol had a beneficial effect on female mouse skeletal muscle.

Peak power of muscles injured by lengthening contractions

Excessive or extreme lengthening contractions have a well-characterized depressive effect on skeletal muscle isometric force. In addition to producing force, active muscles must often shorten in order to meet the power requirements of locomotion and other physical activities. However, the impact of lengthening contractions on muscle power is poorly understood. We evaluated the effect of 20 isometric contractions or 20 lengthening contractions (20% strain at 1.5 fiber lengths/s) on the force-velocity-power relationships of mouse soleus muscles in vitro at 35 degrees C. Pre- and posttreatment data were obtained as the muscles shortened through their optimal length (Lo). The isometric treatment did not alter Lo, the curvature of the force-velocity relationship (a/Po), or soleus maximal shortening velocity (Vmax), whereas peak force (Po) displayed a slow, time-dependent decline of 10% across the experiments. Following the lengthening treatment, Lo increased by 6%, a/Po increased by 22%, and Vmax and Po fell by 24% and 26%, respectively. Under optimal conditions for producing power, muscles damaged by lengthening contractions attained 22% less force and shortened 20% more slowly than before damage. Consequently, soleus peak power fell 37% after lengthening, a 2.5-fold greater decline than noted for the isometric treatment. Under the conditions studied here, the excessive power loss following lengthening contractions was due to force and velocity deficits of approximately equal relative magnitude. Because power represents the ability of the muscle to perform work, reductions in both force and shortening velocity should be considered when evaluating and treating lengthening-induced skeletal muscle injuries.

Impact of stretch-shortening cycle rest interval on in vivo muscle performance

INTRODUCTION

Overuse and overtraining models have implicated both metabolic and mechanical disturbances as contributors to muscle damage and performance decrement but have produced equivocal results. The purpose of the present study was to investigate the impact of rest interval between sets of stretch-shortening cycles (SSC) on static and dynamic muscle performance

METHODS

Animals were randomly assigned to groups (N = 8 per group) of 10-s, 1-min, or 5-min rest between sets of isometric contractions (10-s, 1-min, or 5-min CON), or SSC (10-s, 1-min, or 5-min INJ). The dorsiflexor muscles were exposed in vivo to either seven sets of 10 SSC (500 degrees . s) or seven sets of isometric contractions. Performance was characterized by isometric exertions and positive, negative, and net work, at pretest, during the sets of SSC, and 48 h postexposure

RESULTS

The isometric force at 48 h after the 10-s and 5-min INJ groups were statistically different from the 1-min group (P < 0.05), whereas there was no difference in the CON groups. Negative work of the INJ groups were statistically lower at 48 h than pretest values (P < 0.05), whereas there was no change in positive work. Of the real-time parameters, there was a difference in minimum force and positive work (P < 0.05) with treatment with the 10-s INJ group being most affected.

CONCLUSION

SSC conducted at shorter work-rest cycles resulted in a more profound isometric force decrement 48 h postexposure, and in real-time changes in isometric prestretch force and positive work. These results indicate that short rest intervals between athletic or vocational tasks of heightened physical exertion (i.e., high intensity) may adversely affect performance and increase injury susceptibility.

Excitation-induced Ca2+ influx and muscle damage in the rat: loss of membrane integrity and impaired force recovery

Prolonged or unaccustomed exercise leads to loss of contractility and muscle cell damage. The possible role of an increased uptake of Ca(2+) in this was explored by examining how graded fatiguing stimulation, leading to a graded uptake of Ca(2+), results in progressive loss of force, impairment of force recovery, and loss of cellular integrity. The latter is indicated by increased [(14)C]sucrose space and lactic acid dehydrogenase (LDH) release. Isolated rat extensor digitorum longus (EDL) muscles were allowed to contract isometrically using a fatiguing protocol with intermittent stimulation at 40 Hz. Force declined rapidly, reaching 11% of the initial level after 10 min and stayed low for up to 60 min. During the initial phase (2 min) of stimulation (45)Ca uptake showed a 10-fold increase, followed by a 4- to 5-fold increase during the remaining period of stimulation. As the duration of stimulation increased, the muscles subsequently regained gradually less of their initial force. Following 30 or 60 min of stimulation, resting (45)Ca uptake, [(14)C]sucrose space, and LDH release were increased 4- to 7-fold, 1.4- to 1.7-fold and 3- to 9-fold, respectively (P < 0.001). The contents of Ca(2+) and Na(+) were also increased (P < 0.01), a further indication of loss of cellular integrity. When fatigued at low [Ca(2+)](o) (0.65 mm), force recovery was on average twofold higher than that of muscles fatigued at high [Ca(2+)](o) (2.54 mm). Muscles showing the best force recovery also had a 41% lower total cellular Ca(2+) content (P < 0.01). In conclusion, fatiguing stimulation leads to a progressive functional impairment and loss of plasma membrane integrity which seem to be related to an excitation-induced uptake of Ca(2+). Mechanical strain on the muscle fibres does not seem a likely mechanism since very little force was developed beyond 10 min of stimulation.

MRI evaluation of topical heat and static stretching as therapeutic modalities for the treatment of eccentric exercise-induced muscle damage

The aim of this study was to monitor the effects of topical heat and/or static stretch treatments on the recovery of muscle damage by eccentric exercise. For this purpose, 32 untrained male subjects performed intense eccentric knee extension exercise, followed by 2 weeks of treatment (heat, stretch, heat plus stretch) or no treatment (control, n=8/group). Isometric strength testing, pain ratings, and multi-echo magnetic resonance imaging of the thigh were performed before and at 2, 3, 4, 8, and 15 days following the exercise. Increased T2 relaxation time, muscle swelling, pain ratings, and strength loss confirmed significant muscle damage during the post-exercise period. Pain ratings and muscle volume recovered to baseline by 15 days, although muscle strength remained lower [77 (4) vs. 95 (3) kg pre-exercise, mean (SE)] and T2 values higher [32.2 (0.8) vs. 28.6 (0.2) ms pre-exercise]. Our results indicate that heat and/or static stretching does not consistently reduce soreness, swelling or muscle damage. The practical implication of our findings is that clinicians should be aware that prescribing heat and/or static stretching following intense eccentric or unaccustomed exercise will not enhance the recovery of damaged muscles.

Contraction-induced muscle damage in humans following calcium channel blocker administration

Following contraction-induced damage of skeletal muscle there is a loss of calcium homeostasis. Attenuating the damage-induced rise in myocellular calcium concentration may reduce proteolytic activation and attenuate other indices of damage; calcium channel blockers have been shown to be effective in this regard. The effect of administration of a calcium channel blocker (CCB), amlodipine, on indices of muscle damage following a unilateral 'damage protocol', during which subjects performed 300 maximal isokinetic (0.52 rad s(-1)) eccentric contractions with the knee extensors was investigated. The design was a randomized, double-blind crossover. On one occasion, prior to the damage protocol, subjects consumed CCB for 7 days prior to and for 7 days following the damage protocol. Biopsies were taken from the vastus lateralis prior to (baseline) and following the damage protocol at 4 h and 24 h post-damage. Isometric peak knee extensor torque was reduced (P < 0.05) immediately post-, 24 h post- and 48 h post-damage protocol compared to pre-exercise values with no effect of treatment. Desmin disruption was attenuated (P < 0.05) with CCB versus placebo at 4 h post-damage. Z-band streaming was significantly (P < 0.05) elevated compared to baseline at both times post-damage, but was lower with CCB at 4 h (P < 0.05). Damage resulted in increased inflammatory cell (macrophage) infiltration into skeletal muscle at both 4 h and 24 h post-damage, with no effect of CCB. Neutrophil number was elevated by the damage protocol, but was higher at 24 h post-damage in the CCB condition (P < 0.05). Creatine kinase (CK) activity was higher (P < 0.05) at 24 h and 48 h following the damage protocol compared to baseline, with no effect of treatment. In conclusion, the reduction in desmin disruption and Z-band streaming indicates that CCB attenuated, or delayed, the contraction-induced damage to sarcomeric proteins.

Effect of eccentric contraction-induced injury on force and intracellular pH in rat skeletal muscles

The effect of eccentric contraction on force generation and intracellular pH (pH(i)) regulation was investigated in rat soleus muscle. Eccentric muscle damage was induced by stretching muscle bundles by 30% of the optimal length for a series of 10 tetani. After eccentric contractions, there was reduction in force at all stimulation frequencies and a greater reduction in relative force at low-stimulus frequencies. There was also a shift of optimal length to longer lengths. pH(i) was measured with a pH-sensitive probe, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein AM. pH(i) regulation was studied by inducing an acute acid load with the removal of 20-40 mM ammonium chloride, and the rate of pH(i) recovery was monitored. The acid extrusion rate was obtained by multiplying the rate of pH(i) recovery by the buffering power. The resting pH(i) after eccentric contractions was more acidic, and the rate of recovery from acid load post-eccentric contractions was slower than that from postisometric controls. This is further supported by the slower acid extrusion rate. Amiloride slowed the recovery from an acid load in control experiments. Because the Na(+)/H(+) exchanger is the dominant mechanism for the recovery of pH(i), this suggests that the impairment in the ability of the muscle to regulate pH(i) after eccentric contractions is caused by decreased activity of the Na(+)/H(+) exchanger.

Excitation-induced Ca(2+) influx in rat soleus and EDL muscle: mechanisms and effects on cellular integrity

In rat skeletal muscle, electrical stimulation increases Ca(2+) influx leading to progressive accumulation of calcium. Excitation-induced Ca(2+) influx in extensor digitorum longus (EDL; fast-twitch fibers) and soleus muscle (slow-twitch fibers) is compared. In EDL and soleus, stimulation at 40 Hz increased (45)Ca uptake 34- and 21-fold and (22)Na uptake 17- and 7-fold, respectively. These differences may be related to the measured 70% higher concentration of Na(+) channels in EDL. Repeated stimulation at 40 Hz elicited a delayed release of lactic acid dehydrogenase (LDH) from EDL (11-fold increase) and soleus (5-fold increase). Continuous stimulation at 1 Hz increased LDH release only from EDL (18-fold). This was associated with increased Ca(2+) content and was augmented at high extracellular Ca(2+) concentration ([Ca(2+)](o)) and suppressed at low [Ca(2+)](o). The data support the hypothesis that excitation-induced Ca(2+) influx is mediated in part by Na(+) channels and that the ensuing increase in intracellular Ca(2+) induces cellular damage. This is most pronounced in EDL, which may account for the repeated observation that prolonged exercise leads to preferential damage to fast-twitch fibers.

Decreased EMG median frequency during a second bout of eccentric contractions

PURPOSE

Others have reported preferential recruitment of fast motor units in muscles during performance of eccentric contractions and there is evidence that fast muscle fibers are more susceptible to eccentric contraction-induced injury. We tested the hypothesis that during a second bout of maximal eccentric contractions 1 wk after the first, there would be a reduction in the electromyographic (EMG) median frequency (MF) with minimal change in the EMG root-mean-square (RMS), indicating greater reliance on slower motor units. This could provide an explanation for the enhanced resistance to eccentric contraction-induced injury after a single bout of eccentric exercise.

METHODS

Human subjects performed 50 maximal voluntary eccentric (N = 10) or concentric (N = 10) contractions of the anterior crural muscles on two occasions separated by 1 wk. To determine whether MF changes during the second bout could be a consequence of injury to fibers in fast motor units, the anterior crural muscles of mice were electrically stimulated to perform 50 maximal eccentric (N = 10) or concentric (N = 9) contractions on two occasions separated by 1 wk. In both the humans and mice, torque production and tibialis anterior muscle RMS and MF were measured during the two exercise bouts.

RESULTS

In human tibialis anterior muscle, MF was 30% lower (P < 0.01) during the second eccentric bout although RMS was the same. In the mice, RMS and MF were unchanged at any time after the first eccentric bout despite torque deficits similar to those observed in the humans.

CONCLUSIONS

The data indicate that with repetition of maximal voluntary eccentric contractions, there is an increased activation of slow motor units and a concomitant decrease in activation of fast units.

Dissociation of force production from MHC and actin contents in muscles injured by eccentric contractions

The primary purpose of this study was to determine the relationship between myosin heavy chain (MHC) and actin contents and maximum isometric tetanic force (Po) in mouse extensor digitorum longus (EDL) muscles following eccentric contraction-induced injury. Po and protein contents were measured in injured (n = 80) and contralateral control (n = 80) EDL muscles at the following time points after in vivo injury: sham, 0, 0.25, 1, 3, 5, 14, and 28 days. Po was reduced by 37 +/- 2.3% to 49 +/- 3.8% (p < or = 0.05), while MHC and actin contents were unaltered from 0 to 3 days after injury. Whereas Po partially recovered between 3 and 5 days (from -49 +/- 3.8% to -35 +/- 3.6%), MHC and actin contents in the injured muscles declined by 19 +/- 4.9% and 20 +/- 5.3%, respectively, by 5 days compared with control muscles. Decrements in Po were similar to the reductions in MHC and actin contents at 14 (approximately 24%) and 28 (approximately 11%) days. Evaluation of myofibrillar and soluble protein fractions indicated significant reductions in the content of major proteins at 5 and 14 days. Immunoblots of heat shock protein 72 revealed elevations starting at 0.25 days, peaking during 1-3 days, and declining after 5 days. These findings indicate that decreased contractile protein content is not related to the initial decrease in Po. However, decreased MHC and actin contents could account for 58% of the Po reduction at 5 days, and for nearly all the decrements in Po from 14 to 28 days.

Quantitative measurement of resting skeletal muscle [Ca2+]i following acute and long-term downhill running exercise in mice

Alteration of resting free intracellular [Ca2+] ([Ca2+]i) homeostasis has been implicated in the aetiology of skeletal muscle fibre injury following damaging pliometric (lengthening or 'eccentric') contractions. Quantitative measurements of resting [Ca2+]i in skeletal muscles following acute or long-term exercise involving such injurious contractions have not been performed. We tested the hypothesis that, following an acute bout of pliometric exercise, the maximum force production (Po) of isolated skeletal muscles would be significantly reduced and that this deficit in force would be accompanied by an elevation in resting skeletal muscle [Ca2+]i. Further, we tested whether long-term pliometric exercise training would protect skeletal muscles from contraction-induced injury. Adult male mice were randomly assigned to either, control, 24-hour, 48-hour, or trained groups. The 24-hour and 48-hour group animals were subjected to a single acute downhill treadmill running bout (decline 16 degrees, at a rate of 13 m/min, for 60 min) and sacrificed at 24 or 48 h, respectively. Trained animals underwent a 14 week endurance training program consisting of a daily (5 days/week) downhill running session, under identical conditions to that of the acutely exercised groups. The sedentary control animals remained in their cages. For each animal, Po was determined in the fast-twitch EDL and slow-twitch soleus muscles from one hindlimb and quantitative measurements of [Ca2+]i were made in the contralateral muscles using fluorescence digital imaging microscopy in conjunction with Fura-2. Po was lower in the EDL and soleus muscles from the 48-hour group compared with the control group animals. Po was higher in the EDL muscles of the trained group compared with the 48-hour group. No significant difference in Po was detected in either muscle from the 24-hour or trained groups compared with muscles from control mice. In EDL muscles, [Ca2+]i was elevated in the 48-hour group compared with the control and trained group animals, but was not different between the 24-hour group compared with control mice. [Ca2+]i was not different in the soleus muscles among the 48-hour, trained or control group mice, but was increased in muscles from the 48-hour group compared with the 24-hour group. Endurance downhill running training conferred protection to recruited skeletal muscles against the effects of an acute bout of repeated pliometric contractions, as evidenced by [Ca2+]i and Po values similar to muscles from unexercised control mice.

Redistribution of cell membrane probes following contraction-induced injury of mouse soleus muscle

Our aim was to study how mouse skeletal muscle membranes are altered by eccentric and isometric contractions. A fluorescent dialkyl carbocyanine dye (DiOC18(3)) was used to label muscle membranes, and the membranes accessible to the dye were observed by confocal laser scanning microscopy. Experiments were done on normal mouse soleus muscles and soleus muscles injured by 20 eccentric or 20 isometric contractions. Longitudinal optical sections of control muscle fibers revealed DiOC18(3) staining of the plasmalemma and regularly spaced transverse bands corresponding in location to the T-tubular system. Transverse optical sections showed an extensive reticular network with the DiOC18(3) staining. Injured muscle fibers showed distinctively different staining patterns in both longitudinal and transverse optical sections. Longitudinal optical sections of the injured fibers revealed staining in a longitudinally-oriented pattern. No correlations were found between the abnormal DiOC18(3) staining and the reductions in maximal isometric tetanic force or release of lactate dehydrogenase (P > or = 0.32). Additionally, no difference in the extent of abnormal staining was found between muscles performing eccentric contractions and those performing the less damaging isometric contractions. However, many fibers in muscles injured by eccentric contractions showed swollen regions with marked loss of membrane integrity and an elevated free cytosolic calcium concentration as observed in Fluo-3 images. In conclusion, a loss of cell membrane integrity results from contractile activity, enabling DiOC18(3) staining of internal membranes. The resulting staining pattern is striking and fibers with damaged cell membranes are easily distinguished from uninjured ones.

Associations between muscle soreness, damage, and fatigue

Eccentric exercise results in muscle soreness, structural damage, prolonged losses in strength and range of motion, and neuromuscular dysfunction. Greater and longer lasting fatigue occurs after eccentric compared with concentric and isometric exercise. Higher forces are achieved during eccentric contractions with less ATP usage and greater increases in temperature. Although mechanisms involved in the damage and repair process are not well understood, active strain during eccentric contractions is suggested to cause the initial damage which increases over 2-3 days, followed by regeneration.