Streptomycin and EDTA decrease the number of desmin-negative fibers following stretch injury

Italian consensus conference on guidelines for conservative treatment on lower limb muscle injuries in athlete

Provide the state of the art concerning (1) biology and aetiology, (2) classification, (3) clinical assessment and (4) conservative treatment of lower limb muscle injuries (MI) in athletes. Seventy international experts with different medical backgrounds participated in the consensus conference. They discussed and approved a consensus composed of four sections which are presented in these documents. This paper represents a synthesis of the consensus conference, the following four sections are discussed: (i) The biology and aetiology of MIs. A definition of MI was formulated and some key points concerning physiology and pathogenesis of MIs were discussed. (ii) The MI classification. A classification of MIs was proposed. (iii) The MI clinical assessment, in which were discussed anamnesis, inspection and clinical examination and are provided the relative guidelines. (iv) The MI conservative treatment, in which are provided the guidelines for conservative treatment based on the severity of the lesion. Furthermore, instrumental therapy and pharmacological treatment were discussed. Knowledge of the aetiology and biology of MIs is an essential prerequisite in order to plan and conduct a rehabilitation plan. Another important aspect is the use of a rational MI classification on prognostic values. We propose a classification based on radiological investigations performed by ultrasonography and MRI strongly linked to prognostic factors. Furthermore, the consensus conference results will able to provide fundamental guidelines for diagnostic and rehabilitation practice, also considering instrumental therapy and pharmacological treatment of MI. Expert opinion, level IV.

Effects of Streptomycin Administration on Increases in Skeletal Muscle Fiber Permeability and Size Following Eccentric Muscle Contractions

The purpose of this study was to investigate the preventive effect of streptomycin (Str) administration on changes in membrane permeability and the histomorphological characteristics of damaged muscle fibers following eccentric contraction (ECC ). Eighteen 7-week-old male Fischer 344 rats were randomly assigned to three groups: control (Cont), ECC, and ECC with Str (ECC + Str). The tibialis anterior (TA) muscles in both ECC groups were stimulated electrically and exhibited ECC. Evans blue dye (EBD), a marker of muscle fiber damage associated with increased membrane permeability, was injected 24 hr before TA muscle sampling. The number of EBD-positive fibers, muscle fiber cross-sectional area (CSA), and roundness were determined via histomorphological analysis. The ECC intervention resulted in an increased fraction of EBD-positive fibers, a larger CSA, and decreased roundness. The fraction of EBD-positive fibers was 79% lower in the ECC + Str group than in the ECC group. However, there was no difference in the CSA and roundness of the EBD-positive fibers between the two ECC groups. These results suggest that Str administration can reduce the number of myofibers that increase membrane permeability following ECC, but does not ameliorate the extent of fiber swelling in extant EBD-positive fibers. Anat Rec, 301:1096-1102, 2018. © 2018 Wiley Periodicals, Inc.

Pathways of Ca²⁺ entry and cytoskeletal damage following eccentric contractions in mouse skeletal muscle

Muscles that are stretched during contraction (eccentric contractions) show deficits in force production and a variety of structural changes, including loss of antibody staining of cytoskeletal proteins. Extracellular Ca(2+) entry and activation of calpains have been proposed as mechanisms involved in these changes. The present study used isolated mouse extensor digitorum longus (EDL) muscles subjected to 10 eccentric contractions and monitored force production, immunostaining of cytoskeletal proteins, and resting stiffness. Possible pathways for Ca(2+) entry were tested with streptomycin (200 μM), a blocker of stretch-activated channels, and with muscles from mice deficient in the transient receptor potential canonical 1 gene (TRPC1 KO), a candidate gene for stretch-activated channels. At 30 min after the eccentric contractions, the isometric force was decreased to 75 ± 3% of initial control and this force loss was reduced by streptomycin but not in the TRPC1 KO. Desmin, titin, and dystrophin all showed patchy loss of immunostaining 30 min after the eccentric contractions, which was substantially reduced by streptomycin and in the TRPC1 KO muscles. Muscles showed a reduction of resting stiffness following eccentric contractions, and this reduction was eliminated by streptomycin and absent in the TRPC1 KO muscles. Calpain activation was determined by the appearance of a lower molecular weight autolysis product and μ-calpain was activated at 30 min, whereas the muscle-specific calpain-3 was not. To test whether the loss of stiffness was caused by titin cleavage, protein gels were used but no significant titin cleavage was detected. These results suggest that Ca(2+) entry following eccentric contractions is through a stretch-activated channel that is blocked by streptomycin and encoded or modulated by TRPC1.

Long-term blocking of calcium channels in mdx mice results in differential effects on heart and skeletal muscle

The disease mechanisms underlying dystrophin-deficient muscular dystrophy are complex, involving not only muscle membrane fragility, but also dysregulated calcium homeostasis. Specifically, it has been proposed that calcium channels directly initiate a cascade of pathological events by allowing calcium ions to enter the cell. The objective of this study was to investigate the effect of chronically blocking calcium channels with the aminoglycoside antibiotic streptomycin from onset of disease in the mdx mouse model of Duchenne muscular dystrophy (DMD). Treatment in utero onwards delayed onset of dystrophic symptoms in the limb muscle of young mdx mice, but did not prevent degeneration and regeneration events occurring later in the disease course. Long-term treatment had a positive effect on limb muscle pathology, reduced fibrosis, increased sarcolemmal stability, and promoted muscle regeneration in older mice. However, streptomycin treatment did not show positive effects in diaphragm or heart muscle, and heart pathology was worsened. Thus, blocking calcium channels even before disease onset does not prevent dystrophy, making this an unlikely treatment for DMD. These findings highlight the importance of analyzing several time points throughout the life of the treated mice, as well as analyzing many tissues, to get a complete picture of treatment efficacy.

Eccentric exercise in vivo: strain-induced muscle damage and adaptation in a stable system

The muscle tendon unit is a stable system, designed to operate eccentrically with efficiency and resiliency. Fiber strains, although minimized by tendon compliance during exercise, are essential components to decoding the mechanical and chemical signals during exercise. Subsequent cellular adaptations minimize the subsequent "dose" of stress and strain and serve to limit the exacerbation of damage into injury.

Changes in calpain activity, muscle structure, and function after eccentric exercise

PURPOSE

The aim of this study was to investigate changes in muscle function, muscle structure, and calpain activity after high-force eccentric exercise.

METHODS

Eleven healthy males performed 300 maximal voluntary eccentric actions with knee extensors in one leg. Maximal force-generating capacity was measured before exercise and regularly during the next 7 d. Biopsies from musculus vastus lateralis were taken in both control and exercised legs 0.5, 4, 8, 24, 96, and 168 h after exercise for evaluation of myofibrillar structure, extracellular matrix proteins, and calpain activity.

RESULTS

In the exercised leg, peak torque was reduced by 47 +/- 5% during exercise and was still 22 +/- 5% lower than baseline 4 d after the exercise. Calpain activity was three times higher in the exercised leg compared with the control leg 30 min after exercise. Myofibrillar disruptions were observed in 36 +/- 6% of all fibers in exercised muscle and in 2 +/- 1% of fibers in control muscle. The individual reductions in peak torque correlated with the proportion of fibers with myofibrillar disruptions (r = 0.89). The increase in calpain activity was not correlated to the proportion of fibers with myofibrillar disruptions. Nevertheless, the characteristics of the myofibrillar disruptions mimicked calpain-mediated degradation of myofibrils. Tenascin-C and the N-terminal propeptide of procollagen type III showed increased staining intensity on cross-sections 4-7 d after the exercise.

CONCLUSIONS

Myofibrillar disruptions seem to be a main cause for the long-lasting reduction in force-generating capacity after high-force eccentric exercise. The increase in calpain activity, but the lack of a relationship between calpain activity and the amount of muscle damage, suggests multiple roles of calpain in the damage and repair process.

Stretch-activated ion channel blockade attenuates adaptations to eccentric exercise

PURPOSE

The purpose of this study was to test the hypothesis that stretch-activated ion channel (SAC) function is essential for the repeated bout effect (RBE) in skeletal muscle. Specifically, we investigated if daily injections of streptomycin (a known SAC blocker) would abrogate the muscle's adaptive resistance to the damaging effects of eccentric exercise over a 4-wk period. Furthermore, we hypothesized that the lack of an RBE would be due to the lack of functional adaptations that typically result from repeated bouts of eccentric exercise, including increased peak isometric torque, muscle hypertrophy, and rightward shift of the torque-angle relationship.

METHODS

Twelve New Zealand white rabbits were each subjected to 12 bouts of eccentric exercise over a 4-wk period while receiving either daily injections of streptomycin or sham injections.

RESULTS

Although blocking the SAC function completely eliminated the expected adaptive response in biomechanical parameters during the exercise regimen, there remained evidence of an acquired RBE, albeit with an attenuated response when compared with the muscles with intact SAC function.

CONCLUSION

Blocking sarcolemmal SAC eliminates functional adaptations of muscle after eccentric exercise. In the absence of SAC function, muscles subjected to chronic eccentric exercise still exhibit some degree of the RBE. As such, it appears that the signaling cascade that results in functional, biomechanical adaptations associated with the RBE during eccentric exercise is dependent upon intact SAC function.

Transforming growth factor-beta following skeletal muscle strain injury in rats

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine implicated in inflammatory processes, wound healing, and fibrosis. In muscle diseases (i.e., dystrophy and inflammatory myopathy) and in animal models of muscle injury (i.e., produced by cardiotoxin, laceration, and eccentric contractions), increased TGF-beta was associated with muscle fibrosis and healing. Although TGF-beta transcript abundance was increased following injury, many studies presume that TGF-beta protein was also active as evident by increases in collagen transcript abundance. The purpose was to determine whether TGF-beta protein is present and active 48 h following injury. Using female rats, muscle strains were produced by stretching (50 stretches) the plantar flexor muscles. Forty-eight hours following injury, the medial gastrocnemius was removed and compartmentalized into five equal segments. Damaged myofibers with intracellular concanavalin A staining were counted. The percentage of damaged myofibers was significantly greater in the distal-most segment. TGF-beta was assessed by using immunohistochemistry, RT-PCR, and immunoblot analysis. Immunohistochemistry revealed the presence of TGF-beta1 in areas of myofiber injury, whereas TGF-beta2 was not detected. Increases in TGF-beta1 and TGF-beta2 transcript abundance following strain injury were documented by RT-PCR analysis. Increases in TGF-beta1 and TGF-beta2 precursor abundance were observed following strain injury by using immunoblot analysis but there was no change in active TGF-beta abundance. Although there was no correlation between the amount of cellular injury and TGF-beta transcript and protein abundance, elevated levels of TGF-beta1 and TGF-beta2 precursor proteins were present in strain-injured skeletal muscles 48 h after injury.