Skeletal Muscle Contractility Research Articles

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.

Different effects of ATP on the contractile activity of mice diaphragmatic and skeletal muscles

Apart from acetyl-choline (Ach), adenosine-5′-trisphosphate (ATP) is thought to play a role in neuromuscular function, however little information is available on its cellular physiology. As such, effects of ATP and adenosine on contractility of mice diaphragmatic and skeletal muscles (m. extensor digitorum longa—MEDL) have been investigated in in vitro experiments. Application of carbacholine (CCh) in vitro in different concentrations led to pronounced muscle contractions, varying from 9.15 ± 4.76 to 513.13 ± 15.4 mg and from 44.65 ± 5.01 to 101.46 ± 9.11 mg for diaphragm and MEDL, respectively. Two hundred micromolars of CCh in both muscles caused the contraction with the 65% (diaphragm) to 75% (MEDL) of maximal contraction force—this concentration was thus used in further experiments. It was found that application of ATP (100 μM) increased the force of diaphragmatic contraction caused by CCh (200 μM) from 335.2 ± 51.4 mg ( n = 21) in controls to 426.5 ± 47.8 mg ( n = 10; P < 0.05), but decreased the contractions of MEDL of CCh from 76.6 ± 6.5 mg ( n = 26) in control to 40.2 ± 9.0 mg ( n = 8; P < 0.05). Application of adenosine (100 μM) had no effect on CCh-induced contractions of these muscles. Resting membrane potential (MP) measurements using sharp electrodes were done at 10, 20 and 30 min after the application of ATP and adenosine. Diaphragm showed depolarization from 75 ± 0.6 down to 63.2 ± 1.05, 57.2 ± 0.96 and 53.6 ± 1.1 mV after 10, 20 and 30 min of exposition, respectively (20 fibers from 4 muscles each, P < 0.05 in all three cases). Adenosine showed no effect on diaphragmatic MP. Both agents were ineffective in case of MEDL. The effects of ATP in both tissues were abolished by suramin (100 μM), a P2-receptor antagonist, and chelerythrin (50 μM), a specific protein-kinase C (PKC) inhibitor, but were not affected by 1 H-[1,2,4]-oxadiazolo-[4,3-α]-quinoxalin-1-one (ODQ, 1 μM), a guanylyl-cyclase inhibitor, or by adenosine-3,5-monophosphothioate (Rp-cAMP, 1 μM), a protein-kinase A (PKA) inhibitor. Besides the action on contractile activity, ATP (100 μM) led to a significant ( P < 0.001) depolarization of diaphragm muscle fibers from 74.5 ± 2.3 down to 64 ± 2.1, 58.2 ± 2.2 and 54.3 ± 2.4 mV after 10, 20 and 30 min of incubation, respectively. Incubation of MEDL with the same ATP concentration showed no significant change of MP. Denervation of the muscles for 28 days led to a decrease of CCh-induced contractions of diaphragm down to 171.1 ± 34.5 mg ( n = 11, P < 0.05), but increased the contractile force of MEDL up to 723.9 ± 82.3 mg ( n = 9, P < 0.01). Application of ATP elevated the contractility of denervated diaphragm caused by CCh up to normal values (311.1 ± 79.7 mg, n = 6, P > 0.05 versus control), but did not significantly affect of contractility of MEDL, which became 848.1 ± 62.7 mg ( n = 6). These results show that the effects of ATP on both diaphragmatic and skeletal muscles are mediated through P2Y receptors coupled to chelerytrin-sensitive protein-kinase C.

Tissue-Engineered Axially Vascularized Contractile Skeletal Muscle

As tissue-engineered muscle constructs increase in scale, their size is limited by the need for a vascular supply. In this work, the authors demonstrate a method of producing three-dimensional contractile skeletal muscles in vivo by incorporating an axial vascular pedicle. Primary myoblast cultures were generated from adult F344 rat soleus muscle. The cells were suspended in a fibrinogen hydrogel contained within cylindrical silicone chambers, and situated around the femoral vessels in isogeneic adult recipient rats. The constructs were allowed to incubate in vivo for 3 weeks, at which point they were explanted and subjected to isometric force measurements and histologic evaluation. The resulting three-dimensional engineered skeletal muscle constructs produced longitudinal contractile force when electrically stimulated. Length-tension, force-voltage, and force-frequency relationships were similar to those found in developing skeletal muscle. Desmin staining demonstrated that individual myoblasts had undergone fusion to form multinucleated myotubes. Von Willebrand staining showed that the local environment within the chamber was richly angiogenic, and capillaries had grown into and throughout the constructs from the femoral artery and vein. Three-dimensional, vascularized skeletal muscle can be engineered in vivo. The resulting tissues have histologic and functional properties consistent with native skeletal muscle.

The ubiquitin–proteasome system and skeletal muscle wasting

The ubiquitin-proteasome system (UPS) is believed to degrade the major contractile skeletal muscle proteins and plays a major role in muscle wasting. Different and multiple events in the ubiquitination, deubiquitination and proteolytic machineries are responsible for the activation of the system and subsequent muscle wasting. However, other proteolytic enzymes act upstream (possibly m-calpain, cathepsin L, and/or caspase 3) and downstream (tripeptidyl-peptidase II and aminopeptidases) of the UPS, for the complete breakdown of the myofibrillar proteins into free amino acids. Recent studies have identified a few critical proteins that seem necessary for muscle wasting {i.e. the MAFbx (muscle atrophy F-box protein, also called atrogin-1) and MuRF-1 [muscle-specific RING (really interesting new gene) finger 1] ubiquitin-protein ligases}. The characterization of their signalling pathways is leading to new pharmacological approaches that can be useful to block or partially prevent muscle wasting in human patients.

Soleus and EDL muscle contractility across the lifespan of female C57BL/6 mice

All previous aging research on the contractility of rodent skeletal muscle has been conducted on male rodents. Because males and females age differently, we undertook this study to determine if and when age-related decrements in skeletal muscle contractility occur in female mice. Soleus and extensor digitorum longus (EDL) muscles from female C57BL/6 mice aged ∼4, 8, 16, 24 and 28 mo were assessed in vitro for contractility and subsequently contractile protein content. EDL muscle was resistant to age-related changes in force generation but displayed characteristics of becoming more slow-twitch like. Maximal isometric tetanic force (P o) generated by soleus muscle declined with age. Soleus muscle size and contractile protein contents were not affected by age and thus could not explain the age-related force decrements. Soleus muscle specific P o declined with age being ∼26% lower in muscles of 16–28 mo-old mice indicating that a deterioration in soleus muscle quality of female mice occurred beginning around the age of ovarian failure. Thus this study provides essential, comprehensive baseline data for future studies on age-related muscle dysfunction in the female mouse.

The ubiquitin–proteasome system and skeletal muscle wasting

The ubiquitin–proteasome system (UPS) is believed to degrade the major contractile skeletal muscle proteins and plays a major role in muscle wasting. Different and multiple events in the ubiquitination, deubiquitination and proteolytic machineries are responsible for the activation of the system and subsequent muscle wasting. However, other proteolytic enzymes act upstream (possibly m-calpain, cathepsin L, and/or caspase 3) and downstream (tripeptidyl-peptidase II and aminopeptidases) of the UPS, for the complete breakdown of the myofibrillar proteins into free amino acids. Recent studies have identified a few critical proteins that seem necessary for muscle wasting {i.e. the MAFbx (muscle atrophy F-box protein, also called atrogin-1) and MuRF-1 [muscle-specific RING (really interesting new gene) finger 1] ubiquitin–protein ligases}. The characterization of their signalling pathways is leading to new pharmacological approaches that can be useful to block or partially prevent muscle wasting in human patients.

Melatonin counteracts inducible mitochondrial nitric oxide synthase‐dependent mitochondrial dysfunction in skeletal muscle of septic mice

Mitochondrial nitric oxide synthase (mtNOS) produces nitric oxide (NO) to modulate mitochondrial respiration. Besides a constitutive mtNOS isoform it was recently suggested that mitochondria express an inducible isoform of the enzyme during sepsis. Thus, the mitochondrial respiratory inhibition and energy failure underlying skeletal muscle contractility failure observed in sepsis may reflect the high levels of NO produced by inducible mtNOS. The fact that mtNOS is induced during sepsis suggests its relation to inducible nitric oxide synthase (iNOS). Thus, we examined the changes in mtNOS activity and mitochondrial function in skeletal muscle of wild-type (iNOS(+/+)) and iNOS knockout (iNOS(-/-)) mice after sepsis. We also studied the effects of melatonin administration on mitochondrial damage in this experimental paradigm. After sepsis, iNOS(+/+) but no iNOS(-/-) mice showed an increase in mtNOS activity and NO production and a reduction in electron transport chain activity. These changes were accompanied by a pronounced oxidative stress reflected in changes in lipid peroxidation levels, oxidized glutathione/reduced glutathione ratio, and glutathione peroxidase and reductase activities. Melatonin treatment counteracted both the changes in mtNOS activity and rises in oxidative stress; the indole also restored mitochondrial respiratory chain in septic iNOS(+/+) mice. Mitochondria from iNOS(-/-) mice were unaffected by either sepsis or melatonin treatment. The data suggest that inducible mtNOS, which is coded by the same gene as that for iNOS, is responsible for mitochondrial dysfunction during sepsis. The results also suggest the use of melatonin for the protection against mtNOS-mediated mitochondrial failure.

GLYCOGEN CONTENT AND CONTRACTILE RESPONSIVENESS TO T‐SYSTEM DEPOLARIZATION IN SKINNED MUSCLE FIBRES OF THE RAT

1. Glycogen content (determined microfluorometrically), response capacity to transverse tubular (T) system depolarization and the relationship between these two parameters were examined in single, mechanically skinned fibres from rat extensor digitorum longus (EDL) muscle in the presence of high and constant concentrations of ATP and creatine phosphate. 2. The mean total glycogen content (tGlyc) in freshly dissected fibres was 58.1 +/- 4.2 mmol glucosyl units/L fibre (n = 53). 3. A large proportion of tGlyc was retained in the skinned fibres (SFGlyc) after 2 and 30 min exposure to an aqueous relaxing solution (73.1 +/- 2.8 and 64 +/- 12.3%, respectively). 4. When fibres were incubated for 30 min in a high (30 micromol/L)-Ca2+ solution, the proportion of SFGlyc was markedly lower (approximately 28%), which suggests that rat skinned fibres contain a Ca2+-sensitive glycogenolytic system. 5. In rat skinned fibres, T-system depolarization-induced Ca2+ release was not accompanied by a detectable loss of fibre glycogen and there was no correlation between response capacity and initial SFGlyc, indicating that other factors, unrelated to glycogen depletion, ultimately limited the capacity of rat skinned fibres to respond to T-system depolarization. 6. It is concluded that rat mechanically skinned fibre preparations are well suited for studies of glycogenolysis at a cellular level and that, with further refinement of the depolarization protocol, they may be suitable for studies of the non-metabolic role of glycogen in mammalian skeletal muscle contractility.

Skeletal muscle contractility is preserved in COPD patients with normal fat‐free mass

Peripheral muscle dysfunction often occurs in patients with chronic obstructive pulmonary disease (COPD). The muscle dysfunction may be caused by a loss of force-generating capacity, resulting from a loss of muscle mass, as well as by other alterations in contractile properties of skeletal muscle. The maximal isometric voluntary strength and fatigability were determined in hand-grip and quadriceps muscles from nine male COPD patients (FEV(1) 30-50% predicted) and control subjects matched for fat-free mass (FFM), physical activity level and age. Contractile properties and fatigability of the quadriceps muscle were also studied with electrically evoked isometric contractions. The maximal voluntary force (MVC) and fatigability of the handgrip muscle did not differ between the COPD patients and control subjects. Also the MVC of the quadriceps muscle and the rate of force rise, contraction time, force-frequency relationship and fatigability, as determined with electrically evoked contractions, were similar in patients with COPD and control subjects. Skeletal muscle strength, contractile properties and fatigability are preserved in patients with moderate COPD and a normal FFM and activity level. This suggests that skeletal muscle dysfunction does not take place during moderate COPD until cachexia and/or a decline in physical activity occur.

Effects of lengthening contraction on calcium kinetics and skeletal muscle contractility in humans

We have tested the hypothesis that the altered muscle contractility after lengthening contractions (LC) is caused by altered calcium (Ca2+) kinetics. Subjects (n = 8) performed 100 drop jumps and muscle contractility was measured pre- and post-exercise by maximal voluntary contraction (MVC) and transcutaneous electrical stimulation (1, 20 and 50 Hz). Muscle biopsies were analysed for muscle metabolites, rates of SR Ca(2+) uptake (CaU) and release (CaR) and myosin heavy chain (MHC) composition. The rates of torque relaxation and CaU were positively related to muscle fibre type composition (% MHC II). Muscle creatine (Cr) decreased and the ratio between phosphocreatine (PCr) and Cr increased 3 and 24 h post-exercise (P < 0.05 vs. pre-exercise). LC resulted in reduced MVC (-19%), twitch torque (-41%) and 20/50 Hz torque ratio (-30%) and a faster relaxation rate (P < 0.05). The contractile parameters recovered partially but remained altered 24 h post-exercise (P < 0.05). The average CaR was unchanged after LC (P > 0.05). However, the response varied between subjects and the relative post-exercise CaR was significantly related to the degree of LFF (post/pre 20/50 Hz force ratio) and to the decline in twitch force (post/pre twitch ratio). CaU was lower in seven of eight subjects after LC (P > 0.05). The decline in torque after LC could not be explained by metabolic factors since PCr/Cr ratio increased. The relation between CaR and fatigue suggests that the mechanism of fatigue in part may be attributed to intrinsic changes in the SR Ca2+ release channel. The faster torque relaxation after LC could not be explained by an increased rate of CaU.

The T‐tubule membrane ATP‐operated P2X4receptor influences contractility of skeletal muscle

Evidence indicates that extracellular ATP may have relevant functions in skeletal muscle, even though the physiological role and distribution of specific signaling pathway elements are not well known. The present work shows that P2X4 receptor, an extracellular ATP-regulated cell membrane channel permeable to Ca2+, is expressed in several tissues of the rat, including skeletal muscle. A specific antibody detected a protein band of approximately 60 kDa. Immunofluorescence demonstrated that P2X4 has an intracellular localization, and confocal analysis revealed that the receptor colocalizes with the T-tubule membrane DHP receptor. Considering that the natural agonist of P2X4 is ATP, we explored if changes of extracellular ATP levels could occur in contracting skeletal muscle to regulate the channel. In vitro experiments showed that substantial ATP is released and rapidly hydrolyzed after electrical stimulation of rat muscle fibers. Results show that the presence of ATP-degrading enzymes (hexokinase/apyrase), inhibitors of P2X receptors or Ca2+-free conditions, all abolished the progressive twitch tension potentiation produced in soleus muscle by low-frequency (0.05 Hz) stimulation. These data reveal that ATP-mediated Ca2+ entry, most likely through P2X4 receptor, may play an important role in modulating the contractility of skeletal muscle.

Malaria Parasites Are Rapidly Killed by Dantrolene Derivatives Specific for the Plasmodial Surface Anion Channel

Dantrolene was recently identified as a novel inhibitor of the plasmodial surface anion channel (PSAC), an unusual ion channel on Plasmodium falciparum-infected human red blood cells. Because dantrolene is used clinically, has a high therapeutic index, and has desirable chemical synthetic properties, it may be a lead compound for antimalarial development. However, dantrolene derivatives would need to preferentially interact with PSAC over the sarcoplasmic reticulum (SR) Ca2+ release channel to avoid unwanted side effects from antimalarial therapy. Furthermore, dantrolene's modest affinity for PSAC (K(m) of 1.2 microM) requires improvement. In this study, we tested 164 derivatives of dantrolene to examine whether these hurdles can be surmounted. A simple screen for PSAC block defined the minimal scaffold needed and identified compounds with > or =5-fold higher affinity. Single-channel patch-clamp recordings on infected human red blood cells with two derivatives also revealed increased blocking affinity that resulted from slower unbinding from a site on the extracellular face of PSAC. We tested these derivatives in a frog skeletal muscle contractility assay and found that, in contrast to dantrolene, they had little or no effect on SR Ca2+ release. Finally, these blockers kill in vitro parasite cultures at lower concentrations than dantrolene, consistent with an essential role for PSAC. Because, as a class, these derivatives fulfil the requirements for drug leads and can be studied with simple screening technology, more extensive medicinal chemistry is warranted to explore antimalarial development.

Körperliche Aktivität bei älteren Patienten mit chronischer Herzinsuffizienz

Maximal exercise capacity undergoes a steady decline after the age of 30 by approximately 10 % per decade. As a consequence of this development older people > 65 years of age suffer from the exercise limitation caused by age-associated cardiac, vascular and skeletal muscle changes. These physiologic alterations make older people especially vulnerable for the cardiovascular and peripheral alterations associated with chronic heart failure (CHF). These changes are not phenomenologically different from age-associated changes. Physical activity plays an important role for regaining a considerable part of vasomotor function, skeletal muscle contractility, and cardiac reserve. Up to now there are no prospective trials comparing the effects of physical training between older and younger patients with CHF. However, smaller observational studies indicate that elderly patients benefit equally well from training interventions with regard to functional improvements in proportion to their lower baseline values. In an aging population training aims at maintaining skeletal muscle force and muscle mass as well as locomotor coordination. Ultimately, the goal is to reduce the substantial morbidity among elderly CHF patients which constitute 79 % of all hospital admissions for heart failure.

Intracellular [H+]: a determinant of cell volume in skeletal muscle

Intracellular [H⁺]: a determinant of cell volume in skeletal muscle

Na+-K+ pump stimulation restores carbacholine-induced loss of excitability and contractility in rat skeletal muscle.

Intense exercise results in increases in intracellular Na+ and extracellular K+ concentrations, leading to depolarization and a loss of muscle excitability and contractility. Here, we use carbacholine to chronically activate the nicotinic acetylcholine (nACh) receptors to mimic the changes in membrane permeability, chemical Na+ and K+ gradients and membrane potential observed during intense exercise. Intact rat soleus muscles were mounted on force transducers and stimulated electrically to evoke short tetani at regular intervals. Carbacholine produced a 2.6-fold increase in Na+ influx that was tetrodotoxin (TTX) insensitive, but abolished by tubocurarine, resulting in a significant 36% increase in intracellular Na+, and 8% decrease in intracellular K+ content. The mid region, near the motor end plate, had much larger alterations than the more distal regions of the muscle, and showed a larger membrane depolarization from -73 +/- 1 to -60 +/- 1 mV compared with -64 +/- 1 mV. Carbacholine (10(-4) M) significantly reduced tetanic force to 31 +/- 3% of controls, which underwent significant recovery upon application of Na+-K+ pump stimulators: salbutamol (10(-5) M), adrenaline (10(-5) M) and calcitonin gene-related peptide (CGRP; 10(-7) M). The force recovery with salbutamol was accompanied by a recovery of intracellular Na+ and K+ contents, and a small but significant 4-5 mV recovery of membrane potential. Similar results were obtained using succinylcholine (10(-4) M), indicating that Na+-K+ pump stimulation may prevent or restore succinylcholine-induced hyperkalaemia. The stimulation of the Na+-K+ pump allows muscle to partially recover contractility by regaining excitability through electrogenically driven repolarization of the muscle membrane.

Time of Day Influences the Environmental Effects on Muscle Force and Contractility

To investigate the effects of environmental temperature and the diurnal increase in body temperature on muscle contractile processes, 11 male subjects performed maximal and submaximal isometric contractions of the knee extensors with recording of the electromyographic activity in four different conditions (morning/neutral, morning/moderately warm and humid, afternoon/neutral, and afternoon/moderately warm and humid). The morning experiments were conducted between 0700 and 1900 h, and the afternoon experiments were conducted between 0500 and 0700 h. The mean laboratory temperatures and humidity were 20.5 (+/-1) degrees C + 67 (+/-4)% and 29.5 (+/-0.8) degrees C + 74 (+/-10)% for the neutral and moderately warm and humid conditions, respectively. Results showed a significant diurnal increase in both rectal and skin temperatures whatever the environmental conditions, and an increase in the skin temperature after a 60-min moderately warm exposure. The major finding of this study was an interaction effect of time of day and environmental conditions on the force/electromyographic activity ratio. That suggests that skeletal muscle contractility was differently increased by the passive warm-up effect of a moderately warm exposure, depending on the diurnal variation in body temperature. This conclusion is supported by an increase in force in the morning only after a 60-min warm exposure (+19%) and in a neutral environment only with the diurnal increase in body temperature (+12%). In summary, our data showed that both the warm exposure and the diurnal increase in body temperature influence muscle contractility and consequently muscle strength. However, the improvement in muscle contractility after these two passive warm-ups cannot be combined in order to improve force to a greater level.

Sphingosine 1-phosphate protects mouse extensor digitorum longus skeletal muscle during fatigue

Sphingomyelin derivatives exert various second messenger actions in numerous tissues. Sphingosine (SPH) and sphingosine 1-phosphate (S1P) are two major sphingomyelin derivatives present at high levels in blood. The aim of the present work was to investigate whether S1P and SPH exert relevant actions in mouse skeletal muscle contractility and fatigue. Exogenous S1P and SPH administration caused a significant reduction of tension decline during fatigue of extensor digitorum longus muscle. Final tension after the fatiguing protocol was 40% higher than in untreated muscle. Interestingly, N,N-dimethylsphingosine, an inhibitor of SPH kinase (SK), abolished the effect of supplemented SPH but not that of S1P, suggesting that SPH acts through its conversion to S1P. Moreover, SPH was not effective in Ca(2+)-free solutions, in agreement with the hypothesis that SPH action is dependent on its conversion to S1P by the Ca(2+)-requiring enzyme SK. In contrast to SPH, S1P produced its positive effects on fatigue in Ca(2+)-free conditions, indicating that S1P action does not require Ca(2+) entry and most likely is receptor mediated. The effects of S1P could be ascribed in part to its ability to prevent the reduction (-20 mV) of action potential amplitude caused by fatigue. In conclusion, these results indicate that extracellular S1P has protective effects during the development of muscle fatigue and that the extracellular conversion of SPH to S1P may represent a rheostat mechanism to protect skeletal muscle from possible cytotoxic actions of SPH.

Contractile Skeletal Muscle Tissue-Engineered on an Acellular Scaffold

San Diego and Los Angeles, Calif. Received for publication July 30, 2003. Marc H. Hedrick, M.D. Laboratory for Regenerative Bioengineering and Repair Departments of Surgery and Orthopaedic Surgery UCLA School of Medicine 10833 Le Conte Avenue Los Angeles, Calif. 90095 [email protected]

Contractile skeletal muscle tissue-engineered on an acellular scaffold.

For the reconstructive surgeon, tissue-engineered skeletal muscle may offer reduced donor-site morbidity and an unlimited supply of tissue. Using an acellularized mouse extensor digitorum longus muscle as a scaffold, the authors produced engineered skeletal muscle capable of generating longitudinal force. Eight extensor digitorum longus muscles from adult mice were made acellular using a protocol developed in the authors' laboratory. The acellular muscles were then placed in a bath of 20% fetal bovine serum in Dulbecco's modified Eagle's medium and 100 U/ml penicillin for 1 week at room temperature. C2C12 myoblasts were injected into the acellular muscle matrix using a 26-gauge needle and a 100-microl syringe. The resulting constructs were placed in growth medium for 1 week at 37 degrees C under 5% carbon dioxide, with media changes every 48 hours. The constructs were then placed in differentiation medium for 1 week, with media changes every 48 hours. Isometric contractile force testing of the constructs demonstrated production of longitudinal contractile force on electrical stimulation. A length-tension, or Starling, relationship was observed. Light and electron microscopy studies demonstrated recapitulation of some of the normal histologic features of developing skeletal muscle.

Satellite cell depletion in degenerative skeletal muscle.

Adult skeletal muscle has the striking ability to repair and regenerate itself after injury. This would not be possible without satellite cells, a subpopulation of cells existing at the margin of the myofiber. Under most conditions, satellite cells are quiescent, but they are activated in response to trauma, enabling them to guide skeletal muscle regeneration. In degenerative skeletal muscle states, including motor nerve denervation, advanced age, atrophy secondary to deconditioning or immobilization, and Duchenne muscular dystrophy, satellite cell numbers and proliferative potential significantly decrease, contributing to a diminution of skeletal muscle's regenerative capacity and contractility. This review will highlight the fate of satellite cells in several degenerative conditions involving skeletal muscle, and will attempt to gauge the relative contributions of apoptosis, senescence, impaired proliferative potential, and host factors to satellite cell dysfunction.