Quantitative morphology of stimulation-induced damage in rabbit fast-twitch skeletal muscles

Effects of the purified dry extract of fermented ginseng BST204 on muscle fiber regeneration

Background

Sarcopenia and muscular dystrophy are two muscle diseases. In cancer patients, cancer cachexia induces continuous weight loss and muscle loss due to the disease itself or the use of anticancer drugs. Cachexia occurs in up to 80% of cancer patients. It is recognized as a direct cause of reduced quality of life, contributing to at least 20% of cancer-associated deaths and limiting therapeutic options for cancer patients. Cancer cachexia is associated with multiple chronic or end-stage conditions and develops similarly. There are various options for the treatment of cancer cachexia, but there are still many issues to be solved. Hence, to determine its potential to overcome the muscle wasting during cancer cachexia, we studied the effect of BST204, a refined dry ginseng extract, on muscle fiber regeneration.

Experimental procedure

We checked the muscle regeneration efficacy of BST204. First, BaCl2 and freeze injury models were selected to investigate muscle regeneration after BST204 administration. In addition, after inducing muscle differentiation of C2C12 cells, the efficacy of BST204 was analyzed. In this model, we analyzed the expression of the signal pathway (PI3K-AKT signal) by Western blot and imaging methods.

Results and conclusion

These results showed that BST204 induced muscle fiber regeneration in BaCl2 and freeze injury models. Also, we confirmed that BST204 could regulate the PI3K/AKT signaling pathway and regulate the differentiation of C2C12 cells. These results indicate that BST204 has the potential to facilitate the skeletal muscle regeneration during muscle wasting induced by various factors including cancer cachexia.

Satellite cells maintain regenerative capacity but fail to repair disease-associated muscle damage in mice with Pompe disease

Pompe disease is a metabolic myopathy that is caused by glycogen accumulation as a result of deficiency of the lysosomal enzyme acid alpha glucosidase (GAA). Previously, we showed that adult muscle stem cells termed satellite cells are present at normal levels in muscle from patients with Pompe disease, but that these are insufficiently activated to repair the severe muscle pathology. Here we characterized the muscle regenerative response during disease progression in a mouse model of Pompe disease and investigated the intrinsic capacity of Gaa^-/- satellite cells to regenerate muscle damage. Gaa^-/- mice showed progressive muscle pathology from 15 weeks of age as reflected by increased lysosomal size, decreased fiber diameter and reduced muscle wet weight. Only during the first 15 weeks of life but not thereafter, we detected a gradual increase in centrally nucleated fibers and proliferating satellite cells in Gaa^-/- muscle, indicating a mild regenerative response. The levels of Pax7-positive satellite cells were increased in Gaa^-/- mice at all ages, most likely as result of enhanced satellite cell activation in young Gaa^-/- animals. Surprisingly, both young and old Gaa^-/- mice regenerated experimentally-induced muscle injury efficiently as judged by rapid satellite cell activation and complete restoration of muscle histology. In response to serial injury, Gaa^-/- mice also regenerated muscle efficiently and maintained the satellite cell pool. These findings suggest that, similar to human patients, Gaa^-/- mice have insufficient satellite cell activation and muscle regeneration during disease progression. The initial endogenous satellite cell response in Gaa^-/- mice may contribute to the delayed onset of muscle wasting compared to human patients. The rapid and efficient regeneration after experimental muscle injury suggest that Gaa^-/- satellite cells are functional stem cells, opening avenues for developing muscle regenerative therapies for Pompe disease.

The adaptive response of skeletal muscle: What is the evidence?

Adult skeletal muscle is capable of adapting its properties in response to changing functional demands. This now sounds like a statement of the obvious, and many people assume it has always been this way. A mere 40 years ago, however, the picture was entirely different. In this Review and personal memoir, I outline the scientific context in which the theory was generated, the objections to it from entrenched opinion, and the way those objections were progressively met. The material should be of some historical interest, but, more importantly, it collects together the full range of evidence on which the current paradigm is based. Muscle Nerve 57: 531-541, 2018.

Mechanisms modulating skeletal muscle phenotype

Mammalian skeletal muscles are composed of a variety of highly specialized fibers whose selective recruitment allows muscles to fulfill their diverse functional tasks. In addition, skeletal muscle fibers can change their structural and functional properties to perform new tasks or respond to new conditions. The adaptive changes of muscle fibers can occur in response to variations in the pattern of neural stimulation, loading conditions, availability of substrates, and hormonal signals. The new conditions can be detected by multiple sensors, from membrane receptors for hormones and cytokines, to metabolic sensors, which detect high-energy phosphate concentration, oxygen and oxygen free radicals, to calcium binding proteins, which sense variations in intracellular calcium induced by nerve activity, to load sensors located in the sarcomeric and sarcolemmal cytoskeleton. These sensors trigger cascades of signaling pathways which may ultimately lead to changes in fiber size and fiber type. Changes in fiber size reflect an imbalance in protein turnover with either protein accumulation, leading to muscle hypertrophy, or protein loss, with consequent muscle atrophy. Changes in fiber type reflect a reprogramming of gene transcription leading to a remodeling of fiber contractile properties (slow-fast transitions) or metabolic profile (glycolytic-oxidative transitions). While myonuclei are in postmitotic state, satellite cells represent a reserve of new nuclei and can be involved in the adaptive response.

Hypoxia transiently affects skeletal muscle hypertrophy in a functional overload model

Hypoxia induces a loss of skeletal muscle mass, but the signaling pathways and molecular mechanisms involved remain poorly understood. We hypothesized that hypoxia could impair skeletal muscle hypertrophy induced by functional overload (Ov). To test this hypothesis, plantaris muscles were overloaded during 5, 12, and 56 days in female rats exposed to hypobaric hypoxia (5,500 m), and then, we examined the responses of specific signaling pathways involved in protein synthesis (Akt/mTOR) and breakdown (atrogenes). Hypoxia minimized the Ov-induced hypertrophy at days 5 and 12 but did not affect the hypertrophic response measured at day 56. Hypoxia early reduced the phosphorylation levels of mTOR and its downstream targets P70S6K and rpS6, but it did not affect the phosphorylation levels of Akt and 4E-BP1, in Ov muscles. The role played by specific inhibitors of mTOR, such as AMPK and hypoxia-induced factors (i.e., REDD1 and BNIP-3) was studied. REDD1 protein levels were reduced by overload and were not affected by hypoxia in Ov muscles, whereas AMPK was not activated by hypoxia. Although hypoxia significantly increased BNIP-3 mRNA levels at day 5, protein levels remained unaffected. The mRNA levels of the two atrogenes MURF1 and MAFbx were early increased by hypoxia in Ov muscles. In conclusion, hypoxia induced a transient alteration of muscle growth in this hypertrophic model, at least partly due to a specific impairment of the mTOR/P70S6K pathway, independently of Akt, by an undefined mechanism, and increased transcript levels for MURF1 and MAFbx that could contribute to stimulate the proteasomal proteolysis.

Adaptive change in electrically stimulated muscle: a framework for the design of clinical protocols

Adult mammalian skeletal muscles have a remarkable capacity for adapting to increased use. Although this behavior is familiar from the changes brought about by endurance exercise, it is seen to a much greater extent in the response to long-term neuromuscular stimulation. The associated phenomena include a markedly increased resistance to fatigue, and this is the key to several clinical applications. However, a more rational basis is needed for designing regimes of stimulation that are conducive to an optimal outcome. In this review I examine relevant factors, such as the amount, frequency, and duty cycle of stimulation, the influence of force generation, and the animal model. From these considerations a framework emerges for the design of protocols that yield an overall functional profile appropriate to the application. Three contrasting examples illustrate the issues that need to be addressed clinically.

Up-regulation of ferritin ubiquitination in skeletal muscle of transgenic rats bearing the G93A hmSOD1 gene mutation

In the present study we measured the levels of protein carbolnyls and the H and L subunits of ferritin in three hind limb muscles, [Extensor digitorum longus, Tibialis anterior and Soleus] of transgenic rats bearing the G93A hmSOD1 gene and of their non-transgenic littermates. All of the muscles from the transgenic animals showed significantly higher protein carbonyl levels, compared to the respective muscles from control non-transgenic animals. In two muscles (Tibialis anterior and Soleus) from transgenic rats, both L and H subunits of ferritin were upregulated. Moreover, we observed that the electrophoretic mobility of both ferritin subunits was retarded which indicates their post-translational modification. Ferritin immunoprecipitation experiments show an increased ubiquitination of both H and L ferritin in all muscles from the transgenic animals. Our data show for the first time that ferritin ubiquitination could be responsible for oxidative stress in muscles of rats bearing the G93A hmSOD1, consequently ferritin is not able to control the labile iron pool.

Effects of chronic electrical stimulation on paralyzed expiratory muscles

Following spinal cord injury, the expiratory muscles develop significant disuse atrophy characterized by reductions in their weight, fiber cross-sectional area, and force-generating capacity. We determined the extent to which these physiological alterations can be prevented with electrical stimulation. Because a critical function of the expiratory muscles is cough generation, an important goal was the maintenance of maximal force production. In a cat model of spinal cord injury, short periods of high-frequency lower thoracic electrical spinal cord stimulation (SCS) at the T(10) level (50 Hz, 15 min, twice/day, 5 days/wk) were initiated 2 wk following spinalization and continued for a 6-mo period. Airway pressure (P)-generating capacity was determined by SCS. Five acute, spinalized animals served as controls. Compared with controls, initial P fell from 43.9 +/- 1.0 to 41.8 +/- 0.7 cmH(2)O (not significant) in the chronic animals. There were small reductions in the weight of the external oblique, internal oblique, transverses abdominis, internal intercostal, and rectus abdominis muscles (not significant for each). There were no significant changes in the population of fast muscle fibers. Because prior studies (Kowalski KE, Romaniuk JR, DiMarco AF. J Appl Physiol 102: 1422-1428, 2007) have demonstrated significant atrophy following spinalization in this model, these results indicate that expiratory muscle atrophy can be prevented by the application of short periods of daily high-frequency stimulation. Because the frequency of stimulation is similar to the expected pattern of clinical use for cough generation, the daily application of electrical stimulation could potentially serve the dual purpose of maintenance of expiratory muscle function and airway clearance.

Lesions of rat skeletal muscle after local block of acetylcholinesterase and neuromuscular stimulation

In skeletal muscle, a local increase of acetylcholine (ACh) in a few end plates has been hypothesized to cause the formation of contraction knots that can be found in myofascial trigger points. To test this hypothesis in rats, small amounts of an acetylcholinesterase inhibitor [diisopropylfluorophosphate (DFP)] were injected into the proximal half of the gastrocnemius muscle, and the muscle nerve was electrically stimulated for 30-60 min for induction of muscle twitches. The distal half of the muscle, which performed the same contractions, served as a control to assess the effects of the twitches without DFP. Sections of the muscle were evaluated for morphological changes in relation to the location of blocked end plates. Compared with the distal half of the muscle, the DFP-injected proximal half exhibited significantly higher numbers of abnormally contracted fibers (local contractures), torn fibers, and longitudinal stripes. DFP-injected animals in which the muscle nerve was not stimulated and that were allowed to survive for 24 h exhibited the same lesions but in smaller numbers. The data indicate that an increased concentration of ACh in a few end plates causes damage to muscle fibers. The results support the assumption that a dysfunctional end plate exhibiting increased release of ACh may be the starting point for regional abnormal contractions, which are thought to be essential for the formation of myofascial trigger points.

Partial fast-to-slow conversion of regenerating rat fast-twitch muscle by chronic low-frequency stimulation

Chronic low-frequency stimulation (CLFS) of rat fast-twitch muscles induces sequential transitions in myosin heavy chain (MHC) expression from MHCIIb --> MHCIId/x --> MHCIIa. However, the 'final' step of the fast-to-slow transition, i.e., the upregulation of MHCI, has been observed only after extremely long stimulation periods. Assuming that fibre degeneration/regeneration might be involved in the upregulation of slow myosin, we investigated the effects of CLFS on extensor digitorum longus (EDL) muscles regenerating after bupivacaine-induced fibre necrosis. Normal, non-regenerating muscles responded to both 30- and 60-day CLFS with fast MHC isoform transitions (MHCIIb --> MHCIId --> MHCIIa) and only slight increases in MHCI. CLFS of regenerating EDL muscles caused similar transitions among the fast isoforms but, in addition, caused significant increases in MHCI (to approximately 30% relative concentration). Stimulation periods of 30 and 60 days induced similar changes in the regenerating bupivacaine-treated muscles, indicating that the upregulation of slow myosin was restricted to regenerating fibres, but only during an early stage of regeneration. These results suggest that satellite cells and/or regenerating fast rat muscle fibres are capable of switching directly to a slow program under the influence of CLFS and, therefore, appear to be more malleable than adult fibres.

A specific force deficit exists in skeletal muscle after partial denervation

Skeletal muscle demonstrates a specific force deficit after repair of injured peripheral nerves, microneurovascular muscle transfer, and normal aging. Because atrophy cannot account for deficits in specific force, other, unknown, mechanisms are responsible for the resulting muscle contractile dysfunction under these circumstances. We tested the hypothesis that a subpopulation of denervated fibers is partially or completely responsible for the specific force deficit after partial denervation of the rat extensor digitorum longus muscle (EDL). Adult Fisher rats underwent either sham exposure or partial transection of 80% of the cross-sectional area of the left deep peroneal nerve. After a 2-week recovery period, maximum isometric force (F(0)) was measured in situ and maximum specific force (sF(0)) was calculated for EDL from both control (n = 8) and partial denervation (n = 7) groups. Innervated fiber cross-sectional area (CSA(inn)) was measured directly from whole EDL cross sections after immunohistochemical labeling for neural cell adhesion molecule (NCAM), a marker of muscle fiber denervation. A corrected specific force value (sF(0-inn)) was calculated by normalizing F(0) to CSA(inn). Partial skeletal muscle denervation resulted in significant reductions in muscle mass, F(0), and sF(0). The percentage of muscle fibers expressing NCAM in the extrajunctional sarcolemma increased from 1.0 +/- 0.8% in control to 49 +/- 15% in partially denervated EDL muscles. A 62.7% deficit in EDL specific force was observed after partial denervation. Denervated muscle fibers accounted for 59.3% of this deficit, but sF(0-inn) still differed significantly between control and partially denervated muscles, with a 25.5% difference between groups. In partially denervated muscles, the specific force deficit is partially but not fully explained by a subpopulation of noncontractile, denervated fibers.

Combination of preconditioning and delayed flap elevation: evidence for improved perfusion and oxygenation of the latissimus dorsi muscle for cardiomyoplasty

BACKGROUND

Atrophy and fibrosis of the distal part of the latissimus dorsi muscle (LDM) wrap is a recognized complication of cardiomyoplasty that has been attributed to ischemia. Failure of the muscle wrap contributes to the late attrition seen in clinical cardiomyoplasty. In this study we examined the role of two-staged mobilization and of preconditioning by electrical stimulation on the regional perfusion and oxygenation of the LDM.

METHODS

In a rabbit model (n = 36) the LDM was preconditioned as follows: group A muscles received preconditioning in situ; group B muscles were partially mobilized by dividing the intercostal perforators and then preconditioned; and group C muscles were completely mobilized and wrapped around a silicone-rubber mandrel before conditioning. Controls received no conditioning. The preconditioning regimen consisted of 2 weeks of continuous stimulation at 2.5 Hz. At completion of preconditioning the muscles were fully mobilized and mounted on a muscle-testing apparatus. Purpose-built microelectrodes measured regional PO2 and perfusion using a diffusible gas tracer technique. Muscles were weighed and processed for fiber typing and capillary counting.

RESULTS

All preconditioned muscles demonstrated fiber transformation, with increased fatigue resistance. Perfusion of preconditioned muscles both at rest and during contraction was higher than control in the proximal part of the muscle. Distal regions of group B muscles had higher perfusion and capillary density than any other group (p < 0.05). Distal regions of group C had the lowest perfusion and capillary density, and showed muscle atrophy and histologic evidence of necrosis. During fatigue testing there was a decrease in the PO2 in the distal regions of the control and group C muscles (p < 0.05), whereas it was maintained at resting levels in both group A and B muscles.

CONCLUSIONS

Conditioning in situ improves perfusion of the distal LDM and prevents a fall in tissue PO2 during contraction. Two-stage mobilization further improves distal perfusion and capillary density. In contrast, shortterm elevation followed by conditioning produces impaired distal perfusion, decrease in PO2, and fiber necrosis in the distal muscle. The present study suggests that partial mobilization of the LDM performed at the same time as placement of electrodes for preconditioning may prepare the LDM better for the demands of cardiomyoplasty.

Fiber type-specific expression of major proteolytic systems in fast- to slow-transforming rabbit muscle

The present study investigates the role of two major proteolytic systems in transforming rabbit and rat muscles. The fast-to-slow transformation of rabbit muscle by chronic low-frequency stimulation (CLFS) induces fast-to-slow transitions of intact, mature fibers and replacement of degenerating fibers by newly formed slow fibers. Ubiquitination, an indicator of the ATP-dependent proteasome system, and calpain activity were measured in homogenates of control and stimulated extensor digitorum longus muscles. Calpain activity increased similarly (approximately 2-fold) in stimulated rat and rabbit muscles. CLFS had no effect on protein ubiquitination in rat muscle but led to elevations in ubiquitin protein conjugates in rabbit muscle. Immunohistochemistry was used to study the distribution of micro-calpain and m-calpain and of ubiquitinated proteins in myosin heavy chain-based fiber types. The findings suggest that both proteolytic systems are involved in fiber transformation and replacement. Transforming mature fibers displayed increases in micro-calpain and accumulation of ubiquitin protein conjugates. The majority of these fibers were identified as type IIA. Enhanced ubiquitination was also observed in degenerating and necrotic fibers. Such fibers additionally displayed elevated m-calpain levels. Conversely, p94, the skeletal muscle-specific calpain, decayed rapidly after stimulation onset and was hardly detectable after 4 days of CLFS.

Oxygenation and perfusion of rabbit tibialis anterior muscle subjected to different patterns of electrical stimulation

Dual amperometric microelectrodes were used to measure local pO2 and perfusion at multiple sites in the fast-twitch tibialis anterior muscles of anaesthetized rabbits. Six muscles were stimulated continuously at 10, 5, or 2.5 Hz. For all three frequencies, perfusion declined to about 50% of resting levels and recovered after stimulation. These changes corresponded to a rise followed by a fall in extracellular pO2. The highest levels of pO2 were reached during stimulation at 10 Hz. Eight muscles were stimulated tetanically at 100 Hz for 200 ms with duty cycles that were varied between 1.3 and 20.0%. Perfusion rose to 8.7 +/- 2.0 ml s(-1) 100 g(-1) at a duty cycle of 5% and declined with further increases in duty cycle. pO2 was depressed for duty cycles less than 10% but rose above resting levels at higher duty cycles. It is suggested that the paradoxical combination of elevated pO2 and depressed perfusion is attributable to stimulation conditions that exceed the oxygen transport capacity of a fast muscle.

Chronic contractile activity upregulates the proteasome system in rabbit skeletal muscle

Remodeling of skeletal muscle in response to altered patterns of contractile activity is achieved, in part, by the regulated degradation of cellular proteins. The ubiquitin-proteasome system is a dominant pathway for protein degradation in eukaryotic cells. To test the role of this pathway in contraction-induced remodeling of skeletal muscle, we used a well-established model of continuous motor nerve stimulation to activate tibialis anterior (TA) muscles of New Zealand White rabbits for periods up to 28 days. Western blot analysis revealed marked and coordinated increases in protein levels of the 20S proteasome and two of its regulatory proteins, PA700 and PA28. mRNA of a representative proteasome subunit also increased coordinately in contracting muscles. Chronic contractile activity of TA also increased total proteasome activity in extracts, as measured by the hydrolysis of a proteasome-specific peptide substrate, and the total capacity of the ubiquitin-proteasome pathway, as measured by the ATP-dependent hydrolysis of an exogenous protein substrate. These results support the potential role of the ubiquitin-proteasome pathway of protein degradation in the contraction-induced remodeling of skeletal muscle.

Changes in satellite cell content and myosin isoforms in low-frequency-stimulated fast muscle of hypothyroid rat

Chronic low-frequency stimulation was used to study the effects of enhanced contractile activity on satellite cell content and myosin isoform expression in extensor digitorum longus muscles from hypothyroid rats. As verified by immunohistochemical staining for desmin, vimentin, and myosin heavy chain (MHC) isoforms and by histological analysis, stimulation induced a transformation of existing fast fibers toward slower fibers without signs of fiber deterioration or regeneration. Immunohistochemically detected increases in MHC I and MHC IIa isoforms, as well as reduced numbers of fibers expressing the faster MHC isoforms, mirrored the rearrangement of the thick-filament composition. These changes, especially the upregulation of MHC IIa, were accompanied by an induction of developmental MHC isoforms in the transforming adult fibers. Satellite cell content rose 2.6-, 3.0-, and 3.7-fold over that of corresponding controls (P < 0.05 in all cases) in 5-, 10-, and 20-day-stimulated muscles, respectively. Hypothyroidism alone had no effect on satellite cell content but resulted in a significant reduction in fiber size. The relative satellite cell contents increased (P < 0.05) from 3.8% in euthyroid control muscles to 7.9, 11.5, and 13.8% in the 5-, 10-, and 20-day-stimulated hypothyroid muscles, respectively. In 20-day-stimulated muscles, the relative satellite cell content reached an almost twofold higher level than that of normal slow-twitch soleus muscle. This increase occurred concomitantly with a rise in myonuclear density, most probably because of the fusion of satellite cells with existing fibers.

The dose-related response of rabbit fast muscle to long-term low-frequency stimulation

Rabbit tibialis anterior muscles were stimulated continuously at 2.5 Hz, 5 Hz, or 10 Hz for 10 months. The resulting adaptive transformation was dose-related for contractile speed, myosin isoform composition, and enzyme activities. The "fast-oxidative" state produced by stimulation at 2.5 Hz was stable: even after 10 months, 84% of the fibers were of type 2A. Absence of a secondary decline in oxidative activity in these muscles provided strong evidence of a causal link between myosin transitions and metabolic adaptation. Significant fiber loss occurred only after prolonged stimulation at 10 Hz. The myosin isoform composition of individual muscles stimulated at 5 Hz resembled that of muscles stimulated at either the lower or the higher frequency, behavior consistent with a threshold for fiber type change. In clinical applications such as cardiomyoplasty, muscles could be used more effectively by engineering their properties to combine speed and power of contraction with the necessary resistance to fatigue.

Morphological and functional evidence, and clinical importance, of vascular anastomoses in the latissimus dorsi muscle of the sheep

Mobilisation of the latissimus dorsi muscle as a functional graft necessarily involves division of perforating arteries that enter the distal portion of the muscle, rendering it vulnerable to ischaemic damage when the muscle is stimulated electrically. Using a fluorescent microsphere technique we showed that the blood flow contributed by the thoracodorsal artery decreases in a proximal-to-distal direction, and that of the perforating arteries in a distal-to-proximal direction, but for neither does the flow decline to zero. This is consistent with earlier reports of anastomotic connections between the 2 arterial territories. We went on to use fluorescence microscopy to demonstrate the existence of these vascular anastomoses, the first such evidence obtained under physiological conditions of pressure and flow. In clinical applications, the existence of anastomotic connections offers the prospect of maintaining flow to the distal part of the grafted muscle without the delays inherent in neovascularisation procedures.

Electrical stimulation prevents immobilization atrophy in skeletal muscle of rabbits

OBJECTIVE

To investigate the effect of unilateral cast immobilization with and without surface electrical stimulation (ES) on the tibialis anterior (TA) muscle of rabbits.

DESIGN

Prospective randomized trial.

SETTING

University medical school.

ANIMALS

53 New Zealand White rabbits (aged 54 to 63 days, weight 1.73 to 1.91 kg). METHODS AND INTERVENTION: Random assignment, for a 3-week period, to one of four groups: C group (control group), I group (immobilization group), S group (group of electrical stimulation which was stimulated isometrically at 50 Hz, 30 minutes per day, 5 times a week), and IS group (immobilization group which, like the S-group, received electrical stimulation).

OUTCOME MEASURES

Muscle wet wight, muscle fiber cross-sectional area, muscle fiber types, and muscle capillary supply.

RESULTS

Muscle wet weight decreased significantly in the I group by 19% (p < or = .05), with a corresponding significant reduction in the total muscle fiber cross-sectional area of 26% (p < or = .05). No significant changes were observed in muscle wet weight and muscle fiber cross-sectional area in the S and IS groups. Interstitial fibrosis was observed in the I group and occasionally in the IS group. No significant changes in the total number of muscle fiber types I and II were found in all experimental groups. The capillary supply of the S and IS groups did not change significantly. However, capillary-to-fiber ratio was significantly reduced by 20% with a simultaneously nonsignificant increase in capillary density (capillaries/mm2) of 11% (p > .05) in the I group. Furthermore, muscle fiber regeneration was observed predominantly in the I group.

CONCLUSIONS

In this experimental model, ES effectively prevented immobilization-induced muscle atrophy by minimizing reduction of muscle fiber cross-sectional area, interstitial fibrosis, and impaired blood supply.

Vascular and stimulation delays in dynamic musculoplasty

BACKGROUND

Dynamic musculoplasty is increasingly being used to replace severely damaged muscle. Electrical stimulation induces a change in muscle composition from type II to type I muscle fibers. The aim of this study was to assess the optimal time to start stimulation and the beneficial effect of a vascular delay.

METHODS

Both gracilis muscles in rabbits (n = 3 x 6) were wrapped around a double polyurethane tube, and leads were implanted together with a neurostimulator. Right muscles were electrically stimulated for 11 weeks, and left muscles served as controls. In group 1 the muscle was stimulated immediately after transposition, in group 2 it was stimulated after 6 weeks, and in group 3 a vascular delay was performed and stimulation was started immediately after transposition.

RESULTS

During stimulation the percentage of type 1 muscle fibers increased from a mean of 4.6% to 41.9%. In group 1 there was significantly more change in muscle tissue. There were no significant differences in recorded pressures between groups. In group 1 the increase in amplitude during stimulation was significantly elevated (p < 0.05). The fatigue index in group 1, 259 (SE 130) seconds, was significantly lower than that of group 2, 515 (SE 102) seconds (p < 0.05).

CONCLUSIONS

Compared with the methods used in groups 2 and 3, immediate stimulation (group 1) gives more change in muscle tissue, demanding a higher increase in amplitude to achieve sufficient contraction. The fatigue index is also impaired. Thus vascular delay, as presently applied, does not improve muscle function.

High-energy phosphates and tension production in rabbit tibialis anterior/extensor digitorum longus muscles

The effects of repetitive muscle contraction on energy state and tension production were studied in rabbit tibialis anterior/extensor digitorum longus muscles that had been subjected to 90 days of continuous indirect electrical stimulation at 10 Hz. Anesthetized chronically stimulated and control rabbits were challenged with 15 min of stimulation at 4 and 15 tetani/min. Pi-to-phosphocreatine (PCr) ratio (Pi/PCr) was measured in vivo before, during, and after acute stimulation by 31P-magnetic resonance spectroscopy, and tension was recorded at the same time. Although Pi/PCr was low at rest, it was significantly higher in chronically stimulated muscle than in control muscle (0.20 +/- 0.02 vs. 0.05 +/- 0.01, P < 0.05). Stimulation of control muscle for 15 min at both 4 and 15 tetani/min induced a significant rise in Pi/PCr, whereas the same conditions in chronically stimulated muscle did not produce any significant departure from initial levels. The tension produced by control muscle fell to 93 +/- 3% of its initial value during stimulation at 4 tetani/min and to 61 +/- 7% at 15 tetani/min, respectively. In chronically stimulated muscle, on the other hand, tension was potentiated above its initial level at both stimulation rates (135 +/- 15 and 138 +/- 11%, respectively) and remained significantly elevated throughout each trial. The ability of chronically stimulated muscle to sustain high levels of activity with minimal perturbations in Pi/PCr or decrement in tension is attributable to cellular adaptations that include a well-documented increase in oxidative capacity.

Intermediate filament proteins increase during chronic stimulation of skeletal muscle

Chronic low-frequency electrical stimulation of rabbit fast-twitch skeletal muscle induces increased levels of two intermediate filament proteins, desmin and vimentin, during the first 3 weeks of stimulation. These increases occur over the same timecourse as reported shifts in alpha-actinin expression and increased Z-disc width, but precede the fast-to-slow shifts in contractile proteins, which have been described by others. Desmin and vimentin levels increase during the first 2 weeks of stimulation, at which time the increase in desmin appears to plateau while vimentin continues to increase significantly through 3 weeks of stimulation. Absolute amounts of vimentin are lower than desmin at all time points, however increases in desmin and vimentin levels are strongly correlated during the stimulation period, suggesting that the two proteins are coordinately increased during the initial phases of muscle transformation. We suggest that rapid increases in the expression of intermediate filament proteins, which coincide with alterations in Z-disc structure, may indicate a fortification of the force-bearing ultrastructure of the muscle fibre in response to the increased activity that is induced by stimulation. The presence of vimentin and elevated levels of desmin expression suggest that mature skeletal muscle reverts toward a developmental program of intermediate filament protein expression during fast-to-slow transformation.

Muscle growth in response to mechanical stimuli

The relative merits of the separate and combined uses of stretch and electrical stimulation at 10 Hz in influencing the rates of protein synthesis in vivo, proteolysis, and the growth of the extensor digitorum longus muscle have been investigated after 3 days in the rabbit. Continuous electrical stimulation failed to change muscle protein turnover or growth. Static stretch caused significant adaptive growth, with increases in c-fos, c-jun, and insulin-like growth factor I (IGF-I; 12-fold) mRNA levels, and protein (19%), RNA (128%), and DNA (45%) contents. Both the fractional (138%) and total (191%) rates of protein synthesis increased with stretch, correlating with increased ribosomal capacities. Combining stretch and electrical stimulation increased the mRNA concentration of IGF-I (40-fold). The adaptive growth was greater (35%), with massive increases in the nucleic acids (185 and 300%), ribosomal capacities (230%), and the rates of protein synthesis (345 and 450%). Large increases (i.e., 200-400%) in cathepsins B and L and dipeptidyl aminopeptidase I activities during stretch, with or without stimulation, suggest a role for these enzymes in tissue remodeling during muscle hypertrophy.

Morphological changes during fiber type transitions in low-frequency-stimulated rat fast-twitch muscle

This study investigates morphological adaptations of rat extensor digitorum longus muscle to chronic low-frequency stimulation (10 Hz, 10 h/d, up to 61 +/- 7d). During the early stimulation period (2-4 d), increased basophilia and accumulation of RNA were seen predominantly in type-IIB fibers. Putative satellite cell activation, as indicated by 3H-thymidine incorporation, was also evident during this phase. By 12 d, fiber composition remained unaltered, but there was a decrease in the cross-sectional area of the type-IIB fibers. Following 28 d of low-frequency stimulation, the percentage of type-IIB fibers decreased from 43 +/- 3% to 0%, while type-IID fibers increased from 30 +/- 3% to 60 +/- 6%. The fraction of type-IIA fibers tended to increase (controls 19 +/- 3%; stimulated 29 +/- 4%), whereas that of the type-I fibers was unaltered (4 +/- 1%). At this time, the cross-sectional area of type-IID fibers was unaltered, but that of type-IIA and type-I fibers increased. Further stimulation resulted in a return of type-IID fibers to control levels (23 +/- 5%), and a marked increase in type-IIA fibers (45 +/- 8%). The percentage of type-I fibers increased from 4 +/- 1% to 8 +/- 1%. Throughout each stage of chronic stimulation, there was no histological evidence of fiber degeneration and regeneration. These results indicate that, in contrast to the rabbit, chronic low-frequency stimulation-induced fiber conversion in the rat extensor digitorum longus muscle is entirely due to fiber transformation.

Stimulation-induced expression of slow muscle myosin in a fast muscle of the rat. Evidence of an unrestricted adaptive capacity

Fast muscles of the rat hind limb were stimulated continuously at 10 or 20 Hz for periods of 55-61 days by means of an implantable neuromuscular stimulator. Gel electrophoresis clearly demonstrated the presence in stimulated muscles of slow myosin light and heavy chains, although fast isoforms were still present in all cases. Thus, contrary to previous reports, induction of slow myosin isoforms does occur in this, as in other, mammalian species. The time course of the response to stimulation appears to be more extended than that seen in the rabbit.

Stimulation-induced damage in rabbit fast-twitch skeletal muscles: a quantitative morphological study of the influence of pattern and frequency

The aim of this study was to determine whether muscle fibre degeneration brought about by chronic low-frequency electrical stimulation was related to the pattern and frequency of stimulation. Rabbit fast-twitch muscles, tibialis anterior and extensor digitorum longus, were stimulated for 9 days with pulse trains ranging in frequency from 1.25 Hz to 10 Hz. Histological data from these muscles were analysed with multivariate statistical techniques. At the lower stimulation frequencies there was a significantly lower incidence of degenerating muscle fibres. Fibres that reacted positively with an antineonatal antibody were most numerous in the sections that revealed the most degeneration. The dependence on frequency was generally similar for the two muscles, but the extensor digitorum longus muscles showed more degeneration than the tibialis anterior at every frequency. Muscles subjected to 10 Hz intermittent stimulation showed significantly less degeneration than muscles stimulated with 5 Hz continuously, although the aggregate number of impulses delivered was the same. The incidence of degeneration in the extensor digitorum longus muscles stimulated at 1.25 Hz was indistinguishable from that in control, unstimulated muscles; for the tibialis anterior muscles, this was also true for stimulation at 2.5 Hz. We conclude that damage is not an inevitable consequence of electrical stimulation. The influence of pattern and frequency on damage should be taken into account when devising neuromuscular stimulation regimes for clinical use.