Differential expression of FGF receptors and of myogenic regulatory factors in primary cultures of satellite cells originating from fast (EDL) and slow (Soleus) twitch rat muscles.

In the rat, the fast and slow twitch muscles respectively Extensor digitorum longus (EDL) and Soleus present differential characteristics during regeneration. This suggests that their satellite cells responsible for muscle growth and repair represent distinct cellular populations. We have previously shown that satellite cells dissociated from Soleus and grown in vitro proliferate more readily than those isolated from EDL muscle. Fibroblast growth factors (FGFs) are known as regulators of myoblast proliferation and several studies have revealed a relationship between the response of myoblasts to FGF and the expression of myogenic regulatory factors (MRF) of the MyoD family by myoblasts. Therefore, we presently examined the possibility that the satellite cells isolated from EDL and Soleus muscles differ in the expression of FGF receptors (FGF-R) and of MRF expression. FGF-R1 and -R4 were strongly expressed in proliferating cultures whereas FGF-R2 and R3 were not detected in these cultures. In differentiating cultures, only -R1 was present in EDL satellite cells while FGF-R4 was also still expressed in Soleus cells. Interestingly, the unconventional receptor for FGF called cystein rich FGF receptor (CFR), of yet unknown function, was mainly detected in EDL satellite cell cultures. Soleus and EDL satellite cell cultures also differed in the expression MRFs. These results are consistent with the notion that satellite cells from fast and slow twitch muscles belong to different types of myogenic cells and suggest that satellite cells might play distinct roles in the formation and diversification of fast and slow fibres.     

Interactions between intrinsic regulation and neural modulation of acetylcholinesterase in fast and slow skeletal muscles

1. Initiation of subsynaptic sarcolemmal specialization and expression of different molecular forms of AChE were studied in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle of the rat under different experimental conditions in order to understand better the interplay of neural influences with intrinsic regulatory mechanisms of muscle cells. 2. Former junctional sarcolemma still accumulated AChE and continued to differentiate morphologically for at least 3 weeks after early postnatal denervation of EDL and SOL muscles. In noninnervated regenerating muscles, postsynaptic-like sarcolemmal specializations with AChE appeared (a) in the former junctional region, possibly induced by a substance in the former junctional basal lamina, and (b) in circumscribed areas along the whole length of myotubes. Therefore, the muscle cells seem to be able to produce a postsynaptic organization guiding substance, located in the basal lamina. The nerve may enhance the production or accumulation of this substance at the site of the future motor end plate. 3. Significant differences in the patterns of AChE molecular forms in EDL and SOL muscles arise between day 4 and day 10 after birth. The developmental process of downregulation of the asymmetric AChE forms, eliminating them extrajunctionally in the EDL, is less efficient in the SOL. The presence of these AChE forms in the extrajunctional regions of the SOL correlates with the ability to accumulate AChE in myotendinous junctions. The typical distribution of the asymmetric AChE forms in the EDL and SOL is maintained for at least 3 weeks after muscle denervation. 4. Different patterns of AChE molecular forms were observed in noninnervated EDL and SOL muscles regenerating in situ. In innervated regenerates, patterns of AChE molecular forms typical for mature muscles were instituted during the first week after reinnervation. 5. These results are consistent with the hypothesis that intrinsic differences between slow and fast muscle fibers, concerning the response of their AChE regulating mechanism to neural influences, may contribute to different AChE expression in fast and slow muscles, in addition to the influence of different stimulation patterns.     

Regenerated rat fast muscle transplanted to the slow muscle bed and innervated by the slow nerve,exhibits an identical myosin heavy chain repertoire to that of the slow muscle

 The hypothesis that the limited adaptive range observed in fast rat muscles in regard to expression of the slow myosin is due to intrinsic properties of their myogenic stem cells was tested by examining myosin heavy chain (MHC) expression in regenerated rat extensor digitorum longus (EDL) and soleus (SOL) muscles. The muscles were injured by bupivacaine, transplanted to the SOL muscle bed and innervated by the SOL nerve. Three months later, muscle fibre types were determined. MHC expression in muscle fibres was demonstrated immunohistochemically and analysed by SDS-glycerol gel electrophoresis. Regenerated EDL transplants became very similar to the control SOL muscles and indistinguishable from the SOL transplants. Slow type 1 fibres predominated and the slow MHC-1 isoform was present in more than 90% of all muscle fibres. It contributed more than 80% of total MHC content in the EDL transplants. About 7% of fibres exhibited MHC-2a and about 7% of fibres coexpressed MHC-1 and MHC-2a. MHC-2x/d contributed about 5–10% of the whole MHCs in regenerated EDL and SOL transplants. The restricted adaptive range of adult rat EDL muscle in regard to the synthesis of MHC-1 is not rooted in muscle progenitor cells; it is probably due to an irreversible maturation-related change switching off the gene for the slow MHC isoform. Accepted: 11 June 1996     

Diversity of myosin heavy chain expression in satellite cells from mouse soleus and EDL muscles.

Postnatal myoblasts, the satellite cells, originating from slow and fast skeletal muscle fibres differentiate and fuse into myotubes expressing different phenotype of myosin heavy chain (MyHC) isoforms. Little is known, however, of factors which establish and maintain this phenotypic diversity. We used immunofluorescent labelling and Western blotting to examine the expression of slow and fast MyHC isoforms in myotubes formed in vitro from satellite cells isolated from mouse fast twitch extensor digitorum longus (EDL) and slow twitch soleus muscles. Satellite cells were cultured in serum-rich growth medium promoting myoblast proliferation until cross-striated and self-contracting myotubes were formed. We report that in both cultures myotubes expressed slow as well as fast MyHC isoforms, but the level of slow MyHC was higher in soleus culture than in EDL culture. Hence, the pattern of expression of slow and fast MyHC was characteristic of the muscle fibre type from which these cells derive. These results support the concept of phenotypic diversity among satellite cells in mature skeletal muscles and suggest that this diversity is generated in vitro irrespectively of serum mitogens.     

Differential myogenicity of satellite cells isolated from extensor digitorum longus (EDL) and soleus rat muscles revealed in vitro

Following muscle damage, fast- and slow-contracting fibers regenerate, owing to the activation of their satellite cells. In rats, crush-induced regeneration of extensor digitorum longus (EDL, a fast muscle) and soleus (a slow muscle) present different characteristics, suggesting that intrinsic differences exist among their satellite cells. An in vitro comparative study of the proliferation and differentiation capacities of satellite cells isolated from these muscles is presented there. We observed several differences between soleus and EDL satellite cell cultures plated at high density on gelatin-coated dishes. Soleus satellite cells proliferated more actively and fused into myotubes less efficiently than EDL cells. The rate of muscular creatine kinase enzyme appeared slightly lower in soleus than in EDL cultures at day 11 after plating, when many myotubes were formed, although the levels of muscular creatine kinase mRNA were similar in both cultures. In addition, soleus cultures expressed higher levels of MyoD and myogenin mRNA and of MyoD protein than EDL satellite cell cultures at day 12. A clonal analysis was also carried out on both cell populations in order to determine if distinct lineage features could be detected among satellite cells derived from EDL and soleus muscles. When plated on gelatin at clonal density, cells from both muscles yielded clones within 2 weeks, which stemmed from 3–15 mitotic cycles and were classified into three classes according to their sizes. Myotubes resulting from spontaneous fusion of cells from the progeny of one single cell were seen regardless of the clone size in the standard culture medium we used. The proportion of clones showing myotubes in each class depended on the muscle origin of the cells and was greater in EDL- than in soleus-cell cultures. In addition, soleus cells were shown to improve their differentiation capacity upon changes in the culture condition. Indeed, the proportions of clones showing myotubes, or of cells fusing into myotubes in clones, were increased by treatments with a myotube-conditioned medium, with phorbol ester, and by growth on extra-cellular matrix components (Matrigel). These results, showing differences among satellite cells from fast and slow muscles, might be of importance to muscle repair after trauma and in pathological situations.     

Intramuscular adipose is derived from a non-Pax3 lineage and required for efficient regeneration of skeletal muscles

Ectopic accumulation of adipose in the skeletal muscle is associated with muscle wasting, insulin resistance and diabetes. However, the developmental origin of postnatal intramuscular adipose and its interaction with muscle tissue are unclear. We report here that compared to the fast EDL muscles, slow SOL muscles are more enriched with adipogenic progenitors and have higher propensity to form adipose. Using Cre/LoxP mediated lineage tracing in mice, we show that intramuscular adipose in both EDL and SOL muscles is exclusively derived from a Pax3(-) non-myogenic lineage. In contrast, inter-scapular brown adipose is derived from the Pax3(+) lineage. To dissect the interaction between adipose and skeletal muscle tissues, we used Myf5-Cre and aP2-Cre mice in combination with ROSA26-iDTR mice to genetically ablate myogenic and adipogenic cell lineages, respectively. Whereas ablation of the myogenic cell lineage facilitated adipogenic differentiation, ablation of the adipogenic cell lineage surprisingly impaired the regeneration of acutely injured skeletal muscles. These results reveal striking heterogeneity of tissue-specific adipose and a previously unappreciated role of intramuscular adipose in skeletal muscle regeneration.     

The effect of a unilateral muscle transplantation on the muscle fiber type and the MyHC isoform content in unoperated hind limb slow and fast muscles of the inbred Lewis rats

To reveal the effect of foreign innervation and altered thyroid status on fiber type composition and the myosin heavy chain (MyHC) isoform expression in the rat slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles, a method of heterochronous isotransplantation was developed. In this experimental procedure, the SOL or EDL muscles of young inbred Lewis rats are grafted either into the host EDL or SOL muscles of adult rats of the same strain with normal or experimentally altered thyroid status. To estimate the extent of fiber type transitions in the transplanted muscles, the SOL and EDL muscle from the unoperated leg and unoperated muscles from the operated leg could be legitimately used as controls, but only when the experimental procedure itself does not affect these muscles. To verify this assumption, we have compared the fiber type composition and the MyHC isoform content of unoperated contralateral SOL and EDL muscles and ipsilateral unoperated SOL muscle of experimental rats after unilateral isotransplantation into the host EDL muscle with corresponding muscles of the naive rats of the same age and strain. We provide compelling evidence that the unilateral heterochronous isotransplantation has no significant effect on the fiber type composition and the MyHC isoform content of unoperated muscles of experimental animals. Hence, these muscles can be used as controls in our grafting experiments.     

Stretch-induced myogenin, MyoD, and MRF4 expression and acute hypertrophy in quail slow-tonic muscle are not dependent upon satellite cell proliferation

 The objectives of these studies were to determine if (1) hypertrophy-stimulated myogenic regulatory factor (MRF) mRNA increases occur in the absense of proliferating satellite cells, and (2) acute hypertrophy occurs without satellite cell proliferation. Adult and aged quails were exposed to 0 or 2500 Rads gamma irradiation, and then wing muscles were stretch-overloaded for 3 or 7 days. MRF mRNA levels in stretch-overloaded and contralateral anterior latissimus dorsi (ALD) muscles were determined after 3 days; hypertrophy was determined after 7 days. The elimination of proliferating cells in irradiated muscles was verified histologically by bromodeoxyuridine incorporation. Relative levels of MRF4, MyoD, and myogenin mRNA were elevated 100%–400% in stretch-overloaded ALD muscles from irradiated adult quails indicating that satellite cell proliferation was not a prerequisite for MRF mRNA increases. Myogenin was the only MRF that exhibited mRNA increases that were lowered by irradiation. This suggests that satellite cells contribute only to myogenin mRNA increases in non-irradiated adult muscles following 3 days of stretch-overload. Stretch-overloaded ALD muscles from aged quails had a relative increase in myogenin mRNA of ∼150%. The myogenin increase was the same in non-irradiated and irradiated aged animals and also the same as that in stretch-overloaded muscles from irradiated adult quails. Together, these data indicate that attenuated increases in MRF expression in muscles from aged animals are attributable to lower satellite cell MRF expression. ALD muscle masses and protein contents in adult irradiated quails approximately doubled after 7 days of stretch-overload demonstrating hypertrophy despite the elimination of satellite cell proliferation. Received: 5 June 1998 / Accepted: 19 November 1998     

Differential morphofunctional characteristics and gene expression in fast and slow muscle of rats with monocrotaline-induced heart failure

Heart failure (HF) is characterized by limited exercise tolerance, skeletal muscle atrophy, a shift toward fast muscle fiber, and myogenic regulatory factor (MRF) changes. Reactive oxygen species (ROS) also contribute to target organ damage in this syndrome. In this study, we investigated and compared morphofunctional characteristics and gene expression in Soleus (SOL--oxidative and slow twitching muscle) and in Extensor Digitorum Longus (EDL--glycolytic and fast twitching muscle) during HF. Two groups of rats were used: control (CT) and heart failure (HF), induced by a single injection of monocrotaline. MyoD and myogenin gene expression were determined by RT-qPCR, and MHC isoforms by SDS-PAGE; muscle fiber type frequency and cross sectional area (CSA) were analyzed by mATPase. A biochemical study was performed to determine lipid hydroperoxide (LH), glutathione peroxidase (GSH-Px), and superoxide dismutase (SOD); myography was used to determine amplitude, rise time, fall time, and fatigue resistance in both muscles. HF showed SOL and EDL muscle atrophy in all muscle fiber types; fiber frequency decreased in type IIC and muscle contraction fall time increased only in SOL muscle. Myogenin mRNA expression was lower in SOL and myoD decreased in HF EDL muscle. LH increased, and SOD and GSH-Px activity decreased only in HF SOL muscle. HF EDL muscle did not present changes in MHC distribution, contractile properties, HL concentration, and antioxidant enzyme activity. In conclusion, our results indicate that monocrotaline induced HF promoted more prominent biochemical, morphological and functional changes in SOL (oxidative and slow twitching muscle). Although further experiments are required to better determine the mechanisms involved in HF pathophysiology, our results contribute to understanding the muscle-specific changes that occur in this syndrome.     

Distinct patterns of MMP-9 and MMP-2 activity in slow and fast twitch skeletal muscle regeneration in vivo

Skeletal muscles exhibit great plasticity and an ability to reconstruct in response to injury. However, the repair process is often inefficient and hindered by the development of fibrosis. We explored the possibility that during muscle repair, the different regeneration ability of the fast (extensor digitorum longus; EDL) and slow twitch (Soleus) muscles depends on the differential expression of metalloproteinases (MMP-9 and MMP-2) involved in the remodeling of the extracellular matrix. Our results show that MMP-9 and MMP-2 are present in the intact muscle and are up-regulated after crush-induced muscle injury. The expression and the activity of these two enzymes depend on the type of muscle and the phase of muscle regeneration. In the regenerating Soleus muscle, elevated levels of MMP-9 occurred during the myolysis and reconstruction phase. In contrast, regenerating EDL muscles exhibited decreased MMP-9 levels during myolysis and increased MMP-2 activity at the reconstruction phase. Moreover, satellite cells (mononuclear myoblasts) derived from Soleus and EDL muscles showed no differences in localization or activity of MMP-9 and MMP-2 during proliferation and differentiation in vitro. MMP-9 activity was present during all stages of myoblast differentiation, whereas MMP-2 activity reached its highest level during myoblast fusion. We conclude that MMPs are involved in muscle repair, and that fast and slow twitch muscles exhibit different patterns of MMP-9 and MMP-2 activity.     

Hindlimb suspension suppresses muscle growth and satellite cell proliferation

The effects of long-term hindlimb unweighting by tail suspension on postnatal growth of 20-day rat extensor digitorum longus (EDL) and soleus muscles were studied. Morphological assay indicated that radial growth of soleus myofibers was completely inhibited between 3 and 10 days of suspension and reduced thereafter, leading to a severe attenuation (-76% from control) over the total experimental period. Longitudinal growth rate, however, was accelerated 40% over weight-bearing controls. In addition, myofibers were arranged parallel to the long axis of the muscle, an orientation associated with chronologically younger muscles, suggesting morphological maturation of the soleus muscle had been delayed by suspension. In contrast, radial and longitudinal growth of EDL myofibers were minimally affected under similar conditions and remained within approximately 5% of control at all times. Suspension also influenced the normal changes that occur in satellite cell and myonuclear populations during postnatal growth. Both the number and proliferative activity of satellite cells were severely reduced in individual myofibers after only 3 days in both soleus and EDL muscles. The reduced number of satellite cells within 3 days of initiating hindlimb suspension appeared to be the result of their incorporation into myofibers while the long-lasting reduction appeared to be the added effects of decreased proliferative activity. In the soleus, this reduction in number and proliferation of satellite cells persisted throughout the experimental period and resulted in an overall 43% fewer myonuclei and 45% fewer satellite cells than control at 50 days of age. In contrast, both the total number and mitotic activity of satellite cells in the EDL rapidly returned to weight-bearing control levels by day 10 of suspension, resulting in no overall reduction in myonuclear accretion.     

Myosin Heavy Chain Profiles in Regenerated Fast and Slow Muscles Innervated by the Same Motor Nerve Become Nearly Identical

Plasticity of mature muscles exposed to different activation patterns is limited, probably due to restricted adaptive range of their muscle fibres. In this study, we tested whether satellite cells derived from slow muscles can give rise to a normal fast muscle, if transplanted to the fast muscle bed. Marcaine-treated rat soleus and extensor digitorum longus (EDL) muscles were transplanted to the EDL muscle bed and innervated by the EDL nerve. Six months later expression of myosin heavy chain isoforms was analysed by areal densities of fibres, binding specific monoclonal antibodies, and by SDS gel electrophoresis. Both regenerated muscles closely resembled each other. Their myosin heavy chain profiles were similar to those in fast muscles although they were not identical to that in the control EDL muscle. Since not even regenerated EDL was able to reach the myosin heavy chain isoform profile of mature EDL muscle, our experimental model did not permit studying the adaptive capacity of satellite cells in different muscles in its whole extent. However, the results favour the multipotential myoblast stem cell population in rat muscles and underline the importance of the extrinsic regulation of muscle phenotype.     

Age modifies respiratory complex I and protein homeostasis in a muscle type‐specific manner

Changes in mitochondrial function with age vary between different muscle types, and mechanisms underlying this variation remain poorly defined. We examined whether the rate of mitochondrial protein turnover contributes to this variation. Using heavy label proteomics, we measured mitochondrial protein turnover and abundance in slow‐twitch soleus (SOL) and fast‐twitch extensor digitorum longus (EDL) from young and aged mice. We found that mitochondrial proteins were longer lived in EDL than SOL at both ages. Proteomic analyses revealed that age‐induced changes in protein abundance differed between EDL and SOL with the largest change being increased mitochondrial respiratory protein content in EDL. To determine how altered mitochondrial proteomics affect function, we measured respiratory capacity in permeabilized SOL and EDL. The increased mitochondrial protein content in aged EDL resulted in reduced complex I respiratory efficiency in addition to increased complex I‐derived H₂O₂ production. In contrast, SOL maintained mitochondrial quality, but demonstrated reduced respiratory capacity with age. Thus, the decline in mitochondrial quality with age in EDL was associated with slower protein turnover throughout life that may contribute to the greater decline in mitochondrial dysfunction in this muscle. Furthermore, mitochondrial‐targeted catalase protected respiratory function with age suggesting a causal role of oxidative stress. Our data clearly indicate divergent effects of age between different skeletal muscles on mitochondrial protein homeostasis and function with the greatest differences related to complex I. These results show the importance of tissue‐specific changes in the interaction between dysregulation of respiratory protein expression, oxidative stress, and mitochondrial function with age.     

Apoptosis-inducing factor regulates skeletal muscle progenitor cell number and muscle phenotype

Apoptosis Inducing Factor (AIF) is a highly conserved, ubiquitous flavoprotein localized in the mitochondrial intermembrane space. In vivo, AIF provides protection against neuronal and cardiomyocyte apoptosis induced by oxidative stress. Conversely in vitro, AIF has been demonstrated to have a pro-apoptotic role upon induction of the mitochondrial death pathway, once AIF translocates to the nucleus where it facilitates chromatin condensation and large scale DNA fragmentation. Given that the aif hypomorphic harlequin (Hq) mutant mouse model displays severe sarcopenia, we examined skeletal muscle from the aif hypomorphic mice in more detail. Adult AIF-deficient skeletal myofibers display oxidative stress and a severe form of atrophy, associated with a loss of myonuclei and a fast to slow fiber type switch, both in "slow" muscles such as soleus, as well as in "fast" muscles such as extensor digitorum longus, most likely resulting from an increase of MEF2 activity. This fiber type switch was conserved in regenerated soleus and EDL muscles of Hq mice subjected to cardiotoxin injection. In addition, muscle regeneration in soleus and EDL muscles of Hq mice was severely delayed. Freshly cultured myofibers, soleus and EDL muscle sections from Hq mice displayed a decreased satellite cell pool, which could be rescued by pretreating aif hypomorphic mice with the manganese-salen free radical scavenger EUK-8. Satellite cell activation seems to be abnormally long in Hq primary culture compared to controls. However, AIF deficiency did not affect myoblast cell proliferation and differentiation. Thus, AIF protects skeletal muscles against oxidative stress-induced damage probably by protecting satellite cells against oxidative stress and maintaining skeletal muscle stem cell number and activation.     

Muscle satellite cells are a functionally heterogeneous population in both somite-derived and branchiomeric muscles

Skeletal muscles of body and limb are derived from somites, but most head muscles originate from cranial mesoderm. The resident stem cells of muscle are satellite cells, which have the same embryonic origin as the muscle in which they reside. Here, we analysed satellite cells with a different ontology, comparing those of the extensor digitorum longus (EDL) of the limb with satellite cells from the masseter of the head. Satellite cell-derived myoblasts from MAS and EDL muscles had distinct gene expression profiles and masseter cells usually proliferated more and differentiated later than those from EDL. When transplanted, however, masseter-derived satellite cells regenerated limb muscles as efficiently as those from EDL. Clonal analysis showed that functional properties differed markedly between satellite cells: ranging from clones that proliferated extensively and gave rise to both differentiated and self-renewed progeny, to others that divided minimally before differentiating completely. Generally, masseter-derived clones were larger and took longer to differentiate than those from EDL. This distribution in cell properties was preserved in both EDL-derived and masseter-derived satellite cells from old mice, although clones were generally less proliferative. Satellite cells, therefore, are a functionally heterogeneous population, with many occupants of the niche exhibiting stem cell characteristics in both somite-derived and branchiomeric muscles.     

Analysis of Rat Hindlimb Muscle Proteins by Two-Dimensional Gels Following Spinal Cord Injury

Proteins from extensor digitorum longus (EDL), plantaris (PLN), and soleus (SOL) muscles of adult, female rats were examined by high resolution two-dimensional gel electrophoresis up to 4 weeks following spinal cord transection. The electrophoretograms were analyzed by computer imaging and densitometry. Reproducible and significant changes in the relative concentrations of several proteins in each muscle type were detected. Whereas changes involving the largest number of proteins were observed in SOL, changes in EDL and PLN were also detected. In SOL, approximately 50% of the altered proteins increased in concentration and the remaining decreased: Actin and myosin light chains LCF-1 and LCF-2 were among those proteins whose concentrations increased, whereas myosin light chains LCS-1 and LCS-2 were among those proteins whose concentrations decreased. The present findings regarding the reversal in myosin light chain composition provide biochemical support for previously published data on changes in contractile properties of muscles following spinalization. In EDL, the relative concentration of only one protein was elevated in a time-dependent manner. The concentrations of two protein species in PLN were increased following cord transection. These findings provide new biochemical markers on the effects of spinal cord on gene expression in specific hindlimb skeletal muscles.     

Effect of denervation on the distribution and developmental transition of phosphoglycerate mutase and creatine phosphokinase isozymes in rat muscles of different fiber-type composition

Phosphoglycerate mutase (PGM) and creatine phosphokinase (CK) occur as three isozymes (types MM, MB and BB) in mammals and these exhibit similar transitions during skeletal muscle development. To study the influence of innervation on this transition and on the maintenance of the isozyme phenotype in mature muscle, we have determined the changes produced by sciatic neurectomy in neonatal and adult rat hindlimb muscles. In 40-day-old rats, denervation decreased both PGM and CK activity, the effect being more pronounced in the fast-twitch extensorum digitorum longus (EDL) and gastrocnemius muscles than in the slow-twitch soleus muscle. It also produced a progressive increase in the proportion of MB- and BB-PGM isozymes in EDL and gastrocnemius but not in soleus, and an increase of MB- and BB-CK isozymes in all three muscles. In 5-day-old rats, denervation prevented the developmental increase of PGM and CK activity in all three muscles. Denervation also prevented the normal decrease in the relative amounts of the MB and BB isozymes of both enzymes which occur during postnatal muscle development. These results can be explained by the different effects of denervation upon slow and fast muscles, and by the distinct distribution of PGM and CK isozymes in rat type I and II muscle fibers.     

Effect of exercise on muscle fibre composition and enzyme activities of skeletal muscles in young rats

Young Wistar rats underwent dynamic (D) or static (S) exercise from the 5th to 35th day after birth. Histochemical and biochemical analysis were performed in the extensor digitorum longus (EDL) and the soleus muscle (SOL). Lactate dehydrogenase (LDH) (regulating anaerobic metabolism) and citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HAD) (both regulating aerobic metabolism) activities were determined spectrophotometrically. An increase of the fast oxidative-glycolytic (FOG) muscle fibres was found in the slow SOL muscle in both trained groups, i.e. by 10% in group D and by 7% in group S in comparison with the C group. The EDL muscle fibre distribution did not differ from those of control animals in respect to the slow oxidative (SO) fibre type. A higher percentage of FOG fibres by 19% was found in group D contrary to a decreased number of the fast glycolytic (FG) muscle fibres in this trained group. The greatest increase of CS (EDL 185%, SOL 176%) and HAD (EDL 83%, SOL 178%) activities were found in group D as compared with control group (C). Only small differences were observed in LDH activity. The values of characteristic enzyme activity ratios show that dynamic training resulted in an elevation of oxidative capacity of skeletal muscle, while the static load led preferentially along the glycolytic pathway. It may be concluded that an adaptive response to the training load during early postnatal development is different due to the type of exercise (dynamic or static) and/or the type of skeletal muscle (fast or slow).     

Comparison of red and white muscle wet weight changed by age, denervation and morbid states

[Na]i, [K]i and wet weight of the extensor digitrum longus (EDL) and soleus (SOL) muscles of 9- and 52-week-old rats were measured for 7 days after sectioning of the sciatic nerve. The changes in wet weight of the EDL and SOL muscles of rats over 52 weeks and those of morbid state rats were also measured. There was no significant difference in wet weights between the EDL and SOL muscles in infant rats, but the EDL muscle became much heavier than the SOL muscle with aging. The decrease in rate of growth of wet weight of the EDL and SOL muscles caused by denervation, was greater in young rats than in mature rats. In addition, the rate of decrease was greater in the SOL muscles than in the EDL muscles in both young and mature rats. The [Na]i increased while [K]i was decreased by denervation, and the net Na+ increase and the net K+ loss were greater in young rats than in mature rats. The changing rate was more remarkable in the EDL muscles than in the SOL muscles throughout the aging process. During DOCA treatment over 4 weeks, the decrease of muscle wet weight was greater in the EDL muscles. The mechanisms which serve to maintain normal muscle wet weight in the SOL muscle after denervation or treatment with DOCA, were discussed.