Different metabolic responses to exercise training programmes in single rat muscle fibres

Summary The aim of this report is to elucidate the effects of exercise training on metabolic properties of different muscle fibre types of the rat hindlimb. Single muscle fibres were dissected from soleus (SOL) or extensor digitorum longus (EDL) muscles of Wistar strain male rats trained on a treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic). Then glycolytic and oxidative enzymes (CK, LDH, PFK, PK, SDH, and MDH) activities were measured biochemically. Slow,-type fibres (SO) were hypertrophied following endurance training and fast-twitch fibres (FOG and FG) were hypertrophied following sprint training. In EDL muscles the distribution of the slow-type fibres was reduced following the sprint training. The activity of glycolytic enzymes increased significantly in the fast-type fibres (FOG and FG) following sprint training, while oxidative enzymes activities increased in both fast (FOG and FG) and slow (SO) muscle fibres following the endurance training. Neither glycolytic nor oxidative enzymes' activities always increased equally in all types of fibre following exercise training. Consequently, the metabolic profiles in each type of single muscle fibre were affected differently by different intensities of exercise training. These results suggest that the functional (enzymes activity) and structural (muscle fibre hypertrophy) changes of skeletal muscle fibre following exercise training appeared gradually, and would be controlled by different factors.     

Differences in sodium voltage-gated channel properties according to myosin heavy chain isoform expression in single muscle fibres

The myosin heavy chain (MHC) isoform determines the characteristics and shortening velocity of muscle fibres. The functional properties of the muscle fibre are also conditioned by its membrane excitability through the electrophysiological properties of sodium voltage-gated channels. Macropatch-clamp is used to study sodium channels in fibres from peroneus longus (PL) and soleus (Sol) muscles (Wistar rats, n = 8). After patch-clamp recordings, single fibres are identified by SDS-PAGE electrophoresis according to their myosin heavy chain isoform (slow type I and the three fast types IIa, IIx, IIb). Characteristics of sodium currents are compared (Student's t test) between fibres exhibiting only one MHC isoform. Four MHC isoforms are identified in PL and only type I in Sol single fibres. In PL, maximal sodium current ( I _max), maximal sodium conductance ( g _Na,max) and time constants of activation and inactivation (τ _m and τ _h ) increase according to the scheme I→IIa→IIx→IIb ( P < 0.05). τ _m values related to sodium channel type and/or function, are similar in Sol I and PL IIb fibres ( P = 0.97) despite different contractile properties. The voltage dependence of activation ( V _a,1/2) shows a shift towards positive potentials from Sol type I to IIa, IIx and finally IIb fibres from PL ( P < 0.05). These data are consistent with the earlier recruitment of slow fibres in a fast-mixed muscle like PL, while slow fibres of postural muscle such as soleus could be recruited in the same ways as IIb fibres in a fast muscle.     

Muscle fatigue with prolonged exercise: contractile and biochemical alterations

Alterations in the contractile and biochemical properties of fast and slow skeletal muscle were studied in rats following a prolonged swim to exhaustion. The exercise produced glycogen depletion (less than 1 mg/g tissue) in muscles representative of all three fiber types; the isometric contractile properties were altered in the 84% type I soleus (SOL) and the 60% type IIa extensor digitorium longus (EDL) but not in the 100% type IIb superficial region of the vastus lateralis (SVL). Peak tetanic tension (Po) and the rate of tension development and decline all decreased after prolonged exercise in both the SOL and the EDL. The maximal isotonic shortening velocity was highly correlated with the myofibrillar ATPase activity, and both were relatively resistant to fatigue. Furthermore, the Ca2+ sensitivity of the myofibrils was unaffected by exercise in both fast and slow muscle. The Ca2+ uptake capacity of the sarcoplasmic reticulum (SR) was reduced in both the SOL and the fast-twitch type IIa deep region of the vastus lateralis, whereas the SR ATPase activity was unchanged. Our findings provide evidence that prolonged exercise produces alterations in contractile and biochemical properties of type I and IIa but not type IIb fibers and that muscle fatigue as measured by a decline in Po is not necessarily correlated with glycogen depletion.     

Ultrastructural and metabolic characteristics of single muscle fibres belonging to the same type in various muscles in rats

Summary Single muscle fibres from soleus (SOL) as well as extensor digitorum longus (EDL) muscles from Wistar male rats in relaxing solution were divided into three types by their histochemical features — slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), or fast-twitch glycolytic (FG) fibres. The relationship between ultrastructural profiles (mitochondrial volume, number, and Z-line width) and metabolic profiles (glycolytic and oxidative enzymes' activity) were analysed using the same types of fibres dissected from different SOL and EDL muscles using stereological and biochemical techniques. The Z-line width is specialized in different fibre types. Fast-twitch (FG and FOG) fibres have narrow Z-line width compared to slow-twitch (SO) fibre in SOL and EDL muscles. A significant difference was observed between SOL muscle SO and FOG fibres and EDL muscle SO and FOG fibres. All glycolytic (lactate dehydrogenase (LDH), phosphofructokinase (PFK), pyruvate kinase (PK) and creatine kinase (CK) activities in FOG fibres from EDL muscles were significantly higher (p<0.01) than those dissected from SOL muscles. The oxidative enzyme (succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) activity in SO and FOG fibres dissected from SOL muscle were significantly higher (p<0.01) than those dissected from EDL muscles. Mitochondrial volume and number in SO fibres dissected from SOL muscle were significantly higher (volume,p<0.01, number,p<0.01) than those dissected from EDL muscles. A significant difference was not observed in mitochondrial volume of FOG fibres between SOL and EDL muscles. Significant positive correlation was observed in FOG (p<0.05) and FG (p<0.01) fibres between mitochondrial volume and number dissected from EDL muscle.The results suggest that the same type of single muscle fibres in different muscles have different ultrastructural and metabolic profiles, and these profiles resembled those of the fibre types primarily constituting those muscles.     

Effect of hyperthyroidism on the contractile and histochemical properties of fast and slow twitch skeletal muscle in the rat

Male Wistar rats were made hyperthyroid by intraperitoneal injections on alternate days for 1–6 weeks, of 200 g/kg triiodothyronine (T₃). The effects of this treatment on the contractile properties of the soleus, a slow twitch and the extensor digitorum longus (EDL), a fast twitch skeletal muscle, were studied in vivo in the anaesthetized animal. Post mortem, serial frozen sections of both muscles were stained histochemically for myosin ATPase, succinic dehydrogenase and phosphorylase. Muscle fibres were classified as either slow twitch (SO), fast twitch oxidative glycolytic (FOG) or fast twitch glycolytic (FG).Elevation of plasma T₃ levels is associated with progressive alterations in the muscle fibre populations of both muscles. In the soleus there is conversion of SO to FOG fibres while in the EDL the main effect is FG to FOG conversion. There are also marked changes, mainly confined to the soleus muscle, in contractile properties; progressive increases in isometric twitch and tetanic tension and in the rates of contraction and relaxation during both twitch and tetanus.The effect of T₃ on slow muscle contractility may be related to its effect on muscle phenotype. However, changes in the former precede detectable fibre population changes. T₃ may influence properties such as the Ca²⁺ binding activity of sarcoplasmic reticulum of existing slow twitch fibres before the later changes associated with the interconversion of fibre types occur.     

Determination of metabolic profiles on single muscle fibres of different types

Summary Single muscle fibres separated from extensor digitorum longus (EDL) as well as soleus (SOL) in the Wistar strain male rat in relaxing solution were typed histochemically, then glycolytic and oxidative enzyme activities were determined on the same fibres. Glycolytic enzyme lactate dehydrogenase (LDH), phosphofructokinase (PFK), pyruvate kinase (PK) and creatine kinase (CK) showed highest activities in fast-twitch glycolytic (FG), lower in fast-twitch oxidative glycolytic (FOG) and lowest in slow-twitch oxidative (SO) fibres. Also oxidative enzyme succinate dehydrogenase (SDH) and malate dehydrogenase (MDH) showed highest activities in SO, lower in FOG and lowest in FG fibres. The activities of LDH, PFK, PK and CK in FOG fibres separated from EDL showed higher activity compared to those separated from SOL, whereas the opposite result was obtained for the activities of SDH and MDH. Enzyme activities in a single muscle fibre type were not distinguishable from those of another type, and the activity profiles overlapped over a considerable range. The correlations among the separate enzyme activities of CK, LDH and MDH obtained from the same single fibre overlapped over a considerable range.     

The expression of NFATc1 in adult rat skeletal muscle fibres

Although numerous studies have recently implicated the calcineurin-nuclear factor of activated T-cells (Cn-NFAT) signalling pathway in the regulation of activity-dependent fibre type switching in adult mammalian skeletal muscles, little is known about the endogenous expression of NFAT proteins in the various fibre types present in these muscles. In this study, the immunolocalization of NFATc1 (also known as NFATc or NFAT2) in the extensor digitorum longus (EDL; a mainly fast-twitch muscle) and the soleus (a predominantly slow-twitch muscle) muscles of adult ( approximately 90-day-old) Wistar rats was investigated. The results show that NFATc1 is expressed only in oxidative fibres (i.e. type I and type IIA fibres) that stain intensely for succinate dehydrogenase activity irrespective of whether they are from the fast- or slow-twitch muscle. Thus, 99 +/- 4% (n = 7 rats) of the muscle fibres in the soleus and 42 +/- 2% (n = 7 rats) of those in the EDL expressed NFATc1. In the soleus muscle fibres, NFATc1 was localized mainly in the fibre nuclei, whereas in the EDL fibres it was localized in both the cytoplasm and the nuclei. However, no difference in its localization was observed between type I and type IIA fibres in both muscles. Western blot experiments showed that the soleus expressed more NFATc1 proteins than the EDL. From these results, we suggest that NFATc1 controls the number and distribution of both type I and type IIA fibres, as well as the oxidative capacity of adult mammalian skeletal muscles.     

Effects of running exercise with increasing loads on tibialis anterior muscle fibres in mice

Cross-sectional areas and succinate dehydrogenase (SDH) activities of type identified fibres in the deep, middle and superficial regions of the tibialis anterior muscle in mice were examined after 4 weeks of voluntary running exercise with increasing loads. Nineteen-week-old male mice were assigned randomly to either a control or exercise group. The mean cross-sectional areas of all types (IIa, IIx and IIb) of fibres in the superficial region of the muscle were greater in the exercise group than in the control group. The mean SDH activities of type IIx and type IIb fibres in the middle region and of all types (IIa, IIx and IIb) of fibres in the superficial region of the muscle were greater in the exercise group than in the control group. These results suggest that voluntary running exercise with increasing loads causes hypertrophy and/or an increase in the SDH activity of fibres in the specific muscle region where fibres with a high threshold and a low-oxidative enzyme activity are distributed, and these fibres are recruited to adapt to changes in exercise conditions.     

Morphofunctional responses to anaemia in rat skeletal muscle

Adult male Sprague-Dawley rats were randomly assigned to two groups: control and anaemic. Anaemia was induced by periodical blood withdrawal. Extensor digitorum longus and soleus muscles were excised under pentobarbital sodium total anaesthesia and processed for transmission electron microscopy, histochemical and biochemical analyses. Mitochondrial volume was determined by transmission electron microscopy in three different regions of each muscle fibre: pericapillary, sarcolemmal and sarcoplasmatic. Muscle samples sections were also stained with histochemical methods (SDH and m-ATPase) to reveal the oxidative capacity and shortening velocity of each muscle fibre. Determinations of fibre and capillary densities and fibre type composition were made from micrographs of different fixed fields selected in the equatorial region of each rat muscle. Determination of metabolites (ATP, inorganic phosphate, creatine, creatine phosphate and lactate) was done using established enzymatic methods and spectrophotometric detection. Significant differences in mitochondrial volumes were found between pericapillary, sarcolemmal and sarcoplasmic regions when data from animal groups were tested independently. Moreover, it was verified that anaemic rats had significantly lower values than control animals in all the sampled regions of both muscles. These changes were associated with a significantly higher proportion of fast fibres in anaemic rat soleus muscles (slow oxidative group = 63.8%; fast glycolytic group = 8.2%; fast oxidative glycolytic group = 27.4%) than in the controls (slow oxidative group = 79.0%; fast glycolytic group = 3.9%; fast oxidative glycolytic group = 17.1%). No significant changes were detected in the extensor digitorum longus muscle. A significant increase was found in metabolite concentration in both the extensor digitorum longus and soleus muscles of the anaemic animals as compared to the control group. In conclusion, hypoxaemic hypoxia causes a reduction in mitochondrial volumes of pericapillary, sarcolemmal, and sarcoplasmic regions. However, a common proportional pattern of the zonal distribution of mitochondria was maintained within the fibres. A significant increment was found in the concentration of some metabolites and in the proportion of fast fibres in the more oxidative soleus muscle in contrast to the predominantly anaerobic extensor digitorum longus.     

Oxidative phenotype protects myofibers from pathological insults induced by chronic heart failure in mice

The fiber specificity of skeletal muscle abnormalities in chronic heart failure (CHF) has not been defined. We show here that transgenic mice (8 weeks old) with cardiac-specific overexpression of calsequestrin developed CHF (50.9% decrease in fractional shortening and 56.4% increase in lung weight, P<0.001), cachexia (37.8% decrease in body weight, P<0.001), and exercise intolerance (69.3% decrease in running distance to exhaustion, P<0.001) without a significant change in muscle fiber-type composition. Slow oxidative soleus muscle maintained muscle mass, whereas fast glycolytic tibialis anterior and plantaris muscles underwent atrophy (11.6 and 13.3%, respectively; P<0.05). In plantaris muscle, glycolytic type IId/x and IIb, but not oxidative type I and IIa, fibers displayed significant decreases in cross-sectional area (20.3%, P<0.05). Fast glycolytic white vastus lateralis muscle showed sarcomere degeneration and decreased cytochrome c oxidase IV (39.5%, P<0.01) and peroxisome proliferator-activated receptor gamma co-activator 1alpha protein expression (30.3%, P<0.01) along with a dramatic induction of the MAFbx/Atrogin-1 mRNA. These findings suggest that exercise intolerance can occur in CHF without fiber type switching in skeletal muscle and that oxidative phenotype renders myofibers resistant to pathological insults induced by CHF.     

Limited oxygen diffusion accelerates fatigue development in mouse skeletal muscle

Isolated whole skeletal muscles fatigue more rapidly than isolated single muscle fibres. We have now employed this difference to study mechanisms of skeletal muscle fatigue. Isolated whole soleus and extensor digitorum longus (EDL) muscles were fatigued by repeated tetanic stimulation while measuring force production. Neither application of 10 m m lactic acid nor increasing the [K⁺] of the bath solution from 5 to 10 m m had any significant effect on the rate of force decline during fatigue induced by repeated brief tetani. Soleus muscles fatigued slightly faster during continuous tetanic stimulation in 10 m m [K⁺]. Inhibition of mitochondrial respiration with cyanide resulted in a faster fatigue development in both soleus and EDL muscles. Single soleus muscle fibres were fatigued by repeated tetani while measuring force and myoplasmic free [Ca²⁺] ([Ca²⁺]_i). Under control conditions, the single fibres were substantially more fatigue resistant than the whole soleus muscles; tetanic force at the end of a series of 100 tetani was reduced by about 10% and 50%, respectively. However, in the presence of cyanide, fatigue developed at a similar rate in whole muscles and single fibres, and tetanic force at the end of fatiguing stimulation was reduced by ∼80%. The force decrease in the presence of cyanide was associated with a ∼50% decrease in tetanic [Ca²⁺]_i, compared with an increase of ∼20% without cyanide. In conclusion, lactic acid or [K⁺] has little impact on fatigue induced by repeated tetani, whereas hypoxia speeds up fatigue development and this is mainly due to an impaired Ca²⁺ release from the sarcoplasmic reticulum.     

Profiles of creatine kinase isoenzyme compositions in single muscle fibres of different types

Summary Creatine kinase (CK) isoenzyme compositions of different types of single muscle fibres dissected from soleus (SOL) and extensor digitorum longus (EDL) muscles from rats were examined. CK isoenzymes were separated into cytoplasmic (CK-MM, CK-MB, CK-BB) (muscle, brain and hybrid types, respectively) and mitochondrial (m-CK) isoenzymes. Total CK and CK-MM activities showed the highest activities in fast-twitch glycolytic fibres (FG), lower in fast-twitch oxidative glycolytic (FOG) and the lowest in slow-twitch oxidative (SO) fibres. Conversely, the activity of m-CK was highest in SO, lowest in FG and intermediate in FOG fibres. The activity of CK-MB was highest in SO and lower in FOG and FG fibres. However, the activities of total CK and CK isoenzymes in a single muscle fibre type were not distinguishable from those of another type, and the profiles of CK isoenzyme compositions from the same type of single muscle fibres overlapped over a considerable range. The relationships between the four CK isoenzymes activities in single muscle fibres of different types were not similar. These results suggest that CK isoenzymes of single muscle fibres of different types play different roles in intracellular energy metabolism. Therefore, it is supposed that the CK isoenzyme compositions of single muscle fibres are suitable for their contractive and metabolic properties.     

Effects on skeletal muscle fibres of diabetes and Ginkgo biloba extract treatment

Combined cytophotometric and morphometric analysis of muscle fibre properties and myosin heavy chain electrophoresis were performed on extensor digitorum longus and soleus muscles from healthy rats and rats with streptozotocin-induced diabetes. Moreover, the protective effect of Ginkgo biloba extract, a potent oxygen radical scavenger, on diabetic muscles was investigated. Changes in fibre type-related enzyme activities, fibre type distribution, fibre cross areas and myosin isoforms were found. In muscles of diabetic rats, a metabolic shift was measured mainly in fibres with oxidative metabolism. Fast-oxidative glycolytic fibres showed a shift to more glycolytic metabolism and about a third transformed into fast-glycolytic fibres. Slow-oxidative fibres became more oxidative. Fibre atrophy was measured in diabetic muscles dependent on fibre type and muscle. Different fibre types atrophied to a different degree. Therefore, a decreased area percentage of slow fibres and an increased area percentage of fast fibres of the whole muscle cross section in both muscles were found. This is supported by reduced slow and increased fast myosin heavy chain isoforms. These alterations of diabetic muscle fibres could be due to less motion of diabetic rats and diabetic neuropathy. After treatment with Ginkgo biloba extract, enzyme activities were increased mainly in oxidative fibres of diabetic muscles, which was interpreted as protective effect. Generally, the soleus muscle with predominant oxidative metabolism was more vulnerable to diabetic alterations and Ginkgo biloba extract treatment than the extensor digitorum longus muscle with predominant glycolytic metabolism.     

Non-crossbridge calcium-dependent stiffness in slow and fast skeletal fibres from mouse muscle

We showed previously that force development in frog and FDB mouse skeletal muscle fibres is preceded by an increase of fibre stiffness occurring well before crossbridge attachment and force generation. This stiffness increase, referred to as static stiffness, is due to a Ca²⁺-dependent stiffening of a non-crossbridge sarcomere structure which we suggested could be attributed to the titin filaments. To investigate further the role of titin in static stiffness, we measured static stiffness properties at 24 and 35°C in soleus and EDL mouse muscle fibres which are known to express different titin isoforms. We found that static stiffness was present in both soleus and EDL fibres, however, its value was about five times greater in EDL than in soleus fibres. The rate of development of static stiffness on stimulation increased with temperature and was slightly faster in EDL than in soleus in agreement with previously published data on the time course of the intracellular Ca²⁺ transients in these muscles. The present results show that the presence of a non-crossbridge Ca²⁺-dependent stiffening of the muscle fibre is a physiological general characteristic of skeletal muscle. Static stiffness depends on fibre type, being greater and developing faster in fast than in slow fibres. Our observations are consistent with the idea that titin stiffening on contraction improves the sarcomere structure stability. Such an action in fact seems to be more important in EDL fast fibre than in soleus slow fibres.     

The role of neural and mechanical influences in maintaining normal fast and slow muscle properties

The relative importance of neural and mechanical influences in maintaining normal slow and fast muscle properties remains unclear. To address this issue, we studied the effects of 10 days of hindlimb unloading (HU) with or without tenotomy and/or denervation on the cross-sectional area (CSA), myosin heavy chain (MHC) expression (immunohistochemistry) and composition (gel electrophoresis), and myonuclear number in soleus and plantaris fibers in adult male Wistar rats. In general, the adaptations in fiber type and size were similar using either single fiber gel or immunohistochemical analyses. HU resulted in atrophy of type I and I+IIa/x MHC fibers in the soleus and in type I, I+IIa/x, IIa/x, IIa/x+IIb, and IIb MHC fibers in the plantaris. Addition of tenotomy and/or denervation in HU rats had minimal effects on fiber CSA in the soleus, but fiber CSA in the plantaris further decreased, particularly in fibers expressing only fast MHCs. HU resulted in a de novo appearance of type I+IIa/x+IIb and IIa/x+IIb MHC fibers in the soleus and of type I+IIa/x+IIb MHC fibers in the plantaris.Tenotomy and/or denervation in HU rats had no further effect on the fiber type composition of either muscle. Mean myonuclear number/mm of type I fibers was decreased in the soleus of HU rats, and increased in type I and I+IIa/x fibers in HU plus tenotomy (HU+Ten) rats. In the plantaris, mean myonuclear number/mm of type IIa/x, IIa/x+IIb, and IIb fibers was lower after HU with or without tenotomy and/or denervation. Mean cytoplasmic volume/myonucleus ratio of type I and I+IIa/x fibers in the soleus of the HU group tended to be smaller than in controls. The largest decrease was noted in the HU+Ten group. In the plantaris, this ratio was unaffected by HU alone, but was decreased by addition of tenotomy and/or denervation when all fiber types were combined. These data indicate that the major cause of fiber atrophy and adaptations in myonuclear domain size in the slow soleus of HU rats is the chronic reduction in force generation, whereas the elimination of neuromuscular contact via denervation results in additional fiber atrophy and adaptations in myonuclear domain size in the fast plantaris.     

High temperature does not alter fatigability in intact mouse skeletal muscle fibres

Intense activation of skeletal muscle results in fatigue development, which involves impaired function of the muscle cells resulting in weaker and slower contractions. Intense muscle activity also results in increased heat production and muscle temperature may rise by up to ∼6°C. Hyperthermia is associated with impaired exercise performance in vivo and recent studies have shown contractile dysfunction and premature fatigue development in easily fatigued muscle fibres stimulated at high temperatures and these defects were attributed to oxidative stress. Here we studied whether fatigue-resistant soleus fibres stimulated at increased temperature show premature fatigue development and whether increasing the level of oxidative stress accelerates fatigue development. Intact single fibres or small bundles of soleus fibres were fatigued by 600 ms tetani given at 2 s intervals at 37°C and 43°C, which is the highest temperature the muscle would experience in vivo . Tetanic force in the unfatigued state was not significantly different at the two temperatures. With 100 fatiguing tetani, force decreased by ∼15% at both temperatures; the free cytosolic [Ca²⁺] (assessed with indo-1) showed a similar ∼10% decrease at both temperatures. The oxidative stress during fatigue at 43°C was increased by application of 10 μ m hydrogen peroxide or tert-butyl hydroperoxide and this did not cause premature fatigue development. In summary, fatigue-resistant muscle fibres do not display impaired contractility and fatigue resistance at the highest temperature that mammals, including humans, would experience in vivo . Thus, intrinsic defects in fatigue-resistant muscle fibres cannot explain the decreased physical performance at high temperatures.     

Differentiation of rat skeletal muscle fibres during development and ageing

The purpose of the present study was to determine at which point in the period from embryonic day 21 up to postnatal day (PD) 75, the different fibre types and subtypes are detectable in rat extensor digitorum longus, soleus and gastrocnemius muscles using immunohistochemical, enzyme histochemical and cytophotometrical methods. Moreover, fibre type-specific changes in metabolic profile and changes in fibre type population during postnatal development were analysed. Before birth, no clear differentiation of fibre types was found. At PD 1, slow and fast fibres were typed by antibodies against neonatal, slow and fast myosin heavy chains (MHCs). At PD 8, the different ATPase activities of slow and fast MHCs after alkaline preincubation were detected histochemically. At PD 21, differences in acid stability of ATPase activity of fast MHC isoforms revealed the fast subtypes IIA and IIB (including IIX). At this age, also differences in metabolic properties (oxidative and glycolytic enzyme activities) of fibres were detected for the first time by cytophotometry classifying the fibres into SO, FOG I, FOG II and FG. Before the age of 21 days, the fast fibres were metabolically undifferentiated. During further development and ageing, the population of FG fibres with high glycolytic activity increased at the expense of FOG fibres suggesting FOG to FG transformation. Cytophotometrical measurements revealed that the muscle fibres developed their highest contractile, oxidative and glycolytic activity at PD 21, the time of weaning. In this way, muscle fibres may be prepared for the higher demands for posture and mobility after leaving the nest.     

Ultrastructural and metabolic profiles on single muscle fibers of different types after hindlimb suspension in rats

The relationships between ultrastructural and metabolic profiles in different types of single muscle fiber after hindlimb suspension in rats were examined. Glycolytic (lactate dehydrogenase, LDH; phosphofructokinase, PFK) and oxidative (succinate dehydrogenase, SDH; malate dehydrogenase, MDH) enzyme activities in extensor digitorum longus (EDL) and soleus (SOL) muscles were measured. Relative mitochondrial and lipid droplet volumes were also measured in single muscle fiber of different types. Glycolytic enzyme activity in EDL muscle and oxidative enzyme activity in soleus muscle decreased following suspension for 2 weeks. LDH and PFK activities in fast-twitch (FG, fast-twitch glycolytic; FOG, fast-twitch oxidative glycolytic) fibers and oxidative enzymes in FOG and FG fibers decreased following suspension. Relative mitochondrial volume decreased significantly in all types (SO, slow-twitch oxidative; FOG, and FG) of fibers following suspension. The mitochondrial volume in SO fiber of the control group was significantly (p less than 0.01) higher than that of suspended group; however, SDH and MDH activities were not different between the control and suspended groups. The structural and metabolic changes following hindlimb suspension were influenced by different factors, respectively. Changes in ultrastructural and metabolic profiles in response to the hindlimb suspension differed according to the type of fibers.