Exercise-induced fibre type transitions with regard to myosin,parvalbumin, and sarcoplasmic reticulum in muscles of the rat

Effects of a long-term, high intensity training program upon histochemically assessed myofibrillar actomyosin ATPase, myosin composition, peptide pattern of sarcoplasmic reticulum (SR), and parvalbumin content were analysed in muscles from the same rats which were used in a previous study (Green et al. 1983). Following 15 weeks of extreme training, an increase in type I and type IIA fibres and a decrease in type IIB fibres occurred both in plantaris and extensor digitorum longus (EDL) muscles. In the deep portion of vastus lateralis (VLD), there was a pronounced increase from 10±5% to 27±11% in type I fibres. No type I fibres were detected in the superficial portion of vastus lateralis (VLS) both in control and trained animals. An increase in slow type myosin light chains accompanied the histochemically observed fibre type transition in VLD. Changes in the peptide pattern of SR occurred both in VLS and VLD and suggested a complete transition from type IIB to IIA in VLS and from type IIA to I in VLD. A complete type IIA to I transition in the VLD was also suggested by the failure to detect parvalbumin in this muscle after 15 weeks of training. Changes in parvalbumin content and SR tended to precede the transitions in the myosin light chains. Obviously, high intensity endurance training is capable of transforming specific characteristics of muscle fibres beyond the commonly observed changes in the enzyme activity pattern of energy metabolism. The time courses of the various changes which are similar to those in chronic nerve stimulation experiments, indicate that various functional systems of the muscle fibre do not change simultaneously.     

Adrenaline-mediated glycogenolysis in different skeletal muscle fibre types in the anaesthetized rat

The effect of adrenaline infusion on glycogen breakdown in different muscle fibres types in resting extensor digitorum longus (EDL) and soleus was investigated with histochemical methods. During adrenaline infusion the glycogen content in type IIB and type IIA fibres in EDL, as measured in PAS-stained sections, decreased 24.5% and 11.5% respectively. The glycogen content in type I fibres in EDL and in type I, type IIA and T-fibres in soleus did not change during adrenaline infusion. The present study shows that adrenaline infusion has different effects on glycogen breakdown in the different fibre types in EDL and a different effect on type IIA fibres in EDL and soleus. So far, the reason for these differences is unknown.     

Cortisone-induced changes in myosin heavy chain distribution in respiratory and hindlimb muscles

In this study the effects of administration of cortisone acetate (100 mg kg^-1 body weight subcutaneously for 11 days) on distribution and cross-sectional area of different fibre types of rat skeletal muscles were investigated. Diaphragm, parasternal intercostal (PI), extensor digitorum longus (EDL) and soleus muscles were examined in cortisone treated animals (CA) in comparison with ad libitum controls (CTRL) and pair-fed (PF) controls. Four fibre types (I or slow and IIA, IIX, IIB or fast) were identified on the basis of their myosin heavy chain composition using a set of monoclonal antibodies. In CA rats the reduction of cross-sectional area was above 30% in IIX fibres of diaphragm, IIB fibres of PI and in all fast fibres of EDL. In all muscles slow fibres were spared from atrophy. Significant variations in fibre type distribution were found in the muscles of CA rats when compared to CTRL. The percentage of IIB fibres decreased in EDL, PI and diaphragm. This decrease was accompanied by an increase in the percentage of IIA fibres in the same muscles. No changes in the percentage of slow fibres and of fast IIX fibres were observed in EDL, PI and diaphragm of CA rats in comparison with CTRL. In soleus of CA rats the proportion of IIA fibres was lower than in CTRL. In EDL of PF rats atrophy of IIA fibres and changes in fibre type distribution were similar to those observed in CA rats. In diaphragm, PI and soleus of PF rats no significant decrease in fibre cross-sectional area nor significant changes in fibre distribution were found in comparison with CTRL rats.     

Influence of the interval between removal and freezing of muscle samples on muscle fibre size with special reference to sample size and fibre type

To study the influences of the interval between removal and freezing of a muscle sample on fibre size, rat hindlimb muscles (soleus and extensor digitorum longus, EDL) were cut into three or five pieces and frozen at different intervals, cross-sectioned, stained and analysed morphometrically. As in a previous study (Larsson & Skogsberg 1988), a pronounced interval-dependent change in soleus muscle fibre size was observed, the fibres in soleus muscle samples frozen immediately (within 15 s) being significantly (P less than 0.01-0.001) larger than those samples frozen 4 or 10 min after removal. However, the interval-dependent change was only noted in small muscle samples, i.e. those where the maximum muscle fibre length was 6.5 mm, while samples in which the fibres were approximately twice this length showed no change. In the slow-twitch soleus, type I fibres predominate, and type II fibres were often too few in one or more cross-sections in each animal to allow reliable measurements of fibre size. The fast-twitch EDL was therefore studied, and the results indicated an interval-dependent change in fibre size which was differentiated according to enzyme-histochemical type, i.e. there was a significant change in type I and IIA fibres but not in type IIB. It is concluded that the interval between removal and freezing of small muscle samples, as in percutaneous muscle biopsies in man, is of vital importance for fibre size, and that this interval should be at least 2-4 min.     

Glycogen depletion in intrafusal fibres in rats during short-duration high-intensity treadmill running

AIM: The recruitment patterns of the intrafusal and extrafusal fibres in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats were investigated during brief-intensity exercise by assaying their glycogen content histochemically. METHODS: Six adult male rats were assigned to each of four groups that ran up a 6 degrees incline on a motor-driven treadmill, at 40 m min(-1) for either 0.5, 1, 2, or 4 min. Six adult male rats in the control group did not run. Extrafusal and intrafusal fibres were classified by myosin ATPase staining. Optical densities for glycogen content were evaluated in serial periodic acid Schiff (PAS) stained-sections from the B and C regions of intrafusal fibres. RESULTS: The glycogen content of type IIA fibres in the SOL and EDL muscles decreased significantly in the early phase of exercise whereas the glycogen content of type I fibres in these muscles decreased later than that of type IIA fibres. The glycogen content of bag2 fibres decreased after 1 min of exercise in the SOL muscle and after 2 min of exercise in the EDL muscle. On the other hand, the glycogen content of bag1 and chain fibres decreased significantly after 2 min in the SOL muscle but not in the EDL muscle. CONCLUSION: The results suggest that during brief-intensity exercise, as the glycogen content of type IIA fibres is reduced earlier than that of type I fibres, bag2 fibres are most important early in this type of exercise.     

Glycogen depletion in intrafusal fibres in rats during short‐duration high‐intensity treadmill running

Aim: The recruitment patterns of the intrafusal and extrafusal fibres in the soleus (SOL) and extensor digitorum longus (EDL) muscles of rats were investigated during brief‐intensity exercise by assaying their glycogen content histochemically. Methods: Six adult male rats were assigned to each of four groups that ran up a 6° incline on a motor‐driven treadmill, at 40 m min^?1 for either 0.5, 1, 2, or 4 min. Six adult male rats in the control group did not run. Extrafusal and intrafusal fibres were classified by myosin ATPase staining. Optical densities for glycogen content were evaluated in serial periodic acid Schiff (PAS) stained‐sections from the B and C regions of intrafusal fibres. Results: The glycogen content of type IIA fibres in the SOL and EDL muscles decreased significantly in the early phase of exercise whereas the glycogen content of type I fibres in these muscles decreased later than that of type IIA fibres. The glycogen content of bag₂ fibres decreased after 1 min of exercise in the SOL muscle and after 2 min of exercise in the EDL muscle. On the other hand, the glycogen content of bag₁ and chain fibres decreased significantly after 2 min in the SOL muscle but not in the EDL muscle. Conclusion: The results suggest that during brief‐intensity exercise, as the glycogen content of type IIA fibres is reduced earlier than that of type I fibres, bag₂ fibres are most important early in this type of exercise.     

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.     

Adaptation of rat extensor digitorum longus muscle to gamma irradiation and overload

The right extensor digitorum longus (EDL) muscle of growing male rats was overloaded by ablation of its synergist tibialis anterior (TA) muscle. Four weeks later, the overloaded muscle was heavier and contained larger type IIA, IIX and IIB fibres than either untreated contralateral muscle or control muscle from an untreated animal. The myonuclear-to-myoplasmic volume ratio was maintained in the overloaded muscle. Overloaded EDL muscle, previously subjected to a dose of irradiation sufficient to sterilise satellite cells, and EDL muscle which had been only irradiated, were significantly lighter and contained significantly smaller fibres than controls, though a significant amount of normal EDL muscle growth did occur following either treatment. The myonuclear-to-myoplasmic volume ratio of the irradiated muscles was smaller than in controls. Overloaded muscle, with or without prior irradiation, possessed a smaller proportion of fibres containing IIB myosin heavy chain (MHC) and a larger proportion of fibres containing IIA and IIX MHC; a significant percentage of these fibres coexpressed either type IIA and IIX MHC or type IIX and IIB MHC. Thus in the absence of satellite cell mitosis, muscles of young rats possess a limited capacity for normal growth but not for compensatory hypertrophy. Adaptations in MHC gene expression to chronic overload are completely independent of satellite cell activity.     

Correlation between shortening velocity,force—velocity relation and histochemical fibre-type composition in rat muscles

Summary Isometric and isotonic contractions of three muscles in the rat hind leg (soleus, extensor digitorum longus (EDL) and peroneus longus (PL)) were recordedin situ at 35° C and with nerve stimulation. Additionally, the histochemical muscle fibre-type composition of the three muscles was determined by the method of Guth and Samaha (1970). The data obtained from soleus and EDL muscles were similar to those reported in previous studies. On the basis of twitch contraction time, rate of rise of tetanic tension and maximum shortening velocity, the contraction speed of EDL was 2–3 times higher than in soleus. In the PL muscle, the twitch contraction time, rate of tension rise and shortening velocity were 17 ms, 30Po/s and 12 muscle fibre lengths/s, respectively; the data showed that the contraction speed of PL muscle was intermediate between that of the soleus and EDL muscles. In the case of soleus, more than 75% of the cross-sectional area was occupied by type 1 (slow) fibres; in both EDL and PL muscles more than 90% of the area was occupied by type 2 (fast fibres). However, the two fast muscles (EDL and PL) had different proportions of type 2B fibres; the area occupied by the type 2B fibre complement was less than 5% in PL, whereas it was around 70% in EDL muscle. The differences in shortening velocity and force—velocity relation among the three muscles could be explained on the basis of their respective muscle fibre-type compositions.     

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.     

Are region-specific changes in fibre types attributable to nonuniform muscle hypertrophy by overloading?

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of the muscle length to investigate whether compensatory overload by removal of synergists induces region-specific changes of fibre types in rat soleus and plantaris muscles. In addition, we evaluated fibre cross-sectional area in each region to examine whether fibre recruitment pattern against functional overload is nonuniform in different regions. Increases in muscle mass and fibre area confirmed a significant hypertrophic response in the overloaded soleus and plantaris muscles. Overloading increased the percentage of type I fibres in both muscles and that of type IIA fibres in the plantaris muscle, with the greater changes being found in the middle and distal regions. The percentage of type I fibres in the proximal region was higher than that of the other regions in the control soleus muscle. In the control plantaris muscle, the percentage of type I and IIA fibres in the middle region were higher than that of the proximal and distal regions. With regard to fibre size, type IIB fibre area of the middle and distal regions in the plantaris increased by 51% and 57%, respectively, with the greater changes than that of the proximal region (37%) after overloading. These findings suggest that compensatory overload promoted transformation of type II fibres into type I fibres in rat soleus and plantaris muscles, with the greater changes being found in the middle and distal regions of the plantaris muscle.     

Segmental fibre type composition of the rat iliopsoas muscle

The iliopsoas of the rat is composed of two muscles – the psoas major muscle and the iliacus muscle. The psoas major muscle arises from all the lumbar vertebrae and the iliacus muscle from the fifth and sixth lumbar vertebrae and ilium. Their common insertion point is the lesser trochanter of the femur, and their common action is the lateral rotation of the femur and flexion of the hip joint. Unlike humans, the rat is a quadruped and only occasionally rises up on its hind legs. Therefore, it is expected that the fibre type composition of the rat iliopsoas muscle will be different than that of humans. The iliopsoas muscle of the rat is generally considered to be a fast muscle. However, previous studies of the fibre type composition of the rat psoas muscle showed different results. Moreover, very little is known about the composition of the rat iliacus muscle. The aim of our study was to examine the fibre type composition of the rat iliopsoas muscle in order to better understand the complex function of the listed muscle. The psoas major muscle was examined segmentally at four different levels of its origin. Type I, IIA, IIB and IIX muscle fibres were typed using monoclonal antibodies for myosin heavy chain identification. The percentage of muscle fibre types and muscle fibre cross‐sectional areas were calculated. In our study we showed that in the rat iliopsoas muscle both the iliacus and the psoas major muscles had a predominance of fast muscle fibre types, with the highest percentage of the fastest IIB muscle fibres. Also, the IIB muscle fibres showed the largest cross‐sectional area (CSA) in both muscles. As well, the psoas major muscle showed segmental differences of fibre type composition. Our results showed changes in percentages, as well as the CSAs of muscle fibre types in cranio‐caudal direction. The most significant changes were visible in type IIB muscle fibres, where there was a decrease of percentages and the CSAs from the cranial towards the caudal part of the muscle. From our results it is evident that the rat iliopsoas muscle has a heterogeneous composition and is composed of all four muscle fibre types. Primarily, it is a fast, dynamic muscle with a predominance of fast type IIB muscle fibres with the largest CSAs. The composition of the rat psoas major muscles changes in a cranio‐caudal direction, thus pointing to a more postural role of the caudal part of the muscle.     

Two-dimensional changes of muscle fiber types in growing rat hind limb

In the present study, we examined the changes in two-dimensional distribution of fiber types in the whole area of the rat skeletal muscle and the effect of growth on this distribution. Muscles of rats aged 3 (body weight 58 g), 4 (89 g), 8 (276 g), 12 (312 g), 18 weeks (368 g), and 6 months (450 g) were stained for myofibrillar adenosine triphosphatase (mATPase) with preincubation at pH 4.35. Muscle fibers were classified into type I (slow oxidative), IIA (fast oxidative), IIB (fast glycolytic), and IIX (fast oxidative glycolytic). The x-y coordinates of each fiber were used to analyze the growth-related changes using an image analyzing system. In the tibialis anterior (TA) muscle, type I fibers were predominant in the deep and middle regions at 3 to 4 weeks of age, but became restricted to the deeper region with growth. In the extensor digitorum longus (EDL) muscle, type I fibers were predominant in the deep region at 3 to 8 weeks of age, but decreased gradually with growth and completely disappeared at 6 months of age. Compared with the TA and EDL, type I fibers of the soleus (SOL) muscle were spread throughout the muscle and the number of these fibers tended to increase with growth. Type IIA and IIX fibers of the SOL decreased in number and became restricted to the superficial region with growth. No type IIB fibers were detected in the SOL throughout life. Our results indicated that the growing process influences the distribution, proportion and characteristics of individual muscle fiber types in the rat hind limb muscles.     

Hypoxia-induced fibre type transformation in rat hindlimb muscles

Summary Twelve male Sprague-Dawley rats (21 days old) were randomly assigned into two experimental groups: sea level control (CONT) and hypobaric hypoxia (HYPO). The HYPO rats were kept in an hypobaric chamber maintaining a simulated altitude of 4000 m (61.1 kPa). After 10 weeks of treatment, the rat hindlimb muscles [soleus (SOL) and extensor digitorum longus (EDL)] were subjected to histochemical and electro-mechanical analyses. Results indicated that compared to CONT the HYPO SOL muscle had a significantly greater relative distribution of fast-twitch-oxidative-glycolytic (FOG) fibres (28.9% SEM 2.0 vs 18.3% SEM 1.8,P<0.01) with a significant decrease in slow twitch oxidative fibre distribution (69.5% SEM 2.4 vs 82.9% SEM 3.1,P<0.01). Compared to CONT the HYPO EDL muscle also manifested a significant increase in FOG fibre distribution (51.6% SEM 0.8 vs 46.6% SEM 1.1,P<0.01), but this was accompanied by a significant decrease in fast twitch glucolytic fibres (44.3% SEM 0.9 vs 49.2% SEM 1.7,P<0.05). These histochemical fibre type transformations accompanied significant and expected changes in the electro-mechanical parameters tested in situ, e.g. maximal twitch force, maximal rate of force development, contraction time, half relaxation time, force:frequency curve, and fatigability. It was concluded that chronic hypobaric hypoxia could have a potent influence upon the phenotype expression of muscle fibres.     

Growth hormone induces muscle fibre type transformation in growth hormone-deficient rats

The effect of recombinant human growth hormone (rhGH) on growth and composition of muscle was studied in growth hormone-deficient rats (dw/dw) treated for 10 days with either rhGH (GH) or with placebo (PLA). Age-matched control rats (DW/dw) (AGE) were treated as PLA. Growth rate increased (P < 0.05) when rats were treated with rhGH and plasma insulin-like growth factor-1 concentration was higher (P < 0.05) in GH and AGE than in PLA. The wet weight of the soleus (SOL) and the extensor digitorum longus muscles (EDL) was less in PLA compared to GH and AGE (P < 0.05). In the SOL, the amount of myosin heavy chain (MHC) I was lower (69.1 ± 1.7%) (Mean ± SEM) in PLA compared to both GH (85.3 ± 2.3%) and AGE (76.4 ± 1.6%) (P < 0.05). At the same time the amount of MHC IIA/IIX was higher (30.9 ± 2.2%) in PLA compared to GH (14.7 ± 2.3%) and AGE (23.6 ± 1.6% (P < 0.05)). In EDL, treatment with rhGH did not significantly affect MHC-isoforms or the fibre type composition, but 11% more MHC IIB and 11% less MHC IIA/IIX was observed in PLA compared to AGE (P < 0.05) suggesting a long-term effect of growth hormone. MHC-isoform data were confirmed using histochemistry. In addition, in the SOL, the maximal activity of 3-hydroxyacyl-CoA dehydrogenase (HAD) in GH and AGE was higher (22 and 27%, respectively) than in PLA (P < 0.05). In the EDL, no differences were observed in maximal activity of HAD. In conclusion, the data support a role for growth hormone in muscle fibre growth and differentiation.     

Distribution patterns of muscle fibre types in major muscles of the bull (Bos taurus).

Summary The study describes the variations in distribution and cross-sectional area (fibre size) of three muscle fibre types (I, IIA, IIB) in 34 of the largest muscles of the bull (Bos taurus). The animals had been kept strictly unexercised for one year before slaughter. Representative sampling was done at 15 positions within each muscle, and from 2700 to 4500 fibres were analysed in each muscle. Different intermuscular patterns are described. The overall volume fraction (%) of type I fibres was about 10% higher in the forepart muscles than in the hindpart muscles (41% and 31%, respectively), while the mean content of type IIB fibres was similar. Type I fibres were particularly abundant in antigravity muscles. Of these, the hindlimb muscles contained 50% more type I fibres (by weight) than those of the forelimb. Typical antigravity antagonists contained very few type I fibres. In the thigh cross-section the proportion of type I fibres was highest in the anterior and medial parts, while the IIB fibres tended to be concentrated in the superficial and posterior parts. Intramuscular patterns were revealed, with type I fibres becoming gradually more abundant from superficial to deep regions, while IIB fibres had an opposite distribution. This was particularly evident in the thigh proper and in the scapular region. Within each fasciculus of all the muscles, the I fibres in the muscles of the forepart were on average about 15% larger than those of the muscles in the hindpart. The IIB fibres were on average about 10% larger in the hindpart than in the forepart muscles. A covariation between the proportion of type I and IIB fibres and their cross-sectional area was indicated.     

Matched adaptations of electrophysiological, physiological, and histological properties of skeletal muscles in response to chronic hypoxia

This study tried to differentiate the consequences of chronic hypoxia on the electrophysiological and physiological properties and the histological characteristics of slow and fast muscles in rats. Animals inhaled a 10% O₂ concentration for a 1-month period. Then, slow [soleus (SOL)] and fast [extensor digitorum longus (EDL)] muscles were analyzed in vitro by physiological and electrophysiological measurements and histological analyses. The results were compared to those obtained in corresponding muscles of an age-matched normoxic group. After exposure to hypoxia: (1) in SOL, there was a tendency to elevated F_max, a significant increase in twitch force and tetanic frequency and a shortening of M-wave duration, and a reduced percentage of type I fibres, whereas the proportion of type IIa fibres doubled; (2) in EDL, F_max and tetanic frequency were lowered, the muscle became less resistant to fatigue, and the proportion of type IId/x fibres was halved. Then, after 1 month of hypoxia, in the SOL muscle, both the contractile and histological properties resemble those of a fast muscle. By contrast, the EDL became slower, despite its histology was modestly affected. Reduced muscle use in hypoxia could explain the tendency for deteriorating adaptations in EDL, and the faster properties of SOL could result from hypoxia-induced inhibition of the growth-related fast-to-slow shift in muscle fibre types.     

Effects of prolonged exposure to and physical training in hypobaric conditions on skeletal muscle morphology and metabolic enzymes in rats

Adaptations of skeletal muscle morphology and metabolic enzymes were studied after prolonged training in and exposure to hypobaric (740 –770 mbar) as well as normobaric conditions in rats performing treadmill running training for 10, 21 and 56 days. Animals sacrificed after 91 days served as recovery groups from training and hypobaric exposure for 56 days. The rats were divided into normobaric sedentary (NS) and training (NT) groups and hypobaric sedentary (HS) and training (HT) groups. The weights of extensor digitorum longus (EDL) and soleus (SOL) muscles increased significantly in the 56HS and the 56HT groups compared with the 56NS group, the increase being greatest in the 56HS group. No differences in the mean fibre areas (MFA) of these muscles could be seen, whereas clearly reduced MFAs of type IIA and IIB were observed in the tibialis anterior (TA) muscle. However, fibre area distribution analyses in the EDL and TA muscles showed a higher proportion of larger fibers in the 56HS and 56HT groups than in the respective normobaric groups. On the contrary, in SOL muscles the proportion of smaller fibers was higher in the hypobaric than in normobaric groups at 56 days. Increased activities of citrate synthase and β-hydroxyacyl-CoA-dehydrogenase in SOL and TA muscles in the 56HT group indicate an increase in oxidative capacity. It is concluded that exposure to, and training in moderate hypobaric conditions leads to a positive muscle protein balance which is reflected in increased muscle weights. However, the sites of increased protein synthesis and the possible hyperplasia remain to be studied further. Received : 27 April 1995/Received after revision: 25 October 1995/Accepted: 27 November 1995     

Muscle fibre types in the human vastus lateralis muscles: do symmetrical sites differ in their composition?

Muscle samples obtained from distinct symmetrical sites of left and right vastus lateralis muscles from 10 young healthy males were analysed. A computer-aided system was applied for the determination of muscle fibre types I, IIA, IIB and IIC and for their quantitative analysis. No statistically significant differences were established regarding percentage and surface area of all the four fibre types studied. In 2 cases of IIA and in one case of IIB fibre type significantly different diameters were measured between the left and right muscle. Regarding percentage and surface area no differences were proved between superficial and deep layers of the muscles observed. Fibre diameter but differed significantly in many muscles, there was no rule about whether it was larger in superficial or in deep muscle layers. One biopsy sample can not represent the whole muscle, especially as far as the relative number and surface area of the type IIA and IIB fibres are concerned. A slight asymmetry in fibre diameters of biopsy samples from both legs does not indicate any pathological change and can still be taken for normal.     

Changes in fibre types in rat soleus and plantaris muscles following hypophysectomy and compensatory overload

We investigated whether hypophysectomy could modify the change in muscle fibre types caused by compensatory overload. Male Wistar strain rats were assigned to groups of either normal control (NC), hypophysectomized control (HC), normal compensatory overloaded (NO), or hypophysectomized compensatory overloaded (HO). Compensatory overload was induced by the bilateral removal of the gastrocnemius muscle. Five weeks later, there were losses in the soleus and plantaris muscle weights as a result of hypophysectomy. Compensatory overload increased muscle weights independently of the hypophysectomy. Growth hormone and 3,5,3′-triiodothyronine levels were significantly decreased following hypophysectomy. In the soleus, hypophysectomy increased the percentage of type I fibres at the expense of type IIA fibres. Compensatory overload decreased type IIA fibres under the hypophysectomized condition. In the plantris, the percentage of type IIC fibres was increased at the expense of both type IIA and IIB fibres following hypophysectomy. The decrease in type IIB fibres cause by compensatory overload was induced irrespective of hypophysectomy. The changes in muscle fibre types in the HO group were equal to the sum of the changes in the HC and NO groups. These results suggest that after a period of 5 weeks hypophysectomy may induced decreases in type IIA and IIB fibres in association with the lack of pituitary and thyroid hormones, and that the hypophysectomy could not modify the change in muscle fibre types caused by compensatory overload.