Changes in contractile and metabolic parameters of skeletal muscle as rats age from 3 to 12 months

Laboratory rats are considered mature at 3 months despite that musculoskeletal growth is still occurring. Changes in muscle physiological and biochemical characteristics during development from 3 months, however, are not well understood. Whole muscles and single skinned fibres from fast-twitch extensor digitorum longus (EDL) and predominantly slow-twitch soleus (SOL) muscles were examined from male Sprague–Dawley rats (3, 6, 9, 12 months). Ca²⁺ sensitivity of contractile apparatus decreased with age in both fast- (~?0.04 pCa units) and slow-twitch (~?0.07 pCa units) muscle fibres, and specific force increased (by ~?50% and ~?25%, respectively). Myosin heavy chain composition of EDL and SOL muscles altered to a small extent with age (decrease in MHCIIa proportion after 3 months). Glycogen content increased with age (~?80% in EDL and 25% in SOL) and GLUT4 protein density decreased (~?35 and 20%, respectively), whereas the glycogen-related enzymes were little changed. GAPDH protein content was relatively constant in both muscle types, but COXIV protein decreased?~?40% in SOL muscle. Calsequestrin (CSQ) and SERCA densities remained relatively constant with age, whereas there was a progressive?~?2–3 fold increase in CSQ-like proteins, though their role and importance remain unclear. There was also ~?40% decrease in the density of the Na⁺, K⁺-ATPase (NKA) α1 subunit in EDL and the α2 subunit in SOL. These findings emphasise there are substantial changes in skeletal muscle function and the density of key proteins during early to mid-adulthood in rats, which need to be considered in the design and interpretation of experiments.     

睫状神经营养因子对大鼠去神经骨骼肌的营养作用

目的：通过测定SD大鼠坐骨神经离断后比目鱼肌（SOL）和趾长伸肌（EDL）肌纤维横截面积,观察持续给睫状神经营养因子（CNTF）对神经骨骼肌萎缩的影响。结果：SD大鼠坐骨神经离断后,持续给0．2mg/kg.d的CNTF20天后,损伤侧SOL和EDL肌纤维横截面积分别比实验对照组高27％（P＜0．01）和14％,SOL肌纤维横截面积比EDL增加的幅度大,而给予0．05mg/kg.d的CNTF20天后,动物肌纤维横截面积与实验对照组无明显差异。结论：CNTF可显著改善坐骨神经离断后SD大鼠骨骼肌的萎缩,并且CNTF效应的强弱与用药剂量和肌肉类型有关,0．2mg/kg.dCNTF作用明显强于0．05mg/kg.dCNTF,慢肌（SOL）比快肌（EDL）对CNTF更敏感。     

Different metabolic adaptation of heart and skeletal muscles to moderate-intensity treadmill training in the rat

The effect was investigated of treadmill training of moderate intensity on the fatty acid-binding protein (FABP) content in relation to parameters of oxidative and glycolytic metabolism. To this end, the cytoplasmic FABP content and the activity of β-hydroxyacyl-coenzyme A dehydrogenase (HAD), citrate synthase (CS), and 6-phosphofructokinase (PFK) were measured in heart, fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles (SOL) of male Wistar rats. To investigate the influence of the amount of training (defined as the product of exercise duration, intensity and frequency), two training groups were created that differed in training frequency (HF, high frequency 5 days?·?week^?1, n=9; LF, low frequency 2 days?·?week^?1, n?=?9; the exercise being 20?m?·?min^?1 for 2?h with no gradient, over 6 weeks) and compared with SC, sedentary controls (n?=?7). In heart muscle, the cytoplasmic FABP content was 34% higher in HF than in SC but was the same as in LF. The CS and HAD activities were no different in the three groups, suggesting that the capacity to oxidize fatty acids (FA) was not affected by training. The PFK activity was higher (43%) in HF, suggesting a shift towards carbohydrate utilization. The FABP content and HAD activity did not change in SOL and EDL after training whereas the CS activity increased (27%) in SOL and decreased (21%) in EDL in both training groups. In addition, PFK activity in EDL was much higher (113%) in the HF than in SC group. The HF training was associated with a fine-tuning of FA availability and use in heart muscle, and with a more efficient energy production. It is suggested therefore that cytoplasmic FABP could be an early marker of muscle adaptation to training in heart but not in skeletal muscle. The training reinforced the metabolic profile of the skeletal muscles, in particular that of the fast-twitch glycolytic muscle. We concluded that a large amount of training is needed when the effect on both oxidative and glycolytic parameters is to be studied.     

Physiological and biochemical characteristics of skeletal muscles in sedentary and active rats

Laboratory rats are sedentary if housed in conditions where activity is limited. Changes in muscle characteristics with chronic inactivity were investigated by comparing sedentary rats with rats undertaking voluntary wheel running for either 6 or 12 weeks. EDL (type II fibers) and soleus (SOL) muscles (predominantly type I fibers) were examined. When measured within 1–2 h post-running, calcium sensitivity of the contractile apparatus was increased, but only in type II fibers. This increase disappeared when fibers were treated with DTT, indicative of oxidative regulation of the contractile apparatus, and was absent in fibers from rats that had ceased running 24 h prior to experiments. Specific force production was ~?10 to 25% lower in muscle fibers of sedentary compared to active rats, and excitability of skinned fibers was decreased. Muscle glycogen content was ~?30% lower and glycogen synthase content?~?50% higher in SOL of sedentary rats, and in EDL glycogenin was 30% lower. Na⁺, K⁺-ATPase α1 subunit density was ~?20% lower in both EDL and SOL in sedentary rats, and GAPDH content in SOL?~?35% higher. There were no changes in content of the calcium handling proteins calsequestrin and SERCA, but the content of CSQ-like protein was increased in active rats (by ~?20% in EDL and 60% in SOL). These findings show that voluntary exercise elicits an acute oxidation-induced increase in Ca²⁺ sensitivity in type II fibers, and also that there are substantial changes in skeletal muscle characteristics and biochemical processes in sedentary rats.     

Chronic hypoxia induces prolonged angiogenesis in skeletal muscles of rat

Skeletal muscle capillarity and fibre cross-sectional area were investigated within and between diaphragm (Diaph), extensor digitorum longus (EDL), soleus (SOL) and tibialis anterior (TA) muscles of control and chronic hypoxic (12 % O(2) for 6 weeks) adult male Wistar rats (final body mass approximately 355 g). Cryostat sections were stained for alkaline phosphatase activity to depict all capillaries, and for succinic dehydrogenase to demonstrate regional differences in oxidative capacity within the muscles. Hypoxia-induced angiogenesis occurred in all muscles (P < 0.01), with capillary-to-fibre ratio (C:F) being higher in the more active and oxidative muscles, Diaph (27 %) and SOL (26 %), than phasically active and glycolytic muscles, TA (21 %) and EDL (15 %). Diaph, SOL and EDL maintained fibre size, and hence showed an increased capillary density (CD) and reduced intramuscular diffusion distance (DD), whereas TA showed fibre hypertrophy and maintained CD and DD compared to control muscles. The extent of angiogenesis among different regions of muscle varied so as to suggest that muscle fibre size has an additional influence on capillary growth during chronic systemic hypoxia, which is progressive over an extended period of systemic hypoxia.     

Effects of isoproterenol on rubidium transport in slow- and fast-twitch muscles from euthyroid and hyperthyroid rats

The influence of experimental hyperthyroidism on the catecholamine induced stimulation of rubidium ion transport in the soleus (SOL), a slow-twitch muscle and the extensor digitorum longus (EDL), a fast-twitch skeletal muscle of rats was studied. Thyroxine administration (800 g/kg/day), for ten days induced a rise of ouabainsensitive⁸⁶Rb uptake in SOL muscle, without affecting the ouabain-insensitive uptake, whereas both fractions of⁸⁶Rb uptake were increased in EDL muscle from hyperthyroid rats. Isoproterenol (5 mol/l) caused a two-fold rise in ouabain-sensitive Rb uptake of euthyroid SOL muscle, while in hyperthyroid SOL it could stimulate only the ouabain-insensitive fraction of⁸⁶Rb influx. On the other hand, the stimulating action of isoproterenol on euthyroid EDL muscle was due to an enhancement of ouabain-insensitive Rb uptake, but in hyperthyroid EDL it failed to stimulate the ouabain-insensitive transport and caused a marked rise in ouabain-sensitive Rb uptake.The changes in catecholamine mediated transport properties in SOL muscle may be related to fibre type trans-formation induced by thyroid hormone, although in EDL the changes of catecholamine stimulation are unlikely due to fibre type conversion.Basal and isoproterenol stimulated cAMP levels were significantly reduced in both EDL and SOL muscles from hyperthyroid rats, in contrast with an insignificant decrease in net rubidium uptake caused by isoproterenol at the same concentration.The work was supported by a grant from the Hungarian Ministry of Health (No 17/2-06/072)     

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     

A freeze-fracture study of sarcoplasmic reticulum from fast and slow muscle of the mouse

Sarcoplasmic reticulum (SR) of fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of the mouse was examined by freeze-fracture techniques. A distinctive feature of sarcoplasmic reticulum from the EDL is the presence of hillocks on the A-face within the terminal cisterns. These hillocks are usually arranged in a single row which is deployed parallel to the long axis of the adjacent T-tubule. Center-to-center spacing of hillocks within a row is about 70–75 nm. Hillocks are also found scattered within the collar region. The EDL of ten week mice was characterized by sheet-like terminal and intermediate cisterns, the latter being replaced in 37 week animals by thin tubular longitudinal elements of the SR which contain no hillocks or dimples. Hillocks occur only occasionally in SR from 10 or 20 week SOL muscle. In such cases the hillocks occur singly rather than in rows as in the terminal cisterns of EDL. The predominant form of SR in the SOL contains no hillocks. Total particles in the A-face of EDL-SR (2996 particles/μ²; S.D. = ± 287) slightly exceeded that of SOL-SR (2558 particles/μ²; S.D. = ± 274 8 nm). Packing density of 8 nm particles was slightly higher for EDL (750/μ²) vs. SOL (700/μ²). The possible significance of these features of SR in fast and slow muscle is discussed.     

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.     

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.     

Effects of chronic hypoxia and endurance training on muscle capillarity in rats

Skeletal muscle capillarity expressed as capillary density (CD), and number of capillaries per fibre (C/F), as well as the mean fibre cross-sectional area (FCSA), were determined in the extensor digitorum longus (EDL), plantaris (PLA) and soleus (SOL) muscles of four groups of eight rodents trained on a swimming exercise programme (T) or maintained sedentary (S), at sea level (SL) or at simulated altitude (HA), barometric pressure 61.7 kPa (463 torr) for 12 weeks. It was shown that both HA exposure and endurance training decreased body and skeletal muscles weights (P<0.001). However, neither HA exposure nor endurance training induce any variation in relative importance in the skeletal muscle mass. Altitude exposure and endurance training had increasing effects on CD in all muscles studied (P<0.001). This study confirms the fact that altitude exposure has no direct effect on capillary development. On the other hand, the capillary supply of the several slow- and fast- twitch skeletal muscles studied is increased by endurance training. This real enhancement in capillary network is ascertained by an increase in the C/F ratio (+7%, +26%, +16%, in PLA, EDL, and SOL muscles, respectively at sea level, and +19.5%, +30%, and +14% respectively at HA). These results indicate that the effects of chronic exercise on skeletal muscle capillarity estimated by the C/F ratio, are greater in an hypobaric environment than in a SL environment.     

A freeze-fracture study of sarcoplasmic reticulum from fast and slow muscle of the mouse.

Sarcoplasmic reticulum (SR) of fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of the mouse was examined by freeze-fracture techniques. A distinctive feature of sarcoplasmic reticulum from the EDL is the presence of hillocks on the A-face within the terminal cisterns. These hillocks are usually arranged in a single row which is deployed parallel to the long axis of the adjacent T-tubule. Center-to-center spacing of hillocks within a row is about 70-75 nm. Hillocks are also found scattered within the collar region. The EDL of ten week mice was characterized by sheet-like terminal and intermediate cisterns, the latter being replaced in 37 week animals by thin tubular longitudinal elements of the SR which contain no hillocks or dimples. Hillocks occur only occasionally in SR from 10 or 20 week SOL muscle. In such cases the hillocks occur singly rather than in rows as in the terminal cisterns of EDL. The predominant form of SR in the SOL contains no hillocks. Total particles in the A-face of EDL-SR (2996 particles/mu2; S.D. = +/- 287) slightly exceeded that of SOL-SR (2558 particles/mu2; S.D. = 274 8 NM). Packing density of 8 nm particles was slightly higher for EDL (750/mu2) VS. SOL (700/mu2). The possible significance of these features of SR in fast and slow muscle is discussed.     

Effects of hindlimb suspension and androgen treatment on testosterone receptors in rat skeletal muscles

This study tested the specific and combined effects of testosterone treatment and hindlimb suspension (HS) on the properties of steroid receptors in skeletal muscle. Male rats were either administered weekly high doses of testosterone heptylate (10?mg?·?kg^?1) or olive oil placebo, and were either tail-suspended or acted as controls. After 3 weeks of treatment, three muscles were excised from each animal, soleus (SOL), extensor digitorum longus (EDL), and plantaris. The results showed that the testosterone treatment was unable to minimise the HS-induced atrophy of skeletal muscle. As expected, HS altered the fibre-type composition of SOL muscles (?33% of type I, +188% and +161% of type IIa and intermediate fibres respectively, P?P?相似文献   

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.     

Stromal interaction molecule 1 (STIM1) regulates sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 1a (SERCA1a) in skeletal muscle

Stromal interaction molecule 1 (STIM1) mediates Ca²⁺ movements from the extracellular space to the cytosol through a store-operated Ca²⁺ entry (SOCE) mechanism in various cells including skeletal muscle cells. In the present study, to reveal the unidentified functional role of the STIM1 C terminus from 449 to 671 amino acids in skeletal muscle, binding assays and quadrupole time-of-flight mass spectrometry were used to identify proteins binding in this region along with proteins that mediate skeletal muscle contraction and relaxation. STIM1 binds to sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase 1a (SERCA1a) via this region (called STIM1-SBR). The binding was confirmed in endogenous full-length STIM1 in rabbit skeletal muscle and mouse primary skeletal myotubes via co-immunoprecipitation assay and immunocytochemistry. STIM1 knockdown in mouse primary skeletal myotubes decreased Ca²⁺ uptake from the cytosol to the sarcoplasmic reticulum (SR) through SERCA1a only at micromolar cytosolic Ca²⁺ concentrations, suggesting that STIM1 could be required for the full activity of SERCA1a possibly during the relaxation of skeletal muscle. Various Ca²⁺ imaging experiments using myotubes expressing STIM1-SBR suggest that STIM1 is involved in intracellular Ca²⁺ distributions between the SR and the cytosol via regulating SERCA1a activity without affecting SOCE. Therefore, in skeletal muscle, STIM1 could play an important role in regulating Ca²⁺ movements between the SR and the cytosol.     

Skeletal muscle HSP72 response to mechanical unloading: influence of endurance training

AIMS: It has been shown that increased contractile activity results in heat shock protein 72 (HSP72) accumulation in various skeletal muscles. By contrast, there is no consensus for muscle HSP72 response to muscle disuse for short duration (5-8 days). On the basis of a greater constitutive HSP72 expression in slow-twitch muscles we tested the hypothesis that mechanical unloading for a longer period (2 weeks) would affect this phenotype to a greater extent. Secondly, we evaluated the effects of a physiological muscle heat shock protein (HSP) enhancer (endurance training) on HSP response to unloading and muscle remodelling. METHODS: Adult male Wistar rats were assigned randomly to four groups: (1) sedentary weight-bearing; (2) hindlimb-unloaded (HU) via tail suspension for 2 week; (3) trained on a treadmill (6 week) and (4) trained 6 week and then HU for 2 week. RESULTS: Unloading resulted in a preferential atrophy of slow muscles [soleus (SOL), adductor longus (AL)] and a slow-to-fast fibre transition with no change in HSP72 level. HSP72 levels were significantly lower in fast muscles [extensor digitorum longus (EDL) and plantaris (PLA)], and did not change with mechanical unloading. Endurance training was accompanied by a small (SOL) or a large (EDL, PLA) increase in HSP72 level with no change in AL. Training-induced accumulation of HSP72 disappeared with subsequent unloading in the SOL and PLA whereas HSP72 content remained elevated in EDL. CONCLUSION: The results of this study indicate that (1) after 2 weeks of unloading no change occurred in HSP72 protein levels of slow-twitch muscles despite a slow-to-fast fibre transition; and (2) the training-induced increase of HSP72 content in skeletal muscles did not attenuate fibre transition.     

The effect of exercise on the contractile properties of single skinned fast- and slow-twitch skeletal muscle fibres from the adult rat

The effects of long-term endurance exercise on the contractile properties of single skinned muscle fibres from adult rats, were investigated. Adult (4-month-old) male rats were subjected to a 16-week, high-intensity endurance swimming programme, where animals carried a load (corresponding to 2% of body wt), during all 2-h training sessions. At the conclusion of the training period, muscle fibres isolated from the extensor digitorum longus (EDL), and soleus (SOL), could be classified into distinct classes or fibre types on the basis of their Ca²⁺- and Sr²⁺-activated contractile characteristics. The fast-twitch EDL comprised two fibre populations, while the slow-twitch SOL was found to be composed of three distinct fibre types. Endurance swimming modified the contractile characteristics of fibres from both the EDL and SOL, but exerted greater influence on those of the SOL. This was illustrated by significant increases in the sensitivity to Ca²⁺ and Sr²⁺, and a lower threshold for contraction by these activating ions, in the exercised group. Not one of the total of 272 fibres sampled, exhibited mixed fast- and slow-twitch contractile characteristics, often associated with exercise-induced fibre type transformations. Thus, high-intensity endurance swimming induced changes in some single muscle fibre contractile properties of adult rats, but did not cause major changes in fibre type distribution.     

Effects of hindlimb suspension on contractile properties of young and old rat muscles and the impact of electrical stimulation on the recovery process

The purpose of this study was to investigate the effects of hindlimb suspension (HS) on contractile properties of skeletal muscles of young and old rats and to determine the impact of electrical stimulation (ES) on the quality and degree of recovery of these muscles. After 21 days of HS, young soleus (SOL) muscle became faster, but there was no impact on young extensor digitorum longus (EDL) muscle. Twitch tension (Pt) decreased 61% in young and 70% in old SOL muscles. Specific tetanic tension (Po) decreased 53% in young and 64% in old SOL muscles, but again there was no impact on EDL muscle. After a 14-day period of recovery, contraction time (CT), half-relaxation time (RT1/2), Pt and Po returned to control group values in both young and old SOL muscles. Measurements of the contractile properties of young and old skeletal rat muscles showed ES sometimes to be beneficial but also sometimes to be harmful. A 14-day period of recovery, with or without ES, seemed sufficient for many variables to return to control group values.     

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.     

Contractile properties of skeletal muscle fibre bundles from mice deficient in carbonic anhydrase II

The function of cytosolic carbonic anhydrase (CA) isozyme II is largely unknown in skeletal muscle. Because of this, we compared the in vitro contractile properties of extensor digitorum longus (EDL) and soleus (SOL) fibre bundles from mice deficient in CA II (CAD) to litter mate controls (LM). Twitch rise, 1/2 relaxation time and peak twitch force at 22°C of fibre bundles from CAD EDL [28.4±1.4 ms, 31.2±2.3 ms, 6.2±1.0 Newton/cm² (N/cm²), respectively] and CAD SOL (54.2±7.5 ms, 75.7±13.8 ms, 2.9±0.5 N/cm², respectively) were significantly higher compared to LM EDL (20.5±2.2 ms, 21.9±3.7 ms, 4.5±0.2 N/cm²) and LM SOL (42.8±3.5 ms, 51.4±2.4 ms, 2.1±0.4 N/cm²). However, in acidic Krebs–Henseleit solution, mimicking the pH, PCO₂, and HCO₃ ^– of arterial blood from CAD mice, twitch rise, 1/2 relaxation time, and peak twitch force of fibre bundles from CAD EDL (19.3±0.7 ms, 19.7±2.3 ms, 4.8±0.8 N/cm²) and CAD SOL (41.4±3.6 ms, 51.9±5.5 ms, 2.2±0.7 N/cm²) were not significantly different from LM fibre bundles in normal Krebs–Henseleit solution (EDL: 19.7±1.1 ms, 21.6±0.6 ms, 4.7±0.2 N/cm²; SOL: 42.5±3.1 ms, 51.8±2.6 ms, 1.8±0.3 N/cm²). A higher pH_i during exposure to acidic bathing solution was maintained by CAD EDL (7.37±0.02) and CAD SOL (7.33±0.05) compared to LM EDL (7.28±0.04) and LM SOL (7.22±0.02). This suggests that the skeletal muscle of CAD mice possesses an improved defense of pH_i against elevated pCO₂. In support of this, apparent non-bicarbonate buffer capacity (in mequiv H⁺ (pH unit)^–1 (kg cell H₂O)^–1) as determined by pH microelectrode was markedly increased in CAD EDL (75.7±4.1) and CAD SOL (85.9±3.3) compared to LM EDL (39.3±4.7) and LM SOL (37.5±3.8). Both latter phenomena may be related to the slowed rate of intracellular acidification seen in CAD SOL in comparison with LM SOL upon an increase in PCO₂ of the bath. In conclusion, skeletal muscle from mice deficient in CA II exhibits altered handling of acid–base challenges and shows normal contractile behavior at normal intracellular pH.