Functional regeneration in the hindlimb skeletal muscle of the mdx mouse

Summary The pattern of spontaneous skeletal muscle degeneration and clinical recovery in hindlimb muscles of the mdx mutant mouse was examined for functional and metabolic confirmation of apparent structural regeneration. The contractile properties, histochemical staining and myosin light chain and parvalbumin contents of extensor digitorum longus (EDL) and soleus (Sol) muscles of mdx and age-matched control mice were studied at 3–4 and 32 weeks. Histochemical staining (myofibrillar ATPase and NADH-tetrazolium reductase) revealed no significant change in slow-twitch-oxidative (SO) or fast-twitch-oxidative-glycolytic (FOG) fibre type proportions in mdx Sol apart from the normal age-related increase in SO fibres. At 32 weeks mdx EDL, however, showed significantly smaller fast-twitch-glycolytic (FG) and larger FOG proportions than those in control EDL. These fibre type distributions were confirmed by differential staining with antibodies to myosin slow-twitch and fast-twitch heavy chain isozymes. Frequency distribution of cross-sectional area for each fibre type showed a wider than normal range of areas especially in FOG fibres of mdx Sol, and FG fibres of mdx EDL, supporting previous observations using autoradiography of myofibre regeneration. Isometric twitch and tetanic tensions in Sol were significantly less than in controls at 4 weeks, but by 32 weeks, values were not different from age-matched controls. In mdx EDL at 3 weeks, twitch and tetanus tensions were significantly less, and time-to-peak twitch tensions were significantly faster than in control EDL. By 32 weeks, mdx EDL twitch and tetanus tensions expressed relative to muscle weight continued to be significantly lower than in age-matched controls, despite normal absolute tensions. The maximum velocity of shortening in 32-week mdx EDL was significantly lower than in control EDL. Myosin light chain distribution in mdx Sol exhibited significantly less light chain 2-slow (LC2s) and more light chain 1b-slow(LC1bs) at 32 weeks than age-matched control Sol. Gels of EDL from 32-week-old mdx mice showed significantly less light chain 2-fast-phosphorylated (LC2f-P) and light chain 3-fast (LC3f) and significantly more light chain 1-fast (LC1f) and light chain 2-fast (LC2f), but normal parvalbumin content compared to age-matched controls. These observations suggest that mdx hindlimb muscles are differentially affected by the disease process as it occurs in murine models of dystrophy. However, the uniqueness of mdx Sol and to a lesser extent EDL is that they also undergo an important degree of functional regeneration which is able to compensate spontaneously for degenerative influences of genetic origin. The mdx mutant may therefore be an important model for the study of regeneration by skeletal muscle, and of the nerve-muscle interactions which enable or restrict that regeneration.     

The effect of denervation on the mechanical and electrical responses of fast and slow mammalian twitch muscle

1. The soleus (slow twitch), flexors digitorum and hallucis longus muscles (fast twitch) of the cat were denervated. Isometric contractions and electrical responses were examined 2-160 days after the operation.2. In the first week the time course of the twitch and the ratio of tetanus tension to twitch tension were normal in both muscles. The maximum rate of rise of tension in the tetanus was reduced in fast muscles.3. In the second week all the twitches showed a normal contraction phase but relaxations were interrupted by a repetitive after-contraction. This became less marked after longer periods of denervation.4. During the third and subsequent weeks, the contraction and relaxation phases of the twitches in all muscles became slower than normal. These changes were greater in fast muscles which, nevertheless, remained quite distinct from soleus. The ratio of tetanus tension to twitch tension fell below normal. It is suggested that these changes are brought about by more complete activation of the contractile proteins in a twitch. In flexor hallucis longus the rate of rise of tension in isometric tetani was found to be further reduced. No change was found in soleus.5. In extracellular and intracellular records the initial response was a single action potential. An after-discharge occurred in a proportion of fibres during the relaxation phase of the twitch.6. The intracellularly recorded action potential was smaller and had a longer duration than that of normal muscle. Refractory period increased. Conduction velocity decreased. These changes were greater in fast muscle and differences between fast and slow twitch muscle were less marked than in normal muscle. Fibrillation potentials occurred at the same rate in fast and slow muscle.7. The mechanisms of the mechanical changes are discussed and the possible relevance to the question of motor nerve influence on muscle is indicated.     

Neurotrophic regulation of dynamic properties of skeletal muscle: effects of botulinum toxin and denervation.

In order to determine the role of acetylcholine (ACh) transmission in neurotrophic regulation of dynamic properties of muscle, the effects of botulinum toxin treatment were compared with those of denervation. The extensor digitorum longus (EDL) and soleus muscles of rats were either denervated or injected with botulinum toxin. At times up to 25 days the isometric properties of these muscles were determined. 2. Both botulinum treatment and denervation produced progressive slowing of the time to peak of the twitch (TPT) and half-relaxation time of the twitch (1/2 RT), which was more pronounced in the EDL than in the soleus. 3. Both treatments produced slowing of the relaxation curve following tetanic contraction, more marked in the EDL than in the soleus muscle. This indicates a slowing of relaxation, and suggests a prolongation of the active state of the muscle. 4. The maximum rate of rise of the tetanus did not change significantly in the EDL and soleus muscles after botulinum treatment or denervation. This suggests that there is no major change in the speed of contraction under conditions of botulinum treatment or denervation. 5. The changes produced by botulinum treatment and denervation were virtually identical in all parameters tested. This is interpreted meaning that cholinergic transmission (including muscle usage), or some other factor closely related to cholinergic transmission, accounts for the motor nerve's trophic influence in maintaining these dynamic properties of skeletal muscles.     

Excitation-contraction coupling and contractile properties in denervated rat EDL and soleus muscles

Summary The long-term (up to ten weeks) effects of denervation on isometric tension and potassium (K) contractures were studied in isolated bundles of fibres from rat extensor digitorum longus (EDL) and soleus muscles, at 21° C, bathed in solutions with low concentrations of chloride ions (to reduce the effects of high membrane chloride conductance). The usual increases in twitch time course and twitch for tetanus ratio were attributed to changes in excitation-contraction coupling because both developed between one to three weeks after denervation. Transient changes during the first week in the time course of the twitch and twitch to tetanus ratio in EDL, and post-tetanic twitch size and specific tension in both muscles, were attributed to the surface membrane electrical properties which are maximally altered within three days after denervation. In contrast to results obtained in solutions of normal chloride concentration, the resting membrane potentials of chronically denervated fibres were hyperpolarized and spontaneous action potentials and fibrillations were seen. The time to the peak of the K-contracture was faster than normal and very slow inactivation kinetics appeared in the decay phase. Mechanical repriming was normally slower in EDL than in soleus but similar rates were seen in the two muscles after denervation. These changes in the K-contracture were not due to a change in the relationship between membrane potential and potassium ion concentration. It is concluded that denervation caused changes in the kinetics of mechanical activation, inactivation and repriming and that these aspects of excitation-contraction coupling are normally controlled by an influence of the motor nerve.     

Contractile properties of aneurally regenerated compared with denervated muscles of rat

Summary The time course of aneural regeneration in slow-twitch soleus muscles of young adult rats was studied and compared with the changes following denervation in soleus and fast extensor digitorum longus muscles. Regeneration was induced by auto-grafting after treatment with bupivacaine; isometric contractions were recorded from 5 to 70 days later. Force was detected at 5 days; at 12 days force was maximal (at least 20% of original) and thereafter fell exponentially. Force varied normally with total fibre area, except at 5 and 71 days when force generating capacity was low. Contraction and relaxation in the twitch were longer than normal (maximally at 5 days), and were closer to denervated soleus than EDL; in contrast, the maximal rate of rise of force was as high as that of denervated EDL and much higher than in denervated soleus. It is suggested that the muscle was fundamentally fast contracting, but the twitches were probably slow because of greater than normal activation following a single stimulus — a hypothesis supported by twitch: tetanus ratios that were higher than in denervated muscles. Tetanic force was much more sensitive than normal to changes of muscle length from optimum, despite the fact that the lengths of regenerated muscles were similar to those of contralateral muscles.The properties of denervated soleus gradually approached those of regenerated soleus, probably because of replacement of original fibres by regenerated ones.     

Sarcoplasmic reticulum function in slow- and fast-twitch skeletal muscles from mdx mice

The aim of the present study was to establish whether alterations in sarcoplasmic reticulum function are involved in the abnormal Ca(2+) homeostasis of skeletal muscle in mice with muscular dystrophy ( mdx). The properties of the sarcoplasmic reticulum and contractile proteins of fast- and slow-twitch muscles were therefore investigated in chemically skinned fibres isolated from the extensor digitorum longus (EDL) and soleus muscles of normal (C57BL/10) and mdx mice at 4 and 11 weeks of development. Sarcoplasmic reticulum Ca(2+) uptake, estimated by the Ca(2+) release following exposure to caffeine, was significantly slower in mdx mice, while the maximal Ca(2+) quantity did not differ in either type of skeletal muscle at either stage of development. In 4-week-old mice spontaneous sarcoplasmic reticulum Ca(2+) leakage was observed in EDL and soleus fibres and this was more pronounced in mdx mice. In addition, the maximal Ca(2+)-activated tension was smaller in mdx than in normal fibres, while the Ca(2+) sensitivity of the contractile apparatus was not significantly different. These results indicate that mdx hindlimb muscles are affected differently by the disease process and suggest that a reduced ability of the Ca(2+)-ATPase to load Ca(2+) and a leaky sarcoplasmic reticulum membrane may be involved in the altered intracellular Ca(2+) homeostasis.     

Effects of an anabolic hormone on striated muscle growth and performance

Chronic administration of an anabolic hormone, nandrolone phenylpropionate, in sedentary female rats for 6 weeks gave a 20% increase in body weight and the same proportional increase in all muscles sampled (heart, diaphragm, soleus, TA, EHP and EDL), such that the muscle/body weight ratio was unchanged. Cardiac muscle was unresponsive to treatment. Acute stimulation of EDL via lateral popliteal nerve gave similar values for contraction time, 1/2 relaxation time and twitch: tetanus ratio in both groups suggesting no slowing of the muscle. Fatigue resistance of EDL was improved with 0.29±0.029 vs. 0.46±0.071 of maximum isometric twitch tension being developed after 10 min repetitive stimulation at 4 Hz. This improved endurance was not accompanied by any increase in strength and could not be explained on the basis of cellular hypertrophy, but appears to reflect an increased aerobic capacity of skeletal muscle. The proportion of FOG fibres in EDL increased, 38±1.1% vs. 46±1.1%, and this was paralleled in the other skeletal muscles. Specific hypertrophy of FOG and FG fibres could be conclusively demonstrated in soleus and TA, respectively.     

Effect of cyclopiazonic acid, an inhibitor of the sarcoplasmic reticulum Ca-ATPase, on skeletal muscles from normal and mdx mice

AIM: In this study, we investigated Ca2+ loading by the sarcoplasmic reticulum in skeletal muscle from mdx mice, an animal model of human Duchenne's muscular dystrophy, at two stages of development: 4 and 11 weeks. METHOD: Experiments were conducted on fast- (extensor digitorum longus, EDL) and slow- (soleus) twitch muscles expressing different isoforms of Ca2+-ATPase, which is responsible for the uptake of Ca2+ by the sarcoplasmic reticulum. RESULTS: In sarcoplasmic reticulum vesicles, the ATP-dependent activity and sensitivity to cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum Ca2+-ATPase, were similar in mdx and normal EDL muscle. Furthermore, in chemically-skinned fibres from both normal and mdx muscles, the presence of CPA induced a decrease in Ca2+ uptake by the sarcoplasmic reticulum. However, the sensitivity to CPA was lower in mdx EDL muscle than in normal muscle. In addition, in EDL muscle from 4-week-old mdx mice, the expression of the slow Ca2+-pump isoform (SERCA2a) was significantly increased, without any accompanying change in slow myosin expression. In contrast, the expression and function of the Ca2+-ATPase in mdx soleus muscles at 4- and 11-weeks of development did not differ from those in age-matched controls. CONCLUSION: These findings show that in dystrophic muscle, where the Ca2+ homeostasis was perturbed, the Ca2+ handling by the sarcoplasmic reticulum was altered in fast-twitch muscle, and this was associated with the expression of the slow isoform of SERCA. In these muscles, reduced Ca2+ uptake could then contribute to an elevated concentration of Ca2+ in the cytosol, and also to Ca2+ depletion of the sarcoplasmic reticulum.     

The Effects of Dietary Creatine Supplements on the Contractile Properties of Rat Soleus and Extensor Digitorum Longus Muscles

Daily creatine supplements (0.258 g kg(-1) ) were administered to adult male Wistar rats (n = 7) in the drinking water. Age matched rats (n = 6) acted as controls. After 5-6 days, contractile properties were examined in soleus and extensor digitorum longus (EDL) muscle strips in vitro at 30 degrees C. In soleus muscles, creatine supplements decreased the half-relaxation time of the isometric twitch from 53.6 +/- 4.3 ms in control muscles to 48.4 +/- 5.5 ms but had no effect on twitch or tetanic tension or on twitch contraction time. In EDL muscles twitch tension, tetanic tension, twitch contraction and half-relaxation times were all unaffected by creatine supplements. Creatine supplements increased the fatigue resistance of the soleus muscles but had no effect on that of the EDL muscles. After a 5 min low-frequency fatigue test, tension (expressed as a percentage of initial tension) was 56 +/- 3 % in control soleus muscles, whereas that in the creatine-supplemented muscles was 78 +/- 6 % (P < 0.01). In the EDL muscles, the corresponding values were 40 +/- 2 % and 41 +/- 9 %, respectively. The force potentiation which occurred in the EDL muscles during the initial 20-30 s of the fatigue test was 170 +/- 10 % of initial tension in the control muscles 24 s after the initial stimulus train but was reduced (P < 0.01) to 130 +/- 20 % in the creatine-supplemented muscles. In conclusion, soleus muscle endurance was increased by creatine supplements. EDL endurance was unaffected but force potentiation during repetitive stimulation was decreased. Experimental Physiology (2001) 86.2, 185-190.     

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.     

Metabolic recovery of mouse extensor digitorum longus and soleus muscle

Heat produced by a 1-s isometric tetanus of mouse extensor digitorum longus muscle (EDL; n=6) and a 1.5-s isometric tetanus of soleus muscle (n=7) was measured with thermopiles at 20 °C, and separated into initial heat (I) and recovery heat (R). In EDL the initial heat was 190±40 (SD) mJ g^–1 and in soleus 52±9 (SD) mJ g^–1. The recovery heat production rate immediately following the tetanus was almost zero in both muscles. It rose in 12±6 s (EDL) and in 30±3 s (soleus) to a maximum, to decrease thereafter monoexponentially with a time constant of 30.7±5.7 s (EDL) and 41.7±7.2 s (soleus). The measured recovery ratio (R/I) differed between EDL (0.95±0.14) and soleus (1.54±0.22). The value for soleus muscles was significantly different from the theoretical value of 1.13. EDL muscles were freeze-clamped at rest (n=10) and during the recovery phase, 1 min after the onset of the tetanus (n=10), to determine lactate and creatine phosphate. It was found that no significant amount of net lactate was produced. The amount of creatine phosphate reformed corresponded to the recovery heat produced. The results suggest that metabolic recovery after short tetani of EDL and soleus muscles occurs predominantly through oxidative phosphorylation, but knowledge of respiratory control in the living cell is insufficient to explain its slow onset immediately following contraction and the finding that EDL recovers faster than soleus.     

Effects of long-term cold exposure on contractile muscles of rats

The effects of 20-week cold exposure on contractile properties of soleus and extensor digitorum longus (EDL) muscles and plasma hormone levels were studied in rats. Twenty male Wistar rats (5 week old) were randomly divided into 2 groups (n = 10 each): cage-control and cold-exposed. The rats in the cold-exposed group were immersed in shoulder-deep water (approximately 18 degrees C) for 1 h/d, 5 d/week, for 20 weeks. The temperature and humidity of the animal room with 12:12 h light-dark cycle were maintained at approximately 23 degrees C and 55%, respectively. The rats were pair-fed powdered diets. The electromyogram activities in soleus and EDL were elevated by cold exposure. The body weight and absolute soleus wet weight of the cold-exposed group were significantly less than controls at the end of experiment. The one-half relaxation time and contraction time of EDL were significantly longer in the cold-exposed group than in the control group. The rate of twitch tension development, normalized by the maximum twitch tension, in EDL of the cold-exposed group was less than in the control group. Further, the fatigue resistance of EDL, but not of soleus, in response to train stimulation at 10 Hz was improved by cold exposure. The plasma levels of thyroid hormones, 3,5,3'-triiodothyronine and thyroxine, were significantly greater in cold-exposed group. Similar changes were also seen in the plasma catecholamine levels in the cold-exposed group (p > 0.05). It is suggested that long-term cold exposure causes a shift of the contractile properties of fast-twitch EDL muscle toward the slow-twitch type. The results also indicated that the characteristics of muscles responded more strongly to an increased activity level than to the elevation of plasma hormones.     

Dynamic properties of fast and slow skeletal muscles of the rat after nerve cross-union

1. The properties of self-innervated (S-EDL, S-SOL) and cross-innervated (X-EDL, X-SOL) extensor digitorum longus (EDL) and soleus (SOL) muscles have been determined at various times between 25 and 490 days after operations, and these are compared with the properties of normal muscles from unoperated animals of about the same age.2. The muscle fibres of X-SOL were 1.15 times longer than fibres of N-SOL and S-SOL at about 480 days after operations but the diameter of fibres was the same in the three muscles.3. The length of muscle fibres was the same in X-EDL, N-EDL and S-EDL 480 days after operations but the fibres of X-EDL failed to grow in cross-sectional area after innervation by soleus nerve fibres.4. The twitch: tetanus ratio was altered transitorily in both X-EDL and X-SOL but returned to near normal values within about 300 days after operations.5. The time courses of isometric contractions and the force: velocity properties were virtually the same in normal and self-innervated muscles 480 days after operations. The isometric twitch contraction time was about 13 msec for N-EDL, 25 msec for X-EDL, 34 msec for N-SOL and 15 msec for X-SOL, and the intrinsic speed of shortening of sarcomeres was about 45.1 mu/sec for N-EDL, 22.5 mu/sec for X-EDL, 19.8 mu/sec for N-SOL and 33.8 mu/sec for X-SOL; in these respects there was incomplete transformation of EDL to a slow muscle and of SOL to a fast muscle.6. There was a high degree of correlation between the intrinsic speed of shortening of sarcomeres and the isometric twitch contraction time but there was no correlation between isometric twitch contraction time and twitch: tetanus ratio of self-innervated and cross-innervated muscles 200-480 days after operations, and normal muscles from unoperated animals of the same age.7. The relation between intrinsic speed of shortening of sarcomeres and isometric twitch contraction time was approximately hyperbolic for normal, self-innervated and cross-innervated EDL and SOL muscles 480 days after operations.8. The maximum speed of shortening of whole muscle fibres was the same for X-SOL, S-EDL and N-EDL, and the same for X-EDL, S-SOL and N-SOL. The possibility that a neural influence determines the speed of shortening of whole fibres is discussed.     

Increased susceptibility of EDL muscles from mdx mice to damage induced by contractions with stretch

Summary Absence of dystrophin in mdx muscles may render the muscle more susceptible to damage when submitted to high stress levels. To test this, typically slow (soleus) and fast (EDL) limb muscles of dystrophic (mdx) and normal (C57BL/10) mice were submitted (in vitro) to a series of isometric contractions, followed by a series of contractions with stretches. Muscle injury was assessed by monitoring the force signal. Membrane damage was evaluated by bathing the muscle in Procion Red, a dye that does not penetrate intact fibres, and subsequent analysis by light microscopy. After isometric contractions, only a very small force drop (<3% of maximal isometric force) was observed which indicated that no injury had occurred in soleus and EDL muscles in either mdx or C57 strains. After contractions with a stretch, a force drop of 10% was observed in soleus muscles from both strains and in EDL muscles from C57 mice. However, in mdx mice EDL muscles displayed an irreversible force drop of 40–60%. Histological analysis of the muscles indicates that force drop is associated with membrane damage. These results show that EDL muscles from mdx mice are more vulnerable than their controls, supporting the structural role hypothesis for dystrophin. Furthermore, they suggest that contractions with stretches may contribute to the muscle damage and degeneration observed in DMD-patients.     

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.     

The effect of load on the phenotype of the developing rat soleus muscle

In newborn Wistar rats the load on the soleus muscle was reduced by removing the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles. Eighteen days later the soleus muscles were removed from both the operated and control legs and examined physiologically and histologically. The time course of twitch contraction of the soleus on the operated side was not significantly different from that of control muscles, but the muscles developed less tension. The decreased tension was consistent with a smaller number of muscle fibres. Histochemical and immunocytochemical examination showed that in the operated muscle, fewer fibres reacted with an antibody against slow myosin, while the number of fibres that reacted for alkali-preincubated ATPase, indicative of neonatal or adult fast myosin, was increased. Some fibres expressed both types of myosin. These findings suggest that a reduced load delays the phenotypic expression of slow myosin isoform in the developing soleus muscle.     

Age-dependent fatigue in single intact fast- and slow fibers from mouse EDL and soleus skeletal muscles

In the present work, we investigate age-dependent changes in isometric endurance in response to repetitive stimulation in single intact fast- and slow-twitch muscle fibers from young and old mice. To examine this issue we performed in vitro experiments in manually dissected EDL and soleus muscle fibers. We examined the force generation capacity of fibers in response to two stimulation protocols characterized by different inter-tetanic intervals, named short (1-s) and long interval (3.65-s). Fatigability was measured according to the fatigue index (FI, ratio between the maximum tension recorded in the last over the first tetanus in a train of pulses), the time course of the FI and sag (gradual decrease in force during a partially fused tetanic contraction). Fibers were classified according to the FI using two different criteria previously used in the literature (first criterion: FI > or = 1, 075-099, 0.5-074 and < 0.5; second criterion: FI > or = 1, 0.75-0.99, 0.25-0.74 and < 0.25). The fatigue index distribution recorded in the population of fibers corresponding to EDL and soleus muscles from young and old mice studied with the short and long interval protocols was not statistically different. In summary, these results support the concept that the decline in mechanical performance with aging is not related with changes in fatigability of individual fast- or slow twitch muscles fibers.     

Adaptation of rat extensor digitorum longus to overload and increased activity

Rat extensor digitorum longus (EDL) muscles were overloaded by removal of the synergist tibialis anterior (TA). The weight of the overloaded muscle was increased 15 days after the initial operation and remained higher throughout the period studied (153 days). The times to peak twitch tension and half relaxation remained unaltered, but the twitch and tetanic tensions developed by the overloaded EDL muscles increased. The overloaded EDL muscles became significantly more fatigue resistant. In a separate group of animals the overloaded EDL muscle was also chronically stimulated at 10 Hz. The additional stimulation altered the response of the EDL to overload in that the time to peak twitch tension of the muscle was slightly prolonged. There was no increase in twitch or tetanic tension in spite of the increase in muscle weight, but the electrical stimulation led to a further increase in fatigue resistance above that seen in overloaded muscles. The histochemical and immunocytochemical examination of the muscle revealed that there was a moderate increase in succinate dehydrogenase activity in the muscles overloaded only, but a considerable increase in those overloaded muscles that were also stimulated. There was no obvious change in the number of muscle fibres that reacted with an antibody to slow myosin in either overloaded only or overloaded and stimulated EDL muscles. Thus the addition of continuous activity to overload induced a slowing of contraction and prevented the increase of force usually induced by overload.     

Contractile and histochemical properties of regenerating cross-transplanted fast and slow muscles in the rat

Summary The soleus (SOL) or extensor digitorum longus (EDL) muscles of month-old rats were denervated for 14 days and then cross-transplanted so that the fast muscle was placed into the bed of the slow muscle and vice versa. At 17, 30, 60, and 90 days the transplants were tested for certain contractile and histochemical properties. By 90 days the cross-transplanted SOL showed complete conversion of the full contraction time and nearly complete conversion of the half relaxation time to those of the normal EDL. In contrast, the contraction and relaxation times of the cross-transplanted EDL became considerably slowed, but did not attain the values of the normal SOL. Histochemical staining for ATPase and SDH activity demonstrated similar transformations of fiber types. The degree of transformation of twitch and histochemical characteristics in cross-transplanted muscles was greater than the values reported after cross-innervation of the same muscles. The cross-transplantation model has certain advantages over nerve cross-union experiments because the cross-transplanted muscle is placed in the normal functional environment of the other muscle.Supported by grants from the Muscular Dystrophy Associations of America and a scientific exchange between the Academies of Sciences of Czechoslovakia and the United States.     

The time course of thyroid-hormone-induced changes in the isotonic and isometric properties of rat soleus muscle

Rat thyroidectomy resulted in changes in a number of parameters used to characterise the mechanical and histochemical status of skeletal muscle. Thus thyroidectomy resulted in a prolongation of soleus slow-twitch muscle isometric contraction time and half-relaxation time with a reduced maximum velocity of shortening and maximum rate of development of tetanic tension but no significant change in twitch: tetanus ratio i.e. the ratio of twitch force/unit area to tetanic force/unit area. In addition the percentage of IIA fibres was reduced and the percentage of type I fibres increased. Triiodothyronine, administered to hypothyroid rats, brought about a speeding of these parameters again with no change in twitch: tetanus ratio. There was an increase in the percentage of IIA fibres with a concomitant reduction in the percentage of type I fibres. These changes were induced over 18 days and resulted in isotonic and isometric properties almost identical to those of soleus muscles from chronically hyperthyroid rats; speeding could be detected as early as 2 days after triiodothyronine had been given. Consideration is given to the possibility that changes in myosin isoforms and/or the kinetics of changes in intracellular calcium concentration in activation and relaxation could account for the time course of the observed changes.