The re-expression of two myosin heavy chains in regenerated rat muscle spindles

The vascular supply and tendons of the extensor digitorum longus (EDL) muscles of two adult rats were unilaterally severed and the muscles were allowed to regenerate for 40 days. Serial frozen sections of muscle grafts were cut and stained for enzymes that delineated fiber type, sensory endings and motor endings. MF30 and ALD58, two antibodies which react only to intrafusal fibers in normal rat muscle, were reacted against sections of nerve-intact muscle grafts. Data were compared to that from muscles of normal rats. Encapsulated fibers devoid of sensory innervation and some extrafusal fibers in muscle grafts had a weak to moderate reaction to MF30, but no reaction to ALD58. Regenerated, encapsulated fibers with sensory innervation bound both MF30 and ALD58. These data indicate that afferents which reinnervate regenerated spindles retain the capacity to induce expression of spindle-specific myosin isoforms in rats.     

Increase of elastic fibres in muscle spindles of rats following single or repeated denervation with or without reinnervation

Summary Muscle spindles in the lower lumbrical muscles of rats were studied by transmission electron microscopy following denervation with or without reinnervation. The number and total area of elastic fibres per muscle spindle increased at 3–12 months following various experimental procedures: (1) denervation and reinnervation after a single crush lesion to the sciatic nerve; (2) reinnervation after four-fold repeated crush injuries; and (3) transection and suture of the nerve. The increased number of oxytalan and elaunin fibres, the precursors of mature elastic fibres, within these muscle spindles provided further evidence for their numerical and dimensional increase. An attachment site of elastic fibres at the spindle pole was identified at the inner cells of the outer spindle capsule. The processes of these cells embraced terminating elastic fibres tightly. Attachment of elastic fibres to intrafusal muscle fibres was less conspicuous since they were not similarly embraced but were rather indistinctly, though closely, associated with the basal lamina along longitudinal surface indentations of intrafusal muscle fibres. It is concluded from this series of experiments that muscle spindles, as dynamic mechanoreceptors, maintain their elastic properties even under pathological conditions. The increase of elastic fibres following denervation and reinnervation represents an obviously meaningful reaction that may compensate for loss of tonic properties of muscle spindles without causing stiffness.     

Muscle precursor cells invade and repopulate freeze-killed muscles

Summary A problem with the use of muscle grafting as a therapeutic procedure is to produce a graft functionally adequate to replace a muscle of complex architecture, such as a sphincter muscle. We thought it might be possible to use dead cadaver muscles, repopulated by the patient's own muscle precursor cells (mpc), to reconstruct muscles whose anatomy would be imposed by the framework of dead muscle and whose genetic constitution would be determined by the mpc. Here we show, in the mouse, that an extensor digitorum longus (EDL) muscle, killed by repeated freezing and thawing, repopulated with mpc and grafted into a nu/nu or tolerant AKR host mouse, is capable of supporting muscle formation. By using the allotypic isoenzyme forms of glucose-6-phosphate isomerase as markers, we have shown that the newly regenerated muscle in such grafts is derived mainly from the implanted mpc, but also to some extent from the host mouse's own mpc. By 50–70 days after grafting, new muscle fibres were found to constitute up to 70% of the graft. Many fibres had assumed diameters in the normal range for mouse muscle, often having peripherally placed nuclei. These findings raise the possibility of the therapeutic use of such grafts. To our surprise, dead EDL muscle grafts into which no mpc had been implanted were also the site of good muscle regeneration. New-formed muscle in these grafts was shown to be derived entirely from mpc which must have migrated into the graft from the host. Investigation of the mechanisms underlying this phenomenon should further our knowledge of factors which regulate the proliferation and movement of dormant mpc in adult animals.     

Non-neural and neural expression of myosin heavy chains by regenerated intrafusal fibers of rats.

Expression of myosin heavy chain (MHC) isoforms was studied in rat soleus (SOL) and extensor digitorum longus (EDL) muscles which regenerated in the presence or absence of innervation. Frozen sections of two 5 day denervated SOL and EDL grafts, two 40 day denervated SOL and EDL grafts, and two reinnervated 40 day SOL and EDL grafts were processed for demonstration of motor endplates, sensory endings, myosin adenosine triphosphatase (mATPase) and for expression of 4 MHCs. No qualitative differences in MHC expression were noted between 5 day or 40 day denervated grafts of the SOL and EDL muscles. All regenerated intrafusal and extrafusal myotubes or myofibers reacted to antibodies against neonatal and fast-twitch MHCs, but not to antibodies against slow-twitch and slow-tonic MHCs in these grafts. These data indicate that MHCs expressed by regenerated intrafusal myotubes do not parallel those expressed by myotubes which give rise to the three types of intrafusal fibers during development and that MHC expression by regenerated intrafusal myotubes parallels that of regenerated extrafusal myotubes prior to innervation. However, some regenerated intrafusal fibers in 40 day nerve-intact grafts bound antibodies to slow-twitch and slow-tonic MHCs, thus expressions of these two MHCs are nerve-dependent in regenerated muscle spindles.     

Nestin expression reflects formation, revascularization and reinnervation of new myofibers in regenerating rat hind limb skeletal muscles

This work provides a comprehensive view of expression and distribution patterns of the intermediate filament nestin in rat regenerating skeletal muscles. Regeneration was induced by heterochronous isotransplantation of extensor digitorum longus (EDL) or soleus muscles from 15-day-old rats into the EDL muscle of adult female inbred Lewis rats. Recipients were sacrificed by anesthetic overdose and the host muscles with the graft were excised after 7-, 16-, 21- and 29-day survival, fixed in 10% formalin solution and immunohistochemically stained. In the regenerating skeletal muscle maximal nestin expression was observed in newly formed myoblasts and myotubes and it decreased in the course of skeletal myofiber development. We demonstrate the phenomenon of a peripheral concentration of nestin expression within the differentiating myofibers called 'lateralization', which may represent a mechanism of nestin elimination during the development of skeletal myofibers. Furthermore, nestin has been detected in newly formed endothelial cells of blood vessels growing into the regenerated area and also in peripheral nerves innervating the graft as well as the host muscle. In conclusion, according to our results, nestin represents an ideal marker for skeletal muscle regeneration, as its expression reflects processes of development, revascularization and reinnervation of new myofibers.     

The free autografting of entire limb muscles in the cat: Morphology

Normal or pre-denervated extensor digitorum longus (EDL) muscles were successfully grafted in place of the contralateral EDL muscles in cats. Histological preparations of 57 grafts were examined at intervals from 4 to 170 days after transplantation. The morphological sequence of events in a cat muscle graft is similar to that in the rat except that the time course of regeneration is considerably slower. Surviving original muscle fibers form a thin rim at the periphery of the graft. Beneath this rim, the original muscle fibers undergo fragmentation, starting near the periphery and progressing toward the center. Regenerating muscle fibers take the place of the degenerated muscle fibers. In pre-denervated grafts, the last of the original muscle fibers in the center of the graft have broken down by 41 days whereas in normal muscle grafts the original muscle fibers in the central region persist until 50 days. The main difference between grafts of normal and pre-denervated muscles is the rate of breakdown of the original muscle fibers. Long term grafts of both groups are morphologically very similar.     

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.     

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.     

Skeletal fiber types and spindle distribution in limb and jaw muscles of the adult and neonatal opossum,Monodelphis domestica

The South American opossum, Monodelphis domestica, is very immature at birth, and we wished to assess its potential for studies of jaw muscle development. Given the lack of prior information about any Monodelphis fiber types or spindles, our study aimed to identify for the first time fiber types in both adult and neonatal muscles and the location of spindles in the jaw muscles. Fiber types were identified in frozen sections of adult and 6-day-old jaw and limb muscles by using myosin ATPase and metabolic enzyme histochemistry and by immunostaining for myosin isoforms. The distribution of fiber types and muscle spindles throughout the jaw-closer muscles was identified by immunostaining of sections of methacarnoy-fixed, wax-embedded heads. Most muscles contained one slow (type I) and two fast fiber types (equivalent to types IIA and IIX), which were similar to those in eutherian muscle, and an additional (non-IIB) fast type. In jaw-closer muscles, the main extrafusal fiber type was IIM (characteristic of these muscles in some eutherians), and almost all spindles were concentrated in four restricted areas: one in masseter and three in temporalis. Six-day neonatal muscles were very immature, but future spindle-rich areas were revealed by immunostaining and corresponded in position to the adult areas. Extrafusal and spindle fiber types in Monodelphis share many similarities with eutherian mammalian muscle. This finding, along with the immaturity of myosin isoform expression observed 6 days postnatally, indicates that Monodelphis could provide a valuable model for studying early developmental events in the jaw-closer muscles and their spindles. Anat. Rec. 251:548–562, 1998. © 1998 Wiley-Liss, Inc.     

A quantitative assessment of muscle spindle formation in reinnervated and non-reinnervated grafts of the rat extensor digitorum longus muscle

Free grafting of the rat extensor digitorum longus muscle disrupts the neurovascular supply and results in degeneration and subsequent regeneration of both muscle and nerve. Reinnervation of one group of grafts was prevented by severing and ligating the sciatic nerve. Thirty days after surgery, the number of muscle spindles in each graft was determined by examination of serial sections at the light-microscopic level. Non-operated control muscles contained 37.86 ± 3.02 (s.d.) spindles per muscle. Means for the reinnervated and non-reinnervated grafts were 15.63 ± 3.89 and 18.38 ± 5.15 respectively. At the time of surgery, all of these grafts had been treated with 0.75% bupivacaine, a local anesthetic which, combined with the grafting technique, ensures degeneration of all muscle fibers. An additional group of grafts was not subjected to this treatment and contained a mean of 27.40 ± 2.19 spindles.The embryogenesis of muscle spindles is dependent upon interaction between sensory nerves and developing myotubes. The conditions for the formation of these stretch receptors in regenerating muscle have been less clear. Their formation in non-reinnervated grafts demonstrates that this process, unlike spindle development in the embryo, does not require the presence of nerves. When reinnervation of a graft is allowed, large numbers of nerve fibers have been observed in muscle spindles. It is not yet known whether or not the normal pattern of innervation, essential for proper function of the spindle, is restored.     

Structure, distribution and innervation of muscle spindles in avian fast and slow skeletal muscle

Muscle spindles in 2 synergistic avian skeletal muscles, the anterior (ALD) and posterior (PLD) latissimus dorsi, were studied by light and electron microscopy to determine whether morphological or quantitative differences existed between these sensory receptors. Differences were found in the density, distribution and location of muscle spindles in the 2 muscles. They also differed with respect to the morphology of their capsules and intracapsular components. The slow ALD possessed muscle spindles which were evenly distributed throughout the muscle, whereas in the fast PLD they were mainly concentrated around the single nerve entry point into the muscle. The muscle spindle index (number of spindles per gram wet muscle weight) in the ALD was more than double that of its fast-twitch PLD counterpart (130.5±2.0 vs 55.4±2.0 respectively, n=6). The number of intrafusal fibres per spindle ranged from 1 to 8 in the ALD and 2 to 9 in the PLD, and their diameters varied from 5.0 to 16.0 μm and 4.5 to 18.5 μm, respectively. Large diameter intrafusal fibres were more frequently encountered in spindles of the PLD. Unique to the ALD was the presence of monofibre muscle spindles (12.7% of total spindles observed in ALD) which contained a solitary intrafusal fibre. In muscle spindles of both the ALD and PLD, sensory nerve endings terminated in a spiral fashion on the intrafusal fibres in their equatorial regions. Motor innervation was restricted to either juxtaequatorial or polar regions of the intrafusal fibres. Outer capsule components were extensive in polar and juxtaequatorial regions of ALD spindles, whereas inner capsule cells of PLD spindles were more numerous in juxtaequatorial and equatorial regions. Overall, muscle spindles of the PLD exhibited greater complexity with respect to the number of intrafusal fibres per spindle, range of intrafusal fibre diameters and development of their inner capsules. It is postulated that the differences in muscle spindle density and structure observed in this study reflect the function of the muscles in which they reside.     

A comparative study of muscle spindles in slow and fast neonatal muscles of normal and dystrophic mice

Muscle spindles from the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles of genetically dystrophic mice of the dy^2J/dy^2J strain were compared with age-matched normal animals at neonatal ages of 1–3 weeks according to histochemical, quantitative, and ultrastructural parameters. Intrafusal fibers in both the soleus and EDL exhibited similar regional differences in myosin ATPase activity, and conformed to those noted previously in various adult species. In distal polar regions, all nuclear bag fibers resembled extrafusal fibers of the type 1 variety, whereas in capsular zones they could be divided into two subtypes. Nuclear chain fibers possessed a staining pattern similar to type 2 extrafusal fibers, and in contrast to the bag fibers they exhibited no regional variations. These features were consistently observed in both the normal and dystrophic muscles at all ages. Spindles varied only slightly in their number and distribution in the two types of muscle, and their location followed the neurovascular branching pattern in each. Irrespective of age or genotype, spindles in the soleus were more homogeneously dispersed, but those in the EDL were concentrated along the dorsal aspect of the muscle. No significant differences were noted in the total number of spindles between normal and dystrophic muscles. In addition, no dramatic differences were observed in the muscle spindle index for soleus and EDL. The first obvious disease-related changes were noted in extrafusal fibers of the soleus of 3-week-old mice, and spindles were often located close to these areas of fiber degeneration. Despite alterations in the surrounding tissue, however, spindles appeared morphologically unaltered in dystrophy. These observations indicate that intrafusal fibers of spindles in neonatal mice appear enzymatically and histologically unaffected in incipient stages of progressive muscular dystrophy.     

A quantitative study of satellite cells in regenerated soleus and extensor digitorum longus muscles

Satellite cells quantitated in the rat soleus and extensor digitorum longus (EDL) muscles following a complete regeneration returned to “normal” percentages of myofiber nuclei in both muscles 3 months after injury. Following cross-transplantation, the percentage of satellite cell nuclei in the EDL regenerated in the soleus bed was indistinguishable from the percentage in the soleus. Likewise, the soleus muscle regenerated in the EDL bed had a satellite cell percentage characteristic of the EDL. These results suggest that (1) the proportion of satellite cells is reestablished in a regenerated muscle and (2) the innervating nerve determines the proportion of satellite cell nuclei in a muscle.     

Neural organization of spindles in three hindlimb muscles of the rat

The neuroanatomical organization of the dynamic (bag₁) and static (bag₂ and chain) intrafusal systems was compared by light and electron microscopy of serial sections among 71 poles of muscle spindle in soleus (SOL), extensor digitorum longus (EDL), and lumbrical (LUM) muscles in the rat. Eighty-four percent of 195 fusi-motor (γ) axons to the spindles innervated either the dynamic bag₁ fiber or the static bag₂ and/or chain fibers. Sixteen percent of the γ axons coin-nervated the dynamic and static intrafusal fibers. Some of these nonselective axons were branches of efferents that also gave rise to axons selective to either the dynamic or static types of intrafusal fibers in one or more spindles. Thus activation of individual stem γ efferents might not have a purely dynamic or purely static effect on the integrated afferent outflow from spindles of a hind-limb muscle in the rat. In addition, primary afferents in all muscles had terminations that cross-innervated the dynamic bag₁ and static bag₂ and/or chain intrafusal fibers in individual spindles, an arrangement that may enhance the mixed dynamic/static behavior of afferents when different intrafusal fibers are activated concurrently. Spindles of the slow SOL and fast EDL muscles had similar features, whereas differences were observed in the organization of the proximal (SOL and EDL) and distal (LUM) muscles. Spindles in LUM muscles had fewer static intrafusal fibers, a higher ratio of dynamic to static γ axons, and a higher incidence of skeletofusimotor (β) innervation to intrafusal fibers than spindles in the SOL or EDL muscles. Thus, the relative contribution of dynamic and static systems to muscle afferent outflow may differ among spindles located in different segments of the rat hindlimb. However, the dynamic and static intrafusal systems of spindle were less sharply demarcated in each of the three hindlimb rat muscles than in the cat tenuissimus muscle.     

Satellite cells and myonuclei in long-term denervated rat muscles

Background: The percentage of satellite cells rapidly decreases in aneurally regenerating soleus muscles of rat. Also denervation of intact muscles causes fiber loss and regeneration, but the fate of satellite cells is unknown; myonuclei have been suggested to undergo changes resembling those in apoptotic cells. Methods: Rat soleus and extensor digitorum longus (EDL) muscles were denervated at birth or at age 5 weeks and investigated after periods of up to 38 weeks. At least 400 myonuclei in each muscle were assessed by electron microscopy, and satellite cell nuclei were counted. In sity nick translation and tailing were performed after 30 weeks denervation in order to demonstrate DNA breaks associated with apoptosis. Results: Myotubes indicating regeneration were prominent in the adult denervated soleus and deep layers of EDL muscles after 7 weeks and in the superficial parts of EDL muscle after 16 weeks. The percentage of satellite cell nuclei slowly decreased to less than one fifth of normal after 20–30 weeks. Almost all satellite cells had vanished 10 weeks after neonatal denervation. Degenerating myonuclei in adult, but not in neonatally denervated muscles, remotely resembled apoptotic nuclei of lymphocytes, but no evidence of DNA breaks was found. Conclusion: Denervation of rat skeletal muscles causes, in addition to fiber atrophy, loss of fibers with subsequent regeneration. Proliferation of satellite cells under aneural conditions may lead to exhaustion of the satellite cell pool. This process is more rapid in growing than in adult muscles. Myonuclei in denervated muscles do not show DNA breaks which can be demonstrated by in situ nick translation. © 1995 Wiley-Liss, Inc.     

Stretch receptors in regenerated rat muscle

Although muscle spindles are known to be present in regenerated muscles, it has not previously been reported whether they have any electrophysiological activity. In the present study, rat extensor digitorum longus muscles were traumatized so as to cause all muscle fibers to degenerate; the muscle nerves were either left intact to promote subsequent reinnervation or severed to impede reinnervation. After 2-4 months of regeneration, the muscles were subjected to stretch stimuli and sensory activity was recorded electrophysiologically. Many of the muscles contained stretch-sensitive units that behaved like muscle spindles, although the responses were often highly adaptive and somewhat erratic. In general, the responses from muscles with the nerve left intact were more normal than those from muscles in which the nerve had been severed. Silver staining of the same muscles showed that morphologically recognizable muscle spindles were present, but all were abnormal to varying degrees. These results demonstrate that regenerated muscles can regain some degree of sensory activity in addition to motor functions. The prospect of restored sensory activity may be critical in evaluating the efficacy of graft or transplant procedures for human muscles.     

Axotomy induces fusimotor-free muscle spindles in neonatal rats.

Crushing the nerve to the medial gastrocnemius (MG) muscle at birth and administering nerve growth factor to rats afterwards results in a reinnervated muscle with supernumerary muscle spindles, some of which must have formed de novo. Structure and innervation of spindles in the reinnervated MG muscles were studied in serial 1 micron transverse sections. Two types of spindle-like encapsulations were observed. The prevalent type consisted of one to three small diameter intrafusal fibers with features of nuclear chain fibers or infrequently a nuclear bag fiber. The second type of encapsulation consisted of the small-diameter fibers located in a compartment which abutted a compartment containing a large diameter extrafusal fiber. All intrafusal fibers in spindles of the experimental muscles were innervated by afferents, but most of them (85%) were devoid of efferent innervation. Thus, immature fusimotor neurons may be more susceptible than spindle afferents to cell death after axotomy at birth.     

Modulation of fatty-acid-binding protein content of rat heart and skeletal muscle by endurance training and testosterone treatment

The effects of training and/or testosterone treatment and its aromatization to oestradiol on fatty-acid-binding protein (FABP) content and cytochrome c oxidase activity in heart, soleus and extensor digitorum longus (EDL) muscles were studied in intact adult female rats. One group of rats remained sedentary, whereas the others were trained for 7 weeks. Thereafter the trained rats were divided into control and testosterone-treated groups, with or without an aromatase inhibitor. Testosterone was administered by a silastic implant. Training was continued for 2 weeks. In untreated sedentary rats the immunochemically assayed FABP contents were 497±28, 255±49 and 58±17 g/g wet weight for the heart, soleus, and EDL respectively. In the heart the FABP content was increased after training (29%), testosterone treatment (33%) or both manipulations (53%). In soleus muscle FABP increased only after testosterone treatment (16%), whereas in EDL no changes were found. Inhibiting the aromatase enzyme complex abolished the testosterone-induced effect on FABP content in soleus (suggesting an oestradiol effect) but not in heart muscle. Among the three muscles studied the FABP content was found to be related to the cytochrome c oxidase activity in a non-linear way. In conclusion, it is shown that the FABP contents and mitochondrial activities of heart and skeletal muscle are affected by training and sex hormones and that these effects are different for heart and skeletal muscles.Established Investigator of the Netherlands Heart Foundation     

Regeneration of skeletal muscle in streptozotocin-induced diabetic rats

The present study analyzes the regeneration of skeletal muscle in diabetic rats. Intravenous injection of streptozotocin (STZ) was used to induce diabetes. Six weeks later the extensor digitorum longus (EDL) muscles from diabetic rats were either transplanted into diabetic or normal hosts to initiate regeneration. Normal EDL muscle transplants in normal and diabetic hosts were also performed for comparison. One, 2, 4, and 12 weeks after transplantation, the EDL regenerates were morphologically analyzed. Regeneration and formation of neuromuscular junctions were observed in all transplants, including diabetic regenerates in diabetic hosts. The overall mass and myofiber size of the diabetic EDL regenerate in the diabetic host was significantly reduced in spite of complete regeneration. Recovery of the diabetic muscle mass and the myofiber size was observed after transplantation into normal hosts. A reduction in mass and myofiber size was observed in normal EDL muscles transplanted into diabetic hosts. It is concluded that poor recovery of diabetic muscle is related to metabolic and structural alterations in the diabetic host, rather than to innate capacity of the muscle to per se undergo regeneration and reinnervation. The observed enhancement in recovery of diabetic muscle after transplantation in a normal host and deterioration of normal muscle after transplantation in a diabetic host shows that the host environment determines the success of muscle regeneration.     

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.