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   

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.     

The influence of catecholamines on Na,K transport in slow- and fast-twitch muscles of the rat

The effects of catecholamines on active sodium and potassium transport was compared in slow- (SOL) and fast-twitch (EDL) skeletal muscles of the rat. Stimulation of active Na+-extrusion and K+-uptake induced by adrenaline (6-30 mumol . l-1) and isoprenaline (1-40 mumol . l-1) was markedly greater in slow- than in fast-twitch muscle. In sodium-preloaded muscles the maximal stimulation of 24Na-efflux induced by adrenaline was about 2-fold higher in SOL than in EDL. Isoprenaline caused a 2.4-fold increase in ouabain-sensitive 86Rb influx in SOL muscle, but failed to alter the ouabain-sensitive influx in EDL. The stimulating action of isoprenaline on 86Rb influx in EDL was due to an increase in the ouabain-insensitive fraction of Rb uptake. The effects of catecholamines of fast- and slow-twitch muscles were probably due to the accumulation of cyclic AMP, however the fact that there were no significant differences between the nucleotides levels in fast- and slow-twitch muscle suggests the participation of other mechanism as well. The results presented suggest that cyclic AMP-induced stimulation of ouabain-insensitive transport of cation in the isolated EDL muscle of the rat is similar to that of barnacle muscle.     

Late reinnervation of the rat soleus muscle is differentially suppressed by chronic stimulation and by ectopic innervation

Soleus (SOL) and extensor digitorum longus (EDL) muscles in adult rats were kept denervated for 2 months by four repeated freezes at 2-week intervals of the sciatic nerve. Reinnervation was studied in the absence or presence of chronic muscle stimulation, starting 1 month before reinnervation began. In addition, reinnervation was studied in SOL muscles where a previously transplanted fibular (FIB) nerve had formed ectopic neuromuscular junctions outside the original endplate area. After repeated freezes only, reinnervation was complete judged by tension measurements and histochemical examinations in SOL (n = 7) and EDL (n = 8) muscles. In directly stimulated muscles reinnervation was incomplete, and the force tensions evoked from indirect stimulation was on average 87 (n = 5) and 82% (n = 5) of direct muscle stimulation in SOL and EDL muscles, respectively. Of ectopically innervated SOL muscle fibres, only 26% became reinnervated in 12 muscles. Denervation and reinnervation increased the number of muscle fibres in stimulated (n = 4) and unstimulated (n = 5) EDL muscles by 18 and 15%, respectively. In stimulated (n = 4) and unstimulated (n = 7) SOL muscles, on the other hand, the number of muscle fibres remained normal. The stronger suppression of reinnervation in ectopically reinnervated compared to chronically stimulated SOL fibres indicates that reinnervation can also be suppressed by activity independent influences from the foreign nerve.     

Differential activation of mitogen-activated protein kinase signalling pathways by isometric contractions in isolated slow- and fast-twitch rat skeletal muscle

Activation of mitogen-activated protein (MAP) kinases has been implicated in the signal transduction pathways linking exercise to adaptive changes of muscle protein expression. In the present study, we investigated whether contractions of isolated muscles induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and p38 MAPK in a fibre-type dependent manner. Slow-twitch (soleus) and fast-twitch (epitrochlearis, extensor digitorum longus) rat skeletal muscles were exposed to intermittent tetanic stimulation. Compared with the contralateral non-stimulated muscle, contractions increased ERK1/2 phosphorylation to the same extent in fast- and slow-twitch muscles. Significant increase in phosphorylation of p38 MAPK was observed in the fast-twitch muscles only. The total amount of ERK1/2 and p38 MAPK proteins was higher in the slow-twitch soleus muscle. In conclusion, MAP kinase signalling pathways are differentially activated and expressed in slow- and fast-twitch muscles. In addition, this activation is owing to muscle contraction per se and do not demand additional external influence.     

限食8周对大鼠不同类型骨骼肌收缩功能及线粒体生物合成的影响

目的:探讨限食对不同类型骨骼肌收缩功能和线粒体生物合成的影响,为阐明限食的有益作用及机制提供实验依据。方法:每天按正常大鼠摄食量的60%饲养动物以制备限食8周大鼠模型,麻醉下分离比目鱼肌和趾长伸肌,记录电刺激诱导骨骼肌单次、强直和疲劳收缩,观测线粒体基因细胞色素C氧化酶I亚基(COX I)和核基因β-actin拷贝数的比值以反映线粒体生物合成,检测骨骼肌ATP含量以反映线粒体功能。结果:限食8周对电刺激诱导的比目鱼肌和趾长伸肌的单次和强直收缩均有增强作用,但仅提高比目鱼肌的抗疲劳作用;限食也增加两种肌肉的ATP含量,但对比目鱼肌更明显;限食虽对2种肌肉腺苷酸活化蛋白激酶(AMPK)的磷酸化具有增加作用,但只上调比目鱼肌内线粒体生物合成及其调节基因PGC-1α和NRF的转录。结论:限食8周增强大鼠比目鱼肌和趾长伸肌对电刺激的收缩反应,尤其对富含氧化型肌纤维的比目鱼肌更明显;其机制除与限食促进这2种肌肉AMPK活化、增加ATP供应以外,还与上调比目鱼肌线粒体生物合成及其调控因子有关。     

Glycogen stability and glycogen phosphorylase activities in isolated skeletal muscles from rat and toad

There is increasing evidence that endogenous glycogen depletion may affect excitation–contraction (E–C) coupling events in vertebrate skeletal muscle. One approach employed in physiological investigations of E–C coupling involves the use of mechanically skinned, single fibre preparations obtained from tissues stored under paraffin oil, at room temperature (RT: 20–24°C) and 4°C for several hours. In the present study, we examined the effect of these storage conditions on the glycogen content in three muscles frequently used in research on E–C coupling: rat extensor digitorum longus (EDL) and soleus (SOL) and toad iliofibularis (IF). Glycogen content was determined fluorometrically in homogenates prepared from whole muscles, stored under paraffin oil for up to 6 h at RT or 4°C. Control muscles and muscles stored for 0.5 and 6 h were also analysed for total phosphorylase (Phos_total) and phosphorylase a (Phos a) activities. No significant change was observed in the glycogen content of EDL and SOL muscles stored at RT for 0.5 h. In rat muscles stored at RT for longer than 0.5 h, the glycogen content decreased to 67.6% (EDL) and 78.7% (SOL) of controls after 3 h and 25.3% (EDL) and 37.4% (SOL) after 6 h. Rat muscles stored at 4°C retained 79.0% (EDL) and 92.5% (SOL) of glycogen after 3 h and 75.2% (EDL) and 61.1% (SOL) after 6 h. The glycogen content of IF muscles stored at RT or 4°C for 6 h was not significantly different from controls. Phos_total was unchanged in all muscles over the 6 h period, at both temperatures. Phos a was also unchanged in the toad IF muscles, but in rat muscles it decreased rapidly, particularly in EDL (4.1-fold after 0.5 h at RT). Taken together these results indicate that storage under paraffin oil for up to 6 h at RT or 4°C is accompanied by minimal glycogen loss in toad IF muscles and by a time- and temperature-dependent glycogen loss in EDL and SOL muscles of the rat.     

Sensitivity of rat soleus muscle to a mechanical stimulus is decreased following hindlimb unweighting

Mechanical loading is thought to be an important stimulus regulating muscle mass. However, the responsiveness of a muscle atrophied by a period of mechanical unloading to a subsequently imposed mechanical challenge is not well understood. This study examined the phosphorylation of the mechanically sensitive p54 c-jun NH₂-terminal kinase (JNK) signaling protein in atrophied rat soleus muscle in response to a mechanical challenge in situ (isometric contractions; 100 Hz, 150 ms, once every 1 s for 5 min). Rats underwent either 7 or 14 days of hindlimb suspension (HLS) following which phosphorylation of JNK was measured biochemically. Immunofluorescence analysis revealed that phosphorylated JNK was localized in myonuclei. Baseline JNK phosphorylation measured in non-stimulated soleus muscles of 7- and 14-day HLS groups was 3.0- and 2.8-fold, respectively, the baseline phosphorylation measured in muscle of weight-bearing control animals (CTL). Following a mechanical challenge, JNK phosphorylation in stimulated CTL and 7-day HLS groups was significantly increased by 3.2- and 1.8-fold at the non-stimulated baseline levels, respectively. In stimulated muscle of 14-day HLS, JNK phosphorylation levels did not significantly differ from the baseline levels suggesting that the ability to elicit a mechanically induced phosphorylation of the JNK signaling protein gradually decreases with unweighting and is attenuated after 14-day HLS. Changes in the responsiveness of mechanically sensitive intracellular signaling pathways in atrophic muscle may contribute to the functional impairment experienced by muscle in the absence of weight bearing for prolonged periods.     

'Fast' and 'slow' muscle fibres in hindlimb muscles of adult rats regenerate from intrinsically different satellite cells

Myosin heavy chain (MyHC) expression was examined in regenerating fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of adult rats. Myotoxic bupivacaine was injected into SOL and EDL and the muscles were either denervated or neuromuscularly blocked by tetrodotoxin (TTX) on the sciatic nerve. Three to 10 or 30 days later, denervated SOL or EDL, or innervated but neuromuscularly blocked EDL received a slow 20 Hz stimulus pattern through electrodes implanted on the muscles or along the fibular nerve to EDL below the TTX block. In addition, denervated SOL and EDL received a fast 100 Hz stimulus pattern. Denervated EDL and SOL stimulated with the same slow stimulus pattern expressed different amounts of type 1 MyHC protein (8% versus 35% at 10 days, 13% versus 87% at 30 days). Stimulated denervated and stimulated innervated (TTX blocked) EDL expressed the same amounts of type 1, 2A, 2X and 2B MyHC proteins. Cross-sections treated for in situ hybridization and immunocytochemistry showed expression of type 1 MyHC in all SOL fibres but only in some scattered single or smaller groups of fibres in EDL. The results suggest that muscle fibres regenerate from intrinsically different satellite cells in EDL and SOL and within EDL. However, induction by different extrinsic factors arising in extracellular matrix or from muscle position and usage in the limb has not been excluded. No evidence for nerve-derived trophic influences was obtained.     

Muscle atrophy continues after early cast removal following tendon repair.

We studied soleus (SOL), plantaris (PLN), and gastrocnemius (GST) muscles to determine whether early cast removal minimizes muscle atrophy or permits recovery from atrophy after tendon repair. After right tendocalcaneus (Achilles tendon) was transected and repaired, rabbit right hindlimbs were immobilized with the ankle plantar flexed and the knee flexed to 90 degrees. Rabbits were maintained in the cast and sacrificed at 5, 15, or 21 days postoperatively or the cast was removed on day 5 and the animals sacrificed at day 15 or 21. SOL, PLN, and GST muscles of both limbs were removed and weighed, and then histochemical analyses were performed on SOL and PLN muscles. Immobilization decreased SOL muscle wet weights, mean fiber cross-sectional area, and percentage of Type I fibers and increased the percentage of Type IIc fibers. Ten days after cast removal (i.e., postoperative day 15), SOL muscle atrophy and fiber composition did not differ significantly from continuously immobilized controls. However, 16 days after cast removal (i.e., postoperative day 21), SOL muscle fiber cross-sectional area and fiber composition were near normal, differing significantly from continuously casted controls. At each of the time intervals studied, PLN (containing many glycolytic fibers) did not atrophy as much as SOL (containing mainly oxidative fibers). Our results indicate that 1) early cast removal prevents atrophy of PLN glycolytic fibers, but not oxidative fibers in either PLN or SOL, and 2) early cast removal promotes recovery from atrophy of both oxidative and glycolytic fibers. In spite of the many differences between rabbits and humans, these findings suggest that, although early cast removal may not prevent oxidative muscle fiber atrophy after postoperative immobilization, it may facilitate recovery from atrophy.     

Changes in morphological and functional characteristics of male rat EDL muscle during growth

Summary Morphological and functional changes as well as changes in fibre-type composition were investigated in the left extensor digitorum longus (EDL) muscles of male Wistar rats of approximately 40, 60, 120 and 700 days old. A number of morphological changes occurred in the EDL muscle during growth. While from 40 to 120 days muscle mass and cross-sectional area (CSA) increased by 247 and 192%, changes in muscle and fibre lengths were much smaller (44 and 17%, respectively). Besides morphological changes tetanic force was also found to increase (approximately 307%) up to 120 days. Because this increase in force was greater than the increase in CSA, specific force increased by approximately 29% between 40 and 60 days. Thereafter, specific force stayed rather constant. From 40 until 60 days changes were also found in the force-frequency and force-velocity curve, which indicate a slowing of the muscles (until 60 days). Changes in fibre-type composition of the EDL muscle were found to occur later during growth between 60 and 120 days. In this period an increase in the relative total area of Type IIBd fibres and a decrease in the relative total area of Type IIBm fibres (corresponding to the Type 2X and IIB fibres, respectively), were found; this was apparently due to a conversion of many Type IIBm into Type IIBd fibres and not to a difference in cross-sectional growth between these fibres. Thus it seemed that the changes in the morphological parameters of rat EDL muscle occurred early in life, between 40 and 60 days of age, with a concomitant change in function but without changes in fibre-type composition, which occurred at later age.     

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.     

Effects of chronic stimulation on the size and speed of long-term denervated and innervated rat fast and slow skeletal muscles

This study seeks to identify the mechanisms which motoneurones use to control the contractile force and speed of skeletal muscles. We have stimulated directly slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles of adult rats intermittently at 100 Hz for 1-9 months. The muscles were either chronically denervated, denervated and reinnervated, or normally innervated. The stimulation started either immediately, or more commonly, after 1-9 months of denervation. Stimulation starting several months after denervation increased the mean maximum tetanic tension 37 times in SOL and eight times in EDL. These values represented 40 and 12% of the increases obtained by reinnervation after comparable periods of time. In denervated SOL and EDL muscles stimulated directly for more than 2 months, the mean isometric twitch contraction times were 13 and 12.7 ms, as in normal EDL muscles (13 ms). In innervated SOL muscles stimulated directly for 1-4 months, the mean twitch contraction times were 23.6 ms (normally innervated) and 19.2 ms (reinnervated), which were considerably shorter than in normal control SOL muscles (39.2 ms). Single motor unit recordings revealed that the natural (background) nerve impulse activity was essentially unaffected by the stimulation. Twitch contraction time and percentage of type II fibres in SOL muscles were related. The fastest muscles (denervated and stimulated) consisted of 100% type II fibres (with one exception), the second fastest (reinnervated and stimulated) of 70-50%, the third fastest (normally innervated and stimulated) of 45-0%, the second slowest (reinnervated) of 15-0%, and the slowest muscles (innervated controls) of 5-0% type II fibres.     

Force: velocity pproperties of kitten muscles

1. The characteristics of isometric contractions and force:velocity properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles of neonatal kittens were determined in situ.

2. The mean contraction time is 51 msec for EDL and 70 msec for SOL and the half-relaxation time is 51 msec for EDL and 109 msec for SOL.

3. The average maximum isometric tetanic tension per unit cross-sectional area of muscle is 1·27 kg/cm² for EDL and 1·17 kg/cm² for SOL.

4. The average twitch:tetanus ratio is 0·28 for EDL and 0·119 for SOL; the low value for SOL was found for both indirect and direct stimulation.

5. The average maximum speed of shortening of a sarcomere is 22·8 μ/sec for EDL and 12·7 μ/sec for SOL.

6. These properties of neonatal muscles are compared with those of adult cat muscles and discussed in connexion with differentiation of mammalian muscles into fast and slow types.

     

Overexpression of inducible 70-kDa heat shock protein in mouse improves structural and functional recovery of skeletal muscles from atrophy

Heat shock proteins play a key regulatory role in cellular defense. To investigate the role of the inducible 70-kDa heat shock protein (HSP70) in skeletal muscle atrophy and subsequent recovery, soleus (SOL) and extensor digitorum longus (EDL) muscles from overexpressing HSP70 transgenic mice were immobilized for 7 days and subsequently released from immobilization and evaluated after 7 days. Histological analysis showed that there was a decrease in cross-sectional area of type II myofiber from EDL and types I and II myofiber from SOL muscles at 7-day immobilization in both wild-type and HSP70 mice. At 7-day recovery, EDL and SOL myofibers from HSP70 mice, but not from wild-type mice, recovered their size. Muscle tetanic contraction decreased only in SOL muscles from wild-type mice at both 7-day immobilization and 7-day recovery; however, it was unaltered in the respective groups from HSP70 mice. Although no effect in a fatigue protocol was observed among groups, we noticed a better contractile performance of EDL muscles from overexpressing HSP70 groups as compared to their matched wild-type groups. The number of NCAM positive-satellite cells reduced after immobilization and recovery in both EDL and SOL muscles from wild-type mice, but it was unchanged in the muscles from HSP70 mice. These results suggest that HSP70 improves structural and functional recovery of skeletal muscle after disuse atrophy, and this effect might be associated with preservation of satellite cell amount.     

Resetting of resultant stiffness in ankle flexor and extensor muscles in the decerebrate cat

Summary Flexor (tibialis anterior, TA, and extensor digitorum longus, EDL) and extensor (soleus, SOL) muscles in the decerebrate cat were subjected to length changes and the force responses were measured. Resultant muscular stiffness, which arises from the mechanical reaction of muscle fibers contracting prior to the length change and from a change in force due to reflex action, was calculated by dividing the changes in force by the corresponding length changes. As shown previously in the premammillary preparation, resultant stiffness was usually higher in SOL than in TA or EDL. Following an intercollicular transection in some preparations, resultant stiffness increased markedly for TA but not substantially for SOL. During continuous electrical stimulation in the magnocellular red nucleus in premammillary preparations, resultant stiffness of SOL decreased for a wide range of forces while EDL responses were unaffected. These results show that reflex gain is not determined solely by the level of motoneuronal excitability but also by a descending control from the brainstem, and that the lower resultant stiffness in flexors compared to extensors in the decerebrate cat is set by this control system and not by inherent differences in the strength of autogenetic reflex pathways for the two muscles.     

Intrinsic properties and reflex compensation in reinnervated triceps surae muscles of the cat: effect of activation level

The manner in which activation levels influence intrinsic muscular properties and contributions of the stretch reflex were studied in homogeneous soleus (SOL) and heterogeneous gastrocnemius (G) muscles in the decerebrate cat. Intrinsic mechanical properties were represented by the initial stiffness of the muscle, measured prior to reflex action, and by the tendency of the muscle to yield during stretch in the absence of the stretch reflex. Stiffness regulation by the stretch reflex was evaluated by measuring the extent to which reflex action reduces yielding and the extent to which stiffness depends on background force. Intrinsic mechanical properties were measured in muscles deprived of effective autogenic reflexes using the method of muscular reinnervation. Reinnervated muscles were recruited to force levels comparable to those achieved during natural locomotion. As force declined during crossed-extension reflexes in reinnervated and intact muscles, initial stiffness declined according to similar convex trajectories. The data did not support the hypothesis that, for a given force level, initial stiffness is greatest in populations of predominantly type I motor units. Incremental stiffness (Deltaf/Deltal) of both G and SOL increased in the presence of the stretch reflex. Yielding of SOL (ratio of incremental to initial stiffness) substantially decreased in the presence of the stretch reflex over the full range of forces. In reflexive G, yielding significantly decreased for low to intermediate forces, whereas at higher forces, yielding was similar irrespective of the presence or absence of the stretch reflex. The stretch reflex regulates stiffness in both homogeneous and heterogeneous muscles.     

Changes in the excitability of soleus muscle short latency stretch reflexes during human hopping after 4 weeks of hopping training

Changes in the excitability of the human triceps surae muscle short latency stretch reflexes were investigated in six male subjects before and after 4 weeks of progressive two-legged hopping training. During the measurements the subjects performed 2-Hz hopping with: preferred contact time (PCT) and short contact time. The following reflex parameters were examined before and after the training period: the soleus muscle (SOL) Hoffmann-reflex (H-reflex) at rest and during hopping, the short latency electromyogram (EMG) components of the movement induced stretch reflex (MSR) in SOL and medial gastrocnemius muscle (MG), and the EMG amplitude of the SOL and MG tendon reflexes (T-reflexes) elicited at rest. The main results can be summarized as follows: the SOL T-reflex had increased by about 28% (P < 0.05) after training while the MG T-reflex was unchanged; the SOL MSR (always evident) and the MG MSR (when observable) did not change in amplitude with training, and before training the SOL H-reflex in both hopping situations was significantly depressed to about 40% of the reference value at standing rest (P < 0.05). After training the H-reflex during PCT hopping was no longer depressed. As the value of the measured mechanical parameters (the total work rate, joint angular velocity and the ankle joint work rate) was unchanged after training in both hopping situations, the reflex changes observed could not be ascribed to changes in the movement pattern. To explain the observed changes, hypotheses of changes in the excitability of the stretch reflex caused by the training were taken into consideration and discussed.     

Denervation increases protein tyrosine kinase and protein tyrosine phosphatase activities in fast and slow skeletal muscle

We have investigated the expression and regulation of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs) in fast extensor digitorum longus (EDL) and slow soleus (SOL) in adult rat skele-tal muscles. Biochemical assays revealed significantly greater PTK and PTP activities in SOL than in EDL; these results were confirmed and extended by in-gel assays demonstrating that the PTKs and PTPs detected had higher activity levels in SOL compared to EDL extracts. Although phosophotyrosine proteins were concentrated at the neuromuscular junction (NMJ), PTK and PTP activities were observed in extra-junctional regions of the muscle fiber. Following denervation, we observed significant increases in PTK and PTP activities in both SOL and EDL, and gel-based assays showed an increase in the activities of several PTKs and PTPs. These results suggest that the same PTK and PTPs have different activity levels in fast and slow skeletal muscles and are regulated by nerve-dependent mechanisms. Accepted: 31 May 2001