Sol Muscles Research Articles

The role of ascorbic acid and exercise in chronic ischemia of skeletal muscle in rats

This study was designed to investigate the effects of peripheral arterial insufficiency, exercise, and vitamin C administration on muscle performance, cross-sectional area, and ultrastructural morphology in extensor digitorum longus (EDL) and soleus (Sol) muscles in rats. Adult Wistar rats were assigned to ischemia alone (isch), ischemia-exercised (exe), ischemia-vitamin C (vit C), and ischemia-exercise-vitamin C (vit C + exe) groups. Ischemia was achieved via unilateral ligation of the right common iliac artery. Contralateral muscles within the same animal served as controls. Exercise protocol consisted of 50-min intermittent level running performed every other day for 5 days. Vitamin C (100 mg/kg body wt) was administered intraperitoneally on a daily basis throughout the 14 days of the experiment. With regard to the EDL muscle, ischemia alone reduced muscle strength, which was not recovered after vitamin C administration. Exercise alone following ischemia induced the most severe structural damage and cross-sectional area decrease in the muscle, yet the reduction in tetanic tension was not significant. Exercise in conjunction with vitamin C administration preserved ischemia-induced EDL muscle tetanic tension. In the Sol muscle, a significant reduction in single twitch tension after vitamin C administration was found, whereas the tetanic force of the ischemic Sol was not significantly decreased compared with the contralateral muscles in any group. Ischemic Sol muscle cross-sectional area was reduced in all but the exe groups. In Sol, muscle strength was reduced in the vit C group, and mean cross-sectional area of ischemic Sol muscles was reduced in all groups except the exe group. These results illustrate that mild exercise, combined with a low dose of vitamin C supplementation, may have beneficial effects on ischemic EDL muscle with a smaller effect on the Sol muscle.

Diffusion tensor imaging in evaluation of human skeletal muscle injury

To explore the capability and reliability of diffusion tensor magnetic resonance imaging (DTI) in the evaluation of human skeletal muscle injury. DTI of four patients with gastrocnemius and soleus muscles injuries was compared to eight healthy controls. Imaging was performed using a GE 3.0T short-bore scanner. A diffusion-weighted 2D spin echo echo-planar imaging (EPI) pulse sequence optimized for skeletal muscle was used. From a series of axially acquired diffusion tensor images the diffusion tensor eigenparameters (eigenvalues and eigenvectors), fractional anisotropy (FA), and apparent diffusion coefficient (ADC) were calculated and compared for injured and healthy calf muscles. Two dimensional (2D) projection maps of the principal eigenvectors were plotted to visualize the healthy and pathologic muscle fiber architectures. Clear differences in FA and ADC were observed in injured skeletal muscle, compared to healthy controls. Mean control FA was 0.23 +/- 0.02 for medial and lateral gastrocnemius (mg and lg) muscles, and 0.20 +/- 0.02 for soleus (sol) muscles. In all patients FA values were reduced compared to controls, to as low as 0.08 +/- 0.02. The ADC in controls ranged from 1.41 to 1.31 x 10(-9) m(2)/second, while in patients this was consistently higher. The 2D projection maps revealed muscle fiber disorder in injured calves, while in healthy controls the 2D projection maps show a well organized (ordered) fiber structure. DTI is a suitable method to assess human calf muscle injury.

Heart failure alters MyoD and MRF4 expressions in rat skeletal muscle

Heart failure (HF) is characterized by a skeletal muscle myopathy with increased expression of fast myosin heavy chains (MHCs). The skeletal muscle-specific molecular regulatory mechanisms controlling MHC expression during HF have not been described. Myogenic regulatory factors (MRFs), a family of transcriptional factors that control the expression of several skeletal muscle-specific genes, may be related to these alterations. This investigation was undertaken in order to examine potential relationships between MRF mRNA expression and MHC protein isoforms in Wistar rat skeletal muscle with monocrotaline-induced HF. We studied soleus (Sol) and extensor digitorum longus (EDL) muscles from both HF and control Wistar rats. MyoD, myogenin and MRF4 contents were determined using reverse transcription-polymerase chain reaction while MHC isoforms were separated using polyacrylamide gel electrophoresis. Despite no change in MHC composition of Wistar rat skeletal muscles with HF, the mRNA relative expression of MyoD in Sol and EDL muscles and that of MRF4 in Sol muscle were significantly reduced, whereas myogenin was not changed in both muscles. This down-regulation in the mRNA relative expression of MRF4 in Sol was associated with atrophy in response to HF while these alterations were not present in EDL muscle. Taken together, our results show a potential role for MRFs in skeletal muscle myopathy during HF.

Load-Specific Distribution of Muscle Activity in Human Triceps Surae Muscles

Motor units are arranged topographically within some muscles, and are activated in the order of their size, which enables individual regions to be activated in contraction. The triceps surae muscles are composed of the gastrocnemius and soleus (Sol) muscles. The gastrocnemius contains a higher percentage of type II fibers than that in the Sol. We hypothesized that these two muscles show different distribution characteristics of muscle activity. PURPOSE: This hypothesis was tested by comparing three-dimensional (3-D) distribution of muscle activity in the medial gastrocinemius (MG) and Sol during plantar flexion exercise at two different loads with use of muscle functional magnetic resonance imaging. METHODS: Six male subjects performed seven sets involved 10 repetitions of plantar flexion exercises with load that was 25 or 75% of their 12 repetitions maximum (RM). Transverse relaxation time (T2)-weighted images (repetition time 2500 ms; echo time 25, 80 ms; slice thickness 10 mm; gap 0 mm) were acquired from calf before and after each exercise load. To threshold images, considering as “active muscle” only those pixels showing T2 greater than the mean + 1SD of regions of interest (ROIs) in the pre-exercise images and a T2 lower than the mean + 1 SD of ROIs in the post-exercise images. Thresholded images were 3-D reconstructed, from which area of active muscle was determined along the x-axis (transversely), y-axis (longitudinally), and z-axis (vertically), respectively. RESULTS: The percentage area of activated muscle, which is relative to the anatomical cross-sectional area, was significantly larger after exercise with heavy load (75% of 12RM) than with light load (25% of 12RM) at each axis in the MG. On average, percentage area of activated muscle was 53.5 ± 6.4% in heavy load and 35.5 ± 3.3% in light load at x-axis (P< 0.05), 53.7 ± 6.9% in heavy load and 36.2 ± 3.0% in light load at y- axis (P <0.05), and 54.2 ± 4.8% in heavy load and 37.0 ± 3.7% in light load at z-axis (P <0.05). However, Sol did not show such differences. In the MG, distribution of muscle activity showed similar pattern between heavy and light loads along the x-axis and y-axis, but it varied across along the z-axis (P <0.05); it was higher at the proximal portion with greater load. No interactive effect was found at any axes in the Sol. CONCLUSION: Increase in load of repetitive plantar flexion exercises was accompanied by increased activations in the MG, especially at around the region of proximal portion and at throughout in a transversal and longitudinal axes. These findings suggest that load-specific distribution characteristics of muscle activity would be different between MG and Sol.

Direct corticospinal pathways contribute to neuromuscular control of perturbed stance

The antigravity soleus muscle (Sol) is crucial for compensation of stance perturbation. A corticospinal contribution to the compensatory response of the Sol is under debate. The present study assessed spinal, corticospinal, and cortical excitability at the peaks of short- (SLR), medium- (MLR), and long-latency responses (LLR) after posterior translation of the feet. Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation were individually adjusted so that the peaks of either motor evoked potential (MEP) or H reflex coincided with peaks of SLR, MLR, and LLR, respectively. The influence of specific, presumably direct, corticospinal pathways was investigated by H-reflex conditioning. When TMS was triggered so that the MEP arrived in the Sol at the same time as the peaks of SLR and MLR, EMG remained unaffected. Enhanced EMG was observed when the MEP coincided with the LLR peak (P < 0.001). Similarly, conditioning of the H reflex by subthreshold TMS facilitated H reflexes only at LLR (P < 0.001). The earliest facilitation after perturbation occurred after 86 ms. The TMS-induced H-reflex facilitation at LLR suggests that increased cortical excitability contributes to the augmentation of the LLR peaks. This provides evidence that the LLR in the Sol muscle is at least partly transcortical, involving direct corticospinal pathways. Additionally, these results demonstrate that approximately 86 ms after perturbation, postural compensatory responses are cortically mediated.

Endurance Training: Volume-Dependent Adaptational Changes in Myosin

The purpose of this study was to find the effect of different endurance training volumes on the composition and turnover of myosin. Sixteen-week-old male rats of the Wistar strain were divided into three different volume-based training groups. Changes in myosin heavy chain (MyHC), myosin light chain (MyLC) isoforms' composition, their synthesis rate, as well as myosin binding C-protein synthesis rate, and muscle protein degradation rate were measured. In slow-twitch (ST) soleus (Sol) muscle MyHC I isoform relative content increased and MyHC IIa isoform decreased during excessive increase in the volume of endurance training (ET). In plantaris (Pla) muscle excessive increase in ET volume decreased MyHC I and IIb isoforms, and increased MyHC IIa and IId relative content. In extensor digitorum longus (EDL) muscle the relative content of MyHC IId isoform increased during ET, but excessive increase in training volume decreased it. In Pla muscle the relative content of MyLC 1 (slow) isoform decreased during ET, but excessive increase in ET volume decreased the relative content of MyLC 3 (fast) isoform in both fast-twitch (FT) muscles. Decrease in MyHC and myosin binding C-protein synthesis rate in Pla muscle had significant correlation with ET volume (r = - 0.537, p < 0.05 and r = - 0.727, p < 0.001 subsequently). MyHC I and IIb isoforms and MyLC 3 (fast) isoform in Pla muscle and MyHC IIb, IId and MyLC 3 (fast) isoforms in EDL muscle are the most sensitive to the increase in ET volume. Excessive increase in ET volume leads to a decrease in physical working capacity. The degradation of muscle protein increased during ET in all groups.

Muscle activation and its distribution within human triceps surae muscles

The purposes of this study were 1) to quantify the volume of activated parts within a whole muscle and 2) to examine activated area distributions along the length of muscle. Seven male subjects performed five sets of 10 repetitions of a single-leg calf-raise exercise with the knee fully extended. Transverse relaxation time (T2)-weighted spin echo images were acquired before and immediately after the exercise. A range of pixels with a T2 greater than the mean +1 SD of the region of interest (ROI) from the preexercise image and pixels with a T2 lower than the mean + SD of the ROI from the postexercise image were defined as "active" muscle. The active muscle images were three dimensionally reconstructed, from which the volume of the activated muscle was determined for individual triceps surae (TS) muscles. Our data indicate that approximately 46% of the medial gastrocnemius (MG) muscle was activated during the exercise, with activation of the lateral gastrocnemius (LG) and soleus (Sol) muscles being approximately 35%. In the MG, distal portions had a greater percentage area of activated muscle than the proximal portions (P < 0.05), which was consistent with the results regarding electromyogram activity. In contrast, regional activation differences were not observed in the LG and Sol. These findings suggest that the amounts of activated muscle and its distribution would be different among TS muscles.

Composition and turnover of contractile proteins in volume-overtrained skeletal muscle.

The purpose of this study was to find the composition shift of myosin heavy chain (MyHC) isoforms in overtraining in fast- and slow-twitch skeletal muscles and different changes in MyHC isofom composition, synthesis and turnover rate between 4-week and 6-week overtraining. Male Wistar rats were randomly assigned to 4-week and 6-week endurance training, 4-week and 6-week overtraining groups. Plantaris (Pla), extensor digitorum longus (EDL), and soleus (Sol) muscles were studied. Daily excretion of 3-methylhistidine (3-MeHis) pool as an indicator for protein degradation increased in the 4-week and 6-week overtraining group to 4.04 +/- 0.21 and 4.32 +/- 0.23 %/day subsequently in comparison with the control group (2.16 +/- 14 %/day, p < 0.001). In Pla muscle MyHC I isoform synthesis rate was 33 200 +/- 2150 (after 6-week overtraining 27 100 +/- 1800, p < 0.05), IIa 32 600 +/- 2100; IId 27 300 +/- 1890 and IIb isoform 20 100 +/- 1600 (after 6-week overtraining 15 500 +/- 1400, p < 0.05) dpm/M leucine/min. Actin synthesis rate increased in fast-twitch muscles during 4- and 6-week overtraining, and in soleus muscle during 6-week overtraining. In EDL and Sol muscle MyHC isoform composition during 6-week overtraining did not change significantly. During the 6-week overtraining the relative content of MyHC I and IIb isoforms decreased and IIa and IId isoforms increased in Pla muscle. The initial increase of MyHC IIb isoform after 4-week overtraining shows the higher stability of this isoform in comparison with MyHC I isoform in fast-twitch muscles during high volume exercise.

Voluntary physical activity alterations in endothelial nitric oxide synthase knockout mice.

One of the main factors that control vasoreactivity and angiogenesis is nitric oxide produced by endothelial nitric oxide synthase (eNOS). We recently showed that knocking out eNOS induces an important reduction of mitochondrial oxidative capacity in slow-twitch skeletal muscle. Here we investigated eNOS's role in physical activity and contribution to adaptation of muscle energy metabolism to exercise conditions. Physical capacity of mice null for the eNOS isoform (eNOS-/-) was estimated for 8 wk with a voluntary wheel-running protocol. In parallel, we studied energy metabolism enzyme profiles and their response to voluntary exercise in cardiac and slow-twitch soleus (Sol) and fast-twitch gastrocnemius (Gast) skeletal muscles. Weekly averaged running distance was two times lower for eNOS-/- (4.09 +/- 0.42 km/day) than for wild-type (WT; 7.74 +/- 0.42 km/day; P < 0.01) mice. Average maximal speed of running was also lower in eNOS-/- (17.2 +/- 1.4 m/min) than WT (21.2 +/- 0.9 m/min; P < 0.01) mice. Voluntary exercise influenced adaptation to exercise specifically in Sol muscle. Physical activity significantly increased Sol weight by 22% (P < 0.05) in WT but not eNOS-/- mice. WT Sol muscle did not change its metabolic profile in response to exercise, in contrast to eNOS-/- muscle, in which physical activity decreased cytochrome-c oxidase (COX; -36%; P < 0.05), citrate synthase (-37%; P < 0.06), and creatine kinase (-24%, P < 0.01) activities. Voluntary exercise did not change energy enzyme profile in heart (except for 39% increase in COX activity in WT) or Gast muscle. These results suggest that eNOS is necessary for maintaining a suitable physical capacity and that when eNOS is downregulated, even moderate exercise could worsen energy metabolism specifically in oxidative skeletal muscle.

Differential displacement of the human soleus and medial gastrocnemius aponeuroses during isometric plantar flexor contractions in vivo.

The human triceps surae muscle-tendon complex is a unique structure with three separate muscle compartments that merge via their aponeuroses into the Achilles tendon. The mechanical function and properties of these structures during muscular contraction are not well understood. The purpose of the study was to investigate the extent to which differential displacement occurs between the aponeuroses of the medial gastrocnemius (MG) and soleus (Sol) muscles during plantar flexion. Eight subjects (mean +/- SD; age 30 +/- 7 yr, body mass 76.8 +/- 5.5 kg, height 1.83 +/- 0.06 m) performed maximal isometric ramp contractions with the plantar flexor muscles. The experiment was performed in two positions: position 1, in which the knee joint was maximally extended, and position 2, in which the knee joint was maximally flexed (125 degrees ). Plantarflexion moment was assessed with a strain gauge load cell, and the corresponding displacement of the MG and Sol aponeuroses was measured by ultrasonography. Differential shear displacement of the aponeurosis was quantified by subtracting displacement of Sol from that of MG. Maximal plantar flexion moment was 36% greater in position 1 than in position 2 (132 +/- 20 vs. 97 +/- 11 N.m). In position 1, the displacement of the MG aponeurosis at maximal force exceeded that of the Sol (12.6 +/- 1.7 vs. 8.9 +/- 1.5 mm), whereas in position 2 displacement of the Sol was greater than displacement of the MG (9.6 +/- 1.0 vs. 7.9 +/- 1.2 mm). The amount and "direction" of shear between the aponeuroses differed significantly between the two positions across the entire range of contraction, indicating that the Achilles tendon may be exposed to intratendinous shear and stress gradients during human locomotion.

Effects of chronic heart failure in rats on the recovery of microvascular PO2 after contractions in muscles of opposing fibre type.

Chronic heart failure (CHF) impairs muscle O2 delivery (QO2) and, at a given O2 uptake (VO2), lowers microvascular O2 pressures (PmvO2: determined by the QO2-to-VO2 ratio), which may impair recovery of high-energy phosphates following exercise. Because CHF preferentially decreases QO2 to slow-twitch muscles, we hypothesized that recovery PmvO2 kinetics would be slowed to a greater extent in soleus (SOL: approximately 84% type I fibres) than in peroneal (PER: approximately 14% type I) muscles of CHF rats. PmvO2 dynamics were determined in SOL and PER muscles of control (CON: n= 6; left ventricular end-diastolic pressure, LVEDP: approximately 3 mmHg), moderate CHF (MOD: n= 7; LVEDP: approximately 11 mmHg) and severe CHF (SEV: n= 4; LVEDP: approximately 25 mmHg) following cessation of electrical stimulation (180 s; 1 Hz). In PER, neither the recovery PmvO2 values nor the mean response time (MRT; a weighted average of the time to 63% of the overall response) were altered by CHF (CON: 66.8 +/- 8.0, MOD: 72.4 +/- 11.8, SEV: 69.1 +/- 9.5 s). In marked contrast, SOL PmvO2, at recovery onset, was reduced significantly in the SEV group ( approximately 6 Torr) and PmvO2 MRT was slowed with increased severity of CHF (CON: 45.1 +/- 5.3, MOD: 63.2 +/- 9.4, SEV: 82.6 +/- 12.3 s; P < 0.05 CON vs. MOD and SEV). These data indicate that CHF slows PmvO2 recovery following contractions and lowers capillary O2 driving pressure in slow-twitch SOL, but not in fast-twitch PER muscle. These results may explain, in part, the slowed recovery kinetics (phosphocreatine and VO2) and pronounced fatigue following muscular work in CHF patients.

Recovery of Microvascular Po2 in Rats with Chronic Heart Failure

0181 Chronic heart failure (CHF) impairs muscle O2 delivery (Qo2) and thus reduces microvascular O2 pressures (Po2m: determined by the Qo2-to-O2 utilization ratio), which may impair the recovery of high-energy phosphates following exercise. PURPOSE: Because CHF preferentially impairs Qo2 to slow-twitch muscles (Musch & Terrell, Am. J. Physiol. 262: H411-419, 1992), we hypothesized that recovery Po2m kinetics would be slowed to a greater extent in soleus (SOL: ∼84% type I fibers) than peroneal (PER: ∼14% type I) muscles of CHF rats. METHODS:Po2m dynamics were determined in SOL and PER muscles of control (CON: n = 6; left ventricular enddiastolic pressure, LVEDP: 3±1 mmHg), moderate CHF (MOD: n = 7; LVEDP: 10±2 mmHg) and severe CHF (SEV: n = 4; LVEDP: 28±4 mmHg) CHF following cessation of electrical stimulation (180s; 1 Hz). RESULTS: In PER, neither the recovery Po2m values nor their recovery kinetics (mean response time, MRT; CON: 66.8±8.0, MOD: 72.4±11.8, SEV: 69.1±9.5s) were altered by CHF. In marked contrast, SOL Po2m was reduced significantly in the SEV group (∼6 Torr) and Po2m recovery kinetics were slowed progressively with increased severity of CHF (MRT, CON: 45.1±5.3, MOD: 63.2±9.4, SEV: 82.6±12.3s; P<0.05 CON vs SEV). CONCLUSION: These data indicate that CHF slows Po2m recovery and thus lowers capillary O2 driving pressure in slow-twitch SOL, but not fast-twitch PER muscle with SEV CHF. These results may explain, in part, the slowed recovery kinetics (phosphocreatine and Vo V2) and fatigue following muscular work in CHF patients. Supported by: NIH HL-50306, HL-67619, AG-19228

Recovery of Microvascular Po2 in Rats with Chronic Heart Failure

0181 Chronic heart failure (CHF) impairs muscle O2 delivery (Qo2) and thus reduces microvascular O2 pressures (Po2m: determined by the Qo2-to-O2 utilization ratio), which may impair the recovery of high-energy phosphates following exercise. PURPOSE: Because CHF preferentially impairs Qo2 to slow-twitch muscles (Musch & Terrell, Am. J. Physiol. 262: H411-419, 1992), we hypothesized that recovery Po2m kinetics would be slowed to a greater extent in soleus (SOL: ∼84% type I fibers) than peroneal (PER: ∼14% type I) muscles of CHF rats. METHODS:Po2m dynamics were determined in SOL and PER muscles of control (CON: n = 6; left ventricular enddiastolic pressure, LVEDP: 3±1 mmHg), moderate CHF (MOD: n = 7; LVEDP: 10±2 mmHg) and severe CHF (SEV: n = 4; LVEDP: 28±4 mmHg) CHF following cessation of electrical stimulation (180s; 1 Hz). RESULTS: In PER, neither the recovery Po2m values nor their recovery kinetics (mean response time, MRT; CON: 66.8±8.0, MOD: 72.4±11.8, SEV: 69.1±9.5s) were altered by CHF. In marked contrast, SOL Po2m was reduced significantly in the SEV group (∼6 Torr) and Po2m recovery kinetics were slowed progressively with increased severity of CHF (MRT, CON: 45.1±5.3, MOD: 63.2±9.4, SEV: 82.6±12.3s; P<0.05 CON vs SEV). CONCLUSION: These data indicate that CHF slows Po2m recovery and thus lowers capillary O2 driving pressure in slow-twitch SOL, but not fast-twitch PER muscle with SEV CHF. These results may explain, in part, the slowed recovery kinetics (phosphocreatine and Vo V2) and fatigue following muscular work in CHF patients. Supported by: NIH HL-50306, HL-67619, AG-19228

Downhill Running Induced Myogenic And Cell-Cycle Regulatory Factor mRNA Expression In Mice

0059 Downhill running induces skeletal muscle injury that can activate quiescent muscle satellite cells (SC) in rats. Regulation of SC activity by myogenic and cell cycle regulatory factors is essential for skeletal muscle regeneration. However, little is known about the sequential expression of myogenic and cell cycle regulatory factors after a novel session of downhill running in mice. PURPOSE: To determine the expression of myogenic and cell cycle regulatory factors at the onset and after 72 hr of recovery from downhill running in mice. METHODS: Forty-one C57BL6 mice exercised on a treadmill at 25 m×min−1 and −14 % grade for 150 min or served as non-exercise controls. Mice were sacrificed immediately (0), 12, and 72 h post-run. Soleus muscle mRNA abundance for the myogenic regulatory factors MyoD and myogenin and the cell cycle regulatory factors Cyclin D1 and p21 were assessed by RT-PCR. Data were analyzed by ANOVA. RESULTS: SOL muscle wt to body wt ratio was not different from control at any time point post exercise. Cyclin D1, MyoD, and myogenin mRNA abundance was not significantly different from control at any time point. However, p21 mRNA abundance was 2.5 and 2.8 fold greater at 0 h and 12 h post exercise (p<0.05), and returned to control levels by 72 h. CONCLUSIONS: The absence of Cyclin D1, MyoD, or myogenin induction at 72 h suggests downhill running did not induce detectable SC proliferation in the mouse soleus muscle. However, the early p21 induction may reflect resident macrophage activation in the exercised soleus muscle. Funded by Glenn/AFAR Student Award

Stretch reflex excitability of the anti-gravity ankle extensor muscle in elderly humans.

To examine whether the stretch reflex excitability of the soleus muscle changes with age, stretch reflexes at rest (REST) and during weak voluntary contractions (ACT) were elicited in 18 older and 14 younger subjects. The amplitude of the stretch reflex responses and gain, defined as the gradient of the regression line for the relation between stretch reflex responses against the angular velocity of the applied perturbation, were evaluated in each short-latency (M1) and two long-latency components (M2 and M3). It was found that in the older group, both the amplitude and gain of the M1 component did not change from the REST to the ACT conditions, whereas in the younger group both variables significantly increased from the REST to ACT conditions. The latency of the M1 component was significantly shorter under the REST condition (older vs. younger: 51.8 +/- 7.37 vs. 55.1 +/- 8.69 ms), while no group differences were found in those variables under the ACT condition, suggesting that the muscle-tendon complexes of SOL muscles of the older subjects were less elastic and had less slack, probably due to age-related histochemical alterations. Further, the Hoffman reflex (H-reflex), elicited during the REST condition in 10 older and 11 younger subjects showed no significant differences, suggesting that the soleus motoneuron response to the Ia input was comparable between the two subject groups. The histochemical alterations occurring with the ageing process might augment the short-latency stretch reflex in the SOL muscle without enhancement of motoneuronal excitability, and this effect might be masked when the muscle is voluntarily activated.

Differential activation of mTOR signaling by contractile activity in skeletal muscle.

The cellular mechanisms by which contractile activity stimulates skeletal muscle hypertrophy are beginning to be elucidated and appear to include activation of the phosphatidylinositol 3-kinase signaling substrate mammalian target of rapamycin (mTOR). We examined the time course and location of mTOR phosphorylation in response to an acute bout of contractile activity. Rat hindlimb muscle contractile activity was elicited by high-frequency electrical stimulation (HFES) of the sciatic nerve. Plantaris (Pla), tibialis anterior (TA), and soleus (Sol) muscles from stimulated and control limbs were collected immediately or 6 h after stimulation. HFES resulted in mTOR phosphorylation immediately after (3.4 +/- 0.9-fold, P < 0.01) contractile activity in Pla, whereas TA was unchanged compared with controls. mTOR phosphorylation remained elevated in Pla (3.6 +/- 0.6-fold) and increased in TA (4.6 +/- 0.9-fold, P < 0.05) 6 h after HFES. Interestingly, mTOR activation occurred predominantly in fibers expressing type IIa but not type I myosin heavy chain isoform. Furthermore, HFES induced modest ribosomal protein S6 kinase phosphorylation immediately after exercise in Pla (0.4 +/- 0.1-fold, P < 0.05) but not TA and more markedly 6 h after in both Pla and TA (1.4 +/- 0.4-fold vs. 2.4 +/- 0.3-fold, respectively, P < 0.01). Akt/PKB phosphorylation was similar to controls at both time points. These results suggest that mTOR signaling is increased after a single bout of muscle contractile activity. Despite reports that mTOR is activated downstream of Akt/PKB, in this study, HFES induced mTOR signaling independent of Akt/PKB phosphorylation. Fiber type-dependent mTOR phosphorylation may be a molecular basis by which some fiber types are more susceptible to contraction-induced hypertrophy.

352 Quantitative and qualitative mitochondrial adaptations of substrates utilization in cardiac and skeletal muscles

have recently shown that knocking out of endothelium NOS (eNOS) induces an important reduction of mitochondrial oxidative capacity specifically in slow-twitch muscle (soleus, Sol) (Momken et al. 2002, Biochem J, 368:341-7). To examine the functional consequences of eNOS inhibition, physical capacity of mice, null for eNOS isoform, was estimated using a voluntary wheel-running program. In parallel, we studied energy metabolism enzyme profiles and their response to voluntary exercise in cardiac, Sol and fast-twitch gastrocnemius (Gast) skeletal muscles. Running distance, work, running speed and distance per run (mean distance the mice run each time they use the wheel) over the 8 week program were calculated after continuous recording of the output voltage of the DC generator on a PC computer. Weekly averaged work and running distance were two times lower for eNOS-/mice (608±65 J/day and 4.09±0.42 km/day) than for control (1207±74 J/day, p<0.01 and 7.74±0.42 km/day, p<0.01). The average maximal running speed was 19% (p<0.01) and the distance per run was 50% (p<0.01) lower in eNOS-/-mice. Furthermore, our study showed that physical activity influenced the adaptation to exercise specifically in Sol muscle. Physical activity increased significantly Sol weight by 22% (p<0.05) in wild-type mice but not in eNOS-/mice. Control Sol did not change its metabolic profile, in contrast to eNOS-/muscle where exercise decreased cytochrome c oxidase (COX) (-36%, p<0.05), citrate synthase (-37%, p<0.06) and creatine kinase (-24%, p<0.01) activities. Exercise changed energy enzyme profile neither in heart nor in Gast muscle (the only exception was a 44%, p<0.05 decrease in COX activity in active Gast muscle). Thus, in the absence of eNOS-derived NO, exercise induces a down-regulation of energy metabolism specifically in slow-twitch muscle. These results suggest that eNOS might be important for maintaining a suitable physical capacity and that its down-regulation might participate in intolerance to exercise.

CALCIUM RELEASE AND UPTAKE RATES FROM SARCOPLASMIC RETICULUM FOLLOWING FATIGUING STIMULATION

Skeletal muscle fatigue is characterised by a decrease in force production and a slower rate of tension development and relaxation. There is growing evidence that a decrease in sarcoplasmic reticulum (SR) function, e.g. a decrease in Ca2+ release and uptake rates, occurs during fatigue with short-term, intense stimulation. However, no studies have dealt with prolonged stimulation, or if there is any fiber type specific effect of fatigue on SR Ca2+ release and uptake rates. PURPOSE We tested the effect of short-term intense stimulation and prolonged moderate stimulation on Ca2+ release and uptake rates in the rat soleus (SOL; mainly slow-twitch) and extensor digitorum longus (EDL; mainly fast-twitch) muscles. METHODS Male Sprague-Dawley rats ∼5 wk old were killed by cervical dislocation. EDL and SOL muscles (n=7–10 in each group) were quickly excised and placed in Krebs-Ringer bicarbonate buffer. Muscles were kept resting or electrically stimulated in vitro with either a) 60 Hz for 400 ms every s for 100 s (SHORT) or b) 30 Hz for 400 ms every 2 s for 40 min (LONG). Ca2+ release and uptake rates were analysed on muscle homogenates using Ca2+ fluorescent dye indo-1. RESULTS Resting SR Ca2+ release and uptake rates were around 4-fold higher in EDL compared to SOL (p < 0.01). Ca2+ release rates were inhibited in both EDL and SOL following SHORT (∼25% p < 0.01) and also reduced in both muscle groups following LONG (∼15%, p < 0.05). Ca2+ uptake rates measured in 100nM steps between 200 and 800nM were similarly reduced in both SOL and EDL following SHORT (p < 0.05; concentration and treatment effect). In LONG, Ca2+ uptake rates in SOL showed a minor reduction after stimulation (p < 0.05), whereas no effect was observed in EDL. CONCLUSION These findings demonstrate for the first time that short-term intense stimulation inhibited Ca2+ release and uptake significantly in both slow-twitch and fast-twitch muscles. Prolonged stimulation had a minor but significant effect on Ca2+ release in both muscle groups, with minor or no effect on Ca2+ uptake.

MRI MEASUREMENT OF IN VIVO BEHAVIOR OF HUMAN TRICEPS SURAE DURING VOLUNTARY SUBMAXIMAL ISOMETRIC CONTRACTIONS

A muscle tendon unit complex has traditionally been modeled as a series/parallel arrangement of contractile and passive elastic elements. However, the actual in vivo, measurement of the movement of these elements has been limited. PURPOSE The aim of this study was to develop a model of the human triceps surae complex in terms of the soleus (Sol) and medial gastrocnemius (MG) and how they move relative to one another based on measuring their velocities of movement during a submaximal isometric contraction using MR imaging techniques. METHODS Seven normal subjects performed cyclic isometric contractions at 35% of maximal voluntary contraction (MVC) force level while in a 1.5Tesla Magnetic Resonance Imaging (MRI) scanner (GE, Milwaukee). Subjects performed the contractions timed to an audio metronome. A velocity-encoded cine phase contrast pulse sequence was used to acquire images of the leg in the sagittal plane during the contraction. Velocity was encoded in S/I (superior/inferior) direction. The sagittal images contained MG and Sol tissue velocity information in the form of phase contrast images in which the pixel signal intensity was proportional to the velocity of tissue. RESULTS MG and Sol had significant differences in their movement patterns (p < 0.01, paired t-test). Mean MG contraction velocity was −0.40 cm/s (+/- 0.44) and mean Sol contraction velocity was 0.87 cm/s (+/- 1.1) during the force development phase (see graphs). CONCLUSION During submaximal isometric contraction, the Sol muscle belly lengthened (positive contraction velocity) during the force development phase of the contraction while the MG shortened (negative contraction velocity) during the same phase. These data demonstrate that within the triceps surae muscle complex there are substantial differences in the temporal and regional patterns of shortening and lengthening.Figure: No Caption Available

Overload-induced androgen receptor expression in the aged rat hindlimb receiving nandrolone decanoate.

This study's purpose was to examine whether functional overload with nandrolone decanoate (ND) administration increased muscle mass and steroid receptor concentration in aged rat soleus (Sol) and plantaris (Plan) muscle. ND (6 mg/kg body wt) was administered once a week for 4 wk, whereas control rats received sesame seed oil injections. Functional overload of the hindlimb Sol and Plan was induced by synergistic gastrocnemius muscle ablation at the beginning of the fourth week. Adult (5 mo of age) and aged rats (25 mo of age) were randomly assigned to four groups: control, overload, control-ND, and overload-ND. Seven days of functional overload increased adult Sol muscle mass 27%, whereas the aged Sol muscle mass did not change. The aged overloaded Sol muscle receiving ND significantly increased muscle weight by 35% and total muscle protein by 24%. Aged Plan muscle did not increase muscle weight with overload or ND treatment. Androgen receptor protein was induced by ND treatment and functional Ov, and combining the two treatments induced Sol androgen receptor protein concentration above either alone. Sol glucocorticoid receptor protein concentration increased in overload groups of both ages. ND administration can increase aged Sol muscle mass and protein content after 7 days of functional overload, and the cooperative induction of androgen receptor may be important for this response.