Myosin heavy chain composition of regenerated soleus muscles during hindlimb suspension.

The aim of this study was to determine the MHC profile of regenerated soleus muscles in control (C, n = 8) and hindlimb suspended rats (HS, n = 8). After muscle degeneration was induced by injection of snake venom containing notexin, male rats were either tail suspended for 21 days or submitted to normal weight-bearing activity. Separation and detection of MHCs by sodium dodecyl sulphate-polyarcylamide gel electrophoresis (SDS-PAGE) showed that regenerated soleus muscles from C rats contained only type I and type IIa MHCs. The relative amount of type I MHC was higher in regenerated (93.9 +/- 1.7%) than in untreated muscles (86.5 +/- 2.3%)(P < 0.01). In the HS group, the immunohistochemical analysis showed that the majority of regenerated myofibres reacted positively with the antibody against fast MHCs. SDS-PAGE analysis demonstrated that HS resulted in a shift toward faster MHCs in both intact and regenerated myofibres. Regenerated soleus muscle from HS rats contained approximately 34% type IIa MHC, approximately 37% type IIx/d MHC and approximately 18% type IIb MHC, when type I MHC contributed to only approximately 12% of total myosin. The proportions of fast MHC isoforms in regenerated muscles were higher than those recorded in untreated muscles. Collectively, these results suggest that the shift in the MHC profile associated with hindlimb unweighting in adult undamaged soleus muscles is also related to the heterogeneity of early myoblasts.

Quantitative assessment of degenerative changes in soleus muscle after hindlimb suspension and recovery.

The aim of this study was to quantify the degenerative and regenerative changes in rat soleus muscle resulting from 3-week hindlimb suspension at 45 degrees tilt (HS group, n = 8) and 4-week normal cage recovery (HS-R group, n = 7). Degenerative changes were quantified by microscope examination of muscle cross sections, and the myosin heavy chain (MHC) composition of soleus muscles was studied by sodium dodecyl sulphate polyacrylamide gel electrophoresis. At the end of 3-week hindlimb suspension, histological signs of muscle degenerative changes were detected in soleus muscles. There was a significant variability in the percentage of fibres referred to as degenerating (%dg) in individual animals in the HS group [%dg = 8.41 (SEM 0.5)%, range 4.66%-14.08%]. Moreover, %dg varied significantly along the length of the soleus muscle. The percentage of fibres with internal nuclei was less than %dg in HS-soleus muscles [4.12 (SEM 0.3)%, range 1.24%-8.86%]. In 4-week recovery rats, the greater part of the fibres that were not referred to as normal, retained central nuclei [15.8 (SEM 2.2)%, range 6.2%-21.1%]. A significant increase in the slow isoform of MHG was recorded in the HS-R rats, compared to muscles from age-matched rats (P < 0.01). These results would suggest that a cycle of myofibre degeneration-regeneration occurred during HS and passive recovery, and that the increased accumulation of slow MHC observed in soleus muscles after recovery from HS could be related to the prevalence of newly formed fibres.

Muscle damage induced by running training during recovery from hindlimb suspension: the effect of dantrolene sodium.

We examined the extent of morphological alterations and the myosin heavy chain (MHC) distribution in the rat soleus muscle after a 4-week period of spontaneous recovery or retraining after hindlimb suspension (HS). Moreover, we tested the hypothesis that dantrolene sodium, which affects the flux of calcium over the sarcoplasmic reticulum membrane, was able to attenuate muscle damage. Three groups of rats were submitted to 3 weeks of HS, followed by either 4 weeks of unrestricted cage activity (HC, n = 7), or running training for the same period and were compared to age-matched animals (C, n = 8). Trained rats were treated with either placebo or dantrolene sodium (HTP, HTD, n = 8 each, respectively). Four weeks after HS recovery, the percentage of myofibres with internal nuclei (%in) was determined by histological staining with hematoxylin and eosin. %in was affected by the individual rat (P < 0.001), and was higher in the mid-belly region of the muscle (P < 0.05). Muscle damage, as estimated by %in, was more extensive in trained rats (i.e. HTP and HTD) than in HC animals (23% and 12%, respectively). Moreover, dantrolene sodium tended to exert a protective effect on training-induced muscle injury. A 12% increase in type I MHC was observed in both HTP and HTD rats, in comparison with group C animals (P < 0.001). The relative proportion of type-I MHC was inversely correlated with %in (r = -0.65, P < 0.001). Running recovery led to an increased citrate synthase activity in comparison with that of C or HC rats. In conclusion, the present findings demonstrate that running recovery from HS increases the incidence of muscle damage, and that dantrolene sodium administration has only limited protective effects against exercise-induced muscle injury.

Endurance training affects myosin heavy chain phenotype in regenerating fast-twitch muscle.

The aim of this study was to analyze the effects of treadmill training (2 h/day, 5 days/wk, 30 m/min, 7% grade for 5 wk) on the expression of myosin heavy chain (MHC) isoforms during and after regeneration of a fast-twitch white muscle [extensor digitorum longus (EDL)]. Male Wistar rats were randomly assigned to a sedentary (n = 10) or an endurance-trained (ET; n = 10) group. EDL muscle degeneration and regeneration were induced by two subcutaneous injections of a snake toxin. Five days after induction of muscle injury, animals were trained over a 5-wk period. It was verified that approximately 40 days after venom treatment, central nuclei were present in the treated EDL muscles from sedentary and ET rats. The changes in the expression of MHCs in EDL muscles were detected by using a combination of biochemical and immunocytochemical approaches. Compared with contralateral nondegenerated muscles, relative concentrations of types I, IIa, and IIx MHC isoforms in ET rats were greater in regenerated EDL muscles (146%, P < 0.05; 76%, P < 0.01; 87%, P < 0.01, respectively). Their elevation corresponded to a decrease in the relative concentration of type IIb MHC (-36%, P < 0.01). Although type I accounted for only 3.2% of total myosin in regenerated muscles from the ET group, the cytochemical analysis showed that the proportion of positive staining with the slow MHC antibody was markedly greater in regenerated muscles than in contralateral ones. Collectively, these results demonstrate that the regenerated EDL muscle is sensitive to endurance training and suggest that the training-induced shift in MHC isoforms observed in these muscles resulted from an additive effect of regeneration and repeated exercise.

Role of weight-bearing function on expression of myosin isoforms during regeneration of rat soleus muscles.

The expression of myosin isoforms was studied in regenerated rat soleus muscle during either normal or altered postural activity. Regeneration was induced following injury by venom from the Notechis scutatus scutatus snake. Immunohistochemical analysis showed that, in regenerating soleus muscle after 3 wk of hindlimb suspension, nearly all fibers reacted positively with the myosin heavy chain (MHC) antibody associated with fast-twitch muscle fibers (fast MHC). When 3 wk of recovery with normal weight-bearing activity followed hindlimb suspension, the regeneration soleus muscle exhibited a nearly homogeneous staining with the MHC antibody associated with the slow-twitch muscle fibers (slow MHC). These findings were in accordance with quantitative analysis of the electrophoretic separation of the native myosin isoforms. Immunohistochemical data showed that removal of weight bearing in the 21-day old regenerated soleus muscles resulted in an increase in fast MHC expression. Together, the results of the present study clearly demonstrate that the postural load is an important component in the induction of slow MHC in regenerating muscle and that the control of the expression of MHC in muscle comprising a homogeneous population of fibers deriving from satellite cells appears more homogeneous and more complete than in a nondegenerated one.

Changes in dietary protein intake fail to prevent decrease in muscle growth induced by severe hypoxia in rats.

Muscle growth, fiber size, muscle and liver glycogen, plasma hormones, and muscle glutamine concentration were evaluated in rats chronically exposed (26 days) to a simulated hypobaric altitude (HA; 6,000 m) and fed diets of varying protein concentrations (10, 20, or 40 g protein/100 g of dry matter; LP, MP, and HP, respectively). Values were compared with those measured in animals maintained under normobaric conditions and either fed ad libitum (SL groups) or pair fed equivalent quantities of food consumed by HA animals (PF groups). There was marked anorexia in response to HA exposure for all protein diets (P < 0.001). A specific effect of hypoxia on the decrease in muscle growth has been identified by comparison of the values of the muscle weight-to-body weight ratio between HA and PF groups (P < 0.05 for all dietary protein levels). Plasma insulin concentrations were lower in HA than in SL and PF rats (P < 0.05). Liver glycogen was significantly decreased by exposure to HA (P < 0.001) and high dietary protein content (P < 0.005). Hypoxia per se and decreased food intake had additive effects on soleus muscle glycogen concentrations. An increase in muscle glutamine was observed in rats fed the LP diet in comparison with the MP diet, especially in SL and PF groups (P < 0.05). These results clearly demonstrate that 1) hypobaric hypoxia per se decreases growth rate in rats and 2) increasing the dietary protein intakes in rat had no effect on the depression of muscle growth related to high altitude but had deleterious effects on glycogen deposition in liver and fast muscle.

Effects of chronic low frequency stimulation on structural and metabolic properties of hindlimb suspended rat soleus muscle.

The use of chronic low frequency stimulations (CLFS, 10 Hz bipolar current 8 h.day-1) as a countermeasure against unweighting-induced muscle alterations was investigated in rat soleus muscle during 21 days of hindlimb suspension (HS). It was shown that CLFS was able to minimize the soleus muscle atrophy induced by suspension (-29% in stimulated muscles compared to -56% in the non-stimulated soleus muscle). In parallel, CLFS partly prevented the HS-induced decreases in the cross-sectional area of type I fibres and in the total and myofibril protein contents. Stimulation at low frequency reduced the increase in the fast-myosin expression recorded with unweighting. Moreover, the HS-induced increase in glycolytic capacity was counteracted to a considerable extent by CLFS. In conclusion, the results of this study showed that CLFS can only partly prevent the HS-induced modifications in the soleus muscle. However, the limited effectiveness of CLFS to prevent muscle atrophy emphasized the critical role of reduced load bearing in the induction of soleus muscle atrophy.

Structural and biochemical properties of the rat myocardium after 21 days of head-down suspension.

The effects of 21 d of head-down suspension on the biochemical and structural properties of the myocardium were determined in male rats (HDS, n = 10), and compared with control non-suspended animals (C, n = 10). HDS rats were prepared using Morey's tail-suspension model, and maintained at 45 degrees tilt. At the end of the conditioning period, hearts were excised and dissected into right (RV) and left plus intraventricular septum (LV). We observed that HDS rats had lower LV- and RV-absolute weights than C animals (-8.5%, p < 0.05, and -12%, p < 0.05, respectively). The relative ventricle weights (ventricle weight/body weight, mg.g-1) were unaffected by HDS. Native myosin isoform analysis revealed that HDS did not alter myosin expression in both LV and RV. The capillary bed, examined using histochemical methods, was found to be unaffected by HDS. A significant decrease in the lactate dehydrogenase activity was detected in LV after 21 d of HDS (-16%, p < 0.05). Collectively, these results suggest that the early neurohumoral changes occurring in response to HDS-induced hemodynamic overload are sufficient to prevent any alteration in the biochemical and structural properties of the myocardial tissue.

Effects of growth hormone on rat skeletal muscle after hindlimb suspension.

To examine the effects of growth hormone (GH) on the preferential atrophy of the soleus muscle (SOL) occurring after hindlimb suspension (HS), two groups of male rats received daily injections of 2 IU.kg-1 body mass of recombinant human growth hormone (rhGH). Rats were either suspended by the tail for 21 days (HS-GH, n = 5) or nonsuspended (C-GH, n = 5). The effects of rhGH treatment on SOL and extensor digitorum longus muscles (EDL) were compared in two groups of animals receiving daily injections of saline, either suspended by the tail (HS-SA, n = 5) or nonsuspended (C-SA, n = 5). The results showed that the SOL hypertrophy in response to rhGH administration was mostly observed in C rats (+33%, P < 0.01). This increase in muscle mass was correlated with a concomitant increase in the size of type I fibres (+21%, P < 0.05). Although SOL mass decreased during HS in rhGH treated animals (-44%, P < 0.001), the mean normalized mass of this muscle did not significantly differ between C-SA and HS-GH groups. A statistically significant increase in the absolute mass of EDL occurred with rhGH treatment in C-GH (+12%, P < 0.05). The HS-induced decrease in the percentage distribution of type I fibres in SOL was unaffected by the rhGH treatment. In addition, a decrease in the citrate synthase activity in the whole SOL was observed in the two groups of tail-suspended rats (-31%, P < 0.05; -21%, P < 0.05 in SA and GH animals, respectively).2+ f1p4

Effects of surface electrostimulation on the structure and metabolic properties in monkey skeletal muscle.

Adaptative changes in skeletal muscle following surface electrical stimulation (SES) were investigated in rhesus monkeys. SES was performed on the triceps brachialis muscle (TB) according to an intermittent pattern. The procedure was carried out for 3 wk, using a current with a medium frequency of 60 Hz normally observed in fast motor axons. The histochemical assays performed on biopsies taken from proximal and distal parts of the TB muscle, before and after the SES program, showed that the distribution of fibers typed by ATPase was unaffected. On the other hand, SES led to an overall increase in the mean fiber cross-sectional area (FCSA); P < 0.01 (+13.7%, NS, in proximal portion, vs +31%, P < 0.01 in distal portion). This increase in size occurred in all fiber types. SES was shown to induce an overall increase in capillary to fiber ratio (C/F; +11.06%, NS, in proximal portion, vs +25.93%, P < 0.05 in distal portion). The number of capillaries surrounding fiber Type II (CAFII) was significantly increased by SES (P < 0.05): +3.21%, NS, in proximal portion, versus +21.47%, P < 0.05 in distal portion. Moreover, the number of capillaries surrounding fiber Type I (CAFI) was statistically unaffected by SES. These results suggest that a stimulation of capillary growth may occur following SES-training. Citrate synthase activity was significantly increased after SES. This enhancement in oxidative potential was shown to occur in all fiber types (NADH-diaphorase staining).(ABSTRACT TRUNCATED AT 250 WORDS)

In situ NADH laser fluorimetry during muscle contraction in humans.

The aim of the present study was to use nicotinamide adenine dinucleotide phosphate, reduced (NADH) fluorimetry, to investigate in situ NADH changes during muscle contraction in humans on an isokinetic dynamometer. Thirteen healthy male subjects each performed one maximal voluntary contraction (MVC) with the knee extensor muscle. The NADH muscle fluorescence was monitored by a double beam laser fluorimeter which uses an optical fibre, percutaneously inserted through a needle into the vastus lateral muscle, to guide the light. The NADH fluorescence was continuously measured at a wavelength of 337 nm. To estimate the haemodynamic artefact, blood backscattering was simultaneously determined at a wavelength of 586 nm. The fluorescence signal was recorded before, during and after contractions at 50% of MVC. The fibre was kept out of contact with the muscle during contractions at 100% of MVC and was only put into contact with it at the end of the contraction. At the onset of contractions at 50% of MVC, NADH fluorescence increased rapidly for 3 s and remained stable thereafter until exhaustion. After a muscle measurement had been made, the optical fibre was put successively into solutions of increasing NADH concentration to ascertain the relationship between the muscle fluorescence signal and the muscle NADH level. This procedure yielded estimated mean values for muscle NADH of 0.172 mmol.kg-1, SEM 0.028 and of 0.184 mmol.kg-1, SEM 0.027 after contractions at 50% and 100% of MVC, respectively, from a resting value of 0.087 mmol.kg-1, SEM 0.015. These results indicated that in situ laser fluorimetry could be used to evaluate NADH changes in humans during muscle contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the ablation of the nucleus pulposus on the vibrational behavior of the lumbosacral hinge.

This study was designed to investigate the respective damping properties of the annulus fibrosus and nucleus pulposus of the intervertebral disc during propagation of vibration waves through the osteoligamento-muscular axis of the spine. The study was conducted on a 8-10 kg deeply anesthetized baboon. In the first surgical phase five accelerometers were implanted in the first sacral vertebra and on the anterior side of the four lower lumbar vertebrae. The bioinstrumented animal was placed in a restraining chair and exposed to narrow-bandwidth (0-100 Hz) 0.16 G RMS random vibration. Once data was recorded, the nuclei pulposi of the studied discs were removed by suction, the surrounding annuli remaining intact. The still deeply anesthetized animal was again exposed to the same 0-100 Hz, 0.16 G RMS vibration. Results were analyzed and their reproducibility was tested on three animals.