Effects of immobilization on rat hind limb muscles under non-weight-bearing conditions

Role of NO-synthase in regulation of protein metabolism of stretched rat m. soleus muscle during functional unloading

Gravitational unloading causes atrophy of muscle fibers and can lead to destruction of cytoskeletal and contractile proteins. Along with the atrophic changes, unloaded muscle frequently demonstrates significant shifts in the ratio of muscle fibers expressing fast and slow myosin heavy chain isoforms. Stretching of the m. soleus during hindlimb suspension prevents its atrophy. We supposed that neuronal NO-synthase (NOS) (which is attached to membrane dystrophin-sarcoglycan complex) can contribute to maintenance of protein metabolism in the muscle and prevent its atrophy when m. soleus is stretched. To test this hypothesis, we used Wistar rats (56 animals) in experiments with hindlimb suspension during 14 days. The group of hindlimb suspended rats with stretched m. soleus was injected with L-NAME to block NOS activity. We found that m. soleus mass and its protein content in hindlimb-suspended rats with stretched m. soleus were preserved due to prevention of protein degradation. NOS is involved in maintenance of expression of some muscle proteins. Proliferation of satellite cells in stretched m. soleus may be due to nNOS activity, but maintenance of muscle mass upon stretching is regulated not by NOS alone.

7-Nitroindazole potentiates c-fos expression induced by muscle tendon vibration in the spinal cord

INTRODUCTION

Expression of c-fos initiated by muscle proprioceptive signaling was studied in rats after inhibition of neuronal nitric oxide synthase (nNOS) with administration of 7-nitroindazole (7-NI).

METHODS

Fos-immunoreactive (Fos-ir) neurons were visualized immunohistochemically in the lumbar cord after vibration of the Achilles tendon and/or 7-NI systemic injections.

RESULTS

The total number of Fos-ir interneurons and motoneurons (per slice) was significantly greater in the 7-NI-pretreated and tendon-vibrated (7-NI + Tv) group than in the isolated tendon vibration group (Tv group). The greatest increases in the number of Fos-ir neurons were found in the L4 (+100%) and L5 (+105%) segments (P < 0.05).

CONCLUSIONS

Suppression of NO release after introduction of 7-NI was associated with potentiation of Fos immunoreactivity induced by muscle proprioceptive signaling within distinctive regions of the spinal cord.

Determinants of Disuse-Induced Skeletal Muscle Atrophy: Exercise and Nutrition Countermeasures to Prevent Protein Loss

Muscle atrophy results from a variety of conditions such as disease states, neuromuscular injuries, disuse, and aging. Absence of gravitational loading during spaceflight or long-term bed rest predisposes humans to undergo substantial loss of muscle mass and, consequently, become unfit and/or unhealthy. Disuse- or inactivity-induced skeletal muscle protein loss takes place by differential modulation of proteolytic and synthetic systems. Transcriptional, translational, and posttranslational events are involved in the regulation of protein synthesis and degradation in myofibers, and these regulatory events are known to be responsive to contractile activity. However, regardless of the numerous studies which have been performed, the intracellular signals that mediate skeletal muscle wasting due to muscular disuse are not completely comprehended. Understanding the triggers of atrophy and the mechanisms that regulate protein loss in unloaded muscles may lead to the development of effective countermeasures such as exercise and dietary intervention. The objective of the present review is to provide a window into the molecular processes that underlie skeletal muscle remodeling and to examine what we know about exercise and nutrition countermeasures designed to minimize muscle atrophy.

Clenbuterol induces muscle-specific attenuation of atrophy through effects on the ubiquitin-proteasome pathway

The ubiquitin-proteasome pathway is primarily responsible for myofibrillar protein degradation during hindlimb unweighting (HU). β-Adrenergic agonists such as clenbuterol (CB) induce muscle hypertrophy and attenuate muscle atrophy due to disuse or inactivity. However, the molecular mechanism by which CB exerts these effects remains poorly understood. The aims of this study were to investigate whether CB attenuates HU-induced muscle atrophy through an inhibition of the ubiquitin-proteasome pathway and whether insulin-like growth factor I (IGF-I) mediates this inhibition. Rats were randomized to the following groups: weight-bearing control, 14-day CB-treated, 14-day HU, and CB + HU. HU-induced atrophy was associated with increased proteolysis and upregulation of components of the ubiquitin-proteasome pathway (ubiquitin conjugates, ubiquitin conjugating enzyme E2-14kDa, and 20S proteasome activity). Upregulation of the ubiquitin proteasome occurred in all muscles tested but was more pronounced in muscles composed primarily of slow-twitch fibers (soleus) than in fast-twitch muscles (plantaris and tibialis anterior). Although CB induced hypertrophy in all muscles, CB attenuated the HU-induced atrophy and reduced ubiquitin conjugates only in the fast plantaris and tibialis anterior and not in the slow soleus muscle. CB did not elevate IGF-I protein content in either of the muscles examined. These results suggest that CB induces hypertrophy and alleviates HU-induced atrophy, particularly in the fast muscles, at least in part through a muscle-specific inhibition of the ubiquitin-proteasome pathway and that these effects are not mediated by the local production of IGF-I in skeletal muscle.

Effects of deafferentation on the size and myosin phenotype of muscle fibers on stretching of the rat soleus muscle in conditions of gravitational unloading

The aim of the present work was to assess the contributions of the reflex and local components to preventing decreases in the size and changes in the ratio of fibers containing the slow and fast isoforms of myosin heavy chains during chronic stretching of a postural muscle in rats in conditions of gravitational unloading. A unilateral surgical deafferentation method was used. The results demonstrated that deafferentation of the hindlimb had no effect on preventing reductions in muscle fiber size in conditions of chronic muscle stretching in conditions of gravitational unloading. The results obtained from these experiments did not support the hypothesis that the predominant contribution to preventing the development of atrophic changes comes from activation of muscle afferents in chronic stretching of the unloaded muscle. Deafferentation of both suspended animals and those with normal motor activity led to increases in the proportion of soleus muscle fibers containing the slow isoforms of myosin heavy chain.

Effects of insulin-like growth factor-1/binding protein-3 complex on muscle atrophy in rats

Muscle atrophy and wasting is a serious problem that occurs in patients with prolonged debilitating illness, burn injury, spinal injury, as well as with space flight. Current treatment for such atrophy, which often relies on nutritional supplementation and physical therapy, is of limited value in preventing the muscle wasting that occurs. Considerable recent attention has focused on the use of anabolic growth factors such as insulin-like growth factor (IGF-1) in preventing muscle atrophy during limb disuse or with various catabolic conditions. However, potential side effects such as hypoglycemia appear to be limiting factors in the usefulness of IGF-1 for clinical treatment of muscle wasting conditions. The formulation of IGF-1 used in this study (IGF-1/BP3) is already bound to its endogenous-binding protein (BP3) and, as a result, has a greater specificity of action and significantly less hypoglycemic effect. Using a rat model of hind limb suspension (HLS) for 10 days, we induced marked muscle atrophy that was accompanied by enhanced muscle proteolysis and reduced muscle protein content. When HLS rats were treated with IGF-1/BP3 (50 mg/kg, b.i.d.), they retained greater body and muscle mass. Muscle protein degradation was significantly reduced and muscle protein content was preserved. The rate of protein synthesis, although somewhat reduced in HLS muscle, was not significantly elevated by IGF-1/BP3 treatment. Volume density of HLS-treated muscles were increased compared to untreated HLS rats and the actual number of fibers per area of muscle was likewise increased. The results of the current study suggest that IGF-1/BP3 might be useful for inhibiting muscle proteolysis in catabolic conditions and thus preserving muscle protein content and mass.

Increased antioxidant capacity does not attenuate muscle atrophy caused by unweighting

Previous studies have increased antioxidant capacity in skeletal muscle to attenuate oxidative stress and muscle atrophy during limb immobilization (Appell HJ, Duarte JAR, and Soares JMC. Int J Sports Med 18: 157-160, 1997; Kondo H, Miura M, Nakagaki I, Sasaki S, and Itokawa Y. Am J Physiol Endocrinol Metab 262: E583-E590, 1992). The purpose of this study was to determine the level of oxidative stress in muscle during hindlimb unweighting (HLU) and whether antioxidant supplementation can attenuate the atrophy and changes in contractile properties resulting from 14 days of unweighting. Muscle unweighting caused a 44% decrease in soleus (Sol) and a 30% decrease in gastrocnemius (GS) mass, a 7% decrease in body weight, and 28% decrease in tetanic force in the GS. Protein carbonyls increased by 44% in the Sol with HLU. Antioxidant supplementation did not attenuate the GS or Sol atrophy or the decrease in GS force generation during HLU. Sol and GS protein concentration was not different between groups. The GS was also subjected to three different oxidative challenges to determine whether the supplement increased the antioxidant capacity of the muscle. In all cases, muscles exhibited an increased antioxidant capacity. These data indicate that antioxidant supplementation was not an effective countermeasure to the atrophy associated with HLU.

Nutrition and muscle loss in humans during spaceflight

The protein loss in humans during spaceflight is partly due to a normal adaptive response to a decreased work load on the muscles involved in weight bearing. The process is mediated by changes in prostaglandin release, secondary to the decrease in tension on the affected muscles. On missions, where there is a high level of physical demands on the astronauts, there tends to be an energy deficit, which adds to the muscle protein loss and depletes the body fat reserves. While the adaptive response is a normal part of homeostasis, the additional protein loss from an energy deficit can, in the long run, have a negative effect on health and capability of humans to live and work in space and afterward return to Earth.

Influence of brief daily tendon vibration on rat soleus muscle in non-weight-bearing situation

The purpose of this study was to investigate whether tendon vibration could prevent soleus muscle atrophy during hindlimb unloading (HU). Mechanical vibrations with a constant low amplitude (0.3 mm) were applied (192 s/day) with constant frequency (120 Hz) to the Achilles tendon of the unloaded muscle during the 14-day HU period. Significant reductions in muscle mass (-41%), fiber size, maximal twitch (-54%), and tetanic tensions (-73%) as well as changes in fiber type and electrophoretic profiles and twitch-time parameters (-31% in the contraction time and -30% in the half relaxation time) were found after 14 days of HU when compared with the control soleus. Tendon vibration applied during HU significantly attenuated, but did not prevent, 1) the loss of muscle mass (17 vs. 41%); 2) the decrease in the fiber cross-sectional area of type IIA (-28 vs. -50%) and type IIC (-29 vs. -56%) fibers; and 3) the decrease in maximal twitch (-3 vs. -54%) and maximal tetanic tensions (-29 vs. -73%) and the half relaxation time (1 vs. -30%). Changes in the contraction time and in histological and electrophoretical parameters associated with HU were not counteracted. These findings suggest that tendon vibration can be used as a paradigm to counteract the atrophic process observed after HU.

Sarcomere lesion damage occurs mainly in slow fibers of reloaded rat adductor longus muscles

Sarcomere lesions were previously observed with reloading of rat adductor longus muscles after spaceflight and hindlimb unloading (HU). Spaceflown rats displayed more lesioned fibers in the "slow-fiber" region, suggesting a damage-susceptible fiber type. Unloading induces fast myosin expression in some slow fibers, generating hybrid fibers. We examined whether lesion damage differed among slow-, hybrid-, and fast-fiber types in HU-reloaded adductor longus muscles. Temporal HU for 5, 8, 11, 14, and 17 days revealed that hybrid fiber percent, detected by antimyosin immunostaining, peaked at 29 +/- 12% by 14 days. A 14-day HU followed by 12-14 h of voluntary reloading was performed to induce lesions. chi2 analysis showed that slow fibers were preferentially damaged, accounting for 92 +/- 5% of lesioned fibers; hybrid and fast fibers accounted for 7 +/- 4 and <0.5%, respectively. Atrophy did not explain differential lesion damage across fiber types, as slow and hybrid fibers atrophied to a similar extent. Because active myofiber contractions are requisite for lesion formation, selective recruitment of slow fibers most likely explains their damage susceptibility.

Muscle atrophy associated with microgravity in rat: basic data for countermeasures

Morphological, contractile properties and myosin heavy chain (MHC) composition of rat soleus muscles were studied after 2 weeks of unloading (HS) and after 2 weeks of HS associated with selective deafferentation (HS + DEAF) at the level L4 and L5. The same significant reductions in muscle mass and tetanic tension were found after HS and HS + DEAF. However, the transformation of the slow-twitch soleus muscle towards a faster type characterized by a decrease in twitch time parameters and an increase in fast-twitch type MHC isoforms in HS did not appear in HS + DEAF conditions. Our results also showed that a pattern similar to firing rate of motoneurones innervating slow-twitch muscles inhibited the slow to fast fiber changes observed during HS. Nevertheless, neither the loss of mass or force output in the HS muscles were prevented by electrostimulation. Immobilization in a stretched position during HS maintained the muscle wet weight, mechanical and electrophoretical characteristics close to control values. We concluded that the decrease in mechanical strains imposed on the muscle during unloading was the main factor for the development of atrophy, while the kinetic changes might be predominantly modulated by the nervous command. These basic data suggested that some experimental conditions such as electrostimulation or stretching, could participate in countermeasure programmes.

Insulin attenuates atrophy of unweighted soleus muscle by amplified inhibition of protein degradation

Unweighting atrophy of immature soleus muscle occurs rapidly over the first several days, followed by slower atrophy coinciding with increased sensitivity to insulin of in vitro protein metabolism. This study determined whether this increased sensitivity might account for the diminution of atrophy after 3 days of tall-cast hindlimb suspension. The physiological significance of the increased response to insulin in unweighted muscle was evaluated by analyzing in vivo protein metabolism for day 3 (48 to 72 hours) and day 4 (72 to 96 hours) of unweighting in diabetic animals either injected with insulin or not treated. Soleus from nontreated diabetic animals showed a similar loss of protein during day 3 (-16.2%) and day 4 (-14.5%) of unweighting, whereas muscle from insulin-treated animals showed rapid atrophy (-14.5%) during day 3 only, declining to just -3.1% the next day. Since fractional protein synthesis was similar for both day 3 (8.6%/d) and day 4 (7.0%/d) of unweighting in insulin-treated animals, the reduction in protein loss must be accounted for by a slowing of protein degradation due to circulating insulin. Intramuscular (IM) injection of insulin (600 nmol/L) stimulated in situ protein synthesis similarly in 4-day unweighted (+56%) and weight-bearing (+90%) soleus, even though unweighted muscle showed a greater in situ response of 2-deoxy-[3H]glucose uptake to IM injection of either insulin (133 nmol/L) or insulin-like growth factor-I (IGF-I) (200 nmol/L) than control muscle. These findings suggest that unweighted muscle is selectively more responsive in vivo to insulin, and that the slower atrophy after 3 days of unweighting was due to an increased effect of insulin on inhibiting protein degradation.

Changes in contractile protein mRNA accumulation in response to spaceflight

Ten rats were exposed to 9 days of zero gravity aboard the National Aeronautics and Space Administration SLS-1 space mission (June 1991). Levels of fast and slow isoform mRNAs from six contractile protein gene families were quantified in the flight soleus and extensor digitorum longus (EDL) muscles. The gene families studied were myosin light chain-1 (MLC-1), myosin light chain-2 (MLC-2), troponin (Tn) T, TnI, TnC, and tropomyosin. In the EDL muscle there was no change in slow mRNA levels with a general increase in fast mRNA levels from 23 to 232%. Changes in slow mRNA levels were seen in the flight soleus muscle with TnCslow and TnTslow levels increasing slightly, and MLC-1slow a, MLC-1slow b, TnIslow, alpha-Tmslow, and MLC-2slow levels decreasing. All fast mRNA levels increased in the flight soleus muscle from 170 to 1,100%. We can conclude that exposure to zero gravity results in 1) a general increase in fast mRNA levels in both fast and slow muscles and 2) differing directional changes in slow mRNA accumulation in the soleus muscle.

Influence of chronic stretching upon rat soleus muscle during non-weight-bearing conditions

Morphological, contractile and histochemical properties as well as the myosin heavy chain (MHC) composition of rat soleus muscles were studied after 14 days of non-weight-bearing (NWB) and after immobilization of the foot in dorsiflexion of NWB rats. Significant reductions in soleus mass, fibre sizes and tetanic tension were found after 14 days of NWB. Furthermore, a transformation of the slow-twitch soleus muscle towards a faster type was characterized by a decrease in twitch time parameters, an increase in the fast-twitch type IIA fibre proportion and an increase in fast-twitch type MHC isoforms. Our results showed that the immobilization of the soleus muscle in a lengthened position during NWB not only prevented the loss of muscular mass and force output, but also counteracted the slow to faster shift in contractile and phenotypical parameters normally associated with NWB conditions.

Effect of the antiglucocorticoid RU38486 on protein metabolism in unweighted soleus muscle

Unweighting the hindlimbs by tail suspension of juvenile rats leads to atrophy of the soleus muscle, especially during the third day of unweighting. Although a previous study using adrenalectomized animals suggested a minimal role of glucocorticoids in this atrophy, the inability of adrenalectomized animals to release other adrenal hormones could have been important. Therefore, the influence of oral administration of RU38486 [11-beta-(4-dimethylaminophenyl) 17-beta-hydroxy 17-alpha(prop-1-ynyl) estra 4,9-dien 3-one], a selective glucocorticoid antagonist, on protein metabolism in the unweighted soleus was studied. The effectiveness of RU38486 treatment was demonstrated in hindlimb-suspended rats as the drug abolished the increase in soleus glutamine synthetase activity shown previously to be caused by elevated circulating glucocorticoids. The slower weight gain of suspended rats was unaffected by the drug. After 3 days of unweighting, the difference in protein content from weighted soleus muscle was not diminished significantly by RU38486 (-25%, vehicle only; -18%, RU38486-treated). However, in both weight-bearing and suspended animals, RU38486 seemed to promote protein accretion between days 2 and 3 of the experiment; ie, unweighted muscle seemed to lose less protein. All suspended animals showed slower (-58% to -64%) fractional in vivo rates of synthesis. RU38486 did not affect these percent differences in fractional protein synthesis after either 2 or 3 days of unweighting. The apparent improvement in protein balance likely resulted from a decline in protein degradation in both the weight-bearing (-26%) and unweighted (-35%) soleus.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle protein content and synthesis after voluntary running and subsequent unweighting

The effects of exercise training with or without subsequent unweighting on wet weight, protein content, and in vivo fractional protein synthesis were studied in soleus and plantaris muscles of juvenile female Sprague-Dawley rats under the following four conditions: normal weight bearing (N), voluntary-activity wheel running (WR) for up to 4 weeks, mechanical unweighting for 7 days via hindlimb suspension (HS), or wheel running followed by 7 days of hindlimb suspension (WR-HS). Fractional protein synthesis was determined by the 3H-phenylalanine flooding method. Increases (P < .05) in wet weight and protein content were detected in the soleus after just 1 week of running, with no increase in fractional protein synthesis. Two weeks of running were required for an increase in protein synthesis in this muscle. Significant increases in these parameters were first observed in the plantaris after 2 weeks of running. Maximal increases occurred by 3 weeks in both muscles. Reductions (P < .05) in soleus and plantaris parameters were observed in both HS and WR-HS groups compared with N and WR groups, respectively. However, protein content and fractional synthesis were maintained at significantly higher levels in WR-HS muscles compared with HS muscles. These results indicate that (1) wheel training represents a noninvasive method for inducing rapid hypertrophy of the skeletal muscles studied, in part by increasing fractional protein synthesis; (2) unweighting decreases protein content and synthesis to the same extent whether the muscles are trained; and (3) previously hypertrophied muscles display higher protein contents and fractional protein synthesis following unweighting compared with unweighted muscles of untrained animals.

Compensatory effects of chronic electrostimulation on unweighted rat soleus muscle

The purpose of this study was to investigate the effects of electrostimulation in counteracting the transformation of the unweighted rat soleus muscle. The stimulation resembled the firing patterns of normal slow motor units and was imposed during hindlimb suspension. For the 10-day hindlimb suspended rats, the transformation of the slow soleus muscle towards a faster type was characterized by a decrease in the time to peak tension and the half-relaxation time of the twitch, a reduction in the P20/P0 index, i.e. the ratio of the subtetanic tension at 20 Hz relative to the tetanic tension, and a decrease in the percentage distributions of type I fibres accompanied by an increase of type IIa and IIc fibres. These changes were prevented by electrostimulation since, for the parameters mentioned above, no significant difference was observed in the soleus of the suspended rats that received electrostimulation when compared with the control rats. Nevertheless, neither the loss of mass nor the decrease in force output in the suspended rats were prevented by electrostimulation. The present results suggest a positive compensation of the suspension-induced alterations in the contractile and histochemical properties of the soleus muscle by means of chronic electrostimulation, which, however, do not prevent atrophy or the loss of contractile force.

Rat tail suspension causes a decline in insulin receptors

Decreased muscular activity results in weakness and muscular atrophy. Coincident with this protein catabolic state is glucose intolerance and hyperinsulinemia. Rats were tail suspended for 7 to 14 days to accomplish unloading of the hindlimbs. Insulin resistance was documented in these animals by a 14 day tail suspension-related 26% increase in serum glucose in spite of a 253% increase in serum insulin concentration. Microsomal membranes were prepared from hindlimb muscles and specific binding of insulin and insulin-like growth factor I (IGF-I) were determined in these membranes. Insulin binding was decreased by 27% at 7 days and by 21% at 14 days. In contrast, IGF-I binding was unchanged at 7 days and was increased by 24% at 14 days. Liver membrane insulin receptors also had declined by 14 days of suspension, suggesting that the change in insulin receptors was a generalized, humorally-mediated phenomenon. These data suggest that tail suspension in rats results in insulin resistance, hyperinsulinemia, a decline in insulin receptors in liver and muscle, and a relative increase in muscle membrane IGF-I receptors. These data are consistent with the hypothesis that resistance to insulin's effects on protein metabolism in skeletal muscle may contribute to the protein catabolism associated with decreased muscular activity.

Time course of the response of myofibrillar and sarcoplasmic protein metabolism to unweighting of the soleus muscle

Contributions of altered in vivo protein synthesis and degradation to unweighting atrophy of the soleus muscle in tail-suspended young female rats were analyzed daily for up to 6 days. Specific changes in myofibrillar and sarcoplasmic proteins were also evaluated to assess their contributions to the loss of total protein. Synthesis of myofibrillar and sarcoplasmic proteins was estimated by intramuscular (IM) injection and total protein by intraperitoneal (IP) injection of flooding doses of 3H-phenylalanine. Total protein loss was greatest during the first 3 days following suspension and was a consequence of the loss of myofibrillar rather than sarcoplasmic proteins. However, synthesis of total myofibrillar and sarcoplasmic proteins diminished in parallel beginning in the first 24 hours. Therefore sarcoplasmic proteins must be spared due to a decrease in their degradation. In contrast, myofibrillar protein degradation increased, thus explaining the elevated degradation of the total pool. Following 72 hours of suspension, protein synthesis remained low, but the rate of myofibrillar protein loss diminished, suggesting a slowing of degradation. These various results show (1) acute loss of protein during unweighting atrophy is a consequence of decreased synthesis and increased degradation of myofibrillar proteins, and (2) sarcoplasmic proteins are spared due to slower degradation, likely explaining the sparing of plasma membrane receptors. Based on other published data, we propose that the slowing of atrophy after the initial response may be attributed to an increased effect of insulin.

Effects of chronic electrostimulation on rat soleus skinned fibers during hindlimb suspension

In order to counteract the changes of the contractile properties of the rat soleus occurring during 10 days of hypokinesia-hypodynamia, due to hindlimb suspension (HS), two different patterns of electrostimulation were applied to the tibial nerve. The contractile properties of single chemically skinned muscle fibers were investigated using the tension-pCa relationship characteristics, the similar or different calcium and strontium affinities, and by measuring the P/tmax kinetic parameters. Our results showed that a pattern similar to firing rates of motoneurons innervating slow twitch muscles inhibited the slow to fast fiber changes observed during HS, whereas a pattern similar to firing rates of motoneurons from fast twitch muscles seemed to favor these changes. Since neither pattern maintained the isometric contractile force developed by the soleus fibers, we concluded that the decrease in mechanical strains imposed on the muscle during unloading was the main factor for the development of atrophy, while the kinetic changes might be predominantly modulated by the nervous command.

Effect of spaceflight on human protein metabolism

Nitrogen balance and the whole body protein synthesis rate were measured before, during, and after a 9.5-day spaceflight mission on the space shuttle Columbia. Protein synthesis was measured by the single-pulse [15N]glycine method. Determinations were made 56, 26, and 18 days preflight, on flight days 2 and 8, and on days 0, 6, 14, and 45 postflight. We conclude that nitrogen balance was decreased during spaceflight. The decrease in nitrogen balance was greatest on the 1st day when food intake was reduced and again toward the end of the mission. An approximately 30% increase in protein synthesis above the preflight baseline was found for flight day 8 for all 6 subjects (P < 0.05), indicating that the astronauts showed a stress response to spaceflight.

Active muscle length reduction progressively damages soleus in hindlimb-suspended rabbits

This study describes the morphologic changes in rabbit soleus muscle following hindlimb suspension (HS) for 1 to 4 weeks (group A); or following HS with hindfeet passively dorsiflexed, by means of an elastic band, for 1 to 2 weeks (group B). In the latter, elastic band use allowed phasic contractions of foot extensor muscles against resistance and prevented 35% chronic soleus shortening, which occurred in group A animals. In group A, the soleus revealed progressive muscle atrophy and myofibrillar damage. Myofibrils underwent dissolution, muscle regeneration was ineffective, and adipose tissue developed from about 2-week suspension onward. Conversely, passive dorsiflexion of unloaded hindfeet was essential in maintaining mass and structural muscle integrity in the soleus of group B. It is hereby demonstrated that HS-induced soleus damage in the rabbit is progressive, and can be prevented, avoiding long-term shortening of soleus and its phasic unloaded contractions. Soleus sensitivity to unloading conditions, such as HS, tenotomy, and hypogravity, may depend on the particular physiology of this tonic antigravity muscle, engaged mainly in developing long-lasting isometric contractions in a stretched length.

Immobilization-induced muscle atrophy is not reversed by lengthening the muscle

In clinical practice, repaired tendocalcaneus (Achilles tendon) ruptures are often protected in immobilization casts for 4 weeks in the fully plantar flexed position and for up to another 4 weeks after returning the ankle to joint neutral. Moderate to severe muscle atrophy occurs within 4 weeks of immobilization in plantar flexion, but it is not known if this atrophy is minimized or reversed following restoration of joint neutral position. We tested the hypothesis that the extent of atrophy could be reduced by returning the ankle to joint neutral after 4 weeks of immobilization. Eighteen rabbits were anesthetized, and their right hind-limbs were casted with the knee flexed 90 degrees and the ankle fully plantar flexed. Three animals each were studied after 3, 4, 6, or 8 weeks of immobilization. After 4 weeks of immobilization, the immobilization casts of the remaining six rabbits were modified to return the ankle to joint neutral for another 2 or 4 weeks. For muscle studies, the animals were anesthetized, and the soleus (SOL), plantaris (PLN), and gastrocnemius (GST) muscles were removed and weighed; the SOL and PLN were quick frozen and processed for histochemical fiber typing and fiber cross-sectional area measurement. All three muscles showed significantly reduced muscle weight to body weight ratios after 3 weeks of immobilization. SOL was the most affected, and GST was least affected. There was no significant further atrophy through 8 weeks of immobilization. The atrophy correlated with a significant reduction of mean fiber area (MFA) for Types I, IIo, and IIc fibers in SOL and PLN. In PLN, Type IIg fiber area was not significantly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Myofibrillar disruption in the rabbit soleus muscle after one-week hindlimb suspension

Relevant muscle- and species-specific differences may be found in the reaction of muscles to hindlimb suspension. This problem has been studied in 5 rabbits following a one-week hindlimb suspension, and in 5 ground-based controls. The soleus and the tibialis were prepared for light and transmission electron microscopy. In suspension the animals occasionally extended and flexed the hindlimbs, but, when standing still, their hindfeet were plantar-flexed to an angle of 180 degrees. In this position the length of the soleus was determined to be 35% less than in controls, whereas that of the tibialis was 30% more. Histologically, the tibialis fibers usually exhibited a preserved sarcomeric pattern, whereas soleus fibers displayed a regular sequence of areas of shortened sarcomeres, alternating with areas of myofibrillar disruption. These findings demonstrated that hindlimb suspension induces a focal breakdown of the soleus myofibrils, probably dependent on the reduced longitudinal tension of the suspended soleus and its phasic contractions against no load. It is conceivable that similar factors could also be responsible for soleus muscle atrophy induced by hypogravity as well as by other clinical conditions during which a stressful plantar flexion of the feet occurs against no load.

Insulin effects in denervated and non-weight-bearing rat soleus muscle

Previous reports indicated that glucose uptake in denervated muscle is resistant to insulin, while in non-weight-bearing (unweighted) muscle this effect of insulin is enhanced. To extend the comparison of these differences, insulin effects on amino acid uptake and protein metabolism were studied in soleus muscles subjected to denervation or unweighting. Denervated muscle showed insulin resistance of both 2-deoxy[1,2-3H]glucose and alpha-[methyl-3H]aminoisobutyric acid uptake whereas unweighted muscle showed an increased or normal response, respectively. Atrophy was greater in denervated than in unweighted muscle, apparently due to faster protein degradation. The stimulation of protein synthesis and the inhibition of protein degradation by insulin was generally less in denervated than in unweighted muscle. Since metabolic measurements in denervated-unweighted muscles did not differ from those in denervated-weight-bearing muscles, effects of denervation must be independent of leg posture.

Different mechanisms of increased proteolysis in atrophy induced by denervation or unweighting of rat soleus muscle

Mechanisms of accelerated proteolysis were compared in denervated and unweighted (by tail-cast suspension) soleus muscles. In vitro and in vivo proteolysis were more rapid and lysosomal latency was lower in denervated than in unweighted muscle. In vitro, lysosomotropic agents (eg, chloroquine, methylamine) did not lessen the increase in proteolysis caused by unweighting, but abolished the difference in proteolysis between denervated and unweighted muscle. Leucine methylester, an indicator of lysosome fragility, lowered latency more in denervated than in unweighted muscle. 3-Methyladenine, which inhibits phagosome formation, increased latency similarly in all muscles tested. Mersalyl, a thiol protease inhibitor, and 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), which antagonizes sarcoplasmic reticulum release of Ca2+, reduced accelerated proteolysis caused by unweighting without diminishing the faster proteolysis due to denervation. Calcium ionophore (A23187) increased proteolysis more so in unweighted than control muscles whether or not Ca2+ was present. Different mechanisms of accelerated proteolysis were studied further by treating muscles in vivo for 24 hours with chloroquine or mersalyl. Chloroquine diminished atrophy of the denervated but not the unweighted muscle, whereas mersalyl prevented atrophy of the unweighted but not of the denervated muscle, both by inhibiting in vivo proteolysis. These results suggest that (1) atrophy of denervated, but not of unweighted, soleus muscle involves increased lysosomal proteolysis, possibly caused by greater permeability of the lysosome, and (2) cytosolic proteolysis is important in unweighting atrophy, involving some role of Ca2(+)-dependent proteolysis and/or thiol proteases.

Ecdysteroids affect in vivo protein metabolism of the flight muscle of the tobacco hornworm (Manduca sexta)

Ecdysteroid growth promotion of the dorsolongitudinal flight muscle of Manduca sexta was studied by measuring in vivo protein metabolism using both "flooding-dose" and "non-carrier" techniques. These procedures differ in that the former method includes injection of non-labelled phenylalanine (30 micromoles/insect) together with the [3H]amino acid. Injected radioactivity plateaued in the haemolymph within 7 min. With the flooding-dose method, haemolymph and intramuscular specific radioactivities were similar between 15 min and 2 h. Incorporation of [3H]phenylalanine into muscle protein was linear with either method between 30 and 120 min. Fractional rates (%/12 h) of synthesis with the flooding-dose technique were best measured after 1 h because of the initial delay in radioactivity equilibration. Estimation of body phenylalanine turnover with the non-carrier method showed 24-53%/h which was negligible with the flooding-dose method. Since the two methods yielded similar rates of protein synthesis, the large injection of non-labelled amino acid did not alter the rate of synthesis. Because the flooding-dose technique requires only a single time point measurement, it is the preferred method. The decline and eventual cessation of flight-muscle growth was mostly a consequence of declining protein synthesis though degradation increased between 76-86 h before eclosion and was relatively rapid. This decline in muscle growth could be prevented by treating pupae with 20-hydroxyecdysone (10 micrograms/insect). Protein accretion was promoted by a decline of up to 80% in protein breakdown, which was offset in part by a concurrent though much smaller decrease in protein synthesis. Therefore, ecdysteroids may increase flight-muscle growth by inhibiting proteolysis.

Effects of stretching and disuse on amino acids in muscles of rat hind limbs

Effects of stretching on muscle amino acids were tested in unloaded soleus by casting the foot in dorsiflexion on one limb of tail-casted, hindquarter-suspended rats. For comparison with unloading, amino acids also were measured in shortened extensor digitorum longus (EDL) in the same casted limb and in denervated leg muscles. Concentrations of tyrosine and glutamate were lower, while aspartate, ammonia, and the ratio of glutamine to glutamate were greater in the stretched than in the freely moving, unloaded soleus, but stretched did not differ from weight-bearing, control muscle. Therefore, stretching the soleus muscle prevented changes in certain amino acids due to unloading. Aspartate, ammonia, glutamine, and the ratio of glutamine to glutamate were lower in the shortened EDL than in the freely moving muscle of the contralateral limb, or in the control muscle. When denervated, these leg muscles also showed lower aspartate, ammonia, and ratio of glutamine to glutamate relative to innervated muscles. Since muscle shortening or denervation produced amino acid changes that mimicked the effects of unloading on the soleus, these responses must reflect the effect of muscle disuse. These data suggested that lower ammonia might cause the lower ratio of glutamine to glutamate with disuse. Because the fresh muscle energy charge, one factor which controls AMP deaminase, generally was not affected by disuse, altered deamination of glutamate via glutamate dehydrogenase may explain the variations in muscle ammonia.(ABSTRACT TRUNCATED AT 250 WORDS)