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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

Comparison of a space shuttle flight (STS-78) and bed rest on human muscle function

The purpose of this investigation was to assess muscle fiber size, composition, and in vivo contractile characteristics of the calf muscle of four male crew members during a 17-day spaceflight (SF; Life and Microgravity Sciences Spacelab Shuttle Transport System-78 mission) and eight men during a 17-day bed rest (BR). The protocols and timelines of these two investigations were identical, therefore allowing for direct comparisons between SF and the BR. The subjects' age, height, and weight were 43 +/- 2 yr, 183 +/- 4 cm, and 86 +/- 3 kg for SF and 43 +/- 2 yr, 182 +/- 3 cm, and 82 +/- 4 kg for BR, respectively. Calf muscle strength was examined before SF and BR; on days 2, 8, and 12 during SF and BR; and on days 2 and 8 of recovery. Muscle biopsies were obtained before and within 3 h after SF (gastrocnemius and soleus) and BR (soleus) before reloading. Maximal isometric calf strength and the force-velocity characteristics were unchanged with SF or BR. Additionally, neither SF nor BR had any effect on fiber composition or fiber size of the calf muscles studied. In summary, no changes in calf muscle strength and morphology were observed after the 17-day SF and BR. Because muscle strength is lost during unloading, both during spaceflight and on the ground, these data suggest that the testing sequence employed during the SF and BR may have served as a resistance training countermeasure to attenuate whole muscle strength loss.

Non-NASA Center↗
NASA NTRS2000

Skeletal muscle Ca(2+)-independent kinase activity increases during either hypertrophy or running

Spikes in free Ca(2+) initiate contractions in skeletal muscle cells, but whether and how they might signal to transcription factors in skeletal muscles of living animals is unknown. Since previous studies in non-muscle cells have shown that serum response factor (SRF) protein, a transcription factor, is phosphorylated rapidly by Ca(2+)/calmodulin (CaM)-dependent protein kinase after rises in intracellular Ca(2+), we measured enzymatic activity that phosphorylates SRF (designated SRF kinase activity). Homogenates from 7-day-hypertrophied anterior latissimus dorsi muscles of roosters had more Ca(2+)-independent SRF kinase activity than their respective control muscles. However, no differences were noted in Ca(2+)/CaM-dependent SRF kinase activity between control and trained muscles. To determine whether the Ca(2+)-independent and Ca(2+)/CaM-dependent forms of Ca(2+)/CaM-dependent protein kinase II (CaMKII) might contribute to some of the SRF kinase activity, autocamtide-3, a synthetic substrate that is specific for CaMKII, was employed. While the Ca(2+)-independent form of CaMKII was increased, like the Ca(2+)-independent form of SRF kinase, no alteration in CaMKII occurred at 7 days of stretch overload. These observations suggest that some of SRF phosphorylation by skeletal muscle extracts could be due to CaMKII. To determine whether this adaptation was specific to the exercise type (i.e., hypertrophy), similar measurements were made in the white vastus lateralis muscle of rats that had completed 2 wk of voluntary running. Although Ca(2+)-independent SRF kinase was increased, no alteration occurred in Ca(2+)/CaM-dependent SRF kinase activity. Thus any role of Ca(2+)-independent SRF kinase signaling has downstream modulators specific to the exercise phenotype.

NASA Program Fundamental Space Biology↗
NASA NTRS2001

Functional and structural adaptations of skeletal muscle to microgravity

Our purpose is to summarize the major effects of space travel on skeletal muscle with particular emphasis on factors that alter function. The primary deleterious changes are muscle atrophy and the associated decline in peak force and power. Studies on both rats and humans demonstrate a rapid loss of cell mass with microgravity. In rats, a reduction in muscle mass of up to 37% was observed within 1 week. For both species, the antigravity soleus muscle showed greater atrophy than the fast-twitch gastrocnemius. However, in the rat, the slow type I fibers atrophied more than the fast type II fibers, while in humans, the fast type II fibers were at least as susceptible to space-induced atrophy as the slow fiber type. Space flight also resulted in a significant decline in peak force. For example, the maximal voluntary contraction of the human plantar flexor muscles declined by 20-48% following 6 months in space, while a 21% decline in the peak force of the soleus type I fibers was observed after a 17-day shuttle flight. The reduced force can be attributed both to muscle atrophy and to a selective loss of contractile protein. The former was the primary cause because, when force was expressed per cross-sectional area (kNm(-2)), the human fast type II and slow type I fibers of the soleus showed no change and a 4% decrease in force, respectively. Microgravity has been shown to increase the shortening velocity of the plantar flexors. This increase can be attributed both to an elevated maximal shortening velocity (V(0)) of the individual slow and fast fibers and to an increased expression of fibers containing fast myosin. Although the cause of the former is unknown, it might result from the selective loss of the thin filament actin and an associated decline in the internal drag during cross-bridge cycling. Despite the increase in fiber V(0), peak power of the slow type I fiber was reduced following space flight. The decreased power was a direct result of the reduced force caused by the fiber atrophy. In addition to fiber atrophy and the loss of force and power, weightlessness reduces the ability of the slow soleus to oxidize fats and increases the utilization of muscle glycogen, at least in rats. This substrate change leads to an increased rate of fatigue. Finally, with return to the 1g environment of earth, rat studies have shown an increased occurrence of eccentric contraction-induced fiber damage. The damage occurs with re-loading and not in-flight, but the etiology has not been established.

Review, Tutorial↗
NASA NTRS1994

The effects of space flight on the contractile apparatus of antigravity muscles: implications for aging and deconditioning

Previous studies have shown that the unloading of skeletal muscle, as occurring during exposure to space flight, exerts a profound effect on both the mass (cross sectional area) of skeletal muscle fibers and the relative expression of protein isoforms comprising the contractile system. Available information suggests that slow (type I) fibers, comprising chiefly the antigravity muscles of experimental animals, in addition to atrophying, undergo alterations in the type of myosin heavy chain (MHC) expressed such that faster isoforms become concomitantly expressed in a sub-population of slow fibers when insufficient force-bearing activity is maintained on the muscle. Consequently, these transformations in both mass and myosin heavy chain phenotype could exert a significant impact on the functional properties of skeletal muscle as manifest in the strength, contractile speed, and endurance scope of the muscle. To further explore these issues, a study was performed in which young adult male rats were exposed to zero gravity for six days, following which, the antigravity soleus muscle was examined for a) contractile properties, determined in situ and b) isomyosin expression, as studied using biochemical, molecular biology, and histochemical/immunohistochemical techniques.

Flight Experiment↗
NASA NTRS2003

Ubiquitin-protein ligases in muscle wasting: multiple parallel pathways?

PURPOSE OF REVIEW: Studies in a wide variety of animal models of muscle wasting have led to the concept that increased protein breakdown via the ubiquitin-proteasome pathway is responsible for the loss of muscle mass seen as muscle atrophy. The complexity of the ubiquitination apparatus has hampered our understanding of how this pathway is activated in atrophying muscles and which ubiquitin-conjugating enzymes in muscle are responsible. RECENT FINDINGS: Recent experiments have shown that two newly identified ubiquitin-protein ligases (E3s), atrogin-1/MAFbx and MURF-1, are critical in the development of muscle atrophy. Other in-vitro studies also implicated E2(14k) and E3alpha, of the N-end rule pathway, as playing an important role in the process. SUMMARY: It seems likely that multiple pathways of ubiquitin conjugation are activated in parallel in atrophying muscle, perhaps to target for degradation specific classes of muscle proteins. The emerging challenge will be to define the protein targets for, as well as inhibitors of, these E3s.

Review↗
NASA NTRS2000

Time course of vasodilatory responses in skeletal muscle arterioles: role in hyperemia at onset of exercise

At the onset of dynamic exercise, muscle blood flow increases within 1-2 s. It has been postulated that local vasodilatory agents produced by the vascular endothelium or the muscle itself contribute to this response. We hypothesized that only vasodilators that act directly on the vascular smooth muscle could produce vasodilation of skeletal muscle arterioles in <2 s. To test this hypothesis, we determined the time course of the vasodilatory response of isolated skeletal muscle arterioles to direct application of potassium chloride, adenosine, acetylcholine, and sodium nitroprusside. Soleus and gastrocnemius muscles were dissected from the hindlimbs of male Sprague-Dawley rats. First-order arterioles (100-200 microm) were isolated, cannulated on micropipettes, and pressurized to 60 cmH(2)O in an organ bath. Vasodilatory agents were added directly to the bath, and diameter responses of the arterioles were recorded in real time on a videotape recorder. Frame-by-frame analysis of the diameter responses indicated that none of the vasodilator agents tested produced significant diameter increases in <4 s in either soleus or gastrocnemius muscle arterioles. These results indicate that, although these local vasodilators produce significant vasodilation of skeletal muscle resistance arterioles, these responses are not rapid enough (within 1-2 s) to contribute to the initiation of the exercise hyperemic response at the onset of dynamic exercise.

NASA Discipline Musculoskeletal↗
NASA NTRS2000

Structural and functional remodeling of skeletal muscle microvasculature is induced by simulated microgravity

Hindlimb unloading of rats results in a diminished ability of skeletal muscle arterioles to constrict in vitro and elevate vascular resistance in vivo. The purpose of the present study was to determine whether alterations in the mechanical environment (i.e., reduced fluid pressure and blood flow) of the vasculature in hindlimb skeletal muscles from 2-wk hindlimb-unloaded (HU) rats induces a structural remodeling of arterial microvessels that may account for these observations. Transverse cross sections were used to determine media cross-sectional area (CSA), wall thickness, outer perimeter, number of media nuclei, and vessel luminal diameter of feed arteries and first-order (1A) arterioles from soleus and the superficial portion of gastrocnemius muscles. Endothelium-dependent dilation (ACh) was also determined. Media CSA of resistance arteries was diminished by hindlimb unloading as a result of decreased media thickness (gastrocnemius muscle) or reduced vessel diameter (soleus muscle). ACh-induced dilation was diminished by 2 wk of hindlimb unloading in soleus 1A arterioles, but not in gastrocnemius 1A arterioles. These results indicate that structural remodeling and functional adaptations of the arterial microvasculature occur in skeletal muscles of the HU rat; the data suggest that these alterations may be induced by reductions in transmural pressure (gastrocnemius muscle) and wall shear stress (soleus muscle).

Non-NASA Center↗
NASA NTRS1999

Is skeletal muscle ready for long-term spaceflight and return to gravity?

It is now clear that prevention of muscle debilitation during spaceflight will require a broader approach than simple exercise aimed at strengthening of the muscle fibers. The levels of several hormones and receptors are altered by unloading and must be returned to homeostasis. Pharmacotherapy and gene transfer strategies to raise the relative level of structural proteins may minimize the problems faced by astronauts in readapting to Earth-gravity. Up to now, we have only minimally exploited microgravity for advancing our understanding of muscle biology. A research laboratory in the space station with a centrifuge facility (gravity control) is essential for conducting basic research in this field. Microgravity has proven an excellent tool for noninvasively perturbing the synthesis of muscle proteins in the search for molecular signals and gene regulatory factors influencing differentiation, growth, maintenance and atrophy of muscle. Understanding the relation between blood flow and interstitial edema and between workload and subsequent structural failure are but two important problems that require serious attention. The roles of hormones and growth factors in regulating gene expression and their microgravity-induced altered production are other urgent issues to pursue. These types of studies will yield information that advances basic knowledge of muscle biology and offers insights into countermeasure design. This knowledge is likely to assist rehabilitation of diseased or injured muscles in humans on Earth, especially individuals in the more vulnerable aging population and persons participating in strenuous sports. Will the skeletal muscle system be prepared for the increased exposure to microgravity and the return to gravity loading without injury when space station is operational? The answer depends in large part on continued access to space and funding of ground-based models and flight experiments. The previous two decades of spaceflight research have described the effects of microgravity on multiple systems. The next generation of experiments promises to be even more exciting as we are challenged to define the cellular and molecular mechanisms of microgravity-induced changes.

manned↗
NASA NTRS1999

Control of skeletal muscle perfusion at the onset of dynamic exercise

At the onset of exercise there is a rapid increase in skeletal muscle vascular conductance and blood flow. Several mechanisms involved in the regulation of muscle perfusion have been proposed to initiate this hyperemic response, including neural, metabolic, endothelial, myogenic, and muscle pump mechanisms. Investigators utilizing pharmacological blockade of cholinergic muscarinic receptors and sympathectomy have concluded that neither sympathetic cholinergic nor adrenergic neural mechanisms are involved in the initial hyperemia. Studies have also shown that the time course for vasoactive metabolite release, diffusion, accumulation, and action is too long to account for the rapid increase in vascular conductance at the initiation of exercise. Furthermore, there is little or no evidence to support an endothelium or myogenic mechanism as the initiating factor in the muscle hyperemia. Thus, the rise in muscle blood flow does not appear to be explained by known neural, metabolic, endothelial, or myogenic influences. However, the initial hyperemia is consistent with the mechanical effects of the muscle pump to increase the arteriovenous pressure gradient across muscle. Because skeletal muscle blood flow is regulated by multiple and redundant mechanisms, it is likely that neural, metabolic, and possibly endothelial factors become important modulators of mechanically induced exercise hyperemia following the first 5-10 s of exercise.

Review, Tutorial↗
NASA NTRS1992

Mechanisms of nascent fiber formation during avian skeletal muscle hypertrophy

This study examined two putative mechanisms of new fiber formation in postnatal skeletal muscle, namely longitudinal fragmentation of existing fibers and de novo formation. The relative contributions of these two mechanisms to fiber formation in hypertrophying anterior latissimus dorsi (ALD) muscle were assessed by quantitative analysis of their nuclear populations. Muscle hypertrophy was induced by wing-weighting for 1 week. All nuclei formed during the weighting period were labeled by continuous infusion of 5-bromo-2'-deoxyuridine (BrdU), a thymidine analog, and embryonic-like fibers were identified using an antibody to ventricular-like embryonic (V-EMB) myosin. The number of BrdU-labeled and unlabeled nuclei in V-EMB-positive fibers were counted. Wing-weighting resulted in significant muscle enlargement and the appearance of many V-EMB+ fibers. The majority of V-EMB+ fibers were completely independent of mature fibers and had a nuclear density characteristics of developing fibers. Furthermore, nearly 100% of the nuclei in independent V-EMB+ fibers were labeled. These findings strongly suggest that most V-EMB+ fibers were nascent fibers formed de novo during the weighting period by satellite cell activation and fusion. Nascent fibers were found primarily in the space between fascicles where they formed a complex anastomosing network of fibers running at angles to one another. Although wing-weighting induced an increase in the number of branched fibers, there was no evidence that V-EMB+ fibers were formed by longitudinal fragmentation. The location of newly formed fibers in wing-weighted and regenerating ALD muscle was compared to determine whether satellite cells in the ALD muscle were unusual in that, if stimulated to divide, they would form fibers in the inter- and intrafascicular space. In contrast to wing-weighted muscle, nascent fibers were always found closely associated with necrotic fibers. These results suggest that wing-weighting is not simply another model of regeneration, but rather produces a unique environment which induces satellite cell migration and subsequent fiber formation in the interfascicular space. De novo fiber formation is apparently the principal mechanism for the hyperplasia reported to occur in the ALD muscle undergoing hypertrophy induced by wing-weighting.

Non-NASA Center↗
NASA NTRS2000

GLUT-3 expression in human skeletal muscle

Muscle biopsy homogenates contain GLUT-3 mRNA and protein. Before these studies, it was unclear where GLUT-3 was located in muscle tissue. In situ hybridization using a midmolecule probe demonstrated GLUT-3 within all muscle fibers. Fluorescent-tagged antibody reacting with affinity-purified antibody directed at the carboxy-terminus demonstrated GLUT-3 protein in all fibers. Slow-twitch muscle fibers, identified by NADH-tetrazolium reductase staining, possessed more GLUT-3 protein than fast-twitch fibers. Electron microscopy using affinity-purified primary antibody and gold particle-tagged second antibody showed that the majority of GLUT-3 was in association with triads and transverse tubules inside the fiber. Strong GLUT-3 signals were seen in association with the few nerves that traversed muscle sections. Electron microscopic evaluation of human peripheral nerve demonstrated GLUT-3 within the axon, with many of the particles related to mitochondria. GLUT-3 protein was found in myelin but not in Schwann cells. GLUT-1 protein was not present in nerve cells, axons, myelin, or Schwann cells but was seen at the surface of the peripheral nerve in the perineurium. These studies demonstrated that GLUT-3 mRNA and protein are expressed throughout normal human skeletal muscle, but the protein is predominantly found in the triads of slow-twitch muscle fibers.

NASA Discipline Regulatory Physiology↗
NASA NTRS1998

Comparison of soleus muscles from rats exposed to microgravity for 10 versus 14 days

The effects of two different duration space-flights on the extent of atrophy, fiber type composition, and myosin heavy chain (MHC) content of rat soleus muscles were compared. Adult male Fisher rats (n=12) were aboard flight STS-57 and exposed to 10 days of microgravity and adult ovariectomized female Spraque-Dawley rats (n=12) were aboard flight STS-62 for 14 days. Soleus muscles were bilaterally removed from the flight and control animals and frozen for subsequent analyses. Muscle wet weights, fiber types (I, IC, IIC, and IIA), cross-sectional area, and MHC content were determined. Although a significant difference was found between the soleus wet weights of the two ground-based control groups, they were similar with regard to MHC content (ca 90% MHCI and ca 10% MHCIIa) and fiber type composition. Unloading of the muscles caused slow-to-fast transformations which included a decrease in the percentage of type I fibers and MHCI, an increase in fibers classified as type IC, and the expression of two fast myosin heavy chains not found in the control rat soleus muscles (MHCIId and MHCIIb). Although the amount of atrophy (ca 26%) and the extent of slow-to-fast transformation (decrease in the percentage of MHCI from 90% to 82.5%) in the soleus muscles were similar between the two spaceflights, the percentages of the fast MHCs differed. After 14 days of spaceflight, the percentage of MHCIIa was significantly lower and the percentages of MHCIId and MHCIIb were significantly higher than the corresponding MHC content of the soleus muscles from the 10-day animals. Indeed, MHCIId became the predominant fast MHC after 14 days in space. These data suggest fast-to-faster transformations continued during the longer spaceflight.

short duration↗
NASA NTRS1995

Increase in ubiquitin-protein conjugates concomitant with the increase in proteolysis in rat skeletal muscle during starvation and atrophy denervation

The rapid loss of skeletal-muscle protein during starvation and after denervation occurs primarily through increased rates of protein breakdown and activation of a non-lysosomal ATP-dependent proteolytic process. To investigate whether protein flux through the ubiquitin (Ub)-proteasome pathway is enhanced, as was suggested by related studies, we measured, using specific polyclonal antibodies, the levels of Ub-conjugated proteins in normal and atrophying muscles. The content of these critical intermediates had increased 50-250% after food deprivation in the extensor digitorum longus and soleus muscles 2 days after denervation. Like rates of proteolysis, the amount of Ub-protein conjugates and the fraction of Ub conjugated to proteins increased progressively during food deprivation and returned to normal within 1 day of refeeding. During starvation, muscles of adrenalectomized rats failed to increase protein breakdown, and they showed 50% lower levels of Ub-protein conjugates than those of starved control animals. The changes in the pools of Ub-conjugated proteins (the substrates for the 26S proteasome) thus coincided with and can account for the alterations in overall proteolysis. In this pathway, large multiubiquitinated proteins are preferentially degraded, and the Ub-protein conjugates that accumulated in atrophying muscles were of high molecular mass (> 100 kDa). When innervated and denervated gastrocnemius muscles were fractionated, a significant increase in ubiquitinated proteins was found in the myofibrillar fraction, the proteins of which are preferentially degraded on denervation, but not in the soluble fraction. Thus activation of this proteolytic pathway in atrophying muscles probably occurs initially by increasing Ub conjugation to cell proteins. The resulting accumulation of Ub-protein conjugates suggests that their degradation by the 26S proteasome complex subsequently becomes rate-limiting in these catabolic states.

NASA Discipline Number 40-40↗
DOE OSTI2026

Exploring the effectiveness of a back-supporting exosuit: Trunk muscle activity and user experience in controlled and real-world shoveling scenarios

Introduction: This study evaluates the effectiveness of a passive wearable exosuit (HeroWear Apex) in reducing lumbar muscle effort while shoveling. Method: Two experiments were conducted, involving: (1) moving calibrated sandbags at a predefined pace in a laboratory, and (2) moving loose dirt in an in-field setting. Studies were designed to emulate real-world shoveling conditions at Department of Energy - Environmental Management sites. Muscle activity of the lumbar and oblique muscles was analyzed, along with user perceptions. Due to the asymmetric nature of shoveling, analysis of muscle activity was split between the weighted and unweighted sides, with the weighted side being defined as the side of the body closest to the head of the shovel in a neutral posture. Results: While donning the device, both experiments showed a significant decrease in muscle activity for at least one lumbar muscle on the weighted side. Participants rated the device with a high usability score, and perceived exertion ratings were significantly lower while wearing the exosuit. While opinions varied regarding the device’s helpfulness, participants felt the device was comfortable and did not hinder motion during the task. Practical applications: The reduction in back muscle activity associated with wearing the exosuit has the potential to reduce muscle fatigue resulting from repetitive motions.

Exoskeleton↗
NASA NTRS1982

Effect of limb immobilization on skeletal muscle

Current knowledge and questions remaining concerning the effects of limb immobilization on skeletal muscle is reviewed. The most dramatic of these effects is muscle atrophy, which has been noted in cases of muscles fixed at or below their resting length. Immobilization is also accompanied by a substantial decrease in motoneuronal discharges, which results in the conversion of slow-twitch muscle to muscle with fast-twitch characteristics. Sarcolemma effects include no change or a decrease in resting membrane potential, the appearance of extrajunctional acetylcholine receptors, and no change in acetylcholinesterase activity. Evidence of changes in motoneuron after hyperpolarization characteristics suggests that the muscle inactivity is responsible for neuronal changes, rather than vice versa. The rate of protein loss from atrophying muscles is determined solely by the first-order rate constant for degradation. Various other biochemical and functional changes have been noted, including decreased insulin responsiveness and protein synthesis. The model of limb immobilization may also be useful for related studies of muscle adaptation.

Booth, F. W.↗
NASA NTRS1982

Modulation of the cytosolic androgen receptor in striated muscle by sex steroids

The influence of orchiectomy (GDX) and steroid administration on the level of the cytosolic androgen receptor in the rat levator ani muscle and in rat skeletal muscles (tibialis anterior and extensor digitorum longus) was studied. Androgen receptor binding to muscle cytosol was measured using H-3 methyltrienolone (R1881) as ligand, 100 fold molar excess unlabeled R1881 to assess nonspecific binding, and 500 fold molar excess of triamcinolone acetonide to prevent binding to glucocorticoid and progestin receptors. Results demonstrate that modification of the levels of sex steroids can alter the content of androgen receptors of rat striated muscle. Data suggest that: (1) cytosolic androgen receptor levels increase after orchiectomy in both levator ani muscle and skeletal muscle; (2) the acute increase in receptor levels is blocked by an inhibitor of protein synthesis; and (3) administration of estradiol-17 beta to castrated animals increases receptor binding in levator ani muscle but not in skeletal muscle.

Rance, N. E.↗
NASA NTRS1985

Effects of Muscle Atrophy on Motor Control: Cage-size Effects

Two populations of male Sprague-Dawley rats were raised either in conventional minimum-specification cages or in a larger cage. When the animals were mature (125 to 150 d), the physiological status of the soleus (SOL) and extensor digitorum longus (EDL) muscles of the small- and large-cage animals were compared. Analysis of whole-muscle properties including the performance of the test muscle during a standardized fatigue test in which the nerve to the test muscle was subjected to supramaximal intermittent stimulation shows: (1) the amplitude, area, mean amplitude, and peak-to-peak rate of the compound muscle action potential decreased per the course of the fatigue test; (2) cage size did not affect the profile of changes for any of the action-potential measurements; (3) changes exhibited in the compound muscle action potential by SOL and EDL were substantially different; and (4) except for SOL of the large-cage rats, there was a high correlation between all four measures of the compound muscle action potential and the peak tetanic force during the fatigue test; i.e., either the electrical activity largely etermines the force profile during the fatigue test or else contractile-related activity substantially affects the compound muscle action potential.

Stuart, D. G.↗
NASA NTRS1985

Biochemical response to chronic shortening in unloaded soleus muscles

One leg of tail-casted suspended rats was immobilized in a plantar-flexed position to test whether chronic shortening of posterior leg muscles affected the metabolic response to unloading. The immobilized plantaris and gastrocnemius muscles of these animals showed approximately 20 percent loss of muscle mass in contrast to simply a slower growth rate with unloading. Loss of mass of the soleus muscle during suspension was not accentuated by chronic shortening. Although protein degradation in the isolated soleus muscle of the plantar-flexed limb was slightly faster than in the contralateral free limb, this difference was offset by faster synthesis of the myofibrillar protein fraction of the chronically shortened muscle. Total adenine nucleotides were 17 percent lower (P less than 0.005) in the chronically shortened soleus muscle following incubation. Glutamate, glutamine, and alanine metabolism showed little response to chronic shortening. These results suggest that, in the soleus muscle, chronic shortening did not alter significantly the metabolic responses to unloading and reduced activity.

Jaspers, S. R.↗