Functional and structural adaptations of skeletal muscle to microgravity
1 Department of Biology, Marquette University, Milwaukee, WI 53201, USA,
2 Department of Cellular Biology and Anatomy, Medical College of Wisconsin, Milwaukee, WI 53226, USA and
3 Department of Exercise and Sport Sciences, Oregon State University, Corvallis, OR 97331, USA
*e-mail: Robert.fitts{at}mu.edu
Accepted July 5, 2001
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Summary |
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Key words: microgravity, skeletal muscle, muscle, atrophy, contractile properties.
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Introduction |
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Muscle diameter and force |
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Until recently, the effects of space flight on muscle force or strength were confined to studies of whole-muscle function. While providing important information regarding the extent of atrophy and loss of strength, such studies could not distinguish selective effects on slow versus fast fibers or the cellular mechanism for the loss of function. A cellular analysis of the calf muscles demonstrates a clear difference in the response of rats and humans to microgravity. For both species, fibers from the antigravity slow soleus showed greater atrophy than fibers from the fast-twitch gastrocnemius. However, when fibers within a given muscle were examined, rats, but not humans, showed selective atrophy of the antigravity slow type I fiber (see Fig.1)(Fitts et al., 2000). Data on the vastus lateralis and soleus muscles suggest that in humans the fast type II fibers may be even more susceptible to microgravity-induced atrophy than the slow type I fiber (Fitts et al., 2000). Following an 11-day human space flight, Edgerton et al. (Edgerton et al., 1995) observed a significant decline in the cross-sectional area of fibers from the vastus lateralis, with the decline being greatest in the IIb fibers and least in the type I fibers. Widrick et al. (Widrick et al., 1999) made similar observations for the soleus: following a 17-day flight, the type IIa fiber cross-sectional area declined by 26% compared with a 15% reduction in the cross-sectional area of the slow type I fiber. Fig.1 shows an electron micrograph of soleus muscle fibers obtained pre- and post-flight from astronaut B (Widrick et al., 1999). The space-flight-induced atrophy of the myofibrils is clearly observed.
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Recently, we studied the contractile properties of individual fibers isolated from the soleus and gastrocnemius of four astronauts before and immediately after a 17-day space flight (STS-78). Consistent with previously published whole-muscle studies (Skylab and Mir) and single-fiber analyses of the human vastus lateralis, considerable variability was noted in the degree of cell atrophy and the loss of peak force among astronauts (Widrick et al., 1999). Studies of Mir cosmonauts after 6 months of space flight showed declines of calf plantar flexor volume ranging from 6 to 20%, while maximal voluntary contractions (MVCs) of the same muscle group declined by 2048% (Zange et al., 1997). Recently, a similar result was reported by Lambertz et al. (Lambertz et al., 2000), who found an average 17% decline in the isometric torque measured during an MVC of the human plantarflexor muscle following 90180 days in microgravity. Individual variability was also noted: 12 of the 14 cosmonauts showed decreases in torque ranging from 2 to 37%, while two subjects showed slightly increased MVCs. In our study, we observed a 21% decline in the average peak absolute force of the slow type I fiber following a 17-day space flight, while in individual astronauts the change ranged from 12 to 40%. Currently, it is not known whether the observed variability among individuals is caused by a true difference in susceptibility to microgravity or results from variable amounts of in-flight countermeasure exercise. The average peak force for the post-flight soleus type IIa fibers was 25% lower than the pre-flight value a decline somewhat greater than that observed for the type I fibers.
From a quantitative perspective, the most important cause of the decline in peak force is cell atrophy. When our data were corrected for the reduced mass by expressing fiber force in kNm-2, the 17-day space flight resulted in an average 4% decline in peak force (Widrick et al., 1999). Fig.2 shows the relationship between peak force (mN) and fiber diameter pre- and post-flight for the type I soleus fibers of subject B (the subject with the greatest microgravity-induced decline in fiber size and peak force). Pre-flight, the majority of fibers had diameters greater than 100µm and peak forces greater than 1mN, while the post-flight fiber diameters were mostly less than 100µm with forces less than 1mN. Fig.2 also demonstrates the post-flight increase in the expression of type IIa myosin, as reflected by the increased number of type IIa fibers and the appearance of hybrid (type I/IIa) fibers in this subject.
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Maximal shortening velocity and peak power |
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Widrick et al. (Widrick et al., 1999) studied the effects of a 17-day space flight on the single-fiber contractile properties of fibers isolated from the soleus and gastrocnemius muscles of four crew members. Maximal fiber shortening velocity was determined using the unloaded slack test technique (fiber V0) and calculated from the forcevelocity relationship (Vmax). Importantly, in the soleus, the type I fiber V0 and Vmax increased by 30 and 44%, respectively. Similar increases were observed in the soleus type IIa fiber, in which V0 increased by 55% post-flight. The gastrocnemius fibers were less affected by space flight: the slow type I fibers from this muscle showed a 22% increase in V0, while this variable was unaltered in the fast type IIa fibers.
The data of Widrick et al. (Widrick et al., 1999) indicate that the space-flight-induced increase in the shortening velocity of the plantar flexor muscles is in part due to an increased velocity of the individual fiber types and not simply the result of an increased expression of fast-type myosin. Although we did observe an increase in the number of fibers expressing fast-type myosin post-flight, the significant increase in the velocity of the slow type I and fast type IIa fibers could not be explained by an altered myosin heavy chain isozyme content. The cause of the microgravity-induced increase in fiber V0 and Vmax is unknown. We did observe an increase in the content of the myosin light chain 3 in the slow soleus fibers post-flight; however, the increase did not show a significant correlation with V0. Widrick et al. (Widrick et al., 1999) proposed that the increased fiber V0 might be caused by a selective loss of the thin filament actin. Riley et al. (Riley et al., 2000) demonstrated that microgravity did indeed cause a selective loss of actin relative to myosin, and proposed that this change should increase the spacing between the thick and thin filaments. As a result, the cycling cross-bridges would be expected to detach sooner which, in turn, would reduce the internal drag that develops during the final portion of the cross-bridge stroke. The reduced drag would allow for an increased fiber V0. The thin filament concentration in the A-band was reduced by 26% post-flight, which resulted from a 17% reduction in the number of thin filaments and a 9% increase in the number of filaments too short to penetrate into the overlap A-band region. Fig.3 presents a schematic representation of the thin and thick filament layout in a normal and an atrophic sarcomere.
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Substrate and metabolic changes with microgravity and their impact on fatigue |
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We have observed that both hindlimb unloading in rats and bed rest in humans increased the resting muscle glycogen content of the soleus (Grichko et al., 2000). However, 17 days of space flight had no effect on soleus glycogen content in humans (R. H. Fitts, V. P. Grichko and M. L. De La Cruz, unpublished observations). This result can be observed in Fig.1. The small particles surrounding the myofilaments are glycogen particles, and equal numbers are observed in the pre-flight (Fig.1A) and post-flight (Fig.1B) micrographs. The lack of a significant increase in muscle glycogen content in this study may have resulted from an inadequate caloric intake. Stein et al. (Stein et al., 1999) reported that the crew from this flight were in negative caloric balance. Thus, with an adequate caloric intake, space flight may induce an increase in resting muscle glycogen content similar to that observed following bed rest.
The cause of the increased reliance on carbohydrates following space flight and models of weightlessness is unknown (Baldwin et al., 1993; Grichko et al., 2000). As stated above, the increase does not seem to be caused by an altered aerobic enzyme capacity. Our working hypothesis is that it results from a combination of a change in substrate regulation at the onset of exercise and an inhibition of carnitine palmitoyltransferase (CPTI), the rate-limiting enzyme for the oxidation of long-chain fatty acids. We have previously shown that steady-state skeletal muscle blood flow is not altered by hindlimb unloading (McDonald et al., 1992). However, recent evidence suggests that the rate of increase in blood flow with the onset of exercise may be depressed. Hindlimb unloading has been shown to reduce endothelium-dependent dilation in soleus feed arteries, and soleus blood flow measured using the microsphere technique was depressed in the first minute of exercise following 28 days of hindlimb unloading (Jasperse et al., 1999)(M. D. Delp, personal communication). Thus, the possibility exists that models of weightlessness and space flight may reduce the rate at which muscle blood flow increases with the onset of exercise. This, in turn, would increase the rate of creatine phosphate and ATP hydrolysis, increase ADP, AMP and inorganic phosphate production and stimulate glycolysis. The elevated glycolytic rate would increase the production of acetyl-CoA and malonyl-CoA. The latter is a known inhibitor of CPTI and, thus, fatty acid oxidation would be reduced. In addition, space flight and models of weightlessness may reduce the activity of adenosine-5'-monophosphate-activated protein kinase (AMPK). This enzyme appears to be a metabolic master switch controlling the activity of various metabolic pathways (Winder and Hardie, 1999). Of importance here, a reduced AMPK activity would reduce the inactivation of the target protein acetyl-CoA carboxylase (ACC), which would allow levels of the product of ACC (malonyl-CoA) to remain high. It is also possible that the exercise-induced mobilization of fatty acids from the adipose sites may be depressed by space flight or models of weightlessness. This would reduce fatty acid oxidation by limiting their delivery to and uptake by the skeletal muscle.
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Muscle fiber damage following space flight |
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Although the problem of fiber damage post-flight has not been studied in man, it probably occurs because astronauts complain of persistent muscle soreness for up to months post-flight. The microgravity environment has apparently altered the muscle so that it is more susceptible to eccentric-induced contractile damage post-flight. Following weightlessness and hindlimb unloading in rats, the slow type I fibers showed preferential damage, which was attributed to the selective recruitment of this fiber type (Riley et al., 1996). The causative factors of fiber damage are not known, but damage might in part relate to cell atrophy and the selective loss of the contractile protein actin (Fitts et al., 2000). Post-flight, any load and strain on the atrophied fiber would be relatively greater and thus more likely to cause damage. At the fibril level, the load would be distributed to fewer actin filaments, which might increase their susceptibility to damage. Other possibilities include changes in the protein titin, the cytoskeletal protein desmin and/or alterations in the dystrophinglycoprotein complex. Titin is a protein known to extend from the Z-line to the M-line of the A-band. It is thought to play a role in establishing resting tension and in the orientation of the thick filament myosin (Labeit et al., 1997). Thus, alterations in this protein could cause the thick filament to move from the center of the sarcomere towards the Z-line. A breakdown in desmin and/or reduction or absence of a single component of the dystrophinglycoprotein complex could result in greater susceptibility of the sarcolemma to contraction-induced damage (Chopard et al., 2001; Lieber et al., 1996).
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Concluding remarks |
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In rats (and perhaps humans), space flight and models of weightlessness increase resting muscle glycogen content, and glycogen depletion and lactate production are accelerated during activity. The increased dependence on glycogen is associated with a reduced ability to oxidize free fatty acids. The reduced ability to oxidize fats may be caused by substrate inhibition of CPTI, the rate-limiting enzyme in fatty acid oxidation.
Finally, muscle fiber damage following space flight has been observed in rats. Although some damage may occur in space, the majority occurs following re-loading upon return to earth. The lesions are similar to those observed following eccentric contractions. The etiology of the increased sensitivity to cell damage following microgravity is unknown, but may be caused by the cell atrophy and reduced actin content and/or to the selective loss of other proteins such as titin, desmin and dystrophin.
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Acknowledgments |
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References |
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