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J Appl Physiol 90: 1825-1832, 2001;
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Vol. 90, Issue 5, 1825-1832, May 2001

Plasticity of monkey triceps muscle fibers in microgravity conditions

P. Kischel, L. Stevens, V. Montel, F. Picquet, and Y. Mounier

Laboratoire de Plasticité Neuromusculaire, Université des Sciences et Technologies de Lille 1, F-59655 Villeneuve d'Ascq Cedex, France


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

We examined the changes in functional properties of triceps brachii skinned fibers from monkeys flown aboard the BION 11 satellite for 14 days and after ground-based arm immobilization. The composition of myosin heavy chain (MHC) isoforms allowed the identification of pure fibers containing type I (slow) or type IIa (fast) MHC isoforms or hybrid fibers coexpressing predominantly slow (hybrid slow; HS) or fast (hybrid fast) MHC isoforms. The ratio of HS fibers to the whole slow population was higher after flight (28%) than in the control population (7%), and the number of fast fibers was increased (up to 86% in flight vs. 12% in control). Diameters and maximal tensions of slow fibers were decreased after flight. The tension-pCa curves of slow and fast fibers were modified, with a decrease in pCa threshold and an increase in steepness. The proper effect of microgravity was distinguishable from that of immobilization, which induced less marked slow-to-fast transitions (only 59% of fast fibers) and changed the tension-pCa relationships.

skinned fibers; atrophy; contractile proteins; myosin heavy chain; calcium affinity


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

PREVIOUS FINDINGS ON ANIMALS exposed to microgravity or simulated non-weight-bearing conditions have shown a marked atrophy of some hindlimb muscles. For instance, studies on rat muscles after spaceflights demonstrated that microgravity induced a spectacular atrophy of slow-twitch extensors such as the soleus. Our experiments performed after those flights showed that this atrophy was characterized by a decrease in fiber diameter and isometric force and was accompanied by slow-to-fast changes in contractile properties (15, 27, 29) and in myosin isoform composition (30). These results were confirmed by numerous studies (1, 6, 13).

Data from primates flown on biosatellites have provided an excellent opportunity to study the effects of microgravity on the neuromuscular system (3). Indeed, monkeys appear to be a suitable model for investigating muscle contractile and biochemical properties, as well as other activities, such as work capacity and electromyogram recording. Previous studies, particularly histochemical studies, have shown that extensor and flexor muscles from monkey upper limbs have different functions (23). For instance, extensors have a relatively greater potential for force production and force maintenance than flexors.

In this study, we examined the effects of microgravity induced by exposure of monkeys to the 14-day BION 11 spaceflight on the triceps brachii muscle (caput medialis). The triceps muscle, an upper limb extensor, is composed of a large proportion of slow fibers in its deepest zone. Our objective was to characterize, for the first time, the contractile properties of the extensor triceps muscle. Moreover, this work permitted analysis of the effects of microgravity in this muscle and comparison of these effects with 1) data obtained on extensors from other species such as rat soleus muscles and 2) data obtained on monkey soleus muscle, a hindlimb extensor, and data reported by Fitts' group (12, 37) after the same spaceflight. Finally, this spaceflight offered the opportunity to distinguish some specific effects of microgravity from those resulting from confinement or immobilization conditions generally associated with the space environment.

We have demonstrated that functional modifications occurred after this spaceflight. These modifications consisted of an atrophic process more marked in slow fibers, a decrease in force associated with a decrease in Ca2+ affinity of the contractile system, and slow-to-fast changes in myosin-based fiber typing. These effects were distinguishable from those due to arm immobilization, since in the latter case, no atrophy and no change in the steepness of the tension-pCa (T-pCa) relationships were observed. Finally, we have discussed the implication of the different regulatory contractile proteins.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animals. The experiments were performed on rhesus monkeys (Macaca mulatta) weighing 3.7-4.8 kg at the time they were selected (~5 mo before the flight). They were divided into four groups. The first (Cont) group included 11 monkeys on which biopsies were obtained 5 mo before launch. On the basis of results after the preliminary tests required by the different investigators, two monkeys (357 and 484) were selected for flight aboard the BION 11 satellite for 14 days (designated the Flight group). One day immediately after the satellite landed, a second biopsy was obtained. The third group consisted of cage growth-control (Growth) monkeys from the initial control pool; with this group it was possible to differentiate the eventual proper effects of growth, with 5 mo separating the control and the flight biopsies. Four animals were selected and kept in cages: biopsies were obtained from two monkeys (470 and 513) at the time of launch and from two others (474 and 503) 2 days after landing. The fourth group, arm-immobilized (Immo) animals (447, 501, and 534), were used as ground-based simulation controls; they were placed in a mock-up flight capsule simulating flight conditions (e.g., temperature and sound). Not only were these animals subjected to restraint conditions in the seats, but also the upper part of the right arm was maintained close to the body. The aim of this arm immobilization was to oblige the monkeys to work with the left arm during the flight to perform different tests necessary for electromyogram recordings. Therefore, the right arm was subjected to microgravity and reduced activity.

Biopsies. Muscle biopsies (~20-30 mg) were surgically obtained from the triceps brachii (caput medialis) from the upper limb. All the biopsies were obtained from the deepest portion of the midbelly zone of the triceps. In normal conditions, the deep triceps contained a majority of slow fibers. Another investigator (7) used ATPase staining of the same biopsies to measure the proportion of type I fibers in the Cont monkeys (n = 11, 88.33 ± 3.45%) and the percentages of type IIa and IIb fibers (9.95 ± 3.15 and 1.7 ± 0.57%, respectively). For the Flight monkeys, the proportions of type I, IIa, and IIb fibers were 82.4, 14.8, and 2.8, respectively (for monkey 357) and 88.0, 10.0, and 2.0, respectively (for monkey 484).

Experimental procedures and solutions. The biopsies were chemically skinned by exposure to a "skinning solution" (38). The skinned biopsies were stored at -20°C in a 50% glycerol-50% skinning solution. Experimental procedures and solutions have been described in detail in previous studies (18, 28). Briefly, a single skinned fiber was isolated from a biopsy and transferred to the experimental chamber. A ~1.5-mm-long segment was mounted between a small fixed forceps and a force transducer (Fort 10, WPI, Aston, UK). All the experiments were performed at 19 ± 1°C. With the diffraction of an He-Ne laser beam, the sarcomere length was adjusted to 2.90 µm and controlled during the experiments. At this sarcomere length, the maximal isometric force (Po) could be elicited. The diameter of each fiber was measured using a binocular micrometer (×80). The skinned fiber was activated with various concentrations of Ca2+, buffered with EGTA, and expressed as pCa values (i.e., negative logarithm of Ca2+ concentration). The composition of the solutions was calculated by the computer program of Fabiato (9). After exposure to each pCa, a tension P was recorded and immediately followed by a maximal tension (Po) elicited using a fully activating Ca2+ concentration (pCa 4.2). This allowed the calculation of the relative isometric force (P/Po). Then, a relaxing solution was applied, and the fiber was washed to eliminate the EGTA traces from the relaxing solution before initiation of a new tension activation. The relations between P/Po and the different pCa were given by the T-pCa curves. After a succession of Ca2+ activations, a new range of tensions was obtained using another divalent cation, Sr2+, and T-pSr curves were established as for the T-pCa curves. This protocol was performed to identify the fiber type (see below). From the T-pCa and T-pSr relationships, several characteristics could be derived, including the Ca2+ and Sr2+ concentrations that induce 50% of maximum Ca2+ and Sr2+ tension responses (pCa50 and pSr50, respectively). These characteristics described the affinity of the contractile system for Ca2+ and Sr2+. Other parameters derived from the T-pCa curves were used: the threshold for activation by Ca2+ (pCathr) and the steepness of the curves, indicated by the Hill coefficient (nH). The protocol for data fitting to the Hill equation has been previously described (15).

Fiber type identification. As described in our previous studies (29, 34), fiber type was determined according to the difference in Ca2+ and Sr2+ activation characteristics between fast (F) and slow (S) skeletal muscle fibers. Indeed, it is generally assumed that F skeletal muscle fibers are less sensitive to Sr2+ than S fibers (10, 16, 32). The pCa50-pSr50 difference (or Delta  criterion) is used to reflect the relative affinity of a fiber to Ca2+ and Sr2+. Thus, in our experiments, a fiber showing a Delta  of 0.1-0.3 was identified as the S type. On the contrary, an F-type fiber could be characterized by Delta  close to 1. Moreover, the pCathr, pCa50, and nH parameters also permitted identification of the fiber type. In S fibers, pCathr and pCa50 values were higher (lower Ca2+ concentrations) and nH was lower than in F fibers.

After physiological measurements (T-pCa and T-pSr), the myosin heavy chain (MHC) composition of the fibers from the biopsies of the different monkeys was determined on a 7.5% polyacrylamide slab gel (35). After the gel run, the gel slabs were silver stained. This protocol permitted identification of four MHC isoforms in rat muscles: the S type I isoform and the three F type IIa, IId/x, and IIb isoforms. The electrophoresis was used here only to determine the fiber type, i.e., the proportions of pure and hybrid slow (HS) and hybrid fast (HF) fibers.

Statistical analysis. Values are means ± SE. The results were analyzed statistically using a one-way ANOVA followed by a Bonferroni test to estimate differences among means. The acceptable level of significance was set at P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Because we did not have previous data relative to the characteristics of monkey triceps fibers, we chose to constitute large-size samples of S and F fibers and, therefore, to search F fibers, although they were rarely present (~12%; Table 1; see Experimental procedures and solutions) in the deep part of the triceps from Cont monkeys. Therefore, the pools of S and F fibers were similar (52 and 48 fibers, respectively), and to give a correct typing of the triceps muscle, we reported in Table 1 the data from Desplanches et al. (7) obtained from the same triceps biopsies. For the Immo and Flight groups, the fibers were taken randomly within the different biopsies.

                              
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  		Table 1.   Fiber type proportions of control, arm-immobilized, and flight monkeys

Fiber type identification: data on control monkeys. A total of 117 fibers was examined (>= 10 fibers were isolated from each of the 11 Cont monkeys).

Fibers were functionally classified as S or F according to the Delta  criterion. Electrophoretic analysis revealed four groups defined according to their composition in slow and fast MHC (Fig. 1). S and F fibers expressed only the slow type I and fast type IIa MHC isoforms, respectively. The two other groups were identified as composed of hybrid fibers, which coexpressed the type I and IIa MHC isoforms; they were called HS and HF according to the predominant isoform, S or F, respectively. In these Cont monkeys (Table 1), the proportion of HS fibers within the whole slow population (WSP, HS + S fiber types) was low and equal to ~7%. On some biopsies we did not find any HS fibers. The ratio of the number of HF fibers to the whole fast population (WFP, HF + F fiber types) was ~21%; these fibers were present in all biopsies.


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  		Fig. 1.   Representative profiles for slow (S), hybrid slow (HS), hybrid fast (HF), and fast (F) fibers as determined by SDS-PAGE (7.5% slab gel) of types I and II myosin heavy chain (MHC).

No variability between the different animals was found. Indeed, we obtained similar Delta  criterion values for the relative Ca2+-Sr2+ activation properties for the different control monkeys, in agreement with the data of fiber typing established by Desplanches et al. (7) (see METHODS). Therefore, for each fiber type (S, HS, HF, and F), we pooled the functional data we had obtained in the 11 Cont monkeys.

Functional properties of control monkey triceps fibers. The results are summarized in Table 2. Diameter was not significantly different between S, HF, and F fiber populations. The HS fibers presented the lowest mean diameter, although it was significantly different only from that of F fibers. All populations of fibers exhibited similar Po. However, when expressed per cross-sectional area, Po of the F group appeared lower than that of the S group. The Ca2+ activation properties of S and F types were not significantly different (Table 2). The pCathr was clearly lower for F and HF than for S and HS fibers. The pCa50 values were significantly lower for F and HF fibers than for S fibers. Thus the T-pCa relationships of F and HF fibers were shifted toward higher Ca2+ concentrations (Fig. 2, C and D) than the T-pCa curves of the S and HS fibers (Fig. 2, A and B). Moreover, the F and HF curves were steeper than the S and HS curves, as shown by significantly larger nH values. All Ca2+ activation parameters (pCathr, pCa50, and nH) exhibited very close values within a whole population defined by the Sr2+ test (WSP or WFP). Thus we did not observe any difference between the data obtained for the S and HS fibers or for the F and HF fibers. Because this result remained true regardless of the experimental group (Growth, Immo, or Flight), we gathered the data of functional characteristics for S and HS fibers in a single group and for F and HF fibers in another single group (Fig. 2, E and F). Similar T-pCa relationships were obtained for S + HS or F + HF fibers from all the control monkeys pooled together (n = 11) or from individual groups used as controls in Growth (n = 4), Immo (n = 3), and Flight (n = 2) experiments. The groups were chosen to constitute the controls (or Pre data) of the different experimental groups. Therefore, Pre and Post data were obtained in the same animals, with 5 mo separating the two biopsies. The possible growth effect is stated below.

                              
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  		Table 2.   Contractile parameters of the control triceps muscle fibers



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  		Fig. 2.   Tension-pCa (T-pCa) curves of S, HS, F, and HF fibers from triceps muscles of 11 control monkeys. , Pooled data from all control monkeys; open circle , black-triangle, and triangle , data from separated groups used as controls before growth, immobilization, and flight experiments, respectively. Curves are drawn according to the Hill equation. Values are means ± SE; n, number of fibers. P/Po, relative isometric force.

Growth effect analysis. For the Growth group, no differences in diameter and Po (kN/m2) values were observed in the 5-mo delay between the period of animal selection and the flight. This result is shown for S (n = 23 for Pre and Post growth) and F fibers (n = 21 for Pre and Post growth) in Fig. 3, A and D. T-pCa relationships obtained for WSP and WFP 5 mo before launch are reported in Fig. 2, E and F. T-pCa curves for the biopsies at landing were not different from those at the time of animal selection (data not illustrated). Thus all the studied parameters indicated that there was no growth effect between the time the monkeys were selected and the flight.


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  		Fig. 3.   Fiber diameters and maximal tensions (Po) of the whole slow population (WSP, including S + HS fibers) and the whole fast population (WFP, including F + HF fibers) in control experiments before (Pre) and after (Post) growth (A and D), before and after arm immobilization (B and E), and before and after flight conditions (C and E). *Significant difference between Pre and Post, P < 0.05.

Effects of arm immobilization. Data were collected from three monkeys (447, 501, and 534) whose right arms were submitted to immobilization. For these three monkeys, the HS-to-WSP and HF-to-WFP ratios, established on 35 fibers, before arm immobilization were 7.6 and 19%, respectively; these values are close to those reported above for the 11 Cont monkeys. After immobilization, the percentage of hybrid fibers increased (Table 1). Indeed, the HS-to-WSP ratio reached 33% and the HF-to-WFP ratio was ~32%. Moreover, the proportion of the four fiber types established from the 37 fibers taken randomly in the three biopsies of the Immo monkeys showed an increase in the HF and F fiber proportions compared with data reported by Desplanches et al. (7) (Table 1).

After immobilization, there were no significant differences in fiber diameter and maximal force, regardless of fiber type (Fig. 3, B and E). On the contrary, changes were found in Ca2+ activation parameters: pCathr and pCa50 were decreased for the S and F fibers (Table 3). Therefore, the T-pCa relationships were shifted toward higher Ca2+ concentrations (Fig. 4). The nH values were very close before and after immobilization, and, as a consequence, the T-pCa curves of the two fiber types were parallel.

                              
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  		Table 3.   Contractile parameters of the pre- and post-arm-immobilized and -flight triceps muscle fibers



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  		Fig. 4.   Tension-pCa relationships in control conditions, after arm immobilization, and in spaceflight. A: S + HS fibers. B: F + HF fibers. Dotted curves in A and B correspond to the curves in Fig. 2, E and F, respectively.

Effects of flight conditions. After flight, a primary result is the increased proportion of HS fibers (25 and 28% HS-to-WSP ratio for monkeys 357 and 484, respectively; Table 1) compared with that before flight (6.6%). A secondary effect of weightlessness is the decrease in the proportion of S and HS fibers relative to the whole content in fibers at the expense of a large increase in F fibers, especially pure F fibers. Monkeys 357 and 484 exhibited the same shift from an S to an F typing.

After flight, diameter and maximal forces significantly decreased in S fibers but did not change in F fibers (Fig. 3, C and F). For both fiber types, the main change in Ca2+ activation parameters was observed in the pCathr and the slopes of the T-pCa relationships (Table 3). The values of pCathr were decreased after flight, with a larger effect for the S fibers. The nH was significantly increased for S and F fibers (Table 3, Fig. 4).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

For the first time, data on contractile properties of the triceps brachii, a monkey forelimb muscle, in control and weightlessness conditions are reported. Microgravity induces a slow-to-fast shift of the triceps phenotype, as demonstrated by a large increase in the HS and pure F populations to the detriment of the pure S fiber content. Although only S fibers appeared atrophied and exhibited a decrease in their Ca2+ sensitivity after weightlessness, the cooperativity between the proteins of the thin filament increased in all fibers, indicating an effect of weightlessness on many proteins involved in the contractile process. Moreover, we have provided evidence that, in a situation such as spaceflight, in which immobilization is accompanied by microgravity, with both conditions inducing a slow-to-fast shift in the triceps contractile properties, the proper effect of weightlessness can be identified. Indeed, most of the previous results after exposure to microgravity were obtained in rats. Therefore, it was difficult to assess the hypokinetic factor; in this study, however, the immobilization of the monkey could be strictly simulated and controlled.

Muscle fiber types in monkey triceps. According to electrophoretic analysis, four fiber types were found in the triceps muscle: pure S and F fibers and hybrid fibers, which coexpressed S and F myosin isoforms. In control monkeys, few HS fibers (7%) were found within the WSP, whereas the proportion of HF fibers (21%) within the WFP was more important. Moreover, differences in the Ca2+ activation properties between S and F fibers were found and appeared similar to those described previously, especially in rat muscle (18). Thus the pCa threshold and pCa50 values were lower and nH was higher for F than for S fibers. Ca2+ activation properties in HS and HF fibers were similar to those in S and F fibers, respectively. This suggested that the Ca2+ activation properties were dictated by a predominant type of isoform (S or F) and that the hybrid fibers might modulate other functional properties. For instance, they might contribute to the continuum in the maximal shortening velocity changes observed in rat soleus fibers (34).

The maximal specific force developed by the triceps S fibers appeared clearly larger than that produced by F fibers. This might seem surprising, since only a slight difference or no differences between fiber types have generally been reported (when a difference existed, the larger forces were developed by F fibers). Indeed, Fitts et al. (11) reported no significant differences in Po between type I and type II fibers from soleus and gastrocnemius muscles of rhesus monkeys, in agreement with our previous data (5) that described no statistical differences between the specific Po values of the S, HS, HF, and F fibers in the rhesus soleus muscles. The difference between S and F fiber Po values observed here for the triceps muscle did not appear to result from differences in the size of the two fiber types. In the triceps muscle, diameters of S and F fibers were comparable (60 ± 1.6 and 66.6 ± 1.9 µm, respectively), as observed for S and F fibers in monkey soleus muscle (5, 11). However, these values appeared slightly lower than those reported previously for the S fibers of the extensor soleus muscle of the monkey (5, 11) and rat (29) hindlimbs. This suggested that data obtained from the monkey forelimb triceps, although it is an extensor muscle, cannot be integrated in the scheme already proposed (37), which predicted that type I fiber diameters increased as species size increased. These type I fibers developed specific forces with amplitudes (126.5 ± 9.7 kN/m2) similar to those found in type I fibers of other species (see Table 2 in Ref. 37). A possible explanation can be proposed if we take into account that S fibers in the triceps were found in the deepest part of the muscle close to the bone. It might be supposed that these fibers were submitted to higher mechanical strains and, thus, were trained to develop larger forces than the F fibers.

Effects of immobilization. After immobilization, the WSP, including S and HS fibers, attained 40%. Compared with the proportion of type I fibers (mean 88%) established for the control animals by ATPase staining, immobilization induced an S-to-F shift of the triceps phenotype. Moreover, the proportion of HS fibers within the WSP clearly increased by a factor of 4.5 after immobilization. Our results appeared to be in agreement with previous data (25, 31) that reported, in rat soleus muscles immobilized in a neutral position, an increase in the proportion of fast type IIA MHC in association with a decrease in type I MHC, whereas type IIB MHC appeared insensitive to immobilization. Therefore, the transitions appeared relatively limited to a change in the properties of the two isoforms of MHC, I and IIA, normally expressed in the muscle. It is now well known that the length at which a muscle is immobilized determines its degree of atrophy (17, 21): a neutral position produces more moderate changes than those induced by shortening (25), as in antigravity extensor muscles in real or simulated microgravity conditions (21).

In our experimentation on ground-based monkeys, the arm immobilization did not induce any atrophy (no change in fiber diameters and maximal tensions) in S and F fibers compared with control animals. This can be related to the neutral position of the immobilized triceps. A similar result was obtained for a forelimb extensor, the soleus, after restraint of monkeys seated in a replica of a flight chair in ground-based experiments performed in parallel with a 14-day spaceflight, Cosmos 2044 (2). Fiber diameters of the soleus and median gastrocnemius remained unchanged. In the same way, no change in muscle mass (i.e., no atrophy) was reported in the rat soleus immobilized in a neutral position (31), although other data (26) reported decreases in the cross-sectional areas of slow oxidative and fast oxidative-glycolytic soleus fibers after 4 wk of immobilization. The manner in which the animals were restrained, the duration of restraint, and interspecies variation in susceptibility to the length at which the muscle was maintained might explain differences in the results.

After immobilization, the Ca2+ affinity of the contractile system decreased, since pCa threshold and pCa50 were significantly reduced in S as well as in F fibers. However, in both fiber types the nH values remained unchanged. Therefore, the T-pCa relationships after immobilization were shifted toward higher Ca2+ concentrations, remaining parallel to the control curves. A slight shift has already been described for the T-pCa curve of S soleus fibers after 17 days of bed rest in humans (36). In our conditions of arm immobilization, the amplitude of the shift was larger for S than for F fibers (0.17 vs. 0.09 pCa unit).

Effects of weightlessness. The WSP appeared clearly lower after flight than in the control condition, since its proportion in monkeys 357 and 484 represented 13 and 31% of the total number of fibers, respectively. Even if the comparison with control data uses the reference of ATPase staining for monkeys 357 and 484 (personal communication, D. Desplanches), it is clear that there was a large S-to-F transition of the triceps muscle after microgravity. Thus the whole F proportion was enhanced after flight, and the changes appeared more extensive than those resulting from the arm immobilization. Indeed, according to Table 1, the proper effect of microgravity seems to be the reinforcement of the pure F fiber content. Obviously, for this distinction it was assumed that the arm immobilization effect was similar at 1 and 0 G, an assumption that cannot be verified. Moreover, the S-to-F changes also concerned the HS-to-WSP ratio, which was increased by a factor of ~3.5.

After weightlessness, fibers of the WSP exhibited decreases in diameter and tensions. Therefore, the force decrease after microgravity was not simply due to fiber atrophy, i.e., the loss in total content of myofibrillar proteins. It could also be attributed to a decrease in force per cross bridge or to changes in the myofilament lattice, since myofibrillar volume per contractile unit is maintained or only slightly decreased after spaceflight (7). F fibers did not appear to be affected by spaceflight conditions. Similar results relating differential atrophy of S and F fibers during this spaceflight were obtained in monkey soleus muscle (12). On the contrary, a decrease in fiber diameter has been reported (3) in an fast flexor, similar to the tibialis anterior, in a monkey after a 14-day spaceflight, while the soleus and the median gastrocnemius were much less affected. We have no interpretation of this discrepancy, especially for the soleus muscle. More data from flight monkeys would be necessary. Most of the results describing how muscles respond to microgravity have been obtained from hindlimb muscles of rats flown aboard different biosatellites or Spacelab Life Sciences (SLS) flights. The results we obtained after 2 wk of spaceflight (29), as well as those we obtained after 14 days of simulated microgravity using the tail-suspended rat model (28) and data from many other groups (8, 20, 33), showed that the slow extensor muscles such as the soleus exhibited a large fiber atrophy and loss of force. Within the slow soleus, S and F fibers were atrophied, the larger effect being observed in fibers that remained slow. Fast extensors, e.g., the plantaris or the extensor digitorum muscle, showed no atrophy, but the tibialis anterior exhibited a decrease in absolute maximal force correlated to the decrease in fiber diameter after the SLS-2 flight (30). Therefore, there were some similarities as well as some differences between similar muscles in rats and monkeys. The degree to which the differences are attributable to the responsiveness of the species, to hind- or forelimbs, or to differences in the experimental conditions (i.e., free-floating rats compared with chaired monkeys) remains hypothetical.

Ca2+ activation properties of the S fibers of the triceps appeared more affected by weightlessness. Indeed, the pCathr was significantly decreased by 0.29 pCa unit, while the slighter decrease for F fibers (0.10 pCa unit) was not significant compared with control values. The spectacular effect of microgravity consisted in a large increase in the steepness of the T-pCa curves (higher nH values) for the S as well as the F fibers compared with the curves from control or immobilized animals. This large extent of straightening of the flight fiber curves might contribute to masking of the amplitude of the decrease in Ca2+ affinity (lower pCa50), which can, nevertheless, be detected for S fibers. Therefore, because it has already been well demonstrated that pCa50 can be related to troponin C properties (14), our data indicated that other regulatory proteins might participate in the changes of the Ca2+ activation properties. Indeed, the increase in nH suggested that proteins such as troponin T or tropomyosin implied in the cooperativity process along the thin filament (19) might have been transformed in the adaptation process of the contractile system to the absence of gravity during the BION 11 flight. Increases in nH have been clearly related to the proportions of fast troponin T and tropomyosin isoforms (22, 24), and changes in the troponin T and troponin I isoforms have already been described in simulated microgravity conditions (4).

In conclusion, despite complex factors due to the association of microgravity with inevitable other environmental conditions, the changes in muscular properties specifically due to weightlessness can be discriminated and suggested that many proteins of the contractile system participated in the functional adaptation of the muscle.


    ACKNOWLEDGEMENTS

The authors thank Dr. D. Desplanches for providing histochemical data.


    FOOTNOTES

This work was supported by Centre National d'Etudes Spatiales Grant 2000/3027 and Fonds Européen de Développement Economique Régional Grant F007.

Address for reprint requests and other correspondence: Y. Mounier, Laboratoire de Plasticité Neuromusculaire, Université des Sciences et Technologies de Lille 1, Bâtiment SN4, 59655 Villeneuve d'Ascq Cedex, France (E-mail : Yvonne.Mounier{at}univ-lille1.fr).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 26 July 2000; accepted in final form 2 December 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Baldwin, KM, Herrick RE, Ilyina-Kakueva EI, and Oganov V. Effects of zero-gravity on myofibril content and isomyosin distribution in rodent skeletal muscle. FASEB J 4: 79-83, 1990[Abstract].

2.   Bodine-Fowler, SC, Pierotti DJ, and Talmadge RJ. Functional and cellular adaptation to weightlessness in primates. J Gravit Physiol 2: P43-P46, 1995[Medline].

3.   Bodine-Fowler, SC, Roy RR, Rudolph W, Haque N, Kozlovskaya IB, and Edgerton VR. Spaceflight and growth effects on muscle fibers in the rhesus monkey. J Appl Physiol 73, Suppl: 82S-89S, 1992.

4.   Campione, M, Ausoni S, Guezennec CY, and Schiaffino SJ. Myosin and troponin changes in rat soleus muscle after hindlimb suspension. J Appl Physiol 74: 1156-1160, 1993[Abstract/Free Full Text].

5.   Cordonnier, C, Stevens L, Picquet F, and Mounier Y. Structure-function relationship of soleus muscle fibres from the rhesus monkey. Pflügers Arch 430: 19-25, 1995[Web of Science][Medline].

6.   Desplanches, D, Mayet MH, Ilyina-Kakueva EI, Sempore B, and Flandrois R. Skeletal muscle adaptation in rats flown on Cosmos 1667. J Appl Physiol 68: 48-52, 1990[Abstract/Free Full Text].

7.   Desplanches, D, Mayet-Sornay MH, and Hoppeler H. Structural changes in arm muscles with weightlessness. In: Meeting on the Biosatellite 11 Results. Sunnyvale, CA: NASA, 1998.

8.   Edgerton, VR, and Roy RR. Neuromuscular adaptation to actual and simulated spaceflight. In: Handbook of Physiology. Environmental Physiology. The Gravitational Environment. Bethesda, MD: Am. Physiol. Soc, 1996, sect. 4, vol. III, chapt. 32, p. 721-763.

9.   Fabiato, A. Computer programs for calculating total from specified free or free from specified total ionic concentrations in aqueous solutions containing multiple metals and ligands. Methods Enzymol 157: 378-417, 1988[Web of Science][Medline].

10.   Fink, RH, Stephenson DG, and Williams DA. Calcium and strontium activation of single skinned muscle fibres of normal and dystrophic mice. J Physiol (Lond) 373: 513-525, 1986[Abstract/Free Full Text].

11.   Fitts, RH, Bodine SC, Romatowski JG, and Widrick JJ. Velocity, force, power, and Ca2+ sensitivity of fast and slow monkey skeletal muscle fibers. J Appl Physiol 84: 1776-1787, 1998[Abstract/Free Full Text].

12.   Fitts, RH, Romatowski JG, Blaser C, De la Cruz L, Gettelman GJ, and Widrick JJ. Effect of space flight on the isotonic contractile properties of single skeletal muscle fibers in the rhesus monkey. J Gravit Physiol 7: 37-38, 2000.

13.   Gardetto, PR, Schluter JM, and Fitts RH. Contractile function of single muscle fibers after hindlimb suspension. J Appl Physiol 66: 2739-2749, 1989[Abstract/Free Full Text].

14.   Gulati, J, Scordilis S, and Babu A. Effect of troponin C on the cooperativity in Ca2+ activation of cardiac muscle. FEBS Lett 236: 441-444, 1988[Web of Science][Medline].

15.   Holy, X, and Mounier Y. Effects of short spaceflights on mechanical characteristics of rat muscles. Muscle Nerve 14: 70-78, 1991[Web of Science][Medline].

16.   Kerrick, WG, Malencik DA, Hoar PE, Potter JD, Coby RL, Pocinwong S, and Fischer EH. Ca2+ and Sr2+ activation: comparison of cardiac and skeletal muscle contraction models. Pflügers Arch 386: 207-213, 1980[Web of Science][Medline].

17.   Leterme, D, Cordonnier C, Mounier Y, and Falempin M. Influence of chronic stretching upon rat soleus muscle during non-weight-bearing conditions. Pflügers Arch 429: 274-279, 1994[Web of Science][Medline].

18.   Mounier, Y, Holy X, and Stevens L. Compared properties of the contractile system of skinned slow and fast rat muscle fibres. Pflügers Arch 415: 136-141, 1989[Web of Science][Medline].

19.   Nassar, R, Malouf NN, Kelly MB, Oakeley AE, and Anderson PA. Force-pCa relation and troponin T isoforms of rabbit myocardium. Circ Res 69: 1470-1475, 1991[Abstract/Free Full Text].

20.   Ohira, Y, Jiang B, Roy RR, Oganov V, Ilyina-Kakueva E, Marini JF, and Edgerton VR. Rat soleus muscle fiber responses to 14 days of spaceflight and hindlimb suspension. J Appl Physiol 73, Suppl: 51S-57S, 1992.

21.   Ohira, Y, Yasui W, Roy RR, and Edgerton VR. Effects of muscle length on the response to unloading. Acta Anat (Basel) 159: 90-98, 1997[Web of Science][Medline].

22.   Reiser, PJ, Greaser ML, and Moss RL. Tension/pCa characteristics and regulatory proteins of single fibers from chicken neonatal and adult fast and slow skeletal muscles (Abstract). Biophys J 51: 222A, 1987.

23.   Roy, RR, Bello MA, Powell PL, and Simpson DR. Architectural design and fiber-type distribution of the major elbow flexors and extensors of the monkey (cynomolgus). Am J Anat 171: 285-293, 1984[Web of Science][Medline].

24.   Schachat, FH, Diamond MS, and Brandt PW. Effect of different troponin T-tropomyosin combinations on thin filament activation. J Mol Biol 198: 551-554, 1987[Web of Science][Medline].

25.   Simard, CP, Spector SA, and Edgerton VR. Contractile properties of rat hind limb muscles immobilized at different lengths. Exp Neurol 77: 467-482, 1982[Web of Science][Medline].

26.   Spector, SA, Simard CP, Fournier M, Sternlicht E, and Edgerton VR. Architectural alterations of rat hind-limb skeletal muscles immobilized at different lengths. Exp Neurol 76: 94-110, 1982[Web of Science][Medline].

27.   Stevens, L, and Mounier Y. Ca2+ movements in sarcoplasmic reticulum of rat soleus fibers after hindlimb suspension. J Appl Physiol 72: 1735-1740, 1992[Abstract/Free Full Text].

28.   Stevens, L, Mounier Y, Holy X, and Falempin M. Contractile properties of rat soleus muscle after 15 days of hindlimb suspension. J Appl Physiol 68: 334-340, 1990[Abstract/Free Full Text].

29.   Stevens, L, Mounier Y, and Holy X. Functional adaptation of different rat skeletal muscles to weightlessness. Am J Physiol Regulatory Integrative Comp Physiol 264: R770-R776, 1993[Abstract/Free Full Text].

30.   Stevens, L, Picquet F, Catinot MP, and Mounier Y. Differential adaptation to weightlessness of functional and structural characteristics of rat hindlimb muscles. J Gravit Physiol 3: 54-57, 1996[Medline].

31.   Szczepanowska, J, and Jacubiec-Puka A. Myosin heavy chains in striated muscle after immobilisation. Basic Appl Myol 2: 97-105, 1992.

32.   Takagi, A, and Endo M. Guinea pig soleus and extensor digitorum longus: a study on single-skinned fibers. Exp Neurol 55: 95-101, 1977[Web of Science][Medline].

33.   Thomason, DB, and Booth FW. Atrophy of the soleus muscle by hindlimb unweighting. J Appl Physiol 68: 1-12, 1990[Abstract/Free Full Text].

34.   Toursel, T, Stevens L, and Mounier Y. Evolution of contractile and elastic properties of rat soleus muscle fibres under unloading conditions. Exp Physiol 84: 93-107, 1999[Abstract].

35.   Wada, M, Hamalainen N, and Pette D. Isomyosin patterns of single type IIB, IID and IIA fibres from rabbit skeletal muscle. J Muscle Res Cell Motil 16: 237-242, 1995[Web of Science][Medline].

36.   Widrick, JJ, Norenberg KM, Romatowski JG, Blaser CA, Karhanek M, Sherwood J, Trappe SW, Trappe TA, Costill DL, and Fitts RH. Force-velocity-power and force-pCa relationships of human soleus fibers after 17 days of bed rest. J Appl Physiol 85: 1949-1956, 1998[Abstract/Free Full Text].

37.   Widrick, JJ, Romatowski JG, Karhanek M, and Fitts RH. Contractile properties of rat, rhesus monkey, and human type I muscle fibers. Am J Physiol Regulatory Integrative Comp Physiol 272: R34-R42, 1997[Abstract/Free Full Text].

38.   Wood, DS, Zollman J, Reuben JP, and Brandt PW. Human skeletal muscle: properties of the chemically skinned fiber. Science 187: 1075-1076, 1975[Abstract/Free Full Text].

J APPL PHYSIOL 90(5):1825-1832
8750-7587/01 $5.00 Copyright © 2001 the American Physiological Society


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