The purpose of this investigation was to study the effects of a 17-day spaceflight on the contractile properties of individual fast- and slow-twitch fibers isolated from biopsies of the fast-twitch gastrocnemius muscle of four male astronauts. Single chemically skinned fibers were studied during maximal Ca2+-activated contractions with fiber myosin heavy chain (MHC) isoform expression subsequently determined by SDS gel electrophoresis. Spaceflight had no significant effect on the mean diameter or specific force of single fibers expressing type I, IIa, or IIa/IIx MHC, although a small reduction in average absolute force (Po) was observed for the type I fibers (0.68 ± 0.02 vs. 0.64 ± 0.02 mN, P < 0.05). Subject-by-flight interactions indicated significant intersubject variation in response to the flight, as postflight fiber diameter and Po where significantly reduced for the type I and IIa fibers obtained from one astronaut and for the type IIa fibers from another astronaut. Average unloaded shortening velocity [V o, in fiber lengths (FL)/s] was greater after the flight for both type I (0.60 ± 0.03 vs. 0.76 ± 0.02 FL/s) and IIa fibers (2.33 ± 0.25 vs. 3.10 ± 0.16 FL/s). Postflight peak power of the type I and IIa fibers was significantly reduced only for the astronaut experiencing the greatest fiber atrophy and loss of Po. These results demonstrate that 1) slow and fast gastrocnemius fibers show little atrophy and loss of Po but increased V o after a typical 17-day spaceflight, 2) there is, however, considerable intersubject variation in these responses, possibly due to intersubject differences in in-flight physical activity, and 3) in these four astronauts, fiber atrophy and reductions in Po were less for slow and fast fibers obtained from the phasic fast-twitch gastrocnemius muscle compared with slow and fast fibers obtained from the slow antigravity soleus [J. J. Widrick, S. K. Knuth, K. M. Norenberg, J. G. Romatowski, J. L. W. Bain, D. A. Riley, M. Karhanek, S. W. Trappe, T. A. Trappe, D. L. Costill, and R. H. Fitts. J Physiol (Lond) 516: 915–930, 1999].
- muscle atrophy
- muscle disuse
- nonweight bearing
- myosin heavy chain
skeletal musclesare heterogeneous tissues comprised of slow and fast fibers that perform different roles during locomotion and other physical activities (12, 24). Stevens et al. (28) and Gardetto et al. (10) studied adult rats before and after 14 days of spaceflight and hindlimb suspension, respectively, and found that fiber atrophy and loss of peak Ca2+-activated force was greater for the slow type I fibers of the soleus than the fast type II fibers from the gastrocnemius. Spaceflight had no effect on the rat extensor digitorum longus fiber size or peak force (Po; see Ref.28). This seems to correspond well with the observations that isolated whole rat skeletal muscles with the greatest content of slow fibers show the most atrophy and greatest changes in function after nonweight bearing (2, 6, 8, 19, 37).
Human skeletal muscle fibers may follow a different pattern of atrophy during prolonged nonweight bearing (9). For instance, Edgerton et al. (3) observed greater atrophy in human fast compared with slow vastus lateralis fibers after 11 days in microgravity. In terms of muscle function, Widrick et al. (33) found greater atrophy of soleus type IIa vs. soleus type I fibers after a 17-day spaceflight, and, because of their smaller cross-sectional area, the fast fibers showed the greatest force deficit when directly activated with Ca2+. However, the soleus is composed of ∼90% slow fibers, limiting the total number of fast fibers that can reasonably be obtained for study from this muscle.
In the present study, we obtained human gastrocnemius muscle samples before and after a 17-day spaceflight. Because this muscle is composed of approximately one-half slow and one-half fast fibers (13), we were able to test the hypotheses that1) fast fibers show greater atrophy and functional changes during nonweight bearing and 2) fibers from the gastrocnemius (both slow and fast) are less affected by weightlessness than soleus fibers.
The subjects in this study were four male crew members aboard the National Aeronautics and Space Administration (NASA) Life and Microgravity Spacelab Space Shuttle mission (STS-78). The experimental protocol was reviewed and approved by the appropriate committees at each institution taking part in this study. All subjects were informed of the experimental procedures and consented to participate in writing. The physical characteristics of the astronauts, the standardized in-flight physiological testing conducted during the flight, and the countermeasures program performed by the astronauts have all been described previously (33).
Cell physiological studies.
Samples of the gastrocnemius were obtained using the percutaneous needle biopsy technique. Preflight samples were obtained 45 days before the June 20, 1996, launch of the Shuttle, and the postflight samples were obtained within 3 h after the Shuttle landed at the Kennedy Space Center on July 7, 1996. Muscle samples were placed on saline-soaked gauze and divided longitudinally into several portions, with one portion immediately placed in cold skinning solution (for composition, see below) and shipped overnight (4°C) to Marquette University. There, the sample was placed in fresh skinning solution and stored at −20°C. Over the next 28 days, single fiber segments were isolated from these bundles for physiological analysis.
The skinning solution contained 125 mM potassium propionate, 20 mM imidazole (pH 7.0), 2 mM EGTA, 4 mM ATP, 1 mM free Mg2+, and 50% glycerol (vol/vol). The relaxing and Ca2+-activating solutions used for the physiological studies contained 7 mM EGTA, 20 mM imidazole, 14.5 mM creatine phosphate, 1 mM free Mg2+, 4 mM ATP, KOH for adjustment of pH to 7.0, and KCl to attain a final ionic strength of 180 mM. The free Ca2+ concentrations ([Ca2+]) of the relaxing and activating solutions were pCa 9.0 and 4.5, respectively (where pCa = −log free [Ca2+]). The concentrations of each ligand, metal, and metal-ligand complex were determined from an iterative computer program (5) using stability constants adjusted for the temperature, pH, and ionic strength of our experiments.
On the day of an experiment, a muscle bundle was transferred from skinning solution to cold (4°C) relaxing solution. As previously described (10, 35), a 5- to 6-mm segment of a single muscle fiber was isolated and transferred to a small glass-bottomed chamber where the fiber ends were securely fastened to a force transducer (model 400; Cambridge Technology, Watertown, MA) and motor (model 300B; Cambridge Technology). The mounted fiber was briefly bathed in relaxing solution containing 0.5% Brij-58 (polyoxyethylene 20 cetyl ether; Sigma Chemical, St. Louis, MO).
The transducer, motor, and mounted fiber were placed on the stage of an inverted microscope. Sarcomere length was adjusted to 2.5 μm using a calibrated eyepiece micrometer. Fiber length (FL), defined as the length of the fiber suspended between the force transducer and motor, was measured as the fiber was advanced across the magnified field of view by a digital micrometer. A Polaroid photo was taken of the fiber while it was briefly suspended in air (<5 s). Fiber diameter was then calculated as the mean of three width measurements obtained from the photo based on the assumption that the fiber forms a circular cross section when removed from solution (18).
Analog outputs from the transducer and servomotor were monitored on a digital oscilloscope before being amplified, digitized, and interfaced to a personal computer. Custom software performed on-line analysis of force and position data (33). During data collection, each individual fiber was transferred from relaxing to activating (pCa 4.5) solution and allowed to attain Po. Fibers were then subjected to a series of rapid slack length steps for determination of unloaded shortening velocity (V o), sinusoidal length changes to quantify peak elastic modulus or stiffness (Eo), and/or a series of isotonic load clamps for the determination of the fiber force-velocity-power properties. For the slack steps, the linear relationship between the time for tension redevelopment vs. slack step distance was used to determine fiberV o. Eo was calculated as the change in force divided by the change in FL during oscillation at a frequency of 1.5 kHz and an amplitude of 0.05% of FL. To determine Eo attributed to the active state, stiffness measured in pCa 9.0 relaxing solution was subtracted from the pCa 4.5 value.
The force and V o data points from the isotonic contractions of each single fiber were fit by the hyperbolic Hill equation, (P + a)(V +b) = (Po + a)/b, where P is force, V is shortening velocity, Pois force when V = 0, and a and bare constants having dimensions of force and velocity, respectively. The R 2 of the fitted curve exceeded 0.98 for each fiber. The hyperbole fit to the force-velocity data could be described by the following three parameters:V max, the velocity-axis intercept,a/Po, a measure of the curvature of the relationship, and Po, the force-axis intercept or the peak isometric force produced by the fiber during the experiment. These three parameters, and the peak power calculated from the parameters (38), were used in statistical analysis. For graphical purposes, composite force-power relationships for groups of fibers were derived from the mean parameters of the fibers as described previously (34, 36). Detailed descriptions and illustrations of all of these procedures have been presented in our previous work (34,36). All experiments were conducted at 15°C.
After the physiological measurements, the fiber segment was removed from the experimental apparatus, solubilized in 10 μl of an SDS sample buffer, and stored at −80°C. Fibers were later run on 5 and 12% SDS-PAGE and silver stained to identify fiber myosin heavy chain (MHC) and myosin light chain (MLC) isoform content, respectively (17, 35). A computer-based image analysis system and software (SigmaGel; Jandel Scientific Software) were used to quantify the relative density of each MLC isoform (35).
Immediately after the muscle biopsy procedure, a portion of each muscle sample (∼1 mm diameter × ∼4 mm long) was pinned out straight to flat plastic sticks and immersion fixed in 20 ml of 4% glutaraldehyde, 2% paraformaldehyde in 0.1 M cacodylate buffer (pH 7.2) with 5 mM CaCl2 for 2 h at room temperature, followed by 24–48 h at 4°C. The samples were shipped overnight at 4°C to the Medical College of Wisconsin where they were rinsed in 0.1 M cacodylate buffer with 5 mM CaCl2 for 1 h and postfixed for 2 h at room temperature in 1.3% OsO4 in 0.1 M cacodylate buffer with CaCl2. After being rinsed in buffer without CaCl2, each bundle was dehydrated in graded ethanols to 100%, cleared in propylene oxide, infiltrated with epoxy resin, and cut into four equal pieces before polymerization at 60°C. Longitudinal and cross-thin sections (70 nm) were cut and poststained with uranyl acetate and lead citrate before examination and photography in a Jeol 100 CXII electron microscope. Morphological changes induced by spaceflight were identified by comparing the ultrastructural features of the postflight test samples with those of the preflight control tissue from the same individual.
Type I and IIa fiber types were identified ultrastructurally in longitudinal sections based on relative Z bandwidths, mitochondrial content, and sarcoplasmic reticulum content (1, 26). Classified fibers were recut in cross section for measurement of packing densities of thick and thin filaments (filament number per unit area) as described previously (25). Five fibers per subject were assayed morphologically for each of two conditions (pre- and postflight). For each subject, the percentage changes in thick and thin filaments in the A-band after spaceflight were calculated by expressing the postflight values as percentages of the preflight values.
Contractile properties were analyzed with a two-way ANOVA with main effects of subject and spaceflight. When ANOVA revealed a significant subject-by-spaceflight interaction, pre- and postflight means for each subject were compared with a t-test. Relationships between functional properties and protein isoform expression were evaluated with regression analysis. A paired t-test was used to compare pre- and postflight thin filament densities in the overlap A-band region. Statistical significance was accepted atP < 0.05.
Fiber MHC composition.
Adult human skeletal muscle fibers express the following three isoforms of MHC: type I, IIa, and IIx (4, 27). We obtained functional data on 131 gastrocnemius fibers expressing the type I MHC isoform and 79 fibers expressing the type IIa isoform. Only three fibers were isolated that expressed the type IIx MHC isoform exclusively (all from postflight biopsies), but we did study 40 fibers that coexpressed both the type IIa and the IIx isoform. An additional seven fibers were identified that coexpressed the type I and IIa MHC isoform and, in one case, all three adult MHC isoforms (a preflight fiber). Because of the limited sample sizes for the type I/IIa and type IIx fiber populations, statistical analysis was performed on fibers expressing the type I, type IIa, and type IIa/IIx MHC isoforms.
Fiber diameter and peak Ca2+-activated force.
The average diameter and peak Ca2+-activated force of fibers expressing type I, IIa, or IIa/IIx MHC are presented in Table1. There were no significant main effects of spaceflight, indicating no change in average fiber diameter for the type I, IIa, and IIa/IIx fibers after the flight. However, ANOVA indicated significant subject times spaceflight interactions for the diameter of the type I and IIa fiber populations. To illustrate these interactions, average pre- and postflight fiber diameters for each astronaut have been plotted in Fig. 1. Intersubject variation can be observed in the preflight fiber diameter of the type I and type IIa fibers. However, on closer examination, a consistent pattern emerged for both groups of fibers. Preflight type I fibers obtained from subject B were the largest in diameter; those from subjects D and A were intermediate in diameter; and those obtained from subject C were the smallest in average diameter. The same pattern was observed for the type IIa fibers, i.e., subject B > subjects A and D > subject C. There was also a clear pattern in the response of fiber diameter to the spaceflight. Forastronaut D, postflight type I and IIa fibers had significantly smaller diameters than preflight fibers (Fig. 1).Astronaut B also showed significant postflight atrophy of type IIa but not type I fibers, whereas the diameter of the type I and IIa fibers from astronauts A and C were unaffected by the spaceflight.
There was a significant reduction in the overall average peak Ca2+-activated force (Po, mN) of the type I fibers after the flight. However, this change was relatively small (−6%). There was no overall change in the Po of the type IIa or the type IIa/IIx fibers, but there was a significant subject times flight interaction for the type I and IIa fibers (Fig.2). Because there was no overall change in specific force (kN/m2), postflight absolute Po tended to follow the same pattern of response observed for postflight fiber diameter. Thus type I and IIa fibers obtained fromastronaut D and type IIa fibers from astronaut Bproduced significantly less force after the flight (Fig. 2).
Peak stiffness was measured in subsets of the type I, IIa, and IIa/IIx fibers (Table 2). There were no differences in the specific force of pre- and postflight fibers making up this analysis. Peak stiffness of the type I fibers declined by an average of 9% after the spaceflight while Po/Eo remained unchanged. There were no significant differences between pre- and postflight Eo or Po/Eo for any of the fast fibers. However, for both the type I and the IIa/IIx fibers, significant subject times spaceflight interactions were observed for Eo and Po/Eo.
Average V o was greater after the flight for both type I and IIa fibers, increasing 27 and 33%, respectively (Table 1). In the case of the type I fibers, a significant subject times flight interaction indicated that subjects showed different responses to the flight. Inspection of individual subject means indicated that, while all subjects showed an increase in V o, the magnitude of the increase varied by 10-fold on a percentage basis (9, 76, 31, and 7%, for astronauts A, B,C, and D, respectively), with the changes observed for subjects B and C reaching statistical significance. No interaction was noted for the response of the type IIa fibers.
We quantified the MLC composition of the majority of the type I and IIa fibers compiled in Table 1. In 58 preflight fibers and 66 postflight fibers expressing type I MHC, V o averaged 0.60 ± 0.03 FL/s before the flight and 0.75 ± 0.03 FL/s after the flight. All of these fibers expressed slow isoforms of MLC-1 and MLC-2. In a subgroup of fibers expressing type IIa MHC and the fast isoforms of MLC-1 and MLC-2, V o averaged 2.33 ± 0.265 FL/s (N = 27) before the flight and 3.10 ± 0.16 FL/s (N = 51) after the flight. Thus the change in V o could not be attributed to differences in the expression of slow and fast MLC-1 or MLC-2 isoforms.
We also calculated the ratio of MLC-3 to MLC-2 to examine potential changes in MLC-3 and their impact on V o(15). In the type I fibers, the overall average ratio of MLC-3 to MLC-2 was 0.32 ± 0.02 preflight and 0.24 ± 0.02 after the flight (P < 0.05). This change was significant, but, in the opposite direction, one would expect, if increased MLC-3 expression was responsible for the postflight increase in fiber V o (11, 29). Additionally, no correlation was found between the MLC-3-to-MLC-2 ratio andV o for either the preflight (r = −0.01, P > 0.05) or the postflight (r= 0.12, P > 0.05) fibers. For the type IIa fibers, the ratio of MLC-3 to MLC-2 was similar for preflight (0.66 ± 0.04) and postflight fibers (0.59 ± 0.02), despite the greaterV o of the latter. Again, no correlation was observed between MLC-3 expression and V o for either the preflight IIa fibers (r = 0.23,P > 0.05) or the postflight IIa fibers (r = −0.12, P > 0.05).
Electron microscopic analysis.
Cross-sectional analysis revealed that the type I fiber thin filament density (normalized for the sarcomere length of the fixed fiber) was significantly reduced by microgravity in all four astronauts. The group mean declined from 2,032 ± 134 to 1,821 ± 142 filaments/m2 or 10.4%. The reduced thin filament density of a postflight type I fiber is evident in Fig.3. For this astronaut, the average thin filament decline was 13%. Although microgravity had no significant effect on the average thin filament density in the type IIa or IIa/IIx fibers, an inverse correlation between fiber V oand thin filament density was observed for both the slow type I and fast type IIa fibers. The correlation was highest for subjects B and C where as a group R 2= 0.83 and 0.63 for the type I and IIa fibers, respectively. These two astronauts showed the largest increase in type I and IIa fiberV o and the lowest thin filament density postflight. Postflight, the type I fiber thin filament density averaged 1,556 ± 39 and 1,598 ± 93 for astronauts B andC and 2,097 ± 80 and 2,032 ± 97 forastronauts A and D.
Isotonic contractile properties.
The parameters describing the force-velocity relationships for the type I, IIa, and IIa/IIx fibers are compiled in Table3, and the corresponding composite force-power relationships are presented in Fig.4. Po was 6% lower in the postflight type I fibers (P < 0.05) and 15% lower in the postflight type IIa fibers, although this latter change was not statistically significant. V max, determined by extrapolating the force-velocity relationship to a force of zero, was greater for the type I and IIa fibers after the flight. For the type I fibers, the relative change in V max was +27%, which is identical to the change noted in Table 1 forV o. For the type IIa fibers,V max was 11% greater after the flight, considerably less than the +33% change in V onoted for these fibers. This difference is probably related to the underestimation of maximal shortening velocity of fast fibers by extrapolation of the force-velocity relationship (14). There were no alterations in the curvature of the force-velocity relationship for any group of fibers after the spaceflight.
The average reduction in fiber Po negated the increasedV o of the postflight fibers, resulting in no change in the average peak power of the type I fibers. Likewise, there was no difference in peak power of the type IIa or IIa/IIx fiber populations after the flight. Nevertheless, for both the type I and IIa fibers, the responses of individual astronauts differed significantly. In terms of absolute peak power (μN · FL · s−1), fibers obtained fromsubject D were most affected by the spaceflight. Type I and IIa fibers from this individual showed an average 18% (P < 0.05) and 16% (P = 0.06) decrease, respectively, in peak power after the flight (Fig. 4). In contrast, the peak power of postflight type I fibers from subject B and type IIa fibers from subject A were 36% (P < 0.05) and 29% (P = 0.06) greater, respectively, than the corresponding preflight value. Although there was no effect of spaceflight on average normalized peak power (kN · m−2 · FL · s−1), post hoc analysis of a significant subject-by-treatment effect showed that postflight type I fibers from astronaut B produced 53% greater normalized peak power than the preflight fibers (P < 0.05). This increase in peak power was due primarily to the greater V o of the postflight fibers.
Changes in fiber diameter and Po.
The 17-day spaceflight had no significant effect on the mean diameters of gastrocnemius fibers expressing type I, IIa, or IIa/IIx MHC. However, significant subject times flight interactions revealed that individual astronauts responded very differently to the spaceflight mission. We observed that the average diameter of type I and IIa fibers obtained from astronauts A and C were unchanged after the flight. In contrast, astronauts B and Dshowed postflight atrophy of 6% (P = 0.07) and 13% (P < 0.05), respectively, for type I fibers (equivalent to reductions in fiber cross-sectional area of 12 and 24%) and of 9% (P < 0.05) and 7% (P < 0.05) for IIa fibers (reductions in fiber cross-sectional area of 17 and 14%). Previously, Edgerton et al. (3) found average reductions in cross-sectional area of 16, 23, and 36% for histochemically identified type I, IIA, and IIB vastus lateralis fibers, respectively, obtained from five astronauts after an 11-day spaceflight. These authors noted that “the amount of muscle atrophy as a result of the flight varied widely among the individuals studied.” Although it appears that there was greater intersubject variation in the present flight, the 12–24% reductions in the CSA of type I fibers and the 14–17% reduction in CSA of the type IIa fibers from astronauts B and D are in good agreement with the work of Edgerton and co-workers. The current results and those of Edgerton et al. (3) are different from those obtained from rats flown in space where selected atrophy of slow fibers has consistently been observed (9, 28). In humans, fast type II fibers are at least as susceptible as slow type I fibers to microgravity-induced fiber atrophy.
Because the spaceflight had no effect on peak Ca2+-activated force per fiber cross-sectional area, Po during maximal Ca2+ activation changed in proportion to changes in fiber cross-sectional area. Thusastronauts A and C showed little change in fiber Po, whereas astronauts B (type IIa fibers) andD (type I and IIa fibers) showed reductions in absolute Ca2+-activated force. The lack of a spaceflight effect on the group means for fiber diameter (type I and IIa fibers) and the small 6% decline in the type I fiber Po was the result of the opposing responses of the following two groups of astronauts: those that experienced fiber atrophy and a corresponding loss of force and those who did not experience these changes.
The design of the present study makes it difficult to determine the factors or characteristics that distinguished subjects experiencing fiber atrophy and reductions in fiber Po from those who did not. One important source of intersubject variation could be the amount and/or type of physical activity performed by the astronauts during the flight. Each of the astronauts participated in physiological testing during the flight (31). It is unlikely that this activity explains our results, since the testing was standardized between subjects. However, in compliance with NASA regulations for this flight, astronauts also performed routine in-flight exercise countermeasures. This activity was performed on an individual basis and was not recorded. Additionally, some variability is inherent in the experimental design, as variability exists between fibers of a given type within a muscle, and the same fibers cannot be tested pre- and postflight.
Changes in fiber Vo and impact on fiber power.
Average unloaded shortening velocities of fibers expressing type I and IIa MHC were elevated by 27 and 33%, respectively, after this flight. These results are consistent with our previous observations of an average 30% increase in the V o of postflight type I soleus fibers obtained from the same four astronauts (33) and a 34% increase in the V oof human type I soleus fibers obtained after 17 days of bed rest (35). Increases of 30–60% in theV o of rat type I soleus fibers after 7–28 days of hindlimb suspension have consistently been reported by a number of laboratories (10, 20, 30, 32). However, in this animal model, it is only the type I soleus fibers that display an increase inV o; V o of both slow and fast gastrocnemius fibers did not increase after 14 days of hindlimb suspension (10). This result likely represents a species and/or model difference (i.e., spaceflight vs. hindlimb suspension).
Consistent with our previous work (33, 35), we found no relationship between changes in MLC isoform composition and the elevation in V o, making it unlikely that changes in MLC expression could explain the elevated V oof the postflight fibers. It seems unlikely that small amounts of undetected fast MHC in the slow fibers or IIx MHC in the IIa fibers could account for the elevated postflight V o. The resolution of our gel system is ∼2.5% of total myosin (17), an amount too small to have an appreciable effect onV o (7, 16, 21).
In this study, we observed a significant decline in the thin filament density for the slow type I fibers in all four astronauts. A selective loss of the actin filament could increase the lattice spacing between the thick and the thin filament. We have previously hypothesized that this in turn might cause the myosin head to detach sooner in the power stroke, reducing internal drag and increasing V o(33). For the soleus slow type I fiber, Riley et al. (22, 23) found significant correlations between the decline in thin filament density and the elevated fiber shortening velocities postflight. In this study of gastrocnemius cell function, the correlations between the microgravity-induced increase in fiberV o and the decline in thin filament density were not as strong as previously observed in the soleus. This suggests that, in addition to an altered filament lattice spacing, other unidentified mechanisms may be important in the increased V oobserved after spaceflight. Alternatively, thin filament density may need to fall below a critical value to elicit increases inV o. In support of this idea, astronauts B and C had the lowest thin filament density, the highest slope in the relationship between the decline in thin filament density and the increase in fiber V o, and the greatest increase in V o postflight.
Because subjects experienced different degrees of fiber atrophy and loss of peak Ca2+-activated force, the impact that elevated shortening velocities had on peak power also varied between the four astronauts. As an example, type I and IIa fibers from astronauts B and D showed losses in average peak Ca2+-activated force. Because postflight fibers fromastronaut B showed relatively large increases in fiberV o, average peak power was similar to, or even greater than, the power of the preflight fibers. Astronaut D, on the other hand, showed a loss in peak power, since relatively smaller increases in shortening velocities could not totally compensate for the loss of force production.
Comparison of the responses of type I gastrocnemius and type I soleus fibers to spaceflight.
It is interesting to compare the present data with our previous results obtained on type I soleus fibers from the same astronauts (33). In Figs. 5 and6, we have plotted individual results for the slow gastrocnemius and soleus fibers. Even though the soleus and gastrocnemius fibers expressed the same MHC isoform, it is obvious that they differed in their response to this particular spaceflight. Type I soleus fibers displayed on average an 8% reduction in diameter (15% reduction in cross-sectional area) and a 21% drop in Po(33). In contrast, the gastrocnemius type I fibers showed no overall atrophy and only a small 6% reduction in absolute Po. We conclude that, on this flight, the anatomical location of a type I muscle fiber played a major role in determining how that fiber responded to prolonged nonweight bearing. In terms of fiber atrophy and peak Ca2+-activated force (but notV o), slow soleus fibers were affected by this flight to a much greater extent than slow gastrocnemius fibers. Why the type I soleus fibers appeared more susceptible to nonweight bearing than the type I gastrocnemius fibers is unclear. One possibility may be a result of changes in muscle recruitment patterns during spaceflight. Rhesus monkeys experience a shift in ankle extensor activity within the first few days of spaceflight that produces a reduction in soleus activity and an increase in gastrocnemius activity during plantar flexion tasks (12, 25). If a similar shift in recruitment patterns occurs for humans during spaceflight, it could contribute to the observed differential atrophy of soleus and gastrocnemius fibers, since soleus fibers might experience a greater decline in recruitment than gastrocnemius fibers. The greater relative recruitment of the normally phasic gastrocnemius might even underlie the tendency for some astronauts to have larger-diameter postflight gastrocnemius fibers. This potential mechanism implies that the slow soleus fibers are not inherently more susceptible to nonweight bearing but rather are recruited less during nonweight bearing and therefore experience greater atrophy and loss of function. However, this explanation remains speculative, awaiting evidence that human muscle recruitment patterns are altered in this manner during spaceflight.
A second possibility to explain the differential atrophy of the soleus vs. the gastrocnemius fibers relates to their baseline diameter. For all of the present astronauts, type I fibers from the soleus were larger in diameter than type I fibers obtained from the gastrocnemius (Fig. 5). This is noteworthy, since astronauts with the largest diameter preflight fibers are those astronauts who appear to experience the greatest fiber atrophy after spaceflight (3, 33). An apparent selective loss of large-diameter type I soleus fibers has also been reported after 17 days of bed rest (35). Thus, although the mechanism is unknown, larger fibers appear more likely to undergo atrophy compared with smaller fibers. The greater atrophy of the type I soleus fibers from the present astronauts could therefore be related to the larger diameter of these fibers at the start of the study.
In summary, the diameter and functional properties of the type I and IIa muscle fibers making up the gastrocnemius responded in a similar manner to a 17-day spaceflight. Overall, these fibers showed little atrophy, minor losses in peak Ca2+-activated force, and modest increases in V o. Nevertheless, there was considerable variation in these responses between subjects, with two subjects displaying considerable atrophy of type I and IIa fibers. The functional properties of type I gastrocnemius fibers were affected less by this flight compared with the type I fibers obtained from the soleus of the same four astronauts (33). This indicates that the anatomical location of a muscle cell, and not only its MHC isoform composition, is an important determinant of the response of that cell to a typical spaceflight.
We are grateful to M. Karhanek and G. Gettleman for assistance during this project. We thank the crew of the LMS Spacelab Flight and the scientists at NASA Johnson Space Center, particularly Mel Buderer, Stuart Johnston, and Ladonna Miller.
This study was supported by National Aeronautics and Space Administration grant NAS9-18768 to R. H. Fitts.
Address for reprint requests and other correspondence: R. H. Fitts, Marquette Univ., Dept. of Biology, Wehr Life Sciences Bldg., P.O. Box 1881, Milwaukee, WI 53201-1881 (E-mail:).
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- Copyright © 2001 the American Physiological Society