J. Appl. Physiol.
88: 567–572, 2000.
Decreased thin filament density and length in human
atrophic soleus muscle fibers after spaceflight
DANNY A. RILEY,1 JAMES L. W. BAIN,1 JOYCE L. THOMPSON,1 ROBERT H. FITTS,2
JEFFREY J. WIDRICK,2 SCOTT W. TRAPPE,3 TODD A. TRAPPE,3 AND DAVID L. COSTILL3
1Department of Cell Biology, Neurobiology, and Anatomy, Medical College of Wisconsin,
Milwaukee 53226; 2Department of Biology, Marquette University, Milwaukee, Wisconsin 53201;
and 3Human Performance Laboratory, Ball State University, Muncie, Indiana 47306
skeletal muscle; electron microscopy; myofilaments; unloading; atrophy
ASTRONAUTS RETURNING to Earth after spaceflight show
skeletal muscle weakness, fatigue, and lack of coordination and delayed-onset muscle soreness (3, 6, 23). The
muscle soreness, which occurs without strenuous exercise, indicates increased susceptibility of atrophic
muscle fibers to weight-bearing-induced damage. The
antigravity soleus and adductor longus muscles of rats
either flown in space or subjected to hindlimb suspension unloading for 2 wk exhibited extensive segmental
necrosis of muscle fibers 1–2 days after return to a
weight-bearing condition (12, 13, 18–21, 23). Possible
factors contributing to damage susceptibility may be
weakened structural proteins, increased Ca21-activated protease activity, and compromised microcirculation (15, 18, 19, 21, 23 26). After 17 days of bed rest,
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soleus muscle fibers exhibited a 23% decrease in thin
filament concentration in the overlap A band region and
no reduction in muscle fiber specific tension (force/cross
sectional area) (17). This raised the force per remaining
thin filaments an estimated 30%. The higher load per
filament may have weakened the sarcomere structure.
The present study examined myofilament concentrations and thin filament lengths in the soleus muscles of
astronauts participating in the 17-day Life and Microgravity Sciences Spacelab Mission, which had been
simulated earlier by bed rest (17). On the basis of
bed-rest findings, spaceflight unloading is expected to
generate a disproportionate loss of thin filaments, and
this decrease is anticipated to correlate directly with
the elevated shortening velocity demonstrated physiologically for soleus fibers from the same astronauts
(17, 31).
METHODS
Test subjects. Four male astronauts, averaging 43 6 4 yr
old, 183 6 8 cm tall, and 86 6 6 kg body wt, were biopsied
before spaceflight and after landing subsequent to a 17-day
exposure to microgravity during the Life and Microgravity
Sciences Spacelab Mission (STS-78, June 1996). Subjects
were designated A–D for anonymity. An open-incision, needle
biopsy was obtained 45 days before launch from the left
soleus muscle of each individual as a preflight control.
Preflight muscle adaptations were minimized by requesting
the crew to maintain a constant daily activity pattern from 90
days before and up to the day before launch. Within 3 h after
landing, a postflight biopsy was removed from the right
soleus. Physical activity was minimized postflight by restricting the subjects to wheelchairs until biopsy, because return to
weight bearing can induce secondary degenerative changes in
atrophic muscles (13, 18, 19, 21, 23). Before (90, 60, 30, and 15
days before launch), during (day 2 or 3, day 8 or 9, and day 12
or 13), and after flight (2 and 8 days after landing), subjects
underwent physiological testing with use of an isokinetic
dynamometer and cycle ergometry. Each testing session
consisted of determination of isometric and isokinetic torque
of the right ankle extensors and incremental supine cycle
ergometry performed at work rates up to 85% of preflight
maximum O2 consumption (31). These sessions were identical
to those described in detail for an earlier 17-day bed-rest
simulation of this mission (17, 30). Individuals performed ad
libitum aerobic exercise inflight, which was not possible to
document. Edgerton et al. (7) reported no effect of this type of
exercise on muscle atrophy. Heavy resistance exercise is
necessary for preventing atrophy (1). The human use protocol
was approved by the Institutional Review Boards of the
8750-7587/00 $5.00 Copyright r 2000 the American Physiological Society
567
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Riley, Danny A., James L. W. Bain, Joyce L. Thompson, Robert H. Fitts, Jeffrey J. Widrick, Scott W. Trappe,
Todd A. Trappe, and David L. Costill. Decreased thin
filament density and length in human atrophic soleus muscle
fibers after spaceflight. J. Appl. Physiol. 88: 567–572, 2000.—
Soleus muscle fibers were examined electron microscopically
from pre- and postflight biopsies of four astronauts orbited for
17 days during the Life and Microgravity Sciences Spacelab
Mission (June 1996). Myofilament density and spacing were
normalized to a 2.4-µm sarcomere length. Thick filament
density (,1,062 filaments/µm2 ) and spacing (,32.5 nm) were
unchanged by spaceflight. Preflight thin filament density
(2,976/µm2 ) decreased significantly (P , 0.01) to 2,215/µm2 in
the overlap A band region as a result of a 17% filament loss
and a 9% increase in short filaments. Normal fibers had 13%
short thin filaments. The 26% decrease in thin filaments is
consistent with preliminary findings of a 14% increase in the
myosin-to-actin ratio. Lower thin filament density was calculated to increase thick-to-thin filament spacing in vivo from
17 to 23 nm. Decreased density is postulated to promote
earlier cross-bridge detachment and faster contraction velocity. Atrophic fibers may be more susceptible to sarcomere
reloading damage, because force per thin filament is estimated to increase by 23%.
568
THIN FILAMENTS AND SPACEFLIGHT
Fig. 1. Longitudinal (A) and cross-sectional (B–D) views of sarcomeres to illustrate 3 regions (b–d) at which thin filament densities
were measured. B: near Z band, thin filaments show a square-array
pattern induced by Z band lattice structure. C: in mid-I band, thin
filaments are organized toward hexagonal packing imposed by adjacent interaction with thick filaments. D: thin filaments exhibit
regular packing as they interdigitate with thick filaments in overlap
A band region. Longitudinal section, magnification 324,000; horizontal bar, 0.5 µm. Cross sections, magnification 3194,300; horizontal
bar, 0.1 µm.
in the A band region than near the Z band. Thick filament
concentration was assessed in the overlap A band region.
Micrographs were overlain with a transparency of grid squares
of 0.0056 µm2 at 3201,000. The number of filaments counted
per 0.0056 µm2 was multiplied by 178.57 to compute filament
density as number per micrometer squared. The grid squares
were positioned at random in the sampling regions of centrally located myofibrils. Peripheral myofibrils were avoided,
because sarcomere structure was normally variable and
sometimes incomplete, especially near myonuclei. To avoid
biasing filament counts downward, grid placement was shifted
when the myofilaments were not oriented in cross section or
the predominant feature was an aggregation of glycogen. Five
fibers per subject were assayed morphologically for each of
two conditions (pre- and postflight). For thin filaments, three
grid squares were chosen that contained filaments in a
square-array pattern near the Z band. Three grid squares
were also used in the mid-I band and overlap A band regions.
Myofilament numbers per grid square were counted following
Gunderson’s rules for sampling (10). Within the A band
sampling region, center-to-center spacing distances for the
nearest neighbors of thick-to-thick and thick-to-thin filaments were measured using computer-assisted digitizing
morphometry. Unless otherwise indicated, values are
means 6 SE.
RESULTS
Myofibrils of soleus fibers in the preflight controls
were large (wide) and minimally separated by sarcoplasm, which contained glycogen-like particles and
occasional lipid droplets (Fig. 2A). Mitochondria were
numerous and resided at the level of the I band (Fig.
2A). After 17 days of spaceflight, muscle fibers decreased in cross-sectional area an average of 15% (31).
Myofibrils were noticeably thinner, as evidenced by the
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National Aeronautics and Space Administration Johnson
Space Center, Ball State University, Marquette University,
and the Medical College of Wisconsin, and informed written
consent was obtained before participation.
Tissue processing. With dissecting microscope magnification, individual biopsies were subdivided on a saline-soaked
gauze into portions for single-fiber contractile physiology and
biochemistry, histochemistry, and electron microscopy (17,
27, 30, 31). For the present study, bundles of muscle fibers (,1
mm diameter, ,4 mm long) were pinned out straight to flat
plastic sticks and immersion fixed in 20 ml of 4% glutaraldehyde and 2% paraformaldehyde in 0.1 M cacodylate buffer
(pH 7.2) with 5 mM CaCl2 for 2 h at room temperature and
then for 24–48 h at 4°C. The samples were shipped overnight
at 4°C to the Medical College of Wisconsin for rinsing in 0.1 M
cacodylate buffer with 5 mM CaCl2 for 1 h and postfixing for
2 h at room temperature in 1.3% OsO4 in 0.1 M cacodylate
buffer with CaCl2. After each bundle was rinsed in buffer
without CaCl2, it 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 (17).
Longitudinal and cross thin sections (70 nm) were cut and
poststained with uranyl acetate and lead citrate before they
were examined and photographed in a JEOL 100 CXII
electron microscope. Some sections were poststained with
0.5% tannic acid to accentuate glycogen. 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. Fibers damaged by dissection were excluded. The
means of the pre- and postflight measures were analyzed by a
paired-sample t-test, with each subject serving as his own
control. Bed-rest data supported an a priori prediction of
decreased thin filament densities in the postflight condition
and permitted a one-tailed t-test.
Quantitation of thick and thin filament density. The packing densities of thick and thin filaments (filament number per
unit area) were directly altered by sarcomere length; i.e.,
higher density and closer spacing (interfilament distance)
resulted as sarcomeres were lengthened, and the opposite
occurred in shortened sarcomeres (2, 17). In the present study
the sarcomere lengths across all aldehyde-preserved fibers
ranged from 1.8 to 2.6 µm [2.3 6 0.2 (SD) µm]. This made it
necessary to analyze each fiber first in cross section for
filament density and then to recut it longitudinally for
sarcomere length determination, as performed previously
(17). Longitudinal semithin sections (0.5 µm) from the reoriented blocks were sectioned and stained with toluidine blue
for determination of sarcomere length, as defined by the
average of the four adjacent sarcomeres in series with the
sarcomeres sampled in cross section. To compare density and
spacing values from fibers at different lengths, raw measurements were normalized to a standard sarcomere length of 2.4
µm, as conducted previously for the companion bed-rest study
(17). Density is directly proportional to sarcomere length, and
spacing is inversely proportional to the square root of the
sarcomere length (2, 17). To detect short thin filaments, thin
filament concentration was determined for three regions of
the sarcomere in cross section at a final magnification of
3201,000: region 1, in the I band within ,150 nm of thin
filament origin at the Z band; region 2, about mid-I band; and
region 3, where thin filaments first overlap with thick filaments in the A band (Fig. 1, A–C). If all thin filaments were
long enough to originate from the Z band (region 1) and
penetrate the A band (region 3), then the thin filament
densities would be the same at the three measurement sites.
The presence of short filaments manifests as a lower density
THIN FILAMENTS AND SPACEFLIGHT
shorter transverse lengths of the Z bands (Fig. 2B).
Shorter Z bands indicated atrophy (19). Sarcomeres
looked remarkably intact, with no patchy loss of myofilaments. Postflight fibers had more lipid droplets than
controls (Fig. 2B). The mitochondria and glycogen-like
particles between myofibrils appeared similar in
amounts to controls.
As expected from the bed-rest study, myofilament
density was directly modulated by sarcomere length
(Fig. 3) (17). After normalization for the sarcomere
length differences of each muscle fiber to a standard
length of 2.4 µm, thick filament density was found
unchanged by spaceflight: 1,071 6 9 and 1,054 6 7
filaments/µm2 pre- and postflight, respectively. Thick
filament spacing was also unaltered: 33.8 6 1.4 and
31.1 6 1.9 nm pre- and postflight, respectively. The
mean thick-to-thin filament spacing was not significantly different in these aldehyde-fixed fibers: 17.3 6
0.7 and 18.4 6 0.7 nm pre- and postflight, respectively.
After spaceflight the density of thin filaments was
lower in each of the three regions measured (Fig. 1,
B–D). The average density near the Z band was
significantly (P , 0.05, t 5 3.007, df 5 3) decreased 17%
compared with the preflight control, the mid-I band
value was lower by 21%, and the density in the A bands
was significantly (P , 0.01, t 5 5.508, df 5 3) decreased
by 26% (Table 1).
To assess whether thin filaments arising from the Z
band were long enough to project into the A band, the
thin filament density near the Z band was compared
with that in the overlap A band region for each fiber.
The occurrence of short filaments in preflight control
fibers manifested as a 13% mean lower density in the
overlap A band region than near the Z band. In
postflight fibers, thin filament density was 22% lower
in the overlap A region than near the Z band, indicating
that after spaceflight the percentage of short thin
filaments increased from 13 to 22%, an additional 9%.
The thin filament concentration in the postflight overlap A band region was decreased by 26% compared with
the preflight overlap A band (Table 1). The 26% decrease was the sum of 17% missing thin filaments plus
9% short thin filaments (Table 1).
DISCUSSION
The present results demonstrate that 17 days of
spaceflight unloading decrease myofibrillar proteins, as
evidenced by thinner myofibrils in the soleus muscle
fibers of astronauts. This loss is consistent with previous reports of selective diminution of contractile pro-
Fig. 3. Scatterplot of thick filament density and sarcomere length for all fibers sampled in pre- and postflight
soleus muscles demonstrates that density is directly
proportional to sarcomere length.
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Fig. 2. Myofibrils appear intact in longitudinal sections of soleus
muscle fibers of astronaut D before (A) and after (B) 17 days of
spaceflight. Atrophy of postflight fibers is evident by 39% decrease in
Z band length in thinner myofibrils. Lipid droplets (L) are more
numerous. Magnification 316,800; horizontal bar, 0.5 µm.
569
570
THIN FILAMENTS AND SPACEFLIGHT
Table 1. Thin filament density before and after spaceflight
I Band Density, filaments/µm2
A Band Density, filaments/µm2
Subject
Pre
Post
%D
Pre
Post
%D
A
B
C
D
3,043 6 63
3,036 6 37
3,914 6 169
3,711 6 140
2,989 6 153
2,464 6 54
2,946 6 125
2,932 6 51
22
219
225
221
2,796 6 35
2,821 6 94
3,457 6 130
2,829 6 61
2,400 6 60
2,021 6 72
2,389 6 106
2,050 6 73
214
228
231
228
Mean 6 SE
3,426 6 227
2,833 6 124*
217
2,976 6 161
2,215 6 104†
226
Thin filament number was counted in I band within ,150 nm near Z line and in A band near A-I border. Filament density values were
normalized to a sarcomere length of 2.4 µm. Values for each subject represent averages for 5 fibers/condition. Pre, before spaceflight; Post, after
spaceflight. Post mean density values for I and A bands are significantly different from their Pre values: * P , 0.05; † P , 0.01, respectively
(paired-samples t-test). Subject C exhibited largest decrease in thin filament density and increase (57%) in average velocity of shortening,
whereas subject A showed smallest decrease in thin filament density and increase (16%) in shortening velocity (31).
fast- than in slow-twitch fibers in fish skeletal muscles
(9, 28). Longer periods of spaceflight or bed rest are
needed to determine whether the observed alterations
in soleus thin filaments represent transient adaptations in the early phase of unloading atrophy.
It is not known whether short and full-length thin
filaments are distinct populations or short filaments
are the expected variation in filament length due to a
dynamic state of assembly and disassembly. Unloading
atrophy may increase filament turnover (instability)
and account for the increased percentage of short
filaments. In skeletal muscle, thin filament maximum
length is determined by nebulin (14, 29). Normal
cardiac myocytes lack nebulin, and their thin filament
lengths (0.4–1.6 µm) range more widely than those of
skeletal muscle fibers (14). Stability of thin filaments is
regulated by cap-Z and tropomodulin actin-capping
proteins (8). Transfection overexpression of tropomodulin results in abnormally short thin filaments (25).
Functionally, a wide distribution of thin filament lengths
means that tension output is less affected by changes in
Fig. 4. Schematic representation of in vivo status of thin filament
packing density and spacing in one-half of a sarcomere of a normal
weight-bearing muscle and in one-half of a sarcomere from an
atrophic muscle removed from weight bearing. For normal preflight
controls, 13% of thin filaments were not long enough (,0.5 µm) to
penetrate A band in a 2.4-µm sarcomere. Subsequent to atrophy after
spaceflight unloading, short thin filaments increased by 9%, and 17%
of thin filaments were lost. These changes summated to produce a
26% decrease of thin filament density in overlap A band region
completely, generating a structurally weaker sarcomere. Lower density increases average thick-to-thin filament spacing by 35%, which is
postulated by others to cause earlier detachment, less drag, and
faster cycling of cross bridges, leading to increased velocity of
shortening (31).
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teins during unloading atrophy and the 15% decrease
in cross-sectional area of fibers from the same astronaut biopsies (5, 31). Analysis of myofilament packing
density indicated that the loss of contractile proteins
was not uniform. Thick filament number per micrometer squared remained normal, but thin filament concentration was significantly decreased (26%) in the overlap
A band region. The majority of fibers sampled ultrastructurally were slow-twitch fibers, as judged from the
reported percentages of slow-twitch, slow-twitch/fasttwitch hybrid, and fast-twitch fiber types in the control
and test solei (31). Spaceflight decreased the percentage of pure slow-twitch fibers, i.e., those containing
only slow myosin when screened by gel electrophoresis,
from 91 to 80% (31). Morphological classification of
fiber types was not attempted, because discrimination
is unreliable in cross-sectioned human muscle fibers
(4). Inspection of filament density values for individual
fibers within each subject did not reveal a few fibers
with extreme changes skewing the group means. These
data indicate that if hybrid slow-twitch/fast-twitch and
fast-twitch fibers were sampled, they changed in a
manner similar to slow-twitch fibers and exhibited a
disproportionate loss of thin filaments. This prediction
is indirectly supported by our physiological study of
soleus in which fast-twitch type IIa fibers showed
atrophy, force, and velocity changes in the same directions and equal to or greater in magnitude than those of
slow-twitch type I fibers (31). To obtain direct correlation in future studies, techniques have been developed
for examination of the same fiber physiologically, biochemically, and ultrastructurally.
The increasing percent reductions in thin filaments
near the Z band (17%), mid-I band (21%), and overlap A
band (26%) regions are consistent with an increase in
short thin filaments and a loss of whole filaments
during spaceflight. This pattern is remarkably comparable to that (16, 22, and 23%, respectively) observed
after 17 days of bed rest (17). The combination of 7–9%
more short filaments plus 16–17% missing thin filaments accounts for the 23–26% decrease in thin filament packing density in the overlap A band region after
spaceflight or bed rest unloading for 17 days (17).
Variation in thin filament structure exists in normal
muscles. Short thin filaments are common in normal
soleus fibers of rats, and thin filaments are shorter in
THIN FILAMENTS AND SPACEFLIGHT
susceptibility of atrophic muscles to reloading damage
(20, 23). The adaptation of unloaded slow-twitch fibers
to reduce thin filaments may increase susceptibility to
sarcomere damage during reloading. The present findings suggest that inflight countermeasures to prevent
the accelerated loss of thin filaments may be necessary
to avert delayed-onset muscle soreness experienced by
astronauts.
We thank Joyce Witebsky for conducting the statistical analysis of
the data.
This research was supported by National Aeronautics and Space
Administration Grants NAS-918768 (R. H. Fitts) and NAG-2956
(D. A. Riley) and National Institute of Neurological Disorders and
Stroke Grant U01 NS-33472 (D. A. Riley).
Address for reprint requests and other correspondence: D. A. Riley,
Dept. of Cell Biology, Neurobiology, and Anatomy, Medical College of
Wisconsin, 8701 Watertown Plank Rd., Milwaukee, WI 53226 (E-mail:
[email protected]).
Received 24 June 1999; accepted in final form 1 October 1999.
REFERENCES
1. Bamman, M. M., M. S. F. Clarke, D. L. Feeback, R. J.
Talmadge, B. R. Stevens, S. A. Lieberman, and M. C.
Greenisen. Impact of resistance exercise during bed rest on
skeletal muscle sarcopenia and myosin isoform distribution. J.
Appl. Physiol. 84: 157–163, 1998.
2. Brandt, P. W., E. Lopez, J. P. Reuben, and H. Grundfest. The
relationship between myofilament packing density and sarcomere length in frog striated muscle. J. Cell Biol. 33: 255–263,
1967.
3. Buchanan, P., and V. A. Convertino. A study of the effects of
prolonged simulated microgravity on the musculature of the
lower extremities in man: an introduction. Aviat. Space Environ.
Med. 60: 649–652, 1989.
4. Cullen, M. J., and D. Weightman. The ultrastructure of
normal human muscle in relation to fibre type. J. Neurol. Sci. 25:
43–56, 1975.
5. Diffee, G. M., V. J. Caiozzo, R. E. Herrick, and K. M.
Baldwin. Contractile and biochemical properties of rat soleus
and plantaris after hindlimb suspension. Am. J. Physiol. Cell
Physiol. 260: C528–C534, 1991.
6. Edgerton, V. R., and R. R. Roy. Neuromuscular adaptation to
actual and simulated weightlessness. In: Advances in Space
Biology and Medicine, edited by S. L. Bonting. Greenwich, CT:
JAI, 1994, vol. 4, p. 33–67.
7. Edgerton, V. R., M. Y. Zhou, Y. Ohira, H. Klitgaard, B. Jiang,
G. Bell, B. Harris, B. Saltin, P. D. Gollnick, R. R. Roy, M. K.
Day, and M. Greenisen. Human fiber size and enzymatic
properties after 5 and 11 days of spaceflight. J. Appl. Physiol. 78:
1733–1739, 1995.
8. Fowler, V. M. Regulation of actin filament length in erythrocytes
and striated muscle. Curr. Opin. Cell Biol. 8: 86–96, 1996.
9. Granzier, H. L. M., H. A. Akster, and H. E. D. J. Ter Keurs.
Effect of thin filament length on the force-sarcomere length
relation of skeletal muscle. Am. J. Physiol. Cell Physiol. 260:
C1060–C1070, 1991.
10. Gundersen, H. J. G. Notes on the estimation of the numerical
density of arbitrary profiles: the edge effect. J. Microsc. 11:
219–223, 1977.
11. Jakubiec-Puka, A., J. Szczepanowska, and U. Wieczorek.
Myosin heavy chains in striated muscle immobilized in a shortened position. Basic Appl. Myogr. 5: 147–153, 1995.
12. Krippendorf, B. B., and D. A. Riley. Distinguishing unloadingversus reloading-induced changes in rat soleus muscle. Muscle
Nerve 16: 99–108, 1993.
13. Krippendorf, B. B., and D. A. Riley. Temporal changes in
sarcomere lesions of rat adductor longus muscles during hindlimb reloading. Anat. Rec. 238: 304–310, 1994.
14. Kruger, M., J. Wright, and K. Wang. Nebulin as a length
regulator of thin filaments of vertebrate skeletal muscles: corre-
Downloaded from http://jap.physiology.org/ by 10.220.33.3 on June 14, 2017
muscle cell length, because the plateau of the lengthtension curve is broadened (24). It is of interest to
determine whether unloading atrophy alters the levels
of thin filament regulatory proteins, consistent with a
higher percentage of short filaments.
Density and interfilament spacing of thick filaments
were unchanged by spaceflight. Thick-to-thin filament
spacing was also unchanged. This latter result was, at
first, puzzling, because the 26% reduction in thin
filament density in the overlap A band region was
calculated to increase average spacing between thick
and thin filaments by 35%. A similar lack of change in
thick-to-thin filament spacing was obtained for 17 days
of bed rest (17). In both studies, spacing was measured
for chemically fixed fibers, in which glutaraldehyde
fixation cross-linked myosin cross bridges to actin.
Therefore, with the assumption of no change in crossbridge length, the nearest-neighbor thick-to-thin filament distance should be similar in pre- and postflight
aldehyde-fixed fibers (17). The lack of increased distance between thick and thin filaments in aldehydefixed fibers does not rule out a wider separation in vivo
with functional consequences. Fewer thin filaments
means a greater distance from the thick filament to the
nearest thin filament in vivo. This is depicted schematically in Fig. 4. Comparison of our morphological data
with physiological results from the same astronaut
biopsies indicates that the increased average velocity
(32%) and decreased thin filament density (26%) may
be causally related (31). The spaceflight and bed-rest
studies together support the hypothesis that decreased
thin filament density increases the velocity of shortening of slow-twitch fibers, which do not express fast
myosin heavy chains (17, 30, 31).
After spaceflight, the force per cross-sectional area
(specific tension) of atrophic soleus fibers was mildly
decreased 7–9%, and specific tension remained at
control levels after 17 days of bed rest (30, 31). This
suggests that the normal thick-to-thin filament ratio of
6:1 supplies an excess of actin-binding sites for myosin
cross bridges, and reducing the ratio to ,5:1 during
atrophy continues to provide sufficient cross-bridge
binding to maintain specific tension. In the soleus
muscle fibers of rat hindlimb unloaded for 1 wk, thick
filaments were disproportionately lost, which decreased
the number of cross bridges, and specific tension was
reduced (17%) (16, 22). Alterations in thick-to-thin filament ratios would be expected to change myosinto-actin protein ratios. The myosin-to-actin ratio
decreased in rat soleus muscles immobilized at a
shortened length, which produced a disproportionate
loss of thick filaments (11). Slow-twitch fibers from the
same biopsy studied in the present study showed a 14%
increase in the myosin-to-actin ratio, consistent with a
greater loss of actin thin filaments during spaceflightinduced atrophy (unpublished observations).
Reduced thin filament density and near-normal force
generation exist in atrophic fibers after spaceflight and
bed rest (17, 30, 31). This means that the average stress
per remaining thin filament is 23–30% higher than
normal. Elevated stress per filament may increase
571
572
15.
16.
17.
18.
19.
20.
22.
lation of thin filament length, nebulin size, and epitope profile. J.
Cell Biol. 115: 2199–2212, 1991.
McDonald, K. S., M. D. Delp, and R. H. Fitts. Effect of
hindlimb unweighting on tissue blood flow in the rat. J. Appl.
Physiol. 72: 2210–2218, 1992.
McDonald, K. S., C. A. Blaser, and R. H. Fitts. Force-velocity
and power characteristics of rat soleus muscle fibers after
hindlimb suspension. J. Appl. Physiol. 77: 1609–1616, 1994.
Riley, D. A., J. L. W. Bain, J. L. Thompson, R. H. Fitts, J. J.
Widrick, S. W. Trappe, T. A. Trappe, and D. L. Costill.
Disproportionate loss of thin filaments in human soleus muscle
after 17 days of bed rest. Muscle Nerve 21: 1280–1289, 1998.
Riley, D. A., S. Ellis, C. S. Giometti, J. F. Y. Hoh, E. I.
Ilyina-Kakueva, V. S. Oganov, G. R. Slocum, J. L. W. Bain,
and F. R. Sedlak. Muscle sarcomere lesions and thrombosis
after spaceflight and suspension unloading. J. Appl. Physiol. 73,
Suppl.: 33S–43S, 1992.
Riley, D. A., S. Ellis, G. R. Slocum, T. Satyanarayana,
J. L. W. Bain, and F. R. Sedlak. Hypogravity-induced atrophy
of rat soleus and extensor digitorum longus muscles. Muscle
Nerve 10: 560–568, 1987.
Riley, D. A., S. Ellis, G. R. Slocum, F. R. Sedlak, J. L. W. Bain,
B. B. Krippendorf, C. T. Lehman, M. Y. Macias, J. L.
Thompson, K. Vijayan, and J. A. DeBruin. Inflight and
postflight changes in skeletal muscles of SLS1 and SLS2 spaceflown rats. J. Appl. Physiol. 81: 133–144, 1996.
Riley, D. A., E. I. Ilyina-Kakueva, S. Ellis, J. L. W. Bain, G. R.
Slocum, and F. R. Sedlak. Skeletal muscle fiber, nerve, and
blood vessel breakdown in space-flown rats. FASEB J. 4: 84–91,
1990.
Riley, D. A., G. R. Slocum, J. L. W. Bain, F. R. Sedlak, T. E.
Sowa, and J. W. Mellender. Rat hindlimb unloading: soleus
histochemistry, ultrastructure, and electromyography. J. Appl.
Physiol. 69: 58–66, 1990.
23. Riley, D. A., J. L. Thompson, B. B. Krippendorf, and G. R.
Slocum. Review of spaceflight- and hindlimb suspension unloading-induced sarcomere damage and repair. Basic Appl. Myogr. 5:
139–145, 1995.
24. Robinson, T. F., and S. Winegrad. Variation of thin filament
length in heart muscle. Nature 267: 74–75, 1977.
25. Sussman, M. A., S. Baque, C. S. Uhm, M. P. Daniels, R. L.
Price, D. Simpson, L. Terracio, and L. Kedes. Altered
expression of tropomodulin in cardiomyocytes disrupts the sarcomeric structure of myofibrils. Circ. Res. 82: 94–105, 1998.
26. Thompson, J. L., E. M. Balog, R. H. Fitts, and D. A. Riley.
Five myofibrillar lesion types in eccentrically challenged, unloaded rat adductor longus muscle—a test model. Anat. Rec. 254:
39–52, 1999.
27. Trappe, S. W., T. A. Trappe, D. L. Costill, G. A. Lee, J. J.
Widrick, and R. H. Fitts. Effect of spaceflight on human calf
muscle morphology and function (Abstract). Med. Sci. Sports
Exer. 29: S190, 1997.
28. Treaeger, L., and M. A. Goldstein. Thin filaments are not of
uniform length in rat skeletal muscle. J. Cell Biol. 96: 100–103,
1983.
29. Trinick, J. Titin and nebulin: protein rulers in muscle? Trends
Biochem. Sci. 19: 405–409, 1994.
30. Widrick, J. J., J. G. Romatowski, J. L. W. Bain, S. W. Trappe,
T. A. Trappe, J. L. Thompson, D. L. Costill, D. A. Riley, and
R. H. Fitts. Effect of 17 days of bed rest on peak isometric force
and maximal shortening velocity of human soleus fibers. Am. J.
Physiol. Cell Physiol. 273: C1690–C1699, 1997.
31. Widrick, J. J., J. G. Romatowski, S. T. Knuth, K. M.
Norenberg, J. L. W. Bain, D. A. Riley, S. W. Trappe, T. A.
Trappe, D. L. Costill, and R. H. Fitts. Effect of a 17-day
spaceflight on contractile properties of human soleus muscle
fibres. J. Physiol. (Lond) 516: 915–930, 1999.
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THIN FILAMENTS AND SPACEFLIGHT