Spaceflight
(SF) has been shown to cause skeletal muscle atrophy; a loss in force
and power; and, in the first few weeks, a preferential atrophy of
extensors over flexors. The atrophy primarily results from a reduced
protein synthesis that is likely triggered by the removal of the
antigravity load. Contractile proteins are lost out of proportion to
other cellular proteins, and the actin thin filament is lost
disproportionately to the myosin thick filament. The decline in
contractile protein explains the decrease in force per cross-sectional
area, whereas the thin-filament loss may explain the observed
postflight increase in the maximal velocity of shortening in the type I
and IIa fiber types. Importantly, the microgravity-induced decline in
peak power is partially offset by the increased fiber velocity. Muscle
velocity is further increased by the microgravity-induced expression of
fast-type myosin isozymes in slow fibers (hybrid I/II fibers) and by
the increased expression of fast type II fiber types. SF increases the
susceptibility of skeletal muscle to damage, with the actual damage
elicited during postflight reloading. Evidence in rats indicates that
SF increases fatigability and reduces the capacity for fat oxidation in
skeletal muscles. Future studies will be required to establish the
cellular and molecular mechanisms of the SF-induced muscle atrophy and functional loss and to develop effective exercise countermeasures.
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INTRODUCTION |
SPACE
EXPLORATION OF THE PAST 30 years, and particularly the Skylab,
Spacelab, Cosmos, and Mir flights, have identified major biological
changes with weightlessness in a variety of organ systems (15, 21, 35). In the
next few years, the International Space Station (ISS) will become
operational. One of the primary objectives of ISS research will
be to study and gain a clear understanding of microgravity-induced
changes in biological processes with the ultimate goal of developing
effective countermeasures such that exploration of Mars and points
beyond can become a reality. One of the most affected systems is the
neuromuscular system (15, 21,
35). Weightlessness has been shown to cause atrophy,
reduced functional capacity, and increased fatigue in limb skeletal
muscles, with the greatest change observed in antigravity muscles such as the soleus (Sol) (11, 73, 78,
99).
The purpose of this review is to identify the major effects of space
travel on limb muscle structure and function, highlight those that
present potential limitations to prolonged space travel and/or the
crews' ability to readjust to 1 G, and discuss potential countermeasures. Special emphasis will be placed on how alterations in
structure affect function and on publications that have addressed the
cellular and molecular causes of the microgravity-induced muscle
changes. The review will concentrate on spaceflight (SF) results and
contrast and compare changes observed in rats and humans. Selected data
from models of weightlessness [hindlimb unloading (HU) in rats and bed
rest in humans] will be discussed to support hypotheses for which
insufficient space data exist. No attempt will be made to provide a
complete review of the topic. For this, the reader is referred to a
number of recent reviews in which the effects of SF on limb muscles in
rodents (21, 25, 71,
75) and humans (15, 19,
21, 85) have been discussed in great detail.
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MICROGRAVITY AND LIMB MUSCLE MASS |
Skeletal muscle mass in rats .
Since the mid-1970s, it has been recognized that SF induces substantial
atrophy of mammalian muscle, especially in muscles that play an
antigravity, postural role in a 1-G environment. Early results from the
Cosmos biosatellite program revealed rodent Sol muscle mass losses of
>30% after 20-22 days in space (44). It is now
known that this atrophic response occurs quite rapidly in microgravity,
with reductions in rodent Sol mass of up to 37% observed after only
4-7 days of SF (11, 20, 49,
58). In rats, it has consistently been observed that
antigravity slow muscles such as the Sol and adductor longus (AL)
atrophy more than primarily fast muscles and that extensors are more
affected than flexors. For example, the slow type I fiber shows greater SF-induced atrophy than the fast type II fiber, and fibers from extensor muscles are more affected than those from flexors (Fig. 1). Edgerton and colleagues
(47, 68) evaluated fiber size in Sol, medial
gastrocnemius (MG), and tibialis anterior (TA) muscles after a 14-day
SF (Cosmos 2044). Slow type I fibers from the ankle extensor
muscles showed the greatest atrophy, with Sol fibers atrophying more
than MG fibers (Fig. 1). Both slow and fast fibers of the TA showed a
shift toward larger fibers postflight. Tischler et al.
(94) demonstrated that the slow extensor muscles of young
rats (26 days old) also show an increased susceptibility to SF-induced
atrophy. The muscle weights of the Sol, plantaris, and gastrocnemius
decreased by 38, 24, and 16%, respectively, after a 5.4-day SF [Space
Transport System (STS)-48], whereas the TA and extensor digitorum
longus (EDL) muscles showed no change. Studies from this same flight
showed that the muscle atrophy was associated with an increase in
interstitial fluid volume (IFV) (41). The authors
concluded that the loss of muscle mass and contractile protein was at
least in part responsible for the increased IFV.
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Fig. 1.
Percent change in fiber cross-sectional area preflight
compared with postflight for slow type I and fast II fibers from the
rat soleus (Sol), medial gastrocnemius (MG), and tibialis anterior (TA)
and from the human vastus lateralis (VL), Sol, and gastrocnemius
(Gastroc). Rat Sol, MG, and TA data are from the 14-day Cosmos 2044 flight (Refs. 59, 41, and 41, respectively). Human VL data are from an
11-day flight (22), whereas the Sol and Gastroc data are
from the 17-day STS-78 flight (Ref. 99 and Widrick, Knuth, Norenberg,
Romatowski, Bain, Riley, Karhanek, Trappe, Trappe, Costill, and Fitts,
unpublished observations, respectively). * P < 0.05.
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Skeletal muscle mass in humans.
Until recently, the only evidence on the extent of human limb muscle
atrophy came from gross anthropometric measurements and limb magnetic
resonance imaging (MRI) (46, 55). LeBlanc et al. (55) used MRI to assess leg and back muscle volumes
before and after an 8-day SF. They observed equal volume losses for the calf and quadriceps (
6.3 and 6.0%), whereas the anterior
compartment of the lower leg showed less loss (3.9%). The hamstrings
and intrinsic lumbar muscles underwent the largest declines of 8.0 and 10.3%, respectively. Studies of Mir cosmonauts after 6 mo of SF showed declines in the calf plantar flexors varying from 6 to
20% (102). These data and functional results to be
described below (see SF EFFECTS ON SKELETAL MUSCLE CONTRACTILE
FUNCTION) illustrate that considerable individual variation
exists. Up to now, it has been impossible to determine how much
of the variability reflects true individual differences as opposed to
effects caused by different types and/or degrees of exercise
countermeasure or different flight durations. Two recent human studies
have evaluated cell size changes in response to 5, 11, and 17 days of
SF (22, 99). Consistent with the rat data,
fibers within the Sol were more affected than gastrocnemius fibers
(Fig. 1). However, unlike in rats exposed to microgravity, type I
fibers were not more susceptible to atrophy than type II fibers.
Edgerton et al. (22) observed a trend for fiber atrophy
after 5 days in space and significant reductions in fiber
cross-sectional area (CSA) after an 11-day flight with type IIb > IIa > I (Fig. 1). Widrick et al. (99) reported a
similar finding for the Sol, for which, after a 17-day SF, type IIa
fiber CSA declined 26% compared with a 15% reduction in the slow type
I fiber (Fig. 1). The SF-induced atrophy was particularly evident when
myofibrils were analyzed at the electron microscope level
(76, 99). The postflight Sol type I fiber atrophy is reflected by the 39% decrease in Z-band length and thinner
myofibrils in the post- compared with the preflight longitudinal electron microscope sections (Fig. 2).
This study also demonstrated the considerable subject variability that
exists in response to SF. The percent decline in fiber diameter in the
slow type I fiber of the Sol in four crew members ranged from 2 to 19%
(99).
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Fig. 2.
Electron micrographs of longitudinal sections of slow muscle fibers
obtained from soleus muscles of subject C before
(A) and after (B) a 17-day spaceflight
(99). Preflight control fiber has wide myofibrils, whereas
myofibrils postflight are thinner, indicating atrophy. Mitochondria and
glycogen-like granules are similar in both fibers, but lipid droplets
are more frequent postflight. Bar = 1.5 µm.
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One explanation for the greater microgravity-induced atrophy of the
type IIa fiber in humans and the type I fiber in rats may relate to the
average preflight fiber size. In rats, the type IIa fiber type is
considerably smaller than the type I fiber, whereas in humans the type
IIa fiber is slightly larger than the type I fiber (31).
In their review paper, Edgerton and Roy (21) commented
that one consistent observation was that the greater the preflight
fiber size the greater was the degree of atrophy for a given fiber type
postflight. This generalization appears to hold within a species and
fiber type as well. In the Widrick et al. (99) study, the
astronaut with the greatest type I fiber atrophy had the largest
average fiber size preflight.
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QUALITATIVE AND QUANTITATIVE ALTERATIONS IN CELL PROTEINS |
Fiber type.
The fiber-type changes in response to SF in rats has recently been
reviewed by Edgerton and Roy (21). The available data indicate that, within the first week of SF, the number of slow fibers
in the antigravity muscles of the rat decreases, whereas the number of
fibers containing fast-type myosin increases. For example, after a
7-day flight, Martin et al. (58) observed a 39-50 and
a 20-46% increase in the number of dark ATPase (fast) fibers in
the Sol and AL muscles, respectively. In contrast, the 7-day SF had no
effect on the percent fiber type distribution in the fast plantaris and
superficial region of the MG (extensor muscles) or the fast EDL (a
flexor muscle). Longer flights of 12.5 and 14 days showed the same
result: an increased number of fast fibers in the slow antigravity
muscles with no alterations in the fast-twitch muscles
(47, 63, 68). Ohira et al.
(68) reported that the percentage of Sol fibers showing a
positive reaction with both slow and fast myosin heavy chain (MHC)
antibodies after a 14-day SF increased from 0 to 16%, whereas those
reacting only to slow MHC declined from 90 to 76%. Thus SF in rats
increased the number of hybrid fibers in antigravity slow muscles by
inducing fast myosin expression in slow fibers, whereas it had little
or no effect on the number of pure fast fibers. The data of Caiozzo et
al. (12) suggest that the increased number of hybrid
fibers may be caused by type IIx myosin expression because a 6-day SF increased this isozyme, whereas the type IIa isoform significantly decreased. Haddad et al. (39) expressed the MHC type as a
percentage of the total myosin pool. After a 9-day SF, they observed a
trend for the type I MHC to decrease in both the vastus intermedius (VI) and the deep red region of the vastus lateralis (RVL). However, the only significant change was a decline in IIa/IIx MHC in the RVL
from the control value of 61.1% to the postflight value of 50.7%. In this muscle there was a corresponding increase in
the IIb MHC from a control value of 29.2% to the postflight value of
42.9%. The antibody used could not separate the type IIa and IIx MHCs,
but it seems likely, on the basis of the data of Caiozzo et al.
(12), that the decline in the IIa/IIx MHC was caused by a
drop in the IIa and not the IIx myosin.
In humans, Edgerton et al. (22) reported an 11-day SF to
decrease the slow type I fiber from 48 to 40%, whereas it increased the fast type IIa from 32 to 41% in the vastus lateralis (VL). They
observed no change in the percentage of fast type IIb fibers (20%
preflight vs. 19% postflight). Immunohistochemical and SDS-gel electrophoresis techniques have demonstrated that human muscle contains
little or no type IIb myosin (23, 83). Thus
the histochemically identified type IIb fibers in the Edgerton et al.
study were likely type IIx fibers. Recently, Widrick et al.
(99) studied the effects of a 17-day SF on the properties
of single fibers isolated from the Sol. On average, 91% of the pre-
and 79% of the postflight fibers expressed type I MHC. The results
indicated a significant SF-induced decline in the percentage of fibers
expressing type I MHC and a corresponding increase in fibers containing
type I/IIa or IIa MHC. Similar to fiber CSA, considerable subject
variability existed for MHC expression. The reduction in the expression
of type I MHC could be attributed entirely to the responses of two of
the four crew members studied.
Total, myofibrillar, and sarcoplasmic protein content.
As reviewed above, rats flown in space show a rapid decline in
the mass of antigravity muscles such as the Sol, and thus one would
expect to see a similar drop in total protein. Consistent with this,
Steffen and Musacchia (84) observed a significant decline
in both myofibril and sarcoplasmic protein after a 7-day SF (Skylab 3)
in the Sol and gastrocnemius but not in the EDL. In the Sol, the
myofibril protein loss was greater than that observed for the
sarcoplasmic proteins. The protein concentration of these muscles was
similar to the 1-G controls, which indicates that the protein loss
mirrored the decline in muscle mass. Haddad et al. (39)
observed similar results after a 9-day SF in which the VI showed
significant atrophy but no change in the total or myofibril protein
concentration in milligrams per gram of tissue. The authors used the
muscle weight, myofibril yield (mg/g), percentage of myosin in
myofibril pool, and relative percentage of MHC isoform in the myosin
pool to calculate the content of each myosin isozyme. In the control
VI, there was on average 1.72 and 1.40 mg/muscle of type I and IIa/IIx
MHC, respectively, whereas in the flight VI there was 1.04, 1.13, and
0.05 mg/muscle of type I, IIa/IIx, and IIb MHC, respectively. These
data suggest that the decline in myofibril protein can be attributed to
a loss of the slow type I myosin and, to a lesser extent, of the fast
type IIa/IIx myosin. As reviewed in Fiber type above, the
latter likely reflects a loss in the IIa rather than the IIx protein.
In contrast to 7 days of SF, by 12.5 days a significant decline in
myofibril yields (mg protein/g muscle) was observed in the slow-twitch
VI but not the fast-twitch VL (4). The VI myofibril yield
declined from a control value of 108 to a postflight value of 71 mg/g.
On the basis of densitometric analysis of gel bands of native myosins
separated by pyrophosphate electrophoresis, the authors concluded that
the loss of total myosin in the VI muscle of the flight group was
attributed to reductions in the slow and intermediate (presumably type
IIa) myosin isozymes (4). Interestingly, even though there
was no loss in total myosin content in the VL postflight, this muscle
showed a drop in intermediate myosin that was balanced by an increased
fast (presumably type IIx and IIb) myosin. Clearly, in rats, muscle
wasting is associated with a selective loss of slow-type myosin
protein. In humans, there are no published data on protein content.
However, the loss of force per CSA in slow type I fibers and the
increased number of fast type IIa fibers suggest that slow-type myosin
was also selectively lost in humans (22, 99,
103).
Mechanisms responsible for the myofibril protein loss.
Baldwin et al. (4) suggest that the early decline in
myofibril yields expressed on a muscle basis (mg/g × muscle
weight) indicates that myofibril degradation is an early event in the atrophy of rat hindlimb muscles in response to 0 G. However, rat HU
studies indicate that the earliest event in Sol muscle atrophy was a
decrease in myofibril synthesis (92). Thus, for the first few days of HU, the protein loss was attributed almost entirely to a
reduced synthesis. From day 3 on, the synthesis rate
remained steady, whereas the degradation rate showed a large transient increase. The large majority of the protein loss after the first few
days was attributed to the increased degradation rate
(92). Data from both SF and bed rest suggest that the
primary and perhaps exclusive mechanism for the loss of muscle proteins
in humans is a decline in synthesis. Ferrando et al. (26)
studied this question by using the simulated microgravity model of
6° head-down bed rest. After 14 days of bed rest, they observed an
~50% drop in protein synthesis with no change in protein breakdown
in the VL muscle. The authors found no change in serum cortisol,
testosterone, insulin-like growth factor I (IGF-I), or insulin and
concluded that the decrease in protein synthesis could not be explained by hormonal alterations. These data are supported by the observation of
LeBlanc et al. (56), who found 17 wk of bed rest to have no effect on the level of 3-methylhistidine excretion, an indicator of
muscle breakdown. Additionally, the data of Criswell et al. (16) suggest that muscle atrophy in HU mice was not caused
by a decline in IGF-I mRNA expression and that transgenic mice
overexpressing human IGF-I were not protected from HU-induced muscle atrophy.
Recently, prolonged SF of >3 mo on Mir was shown to reduce whole body
protein synthesis rates by 45% in astronauts and cosmonauts (86). Interestingly, estimates of whole body protein
breakdown demonstrated a reduction in breakdown that paralleled the
reduced synthesis (86). For the six subjects studied, the
decrease in protein synthesis was significantly correlated to a
decreased energy intake. Stein et al. (88) evaluated
whether the decline in muscle protein could be in part explained by an
altered hormonal status. Urinary hormones before, during, and after a
9.5-day [Spacelab Life Science (SLS)-1] and 15-day (SLS-2) SF were
assessed. With the exception of cortisol, which showed a small
increase, there was no effect of flight on the urinary excretion of the
hypothalamic-pituitary-adrenal (HPA) axis hormones, including growth
hormone (GH). These results led the authors to argue against a primary
role for the HPA axis and GH in particular in the regulation of muscle
protein content inflight. The increased cortisol agreed with earlier
data collected on Skylab and confirms that SF likely does increase
serum cortisol. However, the authors argue that the increase does not
play a major role in the SF-induced muscle protein loss
(88). They point out an increased cortisol would have
systemic affects, which could not explain the selective atrophy of
antigravity muscles. Additionally, cortisol primarily stimulates
protein breakdown, and there was no evidence on these flights or the
Mir experiments that myofibril protein breakdown increased.
Collectively, these data suggest that, at least for humans, the primary
mechanism for the decline in myofibril protein content after both SF
and bed rest is a decline in protein synthesis. The data of Stein and
colleagues from both the Mir mission (86) and shuttle
flight STS-78 (87) suggest that the decline in protein
synthesis may be exacerbated by a negative energy balance.
Recently, considerable effort has been extended toward the elucidation
of the molecular changes responsible for the reduced protein synthesis
and muscle atrophy associated with unloading (13,
17, 59, 60, 91). In
rats, it is well established that both HU and SF cause a selective loss
of contractile protein and slow-to-fast transitions in contractile and
regulatory proteins (24, 59). The reduced
protein synthesis appears to result from changes in transcription and
translation (53, 59, 60). Criswell et al. (17) observed 7 days of HU in mice to
significantly decrease Sol troponin I slow (TnIs) mRNA, whereas the
fast troponin I (TnIf) mRNA showed a tendency to increase. The authors
concluded that the decrease in the ratio of TnIs to TnIf resulted from
a large downregulation of the TnIs gene, whereas the TnIf gene
expression was maintained at the control level. The latter would seem
to be unlikely in humans in whom type IIa fiber atrophy with SF was equal to or larger than that observed for the slow type I fiber (22, 99). The answer to the question of
whether fast and slow isozymes are regulated independently, and the
effect that such regulation has on function, will require molecular and
physiological analyses of individual single fibers.
Thomason et al. (92) reported that the 
-MHC mRNA
concentration of the Sol did not decrease with 7 days of HU. From this they concluded that the rapid decrease in myofibril synthesis was
unlikely to be transcriptionally driven. More recently, Ku and Thomason
(53) showed that polysomes isolated from Sol muscles of
rats suspended for 18 h were larger (more ribosomes per mRNA) than
polysomes from control animals. The authors concluded that the
increased polysome size was caused by a slowing of the nascent polypeptide chain elongation. This in turn slowed the translation rate,
reducing protein synthesis. The 70-kDa heat shock protein (HSP70) is
thought to associate with the nascent polypeptide and facilitate
protein translation. Recently, Ku et al. (54) observed a
44 and 28% decrease in HSP70 proteins associated with polysomes after
12 and 18 h of HU, respectively. They suggested that a HU-induced increase in cellular ATP may have enhanced the dissociation of HSP70
proteins from the polysomes, thus slowing ribosome translocation and
elongation rate.
Although Thomason et al. (92) found no decrease in Sol
-MHC mRNA with 7 days of HU, McCarthy et al. (59)
observed a significant decline in this message by 14 days in the mouse
Sol. Their results indicated that a 600-bp region of the promoter
sequence was sufficient to direct the decreased transcription of
-MHC transgenes in response to HU. One complication in the
interpretation of this result was the finding that HU coupled with
ablation of the synergistic muscles prevented Sol atrophy but not the
depressed transgene expression. In a recent paper, McCarthy et al.
(60) found that distinct -MHC promoter sequences
mediate -MHC expression in response to overload and unloading.
Collectively, these data suggest that overload of the Sol during HU was
able to prevent muscle atrophy by overriding the unloading-induced
inhibition of the promoter. A second important observation from this
work was that a negative regulatory element may be involved in the
decreased -MHC expression (60). The increased numbers
of hybrid and fast fiber types after HU in rats and mice and SF in rats
and humans suggest not only a downregulation of the -MHC gene but
also an upregulation of fast-type myosin isoforms. This hypothesis was
supported by the data of Swoap (91), who reported no
detectable IIb myosin mRNA in control Sol but a level for this message
of ~10% of that of the TA in the Sol after 3 wk of HU. The data also
showed that a 295-bp proximal upstream regulatory region of the MHC IIb
gene contained at least one element that was responsive to HU in the Sol.
Although 7 days of HU did not alter the -myosin MHC mRNA, this model
as well as SF have been shown to decrease 
-actin mRNA (92, 93). By 7 days of HU, Babij and Booth
(3) reported a 60 and 29% decline in Sol -actin mRNA
expressed per muscle and per microgram RNA, respectively. After a
14-day SF, rat skeletal muscle -actin mRNA per unit of extractable
RNA decreased 25 and 36% in the VI and lateral gastrocnemius muscles,
respectively. This suggests that myosin and actin content are regulated
independently and that their synthesis rate during SF and models of
weightlessness are controlled by different mechanisms. Consistent with
this hypothesis was the recent finding that a 17-day SF (STS-78)
induced a disproportionate loss of actin filaments in human Sol muscle.
Thick-filament density and spacing were unchanged, whereas
thin-filament density decreased significantly in the overlap A-band
region (99). The decline was the result of a 17% filament
loss and a 9% increase in short filaments that failed to penetrate the
A band (Fig. 3). The thin filament loss
was calculated to increase the thick-to-thin filament spacing in vivo
from 17 to 23 nm (76). The authors hypothesized that this
structural change was responsible for the ~30% postflight increase
in the maximal unloaded shortening velocity (Vo)
and for a reduced peak stiffness in the slow type I fiber
(99).
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Fig. 3.
Schematic representation of in vivo status of
thin-filament packing density and spacing in one-half of a sarcomere of
a normal preflight muscle and in one-half of a sarcomere from an
atrophic muscle after a 17-day spaceflight in humans. For the preflight
controls, 13% of thin filaments were not long enough (<0.5 µm) to
penetrate the A band in a 2.4-µm sarcomere. Subsequent to atrophy
after spaceflight, short thin filaments increased by 9%, and 17% of
the thin filaments were lost. These changes summated to produce a 26%
decrease in thin filament density in overlap A-band region. Reduced
thin filament density was calculated to increase the thick-to-thin
filament spacing by 35%, which is postulated to cause an earlier
detachment, less drag, and faster cycling of cross-bridges, leading to
an increased maximal shortening velocity in the postflight fibers
(62, 76, 99). [Redrawn from
Riley et al. (76).]
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SF EFFECTS ON SKELETAL MUSCLE CONTRACTILE FUNCTION |
Muscle force.
Since the Skylab missions in the early 1970s, it has been known that SF
reduces the peak force of limb skeletal muscles (15, 35). Skylab 2 (28-day mission) showed a greater drop in
thigh vs. arm and extensor vs. flexor torque, with the peak extensor torque of the thigh declining by 20% compared with a 10% loss in
thigh flexor and arm extensor groups. Skylab 3 (59-day mission) showed
an even greater difference between the SF-induced loss in thigh (mean
20% drop) and arm torque (mean 2% drop) than Skylab 2. The
differences were ameliorated on Skylab 4 (84-day mission), in which the
leg exercise countermeasures were credited with reducing the mean loss
of peak thigh torque to 6% (35). In longer Mir flights of
6-mo duration, three cosmonauts showed a 20-48% decline in the
maximal voluntary contraction (MVC) of the calf plantar flexion
(102). The reduced MVC exceeded the percent loss of calf muscle volume determined by MRI, and a correlation between the decline
in muscle volume and MVC was not found. Goubel (33) also
observed the MVC of the calf during plantar flexion to decrease after
1, 3, and 6 mo aboard the Mir, with a range of change from 0.1 to
37.6%. However, the data showed no clear relationship between flight
duration and the fall in MVC.
Greenleaf et al. (35) reviewed the effects of bed rest and
SF on the percent change of TA (ankle dorsiflexion) and triceps surae
(ankle plantar flexion) strength measured isokinetically at four
velocities (0, 60, 120, and 180°/s). A SF duration of 7 days had no
significant effect on either muscle group at any of the four velocities
tested. In contrast, after 237 days in space, both ankle dorsiflexion
and plantar flexion strength were significantly reduced at all
velocities (35). Figure 3 of the Greenleaf et al.
(35) review, which was redrawn from the data of
Grigor'yeva and Kozlovskaya (37), is reproduced here
(Fig. 4). The figure demonstrates two
important observations: 1) 110 days of microgravity reduced
ankle plantar flexion strength as much as 237 days, and 2)
prolonged SF (>110 days) reduced the peak torque of the dorsiflexor
and plantar flexor muscles to approximately the same amount. The former
observation suggests that by 110 days in space a new microgravity
steady state has been obtained, such that flights beyond this duration
would not be expected to elicit additional declines in peak limb muscle
torque. This conclusion is supported by the recently published data of
Antonutto et al. (2) that demonstrated peak force during
two-legged extension to decline by 11, 26, and 29% after 31, 169, and
180 days in microgravity, respectively. The second observation is
consistent with leg volume measurements that demonstrated a similar
degree of muscle wasting in the dorsiflexor and plantar flexor groups
of Mir cosmonauts after 4-6 mo in space (A. D. LeBlanc,
personnel communication). Collectively, the limb muscle torque data
from Skylab 2, Skylab 3, and Mir indicate that the leg extensors lose
torque more rapidly than flexors, but by 4 mo little difference exists
between these groups. The data of Antonutto et al. indicate that the
rate of force recovery is related to the time in space. After a 31-day SF, two-legged peak force was recovered by 6 days postflight, whereas,
after 169-180 days in space, peak force was still depressed by
12-22% after 26 days of recovery.
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Fig. 4.
Comparison of mean percent changes (% ) in plantar flexion and
dorsiflexion isokinetic strengths (0-180°/s) between 110 and 237 days of exposure to microgravity. N, no. of subjects.
[Reprinted from Greenleaf et al. (35), which had
originally been redrawn from Grigor'yeva and Kozlovskaya
(37).]
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Recently, Widrick et al. (99) determined the peak force of
individual slow type I and fast type IIa fibers of the Sol in four
participating crew members obtained from biopsies taken before and
immediately after a 17-day shuttle space flight (STS-78). The average
peak force dropped by 21% in the type I fiber (from 0.99 ± 0.03 to 0.78 ± 0.02 mN) and by 25% in the type IIa fibers (from
1.41 ± 0.14 to 1.06 ± 0.03 mN). The single fibers from the gastrocnemius (both type I and IIa) were less affected than the Sol
fibers (J. J. Widrick, S. T. 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, unpublished observations). When the Sol type I fiber
peak force was analyzed for individual crew members, considerable
differences were apparent in the susceptibility to SF. For example, the
decline in peak force ranged from a low of 12% to a high of 40%. For
the type I fiber, peak force declined out of proportion to the loss in
fiber CSA, resulting in an average 4% drop in the peak force per CSA.
SF had no effect on the type IIa fiber force per CSA. The most likely
explanation for the decline in peak force per CSA was a loss in
myofibril content. Although myofibril content was not measured, the
postflight Sol type I fibers had significantly less peak stiffness,
which is consistent with a decline in the number of strongly bound
cross bridges per unit fiber area.
Narici et al. (65) studied in vivo muscle function on the
same flight (STS-78). Although the MVC of the plantar flexors was not
significantly altered during SF, the peak tetanic torque elicited by
50-Hz direct electrical stimulation showed a progressive decline during
flight (10% loss by flight day 16) that
continued during the first 8 days of recovery. The authors hypothesized that the force drop during recovery was the result of fiber damage after reloading. The single-fiber data of Widrick et al.
(99) and the in vivo direct electrical stimulation data of
Narici et al. (65) are in good agreement and collectively
suggest that significant plantar flexor force is lost by 17 days in
space, and that the Sol contributes more to the decline than the gastrocnemius.
An interesting question is why SF induces greater atrophy and force
decline in the Sol compared with the gastrocnemius. Recent experiments
evaluating the effect of a 14-day SF in rhesus monkeys may shed light
on this question. Recktenwald et al. (74) demonstrated that two flight monkeys (357 and 484) showed a
decreased cycle period and electromyogram (EMG) burst duration of the
primary extensors (Sol and MG) during locomotion postflight, whereas
the burst amplitude of the TA (flexor muscle) increased. Importantly, probability density distributions of EMG amplitude of the Sol and MG
showed a shift toward a higher activation of the MG relative to the Sol
post- compared with preflight (Fig. 5).
Recktenwald et al. (74) also reported that the MG tendon
forces were considerable greater post- compared with preflight during
locomotion. We studied the same two monkeys and found greater fiber
atrophy and force loss in the Sol type I fiber in monkey 357 compared with monkey 484 (29). Of particular importance was
the observation that, inflight, monkey 357 showed a
progressive change from predominately Sol activity to mainly MG
activity over the duration of the flight. In contrast, the Sol/MG
recruitment ratio for monkey 484 was unaltered by SF (J. A. Hodgson personal communication). Clearly the Sol type I fiber
atrophy and reduced functional capacity in monkey 357 was
associated with reduced activation of this muscle. Recktenwald et al.
(74) concluded that microgravity induced a reorganization of motor recruitment to favor fast vs. slow motor units and flexors vs.
extensor muscles. An unanswered question is whether the altered neuronal recruitment pattern was a direct effect of the microgravity environment (perhaps resulting from a reduced muscle afferent input to
the central nervous system) or occurred in response to the Sol muscle
atrophy. The observation that the EMG shift occurred inflight, in which
atrophy and reduced force production of the Sol would not likely
compromise movement or require additional gastrocnemius activation
supports the former (74). In either case, the greater
microgravity-induced atrophy of the Sol compared with the gastrocnemius
has been observed in humans, nonhuman primates, and rodents and can be
attributed both to the removal of the antigravity load-bearing role and
to the reduced activation of the Sol.
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Fig. 5.
Probability density distributions of the EMG amplitudes
in the Sol and MG muscles pre- and postflight (1 day of recovery) for 2 flight monkeys flown on the 14-day Bion 11 mission. Data were recorded
during a sequence of steps at 0.45 m/s. Horizontal axes show EMG
amplitudes with the origin at the back of the plot, obscured by data.
Vertical axes are a logarithmic scale of probability [log (1 + probability)]. [From Recktenwald et al. (74).]
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The hypothesis that microgravity causes a fundamental alteration in
motor control has also been suggested by Antonutto et al.
(1). They observed two-legged muscle power to decline
considerably more than could be explained by the loss in muscle mass.
Additionally, the loss of explosive leg power was associated with a
substantial reduction in the EMG activity of the rectus femoris, VL,
and vastus medialis muscles. These authors concluded that microgravity
induced a basic change in motor control and coordination such that
motor activation of extensor muscles was reduced. An increased bias toward flexor activation would help explain why anterior compartment muscles such as the TA generally show little or no atrophy in rats
(Fig. 1) and a slower time course for atrophy in humans
(55).
Compared with humans, rats appear to show a faster decline in limb
muscle force in response to SF. Caiozzo et al. (11,
12) studied the Sol in situ and observed a 28 and 36%
decline in peak tetanic tension after 6 and 14 days of SF,
respectively. These data confirmed earlier observations by Oganov and
colleagues (66, 67), who found the in situ
tetanic force of the Sol to drop 22 and ~30% after 20.5 days (Cosmos
690 biosatellite) and 22 days (Cosmos 605 biosatellite) of SF,
respectively. In contrast, the peak force of the EDL muscle
showed no significant change after either flight.
The greater susceptibility of rat muscle to SF compared with human
muscle was substantiated by single fiber studies carried out by Mounier
and colleagues (43, 90). In a 7-day SF
(Cosmos 1667 biosatellite), Holy and Mounier (43) found
the peak force of Sol single fibers to decrease by 28%, whereas fibers
from the fast gastrocnemius were not significantly altered. However, in a 5-day SF (Cosmos 1514 biosatellite), the gastrocnemius fibers did
undergo a significant 22% drop in peak force. In both the 5- and 7-day
flights, the reduced force could be explained by fiber atrophy because
the force per CSA was unaltered by SF. Fibers from the plantaris muscle
showed no significant change after either 5 or 7 days of SF
(43). After a 14-day SF (Cosmos 2044 biosatellite), single
fibers from the rat Sol and gastrocnemius showed even greater atrophy
and loss of force (90). In Sol fibers typed as slow twitch, peak force was reduced to 25% of the preflight value. The
decline was for the most part caused by fiber atrophy because the force
per CSA was not significantly altered by flight. As with the shorter
flights, the gastrocnemius fibers were less affected than the Sol
fibers, and, consistent with the whole muscle experiments, single EDL
fibers were not altered by SF (90). In two 18.5-day flights (Cosmos 936 and 1129 biosatellites), Oganov et al.
(67) and Rapcsak et al. (73) found the peak
tension of Sol fibers to decline by up to 45% of the synchronous
control group, whereas EDL fibers were unaltered. These authors also
found a significant drop in the peak force of fibers isolated from the
triceps and brachialis muscles of the forelimb.
An additional factor affecting fiber force is the Ca2+
sensitivity of the fiber. Holy and Mounier (43) observed
that individual rat Sol fibers postflight (7-day SF) required slightly
higher Ca2+ concentrations for activation threshold [log
Ca2+ concentration (pCa) 6.40 postflight compared with pCa
6.60 preflight], and the Ca2+ concentration required for
one-half peak force (pCa50) was also slightly increased.
However, the data are inconclusive because no statistical analyses were
performed and the fibers were not typed as slow or fast twitch. Thus
the slight right shift in the force-pCa relationship may have resulted
from an increase in the number of fast fibers postflight. In a
subsequent study by the same group in which the fibers were typed, the
authors found a 14-day SF to have no significant effect on the slow Sol
fiber activation threshold or pCa50 (90). We
recently studied the effect of a 17-day SF on the force-pCa
relationship of single Sol type I fibers isolated from four astronauts
(99). The flight had no significant effect on fiber
activation threshold or the slope of the relationship either below or
above pCa50. However, in the astronaut with the greatest
fiber atrophy, the slow Sol fibers required 29% more free
Ca2+ to obtain half-maximal activation postflight. Thus SF
does, in some individuals, shift the force-pCa relationship of the type I fiber to the right, but the fibers are still considerably more sensitive to Ca2+ than fast-twitch fibers. The observation
that the slope of the force-pCa relationship was unaltered indicates
that SF does not alter the cooperativity in the Ca2+
activation of force.
Isometric twitch duration.
The isometric twitch duration is a reflection of the intracellular
Ca2+ transient and, as such, depends on the rate and amount
of Ca2+ release from the sarcoplasmic reticulum (SR), the
intracellular Ca2+ buffers, and the rate of
Ca2+ reuptake by the SR. The functional components of the
twitch duration are the time to peak tension (TPT) and the relaxation
time. Because of the difficulty in accurately measuring the point at
which force returns to the pretwitch baseline, researchers generally
measure one-half relaxation time (RT1/2). To our knowledge,
there are no published data on the effects of SF on TPT or
RT1/2 in human muscle. However, Caiozzo et al.
(11) studied the rat Sol in situ after a 6-day SF and
reported TPT to significant decrease by 8.6%. The RT1/2
was reduced but not significantly. After a 14-day SF, the same
investigative team found a significant decrease in both the Sol TPT
(19%) and RT1/2 (30%) (12). The most likely explanation for these data is that SF stimulates SR Ca2+
release and reuptake, thus decreasing the duration of the
Ca2+ transient and the TPT and RT1/2. It is
apparent from the data of Caiozzo et al. (11) that the
effect on TPT occurs first within 1 wk of SF.
Although no cellular and molecular studies on SR function have been
conducted on flight tissue, the rat HU model has provided important
results on this system. Fitts et al. (30) demonstrated a
significant 22 and 39% decline in the Sol TPT after 7 and 14 days of
HU, respectively, with no change in RT1/2 at either time point. The 14-day results were confirmed by Schulte et al.
(82), who, in addition, reported that both TPT and
RT1/2 were significantly reduced by 28 days of HU. A
Western blot analysis at this time point indicated a 300% increase in
fast SR Ca2+ pump protein, and an enzymatic analysis
revealed a 170% increase in the Ca2+-dependent SR
ATPase activity (82). The increased fast Ca2+
pump mRNA and protein occurred rapidly and was 250 and 110% higher than control by 4 days of HU. Recently, Peters et al. (69)
demonstrated that the HU-induced increase in fast Ca2+ pump
mRNA in the Sol was caused by an elevated transcription, which occurred
by 2 days of HU. By 10 days of HU, the Sol fast SR pump mRNA was
~75% of that expressed in the control fast-twitch TA muscle. Despite
this, the Sol RT1/2 was not significantly altered until 28 days of HU (82). This suggests that additional factors (perhaps an increased expression of TnIf) were required to elicit the
HU-induced decline in twitch duration (32,
95).
Kandarian et al. (50) found 28 days of HU to significantly
stimulate the T-tubular dihydropyridine (DHP)-receptor mRNA in the Sol
to levels indistinguishable from those expressed in the control
fast-twitch EDL. Because the DHP receptor is an important protein in
the excitation-contraction coupling pathway linking the T-tubular
action potential to SR Ca2+ release, an increased receptor
density might lead to a faster rate of Ca2+ release.
Stevens and Mounier (89) utilized single skinned fibers from the Sol to demonstrate that 15 days of HU increased the
sensitivity of SR Ca2+ release to caffeine and the rate of
SR Ca2+ reuptake. The former observation suggests that HU
directly affected the SR Ca2+ release channel.
Consistent with the shortened twitch duration, Caiozzo et al.
(11, 12) found both 6 and 14 days of SF to
shift the force-frequency curve of the in situ Sol to the right. After
6 days, significant differences were observed at 5-, 15-, 20-, and
30-Hz stimulation (11). Although a statistical analysis
was not carried out, it appeared that the right shift in the
force-frequency curve was greater with a 14-day SF than with the 6-day
SF (12). Schulte et al. (82) observed a
similar response in HU rats: the right shift in the curve was more
evident after 28 vs. 14 days of HU.
Fiber maximal shortening velocity and peak power.
Relatively little information exists regarding the effects of SF on the
maximal shortening velocity (Vmax) or peak
power. However, recently both have been examined in rats and humans
(11, 99). Caiozzo et al. (11,
12) used loaded contractions to calculate
Vmax of the in situ rat Sol and found 6 and 14 days of SF to increase Vmax by 14 and 20%,
respectively. The peak power of the Sol calculated from the
force-velocity data was significantly reduced after both flights. The
decline in power indicates that the postflight increase in muscle
velocity was unable to fully compensate for the reduced
force-generating capacity of the muscle. The authors suggested that the
elevation in muscle velocity might have resulted from the increased
percentage of fast myosin [both MHC and myosin light chain (MLC)
isozymes] in the slow Sol muscle (12).
Goubel (33) studied two astronauts and five cosmonauts who
lived on the Mir from 3 to 6 mo. When plantar flexion shortening velocity was measured during a contraction at 15% of peak power, all
but one of the subjects showed an increased velocity postflight. Recently, Widrick et al. (99) evaluated the effects of a
17-day SF on the Vo and
Vmax of individual slow type I and fast type IIa
fibers isolated from the Sol and gastrocnemius of four astronauts. In
the Sol, the type I fiber Vo and
Vmax increased in all four subjects, and the
postflight increases in both measurements of shortening velocity were
qualitatively similar (30 and 44%, respectively). The
Vo and Vmax of the Sol
type IIa fiber also increased postflight, with the largest change
observed in Vo, which increased from 2.90 ± 0.40 to
4.49 ± 0.44 fiber lengths/s or 55%. For the gastrocnemius muscle, the type I fibers showed a 22% increase in
Vo after SF. This increase was significant but
was less than that observed for the Sol type I fibers. In contrast, the
Vo of the fast type IIa gastrocnemius fibers was
not significantly altered by SF (Widrick, Knuth, Norenberg, Romatowski,
Bain, Riley, Karhanek, Trappe, Trappe, Costill, and Fitts, unpublished observations).
The cause of the SF-induced increase in Vo and
Vmax is unknown. The increase could not be
attributed to the expression of fast MHCs or MLCs in the slow type I
fibers. Although Sol type I fibers from two of the astronauts had an
increased MLC3 content, the level of this MLC isoform was
not correlated with Vo for any of the subjects
(99). The authors hypothesized that the increased Vo and Vmax might have
occurred due to the selective loss of thin filaments (Fig. 3). This
structural change would be expected to increase the distance between
the thin and thick filaments. As a result, the cycling cross bridges
would be expected to detach sooner, which in turn would reduce the
internal drag that develops during the final portion of the
cross-bridge stroke (62).
Mean power-force curves for the Sol type I and IIa fibers are shown in
Fig. 6. The plots clearly show the
SF-induced drop in peak isometric force (point where the curves cross
the x-axis) for both the slow and fast fiber types. During
contractions eliciting peak power, both fiber types generated
significantly less force but shortened at higher velocities such that
the peak power obtained was only slightly less than the preflight value
(Fig. 6) However, in the two astronauts with the largest drop in fiber
force, significant declines in peak power of ~20% were observed in
the type I fibers of the Sol (99). The reduced power was
entirely explained by the fiber atrophy because the peak power per CSA
was unaltered. In contrast to the Sol, SF had no effect on the peak
power of either the slow or fast fiber types of the gastrocnemius
(Widrick, Knuth, Norenberg, Romatowski, Bain, Riley, Karhanek, Trappe,
Trappe, Costill, and Fitts, unpublished observations).
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Fig. 6.
Mean force-power relationships for preflight (pre) and
postflight (post) type I and type IIa fibers isolated from the soleus.
Plots intercept the x-axis at peak isometric force. Plots
demonstrate the significantly greater peak power in type IIa vs. type I
fibers and the postflight decline in both type I and type IIa peak
isometric force. Average peak power (mean value for all 4 astronauts)
for a given fiber type was unaffected by spaceflight (pre- vs.
postflight). Despite this, 2 of the 4 astronauts did show a significant
spaceflight-induced decline in type I fiber peak power
(99). Fl, fiber lengths.
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Recently, Antonutto et al. (1, 2) examined
the effects of SF of from 21 to 180 days on the maximal power of the
lower limbs. In the one astronaut studied after 21 days, peak leg power was reduced by 54%. This was considerably greater than the loss in
single-fiber power after the 17-day SF and suggests that factors other
than fiber atrophy contributed to the change. One possibility suggested
by Antonutto et al. (1) and reviewed above was that microgravity altered the motor unit recruitment pattern. In a separate study, Antonutto et al. (2) observed two-legged
peak power to drop by 32% after 31 days and by 50% after 169-180
days in space. In one astronaut who remained in space for 438 days, the
authors calculated that leg power showed a similar decline to that
observed at 180 days. Thus the time course for the decline in peak
power to a new microgravity steady state was ~6 mo, a value similar
to that observed for muscle mass and force (35).
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SUBSTRATE AND METABOLITE CHANGES WITH SF: IMPACT ON FATIGUE |
Muscle substrate alterations with SF.
HU in rats and bed rest in humans have been shown to increase Sol
muscle glycogen, and Cosmos and Spacelab flights have documented that
rat skeletal muscle glycogen increased with SF (36,
64, 78). We know of no published reports on
the effects of microgravity on human muscle glycogen. Stein et al.
(87) recently published a paper in which they compared the
energy expenditure during a 17-day SF and 17-day bed rest. Energy
balance was unchanged with bed rest, whereas the four astronauts were
found to have an average negative energy balance of 1,355 kcal/day. We
studied the same subjects and found that the post-bed-rest Sol muscle
fibers showed significant increases in glycogen in both the slow type I
and fast type IIa fibers (unpublished data). In contrast, after SF, no
significant difference was observed in the Sol type I fiber glycogen
(pre- vs. postflight), whereas the Sol type IIa fiber glycogen was
significantly depressed postflight (unpublished data). The failure to
observe an increased Sol muscle glycogen postflight was likely the
result of the extremely low inflight dietary intake of 24.6 ± 3.3 kcal · kg
1 · day1
(87). As a result of the low caloric input, the astronauts lost on average 2.6 kg of body weight; however, despite this condition, electron micrographs of the Sol muscle demonstrated a higher content of
lipid droplets in the postflight compared with the preflight muscles
(99). This observation is consistent with the finding of
Musacchia et al. (64), who found an increased triglyceride storage in the rat vastus medialis after a 14-day SF.
Muscle enzymatic alterations with SF.
Considerable data exist regarding the effects of short-term (4- to
22-day) SF on the oxidative and glycolytic enzymes of rat skeletal
muscle (21). The general finding is that the loss of mitochondrial protein with SF is less than that observed for the contractile proteins and for the degree of cell atrophy. As a consequence, the usual observation is for oxidative enzymes of limb
skeletal muscle to either remain unaltered or slightly elevated when
activity is expressed per gram of dry weight (21,
57). This is true for both the slow antigravity muscles
such as the Sol and AL and for fast muscles like the TA and EDL
(47, 58). For example, no change in succinate
dehydrogenase (SDH) activity (per cell volume or gram tissue) of the
Sol was observed after 4, 7, 12.5, or 14 days of SF (48,
58, 63, 68); however, both Miu
et al. (63) and Manchester et al. (57)
calculated that the total amount of oxidative enzymes in the Sol was
less postflight. Riley and co-workers (78,
80) and Bell et al. (10) showed that
mitochondria were selectively lost from the subsarcolemmal regions of
the fiber.
Two single-fiber studies evaluating the effect of SF on rat Sol have
shown glycolytic enzymes to increase after microgravity. Manchester et
al. (57) found an average increase of 28% in selected glycolytic enzymes after a 12.5-day SF, and Chi et al.
(14) reported pyruvate kinase and glycerol-3-phosphate
dehydrogenase (GPDH) to increase by 41 and 56%, respectively, after a
2-wk flight. In contrast to the slow Sol, no significant
changes in glycolytic enzymes were observed for fibers isolated from
the fast TA. In whole muscle analyses, SF has been reported to have no
effect or to increase marker enzymes of glycolysis. For example, after a 7-day SF, Desplanches et al. (20) found no change in Sol
lactate dehydrogenase (LDH) activity, whereas Martin et al.
(58) observed an ~50% increase in GPDH in the Sol and
AL. After 14 days in microgravity, the LDH activity of the rat vastus
medialis was increased 52%, and after 21.5 days the increased LDH
activity in the Sol was associated with an increase in the M form of
the enzyme (64, 72).
The first enzyme in the processing of glucose after its uptake into
skeletal muscle is hexokinase (HK), which converts glucose to glucose
6-phosphate. When single fibers were isolated from the rat Sol and TA
muscles after a 12.5-day SF, they showed a >100% increase in HK
activity (57). When expressed in absolute amounts of
enzyme, the HK activity was still increased postflight by 50 and 25%,
respectively, in the Sol and TA. In a second study by the same group,
Sol HK activity was found to increase by only 47% after a 14-day SF,
and the TA HK activity was unaltered (14).
Considerable less information is available concerning the effects of SF
on enzyme activities in human limb skeletal muscle. Edgerton et al.
(22) found no significant affects of an 11-day SF on the
SDH or GPDH activities of VL fibers. Although the change was not
significant, the slow type I fibers did show an 80% increase in GPDH
activity, and, as a result, GPDH/SDH was significantly higher
postflight. Recently, we evaluated the enzyme profile of single fibers
isolated from the Sol and gastrocnemius muscles of astronauts before
and immediately after a 17-day SF (STS-78). Consistent with the rat
data, microgravity-induced a significant increase in the oxidative
enzymes of the slow type I fibers of the Sol (unpublished
observations). However, surprisingly the Sol type I fiber was
unaltered in regard to HK activity or the glycolytic enzymes glycogen
phosphorylase, glycogen synthase (GS), phosphofructokinase, or LDH
(unpublished observations). The gastrocnemius type I and IIa fibers
contained elevated oxidative enzyme activities postflight, and the
latter also showed an increased HK, GS, and LDH activity (unpublished observations).
Recently, Kell et al. (51) studied the molecular
regulation of the muscle-specific form of the glycolytic enzyme
phosphoglycerate mutase (PGAM-M) in response to 4 wk of HU in the rat.
PGAM-M enzymatic activity and mRNA in the Sol increased 2.5- and
3.5-fold, respectively, compared with control muscles. The proximal
400-bp region of the PGAM-M promoter was increased two to threefold in
the unweighted Sol. These data suggest that transcriptional activation
was important in the upregulation of PGAM-M with unloading.
Substrate utilization and muscle fatigue.
The microgravity environment appears to increase muscle fatigue during
work in space and on return to 1 G (46). Caiozzo et al.
(11) stimulated the rat soleus in situ and observed
significantly greater fatigue in the flight compared with the control
group (Fig. 7). The cause of the
increased fatigue is unknown but may relate to an accelerated glycogen
utilization and a reduced capacity to oxidize fats. Support for this
hypothesis comes from the work of Baldwin et al. (5).
After a 9-day SF, they observed a 37% decline in the ability of both
the high- and low-oxidative regions of the rat vastus muscle to oxidize
long-chain fatty acids. They found no change in the muscle's ability
to oxidize pyruvate or in key marker enzymes of the Krebs cycle or
-oxidative pathway. The authors hypothesized that the inhibition of
long-chain fatty acid oxidation postflight might result from a reduced
ability to activate or translocate fats from the cytosol to the
mitochondria. The latter process is particularly intriguing as it is
catalyzed by carnitine palmitoyltransferase (CPT I), which is thought
to be the rate-limiting step in fatty acid oxidation.
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Fig. 7.
Isometric fatigue data of control ( ) and
flight ( ) soleus muscles. Flight muscles were more fatigable
than control muscles. Data points for both groups of muscles were
fitted by using a second-order polynomial. Values are means ± SD.
* P < 0.05. [From Caiozzo et al.
(11).]
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Similar to the flight condition, we have previously shown HU to
increase the fatigability of the Sol (61). The increased fatigue could not be attributed to an altered steady-state blood flow
(61). Recently, our laboratory found that 2 wk of HU had no effect on the ability of homogenate or mitochondrial fractions of
the Sol, red gastrocnemius, or white gastrocnemius to oxidize either
pyruvate or palmitate (36). However, the
increased fatigue in the Sol of the HU animal was associated with a
greater rate of glycogen depletion and lactate production in the type I
fibers compared with the control group. The failure to observe any
significant decline in palmitate oxidation suggests that the enzymes
required for fat oxidation were not altered. One hypothesis consistent with the data is that glycolysis is activated and fatty acid oxidation inhibited by substrate-level control. Although HU in rats (and perhaps
SF) does not affect steady-state blood flow during exercise, it has
been shown to reduce the endothelium-dependent dilation in Sol feed
arteries (45). Thus the possibility exists that models of
weightlessness and SF may reduce the rate at which muscle blood flow
increases with the onset of exercise. If true, the decrease in creatine
phosphate and ATP, and the increase in ADP, AMP, and Pi,
would be greater in the muscles of the unloaded subjects. The high AMP
and Pi would stimulate glycolysis and the production of
acetyl-CoA. The latter would act as a substrate for malonyl-CoA, thus
reducing the exercise-induced decline in this substrate
(101). Because malonyl-CoA is a known inhibitor of CPT I,
fatty acid oxidation would be reduced. The increased fatigue could be
explained by the elevated Pi and H+, which are
both known to directly inhibit cross-bridge force production
(28).
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SF AND MUSCLE FIBER DAMAGE |
Relationship of inflight changes to muscle cell damage.
Humans returning to Earth after SF frequently experienced muscle
weakness and delayed-onset muscle soreness conditions indicative of
muscle cell damage (81, 102). However, until
the 1993 SLS-2 mission, during which tissues were harvested in space,
it was impossible to separate inflight from postflight changes in cell structure (79). The SLS-2 studies demonstrated that, at
least in rats, fiber lesions were not observed inflight in either the Sol or AL muscles. This experiment proved what had previously been
suspected: muscle fiber damage was primarily a postflight phenomena,
likely resulting from eccentric contractions during reloading in a 1-G
environment (77, 80, 81).
Although SF does not usually induce muscle cell damage, it establishes
a condition within the fibers that increases their susceptibility for
damage during postflight reloading. The question that remains to be
answered is, What are these factors? It is well established that
reloading after either SF or HU in rats causes selective fiber damage
in the slow type I fiber type (81). When the entire AL was
maximally activated by electrical stimulation post-HU, the fast-twitch
type II fibers showed the greatest susceptibility to damage
(unpublished observations). Thus the selective damage of type I fibers
during normal reloading activities such as walking may simply reflect
the preferential recruitment of this fiber type. Any load would be
relatively greater on the atrophied fiber and thus more likely to cause
damage postflight. In addition to the increased relative load and
selective recruitment, SF may induce qualitative changes in cell
proteins in the type I fiber that leads to a more fragile fiber with
reloading. For example, SF has been shown to cause a selective loss of
the actin thin filaments over the myosin thick filaments in the slow
type I fiber (Ref. 76; Widrick, Knuth, Norenberg, Romatowski, Bain,
Riley, Karhanek, Trappe, Trappe, Costill, and Fitts, unpublished
observations). Although the atrophied fiber had a reduced absolute
tension, its specific tension was near normal. We interpreted this to
mean that the normal thick-to-thin filament ratio of 6:1 supplies an excess of actin-binding sites for myosin cross bridges and that reducing the ratio to ~5:1 during SF continues to provide sufficient cross-bridge binding to maintain a near-normal specific tension (Widrick, Knuth, Norenberg, Romatowski, Bain, Riley, Karhanek, Trappe,
Trappe, Costill, and Fitts, unpublished observations). However, during
postflight eccentric contractions, the average stress per remaining
thin filament would significantly increase, which might increase the
susceptibility of the atrophic muscles to reloading damage. A second
possibility would be a selective loss in linking proteins that normally
transmit contractile force to the extracellular matrix. For example,
reduction or absence of a single component of the
dystrophin-glycoprotein complex can result in greater susceptibility to
contraction-induced tearing of the sarcolemma. Currently, it is unknown
whether SF alters this complex. The data reviewed here suggest that
most fiber damage occurs during postflight reloading. However,
extravehicular activity (space walks) during the Hubble telescope
repair mission was reported to produce muscle soreness. This suggests
that some fiber damage in humans might occur inflight. This question
cannot be answered until inflight biopsies are made on the ISS.
Postflight muscle fiber damage.
The SLS-2 mission documented for the first time that at least in rats
muscle fiber damage did not occur inflight (79). At 3 h postflight, no lesions were detected in the Sol or AL, whereas by
4.5 h eccentric contraction-like sarcomere disruptions were detected in 2.8% (SLS-1) and 2.4% (SLS-2) of the myofibers in the
caudal slow region of the AL (79). The lesions were
significantly more prevalent in the AL myofibers of the caudal slow
region than the rostral mixed fast and slow region, an observation
consistent with slow fibers being more susceptible to damage. No
lesions were observed in the Sol at anytime postflight, and by 9 days no lesions remained in the AL, indicating that sarcomere repair was
complete. The postflight posture of the animals seemed to explain the
selective damage of the AL (79). The postflight rats stood
erect less frequently on the hindlegs, and, when standing, weight
bearing was on the heels without normal plantar flexion. Thus Sol
contractile activity was reduced relative to the control rats. When
standing, the postflight rats were postured closer to the ground, which
produced greater flexion of the hip joints. Biomechanically, this
caused the AL muscles, which are hip adductors and extensors, to be
stretched and bear weight.
The earlier Cosmos flights provided important data on the time course
of the lesion development. It was clear from the SLS-2 results that
postflight fiber lesions developed within hours of landing, whereas the
Cosmos 1887 12.5-day flight, in which rats were killed 2 days after
landing, showed that extensive muscle fiber necrosis, activated
macrophages, and satellite cells, and interstitial edema were present
(80). The 14-day Cosmos 2044 flight showed that rats
killed 8-11 h after landing exhibited extensive sarcomere
disruption and edema but minimal tissue necrosis, macrophage, and
satellite cell activation (77). The postflight fiber
lesions appear to be primarily induced by the degree of strain and the
level of force. The high degree of lesions in the Sol (15%) and AL
(44%) muscles after the Cosmos 2044 mission may have been due in part
to the higher G force of parachute landing compared with the SLS-1 and
SLS-2 shuttle landing on a runway. The fiber lesions postflight do not
appear to have been mediated by Ca2+-activiated proteases , but ubiquitin-mediated proteolysis may play a role (77).
| |
EXERCISE COUNTERMEASURES |
Animal studies.
The rat HU model has been used extensively to assess the effectiveness
of various exercise countermeasures (21). Edgerton and Roy
(21) recently reviewed this topic in detail, and thus only
an overview of important features will be presented here. Edgerton and
colleagues (40, 42, 70), Booth
and colleagues (18), and Fitts and colleagues
(9, 96, 100) have all provided convincing evidence that in rats short periods of exercise interspersed throughout the day were more effective than one long daily bout. The
speculation was that intermittent bouts of exercise were more effective
in maintaining protein synthesis at control levels during the
unweighting period (18). For example, during a 7-day HU period, 10-min bouts of standing or slow walking on a treadmill repeated four times daily maintained a near-normal soleus mass (18, 42). The data of Widrick et al.
(96) suggest that this is particularly true for short
periods of unweighting of up to 7 days. When HU was extended to 14 days, four 10-min periods of standing per day reduced the loss of
soleus-to-body weight ratio by 22% and attenuated alterations in type
I fiber diameter and peak force by 36 and 29%, respectively. Clearly a
simple, intermittent-standing protocol was less effective over 14 days
compared with 7 days of HU. In rats, the first week of muscle wasting
with HU is primarily caused by a decline in protein synthesis, whereas
myofibril degradation does not reach its maximum until days
9-15 (92). Perhaps the intermittent-standing
protocol was more effective in preventing the decline in protein
synthesis than it was in reducing degradation.
The only exercise paradigm known to induce fiber hypertrophy is
high-resistance weight training (27). Despite this, very little data exist on the effectiveness of this exercise modality in the
prevention of atrophy incurred by models of unloading or SF. The best
controlled and quantified data come from rat HU studies. Ten minutes of
standing with high-resistance intermittent exercise
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ABSTRACT
INTRODUCTION
