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1 Department of Physical
Therapy, This study was designed to test the hypothesis
that myosin heavy (MHC) and light chain (MLC) plasticity resulting from
hindlimb suspension (HS) is an age-dependent process. By using an
electrophoretic technique, the distribution of MHC and MLC isoforms was
quantitatively evaluated in the soleus muscles from 3- or 12-wk-old
rats after 1-3 wk of HS treatment was maintained. In normal 12- and 15-wk-old rats, the soleus muscles contained a predominance of
MHCI (~94%) with small amounts
of MHCIIa, but not
MHCIId or
MHCIIb. The suspended muscles of
adult rats were characterized by the appearance of MHCIIb and
MHCIId, the latter reaching ~6%
after 3 wk of HS treatment. In contrast to changes in MHC, HS did not
induce a transition in the MLC pattern in the soleus muscles from adult
rats. Compared with adult rats, in juveniles HS had a much more
pronounced effect on the shift toward faster MHC and MLC isoform
expression. The soleus muscles of 6-wk-old rats after 3 wk of HS were
composed of 37.0% MHCI, 19.1%
MHCIIa, 23.7%
MHCIId, and 20.2%
MHCIIb. Changes in MLC isoforms
consisted of an increase in MLC1f
and MLC2f concomitant with a
decrease in MLC2s. These results
indicate the existence of a differential effect of HS on MHC and MLC
transitions that appears to be age dependent. They also suggest that
the suspended soleus muscles from young rats may acquire the intrinsic
contractile properties that are intermediate between those in the
normal soleus and typical fast-twitch skeletal muscles.
development; hindlimb suspension; transition
SKELETAL MUSCLE FIBERS can be envisaged as dynamic
structures because they are capable of changing certain properties in
response to altered functional demands. Generally, increased
contractile activities, e.g., chronic stimulation, the removal of a
synergist muscle, or heavy exercise training, are responsible for
muscular hypertrophy, an increase in maximal tension, and/or a
reduction in contraction speed (19, 22, 27). In contrast, hindlimb suspension (HS), introduced by Morey (18) as a model for weightlessness during spaceflight, leads to muscle atrophy and causes a shift toward
faster contractile characteristics, especially in the muscles with
postural functions such as the soleus muscle (18).
Myosin, the major myofibrillar protein, consists of two heavy (MHC) and
four light (MLC) chains. Both subunits exist as multiple isoforms. The
skeletal muscles of adult rodents contain four major MHC isoforms,
i.e., one "slow," MHCI, and
three "fast,"
MHCIIa, MHCIId/x, and
MHCIIb. They also contain five
major MLC isoforms, i.e., two slow,
MLC1s and
MLC2s, and three fast,
MLC1f,
MLC2f, and
MLC3f. Muscle fiber types that are
categorized on the basis of histochemical staining for myofibrillar
ATPase (mATPase) differ in their MHC composition. Fibers comprising
MHCI,
MHCIIa,
MHCIId/x, and
MHCIIb are referred to as type I,
IIA, IID/X, and IIB, respectively. The importance of both MHC and MLC
isoforms with regard to contractile properties has been demonstrated
(25). Although, on average, the maximum velocity of shortening
(Vmax)
increases in the order type I << type IIA < type IID/X < type IIB, a large scattering of
Vmax exists
within each fiber type (10). This scattering is thought to relate to
the significant role of MLC isoforms in the contractile properties of
muscle fiber (5, 13).
In accordance with previous observations on the relationship between
the contractile properties of muscle fibers and myosin within them,
shifts toward faster contractile characteristics brought about in the
suspended soleus muscle are accompanied by an increase in the amount of
fast-type MHC isoforms (15). On the other hand, the distribution of MLC
isoforms seems to be unaltered, or less affected, by HS compared with
that of MHC. McDonald et al. (14), for instance, investigated single
fibers of adult rat soleus muscles by using one-dimensional
electrophoresis and found no change in MLC composition after 3 wk of HS.
In the rat soleus muscle, differentiation of muscle fibers is barely
evident at birth but proceeds rapidly in the following 2 mo. It has
been shown that the changes in the contractile properties of the soleus
muscle after HS are more pronounced in young rats than in fully grown
rats (1), suggesting that the introduction of HS at a time when the
animals were rapidly growing might have a greater effect on myosin
expression. The present study was undertaken in an attempt to elucidate
a possible age and/or developmental stage dependence of HS with regard
to MHC and MLC isoforms. Experiments conducted in the soleus muscles in
6- and 15-wk-old rats subjected to HS for 3 wk have revealed that,
compared with adult animals, the muscles of juveniles exposed to this
intervention not only contain smaller amounts of
MHCI but also express large
amounts of MHCIId/x and
MHCIIb, which are absent in normal
soleus muscles. In addition, HS imposed on juveniles produces a
significant elevation in the relative distribution of fast MLC isoforms.
Animals.
Forty-four 3- (young; Y)- and forty-six 12-wk-old (adult; A) female
Wistar rats (number at the beginning of the experiments) were used for
this study. They were assigned to two groups: hindlimb-suspended (HS-Y
and HS-A) or non-hindlimb-suspended (non-HS-Y and non-HS-A) rats. All
animals were housed in the same environment (12:12-h light-dark daily
cycle, food and water provided ad libitum). The experiments received
authorization from the Experimental Animal Care Committee of the
University of Tsukuba.
Suspension procedure.
The animals designated for HS were prepared by a noninvasive
tail-traction procedure in which approximately one-half of the tail
remains uncovered, thereby allowing normal thermoregulatory processes
to occur. The portion of exposed tail maintained normal color,
indicating that blood flow was not compromised. A wire was fastened to
the tail with adhesive tape and connected to another wire via a fish
swivel. The other end of the wire was attached to the top
of the cage, and the suspension height was adjusted so that only the
front legs were able to make contact with the floor. In this apparatus,
the rats could rotate a full 360°, allowing them access to food and
water without contact of the hindlimb with the cage floor or walls. The
animals were checked daily for signs of tail lesions, discoloration, or
undue discomfort. The hindlimbs of the HS animals were elevated for 1 (HS-Y4,
n = 9; HS-A13,
n = 9), 2 (HS-Y5,
n = 9;
HS-A14,
n = 10), or 3 (HS-Y6, n = 6;
HS-A15,
n = 9) wk.
Tissue preparation.
The rats in the non-HS group were separated into two groups. The
animals from one group were killed at the age of 3 (non-HS-Y3; n = 10) and 12 (non-HS-A12;
n = 9) wk, i.e., at the beginning of
HS. The remaining animals were killed at the age of 6 (non-HS-Y6; n = 10) and 15 (non-HS-A15;
n = 9) wk, i.e., at the termination of
experimental treatment. The animals were weighed and given an
anesthetic overdose of diethyl ether. The soleus muscles were removed,
trimmed clean of visible fat and connective tissues, and stored at
MHC electrophoresis.
Muscle homogenates were diluted 75-fold with a solution composed of
62.5 mM Tris · HCl (pH 6.8), 2% (mass/vol) SDS, 10%
(vol/vol) glycerol, 5% (vol/vol) 2-mercaptoethanol, and 0.02%
(mass/vol) bromophenol blue. Then, 20 µl were applied to the gel for
electrophoretic separation of MHC isoforms. Electrophoresis in the
presence of SDS was carried out by using a 0.75-mm-thick 7.5%
separating gel and a 3.5% stacking gel. Electrophoresis lasted 36 h at
a constant voltage of 200 V in a cold room (4°C). After
electrophoresis, gels were silver-stained by using a Wako Silver Stain
Kit (Wako). The percent distribution of the various MHC isoforms was
estimated by densitometric evaluation (CS-9000, Shimadzu).
Two-dimensional electrophoresis.
Electrophoresis in the first dimension was performed on a rectangular
gel (1 × 2 × 60 mm) according to O'Farrell (20) by using
1.6 (pH 5.0-8.0) and 0.5% (pH 3.5-10.0) ampholines
(Pharmacia) in 4.2% (mass/vol) polyacrylamide (Atto AE-6050A
apparatus, Atto). Electrophoresis was first run for 20 min at 100 V and
then for another 80 min at 300 V. Separation in the second dimension
was carried out for 90 min at 200 V in a minigel chamber on a
1-mm-thick 15% separating gel and a 3% stacking gel (33). The gels
were stained with 0.25% (mass/vol) Coomassie brilliant blue R250 in 45% (vol/vol) methanol and 10% (vol/vol) acetic acid. After
destaining, the spots corresponding to MLC were excised, placed into
centrifuge tubes containing 1.5 ml 25% (vol/vol) pyridine, and
incubated overnight. This allowed elution of the bound dye to determine relative protein amounts (8). The eluted dye was measured
spectrophotomecally at 605 nm.
Statistical analyses.
A three-way variance analysis was performed to evaluate the influence
of age, HS, and duration of HS. If an overall significant F-value was obtained, a
Scheffé's post hoc analysis was used to isolate the
significantly different means. All comparisons were performed at a 95%
confidence level. All data shown are presented as means ± SD.
Body weight.
As is commonly reported in studies on rat growth (7), the control
animals in each age group in the present study exhibited markedly
different growth rates, with young rats gaining 3.5 g of body weight on
a daily basis, whereas adults grew at 0.2 g/day. With HS, a significant
decrease in body weight gain was observed in juveniles (Table
1). After 3 wk of HS, the body weight in the HS-Y6 group was ~14% lower
than that found in the age-matched control group
(non-HS-Y6). In adult animals, a
significant reduction in body weight was brought about at an early
stage of HS. Compared with that in
non-HS-A12 animals, it decreased
by ~13% during the first week of HS. Thereafter, body weight of the
rats in HS-A groups was not recovered (Table 1).
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
80°C until used for analyses. Small muscle pieces were
homogenized in an all-glass homogenizer in a 40-fold volume of a
solution consisting of 5 M urea, 2 M thiourea, 10 mM sodium pyrophosphate, and 0.1% (vol/vol) 2-mercaptoethanol; 30-50 µl of this homogenate were used for MLC analysis by using two-dimensional electrophoresis.
RESULTS
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
Table 1.
Muscle wet weight of soleus muscles from and body weight of young and
adult, normal, and tail-suspended rats
Muscle weight. Similar to body weight, normal growth caused a remarkable gain of soleus wet weight in juveniles. It was augmented 2.6-fold for 3 wk (Table 1). HS of young animals led to remarkable retardation of normal growth during the same period of development. The muscle weight tended to decrease during 3 wk of HS, although its reduction from 20.3 ± 3.7 to 16.1 ± 3.6 mg was nonsignificant. Increased body weight and unaltered muscle weight resulted in a persistent decline in the ratio of muscle weight to body weight. A conspicuous decrease to 36% in the values of the age-matched control group (non-HS-Y6) was found after 3 wk of HS.
In adult rats, there were significant decreases in muscle weight during HS. Soleus muscle weight in the HS-A13, HS-A14, and HS-A15 groups was 80.6, 72.6, and 53.7% of that in non-HS-A12 animals, respectively (Table 1). The muscles in HS-A15 rats weighed 47.4% less than those in their age-matched control group (non-HS-A15). The ratio of muscle weight to body weight was progressively reduced during HS. As a consequence, the ratio after 3 wk of HS amounted to only 61.8% of the age-matched control value.MHC.
Independent studies by Schiaffino et al. (24) and Bär and Pette
(3) in rat skeletal muscle were able to detect new MHC isoforms,
designated as MHCIIx and
MHCIId, respectively.
Electrophoretic analyses by Termin et al. (30) provided evidence that
MHCIId may be identical to
MHCIIx. In the present study, the
four MHC isoforms detected in rat skeletal muscles were referred to as MHCIIa,
MHCIId,
MHCIIb, and
MHCI, in the order of increasing
electrophoretic mobilities according to the nomenclature system of
Termin et al. (Fig. 1). As illustrated in
Fig. 2, the soleus muscles in 3-wk-old animals expressed 62.2 ± 4.3%
MHCI and 37.8 ± 4.3%
MHCIIa but not
MHCIId or
MHCIIb. A gradual increase in
MHCI, with a concomitant decrease
in MHCIIa, occurred in the
developing soleus between the third and sixth postnatal week. However,
the increase in MHCI from 62.2 ± 4.3 to 68.3 ± 5.2% was not significant
(P = 0.075). The muscles from the rats
subjected to 1 wk of HS exhibited a reduction in
MHCI and the appearance of
MHCIId and
MHCIIb. The decline in
MHCI and the increase in
MHCIId reached a plateau after 2 wk of HS. On the other hand, a persistent increase in
MHCIIb was found during the
experimental period. The muscles in
HS-Y6 were composed of 37.0 ± 1.9% MHCI, 19.1 ± 5.4%
MHCIIa, 23.7 ± 2.0%
MHCIId, and 20.2 ± 2.6%
MHCIIb.
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MLC.
Two-dimensional electrophoresis of extracts from juvenile soleus
muscles revealed the expression of five MLC isoforms (Figs. 4 and 5).
Similar to MHC, significant changes in MLC isoforms were not found in
the developing soleus between the third and sixth postnatal week (Fig.
5). HS induced progressive and marked increases in
MLC1f and
MLC2f, with a concomitant decrease
in MLC2s. The relative amounts of
MLC1f and
MLC2f increased about twofold after 3 wk of HS. The soleus muscles in
HS-Y6 were composed of 38.0 ± 4.2% MLC1s, 26.3 ± 5.6%
MLC2s, 15.1 ± 2.2%
MLC1f, 18.9 ± 6.1%
MLC2f, and 1.7 ± 0.6%
MLC3f, whereas
non-HS-Y6 muscles contained 41.4 ± 5.6% MLC1s, 48.4 ± 6.9% MLC2s, 4.1 ± 2.0% MLC1f, 5.0 ± 3.0%
MLC2f, and 1.1 ± 1.8%
MLC3f. As seen in Fig. 5, the percent distribution of MLC2s,
MLC1f, and
MLC2f isoforms in
HS-Y6 was significantly different
compared with both non-HS-Y3 and
non-HS-Y6.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The primary purpose of the present study was to examine the age dependence of HS-induced muscle phenotype plasticity with regard to MHC and MLC isoform expression. It has been widely accepted that HS enables a transition from slower to faster MHC in rat soleus muscle (32). However, this intervention does not appear to be a powerful stimulus for the upregulation of fast-type MHC isoforms in adult rats, because the distribution of MHC isoforms or fiber types in fully grown animals is reported to be only slightly affected or unaltered. Histochemical analyses by Leterme et al. (12), who explored the effect of HS on the soleus muscles of adult rats, revealed a significant shift in type I fibers from 91.2 to 73.4%. In studies of similar design, Asmussen and Soukup (2), McNulty et al. (16), and Simard et al. (26) found no change in muscle fiber composition. Assuming that there is a general correspondence between mATPase histochemistry and MHC isoforms expressed (30), the present results showing that the relative amount of MHCI in adult rats was unaltered by 3 wk of HS are in accordance with the findings from the latter studies (2, 16, 26) (Fig. 3).
It seems that a shortcoming of histochemical discrimination of fiber types lies in its low sensitivity for detecting MHC isoforms. Some fibers composed of more than one MHC isoform exist, in addition to the majority of fibers expressing only one isoform. As previously shown, an increase in the number of such hybrid fibers is brought about in the muscles in which fiber-type transformation is proceeding (29). Using immunochemical and electrophoretic techniques, Talmadge et al. (28) found that the whole muscle of the rat soleus exposed to 2 wk of HS contained slight but significant MHCIId content (5%) and that MHCIId always coexisted with another MHC isoform in single fibers. Because of the above-mentioned low sensitivity of the mATPase-based histochemical technique, such hybrid fibers must have been identified as type IIA or type I. The expression of MHCIId in the suspended soleus muscle may, therefore, have escaped detection in previous studies using histochemical analyses. The present study found that the suspended soleus muscles in adult animals exhibited the expression of MHCIIb as well as MHCIId (Figs. 1 and 3), extending the findings of Talmadge et al. to another fast MHC isoform.
A major finding in the present study is that, in addition to
decreased MHCI and
MHCIIa, the suspended soleus
muscles in juveniles are composed of large amounts of
MHCIId (23.7%) and
MHCIIb (20.2%) isoforms,
which are absent in the normal soleus (Figs. 1 and 2). MHC transitions
in rodents are shown to occur in a sequential manner, following the
order of MHCIIb 
MHCIId MHCIIa MHCI. According to this sequence
of MHC transition, the changes illustrated in Fig. 2 may result from a
progression in MHC isoform expression from
MHCI 
MHCIIa MHCIId MHCIIb. However, the possibility cannot be ruled out that some fibers have the capacity to adapt in a
manner different from the commonly accepted sequence because a small
but appreciable number of muscle fibers in the suspended soleus muscle
consist of both MHCI and
MHCIId isoforms (28).
The fact that slow-twitch soleus muscles subjected to HS express large amounts of MHCIId and MHCIIb is of interest with regard to its functional significance. A recent single-fiber study has indicated that type II fibers consisting of fast MHC isoforms exhibit up to threefold higher Vmax than do type I fibers containing MHCI (6). As can be seen in Fig. 2, total fast MHC (IIa, IId, and IIb) from 3-wk-suspended muscles in juveniles amounted to 63.0%. On the basis of the findings of positive correlations between Vmax and mATPase activity (4) and between mATPase activity and the relative distribution of fast-twitch fibers comprised in muscles (31), we calculated the Vmax of the soleus muscles in young animals used in this study. The results derived from this calculation indicate that 3-wk-suspended soleus muscles in juveniles are characterized by a 1.62-fold higher Vmax compared with those in the age-matched control animals.
In the present study, we adopted a two-dimensional electrophoretic technique to evaluate the change in MLC pattern because the muscle homogenate contains some protein, the molecular weight of which is similar to MLC. To our knowledge, this is the first study in which the increases in MLC1f and MLC2f in juvenile soleus muscle subjected to HS have been quantitatively measured by this technique (Fig. 5). Recently, it was demonstrated that Vmax in mammalian skeletal muscles correlates with not only MHC but also MLC isoforms (6, 11, 13). Larsson and Moss (11), for example, observed a lower Vmax in type IIA fibers expressing slow regulatory MLC than in the fibers lacking this MLC. In view of these findings, in addition to the increase in fast MHC, the rise in fast MLC observed in the suspended juvenile soleus muscle also appears to contribute to the shift toward faster contractile characteristics.
It is important to point out that disproportionate increases in fast MHC and MLC occurred in the suspended soleus muscles from young rats. Thus the soleus muscles from HS-Y6 consisted of 63.0% fast MHC (IIa, IId, and IIb) in total, whereas the same muscles were comprised of 35.7% fast MLC (1f, 2f, and 3f). These observations raise the possibility that there exists an appreciable proportion of fibers characterized by isomyosins composed of fast MHC and slow MLC. At the least, MHCIIa, for example, has been shown to be capable of associating with slow as well as fast MLC isoforms (9). Further studies with native electrophoresis are needed to elucidate how many isomyosins exist in single fibers in the suspended soleus muscle.
The mechanisms that could produce differences in responses depending on age are unclear. The differentiation of the innervation pattern in rat soleus musecle is complete at the age of 16 days (17). Therefore, it is unlikely that HS that began from the age of 21 days would affect the process of transition from polyneuronal to single innervation. It has been widely recognized that the activity pattern of motoneurons plays an important role in the expression of myosin (21). Riley et al. (23), studying the impact of HS on soleus electromyographic activity in adult rats, found that such activity shifted from tonic to phasic. The change in the activity pattern of motoneurons innervating the soleus muscle may account, at least in part, for the increase in MHCIId and MHCIIb observed in adult rats. One hypothesis explaining pronounced changes in the expression of myosin that occurred in juveniles is that motoneurons in young rats may be more responsive to altered functional demand than those in adult animals.
In conclusion, the alterations in the expression of MHC and MLC isoforms in response to HS in adult and juvenile rats have been compared. The soleus muscles in young rats were much more markedly affected than were those of adults. After 3 wk of HS, the muscles in juveniles contained 63.0% fast MHC and 35.7% fast MLC in total, whereas adult muscles were composed of 10.3% fast MHC and 7.4% fast MLC. These results indicate that there is a differential effect of HS on MHC and MLC transitions that appears to be age dependent. They also suggest that the suspended soleus muscles from young rats may acquire the intrinsic contractile properties that are intermediate between the normal soleus and typical fast-twitch skeletal muscles.
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ACKNOWLEDGEMENTS |
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The authors acknowledge excellent technical assistance for two-dimensional electrophoresis by P. Gachapin and A. Häkkissen.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: S. Katsuta, Institute of Health and Sport Sciences, Univ. of Tsukuba, 1-1-1 Ibaraki 305-0036, Japan.
Address for correspondence: M. Wada, Faculty of Integrated Arts and Sciences, Hiroshima Univ., 1-7-1 Kagamiyama, Higashihiroshima 739-8521, Japan (E-mail: wada{at}ipc.hiroshima-u.ac.jp).
Received 27 April 1998; accepted in final form 5 January 1999.
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RESULTS
DISCUSSION
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) and nonsuspended (
) soleus muscles from
young rats. Hindlimb suspension of rats commenced from 3 wk of age and
lasted for 1, 2, or 3 wk. Percent distribution of MHC isoforms is
expressed as means ± SD. Comparisons were not performed between
nonsuspended muscles of 6-wk-old rats and suspended muscles of 4- and
5-wk-old rats. *, 
Significantly different from nonsuspended
muscles of 3- and 6-wk-old rats, respectively
(P < 0.05).
Significantly different from suspended muscles of 4-wk-old
rats (P < 0.05).
