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Department of Physiological Science and Brain Research Institute, University of California, Los Angeles, California 90095-1527
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Talmadge, Robert J., Roland R. Roy, and V. Reggie Edgerton.
Distribution of myosin heavy chain isoforms in non-weight-bearing rat soleus muscle fibers. J. Appl.
Physiol. 81(6): 2540-2546, 1996.
The effects of
14 days of spaceflight (SF) or hindlimb suspension (HS) (Cosmos 2044)
on myosin heavy chain (MHC) isoform content of the rat soleus muscle
and single muscle fibers were determined. On the basis of
electrophoretic analyses, there was a de novo synthesis of type IIx MHC
but no change in either type I or IIa MHC isoform proportions after
either SF or HS compared with controls. The percentage of fibers
containing only type I MHC decreased by 26 and 23%, and the percentage
of fibers with multiple MHCs increased from 6% in controls to 32% in
HS and 34% in SF rats. Type IIx MHC was always found in combination
with another MHC or combination of MHCs; i.e., no fibers contained type
IIx MHC exclusively. These data suggest that the expression of the
normal complement of MHC isoforms in the adult rat soleus muscle is
dependent, in part, on normal weight bearing and that the absence of
weight bearing induces a shift toward type IIx MHC protein expression in the preexisting type I and IIa fibers of the soleus.
contractile proteins; fiber type; gravity; hindlimb suspension; spaceflight
INTRODUCTION SPACEFLIGHT (SF) and hindlimb suspension (HS) result in
decreased load bearing by the ankle extensor muscles of rats (22, 29,
36). SF and HS also result in muscular atrophy and
increases in the proportion of fibers containing type II (fast) myosin
heavy chain (MHC) isoforms in predominantly slow postural muscles
(i.e., soleus) (18, 22, 29, 36). The slow oxidative regions of mixed
muscles (such as the medial gastrocnemius) also show increases in the
proportion of fibers containing type II MHC in response to HS or SF
(15).
It is important to determine the types of MHC expressed in muscles and
individual fibers under conditions of altered loading because the MHC
molecule plays a major role in regulating the velocity of unloaded
shortening (3-5, 16, 17, 20, 21, 26), the curvature of the
force-velocity relationship (5), and the myofibrillar
adenosinetriphosphatase (ATPase) activity of single fibers (32). The
MHC molecule is also the primary molecule involved in determining the
histochemical myofibrillar ATPase-staining characteristics of a fiber
(25). In rats, at least four adult MHC isoforms are expressed in the
hindlimb muscles. These MHC isoforms have been identified as slow type
I ( There is an increase in the proportion of the type IIx MHC isoform in
the soleus muscle (normally composed of types I and IIa only) of rats
after 6 days of SF (6) or 14-31 days of HS (1, 7, 8, 19, 27).
However, the distribution of the type IIx MHC relative to other
isoforms in individual fibers of unloaded muscles is unknown.
Therefore, in the present study we have determined the MHC composition
of whole rat soleus muscles and single soleus fibers after 14 days of
either SF or HS. We have 1)
quantified the changes in MHC isoform content in the slow soleus
muscle, 2) determined the
distribution of type IIx MHC relative to other MHC isoforms in soleus
muscle fibers, and 3) compared the
effects of SF and HS on the MHC content of single fibers in the rat
soleus muscle.
MATERIALS AND METHODS
-cardiac) and as fast isoforms IIa, IId/x (henceforth IIx), and
IIb (2, 24, 34). Recent data have also demonstrated the existence of
multiple slow type I MHC isoforms (8, 13); however, the physiological
significance of these multiple slow isoforms is not yet known.
70°C
until analyzed. Cross sections (10 µm thick) from the midbelly were
taken on a Reichert-Jung 2800 Frigocut E cryostat microtome and placed
on chrome/alum-coated slides in preparation for
immunohistochemistry. Thirty midbelly cross sections (20 µm thick) were pooled and placed in precooled (20°C)
microcentrifuge tubes and stored at 70°C in preparation for
myofibrillar protein isolation.
Sodium-dodecyl sulfate polyacrylamide gel electrophoresis
(SDS-PAGE).
Isolated myofibrils were prepared from the cross sections that were
stored in microcentrifuge tubes according to a modification (31) of the
procedure of Thomason et al. (35). MHCs were separated by SDS-PAGE
according to Talmadge and Roy (28). The SDS-PAGE gels were stained,
photographed, and scanned with a Pharmacia LKB Ultroscan laser scanning
densitometer for the quantification of MHC isoforms (28).
Immunohistochemistry.
Immunohistochemical analysis of MHC content in individual fibers was
performed as described by Talmadge et al. (31). Briefly, serial cross
sections were stained by using a series of monoclonal antibodies (MAbs;
primary antibody) specific to rat MHC isoforms (see Table
1 for MAb specificity). The avidin-biotin
immunohistochemical procedure was used for the localization of primary
antibody binding. One hundred fibers from the central
region of the cross sections per muscle were analyzed for MHC content
as determined by MAb binding. Stained cross sections were photographed
on an Olympus BH-2 microscope with a Nikon camera attachment.
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-level set at
P < 0.05.
RESULTS
Immunohistochemistry. In control rat soleus, most fibers were labeled with MAb slow but were negative for all other MAbs (e.g., fiber I in Fig. 3). These fibers contained type I MHC only. A smaller proportion of fibers were labeled with MAb 71, but not with F3, D9, or slow, demonstrating that they contained type IIa MHC only (e.g., fiber a in Fig. 3). Some fibers in the control soleus reacted positively with MAbs slow and 71 but not with F3 or D9; therefore, these fibers (not shown) contained both types I and IIa MHCs and were classified as hybrid fibers. Also, a few fibers in the control soleus stained positively with MAbs 71 and D9 but not with slow or F3; these fibers (not shown) contained both types IIa and IIx MHC isoforms. In both the HS and SF rats, more fibers stained positively with D9, indicating an increase in the proportion of fibers with either type IIx or IIb MHCs relative to control. Because in most cases there was no labeling with MAb F3, these fibers contained type IIx MHC. In HS and SF rat soleus, the type IIx MHC was observed in combination with types I, IIa, and/or IIb MHCs (e.g., see fibers Ix and ax in Figs. 4 and 5). No fibers in any group were labeled by MAbs G6 and B6, suggesting that neither embryonic nor neonatal MHCs were present (data not shown).
Only type I MHC was found in 88 ± 3% of the control fibers while 6 ± 3% contained only type IIa, 6 ± 3% contained both types I and IIa, and <1% had both types IIa and IIx MHC isoforms (Fig. 6). Both HS (68 ± 1%) and SF (65 ± 7%) resulted in decreases in the proportion of fibers with only type I MHC (Fig. 6). The proportion of fibers containing only type IIa MHC was <1% after both HS and SF compared with 6% in controls (Fig. 6). HS and SF also resulted in increases in the proportion of hybrid fibers (Fig. 7A), the proportion of fibers containing multiple isoforms of type II MHC (Fig. 7B), and the proportion of fibers containing IIx MHC (Fig. 7C). However, the proportion of fibers containing at least some type I MHC was similar in all three groups (Fig. 7D).
DISCUSSION
The proportion of type IIx MHC in the rat soleus after 2 wk of HS ranges from 0 to 6% and appears to increase thereafter (1, 8, 19, 27). Increases in the mRNA that code for type IIx MHC have been reported for the slow vastus intermedius muscle after 31 days of HS (1). Although mRNA data were not reported for the soleus muscle, it was demonstrated that both the vastus intermedius and the soleus had large increases in type IIx protein after HS (1). Therefore, the HS-induced increase in type IIx protein expression is most likely regulated at the mRNA level. In the only previous study to determine the amounts of type IIx MHC in the soleus after SF, it was reported that 10% of the soleus MHC pool was type IIx after a 6-day flight (6). We observed that only 4% of the MHC in the soleus was type IIx after a 14-day flight. However, after the 6-day flight no type IIb MHC was observed in the soleus (6), whereas after 14 days type IIb MHC was present in some rats and accounted for a mean of 3% of the total MHC pool. Thus it appears as though the additional duration of SF in the present study resulted in a further conversion from type IIx to IIb MHC. Because fibers containing type IIb MHC exclusively have faster contractile velocities than do fibers with type IIx MHC exclusively (4), the additional duration of flight resulted in a further adaptation toward a faster MHC isoform. Increased expression of type IIb MHC mRNA has been reported for predominantly slow muscles (e.g., vastus intermedius) and muscle portions (e.g., red vastus lateralis) after 9 days of flight (12). At this time, MHC mRNA data are not available for the soleus after SF.
The proportion of the MHC isoforms in the soleus after 14 days of HS at both the whole muscle and single-fiber levels was similar to that found after an equal duration of SF in rats treated nearly identically, substantiating the use of HS as a ground-based model for studying MHC adaptations in response to SF. The only obvious difference between HS and SF was the presence of the type IIb MHC isoform in some SF rats but not in HS rats.
After 15 and 30 days of complete spinal cord transection at a midthoracic level in rats, there is an increase in the proportion of type IIx MHC in the soleus muscle (31). Also, after 30 days of spinal transection, some soleus fibers express type IIb MHC. Thus high proportions of hybrid fibers and fibers containing multiple type II MHCs are present after HS and SF (Fig. 6) as well as after spinal cord transection (31). These data suggest that the types of MHC expressed in slow extensor muscles are modulated by the amount and/or pattern of loading.
Despite the projected differences in the amounts of electrical activation of the soleus associated with HS, SF, and spinal cord transection (22, 29), the MHC adaptations that occur are similar. The common feature among these three models is the phenomenon of unloading of the predominantly slow postural muscles of the hindlimb, such as the soleus (22, 29). Thus the high level of expression of multiple MHCs in single fibers, principally being type IIx plus some other isoform(s), appears to be a defining characteristic of the adaptation induced by unloading of slow postural rodent muscle.
The data are consistent with a progression of MHC isoform expression
from type I 
IIa IIx IIb after the imposition
of non-weight bearing by either HS or SF; however, the rate at which these transitions occur appears to be quite rapid and results in fibers
that contain more than two different MHC isoforms at a given time. For
example, in the present study, fibers were found to contain isoforms
I/IIa/IIx, IIa/IIx/IIb, or even all four adult MHCs
simultaneously. Also, it appears that at least some fibers may have the capacity to adapt from type I MHC expression to type IIx
MHC expression without expressing type IIa MHC protein. This observation suggests that the progression from type I IIa
IIx IIb MHC expression by an individual fiber does
not have to occur in a discrete ordered fashion. It is possible that
the colabeling for multiple MHCs in single fibers after unloading reflects only a transition from the expression of one MHC to another at
the stage at which these muscle fibers were analyzed. More prolonged
periods of unloading may result in the MHC content of the individual
fibers becoming uniform. Colabeling of multiple MHCs within a fiber
also could occur as a result of different myonuclei expressing
different MHCs or the expression of multiple MHC isoforms by a single
myonucleus. The latter possibility has been reported to occur under
conditions of chronic electrical stimulation-induced myosin isoform
shifts (Ref. 14 and B. Russell, personal communication).
As shown in Fig. 6, the expression of the type IIx MHC isoform is normally restricted to a very limited population of fibers that also contain type IIa MHC in control rat soleus. However, after either HS or SF the type IIx MHC was found in combination with types I, IIa, and/or IIb MHCs. This finding is similar to that observed after spinal cord transection; i.e., after 15 or 30 days of spinal cord transection in the rat, the type IIx MHC isoform is expressed in combination with MHC isoforms I, IIa, and/or IIb in the soleus muscle (31). Thus type IIx MHC expression does not appear to be restricted to any particular type (fast or slow) of preexisting fiber in the soleus under conditions of chronic non-weight bearing. This observation suggests that at least some of the myonuclei of both fast and slow fibers in the soleus are capable of expressing type IIx MHC under appropriate conditions.
Although type IIx MHC expression apparently is not restricted to a particular type of fiber, it is not expressed by all fibers after 14 days of either HS or SF despite unloading of all fibers. A number of factors could be responsible for this nonhomogeneous expression of type IIx MHC. For example, the amount of activation of individual fibers is unknown and is likely to be highly variable. Previous studies in cats, however, have shown that even when a muscle is rendered completely electrically silent by spinal cord isolation and all fibers in extensor muscles are presumably unloaded, all of the fibers in the muscle do not respond in a similar manner. For example, after 6 mo of spinal cord isolation, the cat soleus (normally 100% slow) is composed of a mixture of fiber types, with some fibers that contain only slow, others that contain only fast, and still others that contain both slow and fast MHCs (10, 33). Thus some fibers adapt more readily than do others despite apparently similar activation and loading conditions. Clearly, factors other than electrical activation and loading play a role in determining the MHC characteristics of a fiber (30).
Physiological significance of hybrid fibers produced by HS and SF. Our data demonstrate that at 14 days after HS the type IIx MHC is always found in combination with at least one other adult MHC isoform. Thus the contractile velocities of the fibers containing type IIx MHC may not be significantly greater than the type IIa fibers normally observed in the soleus. For example, fibers containing both types IIa and IIx MHCs have contractile velocities similar to those of fibers with only type IIa MHC in control rat muscles (9). Other studies have shown that if a muscle fiber contains two MHCs, then the contractile velocity will increase in linear proportion with the percentage of the faster MHC (17, 20). Unfortunately, the proportions of MHC in a fiber could not be determined with the immunohistochemical techniques used in the present study. Also, it was found that some fibers contained both types I and IIx MHCs, with or without type IIa, after HS; to date no study has reported the contractile properties of a fiber with such a MHC composition. However, it is expected that the types I and IIx MHC fibers would show an increased contractile velocity because fibers containing the type IIx isoform exclusively are associated with contractile velocities that are three times faster than those of fibers containing only the type I isoform (4). The increase in the proportion of fibers with faster MHCs would result in faster contractile properties at the whole muscle level (see Ref. 23 for a review). Thus the increase in contractile velocity of the slow soleus muscle after HS (23) appears to be highly correlated with the types of MHC expressed. Perspective. Unloading induced by either SF or HS resulted in the expression of type IIx MHC in both preexisting fast and slow fibers in the soleus muscle of rats. Thus the expression of type IIx MHC is not restricted to a particular fiber type in the unloaded rat soleus. These data strongly imply that load can be an important factor in regulating the MHC characteristics of a fiber. Further, the results of this study indicate that HS is an excellent ground-based model to study the adaptations of the myosin molecule to weightlessness.ACKNOWLEDGEMENTS
We thank V. Oganov and E. Ilyina-Kakueva for providing the muscle samples. We also thank Dr. S. Schiaffino (University of Padova, Italy) for the generous gift of MHC-specific MAbs.
FOOTNOTES
Address for reprint requests: R. J. Talmadge, Dept. of Physiological Science, Univ. of California, 2301 Life Science Bldg., 405 Hilgard Ave., Los Angeles, CA 90095-1527 (E-mail: bobt{at}lscomp.lifesci.ucla.edu).
Received 11 March 1996; accepted in final form 14 August 1996.
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B. Cormery, F. Pons, J.-F. Marini, and P. F. Gardiner Myosin heavy chains in fibers of TTX-paralyzed rat soleus and medial gastrocnemius muscles J Appl Physiol, January 1, 2000; 88(1): 66 - 76. [Abstract] [Full Text] [PDF]
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P. E. Mozdziak, P. M. Pulvermacher, and E. Schultz Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading J Appl Physiol, January 1, 2000; 88(1): 158 - 164. [Abstract] [Full Text] [PDF]
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L. Stevens, K. R. Sultan, H. Peuker, B. Gohlsch, Y. Mounier, and D. Pette Time-dependent changes in myosin heavy chain mRNA and protein isoforms in unloaded soleus muscle of rat Am J Physiol Cell Physiol, December 1, 1999; 277(6): C1044 - C1049. [Abstract] [Full Text] [PDF]
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A. Saitoh, T. Okumoto, H. Nakano, M. Wada, and S. Katsuta Age effect on expression of myosin heavy and light chain isoforms in suspended rat soleus muscle J Appl Physiol, May 1, 1999; 86(5): 1483 - 1489. [Abstract] [Full Text] [PDF]
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N. Cros, J. Muller, S. Bouju, G. Pietu, C. Jacquet, J. J. Leger, J.-F. Marini, and C. A. Dechesne Upregulation of M-creatine kinase and glyceraldehyde3-phosphate dehydrogenase: two markers of muscle disuse Am J Physiol Regulatory Integrative Comp Physiol, February 1, 1999; 276(2): R308 - R316. [Abstract] [Full Text] [PDF]
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V. J. Caiozzo, M. J. Baker, and K. M. Baldwin Novel transitions in MHC isoforms: separate and combined effects of thyroid hormone and mechanical unloading J Appl Physiol, December 1, 1998; 85(6): 2237 - 2248. [Abstract] [Full Text] [PDF]
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E. E. Dupont-Versteegden, J. D. Houle, C. M. Gurley, and C. A. Peterson Early changes in muscle fiber size and gene expression in response to spinal cord transection and exercise Am J Physiol Cell Physiol, October 1, 1998; 275(4): C1124 - C1133. [Abstract] [Full Text] [PDF]
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P. E. Mozdziak, M. L. Greaser, and E. Schultz Myogenin, MyoD, and myosin expression after pharmacologically and surgically induced hypertrophy J Appl Physiol, April 1, 1998; 84(4): 1359 - 1364. [Abstract] [Full Text] [PDF]
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P. C. Geiger, M. J. Cody, Y. S. Han, L. W. Hunter, W.-Z. Zhan, and G. C. Sieck Effects of hypothyroidism on maximum specific force in rat diaphragm muscle fibers J Appl Physiol, April 1, 2002; 92(4): 1506 - 1514. [Abstract] [Full Text] [PDF]
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