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J Appl Physiol 99: 1494-1499, 2005. First published June 30, 2005; doi:10.1152/japplphysiol.00394.2005
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Effects of inactivity on fiber size and myonuclear number in rat soleus muscle

Hui Zhong,1 Roland R. Roy,1 Boonclaire Siengthai,2 and V. Reggie Edgerton1,2

1Brain Research Institute and 2Physiological Science Department, University of California, Los Angeles, California

Submitted 8 April 2005 ; accepted in final form 24 June 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The effects of short-term (4 days) and long-term (60 days) neuromuscular inactivity on myonuclear number, size, and myosin heavy chain (MHC) composition of isolated rat soleus fibers were determined using confocal microscopy and gel electrophoresis. Inactivity was produced via spinal cord isolation (SI), i.e., complete spinal cord transections at a midthoracic and a high sacral level and bilateral deafferentation between the transection sites. Compared with control, there was an increase in the percentage of fibers containing the faster MHC isoforms after 60, but not 4, days of SI. The mean sizes of type I and type I+IIa fibers were 41 and 27% and 66 and 56% smaller after 4 and 60 days of SI, respectively. Thus atrophy occurred earlier than the shift in myosin heavy chain (MHC) profile. The number of myonuclei was ~30% higher in type I than type I+IIa fibers in control soleus, but after 60 days of SI these values were similar. The number of myonuclei per millimeter in type I fibers was significantly lower than control after 60 days of SI, whereas there was no change in type I+IIa fibers. Thus myonuclei were eliminated from fibers containing only type I MHC. Because the magnitude of the loss of myonuclei was less than the level of atrophy, the myonuclear domains of both type I and type I+IIa fibers were significantly lower than control. Thus chronic (60 days) inactivity results in smaller, faster fibers that contain a higher than normal amount of DNA per unit of cytoplasm. The absence of activation of muscle fibers that are normally the most active (pure type I fibers) resulted in most, but not all, fibers expressing some fast MHC isoforms. The results also indicate that a loss of myonuclei is not a prerequisite for sustained muscle fiber atrophy.

spinal cord isolation; myosin heavy chain isoforms; fiber phenotypes; disuse; fiber cross-sectional area

ALTHOUGH IT IS CLEAR THAT activity plays an important role in modulating the size of skeletal muscle fibers (6, 13, 32), the degree that this modulation is dependent on activity remains obscure. Similarly, the role of activity in determining muscle fiber phenotype has not been well defined. Although the effects of "decreased" levels of skeletal muscle activity have been studied, the models used have been limited by representing changed, but not well-defined, absolute levels of activity (14, 41).

Our laboratory previously has shown that spinal cord isolation (SI), i.e., complete transections of the spinal cord at a midthoracic and a high sacral level plus bilateral deafferentation between the transection sites, results in almost complete inactivation of the hindlimb muscles in cats (28, 29) and rats (47). Because the contact between the nerve and muscle remains intact, this preparation has provided a model for defining the effects of activity-independent neural influences on the muscle and muscle fiber properties (15–18, 20, 28, 29, 33, 36, 46). Recently, our laboratory has identified some of the molecular mechanisms underlying the atrophic response to chronic inactivity and concluded that the rapid and marked atrophy associated with inactivity can be attributed to changes in transcription, translation, and protein degradation (17, 18).

Cellular-level mechanisms of adaptation also seem important in defining activity-dependent responsiveness. For example, skeletal muscle fibers are multinucleated cells offering the possibility for the regulation of protein expression not only by a differential change in the pattern and/or amount of gene expression but also by alterations in the total number of myonuclei available for gene expression. Several lines of evidence indicate that when muscle fibers hypertrophy, the number of myonuclei increases (2, 24, 35), and it is generally assumed that satellite cells provide a source for these new myonuclei. A chronic decrease in neuromuscular activity results in a decrease in fiber size and myonuclear number, and it appears that this occurs, at least in part, via apoptotic mechanisms (1, 3). Furthermore, it has been suggested that this modulation in the amount of available DNA might be a factor in regulating the cytoplasmic volume of muscle fibers in response to chronic changes in neuromuscular activity (13).

The purposes of the present study were to determine and compare the effects of short-term (4 days) and long-term (60 days) inactivity on the size, phenotype, and myonuclear domain, i.e., cytoplasmic volume/myonucleus (10, 19), of rat soleus single muscle fibers. On the basis of our laboratory's observations of a more rapid decrease in fiber size compared with myonuclear number (and thus a decrease in myonuclear domain) in the earlier stages of decreased activity (4, 26), we hypothesized that the mean size of the myonuclear domains would be reestablished to control values after a prolonged period of inactivity.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Experimental design and surgical procedures.   The muscles used in the present study were derived from a subset of animals used previously for different analyses (16). Adult female Sprague-Dawley rats (242 ± 2 g body wt) were assigned randomly to either a control or a SI group (n = 5/group per time point). The spinal cord of SI rats was completely transected at a midthoracic and a high sacral spinal cord level and then bilaterally deafferented between the two transection sites. The SI and animal care procedures have been detailed previously (16, 34). At either 4 (SI-4d) or 60 (SI-60d) days after SI surgery, the rats were weighed, anesthetized with pentobarbital sodium (100 mg/kg body wt ip), and decapitated. For the control group in the present study, we randomly selected five control rats across the time points. This procedure was deemed appropriate because there was no significant increase in mean body weight of these adult female rats across the time points (16). The soleus muscle was excised, cleaned of excess connective tissue and fat, and wet weighed. The muscles then were frozen at approximately their in situ physiological length in melting isopentane cooled in liquid nitrogen and stored at –70° C until further analysis.

Single-fiber isolation.   The fiber segments analyzed in the present study were isolated randomly from the proximal one-third of the muscle. Segments of single fibers were mechanically isolated from the muscle sample as described previously (2). Briefly, the muscle was thawed gradually in a –20°C freezer for 5 h, then placed in a –20°C 50% glycerol-50% low-calcium relaxing solution (12), and immediately transferred to a –5°C freezer where it was kept overnight. The following day, the muscle was transferred to a refrigerator at 4°C for 1 h, and then one end of the sample was pinned in a 100% low-calcium relaxing solution in a Sylgard-coated culture dish. Single-fiber segments (40–60/muscle; 3–6 mm in length) were mechanically isolated from each muscle sample with microdissection forceps under a dissection scope. Fibers were placed on gelatin-coated slides and stored at –5°C.

Confocal microscopy.   Slides were removed from the –5°C freezer, air-dried, and stained with acridine orange and propidium iodide as described previously (4). This combination was found to produce the best staining of, and contrast between, the cytoplasm and the myonuclei (Fig. 1). A Sarastro 2000 confocal microscope with an argon laser (Molecular Dynamics, Sunnyvale, CA) and calibrated measurement software (Silicon Graphics, Salt Lake City, UT) were used to analyze fiber cross-sectional area (CSA), myonuclear number, and myonuclear domain. First, the fibers were optically sectioned in steps of 1.0–1.5 µm to create a stack of images encompassing the entire fiber thickness. The stack of images was optically rotated orthogonal to the long axis of the fiber, and the CSA was measured for two different regions within each viewing field. Total myonuclear number was determined from these projections for the portion of the fiber in the field of view (field size ~173 x 173 µm). Sarcomere length was measured for 3 different sets of 10 consecutive sarcomeres to obtain an average sarcomere length for the field. The myonuclear domain for each region of the fiber was determined by multiplying the fiber CSA by the region length (173 µm) and then dividing by the number of myonuclei per field. For each fiber, three separate regions were selected randomly for analysis and then averaged to produce a single value for each fiber. In all cases, the fiber ends, and any fiber region containing fiber damage or adhering connective tissue, were omitted. To correct for differences in the state of stretch, values for myonuclei per millimeter and fiber CSA were multiplied by the average sarcomere length, and divided by 2.5 to normalize to a 2.5-µm sarcomere length.



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  		Fig. 1. Representative confocal microscope images of a fiber segment from a control (A), a 4-day spinal cord-isolated (SI-4d; B), and a 60-day spinal cord-isolated (SI-60d; C) rat. Fibers were stained with a combination of propidium iodide and acridine orange to label fiber myonuclei and cytoplasm. Scale square in the bottom left corner, 10 µm3.

 
Gel electrophoresis.   After confocal analysis, the fibers were destained and dehydrated in 50% ethanol, scraped off the slide, and placed into a microcentrifuge tube containing 12 µl of 1x electrophoresis sample buffer (21). Electrophoresis was carried out with a Bio-Rad (Richmond, CA) Mini-Protean II dual-slab electrophoresis unit essentially as described by Talmadge and Roy (40). The separating gel contained 8% acrylamide and 30% glycerol. The single-fiber samples were heated for 2 min at 80–90°C before being loaded. On each gel, the entire 12-µl sample was loaded in a single lane: in addition, a "standard" lane, i.e., a mixture of rat soleus and medial gastrocnemius homogenate, was loaded to facilitate the accurate determination of MHC isoform expression. Gels were run at 80 V for 24 h in an ice-packed cooler and then stained with rapid Coomassie (Diversified Biotech, Boston, MA). The percentage of each MHC isoform for each fiber was determined by quantitative densitometry of each band. Densitometry was performed using a IS-1000 Alph Innotech densitometer (San Leandro, CA), and the values for each isoform are reported as a percentage of total MHC expression. These procedures resulted in seven MHC combinations (Fig. 2). Fibers coexpressing both type I and type II MHCs were considered hybrid fibers.



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  		Fig. 2. Distribution of the fiber types sampled in the soleus from control, SI-4d, and SI-60d rats. Fiber types are based on myosin heavy chain isoform content. The total number of fibers for each type is shown at the top of each bar. Values are means ± SE. *Significantly different from control, P < 0.05. {dagger}Significantly different from SI-4d, P < 0.05.

 
Statistical analyses.   All values are reported as means ± SE unless indicated differently. For each muscle, a linear mixed model (multivariate ANOVA, SAS software PROC MIXED procedure; SAS Institute, Cary, NC) was used to compare the outcomes among the three study groups, the four types of fibers (only those that are observed in all 3 groups), and the difference among the three groups within each type of fiber. A linear mixed model two-way ANOVA was used to assess the main effects of MHC type and experimental group using an {alpha} level of 0.05 to determine statistical significance. Bonferroni's post hoc tests were used to determine differences between individual groups. Pearson product-moment correlations were calculated to determine the relationship between fiber CSA and myonuclear number within and among groups.


   RESULTS
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 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Body and muscle weights.   Mean initial body weights for the control and SI rats were 243 ± 2 and 241 ± 2 g, respectively. After 4 and 60 days of SI, mean body weight was 19 and 7% (P > 0.05) lower in SI than control rats (Table 1). Mean body weight was 15% greater in SI-60d than SI-4d rats, indicating that the SI rats were growing. After 4 and 60 days of SI, mean absolute soleus weights were 27 and 64% smaller than in control rats, whereas the relative weights (mg/kg) were 10 (P > 0.05) and 62% smaller. Mean absolute soleus weights were 49% smaller in SI-60d than SI-4d rats.


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  		Table 1. Body and soleus weights in control, SI-4d, and SI-60d rats

 
MHC composition of single fibers.   There were seven MHC combinations expressed in the soleus fibers. Only four of these combinations were observed in the control and SI-4d groups: ~90% pure type I and ~0.5 to 5% type I+IIa, I+IIa+IIx and IIa fibers in both groups. The SI-60d rats showed all seven MHC combinations: pure type I (37%), I+IIa (32%), I+IIa+IIx (10%), I+IIx (16%), IIa (2%), IIa+IIx (~1%), and IIx (~2%) fibers (Fig. 2). Thus the most dramatic adaptations in the SI-60d soleus muscles were a marked decrease in pure type I fibers, an increase in I+IIa, and a de novo appearance of fibers containing only IIx MHC. All of the hybrid fibers (fibers coexpressing type I and II MHC isoforms) in control and SI-4d rats contained >50% type I MHC. In the SI-60d group, however, 21% of the hybrid fibers contained <50% type I MHC.

Fiber CSA.   In control rats, the pure type I fibers were ~36% larger than the type I+IIa fibers (Fig. 3). The mean size of type I fibers was affected the most by inactivity: mean fiber CSA was decreased by 41 and 66% after 4 and 60 days of SI, respectively. Type I+IIa and I+IIa+IIx fibers were 55 and 63% smaller in SI-60d than control rats. Furthermore, type I fibers were 42% smaller in SI-60d than SI-4d rats. Interestingly, the mean size of each fiber type was reduced to ~750 µm2 after 60 days of inactivity.



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  		Fig. 3. Bar graphs of the mean (± SE) cross-sectional area for each fiber type in control, SI-4d, and SI-60d rats. *Significantly different from control, P < 0.05. {dagger}Significantly different from SI-4d, P < 0.05. {ddagger}Significantly different from type I fibers, P < 0.05.

 
Myonuclear number.   The mean myonuclear number of pure type I fibers in control rats was 130/mm of fiber length: this value was significantly higher than for all other fiber types (Fig. 4). Four days of inactivity had no significant effect on the myonuclear number in any fiber type. The number of myonuclei in pure type I fibers, however, was lower (25%) in SI-60d than control rats. In general, myonuclear number was similar across all fiber types after 60 days of SI.



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  		Fig. 4. Bar graphs of the mean (± SE) myonuclei per millimeter fiber length for each fiber type of control, SI-4d, and SI-60d rats. *Significantly different from control, P < 0.05. {ddagger}Significantly different from type I fibers, P < 0.05.

 
Myonuclear domain.   The mean myonuclear domain was similar in pure type I and I+IIa fibers of control rats (Fig. 5). Inactivity resulted in a decrease in the myonuclear domain of pure type I fibers after 4 (36%) and 60 (55%) days and of types I+IIa, (58%) and I+IIa+IIx (67%) fibers after 60 days of SI. After 60 days of inactivity, the mean myonuclear domain was similar among all fiber types. The overall large decreases in myonuclear domain reflect the rapid and more marked decreases in fiber size than in myonuclear number.



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  		Fig. 5. Bar graphs of the mean (± SE) myonuclear domain size for each fiber type of control, SI-4d, and SI-60d rats. *Significantly different from control, P < 0.05. {dagger}Significantly different from SI-4d, P < 0.05.

 
Interrelationships between fiber CSA and myonuclear number across groups.   There was a significant correlation between fiber size and myonuclear number (r = 0.49) for all fibers across all groups (Fig. 6). The correlation was similar for fibers from either only the control (r = 0.47) or only the SI-60d (r = 0.45) groups, but it was weaker for the fibers from only the SI-4d (r = 0.31) group. The lower correlation for the 4-day time point reflects the earlier onset of atrophy relative to the decrease in myonuclear number.



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  		Fig. 6. Relationship between fiber cross-sectional area and myonuclei per millimeter fiber length for all fibers sampled for the control (a), SI-4d (b), SI-60d (c), and across all (thick solid line) groups. The regression equations and correlation coefficients are the following: control: y = 0.022x + 75.233, r = 0.47; SI-4d: y = 0.022x + 93.146, r = 0.31; SI-60d: y = 0.049x + 56.011, r = 0.45; and overall: y = 0.021x + 83.928, r = 0.49. Inset: plot of mean ± SD values for the 3 groups.

 

   DISCUSSION
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 ABSTRACT
 METHODS
 RESULTS
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GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
More than two-thirds of slow muscle fiber mass can be attributed to activity-dependent factors.   SI eliminates the neurally mediated influences on the muscle that are dependent on activation to include the mechanical events associated with activation, while leaving intact the neurally mediated influences that are independent of activation, such as activity-independent neurotrophic influences. The SI model provides a baseline for activity-independent influences on skeletal muscle properties. The present study shows that, after a prolonged period of inactivity (60 days), the mean fundamental morphological property, i.e., CSA, of a slow fiber in a slow muscle was ~750 µm2, which is 34% of normal control levels. Thus it appears that the baseline size for rat soleus fibers after 60 days of inactivity is ~30% of normal. In other words, the normal physiological neural activity-dependent signals, and the associated activation of the muscle, imposed on a slow muscle fiber account for ~70% of its size.

Myonuclei number per millimeter fiber length is higher in slow than fast fibers in muscles of control animals.   We observed that fibers containing only slow MHC have significantly more myonuclei than fibers that contain some fast MHC isoforms in the soleus of control rats (Fig. 4). This observation is consistent with previous results from our laboratory; i.e., our group has reported an 18% (4) and 54% (43) higher myonuclei number in slow than fast fibers in male and female rats, respectively. Burleigh (8) also noted that myonuclear density was generally higher in rat and rabbit red (presumably slow) than white (presumably fast) muscles. Others have come to the same conclusion for the soleus and tibialis anterior muscles in rats (38), the anterior and posterior latissimus dorsi muscles in chickens (23), and the soleus muscle in cats (2). All of these data also are consistent with earlier work using stereological techniques that reported that the number of nuclei per unit volume of muscle fiber is higher in slow than fast fibers in the rat semitendinosus muscle (5). Thus there is a general consensus that slow fibers have more myonuclei per millimeter of fiber length.

These differences in nuclear density among the fiber types, both within and across muscles, may be associated with varying fiber properties. For example, slow fibers (and slow motor units) have a relatively high oxidative capacity and resistance to fatigue, and they are normally activated (recruited) before the recruitment of fast fibers (7, 30, 31). Some evidence indicates that the nature of the cytoplasmic proteins, specifically the amount of mitochondrial protein, is a strong determinant of muscle fiber nuclear number (43). When protein synthesis is expressed per unit of DNA, the rate of synthesis is comparable between slow and fast muscles, suggesting that the amount of protein synthesis per nucleus is similar among fiber types (25). The absolute protein synthesis and turnover rates, however, are higher in slow than fast fibers (22), consistent with the higher myonuclear density in slow than fast fibers. In effect, it appears that slow fibers have a smaller myonuclear domain, i.e., a smaller volume of cytoplasm regulated by a single myonucleus, than fast fibers, resulting in the total protein synthesis load on individual myonuclei in slow and fast fibers being similar. Thus assuming that the total metabolic cost of the protein load is a factor that drives protein expression, one might expect a greater number of myonuclei in slow than fast muscle fibers.

Myonuclear domain size is relatively constant among mammalian species.   The myonuclear domain size is quite similar across mammalian species, regardless of muscle weights. For instance, using the same procedures described in the present study, the mean myonuclear domain sizes (cytoplasmic volume/myonucleus) of slow fibers are as follows: ~7,500 µm3 in mouse soleus (39), ~18,000 µm3 in rat soleus (present study), ~14,000 µm3 in rat plantaris (35), ~21,000 µm3 in cat soleus (2), ~14,000 µm3 in cat plantaris (2), 25,000 µm3 in rhesus soleus (37), 11,000 µm3 in human soleus (27), and ~18,000 µm3 in human vastus lateralis (11). There is an approximately fourfold difference in the mean CSA of slow fibers across these same muscles, i.e., from ~600 µm2 in the mouse soleus (39) to ~2,700 µm2 in the human vastus lateralis (11). A recent study also reported a similar myonuclear domain size, i.e., ~15,000 µm3 for slow fibers in the rat diaphragm muscle (44). Thus, although mean fiber size differs by approximately fourfold, the myonuclear domain size differs by approximately threefold across this wide variety of species and by less than twofold in most species studied. Combined, these data suggest a common regulatory principle that links the amount of DNA and the amount of cytoplasm in skeletal muscle.

Significance of a lower rate of loss of myonuclei than cytoplasmic volume with chronic inactivity.   Combining the results of our laboratory's previous time course study (16) and the present study, it is clear that, with inactivity, muscle atrophy occurs earlier and at a higher rate than the loss of myonuclei. Recently, our laboratory reported marked decreases in both total RNA content and concentration in the soleus muscle after only 4 days of SI (17). Because total RNA consists of ~85% ribosomal RNA (a major component of the protein translational machinery), this observation most likely reflects a limitation in the capacity for protein translation and is consistent with the rapid decrease in fiber size observed in muscles of SI rats (Refs. 16, 33, present study). Haddad et al. (17) also observed an increase in DNA concentration and a decrease in DNA content after SI surgery. The increase in DNA concentration is consistent with the larger decrease in muscle fiber size relative to myonuclear number (thus decrease in myonuclear domain) seen in the present study (Figs. 3–5). The decrease in DNA content is consistent with the net loss in myonuclei observed in inactive muscles (Refs. 2, 45, present study), a process that appears to involve apoptotic mechanisms (1, 3). Haddad et al. also reported a progressive decrease in the whole protein-to-DNA ratio in the soleus muscle of SI rats, consistent with a decrease in fiber size and myonuclear domain. Combined, these findings provide a cellular basis for the adaptations in muscle size and myonuclei content in inactive skeletal muscles and indicate that pretranslational, translational, and, perhaps, posttranslational mechanisms may be involved.

MHC composition of hybrid fibers reflects whole muscle MHC adaptations.   A prevalent view is that hybrid fibers are transforming from one phenotype to another. On the basis of the staining pattern of a battery of MHC type-specific monoclonal antibodies, however, Talmadge et al. (42) reported that the rat soleus muscle was composed of ~80, 30, and 10% hybrid fibers 90, 180, and 360 days after a complete low-thoracic spinal cord transection. Moreover, Caiozzo et al. (9) showed the presence of fibers expressing more than one MHC (including hybrid fibers as defined in the present study) in a variety of muscles in control rats. These data suggest that hybrid fibers may not be merely transitional fibers but may represent an alternative homeostatic condition for prolonged periods. The presence of ~60% hybrid fibers after 60 days of inactivity is consistent with this view.

One question addressed in the present study is the extent that the proportion of MHC isoforms in individual fibers is activity dependent. We found that all hybrid fibers in the soleus of control and SI-4d rats contained >50% type I MHC. These data are consistent with previous results from the soleus of control rats (9). After 60 days of inactivity, however, ~20% of the hybrid fibers contained <50% type I MHC. Whether this trend would continue with more prolonged periods of inactivity is unknown. On the basis of the observation that after 90 days of SI the mRNA and protein levels for MHC I are reduced, respectively, to only ~4 and 16% of the total MHC pool (20), however, it is highly likely that the proportion of type I fibers will be lower than type II in the hybrid fibers at this time point and thereafter.

Summary and conclusions.   Chronic inactivity resulted in the rat soleus muscle fibers becoming smaller, becoming faster, and containing a higher than normal complement of DNA, i.e., having a smaller than normal myonuclear domain. The adaptations in fiber size were more rapid than those in myosin type and myonuclear number. The mean number of myonuclei per fiber was lower in SI than control rats, indicating that the adaptation of skeletal muscle fibers to neuromuscular inactivity is modulated not only by changes in the pattern and/or amount of gene expression among existing myonuclei, but also by alterations in the total number of myonuclei available for gene expression modulation. Because the decrease in fiber size preceded the decrease in myonuclei, it appears that the modulation in the pattern and/or amount of gene expression among existing myonuclei represented an early and more rapid adaptive strategy, whereas the modulation in the number of myonuclei represented a second and more chronic adaptive strategy.


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 ABSTRACT
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 GRANTS
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 REFERENCES
 
This research was supported by National Institute of Neurological Disorders and Stroke Grant NS-16333.


   ACKNOWLEDGMENTS
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 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
The authors thank Maynor Herrera for excellent care of the animals. Drs. Robert Elashoff and He-Jing Wang of the UCLA Biostatistics Department provided valuable statistical support for the data analyses.

A portion of this work was presented at the Annual Meeting of the Society of Neuroscience held on October 23–28, 1999 in Miami Beach, FL (48).


   FOOTNOTES
 

Address for reprint requests and other correspondence: R. R. Roy, Brain Research Institute, Univ. of California, Los Angeles, Box 951761, 1320 Gonda Neuroscience and Genetics Bldg., Los Angeles, CA 90095-1761 (e-mail: rrr{at}ucla.edu)

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. Section 1734 solely to indicate this fact.


   REFERENCES
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 DISCUSSION
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 ACKNOWLEDGMENTS
 REFERENCES
 

  1. Allen DL, Linderman JK, Roy RR, Bigbee AJ, Grindeland RE, Mukku V, and Edgerton VR. Apoptosis: a mechanism contributing to remodeling of skeletal muscle in response to hindlimb unweighting. Am J Physiol Cell Physiol 273: C579–C587, 1997.[Abstract/Free Full Text]
  2. Allen DL, Monke SR, Talmadge RJ, Roy RR, and Edgerton VR. Plasticity of myonuclear number in hypertrophied and atrophied mammalian skeletal muscle fibers. J Appl Physiol 78: 1969–1976, 1995.[Abstract/Free Full Text]
  3. Allen DL, Roy RR, and Edgerton VR. Myonuclear domains in muscle adaptation and disease. Muscle Nerve 22: 1350–1360, 1999.[CrossRef][Web of Science][Medline]
  4. Allen DL, Yasui W, Tanaka T, Ohira Y, Nagoaka S, Sekiguchi C, Hinds WE, Roy RR, and Edgerton VR. Myonuclear number and myosin heavy chain expression in rat soleus single muscle fibers after spaceflight. J Appl Physiol 81: 145–151, 1996.[Abstract/Free Full Text]
  5. Atherton GW and James NT. Stereological analysis of the number of nuclei in skeletal muscle fibres. Acta Anat (Basel) 107: 236–240, 1980.[Web of Science][Medline]
  6. Booth FW and Baldwin KM. Muscle plasticity: energy demand and supply processes. In: Handbook of Physiology. Exercise: Regulation and Integration of Multiple Systems. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 12, chapt. 24, p. 1075–1123.
  7. Burke RE, Levine DN, Salcman M, and Tsairis P. Motor units in cat soleus muscle: physiological, histochemical and morphological characteristics. J Physiol 238: 503–514, 1974.[Abstract/Free Full Text]
  8. Burleigh IG. Observations on the number of nuclei within the fibers of some red and white muscles. J Cell Sci 23: 269–284, 1977.[Abstract/Free Full Text]
  9. Caiozzo VJ, Baker MJ, Huang K, Chou H, Wu YZ, and Baldwin KM. Single-fiber myosin heavy chain polymorphism: how many patterns and what proportions? Am J Physiol Regul Integr Comp Physiol 285: R570–R580, 2003.[Abstract/Free Full Text]
  10. Cheek DB. The control of cell mass and replication. The DNA unit—a personal 20 year study. Early Hum Dev 12: 211–239, 1985.[CrossRef][Web of Science][Medline]
  11. Day MK, Allen DL, Mohajerani L, Greenisen MC, Roy RR, and Edgerton VR. Adaptations of human skeletal muscle fibers to spaceflight. J Gravit Physiol 2: P47–P50, 1995.[Medline]
  12. Donaldson SKB. Calcium-activated force-generating properties of mammalian skeletal muscle fibres: histochemically identified single peeled rabbit fibres. J Muscle Res Cell Motil 5: 593–612, 1984.[CrossRef][Web of Science][Medline]
  13. Edgerton VR and Roy RR. Regulation of skeletal muscle fiber size, shape, and function. J Biomech 24: S123–S133, 1992.[CrossRef]
  14. Edgerton VR and Roy RR. Neuromuscular adaptations to actual and simulated spaceflight. In: Handbook of Physiology. Environmental Physiology. Bethesda, MD: Am. Physiol. Soc., 1996, sect. 4, vol. I, chapt. 32, p. 721–764.
  15. Eldridge L and Mommaerts W. Ability of electrically silent nerves to specify fast and slow muscle characteristics. In: Plasticity of Muscle, edited by Pette D. Berlin: de Gruyter, 1980, p. 325–337.
  16. Grossman EJ, Roy RR, Talmadge RJ, Zhong H, and Edgerton VR. Effects of inactivity on myosin heavy chain composition and size of rat soleus fibers. Muscle Nerve 21: 375–389, 1998.[CrossRef][Web of Science][Medline]
  17. Haddad F, Roy RR, Zhong H, Edgerton VR, and Baldwin KM. Atrophy responses to muscle inactivity. I. Cellular markers of protein deficits. J Appl Physiol 95: 781–790, 2003.[Abstract/Free Full Text]
  18. Haddad F, Roy RR, Zhong H, Edgerton VR, and Baldwin KM. Atrophy responses to muscle inactivity. II. Molecular markers of protein deficits. J Appl Physiol 95: 791–802, 2003.[Abstract/Free Full Text]
  19. Hall ZW and Ralston E. Nuclear domains in muscle cells. Cell 59: 771–772, 1989.[CrossRef][Web of Science][Medline]
  20. Huey KA, Roy RR, Baldwin KM, and Edgerton VR. Temporal effects of inactivity on myosin heavy chain gene expression in rat slow muscle. Muscle Nerve 24: 517–526, 2001.[CrossRef][Web of Science][Medline]
  21. Laemmli UK. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227: 680–685, 1970.[CrossRef][Medline]
  22. Lewis SE, Kelly FJ, and Goldspink DF. Pre- and post-natal growth and protein turnover in smooth muscle, heart and slow- and fast-twitch skeletal muscles of the rat. Biochem J 217: 517–526, 1984.[Web of Science][Medline]
  23. Matthew CA and Moore MJ. Numbers of myonuclei and satellite cell nuclei in latissimus dorsi muscles of the chicken. Cell Tissue Res 248: 235–238, 1987.[CrossRef][Web of Science][Medline]
  24. McCall GE, Allen DL, Linderman JK, Grindeland RE, Roy RR, Mukku VR, and Edgerton VR. Maintenance of myonuclear domain size in rat soleus after overload and growth hormone/IGF-I treatment. J Appl Physiol 84: 1407–1412, 1998.[Abstract/Free Full Text]
  25. Millward DJ and Waterlow JC. Effect of nutrition on protein turnover in skeletal muscle. Fed Proc 37: 2283–2290, 1978.[Web of Science][Medline]
  26. Ohira Y, Jiang B, Roy RR, Oganov V, Ilyina-Kakurva E, Marini JF, and Edgerton VR. Rat soleus muscle fiber responses to 14 days of spaceflight and hindlimb suspension. J Appl Physiol 73, Suppl: 51S–57S, 1992.
  27. Ohira Y, Yoshinaga T, Ohara M, Nonaka I, Yoshioka T, Yamashita-Goto K, Shenkman BS, Kozlovskaya IB, Roy RR, and Edgerton VR. Myonuclear domain and myosin phenotype in human soleus after bed rest with or without loading. J Appl Physiol 87: 1776–1785, 1999.[Abstract/Free Full Text]
  28. Pierotti DJ, Roy RR, Bodine-Fowler SC, Hodgson JA, and Edgerton VR. Mechanical and morphological properties of chronically inactive cat tibialis anterior motor units. J Physiol 444: 175–192, 1991.[Abstract/Free Full Text]
  29. Pierotti DJ, Roy RR, Hodgson JA, and Edgerton VR. Level of independence of motor unit properties from neuromuscular activity. Muscle Nerve 17: 1324–1335, 1994.[CrossRef][Web of Science][Medline]
  30. Rivero JL, Talmadge RJ, and Edgerton VR. Fibre size and metabolic properties of myosin heavy chain-based fibre types in rat skeletal muscle. J Muscle Res Cell Motil 19: 733–742, 1998.[CrossRef][Web of Science][Medline]
  31. Rivero JL, Talmadge RJ, and Edgerton VR. Interrelationships of myofibrillar ATPase activity and metabolic properties of myosin heavy chain-based fibre types in rat skeletal muscle. Histochem Cell Biol 111: 277–287, 1999.[CrossRef][Web of Science][Medline]
  32. Roy RR, Baldwin KM, and Edgerton VR. The plasticity of skeletal muscle: effects of neuromuscular activity. In: Exercise and Sport Sciences Reviews, edited by Holloszy JO. Baltimore, MD: Williams & Wilkins, 1991, p. 269–312.
  33. Roy RR, Grossman EJ, Bekmezian A, Talmadge RJ, Kim JA, Zhong H, and Edgerton VR. Persistence of myosin heavy chain-based fiber types in innervated but silenced rat fast muscle. Muscle Nerve 23: 735–747, 2000.[CrossRef][Web of Science][Medline]
  34. Roy RR, Hodgson JA, Lauretz S, Pierotti DJ, Gayek RJ, and Edgerton VR. Chronic spinal cord injured cats: surgical procedures and management. Lab Anim Sci 42: 335–343, 1992.[Web of Science][Medline]
  35. Roy RR, Monke SR, Allen DL, and Edgerton VR. Modulation of myonuclear number in functionally overloaded and exercised plantaris fibers. J Appl Physiol 87: 634–642, 1999.[Abstract/Free Full Text]
  36. Roy RR, Zhong H, Hodgson JA, Grossman EJ, Siengthai B, Talmadge RJ, and Edgerton VR. Influences of electromechanical events in defining skeletal muscle properties. Muscle Nerve 26: 238–251, 2002.[CrossRef][Web of Science][Medline]
  37. Roy RR, Zhong H, Talmadge RJ, Bodine SC, Fanton JW, Kozlovskaya I, and Edgerton VR. Size and myonuclear domains in Rhesus soleus muscle fibers: short-term spaceflight. J Gravit Physiol 8: 49–56, 2001.[Medline]
  38. Schmalbruch H and Hellhammer U. The number of nuclei in adult rat muscles with special reference to satellite cells. Anat Rec 189: 169–176, 1977.[CrossRef][Medline]
  39. Suzuki I, Zhong H, Roy RR, Eldred E, and Edgerton VR. Myosin heavy chain composition, size and myonuclear number in mouse soleus muscle fibers. Soc Neurosci Abstr 167.8, 2001.
  40. Talmadge RJ and Roy RR. Electrophoretic separation of rat skeletal muscle myosin heavy-chain isoforms. J Appl Physiol 75: 2337–2340, 1993.[Abstract/Free Full Text]
  41. Talmadge RJ, Roy RR, Bodine-Fowler SC, Pierotti DJ, and Edgerton VR. Adaptations in myosin heavy chain profile in chronically unloaded muscles. Basic Appl Myol 5: 114–134, 1995.
  42. Talmadge RJ, Roy RR, and Edgerton VR. Persistence of hybrid fibers in rat soleus after spinal cord transection. Anat Rec 255: 188–201, 1999.[CrossRef][Medline]
  43. Tseng BS, Kasper CE, and Edgerton VR. Cytoplasm-to-myonucleus ratios and succinate dehydrogenase activities in adult rat slow and fast muscle fibers. Cell Tissue Res 275: 39–49, 1994.[CrossRef][Web of Science][Medline]
  44. Verheul AJ, Mantilla CB, Zhan WZ, Bernal M, Dekhuijzen PN, and Sieck GC. Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle. J Appl Physiol 97: 1715–1722, 2004.[Abstract/Free Full Text]
  45. Viguie CA, Lu DX, Huang SK, Rengen H, and Carlson BM. Quantitative study of the effects of long-term denervation on the extensor digitorum longus muscle of the rat. Anat Rec 248: 346–354, 1997.[CrossRef][Medline]
  46. Zhong H, Roy RR, Hodgson JA, Talmadge RJ, Grossman EJ, and Edgerton VR. Activity-independent neural influences on cat soleus motor unit phenotypes. Muscle Nerve 26: 252–264, 2002.[CrossRef][Web of Science][Medline]
  47. Zhong H, Roy RR, Kim SJ, Hodgson JA, and Edgerton VR. Is the spinal cord isolation (SI) a model of hindlimb muscle inactivity? Soc Neurosci Abstr 189.9, 2004.
  48. Zhong H, Roy RR, Siengthai B, Grossman EJ, Talmadge RJ, and Edgerton VR. Myonuclear number, size and myosin heavy chain content in rat soleus muscle fibers after inactivity. Soc Neurosci Abstr 25: 1148, 1999.



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