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Program in Physical Therapy, University of Minnesota, Minneapolis, Minnesota 55455
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ABSTRACT |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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This investigation compared how hindlimb
unweighting (HU) affected the contractile function of single soleus
muscle fibers from 12- and 30-mo-old Fischer 344 Brown Norway F1 Hybrid
rats. After 1 wk of HU, functional properties of single
permeabilized fibers were studied, and, subsequently, the fiber type
was established by myosin heavy chain (MHC) analysis. After HU, the
relative mass of soleus declined by 12 and 19% and the relative mass
of the gastrocnemius declined by 15 and 13% in 12- and 30-mo-old
animals, respectively. In 12-mo-old animals, the peak active force
(5.0 ± 0.2 ×10
4 vs. 3.8 ± 0.2 ×104 N) and the
peak specific tension (92 ± 4 vs. 78 ± 3 kN/m2) were significantly
reduced in the MHC type I fibers by 24 and 15%, respectively. In
30-mo-old animals, the peak active force declined by 40% (4.7 ± 0.2 ×104
vs. 2.8 ± 0. 3 ×104 N) and the peak
specific tension declined by 30% (79 ± 5 vs. 55 ± 4 kN/m2). The maximal unloaded
shortening velocity of the MHC type I fibers increased in 12-mo-old
animals (from 1.65 ± 0.12 to 2.59 ± 0.26 fiber lengths/s) and
in 30-mo-old animals (from 0.90 ± 0.09 to 1.50 ± 0.10 fiber
lengths/s) after HU. Collectively, these data suggest that
the effects of HU on single soleus skeletal muscle fiber function occur
in both age groups; however, the single MHC type I fibers from the
older animals show greater changes than do single MHC type I fibers
from younger animals.
contractile properties; inactivity; aging; fiber types; Fischer 344 Brown Norway F1 Hybrid
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INTRODUCTION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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AGING RESULTS in a progressive decrease in muscle
strength (22, 24). The decrease in muscle strength has
been in part attributed to age-related atrophy (18). The mechanisms
contributing to muscle atrophy with aging are not well understood;
however, there is evidence for loss of muscle fibers and
-motoneurons and a decline in single fiber cross-sectional area
(CSA) (3, 22-24). Significant improvements in muscle strength and
increases in muscle mass have been observed after high-intensity
exercise programs (11, 12). The observed muscle hypertrophy and
increases in muscle strength in the elderly suggest that skeletal
muscles from older populations maintain capabilities to adapt to an
exercise stimulus.
Many older individuals experience periods of imposed immobility or inactivity. The skeletal muscle alterations induced by the imposed inactivity are well documented in adult animal populations (1, 5, 7, 18, 21, 42). Animal models have been used to simulate the effects of decreases in physical activity on the musculoskeletal system (5, 7, 13, 38, 40-42). For example, decrements in physical activity have been induced in rats by limiting weight bearing via hindlimb unweighting (HU) (8, 12, 34, 40-42). HU of rats elicits many of the skeletal muscle alterations that occur after periods of imposed bed rest of humans, including skeletal muscle atrophy (8, 18, 21). In adult animal populations, the greatest inactivity-induced change occurs in the antigravity muscles such as the soleus, in which the fiber type composition is predominantly slow-twitch type I (13, 21, 42). The atrophy of the soleus in the adult animal population may result in a reduction in the muscle's ability to generate force [decline in peak active force, normalization of peak force to CSA, or peak specific tension (Po)] (8, 13). The decline in whole-muscle Po in the adult animal population can, in part, be explained by deficiencies at the single-fiber level (32).
Few studies have focused on the inactivity-induced alterations in muscle of older populations (2, 7). After a 2-wk period of HU in aged rats (30 mo of age), significant reductions in soleus muscle mass (40%) and plantaris muscle mass (29%) were observed (7). Recently, significant reductions in soleus relative muscle mass (20%) and single-soleus fiber Po (24%) were observed after 1 wk of HU in 30-mo-old rats (2). The results of the few studies suggest that muscles of older animals are very sensitive to inactivity and adapt to the removal of weight bearing. However, identification of the differences in inactivity-induced responses of 1 wk of inactivity in various age groups remains to be determined. In this study, it was hypothesized that inactivity-induced changes in single slow-twitch type I fibers would be greater in older animals than in single slow-twitch type I fibers in younger animals. To test this hypothesis, the HU animal model, single skeletal permeabilized fibers, and myosin heavy chain (MHC) determination by gel electrophoresis were utilized to evaluate fiber type-specific contractile alterations in 12- and 30-mo-old rats to 1 wk of inactivity.
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MATERIALS AND METHODS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Animal care and HU. An animal care protocol was approved by the Institutional Animal Care and Use Committee of the University of Minnesota and the guidelines of the National Institutes of Health. Fourteen 12-mo-old and fourteen 30-mo-old Fischer 344 Brown Norway F1 Hybrid rats were purchased (National Institute on Aging) and placed in individual cages, housed in an animal facility under a 12:12-h light-dark cycle at 20°C, and maintained on a diet of rodent chow and water ad libitum.
The rats were randomly assigned to the HU or control group. The HU animals (both age groups) were partially unweighted for 1 wk with a harness attached to the proximal two-thirds of the tail (2). The height of unweighting was adjusted such that the hindlimbs were not in contact with any supportive surface.Solutions for single permeabilized fibers.
The composition of all the solutions used in the single permeabilized
fiber experimental protocols was determined by a computer program (9).
The relaxing and activating solutions contained the following: 7.0 mM
EGTA, 5.4 mM MgCl2, 20 mM
imidazole (pH 7.0), 14.5 mM creatine phosphate, 4.7 mM ATP, 150 units/ml creatine phosphokinase, as well as
CaCl2 to achieve pCa
(
log[Ca2+]) 9.0 (relaxing solution) or pCa 4.5 (activating solution). All solutions
contained enough KCl to achieve an ionic strength of 180 mM and were
adjusted to pH 7.0 with KOH. The glycerinating solution contained (in
mM): 125 K-propionate, 2 EGTA, 4 ATP, 1 MgCl2, and 20 imidazole, (pH 7.0)
as well as 50% glycerol (vol/vol) and the protease inhibitor leupeptin
(0.1%, final concentration).
Muscle and single-fiber preparation.
After 1 wk of HU, the HU and control animals were weighed and then
anesthetized with pentobarbital sodium (35 mg/kg body wt ip). The
soleus, lateral and medial heads of the gastrocnemius, plantaris,
extensor digitorum longus, tibialis anterior, and heart muscles were
isolated, rinsed, trimmed free of excess fat and connective tissue, and
weighed. The soleus muscles of the left hindlimbs were then placed in
relaxing (4°C) solution, and small bundles (~50 fibers) were
isolated (source of slow-twitch type I fibers). The bundles were
stretched to approximately in situ length and tied to glass capillary
tubes with 4-0 surgical suture, placed in a glycerinating solution, and
stored at 20°C for up to 4 wk.
Single-fiber experimental protocol. On the day of the experiment, a permeabilized fiber bundle was placed in a dissection chamber containing relaxing solution (4°C), and a single soleus fiber was isolated and transferred to an experimental chamber containing relaxing solution. The temperature-controlled chamber was mounted to the stage of an inverted microscope. The chamber included a spring-mounted stainless-steel plate insert with three wells containing activating or relaxing solution. The bottom of each well was sealed with a glass coverslip, which allowed the mounted fiber to be transluminated for viewing. The fiber (2-3 mm long) was attached by tweezers between an isometric force transducer (model 403, Cambridge, Cambridge, MA; sensitivity 2 mV/mg) and an arm of a servo-controlled direct-current torque motor (model 300H, Cambridge) (2). The fiber was briefly bathed in relaxing solution containing 0.5% (wt/vol) Brij 58 (polyoxyethylene 20 cetyl ether; Sigma Chemical, St. Louis, MO) to inhibit any remaining sarcoplasmic reticulum Ca2+-uptake activity. Any fiber showing a high degree of striation nonuniformity or a damaged region was discarded.
Determination of fiber diameter and optimal length. The single soleus fiber was observed through a Nikon inverted microscope (×600) and with a Panasonic video camera system. The segment length was adjusted to a sarcomere spacing of 2.5 µm in relaxing solution with the use of a calibrated eyepiece. The fiber diameter was determined (from the calibrated eyepiece) as the average of three measurements made along the length of the fiber, and fiber CSA was calculated by assuming a circular cross section. Segment length was determined by moving the microscope stage with a micrometer such that the fiber segment moved across the visual field of the eyepiece. The segment length was determined directly from the micrometer displacement. The experiments were conducted at 15°C.
Determination of peak active force and
Po.
The outputs of the force and position transducers were amplified and
sent to an IBM-compatible microcomputer via a universal input-output
board (LabMaster, 100 kHz analog-to-digital converter) and to an
oscilloscope (Nicolet 310) for immediate examination. Data were
collected, analyzed online, and stored by customized software. The
baseline in relaxing solution was monitored (pCa 9.0), and the fiber
was activated by transfer into activating solution (pCa 4.5). Peak
active force (×104 N)
was determined in each fiber by computer subtraction of the baseline
force (pCa 9.0) from the peak active force (pCa 4.5) (2). The
Po
(kN/m2) was calculated from the
peak active force and fiber CSA.
Determination of maximal shortening velocity
(Vo).
Vo of individual
fibers was determined by the slack test method (2). Briefly, the fiber
was transferred to the activating solution and force was allowed to
develop. Slack (
L) was rapidly introduced into the fiber by movement (complete within 2 ms) of the
torque motor arm when the developed force reached a plateau. The fiber
was returned to relaxing solution. The time interval (t) between the instant at which
slack was introduced and when force started to redevelop was measured
directly from computerized program. The fiber was activated five
different times with five different slack distances.
Vo was determined
as the slope of the linear regression of
L against
t. The individual fiber was
eliminated from the data set if sarcomere nonuniformity developed
during the five trials or if peak active force declined by 20%.
Fiber type identification and MHC analysis.
After the physiological measurements, the MHC composition
of each fiber (2 mm) was determined by SDS-PAGE (15). The fibers were
solubilized in 10 µl of 1% SDS sample buffer (containing 6 mg/ml
EDTA, 0.06 M Tris, 1% SDS, 2 mg/ml bromophenol blue, 15% glycerol,
and 5% 
-mercaptoethanol), and stored at 80°C.
Approximately one-half (1-mm fiber segment) was loaded on gels
consisting of a 3% (wt/vol) acrylamide stacking gel and a 5%
separating gel for MHC analysis. Gels were stained according to the
procedures decribed by Giulian et al. (16). The fibers described in
this study contained only the slow type I MHC because these fibers comprise the majority of soleus fibers under both control and HU
experimental conditions (2, 14, 27, 32).
Statistical analysis.
Data are presented as means ± SE. Mean values were analyzed by
using an ANOVA with two factors. When a significant
F statistic was obtained, the Tukey
post hoc tests were used to locate significant differences among
groups. Statistical significance was accepted at
P 
0.05.
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RESULTS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Measurements of diameter, peak active force, Po, and Vo were made on single skeletal fibers isolated from the soleus after 1 wk of HU in 12- and 30-mo-old animals. All single fibers reported in this study were identified on SDS-PAGE as MHC type I fiber.
Effects of HU on muscle mass. After HU, the 12-mo-old rats displayed reductions between 10 and 26% in absolute muscle masses and 12-15% in relative muscle masses (muscle mass/body mass) for the soleus and gastrocnemius muscles (Table 1). In 30-mo-old animals, the absolute muscle masses displayed reductions between 10 and 24% for the soleus muscle and gastrocnemius muscle. The relative muscle masses decreased between 12 and 19% (Table 1).
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Effect of HU on single-fiber diameter and single-fiber
force-generating capacity.
The single soleus MHC type I fiber diameters, peak active force, and
Po are summarized in Table
2. In the 12-mo-old animals, no significant
reduction in the diameter of single soleus fibers was observed
(approaching signficance at P 0.11). The single-fiber peak active force decreased by 24%. The
Po declined by 15% after HU,
suggesting that the soleus MHC type I fibers had a reduced ability to
generate force per unit CSA.
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60 kN/m2 compared with only 22% of
the MHC type I fibers in the control group.
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Effect of HU on Vo.
Table 3 reports mean values ± SE for
the fiber Vo. In
12-mo-old animals, the mean
Vo of MHC type I
fibers from the soleus muscle from control group was 1.65 ± 0.12 fiber lengths (fl)/s (n = 34). After
HU, the mean Vo
was 56% faster than the control mean
(n = 39). In 30-mo-old animals, the
mean Vo of
control fibers expressing MHC type I from the soleus muscle was 0.90 ± 0.09 fl/s (n = 23). After HU,
the mean Vo was
66% faster than the control mean (n = 21). The increase observed in both age groups appeared to be the result
of a shift in Vo
distribution of single skeletal muscle fibers of HU animals (Fig.
2). For instance, in 12-mo-old animals 67%
of the MHC type I fibers displayed a
Vo 
1.81 fl/s after HU, whereas only 32% of the control groups had a
Vo 1.81 fl/s.
In 30-mo-old animals, after HU 23% of the MHC type I fibers displayed
a Vo 1.81 fl/s, whereas only 4% of the control group of MHC type
I fibers had a Vo 1.81 fl/s. Figure 3 shows representative slack tests for single fibers from control and HU animals of both age
groups.
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DISCUSSION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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In the present study, it was hypothesized that inactivity-induced changes would be greater in single skeletal MHC type I fibers from older animals compared with single skeletal MHC type I fibers from younger animals. The results report a significant decrease in soleus-to-body weight ratio, peak active force, and Po and an increase in Vo of MHC type I fibers from the soleus muscle in both groups of animals. The older animal population appears to be more susceptible to inactivity-induced alterations at the single-cell level.
Muscle mass, single-fiber diameter, and HU. The reduction in the soleus and gastrocnemius mass and a smaller reduction in the plantaris, extensor digitorum longus, and tibialis anterior mass noted in the present study are comparable with previous observations (1, 5, 8, 13, 18, 32, 35, 42). The soleus muscle declines in mass in both age groups; however, the gastrocnemius also demonstrates a decline in mass in both age groups with the extent of gastrocnemius muscle atrophy being greater in the younger animal (26%) than the older animal (17%). This study is consistent with reports of preferential atrophy in muscles that are predominantly "slow" (soleus) to a greater extent than in muscles that are predominantly "fast" (gastrocnemius) (1, 8, 13, 14, 18, 25, 26, 32). The preferential loss of muscle mass between the soleus and gastrocnemius muscles is probably a consequence of the soleus muscle's postural role and its high level of activation with weight bearing during standing compared with the gastrocnemius muscle (1).
The atrophic response observed in both the soleus and gastrocnemius muscles may in part be the result of the reduced working length of the muscles. During HU, the soleus and gastrocnemius muscles are in chronic plantar flexion (34, 35). The chronic plantar flexion that occurs with HU may provide a stimulus to maintain rates of protein synthesis and degradation, thus preserving muscle mass (34, 35). The present study notes a differential atrophy between the muscles that are responsible for ankle extension and the muscles responsible for ankle flexion. Although reductions in muscle mass are noted, the extensor digitorum longus and tibialis anterior (ankle flexors) are more resistant to atrophy than are the ankle extensors in both age groups. This differential response probably reflects the recruitment and the degree of the anatomic stretch-shortening patterns of the extensor digitorum longus and tibialis anterior during standing. The results suggest that the recruitment and stretch-shortening patterns are similar in both age groups. The reduction in soleus MHC type I fiber diameter observed after short-term inactivity in the older animals, which approaches significance in the younger animal, suggests that the individual cells adapt to the removal of weight bearing. Interestingly, the sensitivity to inactivity occurs in both age groups; however, the time course of HU-induced alterations appears to be different between the two age groups. The older animal appears to exhibit the single-cell alterations earlier. An understanding of the time course of change between the various age groups is critical to the development of appropriate countermeasures. The mechanism underlying the whole-muscle atrophy and the single-fiber atrophy may be related to fiber type composition of the muscle, a decreased total muscle protein, a decreased rate of synthesis, and an increased degradation of cell protein (5, 14, 17, 35, 38, 41, 43). Previous work indicates that in some muscles the primary loss of mass was the result of a change in the noncollagenous protein pool, whereas in other muscles a reduction in the collagenous protein pool was an additional contributing factor with removal of weight bearing (25). With aging, there is an increase in connective tissue content in the muscle, and this may have an influence on the extent and time course of the HU-induced muscle mass observed between the age groups (11, 12, 15).Force and HU. The 24% decline in single-soleus fiber peak active force and 15% decline in single-fiber Po reported in the present study from fibers isolated from the adult animals (12 mo old) after 1 wk of inactivity are consistent with McDonald and Fitts (27), who observed a reduction of 18%. The 40% decline in peak active force and the 30% decline in Po observed after HU in MHC type I fibers isolated from the older animals (30 mo old) in this study indicate a greater sensitivity to inactivity in the older animals than the adult animals.
The mechanism responsible for the decline in Po may be a decrease in soleus myofibrillar protein concentration or the number of cross bridges per CSA. Direct evidence for this possibility has been shown by alterations in protein content, single skeletal fiber stiffness, and alteration in myofibril density (17, 28, 29, 41).Fiber Vo and HU. Vo of limb skeletal muscle is hypothesized to be dependent on the rate of ATP hydrolysis by the actomyosin ATPase (4). This is supported by findings of a high correlation between Vo and myofibrillar ATPase in limb skeletal muscle (6, 27, 37). HU resulted in a shift in fiber Vo; however, the cellular basis for this functional change is unknown. All soleus fibers in this study expressed MHC type I and myosin light chains (MLC) slow 1 and slow 2 on SDS-PAGE. A portion of fibers in this study had HU-induced elevated Vo and the same myosin protein profile as did the control group. Reiser et al. (31, 33) have shown a strong quantitative correlation between Vo and MHC composition in single rabbit and chicken fibers. The observed increase of 66% in Vo of single fiber expressing MHC type I in older rats after 7 days of HU and the increase in adult animals of 56% is significant because previous observations in adult animals have shown increases of 29% in 2 wk and 45% in 1 wk (27). Although the mechanism responsible has not been identified, the elevated Vo does not appear to be due to undetected levels of fast MHC in single fibers classified as type I or to any changes in MLC composition.
However, the HU-induced elevation of Vo in MHC type I fibers may be related to an incomplete characterization of myosin subtypes. Evidence has been provided to indicate there are additional MHC isoforms in fibers that have been classically considered to be homogeneous with regard to their MHC isoform composition (19, 20, 36). One example is the heterogeneity of the slow MHC reported in mammalian skeletal muscle. Hughes et al. (20) report three isoforms of MHC type I in rat skeletal muscle that differ in their expression time course during development, and Fauteck and Kandarian (10) report two isoforms under different loading conditions. Therefore, it is possible that an HU-induced change occurs in the expression of the different MHC type I isoforms, causing an increased Vo in the soleus fibers. The alteration in the different MHC type I isoforms may also be the underlying mechanism for the significant reduction in Vo with age. The increased speed in MHC type I fibers could also be related to alterations in other thick and thin filament proteins or in cytoskeletal elements. However, the knowledge regarding HU-induced alterations in other myofilament and cytoskeletal proteins and their influence on shortening velocity is very limited. In summary, young adult and old adult age groups are sensitive to removal of weight bearing, with the older animal showing greater changes at the single-cell level. The cellular changes have the potential to limit function of the older animal.| |
ACKNOWLEDGEMENTS |
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This study was funded in part by the American Heart Association, Minnesota Affiliate; the Minnesota Medical Foundation; and a University of Minnesota Graduate School Grant-in-Aid.
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FOOTNOTES |
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Address for reprint requests: L. V. Thompson, Program in Physical Therapy, Univ. of Minnesota, UMHC Box 388, Minneapolis, MN 55455 (E-mail: Thomp067{at}tc.umn.edu).
Received 26 August 1997; accepted in final form 27 January 1998.
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REFERENCES |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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), type I
fibers from 12-mo-old control (
), type I fibers from 30-mo-old HU
animals (
), type I fibers from 12-mo-old HU animals (
), and type
IIa fibers from plantaris from 12-mo-old animals for comparison (
).
Vo was determined
from slope of fitted line and normalized for fiber length.
Vo values of
fibers represented in graph were 0.99 fl/s for 30-mo-old control, 1.80 fl/s for 12-mo-old control, 1.71 fl/s for 30-mo-old HU, 2.86 fl/s for
12-mo-old HU, and 4.59 fl/s for control type IIa.