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Departments of 1 Anesthesiology and 2 Physiology and Biophysics, Mayo Foundation, Rochester, Minnesota 55905; and 3 Department of Zoology, University of Wisconsin, Madison, Wisconsin 53706-1381
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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In a previous study (J. G. Swallow, T. Garland, Jr., P. A. Carter, W.-Z. Zhan, and G. C. Sieck, J. Appl. Physiol. 84: 69-76, 1998), we found that in house mice both genetic selection (10 generations of artificial selection for high voluntary activity on running wheels) and access to running wheels (7-8 weeks) elicited a modest increase in maximal oxygen consumption. Based on these results, we hypothesized that genetic selection would affect the changes in endurance and oxidative capacity of the medial gastrocnemius (MG) muscle induced by wheel access (training response). Wheel access increased the isotonic endurance of the MG in both genetically selected and random-bred (control) mice. However, this exercise-induced improvement in isotonic endurance of the MG was similar between genetically selected and control mice. Wheel access also increased the succinate dehydrogenase activity of MG muscle fibers in both selected and control lines. However, this exercise-induced increase in succinate dehydrogenase activity was comparable between genetically selected and control animals. Taken together, these results indicate that the modest increase in maximal oxygen consumption associated with genetic selection is not reflected by the training-induced changes in oxidative capacity and endurance of MG muscle fibers.
artificial selection; wheel running; isotonic endurance; fatigability; oxidative capacity
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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IN A PREVIOUS STUDY (21), we proposed the use of laboratory house mice as a novel model to examine both genetic (10 generations of artificial selection for high voluntary activity on running wheels) and environmental effects (7-8 wk of access or no access to running wheels) on physiological activities. The genetically selected mice (20) were representatives of four replicate lines selectively bred for increased total wheel running (revolutions/day). Through 10 generations, the increased wheel running was accomplished mainly by an increase in average speed while running on wheels, rather than by an increase in the number of minutes per day during which any wheel activity occurred (20, 21). The base population was Hsd:ICR laboratory house mice, a widely available and commonly studied random-bred strain (4, 9). The Hsd:ICR strain shows genetically heritable individual variation in both voluntary wheel running (20) and measures of forced locomotor performance ability (6).
Consistent with previous studies of both mice and rats, we found (21)
that wheel access reduced body mass and increased maximum
O2 consumption
(
O2 max) during forced
treadmill exercise. We also found that mice from genetically selected
lines had higher O2 max
than random-bred control lines. The magnitude of effects on
O2 max was similar for
selection and wheel access: each caused a modest increase of ~6%
(21).
In the present study, we used a subset of the same individual mice to test for effects of genetic selection and endurance exercise (voluntary wheel running) on oxidative capacity [succinate dehydrogenase (SDH) activity] and endurance properties of the medial gastrocnemius (MG) muscle. The MG muscle was studied because it contains a mixture of fiber types (3, 25) and it is recruited during locomotion in rodents (14). We hypothesized that access to running wheels would increase isotonic endurance and enhance SDH activity of all fiber types in this hindlimb muscle. We also hypothesized that muscles of mice genetically selected for high wheel-running activity would exhibit a greater improvement in isotonic endurance and oxidative capacity compared with random-bred controls. To our knowledge, this is the first study of muscle function in lines of rodents selectively bred for high activity levels.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animal model. The male house mice (Mus domesticus) used in this study were sampled from an artificial-selection experiment for increased voluntary activity levels on running wheels, including four replicate selected lines and four randomly bred control lines (20). These mice were the result of 10 generations of within-family selection for total wheel-running activity. Mice in each generation were housed individually with access to running wheels and scored for activity over a 6-day period; selection was based on the total number of revolutions run on days 5 and 6. After 10 generations of selective breeding, during the eighth week of wheel access, mice in the present study averaged 17 m/min running speed. Total wheel-running activity in the selected lines was ~76% higher than in the nonselected lines (20).
Details of husbandry conditions for mice studied herein are described in our previous paper (21). Briefly, experiments were performed on four groups of individuals: 1) sedentary control (i.e., nonselected), 2) wheel-access control (i.e., nonselected), 3) sedentary selected, and 4) wheel-access selected. In both wheel-access groups, mice were placed individually, at 22 days of age, in normal housing cages attached to activity wheels of 35.7-cm diameter (1.12-m circumference), so that they had continuous access to wheels. The wheel-exposure period continued for ~8 wk before measurement ofO2 max
(21). Mice were then transported from Madison, WI, to Rochester, MN,
where housing conditions remained the same until termination, which
occurred during the next 2 wk (wheel access continued, but running was
not recorded during this period). In sedentary control and
sedentary-selected groups, four mice were group housed in normal cages
for the same period. All experimental procedures were approved by the
Institutional Animal Care and Use Committees at the University of
Wisconsin and at the Mayo Clinic and were in strict accordance with the
American Physiological Society Guiding Principles in the Care and Use
of Animals.
Muscle contractile and endurance properties.
Animals were anesthetized with ketamine (60 mg/kg im) and xylazine (2.5 mg/kg im), and the right and left MG muscles were excised: one was used
for contractile measurements and the other for
immunohistochemistry. To determine MG contractile properties, muscle
segments were vertically mounted in a glass tissue chamber that was
constantly perfused with Rees-Simpson solution [(in mM) 135 Na+, 5 K+, 2 Ca2+,
Mg2+, 120 Cl
, 25 HCO3 and 0.012 d-tubocurarine]
aerated with 95% O2-5% CO2 and maintained at
26°C (pH 7.4). The Achilles tendon of the MG muscle was glued onto a
thin plastic strip, which was attached to a force transducer and a
dual-mode length-force servo controller (Cambridge Technologies, model
300B). The other end of the MG muscle was fixed by using a surgical
clamp mounted in series to a micropositioner near the base of the
tissue chamber.
2) of the MG muscle at each load was
calculated as the product of isotonic load and shortening velocity, and
the load corresponding to maximum power output (Pmax) was determined.
Isotonic endurance of the MG muscle was assessed at a load
corresponding to Pmax (30% Po). Repetitive
shortening contractions at a constant load of 30% Po were
induced by stimulation at 100 Hz in trains of 350-ms duration repeated
each second. Changes in shortening velocity and power output were
determined. Isotonic endurance was defined as the time required for
Pmax to decline to zero (i.e., the time when the muscle
lost its ability to shorten).
Muscle histochemistry.
The MG muscle was stretched to 1.5 times resting excised muscle length
(approximating Lo) and rapidly frozen in isopentane cooled to its melting point by liquid nitrogen. In each muscle, serial
sections were cut at 10 µm by using a cryostat kept at 
20°C
(model 2800E Frigocut, Reichert-Jung) and reacted with mouse primary
antibodies against different myosin heavy chain (MHC) isoforms. Fibers
were classified as MHCSlow, MHC2A,
MHC2X, and MHC2B. In some cases, only a single
antibody was used, e.g., anti-MHCAll-2X (Schiaffino BF-35).
However, in most cases, pairs of mouse IgG or IgM primary antibodies
were used, e.g., anti-MHCSlow (Novocastra, IgG),
anti-MHC2A (Blau A4.74, IgG or Blau N1.551, IgM),
anti-MHC2B (Schiaffino BFF3, IgM), and
anti-MHCNeo (Novocastra). Primary antibodies were diluted
in PBS containing 0.5% bovine serum albumin (5 mg/ml) and applied to
the muscle section for ~2 h at room temperature. Slides were then
washed in PBS and reacted with Cy3- or Cy5-conjugated secondary
antibodies (goat anti-mouse IgG or goat anti-mouse IgM) for 45 min at
room temperature. The use of antibody pairs allowed for double labeling
of MHC isoform expression in the same section with minimal
cross-reactivity. This was confirmed by adding the opposite secondary
antibody to a section incubated with only IgG or IgM primary antibody.
Sections incubated with only the secondary antibodies served as
controls for nonspecific reactivity of all primary antibodies. The
slides were then washed in PBS and imaged with a Bio-Rad (MRC500/600)
confocal system mounted on an Olympus (BH2) microscope and equipped
with an Ar-Kr laser.
Muscle fiber SDH activity.
The quantitative histochemical procedure for measuring fiber SDH
activity has been previously described (2, 19). Briefly, the reduction
of nitroblue tetrazolium (NBT) to its diformazan (NBT-dfz) was used as
the reaction indicator. Four serial muscle sections were cut at 6-µm
thickness: two sections were reacted with succinate added to the
incubation medium, and two sections were reacted with succinate absent,
to control for nonspecific reduction of NBT. A single end-point
reaction time of 5 min was used, because the SDH reaction has been
shown to be linear within this time period (2, 19). The concentration
of NBT-dfz deposited within a fiber during the SDH reaction was
determined from optical density (OD) measurements by using the
Beer-Lambert
equation
![[NBT − dfz] = OD/<IT>kl</IT>](/content/vol87/issue6/fulltext/2326/img001.gif)
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1 · cm1), and
l is the path length for light absorbance (6 µm). From these
measurements, the maximum velocity of the SDH reaction was determined,
and the mean SDH activity of each fiber was expressed as millimoles
fumarate per liter tissue per minute. The SDH activities of ~125
fibers were analyzed in each muscle sample, representing at least 20 fibers of each MHC phenotype. The total SDH activity of all MG fiber
types combined was estimated by using the following calculation
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Statistical analysis.
Based on an a priori power analysis of data obtained in previous
studies (22, 23, 24), it was determined that seven animals (muscles)
per group were sufficient to detect a 20% change in isotonic endurance
(at P < 0.05) between experimental and control groups at a
level of 0.80. Similarly, based on an a priori power analysis of
previously obtained data on muscle fiber SDH activities (1, 18, 19,
24), it was determined that a minimum sample of 20 fibers of each MHC
phenotype was required in at least six animals to detect a 10% change
(at P < 0.05) between experimental and control groups at a
level of 0.80. A nested two-way analysis of covariance (ANCOVA) was
performed with type III sums of squares in the SAS general linear
models (GLM) procedure. Exercise group (sedentary vs.
wheel access) and linetype (selected vs. control) were the grouping
factors; replicate line was nested within linetype. The foregoing are
mixed models with both random (line) and fixed (linetype and exercise
group) factors. Therefore, effects of linetype were tested over the
mean squares of line-within-linetype, and effects of exercise group
were tested over the mean squares of the interaction of
exercise group × line-within-linetype. Omega squared
(
2) was also calculated as an index of the degree of
relationship between the control and treatment groups. For all
variables where the P value was <0.05, the values of
2 are listed. In all cases, statistical significance was
established at the 0.05 level. All data are presented as means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Average body mass was reduced in mice with wheel access compared with
sedentary animals, but genetically selected mice were not significantly
different from controls (Table 1). This result is the
same as reported previously for these mice when weighed ~2 wk earlier
(21). MG muscle mass was not significantly different across groups
(Table 1).
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Muscle contractile and endurance properties.
A summary of the isometric and isotonic contractile properties of the
MG muscle in the varying groups of mice is provided in Table
2. Neither genetic selection nor wheel
access significantly affected Pt, Po,
Vmax, or Pmax of the MG muscle. In
each group, the force-velocity relationship of the MG was hyperbolic,
and there was no significant difference in shortening velocity at the
varying load clamp levels among the four groups (Fig.
1A). Accordingly, in all groups,
Pmax of the MG muscle was generated at ~30% of
Po (Fig. 1B).
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Muscle fiber histochemistry and morphometry. Four different fiber types were found in the deep portion of the mouse MG muscle, as determined by immunoreactivity to specific antibodies for different MHC isoforms (Table 3). Generally, there was little evidence of coexpression of MHC isoforms in individual MG muscle fibers, although ~3% of fibers in sedentary mice coexpressed MHC2X and MHC2B isoforms. Overall, fibers expressing the MHC2A isoform (type IIa fibers) were the most prevalent in the deep portion of the mouse MG muscle (~42-48%). Fibers expressing the MHCSlow isoform (type I fibers) comprised ~14-21% of all fibers, whereas fibers expressing MHC2X (type IIx fibers) represented ~20-27%, and fibers expressing MHC2B (type IIb fibers) comprised ~8-18% of the total (Table 3). The proportionate composition of the four fiber types in the MG muscle was comparable across groups.
Type IIb MG fibers were the largest in all animal groups, with fiber CSA over twice as large as type I and IIa fibers (P < 0.05; Table 3 and Fig. 5). The CSA of type IIx fibers were intermediate, being larger than both type I and IIa fibers (P < 0.05) but smaller than type IIb fibers (P < 0.05; Table 3 and Fig. 5). The CSA of the four fiber types varied significantly in relation to either wheel access or genetic selection (Table 3).
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Muscle fiber SDH activity.
In all experimental groups, SDH activity was dependent on fiber type,
with the mean SDH activity being highest in type MHC2A and
MHC2X fibers and lowest in type MHC2B fibers
(P < 0.05; Table 3). Access to running wheels in both
genetically selected and control mice significantly enhanced SDH
activities of all four fiber types compared with the sedentary animals
(P < 0.05; Table 3 and Fig. 5). The mean SDH activity
across all fibers of MG muscle was also higher in the wheel-access
group than that in the sedentary group (increase by ~30% in both
control and selected mice; Fig. 6).
However, SDH activity did not differ between genetically selected and
control mice.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The major new finding of this study is that 10 generations of artificial selection for high levels of voluntary wheel-running activity do not affect fiber-type composition, SDH activity, contractile performance, or endurance of the MG muscle in house mice. Prolonged access to a running wheel (7-8 wk) did result in an increase in muscle fiber SDH activity and endurance of the MG muscle during repetitive isotonic activation. However, muscle adaptations to voluntary wheel running were similar between genetically selected and random-bred (control) mice, suggesting that "trainability" of the mouse MG muscle is not influenced by 10 generations of artificial selection.
Evidence of training effect in wheel-access mice.
We found that 7-8 wk of access to running wheels significantly
increased whole body
O2 max (21) and muscle
fiber SDH activity of both selected and random-bred mice. The
significant increases in
O2 max (6%) and SDH
activity (30%) confirm that an aerobic training effect was induced as
a result of access to running wheels. The relative differences in SDH
activities across fiber types in the mouse MG muscle (deep region) are
also in general agreement with our previous observations in the rat
diaphragm (17, 24) and rat MG (1) muscles. SDH activities are highest
in fibers expressing MHC2A and lowest in those expressing
MHC2B.
O2 max during forced
treadmill running (21). In addition, although we have not measured bout
lengths, other studies in both mice (15) and rats (16) reported that
wheel activity typically involves multiple but brief (i.e., <3 min)
bouts of running rather than the continuous running (up a grade) that
characterizes most treadmill training studies. Therefore, the
relatively low intensity and short duration of individual wheel-running
bouts probably explains the modest improvements observed in muscle
oxidative capacity.
The low intensity of wheel-running activity in house mice may also
explain why muscle mass and fiber size, as well as force-generating capacity (i.e., maximum isometric force), did not increase in wheel-access mice. These results are consistent with previous studies
in mouse hindlimb muscles containing a high percentage of fast-twitch
fibers (3, 10). Maximum power output (Table 2) also did not improve
with wheel running in our mice. There are no comparable data in the
literature for this measure of muscle performance.
Improved endurance of the MG muscle. The most obvious functional adaptation to voluntary wheel running in these mice was an increase in the endurance of the MG muscle during repetitive isotonic activation. We assessed endurance under isotonic conditions to provide a measure of fatigue that more closely approximated the physiological conditions associated with locomotor activity (i.e., changing muscle length). In previous studies of mice given access to running wheels, fatigue resistance (induced by repetitive isometric activation) of the predominantly slow-twitch soleus muscle was found to increase (10) or show no change (3, 8), whereas no change in fatigue resistance has been reported in muscles containing predominantly fast-twitch fibers (e.g., extensor digitorum longus, plantaris).
Muscle fatigue may result from an imbalance between energy supply and energy demand. The higher SDH activity in MG fibers of the wheel-access mice suggests an enhancement in the overall energy supply potential during contractile activity (17, 19). Therefore, it is likely that in the wheel-access mice the elevated oxidative capacity of muscle fibers contributed to the enhanced endurance of the MG muscle during repetitive activation.Muscle fiber-type composition. We studied the MG muscle because of its mixed fiber-type composition and its extensive involvement in locomotor activity. Fiber-type proportions in the deep region of the MG muscle in these mice, determined by MHC isoform expression (Table 3), are generally consistent with those reported previously for house mice of the same strain (unpublished observations), as determined by using standard histochemical procedures. We used antibodies specific to MHC isoforms to permit detection of MHC isoform coexpression (19, 26). However, there was minimal evidence of MHC isoform coexpression in MG fibers in wheel-access mice. In sedentary mice, a few fibers coexpressed MHC2X and MHC2B (~3%). It is important to note that, in the present study, there was no evidence suggesting that wheel-running-induced improvement in endurance of MG muscle was associated with alterations in muscle fiber-type composition.
Characterizing voluntary wheel activity in mice.
Rats given access to running wheels reportedly run at speeds close to
or exceeding that at which aerobic capacity is reached (16). It has
therefore been suggested that voluntary wheel running may involve a
preponderance of sprint-type activity rather than endurance activity.
Our results suggest that this idea may not apply to mice tested on
relatively large wheels. First, wheel access had no effect on peak
isometric force or maximum power, factors that theoretically should
influence locomotor speed (13). Second, wheel running elicited an
increase in muscle fiber oxidative capacity. Such adaptations typically
accompany increased muscle endurance, which we did find (Table 2).
Third, average running speed (during week 8) in the
wheel-access groups was ~17 m/min, which is only 50% of the speed at
which the mice reached
O2 max during forced
treadmill running (21). Finally, siblings of the mice studied here did
not differ in forced maximal sprint running speed (unpublished
observations). Consequently, we have no evidence to suggest that wheel
running by mice in the present study could be characterized as
sprinting. This points to the need for further studies of exercise and
muscle physiology in mice.
Effects of genetic selection.
The physiological factors that influence voluntary wheel-running
behavior are multifactorial. "Motivation" will determine how much
an animal runs but within the limits set by its physical abilities to
exercise. Our previous study (21) showed that both genetic selection
and wheel access increased
O2 max by
similar amounts (6%). In the present study, we found that genetic
selection history had no detectable effect on fiber-type composition,
oxidative capacity, or the contractile and endurance properties of the
MG muscle. This suggests that enhanced hindlimb muscle performance is
not the basis for the augmented total wheel-running activity observed
in house mice resulting from 10 generations of artificial selection.
Taken together, these results suggest that the modest increase in
O2 max associated with
genetic selection is not reflected by enhanced oxidative capacity or
improved endurance of the MG muscle.
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ACKNOWLEDGEMENTS |
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The authors express their thanks to Yun Hua Fang and Jon Sieck for their assistance with the histochemical procedures.
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FOOTNOTES |
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This research was supported by National Heart, Lung, and Blood Institute Grants HL-34817 and HL-37680 to G. C. Sieck and National Science Foundation Grants IBN-9111185, IBN-9157268, and IBN-9728434 to T. Garland.
Present address of P. A. Carter: Department of Zoology and Program in Biology, Washington State University, Pullman, WA 99164-4236.
Address for reprint requests and other correspondence: G. C. Sieck, Anesthesia Research, Mayo Clinic, Rochester, MN 55905 (E-mail: gcs{at}siecklab.mayo.edu).
Received 29 July 1998; accepted in final form 16 August 1999.
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TOP
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RESULTS
DISCUSSION
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 O. J. Kemi, J. P. Loennechen, U. Wisloff, and O. Ellingsen Intensity-controlled treadmill running in mice: cardiac and skeletal muscle hypertrophy J Appl Physiol, October 1, 2002; 93(4): 1301 - 1309. [Abstract] [Full Text] [PDF]
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C. L. Dumke, J. S. Rhodes, T. Garland Jr, E. Maslowski, J. G. Swallow, A. C. Wetter, and G. D. Cartee Genetic selection of mice for high voluntary wheel running: effect on skeletal muscle glucose uptake J Appl Physiol, September 1, 2001; 91(3): 1289 - 1297. [Abstract] [Full Text] [PDF]
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D. L. Allen, B. C. Harrison, A. Maass, M. L. Bell, W. C. Byrnes, and L. A. Leinwand Cardiac and skeletal muscle adaptations to voluntary wheel running in the mouse J Appl Physiol, May 1, 2001; 90(5): 1900 - 1908. [Abstract] [Full Text] [PDF]
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I. Girard and T. Garland Jr. Plasma corticosterone response to acute and chronic voluntary exercise in female house mice J Appl Physiol, April 1, 2002; 92(4): 1553 - 1561. [Abstract] [Full Text] [PDF]
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(for all fiber types)
(mean SDH activity × relative contribution to total muscle CSA).
Note that in both control and selected groups, total SDH activity of MG
increased in response to wheel access. * Significantly different
(P < 0.05) from sedentary rat in each group.