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Departments of 2 Kinesiology and Applied Physiology and 1 Molecular, Cellular, and Developmental Biology, University of Colorado at Boulder, Boulder, Colorado 80309; and 3 The Sports Medicine Research Institute and Human Performance Laboratory and 4 Division of Molecular Cardiovascular Biology, Department of Pediatrics, The Children's Hospital Research Foundation, Cincinnati, Ohio 45229
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The goal of this study was to
characterize the genetic contribution to both forced and voluntary
exercise performance and to determine whether performance in these two
paradigms is controlled by similar genetic influences. There
were marked strain differences in treadmill exercise performance, with
Swiss Webster (SW) and FVB/NJ mice showing elevated performance and
C57BL/6J animals showing decreased performance compared with all other
strains. There was no apparent relationship between treadmill
performance and voluntary wheel performance, with the exception of SW
mice, which demonstrated high performances on both the treadmill and the voluntary wheel. Numerous properties were measured to attempt to
understand the basis for these differences in exercise performance. DBA/1J and SW mice exhibited significantly greater cardiac
contractility than all other analyzed strains. Conversely, BALB/cByJ
mice exhibited significantly reduced cardiac contractility compared
with all other strains. Expression of molecular indicators of
hypertrophy (atrial natriuretic factor and 
-myosin heavy chain) was
significantly elevated in DBA/2J myocardium compared with all other
analyzed strains.
heredity; wheel running; treadmill
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INTRODUCTION |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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AEROBIC EXERCISE PERFORMANCE is widely accepted as a complex quantitative trait that can be used to assess cardiac and hemodynamic function. Twin studies have demonstrated a large genetic component to endurance exercise performance (9, 42), and this genetic component is thought to reflect the interaction of many genes that affect both the intrinsic exercise capacity of an organism and its adaptive capability in the face of a prolonged exercise stimulus (7-10, 23, 27, 36). Both forced and voluntary exercise paradigms are commonly used to evaluate exercise performance in laboratory animals (1, 5, 11-13, 17-21, 24, 28-31, 40, 43, 44, 48, 50), and, although the relative performances for these two paradigms must rely on many common variables (cardiac function, pulmonary function, peripheral vasculature, muscle perfusion, and musculoskeletal function), there are also factors that are potentially unique to each paradigm (psychological desire to run, fear of handling, shock avoidance, and pain perception).
Inbred mouse strains are the result of at least 20 generations of
brother-sister mating, and such breeding has been shown to result in
~97.5% homozygosity at any given locus (15, 33). Whereas the individual animals within a strain are virtually
genetically identical, comparison across inbred strains reveals that
there is genetic variation among inbred strains (4). In
fact, previous studies have revealed significant
heterogeneity among strains for many physiological and
psychological variables, including those that are both common and
unique to forced and voluntary exercise paradigms. More specifically,
strain differences have been observed in cardiac output, blood
pressure, blood glucose levels, hormonal levels, circulating
catecholamine levels, enzymatic activity, blood hematocrit, heart rate
response to handling, fear response, avoidance, emotional defecation,
and learning (3, 5, 6, 11, 14, 15, 17, 25, 30, 31, 46,
47). This variability in the face of genetic similarity provides
a model system for the study of the genetics of exercise performance. Such investigations have typically followed two paths: the "candidate gene approach" and the "selective breeding approach." For the first of these approaches, manipulation of a single gene has been shown
to have both positive and negative effects on exercise performance. For
example, transgenic mice with a 200-fold overexpression of the
2-adrenergic receptor demonstrated increased involuntary exercise performance in young animals (19). On the other
hand, mice null for specific skeletal myosin heavy chain (MHC) isoforms demonstrated reduced exercise capacity, as did mice overexpressing a
ventricular myosin regulatory light chain (2, 16).
Breeding studies in both rats and mice have demonstrated up to a 75%
increase in endurance exercise performance through the systematic
mating of animals selected for high-exercise capacity (13, 24,
28).
Strain differences in endurance exercise performance have been identified in rats and mice by using both treadmill-based and voluntary wheel-running-based exercise paradigms (6, 14, 30, 31, 48). Although treadmill and voluntary wheel performance have been compared in rats (29), no study to date has compared the performances of inbred mouse strains for these two exercise paradigms. The specific aim of this study, therefore, was threefold. First, to evaluate both forced and voluntary exercise performance in commonly used inbred mouse strains. Second, to assess cardiac and skeletal muscle morphology along with cardiac function across these inbred strains, and third, to attempt to link interstrain variability in exercise performance with interstrain variability in the above parameters. We demonstrate that, among the seven strains investigated, there are significant strain differences in both voluntary and forced exercise performance and that there is no significant correlation between the results for these two exercise paradigms. Analysis of cardiac structure, function, and gene expression also revealed significant strain differences, but these results did not account for the observed differences in either voluntary or forced exercise performance across strains.
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METHODS |
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Experimental animals. Seven inbred mouse strains were used for these studies: BALB/cByJ, C3H/HeJ, C57BL/6J, DBA/1J, DBA/2J, FVB/NJ (Jackson Laboratories) and Swiss Webster (SW) (Taconic Laboratories). We chose to characterize these strains because, according to the work of Atchley and Fitch (4), there is significant genetic heterogeneity across these seven strains and because these strains have a high frequency of use in scientific research. For instance, C57BL/6J mice account for 14% of all mice used in scientific studies, and, together, C57BL/6J, C3H/HeJ, BALB/cByJ, and DBA/2J account for 50% of all mouse strains used in scientific publications (15).
Experimental animals consisted of three groups of male mice from each strain. Animals in the first study group (n = 4-6 per strain) were used for treadmill exercise tests at 2-4 mo of age and voluntary wheel-running exercise at 5-6 mo. This group was also studied by transthoracic echocardiography at 8 mo of age. After echocardiography, mice were killed, and the heart and calf (gastrocnemius, plantaris, and soleus) were excised and frozen in liquid nitrogen-cooled isopentane. A separate group of mice (n = 6 per strain) was used for isolated work-performing heart preparations at 3 mo of age. All animals were subjected to a 12:12-h light-dark cycle and given water and standard rodent chow ad libitum. Animals were handled according to approved protocols of the University of Colorado.Treadmill exercise tests. Mice were exercised on an eight-lane treadmill with adjustable belt speed (0-50 m/min) as previously described (18). The treadmill apparatus was equipped with adjustable-amperage (0-2 mA) shock bars at the rear of the belt to stimulate each mouse to run, and an air gun was used as an additional stimulus when mice attempted to rest on the shock grid. A double-beam infrared photon detector located above the shock grid allowed for quantification of the number of shock stimuli received by each mouse. Over a 2-wk period, mice were acclimated to treadmill via three 15-min running sessions at a 7° incline [1) zero shock activation and 2 m/min belt speed; 2) mild shock stimulation and 5 m/min belt speed; and 3) high-shock stimulation and 15 m/min belt speed]. After acclimation, two exercise tests were performed: an endurance exercise tolerance test and an exercise stress test. The endurance exercise test consisted of a 30-min treadmill run at 20 m/min with a 7° incline. During the test, the number of beam breaks per minute was recorded and used as an indication of the ability of the mouse to sustain the required workload. Thus for this test, a high number of beam breaks per minute is indicative of poor performance. This test was used as an indicator of muscle endurance and the ability of the physiological systems to deal with a prolonged, constant exercise stimulus (39). Each mouse was tested three times, and the average values across all exercise sessions for each mouse were calculated.
After completion of the exercise tolerance tests, mice were subjected to a graded exercise stress test. Such tests consist of an incremental protocol with increasing workloads and are commonly used to screen for cardiovascular disease and evaluate cardiovascular fitness (39). This test began with a 7° incline and 20 m/min belt speed. Belt speed was then increased linearly by 1.5 m/min every 2 min until 45 m/min were reached. "Failure" was defined as the inability to continue regular treadmill running, despite the extra stimulus of pressurized nitrogen from an air gun. For both treadmill exercise tests, C3H/HeJ mice refused to run under the test conditions utilized and are, therefore, not included in the data analysis for these tests.Voluntary wheel exercise. Mice 5-6 mo of age were housed individually in a cage (47 × 26 × 14.5 cm) containing a wheel, for a 2-wk period. The voluntary wheel system has been previously described (1). Briefly, this system consists of an 11.5-cm-diameter wheel with a 5.0-cm-wide running surface (model 6208, Petsmart, Phoenix, AZ) equipped with a digital magnetic counter (model BC 600, Sigma Sport, Olney, IL) that is activated by wheel rotation. During the 2-wk period, running duration and distance were recorded daily for each animal. Twenty-four-hour wheel-running durations were recorded as hours and minutes, whereas running distance was recorded in kilometers.
RNA analysis.
Mice were weighed and killed after echocardiography by cervical
dislocation. Hearts were placed in phosphate-buffered saline to remove
all residual blood from the cardiac chambers and coronary vessels.
Hearts were then weighed, and left ventricles were excised and frozen
at 
80°C within 10 min of death. Total RNA was extracted from
left-ventricular myocardium with TRIzol Reagent (Sigma Chemical, St.
Louis, MO). mRNA levels of -MHC, atrial natriuretic factor (ANF),
and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were assessed by
slot blot by using previously described oligonucleotide probes
(45). -MHC and ANF mRNA levels were normalized to GAPDH.
Isolated heart preparations.
Isolated work-performing heart preparations were carried out as
previously described (21). Baseline measurements were
taken after the establishment of steady-state conditions. Hearts with rates under 400 beats/min were paced at that rate. Baseline
measurements were taken after the establishment of steady-state
conditions, and preload was altered over a range of cardiac work from
200 to 350 mmHg · ml
1 · min1 to
generate Starling curves for each heart.
Echocardiography. Mice underwent transthoracic echocardiography under light sedation at 8.5 mo of age as previously described (18). Mice were injected intraperitoneally immediately before imaging with successive 0.05- to 0.3-ml doses of 20 mg/ml Avertin until mild sedation was achieved. Chest hair was removed with depilatory cream (Nair, Carter-Wallace, NY), and each mouse was positioned on its stomach on a 1.25-cm-thick acoustic standoff pad. Electrocardiogram leads were fastened to the right front limb, left rear limb, and tail for monitoring heart rate during image acquisition. Images were produced with a Vingmed CFM800 (Vingmed, Horton, Norway) echocardiography machine with a pediatric 10-MHz wide-band annular array transducer operating at frequencies between 9 and 11 MHz. The following parameters were obtained by analysis of M-mode echocardiographic images: left-ventricular internal dimension, anterior and posterior wall dimension (PWD) during systole and diastole, left-ventricular ejection fraction, percent fractional shortening, anterior and posterior wall percent thickening, and posterior wall relaxation slope. Doppler images were also used to determine heart rate and the maximum velocities of blood ejected through the pulmonary valve, mitral valve, aortic valve, and tricuspid valve. Measurements from three consecutive cardiac cycles per animal were averaged.
Statistics.
All the data were analyzed with Statview 5.0 statistical software
(SAS Institute, Cary, NC). Results were analyzed with ANOVA combined
with the Fishers paired least significant difference post hoc test.
Simple correlation analyses were performed with strain mean values for
all possible dependent and independent variable combinations.
Statistical significance was set at P < 0.05. Two
estimates of heritability in the broad sense, the intraclass correlation (rI) and the coefficient of genetic
determination (g2), were calculated for both
voluntary wheel and treadmill exercise performance (15,
34). Of these variables, rI provides an
indication of the proportion of the total variation that may be
accounted for by genetic differences between the strains, whereas the
calculation of g2 provides a similar indication
but corrects for the doubling of genetic variance that occurs with
inbreeding (15). The rI and g2 were calculated as follows:
rI = (MSB MSW)/[MSB + (n 1)
MSW] and g2 = (MSB MSW)/[MSB + (2n 1) MSW], where MSB and
MSW are the between- and within-strain mean square,
respectively, and n is the number of animals per strain
(15). For tests with an unequal n across
strains, n was calculated with the formula n = (1/a 1) (N 
n
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Treadmill exercise tests. For the treadmill endurance test, there were three groups that were significantly different in performance (from best to worst): 1) FVB/NJ and SW; 2) BALB/cByJ, DBA/1J, and DBA/2J; and 3) C57BL/6J (Fig. 1A). C3H/HeJ animals would not run on the treadmill and are thus not included in this comparison. Overall, the range in performances for this test was ~20-fold, indicating that the relatively small genetic variability between these inbred strains can have a significant effect on this complex trait. For this test, broad-sense heritability estimates resulted in an rI of 0.90 and a g2 of 0.82.
For the treadmill-based exercise stress test, FVB/NJ mice performed significantly better than all other strains for this exercise stress test, whereas C57BL/6J mice showed significantly lower performance than all other strains (Fig. 1B). In fact, the mean FVB/NJ maximum speed was 75% greater than that achieved by the C57BL/6J animals. There was no difference in performance among BALB/cByJ, DBA/1J, DBA/2J, and SW. Overall, for both treadmill-based exercise tests, C57BL/6J mice did not perform as well as the other mouse strains. For the exercise stress test, broad-sense heritability estimates were rI = 0.64 and g2 = 0.47. Across strains, the correlation between the two treadmill-based exercise tests was 0.90 (P < 0.01) (see Table 1).Voluntary wheel exercise. SW and C57BL/6J showed significantly greater running duration compared with all other strains, whereas DBA/1J showed the lowest mean running duration (Fig. 2A). For average running distance, C57BL/6J mice showed the greatest levels followed by SW, FVB/NJ, BALB/cByJ, C3H/HeJ, DBA/2J, and DBA/1J (Fig. 2B). Analysis of average speed revealed a similar pattern, with C57BL/6J demonstrating the highest values and DBA/1J the lowest (Fig. 2C). Thus C57BL/6J consistently showed the highest level of voluntary wheel-running performance. This is in marked contrast to the involuntary treadmill exercise tests in which these animals demonstrated the lowest performances of all strains (except for C3H/HeJ, which did not run). We found no significant correlations between voluntary wheel performance and treadmill performance (Table 1 and Fig. 3). Overall, of the two treadmill tests, the endurance test seemed to be a better predictor of voluntary wheel performance (r = 0.41-0.68) than the stress test (r = 0.27-0.44) (Table 1). All in all, these findings indicate that an animal's performance under forced exercise conditions does not necessarily predict how that animal will perform in a voluntary setting. Broad-sense heritability estimates for voluntary wheel-running performance were rI = 0.59, 0.56, and 0.38 and g2 = 0.42, 0.39, and 0.24 for wheel-running duration, distance, and average speed, respectively. Across strains, the correlations between the different aspects of wheel running (duration, distance, and average speed) were all significant (P < 0.01) (See Table 1).
For the six strains with results for both the treadmill and the voluntary wheel (not including C3H/HeJ, which did not run on the treadmill), the overall average running speed on the voluntary wheel was 57.2% of the maximum speed achieved on the treadmill (17.3 m/min on the voluntary wheel vs. 32.0 m/min on the treadmill). This average speed on the voluntary wheel is similar to that previously reported by Zhan et al. (50) for house mice selectively bred for wheel-running activity (17 m/min), whereas the maximum treadmill speed is similar to the treadmill speed at maximal O2 consumption (
O2 max) reported by Swallow et al.
(43) for wheel-access selected house mice (35 m/min) but
lower than the maximum treadmill speed observed for these same mice (55 m/min). Across the strains evaluated in this study, voluntary wheel
average speed ranged from 36.5% of maximum treadmill speed for
BALB/cByJ to 106.9% of maximum treadmill speed for C57Bl/6J (Fig. 3).
In an attempt to assess the inbred strains' capacity to adapt in the
face of a prolonged endurance exercise program, we compared voluntary
wheel-running average speed at the start (mean of days 1,
2, and 3) and end (mean of days 12,
13, and 14) of the 2-wk wheel-running period.
Over the first 3 days of wheel access, SW animals showed a
significantly greater average speed than BALB/cByJ, DBA/1J, and DBA/2J
(Table 2). For the final 3 days of wheel
access, the SW and C3H/HeJ average speed was significantly greater than that of DBA/1J (Table 2). Comparison across the seven inbred strains
revealed that all strains increased their average running speed during
the 2-wk wheel-running period (Table 2). There was interstrain
variability in this response, with DBA/1J showing the lowest response
(31% increase in average speed) and BALB/cByJ showing the greatest
response (69% increase in average speed) (Table 2). These interstrain
differences, however, were not significant.
Cardiac and skeletal muscle mass. Previous selective breeding studies in rats and mice have shown a significant negative correlation between body mass and voluntary wheel-running performance, with high-activity animals showing reduced body mass compared with control animals (13, 28, 44). Across the seven inbred mouse strains analyzed, we found no significant differences in body mass (Table 3). We did, however, observe a significant positive correlation between body mass and voluntary wheel performance for both mean nightly distance and time run on the wheel (r = 0.84, P = 0.04 for body mass vs. both distance and time) and a weak correlation between body mass and average wheel speed (r = 0.73, P = 0.10) (Table 1). There was no significant correlation between body mass and treadmill exercise performance (Table 1).
We investigated heart mass-to-body mass ratios because an enlarged heart can be indicative of either pathological or physiological hypertrophy, which could result in decreased or increased ability, respectively, to respond to an exercise stress. For absolute heart mass, C3H/HeJ showed the lowest values, and SW showed the highest. When heart mass was expressed relative to body mass, C3H/HeJ animals exhibited the lowest value, and DBA/2J animals the highest value (Table 3). We also investigated calf mass as a potential correlate to exercise performance. For absolute calf mass, DBA/2J values were the lowest, and C57BL/6J the highest (Table 3). For relative calf mass, DBA/2J showed the lowest values, and BALB/cByJ demonstrated the highest values (Table 3). There were no significant correlations between heart or calf mass (absolute or relative) and exercise performance, either voluntary or forced (Table 1). There was a weak association between heart mass and wheel-running duration (r = 0.50) and between calf mass and voluntary wheel-running duration, distance, and average speed (r = 0.62, 0.58, and 0.51, respectively) (Table 1). With n = 7 strains analyzed, statistical power to find such relationships is low, and it is possible that, if we had analyzed a larger number of strains, these correlations would have been stronger.Cardiac gene expression.
Changes in gene expression, including genes normally expressed during
fetal development, have been shown to accompany changes in cardiac
geometry and function. To further characterize the diversity within the
inbred mouse strains, we investigated expression of two molecular
markers of pathological cardiac hypertrophy, ANF and -MHC. DBA/2J
mice expressed significantly greater levels of ANF mRNA than did all
other strains, whereas SW mice expressed significantly greater levels
compared with FVB/NJ, C57BL/6J, and BALB/cByJ animals (Fig.
4). DBA/2J mice also expressed
significantly more -MHC mRNA than C3H/HeJ, BALB/cByJ, and SW mice
(Fig. 4). Although both DBA/2J and SW mice expressed more ANF mRNA than did most other strains, SW mice expressed significantly less -MHC mRNA than DBA/2J mice did, suggesting that expression of these two
markers is not coordinate. ANF and -MHC mRNA levels were associated
with relative heart mass, but the correlations did not reach
statistical significance (r = 0.75, P = 0.08 and r = 0.67, P = 0.15 for ANF and
-MHC, respectively). We found no significant correlation between
either ANF or -MHC mRNA level and either forced or voluntary
exercise performance (Table 1).
Hemodynamic function.
To investigate isolated cardiac function in the absence of neurohumoral
signals, we performed isolated work-performing heart preparations on
the selected inbred strains. DBA/1J mice showed a significantly greater
rate of cardiac tension development (+dP/dtmax) than that of all other strains, whereas BALB/cByJ mice exhibited significantly lower +dP/dtmax values than all
other strains except C3H/HeJ mice (Fig.
5A). Overall, there was a
1.7-fold range of +dP/dtmax values from lowest
(BALB/cByJ) to highest (DBA/1J). The kinetics of cardiac relaxation
(dP/dtmin) were also significantly different
across the inbred mouse strains. DBA/1J mice had a significantly greater dP/dtmin than that of all other
strains, whereas BALB/cByJ mice exhibited significantly lower
dP/dtmin values than all other strains (Fig.
5A). Tau is commonly used as a relatively load-independent measure of diastolic function, with lower values reflecting enhanced cardiac function. For this variable, FVB/NJ mice demonstrated significantly lower values than C57BL/6 mice (Fig. 5B). We
did not find any significant correlations between these measures of cardiac function and either forced or voluntary exercise performance, although there did appear to be a weak association between tau and
treadmill performance that may have reached statistical significance with a larger number of analyzed strains (Table 1).
Transthoracic echocardiography.
Given the link between cardiac function and exercise performance,
transthoracic echocardiography was performed on the inbred strains to
assess numerous parameters. We found no significant difference among
strains for the following echocardiographic parameters: anterior wall
dimension, left-ventricular ejection fraction, fractional shortening,
and anterior wall percent thickening. For all other variables, there
were significant differences among strains. Specifically, DBA/2J
exhibited the greatest left-ventricular end-diastolic internal dimension, whereas BALB/cByJ mice had the highest diastolic PWD (Table
4). For posterior wall relaxation slope,
BALB/cByJ values were significantly lower than those seen for DBA/2J,
FVB/NJ, and SW (Table 4). There were also strain differences for aortic
valve velocity, with DBA/2J having the greatest value and BALB/cByJ the
lowest (Table 4). For heart rate, C57BL/6J exhibited the greatest
values, and BALB/cByJ exhibited the lowest values (Table 4). Overall,
C3H/HeJ and BALB/cByJ mice demonstrated reduced cardiac function
compared with the other analyzed strains, whereas DBA/2J mice tended to
exhibit the highest cardiac function. There was no significant
correlation among any of the measured echocardiography variables and
either treadmill or voluntary exercise performance, although there was
a weak correlation between heart rate and average running speed on the
voluntary wheel (Table 1) (P 
0.1). There also appeared to
be an association between end-diastolic PWD and treadmill performance
(r = 0.50-0.56) that may have been statistically significant with a larger number of analyzed strains (Table 1).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The ability to perform endurance exercise depends on the interaction of a large number of physiological systems, including the pulmonary, cardiovascular, musculoskeletal, and autonomic nervous systems. Together, these systems, along with water retention, evaporative cooling, and blood flow distribution, allow an organism to engage in prolonged work output while minimizing deviations from the body's equilibrium state. However, the response of both people and animals to a given exercise stress is quite variable, with some able to adapt quite easily to the increased work demands and others being less successful. In recent years, considerable interest has been generated in elucidating the genetic contribution to both the exercise response and the variability in this response. Studies in humans have identified genetic polymorphisms that may be associated with increased exercise performance (8, 22, 23, 37, 38). In animal models, researchers have employed both a candidate gene approach and a systematic breeding approach. For the first of these methods, the expression of a single gene is altered, and the effect on exercise performance is quantified. Breeding studies select for better performing animals and, over many generations, generate animal lines with marked increases in exercise performance (13, 24, 28, 44).
Despite genetic similarity, inbred mouse strains show significant heterogeneity for many physiological variables (3, 5, 6, 11, 14, 15, 17, 25, 30, 31, 46, 47). This variability combined with genetic similarity makes investigation of the genetics of exercise performance a much more tractable problem in the mouse than in the human. Furthermore, a better understanding of the variability inherent within the common strains of inbred laboratory mice is vital for the optimal design of experiments utilizing either of these two approaches for understanding the genetic contributions to exercise performance. In addition, as transgenic animal models become more frequently employed, awareness of this heterogeneity is important, as this variability can severely impact interpretation of the resulting transgenic phenotype.
Exercise performance. Across the inbred strains analyzed for this study, there were significant strain differences in exercise performance on both the treadmill and the voluntary wheel. Analysis of the treadmill exercise results and the voluntary wheel-running results, however, revealed no significant correlation between the performances of the mouse strains for these two exercise models, although our statistical power to find such a correlation was low. Of the two treadmill tests, performance on the endurance test was more correlated with wheel performance (r = 0.41-0.68) (Table 1) than was performance on the stress test (r = 0.27-0.44) (Table 1). This might be expected, as the running speed during the treadmill endurance test was more similar to the mean running speed seen on the voluntary wheel (20 vs. 17.3 m/min), whereas the speeds during the treadmill stress test (20-45 m/min) were significantly greater than those at which the animals preferred to run on the voluntary wheel. For both treadmill exercise tests, SW and FVB/NJ showed the best performances followed by DBA/1J. C57BL/6J mice were clearly outperformed by all other strains in both treadmill tests. With voluntary wheel running, SW and C57BL/6J performed the best, FVB midrange, and DBA/1J the worst. The lack of correlation between treadmill performance and voluntary wheel performance is most striking for the C57BL/6J animals, which showed the poorest performance on the treadmill yet ran the farthest with the highest average speed on the voluntary wheel. Overall, this lack of correlation in performance for the two exercise paradigms is in agreement with previous data demonstrating that, in rats, treadmill exercise performance did not predict subsequent voluntary wheel-running performance (29).
The high performance of the SW and FVB/NJ animals in the exercise tests is in agreement with previous data showing that SW mice have a higher resting O2 consumption than do C3H/HeJ and C57BL/6J (35). Resting O2 consumption has recently been shown to have a positive genetic correlation withO2 max in mice (12);
therefore, it may be predicted that SW mice also have a relatively high
O2 max. SW animals have also been shown
to have a greater open-field activity than DBA/1J, DBA/2J, C3H/HeJ, and
BALB/cByJ (41), whereas FVB open-field activity has been
shown to be greater than that of DBA/2J, BALB/cByJ, and C3H/HeJ
(31). C3H/HeJ mice consistently performed poorly compared with most other strains for voluntary wheel running (third worst running duration and distance) and refused to run on the treadmill. It
has been reported that C3H/HeJ mice carry the rd (retinal
degeneration) gene and that the mice are blind by 6 wk of age
(32). The loss of sight could adversely affect motor
behavior and thus lead to decreased basal activity and exercise test
performance. Poor visual acuity might also result in an increased fear
of handling and a greater stress response in this strain when subjected
to the treadmill apparatus, and these factors could contribute to the refusal to run on the treadmill by these animals.
Cardiac structure and function.
Analysis of cardiac structure and function revealed that DBA/2J and SW
mice demonstrated high relative cardiac mass, high cardiac function,
and high levels of ANF and -MHC mRNA. At the other end of the
spectrum, C3H/HeJ animals were found to have smaller hearts, lower
function, and relatively low levels of -MHC and ANF mRNA. Moreover,
echocardiographic evidence showed greater diastolic and systolic
ventricular wall thickness in C3H/HeJ compared with all other strains.
These data indicate that C3H/HeJ mice exhibit relatively impaired
fractional shortening compared with other strains and abnormally thick
left ventricles that could be indicative of decreased compliance and,
therefore, impaired contraction and relaxation.
Determinants of endurance exercise performance. We have demonstrated that, among these seven strains of inbred mice, there are significant strain differences in endurance exercise performance for both voluntary and forced exercise paradigms and that there is little correlation between voluntary exercise performance and forced treadmill exercise performance. Although exercise performance, whether forced or voluntary, is a complex trait influenced by the interactions of many genetic factors, we were interested in the relationship of exercise performance to the physiological measures quantified in this study.
Regression analysis revealed a significant positive correlation between body mass and voluntary wheel-running duration and distance (r = 0.84, P < 0.05) (Table 3). This positive correlation is in contrast to previous selective breeding studies that have demonstrated a negative correlation between body mass and voluntary wheel performance (13, 28, 44). It is important to note, however, that there is an inherent difference between comparing across inbred strains (this study) and selective breeding to enhance exercise performance. In the latter case, previous evidence would suggest that, within a more genetically heterogeneous population, reduced body mass is beneficial to endurance exercise performance. In the present study, we found that strains with a higher mean body mass exhibited higher levels of voluntary wheel performance. Given the work of Garland et al. and others (13, 28, 44), however, it is possible that crossbreeding of high-activity strains would result in a reduction in body mass in the animals of future generations. It is well established that endurance exercise training results in a physiological hypertrophy of the myocardium (26), and thus it might be predicted that a mouse strain with a greater intrinsic relative heart mass would have an inherent advantage over a mouse strain with a lower intrinsic relative cardiac mass (assuming that the differences in mass were not due to a pathological hypertrophy). Although there were significant interstrain differences in cardiac mass, both absolute and relative, we did not observe any correlation between cardiac mass and either forced or voluntary endurance exercise performance. There was a weak relationship between heart mass and wheel-running duration (r = 0.5, P = 0.3), and, if we exclude DBA/2J from the analysis, the correlation is much stronger (r = 0.85, P = 0.07). The DBA/2J myocardium is susceptible to cardiac calcification (49), and we did observe this phenomenon in our animals. Such calcification could increase the weight of the myocardium while at the same time adversely affecting exercise performance. This scenario seems possible as the DBA/2J animals exhibited the second highest absolute heart mass and the highest relative heart mass (Table 3) but had only average exercise performances (Figs. 1 and 2). Similarly, we did not observe any significant correlations between either absolute or relative skeletal muscle mass and exercise performance. There appeared to be a relationship between calf mass and wheel-running performance (r = 0.51-0.62), and, with a greater number of analyzed strains, the results may have reached statistical significance. Overall, however, the results are in agreement with the work of Zhan et al. (50), who demonstrated no correlation between gastrocnemius mass and voluntary activity in selectively bred house mice. We did not find any significant correlation between exercise performance and cardiac gene expression or cardiac function. There were significant strain differences in the level of expression of both ANF and-MHC, but
there was no apparent link between the level of expression of these
markers of cardiac hypertrophy and exercise performance. Cardiac
function data analysis also revealed significant strain differences for many variables, but there were no significant correlations between these variables and exercise performance, other than a weak correlation between heart rate and average running speed on the voluntary wheel.
There was a tendency for both the kinetics of cardiac relaxation (tau)
and end-diastolic PWD to be correlated with treadmill performance (Table 1), and these relationships may have been significant with a
larger number of analyzed strains. One important caveat to the
echocardiography data is that our measures of cardiac function were
recorded under sedation and, therefore, may not reflect the actual
cardiac performance during the particular exercise tasks. The
echocardiographic data are further complicated by the fact that,
because of logistical issues, there was a time gap of several months
between when the animals were subjected to the treadmill exercise
testing (~4 mo of age) and when the echocardiographic analysis was
performed (~8 mo of age). Another limitation to our cardiac function
data is that we did not assess cardiac output, which has been
positively correlated with treadmill performance across 11 inbred rat
strains (5). This evidence, combined with our data,
suggests that exercise performance is more likely to be associated with
an organ-level measure of cardiac function than it is with the
individual components that act in concert to determine cardiac function.
Overall, our results reinforce the notion that exercise performance is
a highly complex trait with a multivariant interaction among cardiac
function, lung function, oxygen delivery, and muscle contraction.
Furthermore, each of these components is probably affected by the
expression of multiple interacting genes and by the interaction of less
quantifiable variables, such as motivation and desire. There are many
possible combinations of these factors, each with its own potentially
unique effect on exercise performance. Our findings of significant
strain differences in specific aspects of cardiac structure, function,
and gene expression, combined with strain differences in two models of
endurance exercise performance, provide the groundwork for a more
comprehensive assessment of the genetics of exercise performance
through crossbreeding experiments designed to further elucidate the
individual contributions of these various parameters to the complex,
polygenic trait that is exercise performance.
| |
ACKNOWLEDGEMENTS |
|---|
The authors thank Dr. Alexander Maass for help with the isolation and analysis of cardiac tissue and Dr. J. Timothy Lightfoot for valuable consultation regarding broad-sense heritability estimates.
| |
FOOTNOTES |
|---|
* I. Lerman and B. C. Harrison contributed equally to this work.
This project was supported by National Institute of General Medical Sciences Grant GM-29090 (to L. A. Leinwand). D. L. Allen was supported by a grant from the Muscular Dystrophy Association.
Address for reprint requests and other correspondence: L. A. Leinwand, Dept. of MCD Biology, Campus Box 347, Univ. of Colorado, Boulder, Boulder, CO 80309 (E-mail: Leslie.Leinwand{at}Colorado.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.
10.1152/japplphysiol.01045.2001
Received 17 October 2001; accepted in final form 13 December 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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 A. MAASS, J.P. KONHILAS, B.L. STAUFFER, and L.A. LEINWAND From Sarcomeric Mutations to Heart Disease: Understanding Familial Hypertrophic Cardiomyopathy Cold Spring Harb Symp Quant Biol, January 1, 2002; 67(0): 409 - 416. [Abstract] [PDF]
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