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Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, Colorado
Submitted 28 January 2005 ; accepted in final form 24 February 2005
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ABSTRACT |
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-galactosidase. Phosphorylation of glycogen synthase kinase-3, protein kinase B (Akt), and p70 S6 kinase was increased only in the hearts of the NR group, consistent with the significant increase in cardiac mass. In conclusion, unloaded and loaded cage wheel exercise have a differential impact on cage wheel performance and muscle (cardiac and skeletal) adaptation.
cardiac adaptation; cardiomyocyte signaling
In addition, resistance (strength) training has gained acceptance as an effective therapeutic intervention to attenuate the age-mediated decline in muscle strength (13). Despite studies showing that exercise training under an increased load leads to an increase in skeletal muscle mass (8, 22, 39), the impact of resistance training on cardiac morphology and function is unclear, and conflicting reports exist. In addition, very little is known about the signaling pathways that mediate cardiac and skeletal muscle adaptation to resistance training. For example, some studies showed that resistance training increases left ventricular (LV) cavity dimensions, septal and posterior wall thickness, and cardiac mass (12, 19); other studies in short- and long-term resistance-trained athletes demonstrated no changes in LV morphology or mass (19, 20). The inconsistency of these data probably reflects the fact that the cardiac response to resistance exercise is likely to depend on the duration (long term vs. acute) and type (bodybuilding, power lifting, or high intensity) of training regimen. Moreover, although rodent models of resistance training have been used for the study of skeletal muscle adaptation (8, 22, 39), the majority of published studies examining the impact of resistance training on cardiac adaptation have focused on human subjects, and of course there is human genetic variability. Thus the impact of exercise training under varying loads and intensities on cardiac and skeletal muscle adaptation needs to be further investigated in inbred mouse strains.
In this study, we used the C57Bl/6 mouse strain to determine the impact of different load-bearing voluntary training regimens on cardiac and skeletal muscle adaptation. This type of exercise training mostly involves low to moderate resistances with higher repetitions and, thus, can be characterized as resistance-endurance training. We chose to impose a known load on a voluntary running wheel, rather than alternative resistance models, for two reasons: 1) our laboratory has previously used the voluntary cage wheel paradigm to demonstrate cardiac and skeletal muscle adaptation to exercise in the mouse (1, 17, 25), and 2) the voluntary cage wheel eliminates physical and psychological stressors associated with forced exercise paradigms. As a major component of this study, we wished to explore the signaling pathways that underlie the cardiac adaptation in response to loaded and unloaded exercise. Previous studies suggested that the signaling pathways leading to pathological cardiac adaptation are distinct from those leading to exercise cardiac adaptation (11, 32, 35, 36, 47, 54, 57, 59). For example, transgenic overexpression of calcineurin induced pathologically enlarged hearts that transition to heart failure (36, 58), whereas downstream targets of calcineurin were not activated after two different exercise (physiological) regimens (57). Similarly, myocyte enhancer factor-2 (MEF-2) transcription factor was strongly activated in pathological (31), but not physiological (25), models of cardiac hypertrophy. Considering that cardiac disease (pathological) and voluntary cage wheel exercise (physiological) activate distinct pathways in the cardiac myocyte, we were interested in whether loaded voluntary exercise would operate through similar signaling components or represent a third, independent signaling pathway. We hypothesized that unloaded and loaded exercise would elicit unique responses in the mouse as determined by exercise performance (distance, duration, speed, work, and power), cardiac and skeletal muscle gravimetric analysis, and molecular and biochemical adaptations in the heart.
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METHODS |
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12 wk) for all groups, with no differences between experimental groups. Each group continued running under varying resistance, depending on experimental group, for 50 days until the animals were 132.6 ± 1.3 days (19 wk) of age. The resistance protocols for each group are illustrated in Fig. 1. The load on the wheel was determined by hanging known weights on the wheel until the wheel was slightly displaced. All exercise groups began with no load on the cage wheel for the first week (week 1). However, the "no-load" condition was actually 2 g, which was determined as the load necessary to maintain wheel inertia and frictional load.
Considering a wheel acclimatization period of 1 wk (1, 25), we changed wheel loads at 1-wk intervals, except for higher loads, which we changed after 2 wk. The load on the wheel for each experimental group was as follows (Fig. 1): week 1 at no load (all groups); week 2 at 4 g (LR and HR); week 3 at 5 g (LR and HR); weeks 4 and 5 at 7 g (HR); weeks 6 and 7 at 9 g and then, finally, 12 g (HR). The exposure at 12 g in the HR group was maintained for the final 4 days of the exercise protocol; the LR group was maintained at a load of 5 g for the remaining protocol duration. Exercised and sedentary control animals were euthanized by cervical dislocation under inhaled anesthesia immediately after the end of the specific exercise period. Body mass was measured, and hearts were rapidly excised and washed with a modified ice-cold PBS solution (in mmol/l: 136.9 NaCl, 3.35 KCl, 12 NaH2PO4, and 1.84 KH2PO4, pH 7.4). Hearts were dissected longitudinally in half (to include equal amounts of the LV and right ventricle) and frozen in isopentane cooled with liquid nitrogen or used fresh for biochemical assays.
Skeletal muscle immunohistochemistry and fiber cross-sectional area. Skeletal muscles (soleus, plantaris, gastrocnemius, and tibialis anterior) were dissected and weighed. Muscles were then embedded in Tissue-Tek and snap frozen in liquid nitrogen-cooled isopentane. Frozen sections (8 µm) were cut and stained with antibodies specific to each myosin heavy chain (MyHC) isoform. The following antibodies were used: anti-MyHC-I (1:50 dilution; Novocastra Laboratories), MyHC-IIa (undiluted, prepared from hybridoma SC-71, deposited in the American Type Culture Collection by S. Schiaffino), MyHC-IId/x (undiluted, prepared from hybridoma developed by Joseph F. Y. Hoh), MyHC-IIb (1:2 dilution, from hybridoma BF-F3, deposited in the American Type Culture Collection by S. Schiaffino), antilaminin (catalog no. L9393, Sigma), goat anti-mouse IgG-FITC (1:100 dilution; catalog no. 115-095-071, Jackson ImmunoResearch), goat anti-mouse IgM-FITC (1:100 dilution; catalog no. 115-095-075, Jackson ImmunoResearch), and anti-rabbit Texas red (catalog no. 711-075-152, Jackson ImmunoResearch). Sections were preincubated for 1 h at room temperature in blocking solution [5% normal goat serum and 2% antilaminin antibody in PBS with 0.12% bovine serum albumin (P/BS; catalog no. A7906, Sigma), 0.12% nonfat dehydrated milk, and 0.01% Triton X]. Sections were rinsed three times in P/BS and incubated in primary antibody diluted in P/BS for 1 h at room temperature. Sections were then rinsed twice in P/BS and incubated three times for 5 min in P/BS before secondary antibodies in P/BS were added and incubated for 1 h at room temperature. Sections were again rinsed in P/BS and mounted in 1,4-diazobicyclo(2,2,2)octane (catalog no. D2522, Sigma)-supplemented glycerol gelatin (catalog no. GG-1, Sigma). Fluorescent microscopy images were obtained with a Zeiss microscope equipped with digital image capture, and fiber area was analyzed using the NIH Image software.
RNA analysis.
Total RNA was extracted from one-half of the frozen myocardium using TRIzol reagent (GIBCO-BRL) according to the manufacturer’s protocol. The level of mRNA expression of -MyHC, 
-skeletal actin (S-actin), and atrial natriuretic factor (ANF) mRNA was determined using slot-blot analysis with previously described oligonucleotide probes (23, 55).
MEF-lacZ mice and activity assays.
A subset of mice were backcrossed with a transgenic line of C57Bl/6J mice that harbored a lacZ transgene under the transcriptional control of three MEF-2 consensus DNA-binding sites (7), allowing direct monitoring of MEF-2 activity in vivo (7, 26, 41). To determine MEF-2 activity, freshly excised or frozen hearts from MEF-2-lacZ indicator mice were mechanically disrupted in an ice-cold buffer solution (in mmol/l: 40 Tris, 150 NaCl, and 1 EDTA). The homogenized tissue was centrifuged at 10,000 g (Beckman J2-HS centrifuge) for 5 min at 4°C. -Galactosidase assays (Galacto-Star, Tropix, Bedford, MA) were performed on cardiac extracts from MEF-2 indicator mice as described previously (34) under conditions of linearity with respect to time and protein concentration.
Western blot analysis and gel electrophoresis.
Preparation of heart samples for SDS-PAGE and subsequent Western immunoblotting for detection of proteins and particular sites of phosphorylation on each protein began by homogenization of hearts (minus atria) in a protein extraction buffer for Ca2+/calmodulin-dependent protein kinase (CaMK) assay [in mmol/l: 50 Tris, 0.5 EGTA, 1 EDTA, and 0.5 dithiothreitol, pH 7.0 (for CaMK assay) and 40 Tris, 150 NaCl, and 1 EDTA (for -galactosidase assay)]. The buffer also contained leupeptin, pepstatin, and phenylmethylsulfonyl fluoride (0.1 mmol/l each) to prevent nonspecific proteolysis and sodium pyrophosphate and sodium vanadate (1 mmol/l each) to prevent nonspecific phosphorylation and dephosphorylation, respectively. The homogenized tissue was then centrifuged at 12,000–14,000 g (Beckman J2-HS centrifuge) for 10 min at 4°C. The supernatant was removed, and protein concentration was determined using the Bradford method. SDS-PAGE was performed on the heart extracts, which were then transferred to a polyvinylidene difluoride membrane. The membranes were probed with antibodies specific for calcineurin, Akt, glycogen synthase kinase-3 (GSK-3), and p70 S6 kinase, including the phosphorylated forms of Akt, GSK-3, and p70 S6 kinase. All antibodies were obtained commercially: calcineurin from BD Transduction Laboratories (San Diego, CA), Akt and p70 S6 kinase from Cell Signaling Technology (Beverly, MA), and GSK-3 and phosphorylated GSK-3 from Santa Cruz Biotechnology (Santa Cruz, CA).
Data and statistical analysis.
Values are means ± SE. The weekly values were determined by averaging the values in a 24-h period over a given 7-day period. To calculate average daily external work and power output, the torque necessary to maintain wheel speed at a given load was calculated as 
= mg x r, where is torque, m is wheel load, g is Newton’s conversion factor (9.81 m/s2), and r is radius of the cage wheel. Next, work (W) was calculated as W = x 
, where is angular displacement (2
rad/revolution, 17,952 rad for 1 km). It follows that power (P) was calculated as P = W/t, where t is time. External work and power were adjusted for each individual animal body weight (in kg). The percent change in cardiac mass with exercise was determined by comparing the mass of each exercised animal with the mean cardiac mass of the sedentary group. The difference in cardiac mass was then expressed as a percent change from sedentary animals for each respective animal. The differences between experimental groups were analyzed with a one-way ANOVA followed by Student’s t-test with post hoc Bonferroni’s correction to assess differences among mean values. P < 0.05 was considered significant.
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RESULTS |
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40% (Fig. 2C). Again, within the HR group, wheel-running speeds under added load (9 and 12 g) were significantly lower than wheel-running speeds under no load (Fig. 2C). Thus mice can tolerate up to 25% of their body weight in resistance load without an effect on cage wheel performance as measured by distance, time, and speed on the cage wheel. With larger loads, exercise performance begins to diminish. Interestingly, NR and LR groups continue to increase wheel-running speeds, perhaps because of continued acclimatization and improved aerobic capacity. Exercise performance and aerobic adaptation by the mice were indexed by the amount of work performed and power output (22). We did not determine metabolic changes in energy production in each animal; therefore, the work and power measured in this study were external work and external power. For each experimental group, work and power averaged over the week were calculated with adjustment for the body weight of each animal (Table 1); the pooled data are displayed in Fig. 3. The amount of load required to maintain unloaded wheel friction was 2 g. Thus, in the work and power calculated for the NR group and for week 1 of all other groups, a wheel load of 2 g is taken into consideration. There were no significant differences in work performed (Fig. 3A) or power output (Fig. 3B) among the groups studied during the week (week 1) of "unloaded" cage wheel exposure. Although increasing wheel load to 4 g (week 2) and 5 g (week 3) in the LR and HR groups showed a trend toward augmented work performed, there was no significant change. During this same exercise period, however, power output was significantly elevated in the LR and HR groups compared with the NR group. By week 4, mice in the LR group responded to the 5-g wheel load with improved exercise performance as demonstrated by increased work and power compared with the NR group, a significant difference that was preserved throughout the remaining duration of the exercise protocol. When the wheel load was increased to 7 g (weeks 4 and 5) in the HR group, work performed and power output were significantly higher than in the NR group. However, the addition of 9 g in the HR group reduced the amount of work performed, whereas the amount of power remained greater than the NR group. A 12-g wheel load in the HR group resulted in a significant decrease in work performed compared with the LR group but not the NR group. At a 12-g wheel load, power output remained unchanged. Interestingly, the amount of power generated by each mouse at a wheel load above the inertial load (i.e., not including the NR group) was not different between the experimental groups studied. Thus it appears that the limiting factor for exercise performance in mice is power output.
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-MyHC (33, 40, 56), exercise induces expression of -MyHC (49). Therefore, using RNA slot-blot analysis, we aimed to determine the impact of resistance training on mRNAs encoding ANF, S-actin, and -MyHC (56). The raw data from one example from each experimental group and the pooled results are shown in Fig. 6. None of these hypertrophic markers were elevated by 2 mo of exercise. Instead, the mRNA levels of the indicated markers appeared to be reduced in the NR, LR, and HR groups. Specifically, there was a significant reduction in the amount of S-actin mRNA in all groups (Fig. 6, bottom). In addition, cage wheel exposure under a moderate load (LR) significantly suppressed all the hypertrophic markers.
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-galactosidase is displayed in Fig. 7A. MEF-2 activity in sedentary animals was 22.4 ± 9.2 pg -galactosidase/mg cardiac tissue. In animals subjected to exercise protocols with no added load (NR) or with increasing load (HR), the average level of MEF-2 activity was not different from that in sedentary animals: 12.0 ± 11.5 and 40.8 ± 13.2 pg -galactosidase/mg cardiac tissue in the NR and HR groups, respectively. Animals exposed to a moderate load (LR) for the duration of the exercise period of 8 wk had no detectable MEF-2 activity (0.4 ± 0.1), even compared with sedentary controls.
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2 mo demonstrated a significant reduction in the level of calcineurin expression from that of sedentary controls: 0.37 ± 0.07-, 0.59 ± 0.24-, and 0.54 ± 0.12-fold for NR, LR, and HR, respectively.
Akt, GSK-3, and p70 S6 kinase regulation.
Increases in insulin-like growth factor I (IGF-I) have been observed in pathological (51) and physiological (42, 50) cardiac hypertrophy. Phosphoinositide 3-kinase (PI3K), a downstream target of IGF-I, regulates the activity of Akt (protein kinase B). Farther downstream of the IGF-I-Akt signaling cascade is p70 S6 kinase (27). In addition, recent studies have reported that Akt can inhibit (by phosphorylation) the ubiquitous serine-threonine kinase GSK-3, a negative regulator of cardiac hypertrophy (2, 3, 38). Therefore, we chose to investigate these signaling components in the heart after resistance training. Representative immunoblots and summarized data are displayed in Fig. 8. None of the different exercise protocols had an effect on the total protein expression of Akt, GSK-3, or p70 S6 kinase (Fig. 8A). There was a significant elevation in the phosphorylated forms of Akt, GSK-3, and p70 S6 kinase in the NR group over sedentary controls: 1.86 ± 0.09-, 2.02 ± 0.10-, and 1.63 ± 0.06-fold, respectively (Fig. 8B). There were no significant increases in the phosphorylated proteins in the LR or the HR group.
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DISCUSSION |
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Many physiological factors determine the level of sustainable workload, such as muscle strength and endurance, as well as cardiopulmonary parameters such as anaerobic (lactate) threshold and maximal oxygen uptake (5, 9, 29). It has been demonstrated in elite cyclists that peak power output was highly correlated with the ability to perform (4, 18), while other studies have indicated that power output and cycling performance were more closely related to blood lactate concentrations (5, 6). Moreover, cardiac output and peripheral oxygen capacity contributed to the attainment of maximal workload (29). Although these parameters were not measured in this study, the aforementioned investigations indicate that exercise capacity and power output are tightly coupled.
Although contractile performance of the heart after exercise was not measured, the hearts of animals in all three exercise groups adapted to the exercise stimulus. However, only the mice in the NR group showed a significant increase in absolute and normalized cardiac mass. A previous report from our laboratory demonstrated that, in response to voluntary cage wheel exposure, mice exhibit a significant increase in cardiac mass by 3 wk of cage wheel activity and that this increase in cardiac mass persisted for 
4 wk (1). The animals in the NR group exhibited a quantitative increase in cardiac mass similar to that reported in this previous study (1), despite an exercise duration that was twice as long. Thus cardiac mass in C57Bl/6 mice appears to reach a plateau after 3 wk of exercise and is maintained for 2 mo. Although there are no data for the impact of resistance-endurance training, such as in this study, on the heart in mice, meta-analysis of human exercise data demonstrated that athletes engaged in an analogous exercise regimen that combined dynamic and strength training such as cycling exhibited a significant increase in LV internal dimension and LV wall thickness (45). In the present study, we were unable to perform a detailed analysis of ventricular morphology and were limited to measurement of heart weight only. Nevertheless, it would be difficult to relate these differences to human studies given enormous species differences and the behavioral component of the voluntary exercise paradigm. Future studies must be aimed at determining how the morphological adaptation of the heart (such as changes in ventricular chamber dimensions or wall thickness) affects ventricular function after loaded exercise training in mice.
As mentioned previously, an array of genes, such as ANF, brain natriuretic peptide, and S-actin, is expressed in the heart as a result of pathological stimuli (33, 40). In response to physiological stimuli such as exercise, the induction of these genes is modest at best (1, 10, 24, 48, 49). Although the reduced expression of -MyHC mRNA was predicted on the basis of previous data (10, 48, 49), the suppressed expression of ANF and S-actin in the LR group was not expected. The critical factors contributing to differences from the present study compared with previous studies most likely involve the differences in species (rat vs. mouse), exercise paradigm (voluntary cage wheel vs. treadmill vs. swimming), and exercise duration (2 vs. 4 vs. 8 wk). If we consider the slight elevation of these hypertrophic markers after 3–4 wk of voluntary cage wheel exercise previously demonstrated in our laboratory (1), the implication is that the myocardium responds to an exercise stimulus initially with a modest increase in these genes and that the change in expression may be attenuated with continued exercise. Thus these factors may be important during the induction of hypertrophy but not the maintenance of an enlarged heart. The absence of hypertrophy and the reduced expression in the LR group suggest that the signals required for the induction of these specific markers during hypertrophy are distinct from the signals that suppress the expression of these markers. The suggestion is that exercise under an increased wheel load represents a third, independent physiological stimulus in the heart. Again, future studies must be aimed at determining the precise timing of expression and/or induction of specific signaling components of the exercise-stimulated heart.
As mentioned above, the ability of Ca2+ handling in the myocyte to vary in amplitude and frequency in response to increases in functional demand, such as exercise, makes Ca2+-dependent enzymes candidate molecules for intracellular myocyte signaling in response to exercise. When Ca2+ binds calmodulin, this complex can activate CaMK (15, 43, 46) and a Ca2+/calmodulin-dependent phosphatase, calcineurin (35, 36, 59). When CaMK is inactive, MEF-2 associates with histone deacetylases (HDACs), namely, HDAC-4 and HDAC-5, thereby suppressing MEF-2 activity. On activation, CaMK directly or indirectly phosphorylates HDACs, allowing their export from the nucleus. Thus MEF-2 transcriptional activity is increased by CaMK by removing the inhibitory action of HDACs (30, 31, 43). In addition, the role of calcineurin in physiological hypertrophy has remained unresolved, as evidenced by the contradictory nature of the published studies (11, 47, 57).
To address this pathway, a subset of animals was included in each experimental group harboring a lacZ transgene under the transcriptional control of three MEF-2 consensus DNA-binding sites, allowing monitoring of MEF-2 activity in vivo (7, 26, 41). Long-term moderate exercise (LR group) was the only protocol to have any statistically significant impact on MEF-2 activity. MEF-2 activity in this group was significantly reduced to levels that were below the levels of all groups, including sedentary controls. We previously demonstrated a modest elevation and then a subsequent attenuation of MEF-2 activity during voluntary cage wheel exposure (25). Results of these longer-term runners are consistent with previous results. As with the hypertrophic markers, long-term training under a moderate load affects those factors that suppress MEF-2 activity compared with those that activate them. Nevertheless, on the basis of these and previous data (25), MEF-2 transcriptional activity plays a minor role, if any, in the maintenance of an exercise stimulus.
Calcineurin and its downstream effector nuclear factor of activated T cells (NFAT) have been shown to be important mediators in cardiac hypertrophy (36, 58). Inasmuch as these transgenic-overexpressing animals develop enlarged hearts that transition to heart failure, the role of calcineurin and NFAT in pathological hypertrophy is strongly implicated. Yet, Eto and co-workers (11) demonstrated an increase in calcineurin activity after voluntary exercise in rats. In contrast, Rothermel et al. (47) reported that overexpression of myocyte-enriched calcineurin-interacting protein 1, an inhibitor of the calcineurin pathway, attenuated the cardiac hypertrophic response to an exercise stimulus. Supporting a negligible role of calcineurin in physiological hypertrophy, a recent study used NFAT-luciferase reporter mice to measure calcineurin activity in vivo and found no increase in NFAT activity associated with cardiac hypertrophy after two different exercise regimens (57). Although the measurements of calcineurin protein do not necessarily correlate with activity, our study demonstrated that 2 mo of voluntary cage wheel exposure reduced the abundance of calcineurin protein relative to that in sedentary controls. Consistent with these findings, mice expressing the NFAT-luciferase reporter gene showed reduced luciferase activity at certain time points during the training regimen (57). On the basis of these data, calcineurin expression in response to an exercise stimulus is most likely transcriptionally regulated by elements that have yet to be identified.
The IGF-I signaling axis is believed to be important during developmental and physiological cardiac growth (27). Activation of the IGF-I receptor is coupled to PI3K, which in turn promotes activation of Akt (protein kinase B) via a kinase intermediate. The active, phosphorylated form of Akt targets mammalian target of rapamycin and subsequent downstream phosphorylation of p70 S6 kinse, leading to an increase in protein synthesis and, presumably, cardiac growth. In support of the role of this pathway in physiological cardiac hypertrophy, expression of the catalytic subunit of PI3K- (p110) in an activated form in the heart results in increased phosphorylation of Akt, significant hypertrophy, and normal contractility, consistent with physiological hypertrophy (54). Similarly, expression of a dominant-negative p110 prevented cardiac hypertrophy in response to an exercise stimulus (32). In this study, activation of Akt and p70 S6 kinase, as demonstrated by elevated levels of the phosphorylated forms of each protein compared with sedentary controls, was associated with a significant increase in cardiac mass after unloaded exercise (NR group). Hearts from mice exercising under moderate (LR) or high (HR) resistance did not exhibit hypertrophy or elevated levels of activated proteins. In addition, only hearts from animals in the NR group showed an increase in phosphorylated GSK-3, which is a negative regulator of cardiac hypertrophy and is inactivated by Akt (2, 3, 38). Phosphorylation of GSK-3 is permissive to cardiac hypertrophy and, thus, is consistent with the observed increase in heart mass in the NR group, whereas the absence of activated GSK-3 (phosphorylated) in the other experimental groups is predicted from the lack of significant cardiac hypertrophy.
The differential response of the heart to each type of exercise stimulus was paralleled by a differential response in skeletal muscle. Whereas loaded cage wheel exercise did not increase cardiac mass, exercising under an increased load increased muscle mass, although this increase reached statistical significance only in soleus muscle. The lack of hypertrophy in the skeletal muscles from animals exercising under unloaded conditions was accompanied by a relative increase in the CSA of fibers expressing MyHC type IIa, consistent with previous data examining unloaded voluntary cage wheel exercise (1, 17). The larger CSA of type IIa fibers was also observed in the other experimental groups, whereas the CSA of type IId/x fibers was increased only after the imposition of a moderate (LR group) or a large (HR group) load on the cage wheel. Moreover, animals in the LR group displayed an increase in CSA of type IIb fibers, whereas animals in the HR group exhibited a significant decrease in the CSA of these fibers. Despite a small but significant decrease in the LR group, the relative CSA of type I fibers remained unaffected by cage wheel exercise, consistent with the lack of effect of voluntary, spontaneous cage wheel exercise on type I fiber number (47). These findings may reflect a decrease in the ratio of type II to type I fibers in bodybuilders compared with power lifters in resistance-trained humans (14). Although preliminary studies indicate a shift to more oxidative fibers in animals exercising under a moderately loaded than an unloaded cage wheel (unpublished observations), future studies need to address the fiber type interconversions under each exercise protocol. In addition, the lack of significant cardiac hypertrophy may be explained by these peripheral adaptations, indicating a predominance of peripheral, skeletal muscle adaptation over central, cardiac muscle adaptation in resistance-trained mice, as previously suggested (8, 22, 39).
The initial goal of this study was to determine whether loaded exercise training represented a third, independent stimulus for cardiac and skeletal muscle adaptation. In support of this contention, Table 3 clearly illustrates that each exercise protocol had a unique impact on the parameters measured in this study. Most notably, loaded exercise had a greater impact on skeletal muscle adaptation than unloaded exercise, as determined by muscle mass and CSA. In contrast, unloaded exercise had a greater impact on the IGF signaling pathway within the cardiac myocytes than resistance training. Future investigations need to address the time dependency of signaling factors, including those studied in this present study, to help elucidate the mechanisms underlying the observed differences between exercise protocols. The activation of signaling components at the initiation of exercise training will most likely be phenotypically distinct from those at the time of completion of the exercise protocol, as in this study. Preliminary results from animals that were exposed to a voluntary cage wheel with a moderate load for a short period of time (3 wk) support this idea, with a very modest elevation in MEF-2 transcriptional activity and calcineurin protein level. Nevertheless, the data presented on the differential impact of loaded exercise training on cardiac and skeletal muscle adaptation provide a foundation for future comparisons.
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ACKNOWLEDGMENTS |
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Present address of U. Widegren: Dept. of Surgical Sciences, Sect. of Integrative Physiology, Karolinska Institutet, Stockholm, SE-171 77, Sweden.
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FOOTNOTES |
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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.
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suppresses cardiac hypertrophy in vivo. Proc Natl Acad Sci USA 99: 907–912, 2002. and induces cardiac hypertrophy following pressure overload. J Clin Invest 109: 373–381, 2002.[CrossRef][Web of Science][Medline]
-myosin heavy chain gene leads to dosage effects and functional deficits in the heart. J Clin Invest 98: 1906–1917, 1996.[Web of Science][Medline]
O2 max and running performance in rats. J Appl Physiol 68: 400–403, 1990.O2 max and cardiac output. Med Sci Sports Exerc 36: 1357–1363, 2004.
) plays a critical role for the induction of physiological, but not pathological, cardiac hypertrophy. Proc Natl Acad Sci USA 100: 12355–12360, 2003.-galactosidase. In: Molecular Genetics, edited by Miller JM. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, 1972, p. 352–355.
-cardiac myosin heavy chain gene expression in vitro and in vivo. J Biol Chem 268: 19512–19520, 1993. pathway regulates transcription of atrial natriuretic factor induced by -adrenergic receptor stimulation in cardiac myocytes. J Biol Chem 275: 14466–14475, 2000.
B isoform of Ca2+/calmodulin-dependent protein kinase II regulates atrial natriuretic factor gene expression in ventricular myocytes. J Biol Chem 272: 31203–31208, 1997.This article has been cited by other articles:
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  N. Ichinoseki-Sekine, H. Naito, K. Tsuchihara, I. Kobayashi, Y. Ogura, R. Kakigi, M. Kurosaka, R. Fujioka, and H. Esumi Provision of a voluntary exercise environment enhances running activity and prevents obesity in Snark-deficient mice Am J Physiol Endocrinol Metab, May 1, 2009; 296(5): E1013 - E1021. [Abstract] [Full Text] [PDF]
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 S. Schiekofer, I. Shiojima, K. Sato, G. Galasso, Y. Oshima, and K. Walsh Microarray analysis of Akt1 activation in transgenic mouse hearts reveals transcript expression profiles associated with compensatory hypertrophy and failure Physiol Genomics, October 11, 2006; 27(2): 156 - 170. [Abstract] [Full Text] [PDF]
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|
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|
|
 |
|
 K. Walsh Akt Signaling and Growth of the Heart Circulation, May 2, 2006; 113(17): 2032 - 2034. [Full Text] [PDF]
|
|
|
|
|
|
A. M. Katz and M. R. Zile New Molecular Mechanism in Diastolic Heart Failure Circulation, April 25, 2006; 113(16): 1922 - 1925. [Full Text] [PDF]
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 S. Choi, X. Liu, P. Li, T. Akimoto, S. Y. Lee, M. Zhang, and Z. Yan Transcriptional profiling in mouse skeletal muscle following a single bout of voluntary running: evidence of increased cell proliferation J Appl Physiol, December 1, 2005; 99(6): 2406 - 2415. [Abstract] [Full Text] [PDF]
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P < 0.05 vs. HR in weeks 2–5.
