Intensity-controlled treadmill running in mice: cardiac and skeletal muscle hypertrophy

Aerobic exercise training upregulates skeletal muscle calpain and ubiquitin-proteasome systems in healthy mice

Aerobic exercise training (AET) is an important mechanical stimulus that modulates skeletal muscle protein turnover, leading to structural rearrangement. Since the ubiquitin-proteasome system (UPS) and calpain system are major proteolytic pathways involved in protein turnover, we aimed to investigate the effects of intensity-controlled AET on the skeletal muscle UPS and calpain system and their association to training-induced structural adaptations. Long-lasting effects of AET were studied in C57BL/6J mice after 2 or 8 wk of AET. Plantaris cross-sectional area (CSA) and capillarization were assessed by myosin ATPase staining. mRNA and protein expression levels of main components of the UPS and calpain system were evaluated in plantaris by real-time PCR and Western immunoblotting, respectively. No proteolytic system activation was observed after 2 wk of AET. Eight weeks of AET resulted in improved running capacity, plantaris capillarization, and CSA. Muscle RING finger-1 mRNA expression was increased in 8-wk-trained mice. Accordingly, elevated 26S proteasome activity was observed in the 8-wk-trained group, without accumulation of ubiquitinated or carbonylated proteins. In addition, calpain abundance was increased by 8 wk of AET, whereas no difference was observed in its endogenous inhibitor calpastatin. Taken together, our findings indicate that skeletal muscle enhancements, as evidenced by increased running capacity, plantaris capillarization, and CSA, occurred in spite of the upregulated UPS and calpain system, suggesting that overactivation of skeletal muscle proteolytic systems is not restricted to atrophying states. Our data provide evidence for the contribution of the UPS and calpain system to metabolic turnover of myofibrillar proteins and skeletal muscle adaptations to AET.

Divergent physiological characteristics and responses to endurance training among inbred mouse strains

Both baseline values and adaptive changes in mice can vary depending on the genetic background. We aimed to assess variation in a battery of variables and their adaptations to endurance training in six inbred mouse strains. Males, n = 184, from A/J, BALB/cByJ, C3H/HeJ, C57BL/6J, DBA/2J, and PWD/PhJ strains were assigned to a control or an endurance group (5 weeks swimming exercise). Enzyme activity, histology of soleus (SOL) muscle, swimming endurance, cardiac ventricular and hind limb muscle weight, and femur length were examined. Endurance capacity, morphological and histological variables, and enzyme activity substantially differed among strains. For example, SOL weight was twofold higher and cross-sectional area (CSA) of fibers was ≈ 30% greater in C57BL/6J than in PWD/PhJ strain. The CSA of type 1 fibers were larger than type 2A in PWD/PhJ (P < 0.01); however, the reverse was true in DBA/2J and BALB/cByJ strains (P < 0.05). Swimming endurance in DBA/2J strain was ≈ 9 times better than in BALB/cByJ. Endurance training increased the activity of citrate synthase in gastrocnemius across strains (P < 0.01), however, changes in endurance were strain-specific; the C57BL/6J and DBA/2J strains improved substantially, whereas A/J and BALB/cByJ strains did not. In conclusion, genetic background is a potent determinant of the physiological characteristics and adaptations to training in mice.

Exercise can induce temporary mitochondrial and contractile dysfunction linked to impaired respiratory chain complex activity

Exercise is considered to elicit a physiological response of the heart. Previous studies investigated the influence of repetitive exercise only at the end of the training period. We assessed the impact of 2 exercise protocols, differing in their treadmill inclination, on cardiac and mitochondrial function at different times during the training period. Within 10 weeks, animals trained with 16% incline developed hypertrophy (left ventricular posterior wall thickness: 1.6 ± 0.1 vs 2.4 ± 0.1 mm; P < .05) with normal function (ejection fraction: 75.2% ± 2.5% vs 75.6% ± 2.1%). However, at 6 weeks, there was temporary impairment of contractile function (ejection fraction: 74.5% ± 1.67% vs 65.8% ± 2.3%; P < .05) associated with decreased mitochondrial respiratory capacity (state 3 respiration: 326 ± 71 vs 161 ± 22 natoms/[min mg protein]; P < .05) and a gene expression shift from the adult (α) to the fetal (β) myosin heavy chain isoform. Although peroxisome proliferator-activated receptor gamma coactivator-1α expression was normal, nuclear respiratory factors (NRFs)-1 and -2 were significantly reduced (NRF-1: 1.00 ± 0.16 vs 0.55 ± 0.09; NRF-2: 1.00 ± 0.11 vs 0.63 ± 0.07; P < .05) after 6 weeks. These findings were associated with a reduction of electron transport chain complexes I and IV activity (complex I: 1016 ± 67 vs 758 ± 71 nmol/[min mg protein]; complex IV: 18768 ± 1394 vs 14692 ± 960 nmol/[min mg protein]; P < .05). Messenger RNA expression of selected nuclear encoded subunits of the electron transport chain was unchanged at all investigated time points. In contrast, animals trained with 10% incline showed less hypertrophy and normal function in echocardiography, normal maximal respiratory capacity, and unchanged complex activities at all 3 time points. Repetitive exercise may cause contractile and mitochondrial dysfunction characterized by impaired respiratory chain complex activities. This activity reduction is temporary and intensity related.

Akt1-mediated skeletal muscle growth attenuates cardiac dysfunction and remodeling after experimental myocardial infarction

BACKGROUND

It is appreciated that aerobic endurance exercise can attenuate unfavorable myocardial remodeling following myocardial infarction. In contrast, little is known about the effects of increasing skeletal muscle mass, typically achieved through resistance training, on this process. Here, we utilized transgenic (TG) mice that can induce the growth of functional skeletal muscle by switching Akt1 signaling in muscle fibers to assess the impact of glycolytic muscle growth on post-myocardial infarction cardiac remodeling.

METHODS AND RESULTS

Male-noninduced TG mice and their nontransgenic littermates (control) were subjected to left anterior coronary artery ligation. Two days after surgery, mice were provided doxycycline in their drinking water to activate Akt1 transgene expression in a skeletal muscle-specific manner. Myogenic Akt1 activation led to diminished left ventricular dilation and reduced contractile dysfunction compared with control mice. Improved cardiac function in Akt1 TG mice was coupled to diminished myocyte hypertrophy, decreased interstitial fibrosis, and increased capillary density. ELISA and protein array analyses demonstrated that serum levels of proangiogenic growth factors were upregulated in Akt1 TG mice compared with control mice. Cardiac eNOS was activated in Akt1 TG mice after myocardial infarction. The protective effect of skeletal muscle Akt activation on cardiac remodeling and systolic function was abolished by treatment with the eNOS inhibitor l-NAME.

CONCLUSIONS

Akt1-mediated skeletal muscle growth attenuates cardiac remodeling after myocardial infarction and is associated with an increased capillary density in the heart. This improvement appears to be mediated by skeletal muscle to cardiac communication, leading to activation of eNOS-signaling in the heart.

Exposure to hypergravity during specific developmental periods differentially affects metabolism and vestibular reactions in adult C57BL /6j mice

The development of the posturo-motor control of movement is conditioned by Earth's gravity. Missing or altered gravity during the critical periods of development delays development and induces durable changes in the vestibular, cerebellar, or muscular structures, but these are not consistently mirrored at a functional level. The differences in the time schedule of vestibular and motor development could contribute to this inconstancy. To investigate the influence of gravity on the development of vestibular and locomotor functions, we analysed the performance of adult mice subjected to hypergravity during the time covering either the vestibular or locomotor development. The mice were centrifuged at 2 g from embryonic day (E) 0 to postnatal day (P) 10 (PRE), from P10 to P30 (POST), from E0 to P30 (FULL), and from E7 to P21. Their muscular force, anxiety level, vestibular reactions, and aerobic capacity during treadmill training were then evaluated at the age of 2 and 6 months. The performance of young adults varied in relation to the period of exposure to hypergravity. The mice that acquired locomotion in hypergravity (POST and FULL) showed a lower forelimb force and delayed vestibular reactions. The mice centrifuged from conception to P10 (PRE) showed a higher aerobic capacity during treadmill training. The differences in muscular force and vestibular reactions regressed with age, but the metabolic changes persisted. These results confirmed that early exposure to hypergravity induces qualitative changes depending on the period of exposure. They validated, at a functional level, the existence of several critical periods for adaptation to gravity.

Slowed relaxation and preserved maximal force in soleus muscles of mice with targeted disruption of the Serca2 gene in skeletal muscle

Sarcoplasmic reticulum Ca(2+) ATPases (SERCAs) play a major role in muscle contractility by pumping Ca(2+) from the cytosol into the sarcoplasmic reticulum (SR) Ca(2+) store, allowing muscle relaxation and refilling of the SR with releasable Ca(2+). Decreased SERCA function has been shown to result in impaired muscle function and disease in human and animal models. In this study, we present a new mouse model with targeted disruption of the Serca2 gene in skeletal muscle (skKO) to investigate the functional consequences of reduced SERCA2 expression in skeletal muscle. SkKO mice were viable and basic muscle structure was intact. SERCA2 abundance was reduced in multiple muscles, and by as much as 95% in soleus muscle, having the highest content of slow-twitch fibres (40%). The Ca(2+) uptake rate was significantly reduced in SR vesicles in total homogenates. We did not find any compensatory increase in SERCA1 or SERCA3 abundance, or altered expression of several other Ca(2+)-handling proteins. Ultrastructural analysis revealed generally well-preserved muscle morphology, but a reduced volume of the longitudinal SR. In contracting soleus muscle in vitro preparations, skKO muscles were able to fully relax, but with a significantly slowed relaxation time compared to controls. Surprisingly, the maximal force and contraction rate were preserved, suggesting that skKO slow-twitch fibres may be able to contribute to the total muscle force despite loss of SERCA2 protein. Thus it is possible that SERCA-independent mechanisms can contribute to muscle contractile function.

High intensity interval training alters substrate utilization and reduces oxygen consumption in the heart

AIMS

although exercise training induces hypertrophy with improved contractile function, the effect of exercise on myocardial substrate metabolism and cardiac efficiency is less clear. High intensity training has been shown to produce more profound effects on cardiovascular function and aerobic capacity than isocaloric low and moderate intensity training. The aim of the present study was to explore metabolic and mechanoenergetic changes in the heart following endurance exercise training of both high and moderate intensity.

METHODS AND RESULTS

C57BL/6J mice were subjected to 10 wk treadmill running, either high intensity interval training (HIT) or distance-matched moderate intensity training (MIT), where HIT led to a pronounced increase in maximal oxygen uptake. Although both modes of exercise were associated with a 10% increase in heart weight-to-body weight ratio, only HIT altered cardiac substrate utilization, as revealed by a 36% increase in glucose oxidation and a concomitant reduction in fatty acid oxidation. HIT also improved cardiac efficiency by decreasing work-independent myocardial oxygen consumption. In addition, it increased cardiac maximal mitochondrial respiratory capacity.

CONCLUSION

This study shows that high intensity training is required for induction of changes in cardiac substrate utilization and energetics, which may contribute to the superior effects of high compared with moderate intensity training in terms of increasing aerobic capacity.

Chronic CaMKII inhibition blunts the cardiac contractile response to exercise training

Activation of the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays a critical role modulating cardiac function in both health and disease. Here, we determined the effect of chronic CaMKII inhibition during an exercise training program in healthy mice. CaMKII was inhibited by KN-93 injections. Mice were randomized to the following groups: sham sedentary, sham exercise, KN-93 sedentary, and KN-93 exercise. Cardiorespiratory function was evaluated by ergospirometry during treadmill running, echocardiography, and cardiomyocyte fractional shortening and calcium handling. The results revealed that KN-93 alone had no effect on exercise capacity or fractional shortening. In sham animals, exercise training increased maximal oxygen uptake by 8% (p < 0.05) compared to a 22% (p < 0.05) increase after exercise in KN-93 treated mice (group difference p < 0.01). In contrast, in vivo fractional shortening evaluated by echocardiography improved after exercise in sham animals only: from 25 to 32% (p < 0.02). In inactive mice, KN-93 reduced rates of diastolic cardiomyocyte re-lengthening (by 25%, p < 0.05) as well as Ca(2+) transient decay (by 16%, p < 0.05), whereas no such effect was observed after exercise training. KN-93 blunted exercise training response on cardiomyocyte fractional shortening (63% sham vs. 18% KN-93; p < 0.01 and p < 0.05, respectively). These effects could not be solely explained by the Ca(2+) transient amplitude, as KN-93 reduced it by 20% (p < 0.05) and response to exercise training was equal (64% sham and 47% KN-93; both p < 0.01). We concluded that chronic CaMKII inhibition increased time to 50% re-lengthening which were recovered by exercise training, but paradoxically led to a greater increase in maximal oxygen uptake compared to sham mice. Thus, the effect of chronic CaMKII inhibition is multifaceted and of a complex nature.

Cardiovascular effects of treadmill exercise in physiological and pathological preclinical settings

Exercise adaptations result from a coordinated response of multiple organ systems, including cardiovascular, pulmonary, endocrine-metabolic, immunologic, and skeletal muscle. Among these, the cardiovascular system is the most directly affected by exercise, and it is responsible for many of the important acute changes occurring during physical training. In recent years, the development of animal models of pathological or physiological cardiac overload has allowed researchers to precisely analyze the complex cardiovascular responses to stress in genetically altered murine models of human cardiovascular disease. The intensity-controlled treadmill exercise represents a well-characterized model of physiological cardiac hypertrophy because of its ability to mimic the typical responses to exercise in humans. In this review, we describe cardiovascular adaptations to treadmill exercise in mice and the most important parameters that can be used to quantify such modifications. Moreover, we discuss how treadmill exercise can be used to perform physiological testing in mouse models of disease and to enlighten the role of specific signaling pathways on cardiac function.

Exercise Performance Tests in Mice

Maximal exercise performance is a multifactorial process in which the cardiovascular component, the innervation of the musculature, and the contractile and metabolic properties of skeletal muscle all play key roles. Here, protocols are provided for assessment of maximal running capacity of mice on a treadmill, with a combination of short high-intensity paradigms primarily intended to test for maximal power and cardiovascular function, and longer low-intensity paradigms to assess endurance and oxidative metabolism in skeletal muscle. The coupling of treadmill running to indirect calorimetry, to correlate performance measurements to maximal oxygen consumption, is also described. Curr. Protoc. Mouse Biol. 1:141-154. © 2011 by John Wiley & Sons, Inc.

Dietary whey hydrolysate with exercise alters the plasma protein profile: a comprehensive protein analysis

OBJECTIVE

It has been shown that dietary whey protein accelerates glucose uptake by altering glycoregulatory enzyme activity in skeletal muscle. In the present study, we investigated the effect of dietary whey protein on endurance and glycogen resynthesis and attempted to identify plasma proteins that reflected the physical condition by a comprehensive proteomics approach.

METHODS

Male c57BL/6 mice were divided into four groups: sedentary, sedentary with whey protein hydrolysate, exercise, and exercise with whey protein hydrolysate. The mice in the exercise groups performed treadmill running exercise five times per week for 4 wk. Protein profiling of plasma sample obtained from individuals was performed, as were measurements of endurance performance and the glycogen content of gastrocnemius muscle.

RESULTS

After the training period, the endurance of mice fed the whey diet was improved compared with that of mice fed the control diet. Muscle glycogen content was significantly increased after 4 wk of exercise, and intake of whey protein led to a further increase in glycogen. Apolipoproteins A-II and C-I and β(2)-glycoprotein-1 were found to be altered by training combined with the intake of whey protein, without significant changes induced by exercise or whey protein alone.

CONCLUSION

Results of the present study suggest that these three proteins may be potential biomarkers of improved endurance and glycogen resynthesis and part of the mechanism that mediates the benefits of whey protein.

Exercise training before cardiac-specific Serca2 disruption attenuates the decline in cardiac function in mice

In the heart, function of the sarco(endo)plasmic Ca2+-ATPase (SERCA2) is closely linked to contractility, cardiac function, and aerobic fitness. SERCA2 function can be increased by high-intensity interval training, whereas reduced SERCA2 abundance is associated with impaired cardiac function. The working hypothesis was, therefore, that exercise training before cardiomyocyte-specific disruption of the Serca2 gene would delay the onset of cardiac dysfunction in mice. Before Serca2 gene disruption by tamoxifen, untreated SERCA2 knockout mice ( Serca2flox/flox Tg-αMHC-MerCreMer; S2KO), and SERCA2 FF control mice ( Serca2flox/flox, S2FF) were exercise trained by high-intensity interval treadmill running for 6 wk. Both genotypes responded to training, with comparable increases in maximal oxygen uptake (V̇o2max; 17%), left ventricle weight (15%), and maximal running speed (40%). After exercise training, cardiac-specific Serca2 gene disruption was induced in both exercise trained and sedentary S2KO mice. In trained S2KO, cardiac function decreased less rapidly than in sedentary S2KO. V̇o2max remained higher in trained S2KO the first 15 days after gene disruption. Six weeks after Serca2 disruption, cardiac output was higher in trained compared with sedentary S2KO mice. An exercise-training program attenuates the decline in cardiac performance induced by acute cardiac Serca2 gene disruption, indicating that mechanisms other than SERCA2 contribute to the favorable effect of exercise training.

Mechanisms of exercise-induced improvements in the contractile apparatus of the mammalian myocardium

One of the main outcomes of aerobic endurance exercise training is the improved maximal oxygen uptake, and this is pivotal to the improved work capacity that follows the exercise training. Improved maximal oxygen uptake in turn is at least partly achieved because exercise training increases the ability of the myocardium to produce a greater cardiac output. In healthy subjects, this has been demonstrated repeatedly over many decades. It has recently emerged that this scenario may also be true under conditions of an initial myocardial dysfunction. For instance, myocardial improvements may still be observed after exercise training in post-myocardial infarction heart failure. In both health and disease, it is the changes that occur in the individual cardiomyocytes with respect to their ability to contract that by and large drive the exercise training-induced adaptation to the heart. Here, we review the evidence and the mechanisms by which exercise training induces beneficial changes in the mammalian myocardium, as obtained by means of experimental and clinical studies, and argue that these changes ultimately alter the function of the whole heart and contribute to the changes in whole-body function.

High-intensity exercise training in mice with cardiomyocyte-specific disruption of Serca2

Several lines of evidence indicate that the sarco(endo)plasmic reticulum ATPase type 2 (SERCA2) is essential for maintaining myocardial calcium handling and cardiac pump function. Hence, a reduction in SERCA2 abundance is expected to reduce work performance and maximal oxygen uptake (V̇o2max) and to limit the response to exercise training. To test this hypothesis, we compared V̇o2max and exercise capacity in mice with cardiac disruption of Serca2 (SERCA2 KO) with control mice (SERCA2 FF). We also determined whether the effects on V̇o2max and exercise capacity could be modified by high-intensity aerobic exercise training. Treadmill running at 85–90% of V̇o2max started 2 wk after Serca2 gene disruption and continued for 4 wk. V̇o2max and maximal running speed were measured weekly in a metabolic chamber. Cardiac function was assessed by echocardiography during light anesthesia. In sedentary SERCA2 KO mice, the aerobic capacity was reduced by 50% and running speed by 28%, whereas trained SERCA2 KO mice were able to maintain maximal running speed despite a 36% decrease in V̇o2max. In SERCA2 FF mice, both V̇o2max and maximal running speed increased by training, while no changes occurred in the sedentary group. Left ventricle dimensions remained unchanged by training in both genotypes. In contrast, training induced right ventricle hypertrophy in SERCA2 KO mice. In conclusion, the SERCA2 protein is essential for sustaining cardiac pump function and exercise capacity. Nevertheless, SERCA2 KO mice were able to maintain maximal running speed in response to exercise training despite a large decrease in V̇o2max.

A valid and reproducible protocol for testing maximal oxygen uptake in rabbits

BACKGROUND

Physiological studies of long-term cardiovascular adaptation to exercise require adequate testing procedures to quantify the outcome. Such test procedures are well established in rats and mice. However, the use of these species may have limitations, and to study several physiological parameters mimicking 'the human adaptation' larger animal models may be preferable. Here, we established a valid and reproducible exercise test protocol for measuring maximal oxygen uptake (VO2max) in rabbits.

METHODS AND RESULTS

The VO2max protocol was studied in six adult female New Zealand White rabbits running on a treadmill at inclinations ranging from 0 to 20 degrees. VO2max was reached at all inclinations indicating that the rabbits reach exhaustion independent of inclination. Average VO2max for test and retest were 35.1+/-4.2 and 35.8+/-4.0 ml/kg per min, respectively. Oxygen uptake and heart rate increased linearly with increased running speed. Average running speed at VO2max was 0.51+/-0.09 m/s, and there was an increase oxygen pulse up to the intensity corresponding to VO2max, where it leveled off and declined.

CONCLUSION

This study shows that rabbit is a suitable species for studying responses to training and could be of great importance for showing novel cellular cardiac adaptations to training.

Selecting exercise regimens and strains to modify obesity and diabetes in rodents: an overview

Exercise is part of a healthy lifestyle and frequently is an important component in combating chronic diseases, such as obesity and diabetes. Understanding the molecular events initiated by regular exercise is best studied in laboratory animals, with mice and rats being favoured for a number of reasons. However, the wide variety of rodent strains available for biomedical research often makes it challenging to select an animal strain suitable for studying specific disease outcomes. In the present review we focus on exercise as a management strategy for obesity and diabetes and we discuss: (i) exercise paradigms in humans shown to ameliorate signs and symptoms of obesity and diabetes; (ii) different rodent strains in terms of their advantages, disadvantages and limitations when using specific forms of exercise; (iii) the strengths and weaknesses of commonly used laboratory methods for rodent exercise; and (iv) the unintended consequences of exercise that are often manifested by increased hormonal and oxidative stress responses.

High-intensity aerobic exercise training improves the heart in health and disease

Regular exercise training confers beneficial effects to the heart as well as to the entire body. This occurs partly because exercise training improves skeletal muscle work capacity and reduces resistance, thus increasing conductance in the peripheral circulation. More directly, exercise training also alters extrinsic modulation of the heart and improves the intrinsic pump capacity of the heart. Together, these effects allow for improved exercise capacity. Accumulating evidence suggests that the magnitude of these benefits increases proportionally with the intensity of individual exercise training sessions constituting the exercise training program. It has emerged that regular exercise training also confers beneficial effects to patients at risk for, or who have, established heart dysfunction and disease and, moreover, that exercise training may reduce the dysfunction of the heart itself and, at least, partly restore its ability to effectively function as a pump. The most recent studies in patients with established heart disease suggest that a high relative, yet aerobic, intensity of the exercise training improves the intrinsic pump capacity of the myocardium, an effect not previously believed to occur with exercise training. However, more and larger studies are needed to establish the safety and efficacy of such exercise training in patients with heart disease. Here, we consider the nature of the intensity dependence of exercise training and the causes of the improved heart function.

Erythropoietin elevates VO2,max but not voluntary wheel running in mice

Voluntary activity is a complex trait, comprising both behavioral (motivation, reward) and anatomical/physiological (ability) elements. In the present study, oxygen transport was investigated as a possible limitation to further increases in running by four replicate lines of mice that have been selectively bred for high voluntary wheel running and have reached an apparent selection limit. To increase oxygen transport capacity, erythrocyte density was elevated by the administration of an erythropoietin (EPO) analogue. Mice were given two EPO injections, two days apart, at one of two dose levels (100 or 300 microg kg(-1)). Hemoglobin concentration ([Hb]), maximal aerobic capacity during forced treadmill exercise (VO2,max) and voluntary wheel running were measured. [Hb] did not differ between high runner (HR) and non-selected control (C) lines without EPO treatment. Both doses of EPO significantly (P<0.0001) increased [Hb] as compared with sham-injected animals, with no difference in [Hb] between the 100 microg kg(-1) and 300 microg kg(-1) dose levels (overall mean of 4.5 g dl(-1) increase). EPO treatment significantly increased VO2,max by approximately 5% in both the HR and C lines, with no dosexline type interaction. However, wheel running (revolutions per day) did not increase with EPO treatment in either the HR or C lines, and in fact significantly decreased at the higher dose in both line types. These results suggest that neither [Hb] per se nor VO2,max is limiting voluntary wheel running in the HR lines. Moreover, we hypothesize that the decrease in wheel running at the higher dose of EPO may reflect direct action on the reward pathway of the brain.

A reliable and valid protocol for measuring maximal oxygen uptake in pigs

BACKGROUND

Most knowledge about cellular and molecular adaptation in the heart after exercise training comes from rodent models, and this has substantially improved our knowledge about exercise-induced cardiac adaptations. However, in rodents, the electrophysiological properties of the heart are different from the human heart. Therefore, the need of exercise-training models in larger animal models is obvious. Physiological studies of cardio-respiratory fitness require training regimens that give robust and adequate testing procedures to quantify the outcome.

METHODS

We developed a valid and reproducible protocol for measuring maximal oxygen uptake (VO2max) in young pigs. As previous studies have exercised pigs using horizontal treadmills, we determined whether treadmill inclination may influence the level of peak oxygen uptake (VO2peak) achieved, and whether the true VO2max was reached. Eight young pigs were used. Submaximal and VO2peak were tested at five different inclinations from 13 to 30 degrees .

RESULTS

At submaximal VO2, there was an excellent test-retest at all treadmill inclinations (r = 0.99, coefficient of variation = 1.8%). The level of VO2peak was dependent upon treadmill inclination and the true VO2max, defined as a levelling-off of VO2 despite increased running speed, was only reached a treadmill inclination of 24 degrees . For VO2peak we only observed a significant test-retest correlation when using 19 and 24 degrees inclination of the treadmill (r = 0.88, coefficient of variation = 9.7%).

CONCLUSION

The use of inappropriate treadmill inclination might hide training-induced adaptations if the true VO2max is not reached. This study shows that the present test protocol can be used in future studies of exercise on treadmill, when the aim is to measure submaximal and VO2max in pigs.

Endurance capacity of mice selectively bred for high voluntary wheel running

Mice from four lines bred for high voluntary wheel activity run approximately 3-fold more revolutions per day and have elevated maximal oxygen consumption during forced treadmill exercise, as compared with four unselected control (C) lines. We hypothesized that these high runner (HR) lines would have greater treadmill endurance-running capacity. Ninety-six mice from generation 49 were familiarized with running on a motorized treadmill for 3 days. On days 4 and 5, mice were given an incremental speed test (starting at 20 m min(-1), increased 1.5 m min(-1) every 2 min) and endurance was measured as the total time or distance run to exhaustion. Blood samples were taken to measure glucose and lactate concentrations at rest during the photophase, during peak nightly wheel running, and immediately following the second endurance test. Individual differences in endurance time were highly repeatable between days (r=0.79), and mice tended to run longer on the second day (paired t-test, P<0.0001). Blood glucose following the treadmill test was low for all animals ( approximately 53 mg dl(-1)) and lactate was high ( approximately 6.5 mmol l(-1)), suggesting that exhaustion occurred. The HR lines had significantly higher endurance than the C lines (1-tailed P<0.05), whether or not body mass was used as a covariate in the analysis. The relationship between line means for wheel running and treadmill endurance differed between the sexes, reinforcing previous studies that indicate sex-specific responses to selective breeding. HR mice appear to have a higher endurance capacity than reported in the literature for inbred strains of mice or transgenics intended to enhance endurance.

Interval training normalizes cardiomyocyte function, diastolic Ca2+ control, and SR Ca2+ release synchronicity in a mouse model of diabetic cardiomyopathy

RATIONALE

In the present study we explored the mechanisms behind excitation-contraction (EC) coupling defects in cardiomyocytes from mice with type-2 diabetes (db/db).

OBJECTIVE

We determined whether 13 weeks of aerobic interval training could restore cardiomyocyte Ca(2+) cycling and EC coupling.

METHODS AND RESULTS

Reduced contractility in cardiomyocytes isolated from sedentary db/db was associated with increased diastolic sarcoplasmic reticulum (SR)-Ca(2+) leak, reduced synchrony of Ca(2+) release, reduced transverse (T)-tubule density, and lower peak systolic and diastolic Ca(2+) and caffeine-induced Ca(2+) release. Additionally, the rate of SR Ca(2+) ATPase-mediated Ca(2+) uptake during diastole was reduced, whereas a faster recovery from caffeine-induced Ca(2+) release indicated increased Na(+)/Ca(2+)-exchanger activity. The increased SR-Ca(2+) leak was attributed to increased Ca(2+)-calmodulin-dependent protein kinase (CaMKIIdelta) phosphorylation, supported by the normalization of SR-Ca(2+) leak on inhibition of CaMKIIdelta (AIP). Exercise training restored contractile function associated with restored SR Ca(2+) release synchronicity, T-tubule density, twitch Ca(2+) amplitude, SR Ca(2+) ATPase and Na(+)/Ca(2+)-exchanger activities, and SR-Ca(2+) leak. The latter was associated with reduced phosphorylation of cytosolic CaMKIIdelta. Despite normal contractile function and Ca(2+) handling after the training period, phospholamban was hyperphosphorylated at Serine-16. Protein kinase A inhibition (H-89) in cardiomyocytes from the exercised db/db group abolished the differences in SR-Ca(2+) load when compared with the sedentary db/db mice. EC coupling changes were observed without changes in serum insulin or glucose levels, suggesting that the exercise training-induced effects are not via normalization of the diabetic condition.

CONCLUSIONS

These data demonstrate that aerobic interval training almost completely restored the contractile function of the diabetic cardiomyocyte to levels close to sedentary wild type.

Trans‐sodium crocetinate does not affect oxygen uptake in rats during treadmill running

Trans-sodium crocetinate (TSC), the isomer of the carotenoid compound crocetin, is found markedly to increase survival in hemorrhagic shock subsequent to 50-60% blood loss, mainly via restored resting oxygen consumption (VO(2)), blood pressure and heart rate. The proposed mechanism is that TSC increases oxygen diffusivity, and thus availability, in plasma. If this were found to be a prominent feature in the oxygen transfer from blood to skeletal muscle fiber mitochondria, increased VO(2) during exercise would be expected because of reduced partial pressure of venous oxygen (increased utilization), which we aimed to elucidate in this study. Male Sprague-Dawley rats were intravenously injected with 0.3 mL kg(-1) TSC (40 microg mL(-1)) or placebo and immediately thereafter tested on a ramped treadmill test protocol. Rats were introduced to the experimental protocols beforehand. Administration of TSC had a neutral effect on submaximal and maximal VO(2) (VO(2max)) as well as running performance measured as maximal running time and maximal aerobic running velocity. Thus, in this study we cannot report any effects of TSC on steady-state submaximal VO(2) or VO(2max) at exhaustive exercise.

Exhaustive exercise training enhances aerobic capacity in American alligator (Alligator mississippiensis)

The oxygen transport system in mammals is extensively remodelled in response to repeated bouts of activity, but many reptiles appear to be ‘metabolically inflexible’ in response to exercise training. A recent report showed that estuarine crocodiles (Crocodylus porosus) increase their maximum metabolic rate in response to exhaustive treadmill training, and in the present study, we confirm this response in another crocodilian, American alligator (Alligator mississippiensis). We further specify the nature of the crocodilian training response by analysing effects of training on aerobic [citrate synthase (CS)] and anaerobic [lactate dehydrogenase (LDH)] enzyme activities in selected skeletal muscles, ventricular and skeletal muscle masses and haematocrit. Compared to sedentary control animals, alligators regularly trained for 15 months on a treadmill (run group) or in a flume (swim group) exhibited peak oxygen consumption rates higher by 27 and 16%, respectively. Run and swim exercise training significantly increased ventricular mass (~11%) and haematocrit (~11%), but not the mass of skeletal muscles. However, exercise training did not alter CS or LDH activities of skeletal muscles. Similar to mammals, alligators respond to exercise training by increasing convective oxygen transport mechanisms, specifically heart size (potentially greater stroke volume) and haematocrit (increased oxygen carrying-capacity of the blood). Unlike mammals, but similar to squamate reptiles, alligators do not also increase citrate synthase activity of the skeletal muscles in response to exercise.

Iron injections in mice increase skeletal muscle iron content, induce oxidative stress and reduce exercise performance

Iron accelerates the production of reactive oxygen species (ROS). Excessive levels of ROS are thought to accelerate skeletal muscle fatigue and contribute to the loss of skeletal muscle mass and function with age. Patients with an iron overload disease frequently report symptoms of weakness and fatigue, which is attributed to reduced cardiac function. The contribution of skeletal muscle to these symptoms is unknown. Using a mouse model of iron overload, we determined the extent of iron accumulation in skeletal muscle and the concentrations of the iron storage protein ferritin. The level of oxidative stress, changes in antioxidant enzymes and exercise performance were also assessed. Compared with control mice, the iron overloaded mice had elevated levels of iron in the tibialis anterior muscle and a fourfold increase in ferritin light chain. The oxidative stress product malondialdehyde was increased in the iron group compared with the control group, as was the antioxidant enzyme activity of glutathione reductase and glutathione peroxidase. The iron group performed less work on an endurance test and produced less force in a strength test. Body weight and skeletal muscle weight were lower in the iron group following the intervention. Iron loading reduced the weight of the fast-twitch extensor digitorum longus muscle more than the slow-twitch soleus muscle. In summary, iron accumulation in skeletal muscle may play a significant role in the reduced exercise capacity seen in iron overload disorders and in ageing, and may play an underlying role in skeletal muscle atrophy.

Running speed and maximal oxygen uptake in rats and mice: practical implications for exercise training

Valid and reliable experimental models are essential to gain insight into the cellular and molecular mechanisms underlying the beneficial effects of exercise in prevention, treatment, and rehabilitation of lifestyle-related diseases. Studies with large changes, low variation, and reproducible training outcome require individualized training intensity, controlled by direct measurements of maximal oxygen uptake or heart rate. As this approach is expensive and time consuming, we discuss whether maximal treadmill running speed in a gradually increasing ramp protocol might be sufficient to control intensity without losing accuracy. Combined data from six studies of rats and mice from our lab demonstrated a close correlation between running speed and oxygen uptake. This relationship changed towards a steeper linear slope after endurance training, indicating improved work economy, that is, less oxygen was consumed at fixed submaximal running speeds. Maximal oxygen uptake increased 40-70% after high-intensity aerobic interval training in mice and rats. The speed at which oxygen uptake reached a plateau, increased in parallel with the change in maximal oxygen uptake during the training period. Although this suggests that running speed can be used to assess training intensity throughout a training program, the problem is to determine the exact relative intensity related to maximal oxygen uptake from running speed alone. We therefore suggest that directly measured oxygen uptake should be used to assess exercise intensity and optimize endurance training in rats and mice. Running speed may serve as a supplement to ensure this intensity.

Relationships among running performance, aerobic physiology and organ mass in male Mongolian gerbils

Relationships among individual variation in exercise capacity, resting metabolism and morphology may offer insights into the mechanistic basis of whole-animal performance, including possible performance trade-offs (e.g. burst versus sustainable exercise, resting ;maintenance' costs versus maximal power output). Although there have been several studies of correlations between performance, metabolism and morphology in fish, birds and squamate reptiles, relatively little work has been done with mammals. We measured several aspects of forced and voluntary locomotor performance in Mongolian gerbils (Meriones unguiculatus), along with minimal and maximal aerobic metabolic rates and organ sizes (mainly visceral organs and the musculoskeletal system). Maximal sprint and aerobic speeds and maximal oxygen consumption (VO2max)) during forced exercise were similar to those of other small rodents; basal metabolic rate was below allometric predictions. At all tested speeds, voluntary running had a lower energy cost than forced treadmill running, due primarily to a higher zero-speed intercept of the speed-versus-power (oxygen consumption) relationship during forced running. Incremental costs of transport (slopes of speed-versus-power regressions) were slightly higher during voluntary exercise. Few of the correlations among performance variables, or between performance and organ morphology, were statistically significant. These results are consistent with many other studies that found weak correlations between organismal performance (e.g. VO2max)) and putatively relevant subordinate traits, thus supporting the idea that some components within a functional system may exhibit excess capacity at various points in the evolutionary history of a population, while others constitute limiting factors.

Activation or inactivation of cardiac Akt/mTOR signaling diverges physiological from pathological hypertrophy

Cardiomyocyte hypertrophy differs according to the stress exerted on the myocardium. While pressure overload-induced cardiomyocyte hypertrophy is associated with depressed contractile function, physiological hypertrophy after exercise training associates with preserved or increased inotropy. We determined the activation state of myocardial Akt signaling with downstream substrates and fetal gene reactivation in exercise-induced physiological and pressure overload-induced pathological hypertrophies. C57BL/6J mice were either treadmill trained for 6 weeks, 5 days/week, at 85-90% of maximal oxygen uptake (VO(2max)), or underwent transverse aortic constriction (TAC) for 1 or 8 weeks. Total and phosphorylated protein levels were determined with SDS-PAGE, and fetal genes by real-time RT-PCR. In the physiologically hypertrophied heart after exercise training, total Akt protein level was unchanged, but Akt was chronically hyperphosphorylated at serine 473. This was accompanied by activation of the mammalian target of rapamycin (mTOR), measured as phosphorylation of its two substrates: the ribosomal protein S6 kinase-1 (S6K1) and the eukaryotic translation initiation factor-4E binding protein-1 (4E-BP1). Exercise training did not reactivate the fetal gene program (beta-myosin heavy chain, atrial natriuretic factor, skeletal muscle actin). In contrast, pressure overload after TAC reactivated fetal genes already after 1 week, and partially inactivated the Akt/mTOR pathway and downstream substrates after 8 weeks. In conclusion, changes in opposite directions of the myocardial Akt/mTOR signal pathway appears to distinguish between physiological and pathological hypertrophies; exercise training associating with activation and pressure overload associating with inactivation of the Akt/mTOR pathway.

Endurance exercise promotes cardiorespiratory rehabilitation without neurorestoration in the chronic mouse model of parkinsonism with severe neurodegeneration

Physical rehabilitation with endurance exercise for patients with Parkinson's disease has not been well established, although some clinical and laboratory reports suggest that exercise may produce a neuroprotective effect and restore dopaminergic and motor functions. In this study, we used a chronic mouse model of Parkinsonism, which was induced by injecting male C57BL/6 mice with 10 doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (25 mg/kg) and probenecid (250 mg/kg) over 5 weeks. This chronic parkinsonian model displays a severe and persistent loss of nigrostriatal neurons, resulting in robust dopamine depletion and locomotor impairment in mice. Following the induction of Parkinsonism, these mice were able to sustain an exercise training program on a motorized rodent treadmill at a speed of 18 m/min, 0 degrees of inclination, 40 min/day, 5 days/week for 4 weeks. At the end of exercise training, we examined and compared their cardiorespiratory capacity, behavior, and neurochemical changes with that of the probenecid-treated control and sedentary parkinsonian mice. The resting heart rate after 4 weeks of exercise in the chronic parkinsonian mice was significantly lower than the rate before exercise, whereas the resting heart rate at the beginning and 4 weeks afterward in the control or sedentary parkinsonian mice was unchanged. Exercised parkinsonian mice also recovered from elevated electrocardiogram R-wave amplitude that was detected in the parkinsonian mice without exercise for 4 weeks. The values of oxygen consumption, carbon dioxide production, and body heat generation in the exercised parkinsonian mice before and during the Bruce maximal exercise challenge test were all significantly lower than that of their sedentary counterparts. Furthermore, the exercised parkinsonian mice demonstrated a greater mass in the left ventricle of the heart and an increased level of citrate synthase activity in the skeletal muscles. The amphetamine-induced, dopamine release-dependent locomotor activity was markedly inhibited in the sedentary parkinsonian mice and was also inhibited in the exercised parkinsonian mice. Finally, neuronal recovery from the loss of nigrostriatal tyrosine hydroxylase expression and dopamine levels in the severe parkinsonian mice after exercise was not evident. Taken all together, these data suggest that 4 weeks of treadmill exercise promoted physical endurance, resulting in cardiorespiratory and metabolic adaptations in the chronic parkinsonian mice with severe neurodegeneration without demonstrating a restorative potential for the nigrostriatal dopaminergic function.

Efeitos do exercício físico sobre o estado redox cerebral

A atividade física é conhecida por promover saúde e bem-estar. O exercício também é responsável por aumentar a produção de Espécies Reativas de Oxigênio (ERO) pelo acréscimo do consumo de oxigênio mitocondrial nos tecidos. O desequilíbrio entre a produção de EROs e as defesas oxidantes dos tecidos pode provocar danos oxidativos a proteínas, lipídios e DNA. O dano oxidativo cerebral é um mecanismo etiopatológico comum da apoptose e da neurodegeneração. O fator de crescimento cérebro-derivado desempenha um importante papel neste contexto. Nesta revisão, apresentamos os resultados de diferentes modelos de exercício físico no metabolismo oxidativo e neurotrófico do Sistema Nervoso Central (SNC). Também revisamos estudos que utilizaram suplementação antioxidante para prevenir danos oxidativos exercício-induzido ao SNC. Os modelos de exercício físico mais comuns foram as rodas de correr, a natação e a esteira com configurações de treinamento muito diferentes como a duração e a intensidade. Os resultados do treinamento físico no tecido cerebral são muito controversos, mas geralmente demonstram ganhos na plasticidade sináptica e na função cognitiva com exercícios de intensidade moderada e baixa.

Nitric oxide synthase type-1 modulates cardiomyocyte contractility and calcium handling: association with low intrinsic aerobic capacity

BACKGROUND

The neuronal isoform of nitric oxide synthase (NOS-1) may be an important regulator of cardiac contractility by modifying calcium release and uptake from sarcoplasmic reticulum. Our working hypothesis was that NOS-1 modulates cardiomyocyte contractility more markedly in rat lines with low versus high congenital aerobic fitness.

METHODS AND RESULTS

Rats performed high-intensity interval treadmill running 5 days per week over 8 weeks; age-matched sedentary rats served as controls. At baseline before the training program, aerobic fitness measured as maximal oxygen uptake was 30% higher, and cardiomyocyte contractility measured as fractional shortening 42% higher in high than in low congenital aerobic fitness rats. Training markedly increased aerobic fitness as well as cardiomyocyte contractility, relaxation and corresponding changes in calcium transient in both lines. Selective inhibition of NOS-1 increased cardiomyocyte contractility (12-43%) and calcium transient amplitude (10-28%), prolonged time to 50% relengthening (13-52%) and time to 50% calcium decay (17-35%), in all groups. Interestingly, NOS-1-inhibition abolished the difference in systolic events between low and high congenital aerobic fitness whereas no such findings occurred in diastolic parameters.

CONCLUSION

NOS-1-derived nitric oxide production is a modulator of cardiomyocyte contractile performance and calcium handling in rats. It accounts for some of the difference between rats with low versus high congenital aerobic fitness, whereas it contributes little during adaptation to exercise training.

In Vivo Characterization of Murine Myocardial Perfusion With Myocardial Contrast Echocardiography

Background— The ability to noninvasively evaluate murine myocardial blood flow (MBF) in vivo would provide an important tool for cardiovascular research. Myocardial contrast echocardiography (MCE) has been used to measure MBF; however, it has not been validated in mice. This study assesses whether MCE can evaluate MBF at rest and after vasodilation and measure the maximal augmentation (coronary reserve) of MBF in mice. Wild-type (WT) and nitric oxide synthase 3 (NOS3)–deficient (NOS3 −/− ) mice were studied.

Methods and Results— MCE was performed at baseline and after intravenous infusion of acetylcholine or adenosine. Definity contrast agent was infused, and parasternal views were acquired in real-time mode. Replenishment curves of myocardial contrast were obtained, and rates of signal rise (β) and plateau intensity (A) were calculated. MBF estimated by the product of A and β (Aβ) was compared with that measured with fluorescent microspheres. MCE analysis was feasible in 98% (52/53) of mice. MBF measured by microspheres increased with adenosine and correlated closely with Aβ. There was no difference in MCE-derived MBF between WT and NOS3 −/− mice at rest. Adenosine infusion increased MBF by 3.0±0.6-fold in NOS3 −/− mice and 2.5±0.3-fold in WT ( P =0.58 between genotypes). Acetylcholine induced an increase of 2.4±0.2-fold in MBF in WT mice but did not increase MBF in NOS3 −/− mice ( P <0.0005 versus WT).

Conclusions— MBF, coronary reserve, and vasodilator responses can be evaluated accurately in the intact mouse by MCE. This method demonstrated a preserved coronary response to adenosine but an impaired acetylcholine-induced vasodilation in NOS3 −/− mice compared with WT mice.

Cardiovascular characterization of Pkd2+/LacZ mice, an animal model for the autosomal dominant polycystic kidney disease type 2 (ADPKD2)

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2. Patients with ADPKD have an increased incidence of cardiac valve abnormalities and left ventricular hypertrophy. Systematic analyses of cardiovascular involvement have so far been performed only on genetically unclassified patients or on ADPKD1 patients, but not on genetically defined ADPKD2 patients. Even existing Pkd1 or Pkd2 mouse models were not thoroughly analyzed in this respect. Therefore, the aim of this project was the noninvasive functional cardiovascular characterization of a mouse model for ADPKD2.

METHODS

Pkd2(+/LacZ) mice and wildtype controls were classified into 8 groups with respect to gender, age and genotype. In addition, two subgroups of female mice were analyzed for cardiac function before and during advanced pregnancy. Doppler-echocardiographic as well as histological studies were performed.

RESULTS

Doppler-echocardiography did not reveal significant cardiovascular changes. Heart rate and left ventricular (LV) length, LV mass, LV enddiastolic and LV endsystolic diameters did not differ significantly among the various groups when comparing wildtype and knockout mice. There were no significant differences except for a tendency towards higher maximal early and late flow velocities over the mitral valve in old wildtype mice.

CONCLUSIONS

Non-invasive phenotyping using ultrasound did not reveal significant cardiovascular difference between adult Pkd2(+/LacZ) and WT mice. Due to the lack of an obvious renal phenotype in heterozygous mice, it is likely that in conventional ADPKD knock out mouse models severe cardiac problems appear too late to be identified during the reduced lifespan of the animals.

Effects of Excessive Long-Term Exercise on Cardiac Function and Myocyte Remodeling in Hypertensive Heart Failure Rats

The long-term effects of exercise on cardiac function and myocyte remodeling in hypertension/progression of heart failure are poorly understood. We investigated whether exercise can attenuate pathological remodeling under hypertensive conditions. Fifteen female Spontaneously Hypertensive Heart Failure rats and 10 control rats were housed with running wheels beginning at 6 months of age. At 22 months of age, heart function of the trained rats was compared with heart function of age-matched sedentary hypertensive and control rats. Heart function was measured using echocardiography and left ventricular catheterization. Cardiac myocytes were isolated to measure cellular dimensions. Fetal gene expression was determined using Western blots. Exercise did not significantly impact myocyte remodeling or ventricular function in control animals. Sedentary hypertensive rats had significant chamber dilatation and cardiac hypertrophy. In exercised hypertensive rats, however, exercise time was excessive and resulted in a 21% increase in left ventricular diastolic dimension (P<0.001), a 24% increase in heart to body weight ratio (P<0.05), a 27% increase in left ventricular myocyte volume (P<0.01), a 13% reduction in ejection fraction (P<0.001), and a 22% reduction in fractional shortening (P<0.01) compared with sedentary hypertensive rats. Exercise resulted in greater fibrosis and did not prevent activation of the fetal gene program in hypertensive rats. We conclude that excessive exercise, in the untreated hypertensive state can have deleterious effects on cardiac remodeling and may actually accelerate the progression to heart failure.

Aerobic interval training enhances cardiomyocyte contractility and Ca2+ cycling by phosphorylation of CaMKII and Thr-17 of phospholamban

Cardiac adaptation to aerobic exercise training includes improved cardiomyocyte contractility and calcium handling. Our objective was to determine whether cytosolic calcium/calmodulin-dependent kinase II and its downstream targets are modulated by exercise training. A six-week aerobic interval training program by treadmill running increased maximal oxygen uptake by 35% in adult mice, whereupon left ventricular cardiomyocyte function was studied and myocardial tissue samples were used for biochemical analysis. Cardiomyocytes from trained mice had enhanced contractility and faster relaxation rates, which coincided with larger amplitude and faster decay of the calcium transient, but not increased peak systolic calcium levels. These changes were associated with reduced phospholamban expression relative to sarcoplasmic reticulum calcium ATPase and constitutively increased phosphorylation of phospholamban at the threonine 17, but not at the serine 16 site. Calcium/calmodulin-dependent kinase IIdelta phosphorylation was increased at threonine 287, indicating activation. To investigate the physiological role of calcium/calmodulin-dependent kinase IIdelta phosphorylation, this kinase was blocked specifically by autocamtide-2 related inhibitory peptide II. This maneuver completely abolished training-induced improvements of cardiomyocyte contractility and calcium handling and blunted, but did not completely abolish the training-induced increase in Ca(2+) sensitivity. Also, inhibition of calcium/calmodulin-dependent kinase II reduced the greater frequency-dependent acceleration of relaxation that was observed after aerobic interval training. These observations indicate that calcium/calmodulin-dependent kinase IIdelta contributes significantly to the functional adaptation of the cardiomyocyte to regular exercise training.

Treadmill but not wheel running improves fatigue resistance of isolated extensor digitorum longus muscle in mice

AIM

The present study is the first to compare the physiological impact of either forced treadmill or voluntary wheel running exercise on hindlimb muscle in mice.

METHODS

Male C57BL/6 mice were subjected to either 6 weeks of forced treadmill or voluntary wheel running exercise. Mice in the treadmill running exercise group (TRE; n = 8) ran 1.9 km day(-1) at a speed of 16 m min(-1) against an uphill incline of 11 degrees. In the running wheel exercise group (RWE; n = 8) animals ran 8.8 +/- 0.2 km per day (average speed 42 +/- 2 m min(-1)). After the experimental period, animals were killed and mechanical performance and oxygen consumption of isolated extensor digitorum longus (EDL) muscle were determined during serial electrical stimulation at 0.5, 1 and 2 Hz.

RESULTS

Steady-state half-width time (HWT) of twitch contraction at 0.5 Hz was significantly shorter in TRE and RWE than controls (CON) (41.3 +/- 0.2, 41.3 +/- 0.1 and 44.3 +/- 0.1 s respectively; P < 0.05). The rate of fatigue development and HWT lengthening at 2 Hz was the same in RWE and CON but lower in TRE (1.2-fold and twofold respectively; P < 0.05). EDL oxygen consumption, mitochondrial content and myosin heavy chain (MyHC) composition were not different between the groups.

CONCLUSION

These results indicate that both exercise modalities have an effect on a hindlimb fast-twitch muscle in mice, with the greatest impact seen with forced treadmill running.

Myocyte death and renewal: modern concepts of cardiac cellular homeostasis

The adult mammalian myocardium has a robust intrinsic regenerative capacity because of the presence of cardiac stem cells (CSCs). Despite being mainly composed of terminally differentiated myocytes that cannot re-enter the cell cycle, the heart is not a postmitotic organ and maintains some capacity to form new parenchymal cells during the lifespan of the organism. Myocyte death and formation of new myocytes by the CSCs are the two processes that enable this organ to maintain a proper and uninterrupted cardiac output from birth to adulthood and into old age. CSCs are activated in response to pathological or physiological stimuli, whereby they enter the cell cycle and differentiate into new myocytes (and vessels) that significantly contribute to changes in myocardial mass. The future of regenerative cardiovascular medicine is arguably dependent on our success in dissecting the biology and mechanisms regulating the number, growth, differentiation, and aging of CSCs. This information will generate the means to manipulate CSC growth, survival, and differentiation and, therefore, will provide the tools for the design of more physiologically relevant clinical regeneration protocols. In this article, we review the developments in cardiac cell biology that might, in our opinion, have a broad impact on cardiovascular medicine.

Small gene effect and exercise training-induced cardiac hypertrophy in mice: an Ace gene dosage study

Small gene effects influence complex phenotypes in a context dependent manner. Here we evaluated whether increasing dosage of the angiotensin I converting enzyme ( Ace) gene influence exercise-induced cardiac hypertrophy. Mice harboring one, two, three, and four copies of the Ace gene were assigned to sedentary (S1–4) and swimming exercise-trained (T1–4) groups (1.5 h twice daily, 5 days/wk, 4 wk). Exercising resulted in comparable bradycardia and elevated skeletal muscle citrate synthase activity, while blood pressure remained unchanged. Left ventricle mass index and cardiomyocyte diameter were similar among sedentary mice and the magnitude of their increase associated to exercising was not influenced by the Ace genotype (T1: 12.6 and 17.9%, T2: 15.2 and 13.8%, T3: 16.9 and 20%, T4: 17 and 19%, respectively). Plasma renin activity (PRA) levels were higher in one vs. three or four copies mice (4.89 ± 0.5 vs. 2.43 ± 0.6 and 2.12 ± 01.1 ng/ml Ang I, P < 0.05), while cardiac ACE activity was higher in three vs. two or one copy mice (5,946 ± 590.8 vs. 2,951.5 ± 328.3 and 3,504.1 ± 258.9 μF·min−1·ml−1, P < 0.05). With exercise, PRA remained unchanged in each group, while cardiac immunostaining for Ang II reached comparable levels. In summary: 1) exercise training led to similar aerobic adaptation regardless of the Ace genotype, and 2) higher number of Ace gene copies per se, which alters cardiac ACE activity, did not influence basal cardiac mass or, most importantly, the magnitude of swimming-induced cardiac hypertrophy. Collectively, these data indicate that small isolated genetic disturbances in ACE cardiac levels can be well compensated under physiological perturbations.

Impact of endurance training on murine spontaneous activity, muscle mitochondrial DNA abundance, gene transcripts, and function

We hypothesized that enhanced skeletal muscle mitochondrial function following aerobic exercise training is related to an increase in mitochondrial transcription factors, DNA abundance [mitochondrial DNA (mtDNA)], and mitochondria-related gene transcript levels, as well as spontaneous physical activity (SPA) levels. We report the effects of daily treadmill training on 12-wk-old FVB mice for 5 days/wk over 8 wk at 80% peak O(2) consumption and studied the training effect on changes in body composition, glucose tolerance, muscle mtDNA muscle, mitochondria-related gene transcripts, in vitro muscle mitochondrial ATP production capacity (MATPC), and SPA levels. Compared with the untrained mice, the trained mice had higher peak O(2) consumption (+18%; P < 0.001), lower percentage of abdominal (-25.4%; P < 0.02) and body fat (-19.5%; P < 0.01), improved glucose tolerance (P < 0.04), and higher muscle mitochondrial enzyme activity (+19.5-43.8%; P < 0.04) and MATPC (+28.9 to +32.4%; P < 0.01). Gene array analysis showed significant differences in mRNAs of mitochondria-related ontology groups between the trained and untrained mice. Training also increased muscle mtDNA (+88.4 to +110%; P < 0.05), peroxisome proliferative-activated receptor-gamma coactivator-1alpha protein (+99.5%; P < 0.04), and mitochondrial transcription factor A mRNA levels (+21.7%; P < 0.004) levels. SPA levels were higher in trained mice (P = 0.056, two-sided t-test) and significantly correlated with two separate substrate-based measurements of MATPC (P < 0.02). In conclusion, aerobic exercise training enhances muscle mitochondrial transcription factors, mtDNA abundance, mitochondria-related gene transcript levels, and mitochondrial function, and this enhancement in mitochondrial function occurs in association with increased SPA.

alpha-Neurexins are required for efficient transmitter release and synaptic homeostasis at the mouse neuromuscular junction

Neurotransmission at chemical synapses of the brain involves alpha-neurexins, neuron-specific cell-surface molecules that are encoded by three genes in mammals. Deletion of alpha-neurexins in mice previously demonstrated an essential function, leading to early postnatal death of many double-knockout mice and all triple mutants. Neurotransmitter release at central synapses of newborn knockouts was severely reduced, a function of alpha-neurexins that requires their extracellular sequences. Here, we investigated the role of alpha-neurexins at neuromuscular junctions, presynaptic terminals that lack a neuronal postsynaptic partner, addressing an important question because the function of neurexins was hypothesized to involve cell-adhesion complexes between neurons. Using systems physiology, morphological analyses and electrophysiological recordings, we show that quantal content, i.e. the number of acetylcholine quanta released per nerve impulse from motor nerve terminals, and frequency of spontaneous miniature endplate potentials at the slow-twitch soleus muscle are reduced in adult alpha-neurexin double-knockouts, consistent with earlier data on central synapses. However, the same parameters at diaphragm muscle neuromuscular junctions showed no difference in basal neurotransmission. To reconcile these observations, we tested the capability of control and alpha-neurexin-deficient diaphragm neuromuscular junctions to compensate for an experimental reduction of postsynaptic acetylcholine receptors by a compensatory increase of presynaptic release: Knockout neuromuscular junctions produced significantly less upregulation of quantal content than synapses from control mice. Our data suggest that alpha-neurexins are required for efficient neurotransmitter release at neuromuscular junctions, and that they may perform a role in the molecular mechanism of synaptic homeostasis at these peripheral synapses.

Maximum aerobic performance in lines ofMusselected for high wheel-running activity: effects of selection, oxygen availability and the mini-muscle phenotype

We compared maximum aerobic capacity during forced exercise(V̇O2max) in hypoxia (PO2=14% O2), normoxia (21%) and hyperoxia (30%) of lines of house mice selectively bred for high voluntary wheel running (S lines) with their four unselected control (C) lines. We also tested for pleiotropic effects of the `mighty mini-muscle' allele, a Mendelian recessive that causes a 50% reduction in hind limb muscle but a doubling of mass-specific aerobic enzyme activity, among other pleiotropic effects. V̇O2max of female mice was measured during forced exercise on a motorized treadmill enclosed in a metabolic chamber that allowed altered PO2. Individual variation in V̇O2max was highly repeatable within each PO2, and values were also significantly correlated across PO2. Analysis of covariance showed that S mice had higher body-mass-adjusted V̇O2max than C at all PO2, ranging from +10.7% in hypoxia to +20.8% in hyperoxia. V̇O2maxof S lines increased practically linearly with PO2,whereas that of C lines plateaued from normoxia to hyperoxia, and respiratory exchange ratio (=CO2production/V̇O2max)was lower for S lines. These results suggest that the physiological underpinnings of V̇O2max differ between the S and C lines. Apparently, at least in S lines, peripheral tissues may sustain higher rates of oxidative metabolism if central organs provide more O2. Although the existence of central limitations in S lines cannot be excluded based solely on the present data, we have previously reported that both S and C lines can attain considerably higher V̇O2max during cold exposure in a He-O2 atmosphere, suggesting that limitations on V̇O2max depend on interactions between the central and peripheral organs involved. In addition,mini-muscle individuals had higher V̇O2max than did those with normal muscles, suggesting that the former might have higher hypoxia tolerance. This would imply that the mini-muscle phenotype could be a good model to test how exercise performance and hypoxia tolerance could evolve in a correlated fashion, as previous researchers have suggested.

Transcriptional profiling in mouse skeletal muscle following a single bout of voluntary running: evidence of increased cell proliferation

Skeletal muscle undergoes adaptation following repetitive bouts of exercise. We hypothesize that transcriptional reprogramming and cellular remodeling start in the early phase of long-term training and play an important role in skeletal muscle adaptation. The aim of this study was to define the global mRNA expression in mouse plantaris muscle during (run for 3 and 12 h) and after (3, 6, 12, and 24 h postexercise) a single bout of voluntary running and compare it with that after long-term training (4 wk of running). Among 15,832 gene elements surveyed in a high-density cDNA microarray analysis, 900 showed more than twofold changes at one or more time points. K-means clustering and cumulative hypergeometric probability distribution analyses revealed a significant enrichment of genes involved in defense, cell cycle, cell adhesion and motility, signal transduction, and apoptosis, with induced expression patterns sharing similar patterns with that of peroxisome proliferator activator receptor-gamma coactivator-1alpha and vascular endothelial growth factor A. We focused on the finding of a delayed (at 24 h postexercise) induction of mRNA expression of cell cycle genes origin recognition complex 1, cyclin A2, and cell division 2 homolog A (Schizoccharomyces pombe) and confirmed increased cell proliferation by in vivo 5-bromo-2'-deoxyuridine labeling following voluntary running. X-ray irradiation of the hindlimb significantly diminished exercise-induced 5-bromo-2'-deoxyuridine incorporation. These findings suggest that a single bout of voluntary running activates the transcriptional network and promotes adaptive processes in skeletal muscle, including cell proliferation.

Effects of size, sex, and voluntary running speeds on costs of locomotion in lines of laboratory mice selectively bred for high wheel-running activity

Selective breeding for over 35 generations has led to four replicate (S) lines of laboratory house mice (Mus domesticus) that run voluntarily on wheels about 170% more than four random-bred control (C) lines. We tested whether S lines have evolved higher running performance by increasing running economy (i.e., decreasing energy spent per unit of distance) as a correlated response to selection, using a recently developed method that allows for nearly continuous measurements of oxygen consumption (VO2) and running speed in freely behaving animals. We estimated slope (incremental cost of transport [COT]) and intercept for regressions of power (the dependent variable, VO2/min) on speed for 49 males and 47 females, as well as their maximum VO2 and speeds during wheel running, under conditions mimicking those that these lines face during the selection protocol. For comparison, we also measured COT and maximum aerobic capacity (VO2max) during forced exercise on a motorized treadmill. As in previous studies, the increased wheel running of S lines was mainly attributable to increased average speed, with males also showing a tendency for increased time spent running. On a whole-animal basis, combined analysis of males and females indicated that COT during voluntary wheel running was significantly lower in the S lines (one-tailed P=0.015). However, mice from S lines are significantly smaller and attain higher maximum speeds on the wheels; with either body mass or maximum speed (or both) entered as a covariate, the statistical significance of the difference in COT is lost (one-tailed P> or =0.2). Thus, both body size and behavior are key components of the reduction in COT. Several statistically significant sex differences were observed, including lower COT and higher resting metabolic rate in females. In addition, maximum voluntary running speeds were negatively correlated with COT in females but not in males. Moreover, males (but not females) from the S lines exhibited significantly higher treadmill VO2max as compared to those from C lines. The sex-specific responses to selection may in part be consequences of sex differences in body mass and running style. Our results highlight how differences in size and running speed can account for lower COT in S lines and suggest that lower COT may have coadapted in response to selection for higher running distances in these lines.