Loaded wheel running and muscle adaptation in the mouse

Area of Science:

• Exercise Physiology
• Muscle Adaptation
• Cardiac Physiology

Background:

• Voluntary cage wheel exercise is a common model for studying cardiac and skeletal muscle adaptations in mice.
• Understanding how different exercise loading conditions affect physiological responses is crucial for exercise science.

Purpose of the Study:

• To investigate the impact of varying resistance loads during voluntary cage wheel exercise on mouse performance and muscle adaptation.
• To differentiate the effects of unloaded versus loaded exercise on cardiac and skeletal muscle physiology.

Main Methods:

• Male C57Bl/6 mice underwent 7 weeks of voluntary cage wheel exercise with no-resistance (NR), low-resistance (LR), or high-resistance (HR) loads.
• Measurements included power output, heart mass, skeletal muscle mass (soleus), muscle fiber cross-sectional area, and cardiac gene/protein expression.
• Key molecular markers analyzed included calcineurin, myocyte enhancer factor-2 (MEF-2), and signaling pathways like Akt and p70 S6 kinase.

Main Results:

• Loaded exercise (LR and HR) significantly elevated power output compared to unloaded (NR) exercise.
• Unloaded exercise uniquely increased heart mass, while loaded exercise uniquely increased soleus muscle mass.
• Differential effects on muscle fiber cross-sectional area were observed based on myosin heavy chain expression.
• Cardiac adaptation showed decreased pathological hypertrophy genes and calcineurin in all groups, with reduced MEF-2 activity and altered Akt/p70 S6 kinase phosphorylation in loaded groups.

Conclusions:

• Unloaded and loaded voluntary cage wheel exercise induce distinct adaptations in cardiac and skeletal muscle.
• Exercise resistance level is a critical factor determining the physiological outcomes of exercise training in mice.
• These findings highlight the importance of considering exercise intensity and load in exercise physiology research.