Cardiac and skeletal muscle adaptations to voluntary wheel running in the mouse

In this paper, we describe the effects of voluntary cage wheel exercise on mouse cardiac and skeletal muscle. Inbred male C57/B16 mice (age 8-10 wk; n = 12) ran an average of 4.3 h/24 h, for an average distance of 6.8 km/24 h , and at an average speed of 26.4 m/min. A significant increase in the rati o of heart mass to body mass (mg/g) was evident after 2 wk of voluntary exe rcise, and cardiac atrial natriuretic factor and brain natriuretic peptide mRNA levels were significantly increased in the ventricles after 4 wk of vo luntary exercise. A significant increase in the percentage of fibers expres sing myosin heavy chain (MHC) IIa was observed in both the gastrocnemius an d the tibialis anterior (TA) by 2 wk, and a significant decrease in the per centage of fibers expressing IIb MHC was evident in both muscles after 4 wk of voluntary exercise. The TA muscle showed a greater increase in the perc entage of IIa MHC-expressing fibers than did the gastrocnemius muscle (40 a nd 20%, respectively, compared with 10% for nonexercised). Finally, the num ber of oxidative fibers as revealed by NADH-tetrazolium reductase histochem ical staining was increased in the TA but not the gastrocnemius after 4 wk of voluntary exercise. All results are relative to age-matched mice housed without access to running wheels. Together these data demonstrate that volu ntary exercise in mice results in cardiac and skeletal muscle adaptations c onsistent with endurance exercise.