Exercise-enhanced satellite cell proliferation and new myonuclear accretion in rat skeletal muscle

The effects of increased functional loading on early cellular regenerative events after exercise-induced injury in adult skeletal muscle were examined with the use of in vivo labeling of replicating myofiber nuclei and immuno cyto- and histochemical techniques. Satellite cell proliferation in the sol eus (Sol) of nonexercised rats (0.4 +/- 0.2% of fibers) was unchanged after an initial bout of declined treadmill exercise but was elevated after two (1.0 +/- 0.2%, P less than or equal to 0.01), but not four or seven, daily bouts of the same task. Myonuclei produced over the 7-day period comprised 0.9-1.9% of myonuclei in isolated fibers of Sol, tibialis anterior, and vas tus intermedius of nonexercised rats. The accretion of new myonuclei was en hanced (P less than or equal to 0.05) in Sol and vastus intermedius by the initial exercise followed by normal activity (to 3.1-3.4% of myonudei) and more so by continued daily exercise (4.2-5.3%). Observed coincident with a lower incidence of histological fiber injury and unchanged fiber diameter a nd myonuclei per millimeter, the greater new myonuclear accretion induced b y continued muscle loading may contribute to an enhanced fiber repair and r egeneration after exercise-induced injury.