Fiber-type transitions and satellite cell activation in low-frequency-stimulated muscles of young and aging rats

We examined satellite cell content and the activity of satellite cell proge ny in tibialis anterior muscles of young (15 weeks) and aging (101 weeks) B rown Norway (BN) rats. after they were exposed for 50 days to a standardize d and highly reproducible regime of chronic low-frequency electrical stimul ation. Chronic low-frequency electrical stimulation was successful in induc ing fast-to-slow fiber-type transformation, characterized by a 2.3-fold inc rease in the proportion of IIA fibers and fourfold and sevenfold decreases in the proportion of IID/X and IIB fibers in both young and aging BN rats. These changes were accompanied by a twofold increase in the satellite cell content in both the young and aging groups: satellite cell content reached a level that was significantly higher in the young group (p < .04). The tot al muscle precursor cell content (i.e., satellite cells plus progeny), howe ver, did not differ between groups, because there was a greater number of s atellite cell progeny passing through the proliferative and differentiative compartments of the aging group. The resulting 1.5-fold increase in myonuc lear content was similar in the young and aging groups. We conclude that sa tellite cells and satellite cell progeny of aging BN rats possess an unalte red capacity to contribute to the adaptive response.