Effect of satellite cell ablation on low-frequency-stimulated fast-to-slow fibre-type transitions in rat skeletal muscle

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Abstract
The purpose of this study was to determine whether satellite cell ablation within rat fast-twitch muscles exposed to chronic low-frequency stimulation (CLFS) would limit fast-to-slow fibre-type transitions. Twenty-nine male Wistar rats were randomly assigned to one of three groups. Satellite cells of the left tibialis anterior were ablated by weekly exposure to a 25 Gy dose of γ-irradiation during 21 days of CLFS (IRR-Stim), whilst a second group received only 21 days of CLFS (Stim). A third group received weekly doses of γ-irradiation (IRR). Non-irradiated right legs served as internal controls. Continuous infusion of 5-bromo-2′-deoxyuridine (BrdU) revealed that CLFS induced an 8.0-fold increase in satellite cell proliferation over control (mean ±s.e.m.: 23.9 ± 1.7 versus 3.0 ± 0.5 mm−2, P < 0.0001) that was abolished by γ-irradiation. M-cadherin and myogenin staining were also elevated 7.7- and 3.8-fold (P < 0.0001), respectively, in Stim compared with control, indicating increases in quiescent and terminally differentiating satellite cells; these increases were abolished by γ-irradiation. Myonuclear content was elevated 3.3-fold (P < 0.0001) in Stim, but remained unchanged in IRR-Stim. Immunohistochemical analyses revealed attenuation of fast-to-slow fibre-type transitions in IRR-Stim compared with Stim. Comparable changes were observed at the protein level by SDS-PAGE. It is concluded that although considerable adaptive potential exists within myonuclei, satellite cells play a role in facilitating fast-to-slow fibre-type transitions.