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

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.

Continuous mild heat stress induces differentiation of mammalian myoblasts, shifting fiber type from fast to slow

“…Strength training led to a shift in MyHC isoform composition from MyHC IIx to IIa in human triceps brachii (23). Chronic low-frequency electrical stimulation (CLFS) increases the expression of MyHC I or IIa, whereas it decreases that of MyHC IIx or IIb in human and rat tibialis anterior (26,47). So far, changes of MyHC following heat stress have not been reported.…”

α-Catalytic subunits of 5′AMP-activated protein kinase display fiber-specific expression and are upregulated by chronic low-frequency stimulation in rat muscle

“…A range of stimulation periods was examined that correspond to 1) the early adaptive period (i.e., 1-5 days of CLFS) where we have consistently observed rapid adaptive responses (54,57), and 2) to a later period (i.e., Ն14 days of CLFS) characterized by more gradual adaptive changes (39,54,57) (Fig. 7, A-C).…”

“…7) corresponded to an increase of MHCIIa content from 10% to 40% of the total (please see Ref. 54), which has been shown to reflect increasing levels of MHCIIa expression within IID/X and IIB fiber populations, creating transitional fast hybrid fiber subtypes (39,56,57). This gradual transition through a population of IIA hybrid fibers appears to account for the corresponding 56% and 38% increases in ␣1 and ␣2 contents, respectively (Fig.…”

“…6, A-C. Individual areas (i.e., 9.6 ϫ 10 Ϫ3 mm 2 ) of the medial gastrocnemius and the soleus were examined at 1,000ϫ magnification for ␣1 and ␣2 immunocytochemical staining within the myonuclear compartment, as previously described by us for identification of other myonuclear proteins (39,57). The data are expressed as the proportion of individual areas with one or more incidences of strong positive nuclear staining, relative to the total muscle cross-sectional area (Fig.…”

“…MHCIId/x was identified as those fibers that remained unstained by this panel of antibodies and was validated by comparison of the staining pattern with monoclonal antibody BF-35, which stains all MHC isoforms, except IID/X (63). MHC isoform contents of the white and red gastrocnemius and the soleus muscles were quantitatively evaluated by SDS-PAGE as previously described by us (21,39,54,58).…”