The effect of electrical stimulation on muscle disuse atrophy

Abstract

Skeletal muscle atrophy occurs with decreased mechanical loading resulting in decreased muscle mass and strength. Satellite cells are stem cells for muscle regeneration and are important for normal adaptive functions. Impaired satellite cell proliferation and/or increased apoptosis are possible mechanisms underlying disuse atrophy. Electrical stimulation has been used as a countermeasure to counteract disuse atrophy. The hypothesis is that through optimization of different stimulation protocols, electrical stimulation can attenuate muscle atrophy by influencing satellite cell activity. One hindlimb of male Balb/c mice received electrical stimulation while the contralateral limb served as control during 14-day hindlimb suspension. Different durations (3 h/day or 2×3 h/day) and frequencies (2, 10 or 20 Hz) of stimulation were used. Muscle mass, cross-sectional area, fiber-type composition and maximal tetanic force of soleus were measured. Immunohistochemical staining was used to evaluate satellite cell content, activation, proliferation and differentiation. Cell apoptosis was detected by TUNEL assay. The results showed that stimulation at 2 Hz for 2×3 h/day achieved the best effect in attenuating muscle mass and force. Furthermore, this stimulation parameter led to a 1.2 fold increase in satellite cell proliferation and was effective in rescuing cells from apoptosis. To understand the possible mechanism of the favorable effect from electrical stimulation, mechano-growth factor (MGF), a splice variant of insulin-like growth factor-I was investigated. Hindlimb suspension induced MGF downregulation. In response to electrical stimulation, MGF was upregulated at days 2 and 3 prior to increased satellite cell content and proliferation at day 7. The results suggested that MGF was mechanically sensitive and might be, at least in part, related to the beneficial effects of electrical stimulation on satellite cell proliferation. The function of MGF was further characterized using C2C12 cells. The endogenous MGF was highly expressed during the proliferation phase and gradually decreased as differentiation proceeded. Furthermore treatment with synthetic MGF peptides promoted proliferation in a dose-dependent manner. Using electroporation in an in vivo model, MGF plasmid DNA could be successfully delivered into muscle. The study findings form basis for further investigation into the effects of electrical stimulation for disuse atrophy and the functional role of MGF.