Influence of hindlimb unweighting and intermittent weight bearing on dynamics of nuclei in rat soleus muscle.

The purpose of this study was to examine differences in the inhibitory effect of disuse atrophy as a result of intermittent weight bearing in terms of the dynamics of nuclei in rat soleus muscle. Disuse muscle atrophy was induced by hindlimb suspension for two weeks. Forty-nine male Wistar rats (body weight: 190-228 g) were divided into the control group (CON) and the experimental group. The experimental group was subdivided into four groups: hindlimb suspension alone (HS), weight bearing for 10 minutes × 4 times/day (W10), weight bearing for 20 minutes × 2 times/day (W20), and weight bearing for 40 minutes × one time/day (W40). In addition to histochemical examination, this study examined both cell proliferation and apoptosis in terms of the dynamics of myonuclei immuno-histochemically. The mean cross-sectional area of muscle fibers demonstrated the effect of weight bearing. The number of proliferating myonuclei per 100 muscle fibers was decreased in the experimental groups as compared with CON. Proliferating myonuclei in W10 and W40 were more than HS, indicating the effect of weight bearing. Apoptotic myonuclei was increased in the experimental groups as compared with CON. This parameter in W10 and W40 were statistically not significantly different from CON, suggesting that these weight bearing methods can prevent the loss of myonuclei by apoptosis. However, W20 was not significantly different from HS in terms of the dynamics of myonuclei. This suggests that weight bearing for W20 was ineffective. The results in this study indicated the possibility of inducing different effects by the frequency of weight bearing.