Upregulation of HSP72 in reloading rat soleus muscle after prolonged hindlimb unloading

To clarify the changes of heat shock protein (HSP) 72 in the rat soleus muscle after hind-limb unloading (HU) and during reloading, 7-week-old male Wistar rats were hind-limb-suspended for 9 weeks, thereafter ambulatory recovery was permitted for 8 weeks. The body and absolute soleus weights were significantly lower in the HU than in the age-matched control group after HU and during reloading. The soleus weight relative to body weight was also significantly lower in the HU than in the age-matched control group at the end of 9 weeks of suspension, but returned to the control level after 2 weeks of reloading. The HSP72 content decreased to 38% of the control level after HU and conversely increased to 165 and 175% of the control level after 2 and 4 weeks of reloading, respectively. The HSP72 content returned to the control level after 8 weeks of reloading. Thus our results showed that the expression of HSP72 was downregulated by HU and upregulated temporally over the level of the control during the reloading period, and they suggested that these down- and up-regulations of HSP72 may be related to many factors including mechanical stress or load applied to the muscle.