Changes in calpains and calpastatin in the soleus muscle of Daurian ground squirrels during hibernation

We investigated changes in muscle mass, calpains, calpastatin and Z-disk ultrastructure in the soleus muscle (SOL) of Daurian ground squirrels (Spermophilus dauricus) after hibernation or hindlimb suspension to determine possible mechanisms by which muscle atrophy is prevented in hibernators. Squirrels (n = 30) were divided into five groups: no hibernation group (PRE, n = 6); hindlimb suspension group (HLS, n = 6); two month hibernation group (HIB, n = 6); two day group after 90 ± 12 days of hibernation (POST, n = 6); and forced exercise group (one time forced, moderate-intensity treadmill exercise) after arousal (FE, n = 6). Activity and protein expression of calpains were determined by casein zymography and western blotting, and Z-disk ultrastructure was observed by transmission electron microscopy. The following results were found. Lower body mass and higher SOL muscle mass (mg) to total body mass (g) ratio were observed in HIB and POST; calpain-1 activity increased significantly by 176% (P = 0.034) in HLS compared to the PRE group; no significant changes were observed in calpain-2 activity. Protein expression of calpain-1 and calpain-2 increased by 83% (P = 0.041) and 208% (P = 0.029) in HLS compared to the PRE group, respectively; calpastatin expression increased significantly by 180% (P < 0.001) and 153% (P = 0.007) in HIB and POST, respectively; the myofilaments were well-organized, and the width of the sarcomere and the Z-disk both appeared visually similar among the pre-hibernation, hibernating and post-hibernation animals. Inhibition of calpain activity and consequently calpain-mediated protein degradation by highly elevated calpastatin protein expression levels may be an important mechanism for preventing muscle protein loss during hibernation and ensuring that Z-lines remained ultrastructurally intact.