Effect of hindlimb unloading on resting intracellular calcium in intrafusal fibers and ramp-and-hold stretches evoked responsiveness of soleus muscle spindles in conscious rats

The present study was to investigate and evaluate the dynamic changes of calcium homeostasis of soleus muscle spindle for the exploration of the potential mechanisms of muscle spindle degeneration induced by hindlimb unloading. We systematically observed the changes in immunoreactivity of calbindin D28K (CaBP-D28K), intracellular resting calcium in intrafusal fibers of soleus muscle spindle, and the responsiveness of muscle spindles to ramp-and-hold stretches after short- and long-term (3, 7, 14 d) hindlimb unloading. The immunoreactivity of CaBP-D28K started to decrease after 7-d hindlimb unloading, while its decrease was obviously different compared with the control group 14 d following the hindlimb unloading. The resting calcium concentration was increased significantly at 3 d, and reached the peak level 14 d after the hindlimb unloading. The responsiveness of muscle spindles, assessed by investigating Index3-L2, Index3-L3, and Index5, to ramp-and-hold stretches started to decrease during the period of 7-d hindlimb unloading. All Indexs, in particular Index3-L3 and Index5, were significantly decreased at 14 d after the hindlimb unloading. The data suggest the disturbance of calcium homeostasis in intrafusal fibers during the exposure to hindlimb unloading might gradually influence the structure and function of muscle spindles.