Effects of simulated weightlessness on myosin heavy chain expression of soleus intrafusal muscle fibers in rats.

Objective: To examine the myosin heavy chain expression in intrafusal fibers in rat soleus muscle before and after simulated weightlessness and to analyse the mechanism involved in muscle spindle deterioration during simulated weightlessness.
Method: The rats were divided into 3 groups: 7 d, 14 d tail-suspension group and control group. Control and experimental spindles were examined using monoclonal antibodies specific for myosin heavy chains of slow-tonic (ALD58) and fast-twitch (MF30) chicken muscles by the immunoperoxidase reaction utilizing the ABC (avidin-biotin-complex) method.
Result: The extrafusal muscle fibers did not exhibit immunoreactivity to ALD58 and MF30. Only intrafusal fibers bound these antibodies. The bag 1 fiber and bag 2 fiber reacted to ALD58. The bag 2 fiber and chain fibers stained with MF30. After 7 d suspension the immunoreactivity of the bag 1 fiber and bag 2 fiber to ALD58 decreased, while the immunoreactivity of the bag 2 fiber and chain fibers to MF30 increased. After 14 d suspension the immunoreactivity of the bag 1 fiber and bag 2 fiber to ALD58 decreased remarkably, even disappeared. While all of the intrafusal fibers reacted to MF30 and the immunoreactivity increased obviously.
Conclusion: The expression level and patterns of MHC in intrafusal fibers of rat soleus muscle changed after simulated weightlessness.