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Am J Physiol Cell Physiol 292: C1192-C1203, 2007. First published November 15, 2006; doi:10.1152/ajpcell.00462.2006
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MUSCLE CELL BIOLOGY AND CELL MOTILITY

Differential regulation of myofilament protein isoforms underlying the contractility changes in skeletal muscle unloading

¹Section of Molecular Cardiology, Evanston Northwestern Healthcare, Northwestern University Feinberg School of Medicine, Evanston, Illinois; and ²Department of Aerospace Physiology, Fourth Military Medical University, Xi'an, China

Submitted 29 August 2006 ; accepted in final form 9 November 2006

Weight-bearing skeletal muscles change phenotype in response to unloading. Using the hindlimb suspension rat model, we investigated the regulation of myofilament protein isoforms in correlation to contractility. Four weeks of continuous hindlimb unloading produced progressive atrophy and contractility changes in soleus but not extensor digitorum longus muscle. The unloaded soleus muscle also had decreased fatigue resistance. Along with the decrease of myosin heavy chain isoform I and IIa and increase of IIb and IIx, coordinated regulation of thin filament regulatory protein isoforms were observed: - and -tropomyosin decreased and -tropomyosin increased, resulting in an / ratio similar to that in normal fast twitch skeletal muscle; troponin I and troponin T (TnT) both showed decrease in the slow isoform and increases in the fast isoform. The TnT isoform switching began after 7 days of unloading and TnI isoform showed detectable changes at 14 days while other protein isoform changes were not significant until 28 days of treatment. Correlating to the early changes in contractility, especially the resistance to fatigue, the early response of TnT isoform regulation may play a unique role in the adaptation of skeletal muscle to unloading. When the fast TnT gene expression was upregulated in the unloaded soleus muscle, alternative RNA splicing switched to produce more high molecular weight acidic isoforms, reflecting a potential compensation for the decrease of slow TnT that is critical to skeletal muscle function. The results demonstrate that differential regulation of TnT isoforms is a sensitive mechanism in muscle adaptation to functional demands.

troponin T; fatigue resistance; troponin I; tropomyosin; myosin; hindlimb-suspended rat; Western blot protein quantification

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