Transcriptional regulation of decreased protein synthesis during skeletal muscle unloading.

Muscle atrophy resulting from disuse is associated with marked decrements in protein synthesis. The objective of the present investigation was to determine levels of total muscle RNA and the content and composition of the mRNA fraction as a qualitative assessment of the potential regulatory role of transcriptional alterations in unloaded skeletal muscles. Hindlimb muscle unloading was produced by whole-body suspension of rats for up to 7 days. The soleus, gastrocnemius, and extensor digitorum longus (EDL) were excised from 1-, 3-, and 7-day suspended and pair-fed controls, and RNA was extracted by homogenization in 5 M guanidinium thiocyanate. Total RNA and mRNA contents were lower in soleus and gastrocnemius after 7 days of suspension compared with pair-fed controls, but total RNA and mRNA concentrations (per g muscle and per microgram total RNA, respectively) were unaltered. alpha-Actin mRNA, assessed by dot blot hybridization, was significantly reduced in soleus after 1 (37%), 3 (28%), and 7 (59%) days of suspension and in gastrocnemius after 3 (44%) and 7 (41%) days. However, alpha-actin mRNA was unchanged in the EDL after suspension. Protein synthesis directed by RNA extracted from soleus and EDL indicated marked (30-400%) alterations in mRNAs coding for several small (15- to 25-kDa) proteins. The results of this study suggest that altered transcription and availability of specific mRNAs could contribute significantly to the regulation of protein synthesis during unloading of skeletal muscle.