Contractile activity-induced gene expression in fast- and slow-twitch muscle.

Many proteins that function as transcription factors regulate the transcriptional activity of nuclear genes encoding mitochondrial proteins. Several of these are rapidly inducible with contractile activity, followed by a recovery phase. The aim of the present study was to evaluate the expression of a number of rapidly responding gene products to an acute bout of contractile activity followed by a recovery period in both slow- and fast-twitch muscle. Using an in vitro isolated muscle preparation, extensor digitorum longus (EDL) and soleus muscles were stimulated for 15 min, followed by 30 min recovery. Following stimulation, ATP levels were decreased in both the EDL and soleus (25% and 32%, respectively). We found that phosphorylation of p38 MAP kinase was elevated in both muscle types, with a more dramatic 3.5-fold increase observed in the EDL muscle. Peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) mRNA expression was unchanged as a result of stimulation and recovery, while c-Fos transcript levels were decreased as a result of stimulation, but returned to resting values following recovery. Interestingly, nuclear respiratory factor 1 mRNA levels were unaffected by stimulation, but increased significantly (34%) during the recovery phase. These data suggest that the extent of the induction of transcription factor mRNA to acute exercise, which leads to subsequent muscle adaptations, is transcript specific and dependent on (i) the activation of upstream kinases, (ii) the muscle phenotype, and (iii) the duration of the recovery period.