Skeletal muscle phenotype signaling with ex vivo endurance-type dynamic contractions in rat muscle.

Skeletal muscle phenotype may influence the response sensitivity of myocellular regulatory mechanisms to contractile activity. To examine this, we employed an ex vivo endurance-type dynamic contraction model to evaluate skeletal muscle phenotype-specific protein signaling responses in rat skeletal muscle. Preparations of slow-twitch soleus and fast-twitch extensor digitorum longus skeletal muscle from 4-wk-old female Wistar rats were exposed to an identical ex vivo dynamic endurance-type contraction paradigm consisting of 40 min of stretch-shortening contractions under simultaneous low-frequency electrostimulation delivered in an intermittent pattern. Phosphorylation of proteins involved in metabolic signaling and signaling for translation initiation was evaluated at 0, 1, and 4 h after stimulation by immunoblotting. For both muscle phenotypes, signaling related to metabolic events was upregulated immediately after stimulation, with concomitant absence of signaling for translation-initiation. Signaling for translation-initiation was then activated in both muscle phenotypes at 1-4 h after stimulation, coinciding with attenuated metabolic signaling. The recognizable pattern of signaling responses support how our ex vivo dynamic muscle contraction model can be utilized to infer a stretch-shortening contraction pattern resembling stretch-shortening contraction of in vivo endurance exercise. Moreover, using this model, we observed that some specific signaling proteins adhering to metabolic events or to translation-initiation exhibited phosphorylation changes in a phenotype-dependent manner, whereas other signaling proteins exhibited phenotype-independent changes. These findings may aid the interpretation of myocellular signaling outcomes adhering to mixed muscle samples collected during human experimental trials.NEW & NOTEWORTHY The application of cyclic ex vivo dynamic muscle contractions delivered in an intermittent pattern may be suitable for the exploration of skeletal muscle regulatory responses to endurance-type contractile activity. In the present study, it is demonstrated that the response to such stimulus of some nodal myocellular signaling proteins related to either metabolic or anabolic signaling events may be influenced by muscle phenotype, whereas the response of others appears to be independent of phenotype.