Plantar mechanical stimulation prevents calcineurin-NFATc1 inactivation and slow-to-fast fiber type shift in rat soleus muscle under hindlimb unloading.

The prevailing myosin isoform [myosin heavy chain (MyHC)] in a skeletal muscle determines contractile properties of the muscle. Under actual or simulated microgravity conditions such as human bed rest or rat hindlimb unloading, decrease in expression of MyHC of the slow type [MyHC I(β)] has been observed. It was demonstrated that increasing sensory input by performing plantar mechanical stimulation (PMS) on the soles of the feet results in an increase in neuromuscular activation of the lower limb muscles and may prevent slow-to-fast fiber type shift. The calcineurin-nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) signaling pathway is the main cascade regulating MyHC I(β) expression. The present study was aimed to analyze the states of the calcineurin-NFATc1 signaling cascade under conditions of PMS during rat hindlimb unloading. Male Wistar rats were randomly assigned to vivarium control groups and 1-day unloading (1HS), 3-day unloading (3HS), 1HS+PMS, and 3HS+PMS groups. We found that both 1 day and 3 days of unloading caused decrease in MyHC I(β) mRNA expression and decrease in glycogen synthase kinase-3β phosphorylation (Ser 9) that brought about the kinase activation, and these effects of unloading were prevented by PMS. Three days of unloading also caused increase in expression of calsarcin-2 (myozenin-I), which was found to be the endogenous calcineurin inhibitor. The level of calsarcin-2 expression in the 3HS+PMS group did not differ from the control group. Therefore, we conclude that PMS upregulates the calcineurin-NFATc1 signaling pathway and prevents unloading-induced MyHC I(β) decrease. NEW & NOTEWORTHY It is widely accepted that changes in the myosin phenotype during functional unloading (disuse) are determined by a decreased expression of the myosin heavy chain (MyHC) I(β) gene, and this decrease leads to changes of contractile and fatigue characteristics of soleus muscle. The calcineurin-nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) pathway is one of the most important signaling cascades regulating slow MyHC isoform expression. The present study is the first to show that plantar mechanical stimulation upregulates calcineurin-NFATc1 signaling in soleus muscles of hindlimb-unloaded rats.