Unloading of skeletal muscles triggers rapid changes in molecular signaling, leading to muscle atrophy and functional alterations. Electrical stimulation of muscles is commonly used to counteract these changes, but the precise molecular mechanisms behind its effects remain unclear.

To investigate the early changes in postural soleus muscle under unloading conditions (dry immersion, DI) and the impact of electrical stimulation during unloading, two groups of volunteers (10 men in each) underwent a 6-day DI or a 6-day DI with electrical stimulation (DI ​+ ​ES). Soleus muscle samples were collected 14 days before and 6 days after DI and DI ​+ ​ES.

Six-day DI did not did not cause atrophy of the soleus myofibers or alter protein synthesis parameters, However, it did lead to an increase in atrogin-1 expression, a downregulation of markers for mitochondrial biogenesis and dynamics, and a decline in the mRNA expression of fast oxidative myosin isoform Ⅱa. It also resulted in the downregulation of microRNAs mir-206 and mir-208b, which support slow fiber types. There was an upregulation of CpG methylation in the peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1a) promoter region and an accumulation of Ca2+/calmodulin-dependent protein kinase (p-CaMK Ⅱ), indicating an increase in myoplasmic calcium levels. Electrical stimulation during the 6-day disuse period prevented the disuse-induced decreases in mitochondria-related markers and the content of mir-206 and mir-208b. It also induced a shift in myosin mRNA expression from types Ⅱd/x to Ⅱa, counteracted the accumulation of p-CaMK Ⅱ and CpG methylation in the PGC1a promoter region.

Electrical stimulation upregulated markers of both protein synthesis and proteolysis, as well as resulted in lower cross-sectional area of fast-type fibers compared to pre-DI ​+ ​ES.