Long-term physical inactivity exacerbates hindlimb unloading-induced muscle atrophy in young rat soleus muscle

This is the authors' abstract. We don't add key points for this paper.

This study investigated the effects of long-term physical inactivity in adolescent on subsequent hindlimb unloading-induced muscle atrophy in rat soleus muscle. First, 3-week-old male Wistar rats were assigned to an age-matched control (n=6) or a physical inactivity (n=8) group. Rats in the physical inactivity group were housed in narrow cages with approximately half the usual floor space for 8 weeks to limit range of movement. Whole body energy consumption was measured and the blood, organs, femoral bone, and hindlimb muscles were removed. We found that long-term physical inactivity did not affect the metabolic and physiological characteristics of growing rats. Then, fifty-six 3-week-old male Wistar rats were assigned randomly into control (n=28) and physical inactivity (n=28) groups. After 8 weeks, the rats in both groups underwent hindlimb unloading. The soleus muscles were removed before unloading (0-day), and 1, 3, and 7 days after unloading (n=7 for each). Although the soleus muscle weight was significantly decreased after 7 days of hindlimb unloading in both groups, the decrease was drastic in the inactive group. A significant interaction between inactivity and unloading (p<0.01) was observed according to the 4-hydroxynonenal-conjugated protein levels and the histone deacetylase 4 (HDAC4) and NF-kB protein levels. HDAC4 and NF-kB p65 protein levels in the physical inactivity group increased significantly 1 day after hindlimb unloading, along with the mRNA levels of their downstream targets myogenin and muscle RING finger protein 1 (MuRF1). Subsequent protein ubiquitination was upregulated by long-term physical inactivity (p<0.05).