Myonuclear and satellite cell content of the vastus lateralis and soleus with 70 days of simulated microgravity and the NASA SPRINT exercise program

We previously observed a range of whole muscle and individual slow and fast myofiber size responses (mean: +4 to -24%) in quadriceps (vastus lateralis) and triceps surae (soleus) muscles of individuals undergoing 70 days of simulated microgravity with or without the NASA SPRINT exercise countermeasures program. The purpose of the current investigation was to further explore, in these same individuals, the content of myonuclei and satellite cells, both of which are key regulators of skeletal muscle mass. Individuals completed 6° head-down-tilt bedrest (BR, <i>n</i> = 9), bedrest with resistance and aerobic exercise (BRE, <i>n</i> = 9), or bedrest with resistance and aerobic exercise and low-dose testosterone (BRE + T, <i>n</i> = 8). The number of myonuclei and satellite cells associated with each slow [myosin heavy chain (MHC) I] and fast (MHC IIa) myofiber in the vastus lateralis was not changed (<i>P</i> > 0.05) pre- to postbedrest within the BR, BRE, or BRE + T groups. Similarly, in the soleus, the number of myonuclei associated with each slow and fast myofiber, and the number of satellite cells associated with each slow myofiber were not changed (<i>P</i> > 0.05) pre- to postbedrest within the BR, BRE, or BRE + T groups. It appears that even with relatively large perturbations in muscle mass over a few months of simulated microgravity, or with partially or completely effective exercise countermeasures, human skeletal muscle tightly regulates the abundance of myonuclei and satellite cells. Thus, exercise countermeasures efficacy for skeletal muscle atrophy appears to be independent of myonuclei and satellite cell abundance.<b>NEW & NOTEWORTHY</b> This study showed that after 70 days of simulated microgravity, human skeletal muscle does not alter the number of nuclei or satellite cells associated with slow or fast myofibers in the two muscle groups most negatively influenced by microgravity exposure [i.e., quadriceps (vastus lateralis) and triceps surae (soleus)]. Furthermore, the efficacy of exercise countermeasures for maintaining the mass of these muscles does not appear to be related to the myocellular content of nuclei or satellite cells.