Title: Influence of the NASA SPRINT exercise protocol on myonuclear and satellite cell content of the vastus lateralis and soleus during 70 days of bedrest
Authors: Skiles Chad , Boyd Gerard , Robbins Ethan , Gouw Aaron , Minchev Kiril , Ryder Jeffrey , Ploutz-Snyder Lori 
Published: May 2024

Objectives: The purpose of this study was to assess the effcacy of the NASA SPRINT exercise protocol in preserving myonuclear and satellite cell number of the quadriceps (vastus lateralis) and triceps surae (soleus) muscles following 70 days of bedrest (i.e., simulated microgravity). These two muscles were the focus of this study because they both atrophy significantly with microgravity exposure in the absence of countermeasures. Hypotheses: The SPRINT exercise protocol will preserve myonuclear number (MNN), myonuclear domain (MND), and satellite cell number (SCN) during 70 days of bedrest. Methods: Subjects were randomized into three groups: bedrest only (BR; n=8, 37±3 y), bedrest with endurance and resistance exercise (BRE; n=9, 34±2 y) and bedrest with endurance and resistance exercise and low dose testosterone injections (BRE+T; n=8, 33±3 y). Muscle biopsies were taken before and at the end of bedrest from the vastus lateralis and soleus, and were sectioned, immunofluorescently stained, and analyzed for MNN (myonuclei/fiber), MND (μm2/myonuclei), and SCN (satellite cells/fiber) in slow (myosin heavy chain (MHC) I) and fast (MHC IIa) muscle fibers. SCN was only determined in MHC I fibers for the soleus. Results: MNN, MND, and SCN in MHC I and IIa fibers of the vastus lateralis and soleus were unchanged (p<0.05) from pre- to post-bedrest within BR, BRE, and BRE+T. However, there was a main effect for time (p<0.05) for a decrease in MND for vastus lateralis MHC I fibers (-8%), as well as soleus MHC I (-18%) and IIa (-18%) fibers. Also, there was a trend for a main effect for time (P=0.059) for an increase (+37%) in SCN for vastus lateralis MHC IIa fibers, which was driven primarily by the exercise groups (BR: -4%, BRE: +36%, BRE+T: +67%). Conclusions: Myonuclear number appears to be unaltered during 70 days of simulated microgravity with or without countermeasures exercise. As a result, any changes in myonuclear domain correspond with alterations in muscle mass. Satellite cell number also appears to be relatively refractory to change, but may be influenced in fast muscle fibers under the combined dynamic cellular control mechanisms of exercise and atrophy encountered during relatively long-duration simulated microgravity. Funding Source: This research was supported by the National Aeronautics and Space Administration, NNJ06HF59G; the Institute for Translational Sciences at the University of Texas Medical Branch, supported in part by a Clinical and Translational Science Award (UR1TR000071) from the National Center for Advancing Translational Sciences, National Institutes of Health; the NASA Flight Analogs Project; and the Human Bioenergetic Program at Ball State University. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.