Purpose: 1) To compare the effects of an acute bout of HIIT exercise (treadmill running) on autophagy to MICT exercise in human skeletal muscle 3 hours post exercise, and 2) to test an in vitro model of muscle contraction-induced autophagy using electrical pulse stimulation in C2C12 myotubes. Methods: Study 1: Ten recreationally active males and females completed a bout of high intensity interval training (HIIT) exercise and moderate intensity continuous exercise (MICT) exercise in a fasted state. Muscle biopsies from the vastus lateralis were taken pre and 3 hours post-exercise. Muscle tissue was analyzed for protein expression of markers of autophagy (LC3I, LC3II) and autophagy signaling (p38MAPK). Study 2: C2C12 myoblasts were differentiated into myotubes and underwent 8 hours of low-frequency electrical pulse stimulation (EPS) and starvation (St) and EPS+St conditions with and without bafilomycin A1 (Baf) or non-treatment control conditions (Con). Cells were harvested immediately following EPS and analyzed for protein expression of markers of autophagy (LC3I, LC3II, and p62). Results: Study 1: No differences were detected for LC3I, LC3II, and p38MAPK at the 3h time point vs. pre-exercise in both HIIT and MICT conditions. The LC3II:LC3I ratio increased 3-h post exercise in the HIIT (162.4, SE: 45.9%), which was significantly higher than the 3h timepoint in the MICT (48.8, SE: 9.4%; p\u3c0.05) Study 2: LC3II (324.4, SE: 29.8%) LC3II:I (258.2, SE: 323.3%), and p62 (437.8, SE: 9.7%) were significantly higher in EPS+Baf conditions compared to control (100%, SE: 9.3, 8.8, 22.6%, respectively) and EPS alone (127.7, SE: 24.3%; 95.5, SE: 13.2%, 251.2, SE: 33.2%) . p62 protein expression was also higher in EPS compared to Con and in St+Baf (320.9, SE: 65.9%) compared Con and St (91, SE: 20.3%). There were no differences between EPS and St alone vs. combined EPS+St conditions in LC3I, LC3II, LC3II:I or p62. Conclusions: HIIT stimulates autophagy in a distinct fashion compared to MICT. Additionally, EPS may serve as an in vitro model for muscle contraction-induced autophagy in C2C12 myotubes in fed and fasted conditions.
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