Daniel L. Plotkin1, J. Max Michel1, Joshua S. Godwin1, Paulo H.C. Mesquita1, Bradley A. Ruple1, Mason C. McIntosh1, Cleiton A. Libardi2, Andreas N. Kavazis1, Michael D. Roberts1,3. 1Auburn University, Auburn, AL. 2Federal University of Sao Carlos, Sao Carlos. 3Edward Via College of Osteopathic Medicine-Auburn Campus, Auburn, AL.

BACKGROUND: While the dynamics of muscle protein balance have been explored in the context of many exercise modalities, they have not been studied in a resistance training followed by endurance training (ET) paradigm. Our laboratory recently determined that seven weeks of resistance training (RT) increases vastus lateralis (VL) muscle thickness, and seven weeks of treadmill high-intensity interval training (HIIT) following the RT period reduces VL muscle thickness. Thus, the purpose of this study was to examine calpain and autophagy activity markers after the 7-week RT and HIIT periods. METHODS: Vastus lateralis biopsies and muscle thickness (MT) measures were collected from 11 untrained college-aged males at baseline (PRE), after 7 weeks of RT (MID) and after a subsequent 7 weeks of HIIT (POST). Proteins associated with autophagy and calpain activities were assessed via immunoblotting, and calpain activity was assessed via a commercially available plate-based assay. Data were checked for normality using Shapiro-Wilk tests and analyzed via one-way ANOVAs. RESULTS: Tukey’s Multiple comparisons test showed a significant increase in MT from PRE to MID [2.3 mm, CI95% = (1.1 mm, 3.5 mm), p= 0.001], a significant decrease from MID to POST [1.2 mm, CI95% = (0.30 mm, 2.1 mm), p= 0.012],and a non-significant increase from PRE to POST [1.1 mm, CI95% = (2.1 mm, 2.9 mm), p= 0.213]. Microtubule-associated proteins 1A/1B light chain 3B (LC3 II/I), phosphorylated AMP-kinase (p-AMPKα, Thr172) / pan-AMPKα, and calpain-1 protein abundance did not reach overall ANOVA significance with respective p-values of 0.854, 0.330, 0.365. Phosphorylated-Unc-51 like autophagy activating kinase 1 (p-ULK, Ser555) / pan-ULK protein abundance was not significant between time points. Beclin-1 protein abundance increased from PRE to MID (p= 0.012) and PRE to POST (p = 0.005). Calpain-2 increased from PRE to MID (p = 0.001) and PRE to POST (p = 0.003). Finally, calpain-activity normalized to Calpain-1/2 protein content decreased from PRE to MID (p = 0.001), albeit with no significant differences from PRE to POST and MID to POST. CONCLUSION: These data suggest that an elevation in calpain activity markers (rather than autophagy markers) during seven weeks of HIIT may have been partially responsible for mid-thigh muscle size decreases following a seven-week period of RT.

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