Contraction mode itself does not determine the level of mTORC1 activity in rat skeletal muscle.

Satoru Ato,Kohei Kido,Yuhei Makanae,Satoshi Fujita

doi:10.14814/phy2.12976

Abstract

Resistance training with eccentric contraction has been shown to augment muscle hypertrophy more than other contraction modes do (i.e., concentric and isometric contraction). However, the molecular mechanisms involved remain unclear. The purpose of this study was to investigate the effect of muscle contraction mode on mammalian target of rapamycin complex 1 (mTORC1) signaling using a standardized force‐time integral (load (weight) × contraction time). Male Sprague–Dawley rats were randomly assigned to three groups: eccentric contraction, concentric contraction, and isometric contraction. The right gastrocnemius muscle was exercised via percutaneous electrical stimulation‐induced maximal contraction. In experiment 1, different modes of muscle contraction were exerted using the same number of reps in all groups, while in experiment 2, muscle contractions were exerted using a standardized force‐time integral. Muscle samples were obtained immediately and 3 h after exercise. Phosphorylation of molecules associated with mTORC1 activity was assessed using western blot analysis. In experiment 1, the force‐time integral was significantly different among contraction modes with a higher force‐time integral for eccentric contraction compared to that for other contraction modes (P < 0.05). In addition, the force‐time integral was higher for concentric contraction compared to that for isometric contraction (P < 0.05). Similarly, p70S6K phosphorylation level was higher for eccentric contraction than for other modes of contraction (P < 0.05), and concentric contraction was higher than isometric contraction (P < 0.05) 3 h after exercise. In experiment 2, under the same force‐time integral, p70S6K (Thr389) and 4E‐BP1 phosphorylation levels were similar among contraction modes 3 h after exercise. Our results suggest that mTORC1 activity is not determined by differences in muscle contraction mode itself. Instead, mTORC1 activity is determined by differences in the force‐time integral during muscle contraction.

Highlights

Skeletal muscle is an essential tissue for motion
Exercise at 30% of a 1 repetition maximum (1RM) compared with 90% of a 1RM has been shown to exert a larger exercise volume, a greater increase in p70S6K phosphorylation level, and greater muscle protein synthesis than does 90% at a 1RM in the human study (Burd et al 2010b). These findings indicate that exercise volume load is a critical determining factor in mammalian target of rapamycin complex 1 (mTORC1) activity and muscle protein synthesis levels after resistance exercise
We sought to investigate the mechanism by which different contraction modes affect mTORC1 activity using a matched force-time integral

Summary

Introduction

Skeletal muscle is an essential tissue for motion. The magnitude of the muscle power output is dependent on skeletal muscle mass (Bamman et al 2000). Skeletal muscle mass is an important factor augmenting the exertion of muscle power. Skeletal muscle mass is regulated by the net balance of muscle protein synthesis and muscle protein breakdown. An acute bout of resistance exercise increases the amount of muscle protein synthesis to a greater extent than muscle protein breakdown does. Thereby, the net balance of protein metabolism improves significantly for up to 48 h after exercise (Phillips et al 1997). Repeated bouts of exercise can induce muscle protein accumulation, and subsequently hypertrophy (Adams and Haddad 1996)

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