Activation of mTORC1 signaling and protein synthesis in human muscle following blood flow restriction exercise is inhibited by rapamycin

Abstract

Restriction of blood flow to a contracting muscle during low-intensity resistance exercise (BFR exercise) stimulates mTORC1 signaling and protein synthesis in human muscle within 3 h postexercise. However, there is a lack of mechanistic data to provide a direct link between mTORC1 activation and protein synthesis in human skeletal muscle following BFR exercise. Therefore, the primary purpose of this study was to determine whether mTORC1 signaling is necessary for stimulating muscle protein synthesis after BFR exercise. A secondary aim was to describe the 24-h time course response in muscle protein synthesis and breakdown following BFR exercise. Sixteen healthy young men were randomized to one of two groups. Both the control (CON) and rapamycin (RAP) groups completed BFR exercise; however, RAP was administered 16 mg of the mTOR inhibitor rapamycin 1 h prior to BFR exercise. BFR exercise consisted of four sets of leg extension exercise at 20% of 1 RM. Muscle biopsies were collected from the vastus lateralis before exercise and at 3, 6, and 24 h after BFR exercise. Mixed-muscle protein fractional synthetic rate increased by 42% at 3 h postexercise and 69% at 24 h postexercise in CON, whereas this increase was inhibited in the RAP group. Phosphorylation of mTOR (Ser(2448)) and S6K1 (Thr(389)) was also increased in CON but inhibited in RAP. Mixed-muscle protein breakdown was not significantly different across time or groups. We conclude that activation of mTORC1 signaling and protein synthesis in human muscle following BFR exercise is inhibited in the presence of rapamycin.

Keywords: blood flow restriction exercise; fractional synthetic rate; mammalian target of rapamycin; mammalian target of rapamycin complex 1; rapamycin.

Copyright © 2014 the American Physiological Society.

Figures

Fig. 1.

Infusion study protocol. Study design was identical for both groups. ±Rap indicates the time point where the rapamycin (RAP) group ingested 16 mg of rapamycin, whereas the control (CON) group did not. Timing of blood draws and biopsies are represented by upward-facing arrows. Between hours 6 and 20, subjects were fed and then refasted overnight.

Fig. 2.

Mixed-muscle fractional synthetic rate (FSR) at baseline and 3, 6, and 24 h postexercise, presented as percent per hour (%/h). Error bars represent SE (n = 8/group). *P < 0.05 vs. baseline. No differences were detected between the CON and RAP groups (P > 0.05).

Fig. 3.

Protein phosphorylation status during postexercise recovery of the mammalian/mechanistic target of rapamycin (mTOR) pathway was determined via Western blot analysis during the postexercise recovery period, presented as a fold change from baseline. Error bars represent SE (n = 8/group). *P < 0.05 vs. baseline; #P < 0.05 vs. CON.

Fig. 4.

Representative Western blots at baseline and 3, 6, and 24 h postexercise. S6K1, ribosomal protein S6 kinase 1; Mnk1, MAPK-interacting kinase 1. The representative blots are replicates from the same subject.

Fig. 5.

Muscle fractional breakdown rate (FBR) at baseline and 6 and 24 h postexercise, presented as %/h. Error bars represent SE (n = 8/group).

Fig. 6.

Net balance at baseline and 6 and 24 h postexercise. Net balance was calculated by subtracting FBR from FSR. Error bars represent SE (n = 8/group). *P < 0.05 vs. baseline. No statistical significance was seen between CON and RAP groups.