Reactive hyperemia is not responsible for stimulating muscle protein synthesis following blood flow restriction exercise

Abstract

Blood flow restriction (BFR) to contracting skeletal muscle during low-intensity resistance exercise training increases muscle strength and size in humans. However, the mechanism(s) underlying these effects are largely unknown. We have previously shown that mammalian target of rapamycin complex 1 (mTORC1) signaling and muscle protein synthesis (MPS) are stimulated following an acute bout of BFR exercise. The purpose of this study was to test the hypothesis that reactive hyperemia is the mechanism responsible for stimulating mTORC1 signaling and MPS following BFR exercise. Six young men (24 ± 2 yr) were used in a randomized crossover study consisting of two exercise trials: low-intensity resistance exercise with BFR (BFR trial) and low-intensity resistance exercise with sodium nitroprusside (SNP), a pharmacological vasodilator infusion into the femoral artery immediately after exercise to simulate the reactive hyperemia response after BFR exercise (SNP trial). Postexercise mixed-muscle fractional synthetic rate from the vastus lateralis increased by 49% in the BFR trial (P < 0.05) with no change in the SNP trial (P > 0.05). BFR exercise increased the phosphorylation of mTOR, S6 kinase 1, ribosomal protein S6, ERK1/2, and Mnk1-interacting kinase 1 (P < 0.05) with no changes in mTORC1 signaling in the SNP trial (P > 0.05). We conclude that reactive hyperemia is not a primary mechanism for BFR exercise-induced mTORC1 signaling and MPS. Further research is necessary to elucidate the cellular mechanism(s) responsible for the increase in mTOR signaling, MPS, and hypertrophy following acute and chronic BFR exercise.

Fig. 1.

Infusion study protocol. Doppler ultrasound measurements; blood and muscle samples are indicated by arrows. The study design was identical for both groups.

Fig. 2.

A: femoral artery blood flow measured by Doppler ultrasound. Data are presented as ml/min. Error bars represent SE. *P < 0.05 vs. baseline; #P < 0.05 vs. sodium nitroprusside (SNP). Open symbol represents the cumulative, average basal blood flow as a reference point prior to exercise. BFR, Blood flow restriction. B: total femoral arterial blood flow within the 1st h following exercise, as calculated by the blood flow rate area under the curve (AUC). Presented as liters of blood through the femoral artery over the hour. Error bars represent SE. *P < 0.05 vs. baseline.

Fig. 3.

A: total glucose delivery (blood glucose × blood flow) within the 1st h postexercise, as calculated by the glucose delivery AUC. Presented as mmol of glucose through the femoral artery over the hour. Error bars represent SE. *P < 0.05 vs. baseline. B: total phenylalanine delivery (blood phenylalanine × blood flow) within the 1st h postexercise, as calculated by the phenylalanine delivery AUC. Presented as mmol of phenylalanine through the femoral artery over the hour. Error bars represent SE. *P < 0.05 vs. baseline.

Fig. 4.

Plasma lactate concentrations. Data are presented as mmol/l. Error bars represent SE. At 15 min, 30 min, and 45 min postexercise, lactate values were significantly greater than basal and between trials (P < 0.05). At 1 h postexercise, plasma lactate was still elevated above basal during the BFR trial and was higher during the BFR trial than the SNP trial (P < 0.05). At 1.5 h postexercise, plasma lactate was not different from basal (P > 0.05), although lactate concentrations during the BFR trial were higher than the SNP trial (P < 0.05). No other time or group differences were detected (P > 0.05).

Fig. 5.

Mixed-muscle fractional synthetic rate (FSR) at baseline and 3 h postexercise. Presented in %/h. Error bars represent SE. *P < 0.05 vs. baseline; #P < 0.05 vs. SNP.

Fig. 6.

Protein phosphorylation during postexercise (post ex) recovery of the mammalian target of rapamycin complex 1 (mTORC1) pathway from Western blot analysis (A), S6 kinase 1 (S6K1; B), and ribosomal protein S6 (rpS6; C). Presented as a fold change from baseline. *P < 0.05 vs. baseline; #P < 0.05 vs. SNP.

Fig. 7.

Protein phosphorylation during postexercise recovery of the MAPK pathway from Western blot analysis. Presented as a fold change from baseline. Mnk1, MAPK-interacting kinase 1. *P < 0.05 vs. baseline; #P < 0.05 vs. SNP.

Fig. 8.

mRNA expression of the E3 ligases during postexercise recovery. Presented as fold change from baseline using the 2^−ΔΔCt method. Atrogin1, atrophy F-box protein-1; MuRF1, muscle really interesting new gene finger protein-1. *P < 0.05 vs. baseline.

Fig. 9.

Representative Western blots for all proteins measured. eEF2, eukaryotic elongation factor 2; 4EPB1, 4E-binding protein 1.