Muscle protein breakdown has a minor role in the protein anabolic response to essential amino acid and carbohydrate intake following resistance exercise

Abstract

Muscle protein breakdown (MPB) is increased following resistance exercise, but ingestion of carbohydrate during postexercise recovery can decrease MPB with no effect on muscle protein synthesis (MPS). We sought to determine whether a combination of essential amino acids (EAA) with low carbohydrate or high carbohydrate could effectively reduce MPB following resistance exercise and improve muscle protein net balance (NB). We hypothesized that higher levels of carbohydrate and resulting increases in circulating insulin would inhibit MPB and associated signaling, resulting in augmented NB. Thirteen male subjects were assigned to one of two groups receiving equivalent amounts of EAA (approximately 20 g) but differing carbohydrate levels (low = 30, high = 90 g). Groups ingested nutrients 1 h after an acute bout of leg resistance exercise. Leg phenylalanine kinetics (e.g., MPB, MPS, NB), signaling proteins, and mRNA expression were assessed on successive muscle biopsies using stable isotopic techniques, immunoblotting, and real-time quantitative PCR, respectively. MPB tended to decrease (P < 0.1) and MPS increased (P < 0.05) similarly in both groups following nutrient ingestion. No group differences were observed, but muscle ring finger 1 (MuRF1) protein content and MuRF1 mRNA expression increased following resistance exercise and remained elevated following nutrient ingestion, while autophagy marker (light-chain 3B-II) decreased after nutrient ingestion (P < 0.05). Forkhead box-O3a phosphorylation, total muscle atrophy F-box (MAFbx) protein, and MAFbx and caspase-3 mRNA expression were unchanged. We conclude that the enhanced muscle protein anabolic response detected when EAA+carbohydrate are ingested postresistance exercise is primarily due to an increase in MPS with minor changes in MPB, regardless of carbohydrate dose or circulating insulin level.

Figures

Fig. 1.

Study design. The study consisted of a baseline/basal period, a recovery period lasting 1 h (1 h post-Ex), and a nutrition period lasting 1 h (2 h post-Ex). Indocyanine green was infused in each period to measure blood flow. Blood samples were collected to measure blood glucose, insulin, and phenylalanine concentrations. Muscle biopsies were used to measure muscle protein breakdown, signaling proteins, and gene expression associated with protein breakdown. post-Ex, postexercise; EAA, essential amino acid; LCHO, low carbohydrate; HCHO, high carbohydrate.

Fig. 2.

Plasma insulin concentration (A), phosphorylation of Akt (B), phosphorylation of AMPKα (C), and leg protein breakdown represented as rate of appearance of phenylalanine across the leg (Ra) (D). Data for B and C are expressed relative to a normalization control and as fold change from baseline (B) ± SE, n = 6 per group (except n = 5 EAA+LCHO AMPKα). Insert shows representative Western blot in duplicate for baseline, 1 h post-Ex (1 h) and EAA+LCHO (Lo) and EAA+HCHO (Hi) groups. Line across groups in (D) indicates a time effect (P < 0.1) at 2 h post-Ex. *Significantly different from baseline (P < 0.05); †significantly different from EAA+LCHO (P < 0.05). Insulin and Akt signaling data for EAA+LCHO are a subset of a group of subjects from a previously published study, Dreyer, et al. (14).

Fig. 3.

Muscle ring finger 1 (MURF1) (A) and muscle atrophy F-box (MAFbx) (C) mRNA expression. Data are expressed as fold change after normalization to GAPDH as means ± SE, n = 6 per group. Total protein content is shown of MuRF1 (B) and MAFbx (D). Data are expressed relative to a normalization control and as fold change from baseline ± SE, n = 6 per group (except n = 5 for EAA+HCHO MAFbx). Insert shows representative Western blot in duplicate for baseline, 1 h post-Ex (1 h), and EAA+LCHO and EAA+HCHO groups. Line across groups in B indicates a significant time effect, P < 0.05 at 2 h post-Ex. *Significantly different from baseline (P < 0.05).

Fig. 4.

Total protein content of microtuble-associated protein 1 light-chain 3 (LC3B-I) (A) and LC3B-II (B). Data are expressed relative to a normalization control and as fold change from baseline ± SE, n = 6 per group. Insert shows representative Western blot analysis in duplicate for baseline, 1 h post-Ex (1 h), and EAA+LCHO and EAA+HCHO groups. *Significantly different from baseline (P < 0.05).