Muscle Protein Anabolic Resistance to Essential Amino Acids Does Not Occur in Healthy Older Adults Before or After Resistance Exercise Training

Abstract

Background: The muscle protein anabolic response to contraction and feeding may be blunted in older adults. Acute bouts of exercise can improve the ability of amino acids to stimulate muscle protein synthesis (MPS) by activating mechanistic target of rapamycin complex 1 (mTORC1) signaling, but it is not known whether exercise training may improve muscle sensitivity to amino acid availability.

Objective: The aim of this study was to determine if muscle protein anabolism is resistant to essential amino acids (EAAs) and whether resistance exercise training (RET) improves muscle sensitivity to EAA in healthy older adults.

Methods: In a longitudinal study, 19 healthy older adults [mean ± SD age: 71 ± 4 y body mass index (kg/m2): 28 ± 3] were trained for 12 wk with a whole-body program of progressive RET (60-75% 1-repetition maximum). Body composition, strength, and metabolic health were measured pre- and posttraining. We also performed stable isotope infusion experiments with muscle biopsies pre- and posttraining to measure MPS and markers of amino acid sensing in the basal state and in response to 6.8 g of EAA ingestion.

Results: RET increased muscle strength by 16%, lean mass by 2%, and muscle cross-sectional area by 27% in healthy older adults (P < 0.05). MPS and mTORC1 signaling (i.e., phosphorylation status of protein kinase B, 4E binding protein 1, 70-kDa S6 protein kinase, and ribosomal protein S6) increased after EAA ingestion (P < 0.05) pre- and posttraining. RET increased basal MPS by 36% (P < 0.05); however, RET did not affect the response of MPS and mTORC1 signaling to EAA ingestion.

Conclusion: RET increases strength and basal MPS, promoting hypertrophy in healthy older adults. In these subjects, a small dose of EAAs stimulates muscle mTORC1 signaling and MPS, and this response to EAAs does not improve after RET. Our data indicate that anabolic resistance to amino acids may not be a problem in healthy older adults. This trial was registered at www.clinicaltrials.gov as NCT02999802.

FIGURE 1

Schematic showing the infusion study design. Fractional synthesis rate was determined with the use of muscle biopsy samples taken from participants with postabsorptive status (basal period) and 1 and 3 h from EAA ingestion (treatment period). Bx, biopsy; EAA, essential amino acid; Hr, hour.

FIGURE 2

CONSORT diagram of study recruitment, enrollment, randomization follow-up, and analysis. CONSORT, Consolidated Standards of Reporting Trials; Drop, dropout; POST, posttraining; PRE, pretraining; RET, resistance exercise training.

FIGURE 3

Percentage change (A) in myofiber CSA after 12 wk of resistance exercise training in healthy older adults. Values are percentage changes from the pretraining condition. Data are means ± SEMs. The table (D) presents absolute values for each fiber type. *Different from pretraining, P < 0.05. (B, C, E, F) Representative immunohistochemical images of myofiber CSA and myosin heavy chain distribution: type I (pink), type IIa (green), type I/IIa (orange), and type IIa/IIx (red). Scale bars represent 100 μm. CSA, cross-sectional area; post, posttraining; pre, pretraining.

FIGURE 4

Mixed-muscle protein FSR after an overnight fast (basal period) and after the ingestion of EAAs (3 h EAAs) for both pre- and posttraining conditions in healthy older adults with the use of intracellular enrichment (A) and blood (B) as the precursor. (C) Basal mixed-muscle protein FSR increased after 12 wk of resistance exercise training. (D) Older adults’ FSR response to the ingestion of 6.7 g EAAs compared with that in young subjects [n = 7; 3 men and 4 women; aged 32 ± 2 y (33)] ingesting 10 g EAAs. n = 17 for each condition. Data are means ± SEMs. *Different from basal, P < 0.05; **different from pretraining, P < 0.01. EAA, essential amino acid; FSR, fractional synthetic rate; hr, hour; post, posttraining; pre, pretraining.

FIGURE 5

mTORC1 signaling response to EAA ingestion at baseline and after 12 wk of training in healthy older adults. Skeletal muscle mTOR (A), S6K1 (B), rpS6 (C), and 4E-BP1 (D) phosphorylation reported as the fold-change from basal. n = 17 for all proteins. Data are means ± SEMs. Representative blot images are included (in duplicate) for comparison. *Different from basal, P < 0.05; **Different from pretraining, P < 0.01. EAA, essential amino acid; mTOR, mammalian target of rapamycin; rpS6, ribosomal protein S6; S6K1, 70-kDa S6 protein kinase 1; 4E-BP1, 4E-binding protein 1.

FIGURE 6

(A) Representative IHC images of merged mTOR and LAMP2, mTOR alone (red), and LAMP2 alone (green) show the localization of mTOR with the lysosome before and after EAA ingestion). mTOR associations with LAMP2 are denoted with white arrowheads. Yellow arrowheads denote sites of no association. (B) Quantification of localization presented as the Manders’ overlap coefficient shows the relative overlap of mTOR⁺ and LAMP2⁺ pixels. Values are fold-changes from basal. *Different from pretraining (3 h EAA), P < 0.05. (C) A simple linear regression between the quantification of total mTOR protein content obtained by immunoblot and IHC analysis showed that mTOR total protein content as assessed by immunoblot was significantly correlated with mTOR protein content as assessed by IHC (Pearson's r = 0.626, R² = 0.392, P < 0.0001). (D) Total mTOR protein densitometry values from immunoblot analysis did not differ after 12 wk of training. Data are means ± SEMs. A.U., arbitrary units; EAA, essential amino acid; IHC, immunohistochemistry; LAMP2, lysosome-associated membrane protein 2; mTOR, mammalian target of rapamycin; post, posttraining; pre, pretraining.