Incorporation of Dietary Amino Acids Into Myofibrillar and Sarcoplasmic Proteins in Free-Living Adults Is Influenced by Sex, Resistance Exercise, and Training Status

Abstract

Background Acute exercise increases the incorporation of dietary amino acids into de novo myofibrillar proteins after a single meal in controlled laboratory studies in males. It is unclear whether this extends to free-living settings or is influenced by training or sex. Objectives We determined the effects of exercise, training status, and sex on 24-hour free-living dietary phenylalanine incorporation into skeletal muscle proteins. Methods In a parallel group design, recreationally active males (mean ± SD age, 23 ± 3 years; BMI. 23.4 ± 2.9 kg/m2; n = 10) and females (age 24 ± 5 years; BMI, 23.1 ± 3.9 kg/m2; n = 9) underwent 8 weeks of whole-body resistance exercise 3 times a week. Controlled diets containing 1.6 g/kg–1/d–1 (amino acids modelled after egg), enriched to 10% with [13C6] or [2H5]phenylalanine, were consumed before and after an acute bout of resistance exercise. Fasted muscle biopsies were obtained before [untrained, pre-exercise condition (REST ] and 24 hours after an acute bout of resistance exercise in untrained (UT) and trained (T) states to determine dietary phenylalanine incorporation into myofibrillar (ΔMyo) and sarcoplasmic (ΔSarc) proteins, intracellular mechanistic target of rapamycin (mTOR) colocalization with ulex europaeus agglutinin–1 (UEA-1; capillary marker; immunofluorescence), and amino acid transporter expression (Western blotting). Results The ΔMyo values were ∼62% greater (P < 0.01) in females than males at REST. The ΔMyo values increased above REST by ∼51% during UT and ∼30% in T (both P < 0.01) in males, remained unchanged in females during UT, and were ∼33% lower at T when compared to UT (P = 0.013). Irrespective of sex, ΔMyo and ΔSarc were decreased at T compared to UT (P ≤ 0.026). Resistance training increased mTOR colocalization with UEA-1 (P = 0.004), while L amino acid transporter 1, which was greater in males (P < 0.01), and sodium-coupled neutral amino acid transporter 2 protein expression were not affected by acute exercise (P ≥ 0.33) or training (P ≥ 0.45). Conclusions The exercise-induced incorporation of dietary phenylalanine into myofibrillar and sarcoplasmic proteins is attenuated after training regardless of sex, suggesting a reduced reliance on dietary amino acids for postexercise skeletal muscle remodeling in the T state.

Molecular responses to acute exercise and their relevance for adaptations in skeletal muscle to exercise training

“…Biological sex is another mediating influence on responses to acute exercise and long term exercise training, and studying sex as a biological variable is increasingly emphasized (1173)(1174)(1175)(1176). Phenotypical sex differences exist in skeletal muscle in terms of gene expression (1109,(1177)(1178)(1179)(1180), satellite cell (1181)(1182)(1183), and metabolic and mitochondrial characteristics (1184)(1185)(1186)(1187)(1188)(1189). Additionally, given the influence of endogenous and exogenous sex hormones (1190)(1191)(1192)(1193), and that the responses of several genetic, metabolic and physiological parameters to exercise differs between males and females (1194)(1195)(1196), sex differences at the level of molecular responses to acute exercise and exercise training seem likely (1176).…”

The skeletal muscle fiber periphery: A nexus of mTOR-related anabolism

“… 60 Evidence for the acute modulation of LAT1 and SNAT2 protein expression and trafficking by dietary protein ingestion and exercise in humans is equivocal. 64 , 73 , 90 , 92 , 93 , 94 However, chronic total-body resistance training, irrespective of daily dietary protein supplementation, was shown to increase LAT1 protein content per muscle fiber after 12 weeks. 85 This increase occurred primarily in the sarcoplasm, suggesting that membrane LAT1 content remains unaltered.…”

“…The microvasculature appears to be a central component underpinning the fiber periphery as a region of importance in skeletal muscle anabolism. Feeding and resistance exercise are important anabolic stimuli that redistribute mTOR-LAMP2 complexes toward the sarcolemmal membrane, 57 , 59 , 62 , 66 , 73 in close proximity to capillaries. 57 , 73 Repositioning of mTORC1 to the microvasculature in this manner could ostensibly suggest a greater ability to “sense” extracellular AA in the post-training state.…”

“…Feeding and resistance exercise are important anabolic stimuli that redistribute mTOR-LAMP2 complexes toward the sarcolemmal membrane, 57 , 59 , 62 , 66 , 73 in close proximity to capillaries. 57 , 73 Repositioning of mTORC1 to the microvasculature in this manner could ostensibly suggest a greater ability to “sense” extracellular AA in the post-training state. 73 These complexes are themselves enriched with growth factor receptors, AA transporters, and integrins, suggesting that they could be a nexus where anabolic stimuli convene to regulate mTORC1 activity.…”

“… 57 , 73 Repositioning of mTORC1 to the microvasculature in this manner could ostensibly suggest a greater ability to “sense” extracellular AA in the post-training state. 73 These complexes are themselves enriched with growth factor receptors, AA transporters, and integrins, suggesting that they could be a nexus where anabolic stimuli convene to regulate mTORC1 activity. 58 …”