Abstract
The influx of essential amino acids into skeletal muscle is primarily mediated by the large neutral amino acid transporter 1 (LAT1), which is dependent on the glutamine gradient generated by the sodium-dependent neutral amino acid transporter 2 (SNAT2). The protein expression and membrane localization of LAT1 may be influenced by amino acid ingestion and/or resistance exercise, although its acute influence on dietary amino acid incorporation into skeletal muscle protein has not been investigated. In a group design, healthy males consumed a mixed carbohydrate (0.75 g·kg−1) crystalline amino acid (0.25 g·kg−1) beverage enriched to 25% and 30% with LAT1 substrates L-[1-13C]leucine (LEU) and L-[ring-2H5]phenylalanine (PHE), respectively, at rest (FED: n = 7, 23 ± 5 y, 77 ± 4 kg) or after a bout of resistance exercise (EXFED: n = 7, 22 ± 2 y, 78 ± 11 kg). Postprandial muscle biopsies were collected at 0, 120, and 300 min to measure transporter protein expression (immunoblot), LAT1 membrane localization (immunofluorescence), and dietary amino acid incorporation into myofibrillar protein (ΔLEU and ΔPHE). Basal LAT1 and SNAT2 protein contents were correlated with each other (r = 0.55, p = 0.04) but their expression did not change across time in FED or EXFED (all, p > 0.05). Membrane localization of LAT1 did not change across time in FED or EXFED whether measured as outer 1.5 µm intensity or membrane-to-fiber ratio (all, p > 0.05). Basal SNAT2 protein expression was not correlated with ΔLEU or ΔPHE (all, p ≥ 0.05) whereas basal LAT1 expression was negatively correlated with ΔPHE in FED (r = −0.76, p = 0.04) and EXFED (r = −0.81, p = 0.03) but not ΔLEU (p > 0.05). Basal LAT1 membrane localization was not correlated with ΔLEU or ΔPHE (all, p > 0.05). Our results suggest that LAT1/SNAT2 protein expression and LAT1 membrane localization are not influenced by acute anabolic stimuli and do not positively influence the incorporation of dietary amino acids for de novo myofibrillar protein synthesis in healthy young males.
Highlights
Exogenous amino acid administration increases the uptake of amino acids into skeletal muscle at rest and, to a greater extent, after resistance exercise [1]
The ability of dietary amino acids to stimulate muscle protein synthesis is generally dependent on their delivery to intracellular sensors and effector molecules associated with the mTORC1 pathway [8] of which amino acid transporters at the plasma membrane are an essential but not rate limiting component [9,10]
The influx of essential amino acids into skeletal muscle is primarily mediated by the large neutral amino acid transporter 1 (LAT1), which is dependent on the glutamine gradient generated by the sodium-dependent neutral amino acid transporter 2 (SNAT2) [11,12]
Summary
Exogenous amino acid administration increases the uptake of amino acids into skeletal muscle at rest and, to a greater extent, after resistance exercise [1]. Muscle protein synthesis rates are stimulated within 2 h post-feeding/exercise with rapidly digested amino acid sources [5,6,7] and are underpinned by increases in extracellular and intracellular essential amino acid availability within 30 min of amino acid intake [1,2]. LAT1 seems to be the primary contributor to both leucine and phenylalanine influx into skeletal muscle In addition to their role in inward amino acid transport, membrane-bound transporters have been suggested to have auxiliary roles as extracellular amino acid sensors [16,17,18]. Amino acid transporters appear to play an integral role in the anabolic response to dietary amino acids and exercise, the relationship between the protein expression and membrane content of key amino acid transporters, and the subsequent incorporation of dietary amino acids to support de novo protein synthesis, is lacking
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