Abstract
Leucine is unique among the amino acids in its ability to promote protein synthesis by activating translation initiation via the mammalian target of rapamycin (mTOR) pathway. Previously, we showed that leucine infusion acutely stimulates protein synthesis in fast-twitch glycolytic muscle of neonatal pigs but this response cannot be maintained unless the leucine-induced fall in amino acids is prevented. To determine whether leucine can stimulate protein synthesis in muscles of different fiber types and in visceral tissues of the neonate in the long-term if baseline amino acid concentrations are maintained, overnight fasted neonatal pigs were infused for 24 h with saline, leucine (400 μmol kg−1 h−1), or leucine with replacement amino acids to prevent the leucine-induced hypoaminoacidemia. Changes in the fractional rate of protein synthesis and activation of mTOR, as determined by eukaryotic initiation factor 4E binding protein (4E-BP1) and S6 kinase 1 (S6K1) phosphorylation, in the gastrocnemius and masseter muscles, heart, liver, jejunum, kidney, and pancreas were measured. Leucine increased mTOR activation in the gastrocnemius and masseter muscles, liver, and pancreas, in both the absence and presence of amino acid replacement. However, protein synthesis in these tissues was increased only when amino acids were infused to maintain baseline levels. There were no changes in mTOR signaling or protein synthesis in the other tissues we examined. Thus, long-term infusion of leucine stimulates mTOR signaling in skeletal muscle and some visceral tissues but the leucine-induced stimulation of protein synthesis in these tissues requires sustained amino acid availability.
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Acknowledgments
We thank Marta Fiorotto for helpful discussions, Jerome Stubblefield for care of animals, E. O’Brian Smith for statistical assistance, Adam Gillum for graphics and Linda Weiser for secretarial assistance. Funding for this research was received from the Ajinomoto Amino Acid Research Program, NIH R01 AR-44474, NIH K08 AR051563, and USDA/ARS Cooperative Agreement no. 58-6250-6-001. This work is a publication of the United States Department of Agriculture, Agricultural Research Service (USDA/ARS) Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX.
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Wilson, F.A., Suryawan, A., Orellana, R.A. et al. Differential effects of long-term leucine infusion on tissue protein synthesis in neonatal pigs. Amino Acids 40, 157–165 (2011). https://doi.org/10.1007/s00726-010-0629-9
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DOI: https://doi.org/10.1007/s00726-010-0629-9