Pulsatile leucine administration during continuous enteral feeding enhances skeletal muscle mechanistic target of rapamycin complex 1 signaling and protein synthesis in a preterm piglet model

Authors: RUDAR, MARKO - Auburn University; SURYAWAN, AGUS - Children'S Nutrition Research Center (CNRC); NGUYEN, HANH - Children'S Nutrition Research Center (CNRC); CHACKO, SHAJI - Children'S Nutrition Research Center (CNRC); VONDEROHE, CAITLIN - Children'S Nutrition Research Center (CNRC); STOLL, BARBARA - Children'S Nutrition Research Center (CNRC); Burrin, Douglas - Doug; FIOROTTO, MARTA - Children'S Nutrition Research Center (CNRC); DAVIS, TERESA - Children'S Nutrition Research Center (CNRC)

Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2023
Publication Date: 12/22/2023
Citation: Rudar, M., Suryawan, A., Nguyen, H.V., Chacko, S.K., Vonderohe, C., Stoll, B., Burrin, D.G., Fiorotto, M.L., Davis, T.A. 2023. Pulsatile leucine administration during continuous enteral feeding enhances skeletal muscle mechanistic target of rapamycin complex 1 signaling and protein synthesis in a preterm piglet model. Journal of Nutrition. 154(2):505–515. https://doi.org/10.1016/j.tjnut.2023.12.034.
DOI: https://doi.org/10.1016/j.tjnut.2023.12.034

Interpretive Summary: Infants born prematurely commonly have a slower rate of growth compared to infants born at term. This reduced weight gain is largely due to the reduced growth of their lean tissue, especially skeletal muscle. This altered body composition likely contributes to their increased risk for developing obesity and type 2 diabetes later in life. We recently demonstrated that the capacity of muscle to use nutrients for muscle growth is reduced in the preterm. To identify strategies to promote muscle growth in order to mitigate these poor health outcomes, we examined the effectiveness of supplementing a milk replacer formula with leucine, which is an amino acid that is abundant in milk protein. Pigs were delivered by Cesarean section at a time equivalent to 30 weeks of gestation in the human infant, and advanced from parenteral nutrition to full continuous oral feeds by 4 days. We showed that leucine supplementation enhanced the activation of the intracellular amino acid signaling pathway that regulates protein synthesis in skeletal muscle and enhanced muscle protein synthesis. However, a higher amount of leucine was required in preterm pigs than in pigs born at term to maximally stimulate protein synthesis. These findings support our previous studies which show anabolic resistance to amino acids in skeletal muscle of the preterm. However, targeted nutritional therapies, such as supplementation with leucine at effective doses, applied to preterm infants could support postnatal weight gain and overcome the deficit in lean mass in early childhood. Promotion of lean growth may help minimize the lifelong risk of developing insulin resistance and obesity.

Technical Abstract: Continuous feeding does not elicit an optimal anabolic response in skeletal muscle but is required for some preterm infants. We reported previously that intermittent intravenous pulses of leucine (Leu; 800 µmol Leu·kg–1 ·h–1 every 4 h) to continuously fed pigs born at term promoted mechanistic target of rapamycin complex 1 (mTORC1) activation and protein synthesis in skeletal muscle. Objectives: The aim was to determine the extent to which intravenous Leu pulses activate mTORC1 and enhance protein synthesis in the skeletal muscle of continuously fed pigs born preterm. Pigs delivered 10 d preterm was advanced to full oral feeding >4 d and then assigned to 1 of the following 4 treatments for 28 h: 1) ALA (continuous feeding; pulsed with 800 µmol alanine·kg–1 ·h–1 every 4 h; n ¼ 8); 2) L1 (continuous feeding; pulsed with 800 µmol Leu·kg–1 ·h–1 every 4 h; n ¼ 7); 3) L2 (continuous feeding; pulsed with 1600 µmol Leu·kg–1 ·h–1 every 4 h; n ¼ 8); and 4) INT (intermittent feeding every 4 h; supplied with 800 µmol alanine·kg–1 per feeding; n ¼ 7). Muscle protein synthesis rates were determined with L-[2 H5- ring]Phenylalanine. The activation of insulin, amino acid, and translation initiation signaling pathways were assessed by Western blot. Peak plasma Leu concentrations were 134% and 420% greater in the L2 compared to the L1 and ALA groups, respectively (P < 0.01). Protein synthesis was greater in the L2 than in the ALA and L1 groups in both the longissimus dorsi and gastrocnemius muscles (P < 0.05) but not different from the INT group (P > 0.10). Amino acid signaling upstream and translation initiation signaling downstream of mTORC1 largely corresponded to the differences in protein synthesis. Intravenous Leu pulses potentiate mTORC1 activity and protein synthesis in the skeletal muscles of continuously fed preterm pigs, but the amount required is greater than in pigs born at term.