Intermittent bolus compared with continuous feeding enhances insulin and amino acid signaling to translation initiation in skeletal muscle of neonatal pigs

Authors: SURYAWAN, AGUS - Children'S Nutrition Research Center (CNRC); EL-KADI, SAMER - Children'S Nutrition Research Center (CNRC); NGUYEN, HANH - Children'S Nutrition Research Center (CNRC); 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: 5/18/2021
Publication Date: 6/22/2021
Citation: Suryawan, A., El-Kadi, S.W., Nguyen, H.V., Fiorotto, M.L., Davis, T.A. 2021. Intermittent bolus compared with continuous feeding enhances insulin and amino acid signaling to translation initiation in skeletal muscle of neonatal pigs. Journal of Nutrition. https://doi.org/10.1093/jn/nxab190.
DOI: https://doi.org/10.1093/jn/nxab190

Interpretive Summary: Some infants cannot ingest food properly, and orogastric tube feeding must be provided either as intermittent bolus meals or by continuous infusion. Intermittent bolus feeding is often preferred over continuous feeding but some infants cannot tolerate meal feeds. Using the neonatal pig as a model for the human infant, our laboratory has recently shown that intermittent bolus meals enhance lean growth more than continuous feeding, and this higher muscle mass is attained via enhanced rates of protein synthesis. Therefore, in this study, we aimed to identify insulin and/or amino acid signaling components involved in the increased stimulation of skeletal muscle by intermittent bolus meals compared to continuous feeding in neonatal pigs born at term. To determine this, piglets born at term were fed equal amounts of a sow milk replacer by orogastric tube as intermittent bolus meals or by continuous infusion. After 21 days, skeletal muscle was collected and components in the intracellular signaling pathways that regulate protein synthesis were measured. Our results show that the enhanced rates of muscle protein synthesis and growth with intermittent bolus meals compared to continuous feeding can be explained by enhanced activation of both the insulin and the amino acid signaling pathways that regulate translation initiation in skeletal muscle of the neonate.

Technical Abstract: Nutrition administered as intermittent bolus feeds rather than continuously promotes greater protein synthesis rates in skeletal muscle and enhances lean growth in a neonatal piglet model. The molecular mechanisms responsible remain unclear. We aimed to identify the insulin- and/or amino acid-signaling components involved in the enhanced stimulation of skeletal muscle by intermittent bolus compared to continuous feeding in neonatal pigs born at term. Term piglets (2–3 days old) were fed equal amounts of sow milk replacer [12.8 g protein and 155 kcal/(kg body weight/d)] by orogastric tube as intermittent bolus meals every 4 hours (INT) or by continuous infusion (CTS). After 21 days, gastrocnemius muscle samples were collected from CTS, INT-0 (before a meal), and INT-60 (60 minutes after a meal) groups (n = 6/group). Insulin- and amino acid-signaling components relevant to mechanistic target of rapamycin complex (mTORC) 1 activation and protein translation were measured. Phosphorylation of the insulin receptor, IRS-1, PDK1, mTORC2, pan-Akt, Akt1, Akt2, and TSC2 was 106% to 273% higher in the skeletal muscle of INT-60 piglets than in INT-0 and CTS piglets (P < 0.05), but phosphorylation of PTEN, PP2A, Akt3, ERK1/2, and AMPK did not differ among groups, nor did abundances of PHLPP, SHIP2, and Ubl4A. The association of GATOR2 with Sestrin1/2, but not CASTOR1, was 51% to 52% lower in INT-60 piglets than in INT-0 and CTS piglets (P < 0.05), but the abundances of SLC7A5/LAT1, SLC38A2/SNAT2, SLC38A9, Lamtor1/2, and V-ATPase did not differ. Associations of mTOR with RagA, RagC, and Rheb and phosphorylation of S6K1 and 4EBP1, but not eIF2a and eEF2, were 101% to 176% higher in INT-60 piglets than in INT-0 and CTS piglets (P < 0.05). The enhanced rates of muscle protein synthesis and growth with intermittent bolus compared to continuous feeding in a neonatal piglet model can be explained by enhanced activation of both the insulin- and amino acid-signaling pathways that regulate translation initiation.