Leucine Supplementation Does Not Enhance Translation Initiation of Low Birth‐Weight Neonatal Pigs Despite Greater Sestrin2 Expression in Skeletal Muscle

Lean growth is impaired in low birth‐weight (LBWT) neonatal pigs and correlates with a reduction in protein synthesis rate in skeletal muscle. Translation initiation via mTOR signaling pathway is the rate limiting step in protein synthesis. Leucine binds and inactivates Sestrin2, an inhibitor of the mTORC1 complex. The objective of this study was to determine whether dietary branched‐chain amino acids (BCAA) stimulate translation initiation in skeletal muscle of LBWT pigs. LBWT and normal birth weight (NBWT) were defined when weight at birth was 2 SD below or within 0.5 of the corresponding litter average. Three day old LBWT and NBWT pigs (n=11‐13/group) were assigned to a Control or BCAA sow milk‐replacer formula in a 2×2 factorial arrangement. Formulas were isonitrogenous and contained either 1% BCAA (1:0.68:0.5 leucine:isoleucine:valine) or 0.7% alanine, and were offered at 250 mL·kg body weight−1·day−1 in 5 equal meals. Carotid and jugular catheters were surgically placed on day 21. On day 29, blood samples were taken over 4 h following a meal. Pigs were euthanized 1 h after a final meal. Data were analyzed using MIXED procedure of SAS version 9.4. BCAA supplementation increased BCAA concentration in plasma (P<0.0001) and in muscle free amino acid pool (P<0.001) compared to the Control diet, which similarly increased alanine concentration in plasma and muscle (P<0.0001). Although BCAA supplementation decreased circulating Pro, Ser, Asp, and Lys (P<0.001, 0.05, 0.05, 0.06, respectively), concentrations were not different in skeletal muscle. LBWT pigs expressed less BCAT2 mRNA in the longissimus dorsi than NBWT pigs, but expression was enhanced to normal levels with BCAA supplementation (P<0.05). BCKDHA expression was greater in the liver of LBWT compared to NBWT pigs irrespective of diet (P<0.05). Circulating branched‐chain α‐keto acids increased only in response to BCAA supplementation (P<0.0001). LBWT pigs expressed less Sestrin2 in skeletal muscle (P<0.01), suggesting enhanced sensitivity to leucine. However, despite lower expression of Sestrin2 and increased intramuscular BCAA concentrations, 4EBP1 and S6K1 phosphorylation were not affected by BCAA supplementation. These data suggest that branched‐chain amino acid supplementation did not enhance translation initiation in muscles of neonatal pigs regardless of birth weight.