Activation of Signaling Components for Muscle Protein Accretion by Leucine Supplementation Differ by Fiber-Type in a Preterm Pig Model

Objectives: Postnatal growth of lean mass is commonly blunted in preterm infants and may contribute to long-term morbidities. Previously we showed that leucine supplementation promotes protein synthesis in skeletal muscle of a preterm piglet model in the short-term but long-term anabolic effects and the effect of muscle fiber type have not been elucidated. We hypothesized that chronic leucine supplementation enhances muscle protein accretion in preterm pigs by regulating key signaling components of protein synthesis and protein degradation in muscle. Methods: Pigs delivered by C-section at 105 d gestation transitioned over 7 d from parenteral to enteral feeding and were fed continuously a nutritionally complete milk-replacer to deliver 195 kcal ME/(kg body weight (BW)/d) and 13.5 g protein/(kg BW/d) for 17 d. Pigs were randomly assigned to: 1) Leucine (LEU; 1.6 mmol/(kg BW/4 h); n = 16) or alanine (ALA; 1.6 mmol Ala/(kg BW/4 h); isonitrogenous control; n = 13) supplementation groups. Leucine and alanine were administered i.v. as a pulse for 1 h, every 4 h from 3 to 24 d. Body composition was determined using dual x-ray absorptiometry at 22 d. After 24 d, signaling component activation and protein synthesis rates were determined in muscles with different fiber types 60 min after initiation of the last pulse. Results: Average daily weight gain, final BW, and longissimus dorsi (LD) muscle weight relative to BW were higher (P< 0.05) and total lean mass tended to be higher (P< 0.06) in LEU compared to ALA pigs. Fractional protein synthesis rates in LD and gastrocnemius muscles, but not soleus, diaphragm, or heart, were greater (P< 0.05) in LEU than ALA pigs. Translation initiation signaling responses downstream of mTORC1 (S6K1 and 4EBP phosphorylation and eIF4E-eIF4G abundance) corresponded to the differences in protein synthesis (P< 0.05). There were no differences between groups in indices of non-mTORC1 dependent translation initiation and elongation (eIF2alpha and eEF2 phosphorylation) and protein degradation (MURF, Atrogin, and LC3 abundance). Conclusions: Leucine supplementation during continuous feeding enhances lean growth by stimulating the mTORC1-dependent translation initiation pathway leading to protein synthesis in predominantly glycolytic muscles of the preterm pig. Funding Sources: NIH and USDA.