Leucine Supplementation Does Not Restore Diminished Skeletal Muscle Satellite Cell Abundance and Myonuclear Accretion When Protein Intake Is Limiting in Neonatal Pigs

Abstract

Background Rapid growth of skeletal muscle in the neonate requires the coordination of protein deposition and myonuclear accretion. During this developmental stage, muscle protein synthesis is highly sensitive to amino acid supply, especially Leu, but we do not know if this is true for satellite cells, the source of muscle fiber myonuclei. Objective We examined whether dietary protein restriction reduces myonuclear accretion in the neonatal pig, and if any reduction in myonuclear accretion is mitigated by restoring Leu intake. Methods Neonatal pigs (1.53 ± 0.2 kg) were fitted with jugular vein and gastric catheters and fed 1 of 3 isoenergetic milk replacers every 4 h for 21 d: high protein [HP; 22.5 g protein/(kg/d); n= 8]; restricted protein [RP; 11.2 g protein/(kg/d); n= 10]; or restricted protein with Leu [RPL; 12.0 g protein/(kg/d); n= 10]. Pigs were administered 5-bromo-2’-deoxyuridine (BrdU; 15 mg/kg) intravenously every 12 h from days 6 to 8. Blood was sampled on days 6 and 21 to measure plasma Leu concentrations. On day 21, pigs were killed and the longissimus dorsi (LD) muscle was collected to measure cell morphometry, satellite cell abundance, myonuclear accretion, and insulin-like growth factor (IGF) system expression. Results Compared with HP pigs, postprandial plasma Leu concentration in RP pigs was 37% and 47% lower on days 6 and 21, respectively (P < 0.05); Leu supplementation in RPL pigs restored postprandial Leu to HP concentrations. Dietary protein restriction reduced LD myofiber cross-sectional area by 21%, satellite cell abundance by 35%, and BrdU+ myonuclear abundance by 25% (P < 0.05); Leu did not reverse these outcomes. Dietary protein restriction reduced LD muscle IGF2 expression by 60%, but not IGF1 or IGF1R expression (P < 0.05); Leu did not rescue IGF2 expression. Conclusions Satellite cell abundance and myonuclear accretion in neonatal pigs are compromised when dietary protein intake is restricted and are not restored with Leu supplementation.

Intermittent bolus feeding does not enhance protein synthesis, myonuclear accretion, or lean growth more than continuous feeding in a premature piglet model

“…Despite the clear role for specific nutrients and insulin in promoting mTORC1 activation and protein synthesis in skeletal muscle ( 49 , 50 ), less is known about their role in regulating myonuclear accretion. Previously, we showed that restricting the protein intake of neonatal pigs reduces myonuclear accretion ( 35 ), and that Leu supplementation does not restore SC abundance or myonuclear accretion when protein intake is limiting ( 51 ). Although this implies an essential role for dietary protein in supporting myonuclear accretion in addition to protein synthesis in muscle, it is unclear whether time-dependent differences in insulin and substrate availability between intermittent and continuous feeding patterns over an extended period can affect this process.…”