Mammalian target of rapamycin signaling and ubiquitin proteasome-related gene expression in 3 different skeletal muscles of colostrum- versus formula-fed calves.

The rates of protein turnover are higher during the neonatal period than at any other time in postnatal life. The mammalian target of rapamycin (mTOR) and the ubiquitin-proteasome system are key pathways regulating cellular protein turnover. The objectives of this study were (1) to elucidate the effect of feeding colostrum versus milk-based formula on the mRNA abundance of key components of the mTOR pathway and of the ubiquitin-proteasome system in skeletal muscle of neonatal calves and (2) to compare different muscles. German Holstein calves were fed either colostrum (COL; n = 7) or milk-based formula (FOR; n = 7) up to 4 d of life. The nutrient content in formula and colostrum was similar, but formula had lower concentrations of free branched-chain AA (BCAA) and free total AA, insulin, and insulin-like growth factor (IGF)-I than colostrum. Blood samples were taken from d 1 to 4 before morning feeding and before and 2 h after the last feeding on d 4. Muscle samples from M. longissimus dorsi (MLD), M. semitendinosus (MST), and M. masseter (MM) were collected after slaughter on d 4 at 2 h after feeding. The preprandial concentrations of free total AA and BCAA, insulin, and IGF-I in plasma changed over time but did not differ between groups. Plasma free total AA and BCAA concentrations decreased in COL, whereas they increased in FOR after feeding, resulting in higher postprandial plasma total AA and BCAA concentrations in FOR than in COL. Plasma insulin concentrations increased after feeding in both groups but were higher in COL than in FOR. Plasma IGF-I concentrations decreased in COL, whereas they remained unchanged in FOR after feeding. The mRNA abundance of mTOR and ribosomal protein S6 kinase 1 (S6K1) in 3 different skeletal muscles was greater in COL than in FOR, whereas that of eukaryotic translation initiation factor 4E binding protein 1 (4EBP1) was unaffected by diet. The mRNA abundance of ubiquitin activating enzyme (UBA1) and ubiquitin conjugating enzyme 1 (UBE2G1) enzymes was not affected by diet, whereas that of ubiquitin conjugating enzyme 2 (UBE2G2) was greater (MLD) or tended to be greater (MM) in COL than in FOR. The mRNA abundance of atrogin-1 in MLD and MST was lower in COL than in FOR, whereas that of muscle ring finger protein-1 (MuRF1) was greater (MST) or tended to be greater (MLD). The abundance of MuRF1 mRNA was highest in MST, followed by MLD, and was lowest in MM. The results indicate that colostrum feeding may stimulate protein turnover that may result in a high rate of protein deposition in a muscle type-specific manner. Such effects seem to be mediated by the postprandial increase in plasma insulin.