Abundance of Amino Acid Transporters and mTOR Pathway Components in the Gastrointestinal Tract of Lactating Holstein Cows

SIMPLE SUMMARY: Peptides and amino acids (AAs) arising from feed or microbial protein digestion require transporters in the gastrointestinal tract (GIT) for uptake into the blood. The mTOR pathway, considered the master regulator of protein synthesis, is partly controlled by specific AAs. We measured AA concentrations, mRNA abundance of AA transporters, and genes in the mTOR pathway in epithelia from the rumen, duodenum, jejunum, and ileum of lactating Holstein cows. The concentrations of most AAs and the abundances of AA transporters and mTOR mRNA were greater in the small intestine than in the rumen. As in non-ruminants, the absorption of AAs from the small intestine is partly due to the greater abundance of transporters. Compared with the ruminal epithelium, the greater abundance of mTOR in the small intestine underscored its role in regulating cellular protein synthesis. ABSTRACT: Data from non-ruminants indicate that amino acid (AA) transport into cells can regulate mTOR pathway activity and protein synthesis. Whether mTOR is expressed in the ruminant gastrointestinal tract (GIT) and how it may be related to AA transporters and the AA concentrations in the tissue is unknown. Ruminal papillae and the epithelia of the duodenum, jejunum, and ileum collected at slaughter from eight clinically healthy Holstein in mid-lactation were used. Metabolites and RNA were extracted from tissue for liquid chromatography–mass spectrometry and RT-qPCR analysis. The glycine and asparagine concentrations in the rumen were greater than those in the intestine (p < 0.05), but the concentrations of other AAs were greater in the small intestine than those in the rumen. Among the 20 AAs identified, the concentrations of glutamate, alanine, and glycine were the greatest. The mRNA abundances of AKT1 and MTOR were greater in the small intestine than those in the rumen (p < 0.05). Similarly, the SLC1A1, SLC6A6, SLC7A8, SLC38A1, SLC38A7, and SLC43A2 mRNA abundances were greater (p < 0.05) in the small intestine than those in the rumen. The mRNA abundances of SLC1A5, SLC3A2, and SLC7A5 were greater in the rumen than those in the small intestine (p < 0.05). Overall, the present study provides fundamental data on the relationship between mTOR pathway components and the transport of AAs in different sections of the gastrointestinal tract.