The interaction between lysyl-lysine and arginine in the neonatal small intestine

Dietary arginine is absorbed at the small intestinal brush border via the Na⁺-independent amino acid antiporter, b^0,+ which transports cationic amino acids. Previously, we demonstrated greater arginine absorption when arginine was provided with the di-peptide lysyl-lysine. This phenomenon may be explained by trans-stimulation of b^0,+ antiporter. Peptide transporter 1 (PepT1) facilitates the uptake of lysyl-lysine into enterocytes. Lysyl-lysine may be hydrolyzed to free lysine, increasing its concentration inside the cell. This in turn stimulates b^0,+ antiporter which initiates the exchange of the intracellular lysine for luminal arginine. Using an in situ intestinal perfusion model in sow-fed neonatal piglets, our objectives were to 1) determine whether the lysyl lysine-enhanced arginine absorption led to functional benefits (determined by rate of mucosal protein synthesis), and 2) to determine if greater mucosal protein synthesis in response to arginine uptake was mediated through the mTOR pathway. In piglets (d=16-21, N=6) the small intestine was exposed by laparotomy, and six segments of proximal small intestine were isolated. Segments were continuously perfused, with one of three treatments in PBS: 1) arginine, 2) arginine + lysyl-lysine, or 3) arginine + L-lysine. The same treatments were perfused through two loops in each pig. In one replicate, ³H-arginine was added to measure arginine uptake. In the second replicate, ³H- phenylalanine and L- phenylalanine were added to measure mucosal protein synthesis. Six additional piglets underwent the identical protocol, but with rapamycin (inhibitor of mTOR) delivered intravenously 1 h prior to the study. Arginine + lysyl-lysine resulted in greater arginine uptake, and higher mucosal protein synthesis compared to the other two treatments. Moreover, when rapamycin was infused, protein synthesis was inhibited and was not different amongst treatments. In support of these findings, rapamycin treatment inhibited the phosphorylation of mTOR, interfering with its activation. Thus, the presence of the lysyl-lysine led to a functional advantage of greater mucosal protein synthesis; this was likely mediated through enhanced arginine absorption and the activation of mTOR.