Figure 1.

Interorgan metabolism of citrulline and arginine in pigs. There is no net synthesis of Arg in the liver via the urea cycle due to its rapid hydrolysis by arginase. Therefore, the intestinal-renal axis plays an important role in Arg provision in neonatal and postnatal pigs. Branched-chain amino acids (BCAA) in arterial blood are taken up by skeletal muscle for the synthesis of glutamine, which is released into the circulation. Glutamine in arterial blood, as well as dietary glutamine, glutamate, and proline is utilized by enterocytes of the small intestine for the production of citrulline, the immediate precursor of Arg. Virtually all of the intestine-derived citrulline by-pass the liver and are converted into Arg in the kidneys and extra-renal cells (including endothelial cells and macrophages), whereas 7% to 10% of Arg in the portal vein is extracted by the liver in the first pass. The symbol * denotes the flux in postweaning pigs. This figure is reused from Wu et al. (2007b), with permission from Elsevier.