ENDOGENOUS SYNTHESIS OF ARGININE PLAYS AN IMPORTANT ROLE IN MAINTAINING ARGININE HOMEOSTASIS IN POSTWEANING GROWING PIGS

This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweanin g pigs. Rigs were fed a sorghum-based diet containing 0.98% arginine a nd were used for studies at 75 d of age (28.4 kg body weight). Mitocho ndria were prepared from the jejunum and other major tissues for measu ring the activities of Delta(1)-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (th e enzyme catalyzing the interconversion of P5C into ornithine). For me tabolic studies, jejunal enterocytes were incubated at 37 degrees C fo r 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-g lutamine, 2 mmol/L L-[U-C-14]proline, and 0-200 mu mol/L gabaculine (a n inhibitor of OAT). The activities of P5C synthase, proline oxidase a nd OAT were greatest in enterocytes among all of the tissues studied. Incubation of enterocytes with gabaculine resulted in decreases (P < 0 .05) in the synthesis of ornithine and citrulline from glutamine and p roline. When gabaculine was orally administered to pigs (0.83 mg/kg bo dy weight) to inhibit intestinal synthesis of citrulline from glutamin e and proline, plasma concentrations of citrulline (-26%) and arginine (-22%) decreased (P < 0.05), whereas those of alanine (+21%), ornithi ne (+17%), proline (+107%), taurine (+56%) and branched-chain amino ac ids (+21-40%) increased (P < 0.05). On the basis of dietary arginine i ntake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided greater than or equal to 50.2% of t otal daily arginine requirement. Taken together, our results suggest a n important role for endogenous synthesis of arginine in regulating ar ginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs.