Endogenous Synthesis of Arginine Plays an Important Role in Maintaining Arginine Homeostasis in Postweaning Growing Pigs

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Abstract
This study was conducted to determine whether endogenous synthesis of arginine plays a role in regulating arginine homeostasis in postweaning pigs. Pigs were fed a sorghum-based diet containing 0.98% arginine and were used for studies at 75 d of age (28.4 kg body weight). Mitochondria were prepared from the jejunum and other major tissues for measuring the activities of Δ1-pyrroline-5-carboxylate (P5C) synthase and proline oxidase (enzymes catalyzing P5C synthesis from glutamate and proline, respectively) and of ornithine aminotransferase (OAT) (the enzyme catalyzing the interconversion of P5C into ornithine). For metabolic studies, jejunal enterocytes were incubated at 37°C for 30 min in Krebs-Henseleit bicarbonate buffer containing 2 mmol/L L-glutamine, 2 mmol/L L-[U-14C]proline, and 0–200 μmol/L gabaculine (an inhibitor of OAT). The activities of P5C synthase, proline oxidase and 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 proline. When gabaculine was orally administered to pigs (0.83 mg/kg body weight) to inhibit intestinal synthesis of citrulline from glutamine and proline, plasma concentrations of citrulline (−26%) and arginine (−22%) decreased (P< 0.05), whereas those of alanine (+21%), ornithine (+17%), proline (+107%), taurine (+56%) and branched-chain amino acids (+21–40%) increased (P< 0.05). On the basis of dietary arginine intake and estimated arginine utilization, the endogenous synthesis of arginine in the 28-kg pig provided ≥50.2% of total daily arginine requirement. Taken together, our results suggest an important role for endogenous synthesis of arginine in regulating arginine homeostasis in postweaning growing pigs, as previously shown in neonatal pigs.