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 Delta1-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 degrees 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 micromol/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.