Plasma citrulline and arginine kinetics in juvenile channel catfish, Ictalurus punctatus, given oral gabaculine
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abstract
Experiments with young mammals have indicated that renal arginine synthesis is a major endogenous source of this amino acid. The kidney readily synthesizes arginine from intestinally derived citrulline, which is produced from glutamate, the so-called intestinal-renal axis. The present experiment was conducted to ascertain whether or not citrulline and arginine are synthesized in channel catfish via this pathway by administering gabaculine, an effective inhibitor of ornithine aminotransferase, and then monitoring circulating levels of arginine, citrulline, glutamate and ornithine. A conditioning diet was fed to apparent satiation to triplicate groups of channel catfish initially averaging 150 g/fish. After 2 weeks of acclimation, fish were lightly anesthetized with MS-222 and were intubated with a gelatin capsule containing gabaculine (3 mg/kg body weight) in one dose. Then, 24 and 48 h after gabaculine treatment, blood plasma was obtained and analyzed for amino acids via HPLC. Oral gabaculine administration significantly (p < 0.05) decreased plasma concentrations of citrulline (-41.8%) and arginine (-8.9%). It also increased (p < 0.05) plasma glutamine (+17.5%) and ornithine (+256.6%). A reasonable explanation for the increase in plasma glutamine and the decrease in citrulline and arginine is offered by the inhibition of glutamine degradation and a decrease in synthesis of citrulline caused by gabaculine. Results of this experiment suggest that endogenous synthesis of citrulline occurs in channel catfish, most likely at the small intestine, and it may serve as a de novo source of arginine. Therefore, the physiological needs of fish tissues and organs for arginine appear to be met via both dietary arginine and the endogenous synthesis of this amino acid.
