Plasma citrulline and arginine kinetics in juvenile channel catfish, Ictalurus punctatus, given oral gabaculine

Experiments with young mammals have indicated that renal arginine synthesis is a major endogenous source of this amino acid. The kidney readily synthe sizes 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 synthesi zed in channel catfish via this pathway by administering gabaculine, an eff ective inhibitor of ornithine aminotransferase, and then monitoring circula ting levels of arginine, citrulline, glutamate and ornithine. A conditionin g diet was fed to apparent satiation to triplicate groups of channel catfis h initially averaging 150 g/fish. After 2 weeks of acclimation, fish were l ightly anesthetized with MS-222 and were intubated with a gelatin capsule c ontaining gabaculine (3 mg/kg body weight) in one dose. Then, 24 and 48 h a fter gabaculine treatment, blood plasma was obtained and analyzed for amino acids via HPLC. Oral gabaculine administration significantly (p < 0.05) de creased 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 glutami ne degradation and a decrease in synthesis of citrulline caused by gabaculi ne. Results of this experiment suggest that endogenous synthesis of citrull ine occurs in channel catfish, most likely at the small intestine, and it m ay serve as a de novo source of arginine. Therefore, the physiological need s of fish tissues and organs for arginine appear to be met via both dietary arginine and the endogenous synthesis of this amino acid.