Dietary citrulline improves survival of rainbow trout Oncorhynchus mykiss juveniles challenged with Vibrio anguillarum

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Arginine is a versatile amino acid that plays important roles in various physiological functions. However, supplementation with arginine can also increase urea excretion of the fish. Thus, given than ornithine and citrulline can potentially be used to increase arginine availability or improve fish immunity, this study aimed to compare the effects of dietary supplementation of arginine, ornithine, and citrulline on the survival of rainbow trout Oncorhynchus mykiss against Vibrio anguillarum. Dietary treatment consisted of a control diet (CTRL) and three treatment diets comprising CTRL supplemented with 2% L-arginine (+ARG), L-ornithine (+ORN), or L-citrulline (+CIT). Protein and lipid content of the diets were confirmed at 48.9 and 18.9%, 53.1 and 18.7%, 51.7 and 18.1% and 52.9 and 18.9% for the CTRL, +ARG, +ORN & + CIT diets respectively. Postprandial plasma amino acid levels were measured at 6, 15, and 30 h after feeding. A feeding trial was conducted for 30 days, and at the end of the trial, the rainbow trout were challenged with V. anguillarum. At 24 h post-injection, plasma amino acid and renal gene expression levels were examined. Fish exhibited better weight gain in +ARG treatment compared to +CIT treatment. Similar postprandial levels of plasma arginine were observed in +ARG and + CIT, and the levels in both treatments were significantly higher than those in CTRL. A survival analysis revealed that fish in +CIT (69.2%) were more resistant than those in CTRL (25.0%). There was significantly higher inducible nitric oxide synthase (iNOS) expression in the kidneys of fish in +CIT than those in CTRL. A significant difference in interleukin-1-beta expression was found between +ORN and CTRL treatments, while there was no significant difference in arginase II expression among the treatments. These result shows that citrulline supplementation can be used to increase arginine availability and better at enhancing the immune performance of rainbow trout compared to arginine supplementation when challenged with V. anguillarum.