Metabolic and immune effects of orange-spotted grouper, Epinephelus coioides induced by dietary arginine

Arginine (Arg) as an important substrate for protein metabolism and endocrine influences on fish health directly. To evaluate the effect of dietary Arg on these endocrine and immunity of juvenile orange-spotted grouper (Epinephelus coioides), an experiment adding Arg with 0.00, 0.30, 0.60, 0.90, 1.20, 1.50, and 1.80% in seven diets was conducted. Dietary Arg levels reached 2.13 (Diet 1), 2.42 (Diet 2), 2.71 (Diet 3), 2.95 (Diet 4), 3.20 (Diet 5), 3.48 (Diet 6) and 3.74% (Diet 7), respectively. Each treatment was randomly assigned to triplicate groups of 30 fish (7.52 ± 0.02 g) each tank. The present results indicated that different Arg levels of diets were no significant effects on the fish survival rate of each groups (P > 0.05) after 8 weeks feeding experiment. The highest weight gain and specific growth rate were found in Diet 4, and significantly higher than the other groups (P < 0.05). Hepatic arginase activities were significantly higher in fish fed Diet 6 and Diet 7 compared to the activities of the fish fed first three Diets (P < 0.05). Hepatic ornithine decarboxylase activities of fish fed Diet 1 and Diet 2 were significantly decreased compared to the fish fed Diet 4–Diet 7 (P < 0.05). With ascending dietary Arg, the serum Insulin (INS) and insulin-like growth factor I (IGF-I) levels significantly increased. Gene expression of tissue toll-like receptors 22 in the three tissues were found first up-regulated and then down-regulated with the Arg increasing. Gene expression of major histocompatibility complex II in liver and kidney significantly increased in fish fed Diet 4 and Diet 6, and decreased thereafter. However, gene expression of hepcidin decreased with dietary Arg increased. The present study showed alterations in serum INS and IGF-I levels in response to Arg-induced growth activation. Additionally, the results indicate that Arg has beneficial effects on regulating mRNA expression of immune-associated genes and improving humoral and cellular immunity of fish.