Dietary arginine supplementation decreases plasma cortisol levels and modulates immune mechanisms in chronically stressed turbot (Scophthalmus maximus)

Repeated handling was employed in this study as a chronic stressor to verify whether its attributed immunosuppressive effects could be minimized by dietary arginine supplementation. Therefore, turbot (Scophthalmus maximus) were air-exposed for 3min (mild hypoxia) every other day during 15 or 60days (handled) or left undisturbed (control). In addition, control and handled specimens were fed one of three diets with graded levels of arginine (6, 7 and 11g 16g(-1) N; Arg 6, Arg 7 and Arg 11, respectively). Repeated handling increased plasma cortisol levels in handled fish fed the Arg 6 diet when compared with control groups, while those levels decreased in handled fish fed Arg 7 and Arg 11 diets after both 15 and 60days. Moreover, repeated handling stress decreased nitric oxide production, lysozyme activity and peripheral lymphocyte numbers after 15days, while more profound effects were observed after 60days. Furthermore, circulating monocytes, nitric oxide production and humoral parameters increased in both control and handled fish fed arginine supplements. In conclusion, dietary arginine supplementation may present valuable effects in chronically stressed turbot by decreasing plasma cortisol levels and enhancing most aspects of the innate immune mechanisms assessed after 60days.