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This study investigated the effects of dietary glutamine on growth performance, antioxidant capacity, and osmoregulation in euryhaline fish (Oreochromis niloticus) under hyperosmotic stress. Nile tilapia was fed four diets with varying glutamine concentrations (0 %, 0.75 %, 1.5 %, and 3 %) at 0 or 16 ppt for 42 days. The results demonstrated that dietary supplementation of 0.75 % glutamine significantly enhanced the growth performance of Nile tilapia under both 0 ppt and 16 ppt salinity. Hyperosmotic stress significantly reduced the crude protein and crude lipid contents while elevated the ash content of the fish. Under hyperosmotic stress, dietary glutamine supplementation significantly increased the antioxidant and immune capacities, and attenuated apoptosis in the liver of tilapia. The expression of ion transporter genes in the gills and kidneys were both significantly increased by dietary glutamine. Meanwhile, glutamine supplementation significantly reduced the levels of inorganic ions in serum while increased serum osmolality. Hyperosmotic stress significantly activated the glycolysis, gluconeogenesis, and pentose phosphate pathways in the liver of tilapia. However, supplementation of glutamine under hyperosmotic stress would significantly enhance the ability of gluconeogenesis of tilapia in the liver. The results of this research suggested that dietary glutamine might promote the growth performance of fish by enhancing their antioxidant capacity and osmoregulation capacity under chronic hyperosmotic stress. This work investigates the role of glutamine in osmolarity regulation and extends the understanding of its mechanism of action, which could provide a reference for hyperosmotic adaptation in aquatic animals.

The purpose of this study was to explore the relationship between inositol metabolism and carbohydrate metabolism in Nile tilapia (Oreochromis niloticus) under acute hyperosmotic stress. 50 mg/mL glucose solution and PBS was injected to the fish of experimental group and the control group, respectively. The fish were then transferred to 16 psu brackish water for hyperosmotic stress immediately, and sampled at 0, 1, 3, 6, 12, 24, and 48 h post stress. The hyperosmotic stress significantly increased the osmotic pressure, glucose content of serum, the process of glycolysis and gluconeogenesis in the liver of fish in both groups over time. The expression of genes involved in myo-inositol (MI) synthesis and transport was all induced by hypertonicity in gill, kidney and liver of fish in both groups. The injection of glucose solution significantly decreased MI synthesis in gill, kidney and liver, and relieved the apoptosis of liver under acute hyperosmotic stress. However, glucose injection significantly increased Na+K+-ATPase activity in the gill, as well as serum osmotic pressure, and the decomposition of carbohydrate, indicating that additional glucose promoted osmoregulation ability of fish. The results of this research suggested that during the hyperosmotic stress, the injection of exogenous glucose could not only provide energy that required for osmoregulation, but also participate in the osmoregulation by acting as an osmolyte itself.