Dietary chromium picolinate-promoted oxidative phosphorylation may be a potential mean to improve hypoxia tolerance of juvenile grass carp (Ctenopharyngodon idella)

This study aimed to investigate the effects of dietary chromium picolinate (Cr-Pic) on growth performance, glucose (GLU) metabolism, and hypoxia tolerance of juvenile grass carp (<i>Ctenopharyngodon idella</i>). A total of 2160 juvenile grass carp (10.98 ± 0.01 g) were randomly divided into six groups with six cages each, containing 60 fish per cage. They were fed experimental diets with chromium (Cr³⁺) at 0.00 (basal diet), 0.40, 0.80, 1.20, 1.60, and 2.00 mg/kg (actual contents: 0.25, 0.67, 1.08, 1.63, 2.08, and 2.65 mg/kg, respectively) for 10 weeks. Then, 96 fish were selected and divided into a normoxia group (<i>n</i> = 48) and a hypoxia group (<i>n</i> = 48) for a 96 h hypoxia trial. The growth trial results showed that, compared to the 0.25 mg/kg Cr³⁺ group, the 0.67 to 2.65 mg/kg Cr³⁺ groups enhanced the percent weight gain (PWG; <i>P</i> < 0.001) and increased the protein retention value (<i>P</i> < 0.001), and promoted the growth and development of juvenile grass carp. In the hypoxia group, dietary 1.08 mg/kg Cr³⁺ reduced the activities of serum glutamic pyruvic transaminase (GPT; <i>P</i> < 0.001) and glutamic oxaloacetic transaminase (GOT; <i>P</i> < 0.001), thereby alleviating liver damage. Furthermore, dietary 1.63 mg/kg Cr³⁺ also increased glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation in juvenile grass carp under hypoxia stress, as evidenced by the enhanced activities of liver enzymes, such as hexose kinase (<i>P <</i> 0.001), pyruvate kinase (<i>P <</i> 0.001), and malate dehydrogenase (<i>P <</i> 0.001). Additionally, in the liver, dietary 1.08 to 1.63 mg/kg Cr³⁺ inhibited the expression of proteins related to gluconeogenesis, like glucose-6-phosphatase (<i>P <</i> 0.001) and phosphoenolpyruvate carboxykinase (<i>P <</i> 0.001), and elevated the relative mRNA expression levels of mitochondrially encoded NADH dehydrogenase I (<i>P <</i> 0.001) and adenosine triphosphate (ATP) synthase-coupling factor 6 (<i>P <</i> 0.001), thereby increased ATP production under hypoxia stress. Moreover, in the hypoxia group, dietary 1.08 to 1.63 mg/kg Cr³⁺ up-regulated the liver relative mRNA expression levels of nuclear respiratory factor 1 (<i>P <</i> 0.001) and mitofusin 1 (<i>P <</i> 0.001), and down-regulated the relative mRNA expression levels of mitochondrial fission protein 1 (<i>P <</i> 0.001) and B-cell lymphoma-2-interacting protein 3 (<i>P <</i> 0.001), which contributed to the maintenance of normal mitochondrial function. Regression analysis was conducted utilizing the parameters of PWG, GPT activity, and ATP content, and the dietary Cr³⁺ requirements of juvenile grass carp were 1.02, 1.55, and 1.54 mg/kg, respectively. Overall, this study provided new insights into the effects of Cr³⁺ on GLU metabolism of juvenile grass carp under hypoxia stress, offering a nutritional regulation strategy for improving the ability of fish to resist hypoxia stress.