Effect of dietary vitamin A on growth, serum biochemical index, digestive enzyme activities and glucose and lipid metabolism in juvenile Mylopharyngodon piceus

An 8-week feeding trial was carried out to investigate the effect of dietary vitamin A (178.2, 2 058.9, 18 436.2 IU/kg) on growth performance, serum biochemical index and hepatic glucose and lipid metabolism key enzyme activities and gene expression levels of juvenile black carp (Mylopharyngodon piceus). Using a one-factor experiment design, three isonitrogenous and isoenergetic purified experimental diets (diet 1, diet 2 and diet 3) containing 0, 2 200 and 20 000 IU/kg VA were formulated. Total vitamin A contents in the diets were measured by HPLC (Agilent-1100, Agilent, USA), actual content of vitamin A in each diet were 178.2, 2 058.9 and 18 436.2 IU/kg, respectively. Casein (vitamin free) and gelatin were used as protein sources, rapseed oil was used as lipid source, dextrin was used as carbohydrate source. Total 360 juvenile black carp with initial weight (6.10 ± 0.10) g were randomly divided into 3 groups with 3 replicates per group and 40 black carp per replicate. The results showed as follows: weight gain rate (WGR) and specific growth rate (SGR) were significantly reduced for fish fed diet with 178.2 IU/kg VA. When VA is deficient, concentrations of serum glucose (GLU), triglycerides and low-density lipoprotein (LDL) were decreased, but total cholesterol (TCH) concentration was increased. The activities of hexokinase (HK), phosphofructokinase (PFK) and pyruvate (PK) increased for fish fed diet with 2 058.9 IU/kg VA. Gene expression of liver glucose transporter-2 (GLUT-2), HK, GK, PFK and G6Pase were increased in fish fed diet with 2 058.9 IU/kg VA. However, there were no significant differences in the gene expression of fatty acid transporter protein-1 (FATP-1) for fish fed diet with 2 058.9 IU/kg VA. But the gene expression of Carnitine O-palmitoyltransferase-1 (CPT-1) and Carnitine O-palmitoyltransferase-2 (CPT-2) were significantly influenced. The gene expression of CPT-1 and CPT-2 were inhibited for fish fed VA deficient diet. When fish fed diet with 18 436.2 IU/kg VA, the expression of acetyl-CoA carboxylase-2 (ACC-2) and lipoprotein lipase (LPL) genes were inhibited. Besides, the gene expression of peroxisome proliferator-activated receptor-γ (PPAR-γ) decreased when the VA content was 18 436.2 IU/kg in feed. Overall, fish fed diet with 2 058.9 IU/kg VA could improve growth performance and ability of liver cells to transport glucose of juvenile black carps. Diet with 2 058.9 IU/kg VA could keep a balance between glycolysis and gluconeogenesis, and it also could promote fatty acids synthesis and transport.