Effects of dietary leucine levels on growth, tissue protein content and relative expression of genes related to protein synthesis in juvenile gibel carp (Carassius auratus gibelio var. CAS III)

An 8-week feeding trial was conducted to investigate the impacts of dietary leucine on growth, hepatopancreas and muscle protein content and relative expressions of protein synthesis-related genes in juvenile gibel carp (5.2 g). Triplicate tanks of fish (22 fish per tank) were fed with a low (0.82%), middle (1.90%) or high (2.99%) leucine diet to apparent satiation three times a day. The results showed that the weight gain (WG) and feed efficiency (FE) of the fish fed with the 1.90% leucine diet had the highest values compared to those of other two groups (p < .05). Dietary leucine significantly affected the protein and lipid contents of the hepatopancreas (p < .05), but not muscle or the whole body composition of the fish. The amino acid contents (Arginine, Leucine, Methionine, Phenylalanine, Alanine, Aspartic acid, Glutamic acid and Proline) of the dorsal muscles of fish fed with a 1.90% leucine diet were significantly higher than those fed with a 0.82% leucine diet. Moreover, transcriptional levels of target genes of rapamycin (TOR), ribosomal protein S6 kinase 1 (S6K1), eukaryotic translation initiation factor 4E (eIF4E) and leucyl-tRNA synthetase (LARS) in the hepatopancreas were significantly up-regulated (p < .05), but eukaryotic translation initiation factor 4E-binding protein 2 (4E-BP2) was significantly down-regulated (p < .05) with the increasing dietary leucine levels. However, dietary leucine had no effects on the gene expressions of 4E-BP2 and S6K1 in the muscle of gibel carp. The present results indicate that dietary leucine led to a marked tissue-specific response on protein content in the muscle and hepatopancreas of gibel carp that is related to the TOR signalling pathway.