Dietary leucine requirement of juvenile largemouth bass (Micropterus salmoides) based on growth, nutrient utilization and growth-related gene analyses

Abstract

Leucine is an essential amino acid in fish nutrition, but information is lacking about dietary leucine requirements of largemouth bass (Micropterus salmoides), which is an important freshwater carnivorous species of aquaculture interest. Thus, 450 fish (18.0 ± 0.1 g) were randomly divided into six groups, and offered semi-purified diets supplemented with crystalline L-leucine at 0 g kg−1 (control), 4.0 g kg−1 (Leu-4), 8.0 g kg−1 (Leu-8), 12.0 g kg−1 (Leu-12), 16.0 g kg−1 (Leu-16) and 20.0 g kg−1 (Leu-20) for eight weeks to determine the required amount of dietary leucine for achieving an optimal growth of largemouth bass. The results showed that leucine significantly (P < 0.05) influenced weight gain (WG), feed conversion ratio (FCR) and protein efficiency ratio (PER) of juvenile largemouth bass. WG and FCR presented a quadratic effect (P < 0.05) against the dietary leucine levels. Fish receiving Leu-8 diet had the highest WG (351%) and the lowest FCR (0.81) among all the experimental groups. Whole-body moisture and lipid showed a significant linear trend (P < 0.05), while crude protein showed a significant (P < 0.05) quadratic effect. Moreover, whole-body amino acid compositions, and amino acid, protein and lipid retention efficiencies were influenced quadratically (P < 0.05) by the dietary leucine. Importantly, the optimal leucine level significantly (P < 0.001) up-regulated mRNA expression levels of insulin-like growth factor 1, target of rapamycin, 70-kDa ribosomal protein S6 kinase 1 and eukaryotic translation initiation factor 4E-binding protein 1 genes in fish muscle. All of the target genes in general showed a significant positive quadratic trend (P < 0.001). Overall, leucine positively affected the growth and protein synthesis of largemouth bass by regulating target of rapamycin signaling pathway, and the optimal supplementation level of leucine was 8.0 g kg−1 (Leu-8), corresponding to 25.2 g kg−1 dietary leucine.