Effects of dietary leucine levels on growth, feed utilization, neuro-endocrine growth axis and TOR-related signaling genes expression of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

Abstract

A 9-week growth trial was conducted to evaluate the effects of dietary leucine (Leu) levels on growth, feed utilization, neuro-endocrine growth axis and TOR-related signaling genes expression of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂). Seven isoenergetic (340 kcal per 100 g of dry matter), isoproteic (53.5% of dry matter) and isolipidic (7% of dry matter) diets were formulated to contain graded Leu levels (1.83%, 2.23%, 2.78%, 3.28%, 3.70%, 4.40% and 4.71%, dry matter basis). Each experimental diet was fed to triplicate groups of 12 hybrid grouper juveniles (average initial body weight: 6.92 ± 0.03 g/fish) which were stocked into small floating cages (L 120 cm × W 70 cm × H 50 cm). Fish were fed twice daily (08:00 and 16:00) to apparent satiation.Results showed that weight gain% (WG%) was increased as dietary Leu level increased, reaching a peak value at the 3.28% dietary Leu level, and thereafter, it started to reduce as dietary Leu level continued to rise. Protein productive values (PPVs) of fish fed 3.28% and 3.70% dietary Leu levels were higher than those of fish fed other dietary Leu levels. The quadratic regression analysis of WG% or PPV against dietary Leu levels indicated that the optimal dietary Leu requirement for hybrid grouper was estimated to be 3.25% or 3.41% of dry matter. Fish fed 3.28% and 3.70% dietary Leu levels had higher growth hormone (GH) expression in pituitary than fish fed other dietary Leu levels. For genes of insulin like growth factor 1 (IGF-1) and growth hormone receptor 1 (GHR1) in liver, fish fed 3.28% dietary Leu had higher expression than fish fed other dietary Leu levels. Fish fed 3.28% dietary Leu had higher expression of target of rapamycin (TOR) and S6-kinase 1 (S6K1) in liver than fish fed other dietary Leu levels. Gut morphometric indices of experimental fish were improved by 3.28% or 3.70% dietary Leu supplementations. Generally, optimal dietary Leu requirement for best growth of hybrid grouper was estimated to be 3.25% of dry matter, corresponding to 6.07% % of dietary protein.