Abstract
A study was designed to investigate the influence of leucine (Leu) levels on growth performance, enzyme activities and relative expression of TOR and 4E-BP genes in rainbow trout, Oncorhynchus mykiss (average initial body weight: 1.60 ± 0.25 g/fish; average initial length 5.20 ± 0.30 cm/fish). Six isonitrogenous (450 g/kg) and energetic (20.90 kJ 100 g−1, gross energy) experimental diets in gradation of 2.5 g/kg of dry diets were formulated to contain graded Leu levels (10.0, 12.5, 15.0, 17.5, 20.0 and 22.5 g/kg, drymatter basis). 20 fish were stocked in triplicate groups in 75 L trough connected with flow-through system (2–2.5 L/min) and were fed twice daily (09:00 and 17:00 hours) at 5% BW day−1. Quadratic regression analysis of live weight gain (LWG%), feed conversion ratio (FCR), protein efficiency ratio (PER) and body protein deposition (BPD) data indicated Leu requirement at 17.70, 17.42, 17.25 and 17.57 g/kg dry diet, respectively. Highest (p < .05) protein, lowest moisture and intermediate fat contents were noted at 17.5 g/kg Leu fed diet. Significant variation in haematological parameters was also observed. While no significant (p ˃ .05) differences in plasma total protein, cholesterol, triglycerides, albumin and globulin were noted. However, alkaline phosphatase (ALP), alanine transaminase (ALT), aspartate transaminase (AST), glucose, urea and uric acid levels produced significant differences. Except amylase, higher enzymatic activities were observed at 17.5 g/kg Leu diet. Higher relative expression level of target of rapamycin (TOR) and eukaryotic translation initiation factor 4E-binding protein (4E-BP) mRNA was observed at 17.5 and 15.0 g/kg Leu fed diets. Based on the above data, it is recommended that 17.45 g/kg Leu, corresponding to 38.77 g/kg of dietary protein would be useful for optimum growth and also regulates relative gene expression of TOR and 4E-BP signalling pathway.
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