The experiment was conducted to investigate the effects of dietary threonine (Thr) on growth performance and muscle growth, protein synthesis and antioxidant-related signalling pathways of hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂. A total of 1200 fish (14·19 (se 0·13) g) were randomly distributed into six groups with four replicates each, fed six diets with graded level of Thr (9·5, 11·5, 13·5, 15·4, 17·4 and 19·3 g/kg diets) for 56 d. Results showed (P < 0·05) that dietary Thr (1) increased percentage weight gain, specific growth rate, feed efficiency and protein efficiency ratio; (2) up-regulated growth hormone (GH), insulin-like growth factor 1 (IGF-1), proliferating cell nuclear antigen, myogenic regulation factors (MyoD, Myf5, MyoG and Mrf4) and myosin heavy chain (MyHC) mRNA levels; (3) increased muscle protein content via regulating the protein kinase B/target of rapamycin signalling pathway and (4) decreased malondialdehyde and protein carbonyl contents, increased catalase, glutathione-S-transferase, glutathione reductase and GSH activities, up-regulated mRNA levels of antioxidant enzymes related to NFE2-related factor 2 and γ-glutamylcysteine ligase catalytic subunit. These results suggest that Thr has a potential role to improve muscle growth and protein synthesis, which might be due to the regulation of GH-IGF system, muscle growth-related gene, antioxidative capacity and protein synthesis-related signalling pathways. Based on the quadratic regression analysis of specific growth rate, the Thr requirement of hybrid catfish (14·19–25·77 g) was estimated to be 13·77 g/kg of the diet (33·40 g/kg of dietary protein).

The yellow catfish Pelteobagrus fulvidraco with initial average weight 16.6 ± 0.17 g were fed three extruded diets for 56 days. Fish meal (FM) diet was formulated as the normal control with 380 g FM and 200 g soybean meal (SBM) kg diet. The SBM diet was prepared with 220 g FM and 360 g SBM kg diet without Lys or Met supplementation. The SBM supplement (SBMS) diet was similar to SBM diet and supplemented with Lys and Met to ensure their levels similar to FM diet. The results showed fish fed SBM diet had lower percent weight gain and specific growth rate than the other two groups (P < 0.05). Whole body protein content of fish fed FM and SBMS diets were higher than that of fish fed SBM diet (P < 0.05). The hepatosomatic and intestosomatic indexes of fish fed SBM diet were significantly lower than that of fish fed FM and SBMS diets (P < 0.05). The activities of pepsin in stomach, trypsin, and chymotrypsin in intestine, alkaline phosphatase and creatine kinase in proximal intestine, Na, K-ATPase, and gamma-glutamyl transpeptidase in distal intestine were significantly higher in fish fed FM and SBMS diets compared to SBM diet. The activities of catalase, glutathione-S-transferase, reduced glutathione, superoxide anion scavenging, and hydroxyl radical scavenging in the intestine showed the same changes (P < 0.05). Malondialdehyde and protein carbonyl contents in intestine were significantly decreased in fish fed SBMS diet compared to SBM diet. These results indicated high inclusion of SBM induced intestinal oxidative injury and digestive and antioxidant capacity decrease. The Lys and Met supplementation could ameliorate these adverse effects on yellow catfish.

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The present study evaluated effects of dietary supplementation with tryptophan (Trp) on muscle growth, protein synthesis, and antioxidant capacity in hybrid catfish Pelteobagrus vachelli♀ × Leiocassis longirostris♂. Fish were fed six different diets containing 2.6 (control), 3.1, 3.7, 4.2, 4.7, and 5.6 g Trp kg−1 diet for 56 days, respectively. Results showed that dietary Trp significantly (1) improved muscle protein content, fiber density, and frequency of fiber diameter; (2) up-regulated the mRNA levels of PCNA, myf5, MyoD1, MyoG, MRF4, IGF-I, IGF-II, IGF-IR, PIK3Ca, TOR, 4EBP1, and S6K1; (3) increased phosphorylation levels of AKT, TOR, and S6K1; (4) decreased contents of MDA and PC, and increased activities of CAT, GST, GR, ASA, and AHR; (5) up-regulated mRNA levels of CuZnSOD, CAT, GST, GPx, GCLC, and Nrf2, and decreased Keap1 mRNA level; (6) increased nuclear Nrf2 protein level and the intranuclear antioxidant response element binding ability, and reduced Keap1 protein level. These results indicated that dietary Trp improved muscle growth, protein synthesis as well as antioxidant capacity, which might be partly related to myogenic regulatory factors (MRFs), IGFs/PIK3Ca/AKT/TOR, and Keap1/Nrf2 signaling pathways. Finally, based on the quadratic regression analysis of muscle protein and MDA contents, the optimal Trp requirements of hybrid catfish (21.82-39.64 g) were estimated to be 3.94 and 3.93 g Trp kg−1 diet (9.57 and 9.54 g kg−1 of dietary protein), respectively.