(1) Background:
l-leucine (Leu) plays a positive role in regulating
protein turnover in skeletal muscle in mammal. However, the molecular mechanism for the effects of Leu on muscle growth and
protein deposition is not clearly demonstrated in fish. This study investigated the effects of dietary Leu on growth performance and muscle growth,
protein synthesis, and degradation-related signaling pathways of hybrid catfish (Pelteobagrus vachelli♀ × Leiocassis longirostris♂). (2) Methods: A total of 630 hybrid catfish (23.19 ± 0.20 g) were fed 6 different experimental diets containing graded levels of Leu at 10.0 (control), 15.0, 20.0, 25.0, 30.0, 35.0, and 40.0 g Leu kg-1 for 8 weeks. (3) Results: Results showed that dietary Leu increased percent
weight gain (PWG), specific growth rate (SGR), FI (feed intake), feed efficiency (FE),
protein efficiency ratio (PER), muscle fibers diameter, and muscle fibers density; up-regulated
insulin-like growth factor I (
IGF-I),
insulin-like growth factor I receptor (IGF-IR),
proliferating cell nuclear antigen (
PCNA), myogenic regulation factors (MyoD, Myf5, MyoG, and
Mrf4), and MyHC
mRNA levels; increased
muscle protein synthesis via regulating the AKT/TOR signaling pathway; and attenuated protein degradation via regulating the AKT/FOXO3a signaling pathway. (4) Conclusions: These results suggest that Leu has potential role to improve muscle growth and
protein deposition in fish, which might be due to the regulation of IGF
mRNA expression, muscle growth related gene, and
protein synthesis and degradation-related signaling pathways. Based on the broken-line model, the Leu requirement of hybrid catfish (23.19-54.55 g) for PWG was estimated to be 28.10 g kg-1 of the diet (73.04 g kg-1 of
dietary protein). These results will improve our understanding of the mechanisms responsible for muscle growth and
protein deposition effects of Leu in fish.