A study on methionine-mediated regulation of muscle fiber growth, development and differentiation in the rice field eel (Monopterus albus)

An eight-week feeding trial was performed to determine how methionine regulates muscle growth in the rice field eel Monopterus albus (initial weight 25.08 ± 0.31 g). Six isonitrogenous and isoenergetic experimental feeds were formulated, and different levels of methionine (0, 2 g/kg, 4 g/kg, 6 g/kg, 8 g/kg, or 10 g/kg) were added to the basic diet (110 g/kg fish meal, 400 g/kg soy concentrate meal). The weight gain rate (WGR) of M. albus markedly increased as methionine concentrations increased to 6 g/kg (P < 0.05) and gradually decreased as methionine concentrations increased to 10 g/kg. The feed conversion rate of M. albus showed a trend opposite to the WGR. Muscle glycogen markedly increased as methionine concentrations increased to 2 g/kg (P < 0.05), while the creatine kinase, lactic acid and lactate dehydrogenase contents of muscle markedly decreased as dietary methionine concentrations increased to 2 g/kg (P < 0.05). The content of amino acids, total essential amino acids, total nonessential amino acids and total amino acids in muscle was increased by supplementation with 8 g/kg and 10 g/kg methionine. The springiness of muscle markedly increased as methionine supplementation increased to concentrations of 6 g/kg (P < 0.05). The hardness, gumminess and chewiness of muscle markedly increased as methionine concentrations increased to 8 g/kg (P < 0.05). When supplemented with 8 g/kg and 10 g/kg methionine, the density of muscle fibers and the percentage of muscle fibers under 60 μm were remarkably decreased, while the percentage of muscle fibers greater than or equal to 100 μm was remarkably increased (P < 0.05). When 8 g/kg and 10 g/kg methionine were added, gcn2 and mstn mRNA expression was significantly downregulated (P < 0.05), while gh, ghr, TnT, mhc, mlc, myf5, myod2, myog, pax7 and mef2a mRNA expression was remarkably upregulated (P < 0.05). Correlative analysis of muscle-related gene expression showed that myf5, mlc, and myog expression was negatively correlated with gcn2 expression (P < 0.05), and ghr, mef2b, gh, and mhc expression was negatively correlated with gcn2 expression (P < 0.01), while mstn expression was positively correlated with gcn2 expression (P < 0.01). In conclusion, dietary methionine increased the growth performance of M. albus, especially for muscle growth and development, improved muscle texture and main nutritional factors (amino acids, glycogen), and reduced the content of lactic acid in muscle.