Dietary Supplementation of Astragalus membranaceus Extract Affects Growth Performance, Antioxidant Capacity, Immune Response, and Energy Metabolism of Largemouth Bass ( Micropterus salmoides ).

The present study investigated the effects of Astragalus membranaceus extract (AME) on growth performance, immune response, and energy metabolism of juvenile largemouth bass ( Micropterus salmoides ). Seven diets containing 0%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, and 0.6% AME (Con, AME0.1, AME0.2, AME0.3, AME0.4, AME0.5, and AME0.6 groups) were formulated and fed to M. salmoides for 8 weeks. Final body weight (FBW), feed intake (FI), weight gain (WG), and specific growth rate (SGR) were all significantly higher in AME0.4 group than in Con group ( P < 0.05). Feed conversion rate (FCR) was significantly improved in AME0.5 group compared with Con group ( P < 0.05). Whole-body crude protein contents were significantly increased in AME0.2 group ( P < 0.05). Whole-body crude lipid contents were significantly lower in AME0.2 and AME0.3 groups, while muscle lipid was upregulated by dietary AME ( P < 0.05). Hepatic malondialdehyde (MDA) contents were significantly lowered in AME0.3 and AME0.4 groups, and catalase (CAT) activities were significantly increased in AME0.1 and AME0.2 groups ( P < 0.05). Plasma aspartate aminotransferase (AST) level was significantly lowered in AME0.5, and AME0.6 groups, and alanine aminotransferase (ALT) level was lowered in AME0.5 groups ( P < 0.05). Plasma triglyceride was declined in AME0.6 group, and glucose was decreased by 0.3%-0.5% AME ( P < 0.05). Significantly higher hepatocyte diameter, lamina propria width, and submucosal layer thickness were recorded in AME0.6 groups, while the longest villi height was obtained in AME0.2 and AME0.3 groups ( P < 0.05). The mRNA expression levels of insulin-like growth factor 1 ( igf1 ) revealed the growth-promoting effect of AME. The anti-inflammatory and antiapoptotic effects of AME were demonstrated by transcription levels of interleukin 8 ( il-8 ), tumor necrosis factor-alpha ( tnf-a ), caspase, B-cell lymphoma-xl ( Bcl-xl ), bcl-2 associated x ( Bax ), and bcl-2-associated death protein ( Bad ). The transcription levels of lipid metabolism and gluconeogenesis related genes, including acetyl-CoA carboxylase alpha ( acc1 ), fatty acid synthase ( fasn ), fatty acid binding protein 1 ( fabp1 ), phosphoenolpyruvate carboxykinase 2 ( pepck2 ), and glucose-6-phosphatase catalytic subunit 1a ( g6pc ), were reduced by AME treatment, while the levels of glycolysis-related genes, including glucokinase ( gck ) and pyruvate kinase ( pk ), were the highest in AME0.2 and AME0.3 groups ( P < 0.05). According to polynomial regression analysis of SGR, WG, FCR, whole-body crude lipid, MDA, and ALT, the optimal AME supplementation level was estimated to be 0.320%-0.429% of the diet. These results provided insights into the roles of AME in regulating immunity and metabolism, which highly indicated its potential as immunostimulants and metabolic regulators in diverse aquatic animals.