| Fat is one of the essential energy substances for fish growth,and is being added to an increasing proportion of feeds because of its role on saving protein resources.However,long-term high-fat diet(HFD)can cause reduced growth performance,fat accumulation,oxidative stress,metabolic disorders and nutritional diseases.Bile acids(BA),one of the components of bile,are degradation products of cholesterol.BA have various effects such as promoting growth,emulsifying fat,improving intestinal function,regulating glycolipid metabolism,and regulating BA homeostasis.BA can regulate the growth of mammals by affecting the lipid metabolism and BA metabolism via activating farnesoid X receptor(FXR).However,the regulatory mechanism of BA on lipid metabolism and BA metabolism of fish under HFD need further explore.In this study,five groups were designed: control group(group C,10.6 % fat),high fat group(group HF,17.5 % fat),group HFB1,group HFB2 and group HFB3(adding 300 mg/kg,600 mg/kg and 900 mg/kg BA in HF group).Two experiments were conducted to explore the mechanisms of the effects of BA on the growth and metabolism.The main findings were as follows:1.Compared with the group C,HF diet significantly reduced FBW,WGR and SGR of largemouth bass(P < 0.05),and significantly increased HSI and VSI(P < 0.05).Compared with other groups,600 mg/kg BA significantly increased the FBW,WGR and SGR(P <0.05)and significantly decreased the HSI(P < 0.05).In addition,HFD significantly increased crude fat content in muscle(P < 0.05).Dietary BA addition reduced fat deposition in muscle,and crude fat content in muscle was significantly reduced with increasing levels of BA addition(P < 0.05).Histomorphology showed that hepatocytes nuclei reduced and serious vacuoles degeneration occurred in group HF.BA supplementation increased hepatocyte integrity and decreased the number of vacuolation.Compared with group C,HF diet significantly increased the transaminase activities(ALT and AST)in plasma(P < 0.05).The transaminase activities in plasma decreased gradually with the addition of BA(P <0.05).Compared with group HF,BA supplementation increased SOD and CAT activates in plasma,activated the Nrf2/keap1 pathway,and upregulated the expression of antioxidant enzyme-related genes(sod,cat,gpx)(P < 0.05).In addition,HFD induced inflammatory responses.HFD decreased the m RNA level of anti-inflammatory factors(il-10,tgfβ)and increased the m RNA level of pro-inflammatory factors(il-1β,nf-κb)in the liver and intestine(P < 0.05).BA supplementation upregulated the expression level of il-10 and tgfβ,and decreased the m RNA level of il-1β and nf-κb(P < 0.05).2.HFD significantly increased plasma TG and LDL-C concentration(P < 0.05).When BA were added at 900 mg/kg,TG content in liver and plasma decreased,and plasma LDLC content also decreased(P < 0.05).Fatty acid content was also affected by the HFD and BA.In the group HF,liver and muscle SFA content increased,and PUFA content decreased(P < 0.05).BA supplementation reduced the content of SFA and significantly increased the content of MUFA(P < 0.05).In addition,HF diets did not affect plasma and liver total bile acid(TBA)levels(P > 0.05).BA supplementation significantly increased plasma and liver TBA levels,and there was a dose-effect relationship between TBA levels in the liver and BA addition levels(P < 0.05).when BA was added at 600 mg/kg and 900 mg/kg,fxr and cholesterol 7α-hydroxylase(cyp7a1)expression level were increased,and sterol 27Ahydroxylase(cyp27a1)expression level were decreased in the liver(P < 0.05).Gene expression levels of transport receptors(ntcp,bsep)were significantly increased in the liver and transport receptors(asbt,ostα/β)were significantly increased in the intestine(P < 0.05).Addition of BA activated the FXR/SHP/SREBP1 signaling pathway,significantly decreased the expression of fas and acc2,and upregulated the expression levels of lpl,hsl,and hl(P < 0.05).In addition,fxr increased the expression of cpt1 and acox1 in the liver.In summary,the main conclusions of this study include:(1)High-fat diets significantly inhibited the growth of largemouth bass,and the addition of BA significantly improved growth performance.And in this study,when BA addition level was at 600 mg/kg,the strongest growth-promoting effect was observed.(2)BA could promote lipolysis and β-oxidation through FXR/SHP/SREBP1 signaling pathway and inhibit lipid synthesis.(3)the addition of exogenous bile acids to high-fat diets increased bile acid synthesis and transport in largemouth bass to maintain bile acid homeostasis. |
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