| As one of the intermediates in the tricarboxylic acid cycle,succinic acid plays an important role in biological metabolism.Succinic acid can improve the anti-stress and immunity of animals,and has a significant role in promoting lipase activity and insulin sensitivity;in addition,succinic acid can also be converted into glucose by gluconeogenesis,providing the body with energy.However,the effects of succinic acid on glucose homeostasis and energy metabolism of fish are rately reported.In this study,we established sodium succinate cultured model of zebrafish firstly and fed 1-month old zebrafish a normal diet without sodium succinate(CK)and four normal diets added with sodium succinate(SS0.05%,SS0.1%,SS0.15%,SS0.2%).Satiation feeding for 2 weeks showed that succinic acid did not affect fish survival rate and feed intake,but significantly promoted weight gain,especially0.15% SS group which increased by 21.1%(P=0.006)when compared with CK.Although there was no significant difference in feed conversion efficiency,energy conversion efficiency was significantly increased in the 0.15% SS group,and energy gain increased by 1.08-fold(P=0.007).In addition,the standard metabolic energy in 0.15% SS group was 40.3%(P=0.007)lower than in CK,indicating that the growth-promotion effect of dietary sodium succinate is associated with reduced energy expenditure of the host.Compared with the control group,crude protein,crude fat and energy increased by 2.8%(P=0.029),24.34%(P=0.008)and 19.3%(P=0.024).And gut flora was significantly altered.Gut,hepatic,and muscle TAG analysis showed that gut TAG content was increased by 69.8%(P=0.014)in0.15% SS group,89.2%(P=0.033)in liver,2.8-fold(P= 0.036)in muscle,and muscle fiber area increased by 15.5%(P=0.035).Glucose tolerance test and activity and gene expression of SDH were analyzed.Glucose tolerance of 0.15% SS group was significantly increased.After intraperitoneal injection of glucose,blood glucose began to decline at 1 h and returned to normal level at 6 h.However,compared with CK,blood glucose of the 0.15% SS zebrafish decreased significantly faster than that of the CK,with a decrease of 19.1% at2 h(P=0.021)and 24.7% at 4 h(P=0.032).At 5 h,it was 43.7% faster,with the most significant hypoglycemic effect(P<0.001).In addition,SDH activity and gene expression in gut increased by26.4%(P=0.043)and 70.3%(P=0.009),while increased by 77.3%(P=0.047)and 97.0%(P=0.019)in liver.These results indicate that sodium succinate can maintain zebrafish homeostasis,and can directly enter the body as a tricarboxylic acid circulating substrate for metabolism.To further illustrate how sodium succinate affects zebrafish nutritional metabolism,the metabolism of three major nutrients in gut,liver and muscle were analyzed.Compared with control group: 1.For the gut,G6 Pase and PEPCK activities reduced by 35.9%(P=0.027)and 39.7%(P=0.055),gene expression reduced by 51.4%(P=0.004)and 54.8%(P=0.009);gene expression of ACC and FAS increased by77.9%(P=0.066)and 76.7%(P=0.001),and ATGL decreased by 58.2%(P=0.012);GDH1a and APN reduced by 45.9%(P=0.007)and 38.1%(P=0.017).2.For the liver,G6 Pase and PEPCK activities increased by 87.1%(P=0.009)and 79.2%(P=0.004),and gene expression increased by 1.4-fold(P=0.005)and 45.0%(P=0.036),and Gys was increased by 1.2-fold(P=0.038);gene expression of ACC,FAS and DGAT2 increased by 64.3%(P=0.039),83.6%(P=0.032)and 1.7-fold(P=0.024),and HAD decreased by 53.5%(P=0.04);gene expression of GDH1 a and APN decreased by 31.8%(P=0.018)and 51.3%(P=0.011).3.For the muscle,PFK gene expression increased by 86.7%(P=0.016),Gys increased by 1.9-fold(P=0.045);ACC and FAS increased by 89.7%(P=0.021)and 2.2-fold(P=0.034);CPT1A and ACOX3 decreased by 59.1%(P=0.005)and 80.1%(P=0.001);GDH1a and APN decreased by 33.1%(P=0.004)and 58.6%(P=0.033).The above results showed that adding 0.15% sodium succinate to diet can improve glucose homeostasis and strengthen tricarboxylic acid cycle,providing reference for the role of succinic acid in fish glucose homeostasis and nutritional metabolism. |
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