| Because of its good physiological function and physical properties,carbohydrates are widely used in fish feed.However,the carbohydrate raw materials,mainly starch,will increase the ability of fish body lipid synthesis,so that it can be easily convert carbohydrates into fat.Excessive lipid deposited in tissues,especially in liver,not only affected the quality of farmed fish,but also caused a series of health problems.Porphyra polysaccharides(PPS)has the potential function of preventing hyperlipidemia and hyperglycemia,as well as reducing lipid deposition.Xylose and arabinose,as pentoses,have the potential to inhibit the glucose-based lipid synthesis pathway.In order to explore the application potential of PPS,xylose and arabinose in aquatic feed,especially their effect on lipid metabolism of fish,this thesis carried out a systematic study using rabbitfish Siganus canaliculatus as the research object.In this study,starch was used as the main carbohydrate source,two kinds of feed diets with low carbohydrate(15% starch,BSD)and high carbohydrate(25% starch,HSD)were used.For the feed with high carbohydrate diet,PPS,xylose and arabinose were used to replace 5%,10% and 15% starch in feeds,respectively.The farming experiment was carried out for 60 days.The main results are as follows:1.Study on the application of Porphyra polysaccharides(PPS)in feed(1)PPS has the ability to promote growth,improve immunity and antioxidant capacityHigh starch diet reduced weight gain rate and survival rate of fish,which were increased after PPS was added to HSD,and PPS also reduced the high hepatosomatic index caused by high starch diet(P < 0.05).PPS supplementation increased the low activity of serum immunoglobulin M(Ig M)and lysozyme(LZM),high activity of glutamic-pyruvic transaminase(GPT)and glutamic-oxalacetic transaminase(GOT)as well as high content of malondialdehyde(MDA)caused by high starch diet.Meanwhile,the weak total antioxidant capacity(T-AOC),and low activity of superoxide dismutase(SOD)and catalase(CAT)activities caused by high starch diets got improved by PPS(P < 0.05).(2)PPS has the ability to improve intestinal healthHSD reduced the enzyme activity of intestinal lipase,hepaticlipase,total esterase and trypsin,made the area of foregut internal voids increased,the intestinal wall became thinner,and the intestinal villi were seriously damaged.What’s more,HSD also reduced the diversity of the intestinal bacteria and the content of beneficial bacteria,increased the content of harmful bacteria.After adding PPS,the intestinal digestive enzyme activities of juvenile Siganus canaliculatus were enhanced,the area of foregut internal voids and the damage of intestinal villi were reduced,and the intestinal wall was thickened.Besides that,the diversity of bacteria and the content of beneficial bacteria got more,and the content of harmful bacteria got less.(3)PPS has the abilites to reduce lipid deposition and regulate lipidmetabolismBlood glucose(GLU),serum total cholesterol(TC),triglyceride(TG)and low density lipoprotein cholesterol(LDL-C)increased,while high density lipoprotein cholesterol(HDL-C)decreased with the increase of starch.However,with the addition of PPS,GLU,TG and LDL-C decreased while HDL-C increased(P < 0.05).PPS also reduced the crude fat content in whole fish body,dorsal muscle,abdominal muscle and liver(P < 0.05).Liver oil red O staining showed that HSD significantly increased the number of red lipid droplets in the liver,which got less and were evenly distributed between the nucleus after PPS was added to HSD.The effect was most significant when PPS was added to 2.5%(P < 0.05).What’s more,adding PPS to HSD increased the expression of liver lipid catabolism associated genes(pparα,hsl and lpl)and lipid transport associated genes(cpt-1),in addition,decreased the expression of lipid anabolism associated genes(pparγ,srebp-1,fas and acc)(P < 0.05).2.Effects of xylose and arabinose on growth performance and glucolipid metabolism of juvenile Siganus canaliculatus(1)Xylose and arabinose improved the growth performance of juvenile Siganus canaliculatusWith the addition of xylose and arabinose,the low FBW and WG caused by HSD were decreased,and the HSI increased by HSD was decreased(P < 0.05).(2)Xylose and arabinose reduced the lipid deposition in juvenile Siganus canaliculatusCompared with BSD,HSD increased the lipid content of fish body,liver,dorsal and abdominal muscle,while the addition of xylose and arabinose decreased these indexes.Liver oil red O staining showed that HSD significantly increased the number of red lipid droplets in the liver,which got less and were evenly distributed between the nucleus after xylose and arabinose were added.The effect was most significant when xylose and arabinose were added to 1.25%(P < 0.05).(3)Xylose and arabinose inhibited the activities of enzymes related to glucose metabolism in juvenile Siganus canaliculatusHSD and the levels of xylose and arabinose had no significant effect on the activity of Amylase in liver,while the addition of xylose and arabinose inhibited the activity of Maltase increased by HSD(P < 0.05).Meanwhile,compared with BSD,HSD increased the activities of hexokinase(HK),phosphofructokinase(PFK)and pyruvate kinase(PK),which were all inhibited by xylose and arabinose.In addition,pyruvate dehydrogenase(PDH),which catalyzes pyruvate to acetyl coenzyme,was increased in HSD and was inhibited with the addition of xylose and arabinose(P < 0.05).(4)Xylose and arabinose regulated lipid metabolism in juvenile Siganus canaliculatusThe contents of cpt-1,hsl and lpl enzymes in liver were not increased by HSD,but were increased by xylose or arabinose addition(P < 0.05).Besides that,the expression of lipid anabolism associated genes,such as pparγ,srebp-1,fas and acc,got promoted with the higher starch,the expression of lipid transport associated genes including cpt-1,and lipid catabolism associated genes including pparα,hsl and lpl were all inhibited with HSD.However,xylose and arabinose inhibited the lipid anabolism associated genes,promoted the lipid transport and catabolism associated genes.The effect was most significant when xylose and arabinose were added to 1.25%(P < 0.05).In conclusion,dietary PPS can improve the growth,immune capacity,antioxidant capacity,intestinal health and liver lipid metabolism of juvenile Siganus canaliculatus.Meanwhile,PPS has the abilities to reduce the synthesis of liver lipid,accelerate the decomposition and transport of lipid,prevent the excessive deposition of lipid in fish,so as to promote the healthy growth and disease resistance of fish,reduce the death of farmed fish.In addition,xylose and arabinose are proved to improve the growth performance of juvenile Siganus canaliculatus,and inhibit the glucose-based lipid synthesis pathway and further accelerate the decomposition and transport of lipid by inhibiting the process of starch digestion and glycolysis,so as to reduce the excessive lipid deposition.This study systematiclly explored the effect of Porphyra polysaccharides,xylose and arabinose on juvenile Siganus canaliculatus,preliminary clarified the metabolism related to reducing lipid of these three kinds of carbohydrates,which enriched the the theoretical system of feed nutrition and provided important imformation for solving the problem of excessive lipid deposition in farmed fish. |
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