| The nutritional function and metabolism of carbohydrate in juvenile blalck sea bream were studied in present study by compareing the effe cts of different carbohydrate sources and carbohydrate and lipid ratios on the growth and nutrient metabolism.Carbohydrate metabolism mech anisim were studied in both enzyme activities level and gene expressio n level.This study was mainly composed of four 8-week growth trials with the same culutre condition as they were all conducted in experim ental factory of Xixuan Island in Zhejiang Province Institute of Marine Fisheries.In Experiment I,six carbohydrates with different molecular c omplexity,namely ?-starch,wheat starch,cornstarch,sweet potato star ch,dextrin and glucose,were selected as the source of carbohydrate w ith the common inclusion level of 18%.Six isonitrogenous and isoenerg etic diets were formulated to evaluate the utilization of different carbo hydrate sources in the diet of juvenile black sea bream.In Experiment II and Experiment III,corn starch and wheat starch were used as carb ohydrate sources.Six isonitrogenous and isoenergetic diets were formul ated with diferent carbohydrate/lipid(CHO/L),namely 0.89,1.16,1.50,1.83,2.31 and 2.81 for corn starch and 0.90,1.17,1.49,1.80,2.27 a nd 2.72 for wheat starch,respectively,to explore the utilization of diff erent CHO/L ratio with two types of starch in juvenile black sea brea m.In Experiment IV,the effects of different CHO/L ratio(1.16,1.72 a nd 2.55,respectively)on growth performance and nutritional regulation of juvenile black sea bream were evaluated,using ?-starch as carbohy drate source.In Experiment VI,the postprandial metabolic enzyme kine tics was investigated in enzyme activity and gene transcription level aft er the 8-week acclimatization with three CHO/L ratio diets(namely 1.16,1.72 and 2.55,respectively).The main experimental results obtained in present study are as foll ows:1.Black sea bream,as a carnivorous fish,shows similar utilization ability with different molecule compelexity carbohydrate,while shows d ifferent utilization with different origins of starch,namely worst in swe et potato starch and bset in ?-starch.With the inclusion level of 18%in present study,carbohydrate sources didn' t show any significant diff erence in growth performance of black sea bream.Improved feed intak e can compansate the impared feed utilization to achieve the same gro wth performance in differernt carbohydrate sources diets.Besides,diffe rent carbohydrates sources have an effect on lipid and protein metabol ism in juvenile black sea bream,while no significant effect was observ ed in carbohydrate metabolism.2.Appropriate carbohydrate inclusion in diets of juveile black sea bream can replace dietary lipid to save the cost of feed,and the appr opriate CHO/L ratio shows protein sparing effects by increasing dietary protein retention and utilization in fish rather than by improving fish g rowth performance.The effects of different CHO/L ratios with different starch origins on the metabolism of juvenile black sea bream were diff erent.In general,black sea bream shows better utilization with wheat starch than corn starch with the appropriate CHO/L ratio of 2.72 for wheat starch and 1.50 for corn starch.Effects of different origin starc hes on feed utilization were achieved by differernt feed intake and pro tein utilization in juvenile black sea bream.3.Black sea bream also shows high utilization ability with carbohy drate source of a-starch to repalce certain level of dietary lipid with a ppropriate CHO/L ratio.Black sea bream are adapted to high inclusion levels of ?-starch by regulating the activities of enzymes invloved in metabolism of lipid and carbohydrate,and results in no influence in gr owth performance.However,impaired microstructure of liver and intest ine shows in high CHO/L ratio indicate that culture peirod of 8 weeks may not as long as enough to have effect on growth performance.4.Juvenile black sea bream fed with different dietary CHO/L ratio regulated carbohydrate metabolism involved enzyme both in activities 1 evel gene expression level to deal with different dietary carbohydrate i ntake.In terms of enzyme activity,high carbohydrate group improved PK activity and PFK activity at 5h postprandial and HK activity at 36h postprandial while reduced PEPCK activity at 36h.In terms of gene ex pression level,high carbohydrate group improved FAS expression at 5h postprandial to accelerate the clerance rate of absorbed glucose,and u p-regulation 1h postprandial PK and 36h postprandial PFK gene expres sion as well as down-regulation 5h postprandial G6Pase gene expressio n.The results showed that juvenile black sea bream fed with different dietary CHO/L ratios were able to maintain glucose homeostasis by reg ulating different period postprandial glycolysis and gluconeogenesis.In summary,different molecular structure and origins of carbohydr ates will affect the utilization of carbohydrates and nutrient metabolism in juvenile black sea bream,and they can utilize high inclusion level o f dietary carbohydrate with appropriate CHO/L ratio by improving post prandial glucose metabolism ability in different time. |
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