Effects Of Dietary Vitamin C On Growth Performance,Physiological And Biochemical Indexes And Liver Histology Structure In Lateolabrax Maculatus Reared In High Temperature

An 8-week feeding trial was carried out to evaluate the effects of various dietary ascorbic acid levels on growth performance,physiological and biochemical indexes and liver histology structure in Lateolabrax maculatus reared in high temperature(33oC).A basal diet was formulated without vitamin C addition(D1),and then five other diets(D2-D6)were prepared by supplementing 40,80,160,320 and 640 mg vitamin C/kg diet leading to final concentrations of6.85(D1),46.90(D2),88.24(D3),167.43(D4),329.21(D5)and 658.69(D6)mg vitamin C/kg diet.Quadruplicate groups(30 fish per replicate)of seabass(2.00±0.10 g)were fed the test diets twice daily.L-ascorbyl-2-polyphosphate was used as the vitamin C source.The main results were as follows:(1)Effects on growth,body composition and tissue vitamin C concentration in L.maculatusThe results showed no significant effect of vitamin C on L.maculatus growth performance(P>0.05).The highest feed efficiency was observed in the group fed 46.90 mg vitamin C/kg diet which significantly different from those received 329.21-658.69 mg vitamin C/kg diet.The group offered 46.90 mg vitamin C/kg diet exhibited the highest(P<0.05)protein efficiency ratio and protein retention ratio.While seabass whole-body composition was not significantly(P>0.05)affected by dietary vitamin C levels.Furthermore,liver and muscle ascorbic acid concentrations were progressively enhanced by increasing dietary vitamin C levels.However,serum ascorbic acid concentration was increased according to the level of dietary supplementation and appeared to plateau in the group fed 329.21 mg vitamin C/kg diet(P<0.05).A broken-linear regression analysis based on serum ascorbic acid concentration showed that the minimum vitamin C requirement of L.maculatus was 194.5 mg/kg diet.(2)Effects on immune response,antioxidant capacity and stress tolerance in L.maculatusSerum C3 level significantly(P<0.05)increased at vitamin C concentration of 329.21mg/kg diet and C4 activity was enhanced at 88.24 mg vitamin C/kg diet compared to group fed the basal diet(P<0.05).Also,serum IgM level and lysozyme activity were remarkably increased in groups received ?167.43 and ?88.24 mg vitamin C/kg diet,respectively.Liver total antioxidant capacity was significantly enhanced at over 88.24 mg vitamin C/kg levels and superoxide dismutase activity increased in groups fed 167.43 mg vitamin C/kg diet.Liverglutathione level and catalase activities were enhanced at over 46.90 mg vitamin C/kg and heme oxygenase-1 activity was elevated in groups received 46.90-167.43 mg vitamin C/kg diet.Liver histology was not influenced by dietary vitamin C levels.Expression of inflammatory genes including TNF-?,IL-1?,IL-10 as well as the expression of antioxidant genes such as Nrf2,HO-1,AP-1,NF-k? in liver were significantly up-regulated in the group received 88.24 mg vitamin C/kg diet.Serum cortisol concentration was not significantly affected by dietary vitamin C levels.Heat Shock Proteins content was enhanced at 88.24 mg vitamin C/kg diet,and expression of HSP70 gene was significantly up-regulated in the group received 329.21 mg vitamin C/kg diet.Broken-linear regression analysis based on serum IgM concentration,liver antioxidant capacity and catalase activity showed that the minimum vitamin C requirement of L.maculatus were152.45,94.64 and 68.05 mg/kg diet,respectively.(3)Effects on immune,antioxidant capacity and stress tolerance with acute temperature stress in L.maculatusAt the end of the feeding experiment,8 fishes per tank were captured and exposed to acute temperature stress by transferring the fish from 33 to 27oC.The effects were evaluated on serum immune parameters,HSPs concentrations and liver antioxidant parameters after 24 h and 48 h of exposure.The results indicated the significant(P<0.001)effect of vitamin C level,exposure time and their interaction on serum immune response and liver antioxidant parameters.Serum C3 and IgM level significantly(P<0.001)increased by increasing the exposure time for 48 h than 0 h and opposite trend was found for C4,HSPs level and lysozyme activities(P<0.001).Also,the serum C3,C4 level and lysozyme activity were enhanced by increasing dietary vitamin C level.C3 level was remarkably(P<0.001)higher in the groups fed 167.43 and 658.69 mg vitamin C/kg diet compared to group fed the basal diet.C4 level and lysozyme activities were also elevated(P<0.001)in groups received 88.24-658.69 mg vitamin C/kg diet.While serum HSPs and IgM levels were significantly increased then significantly decreased by increasing dietary vitamin C level.HSPs was enhanced(P<0.01)at group fed 88.24 mg vitamin C/kg diet and IgM level was significantly increased at vitamin C concentration of 46.9-167.43 mg/kg diet(P<0.001).Liver superoxide dismutase and glutathione level and MDA concentration were significantly(P<0.001)increased by exposing fish to acute temperature stress for 24 h and their values decreased significantly(P<0.001)by increasing the exposure time to 48 h.However,the highest liver heme oxygenase-1 activity was detected after 48 h of exposure.Liver glutathione level was remarkably(P<0.001)higher at dietary vitamin C supplementation than that fed the basal diet.While MDA concentration showed an opposite trend(P<0.001).Furthermore,liver superoxide dismutase and heme oxygenase-1 activities were not significantly(P>0.05)affected by dietary vitamin C levels.The above evidence suggested that stress response was caused by actue hypothermia stress,anddietary vitamin C supplementation could improve the capacity to cope with acute hypothermia stress in L.maculatus.