| The basic diet of juvenile blunt snout bream was prepared with fish meal,soybean meal,cottonseed meal and rapeseed meal as protein source,and with soybean oil and soybean phospholipid as lipid source.Vitamin C was added to the basic diet to get six isonitrogen isoenergetic diets with Vc levels of 25.35 mg/kg,150.65 mg/kg,285.39mg/kg,573.79mg/kg,1133.79 mg/kg and 2231.98mg/kg respectively.Each experimental diet was randomly fed to 3 groups of juvenile blunt snout bream(12.85±0.06g).After 60 days of feeding trail,Aeromonas hydrophila(A.h)was oral administrated for artificial infection.The results showed that:1.Effects of Vc on growth,body composition,serum biochemical indexes and digestive enzyme activity of juvenile blunt snout bream.With the increase of vitamin C in diet,the specific growth rate(SGR),protein efficiency(PER)and weight gain rate(WGR)increased at first and then flattening trend.While the Vc level was in the range of 150.65～573.79 mg/kg,the WGR and SGR was significantly higher than that of the control group(P<0.05),and the feed conversion ratio(FCR)was significantly lower than that of the control group(P<0.05).Compared with the control group,the moisture content of the whole fish in the 150.65～1133.79mg/kg Vc groups decreased significantly(P<0.05),while the content of crude protein and crude fat in 573.79mg/kg Vc group increased significantly(P<0.05).There was no significant difference in ash content between groups(P>0.05).Compared with the control group,the serum total cholesterol(TC)content in the 285.39mg/kg Vc group and the serum triglyceride(TG)content in the 573.79mg/kg Vc group decreased significantly(P<0.05).Meanwhile,thetotal protein(TP)content,glucose(GLU)content,blood ammonia(Amonnia)content,y-glutamyl transferase(y-GT)content,glutamic-oxalotransferase(AST)content,glutamic-pyruvic transaminase(ALT)content and ALT/ALT ratio in serum were not significantly different among groups(P>0.05).Compared with the control group,the intestinal trypsin(Try)activity in the 1133.79mg/kg Vc group,the intestinal chymotrypsin(Chy)activity in the 150.85～285.39mg/kg Vc group,the intestinal amylase(AMS)activities in the 150.65～1133.79mg/kg Vc group and the intestinal lipase(LPS)activity in the 285.79～2231.98mg/kg Vc group were significantly improved(P<0.05).Taking weight gain rate as growth evaluation index and regression analysis of broken line model,it was found that the optimal Vc requirement for juvenile Wuchang bream was 250.18 mg/kg.2.Effects of Vc on serum,intestinal antioxidation and immune capacity of juvenile blunt snout bream.The results showed that:compared with the control group,the content of serum albumin(ALB)in the 1133.79mg/kg Vc group and C3 content in the 573.79～1133.79mg/kg Vc group were significantly improved(P<0.05).However,there was no significant difference in the content of serum inalkaline phosphatase(ALP)among the groups(P>0.05).When the Vc level exceeded 285.39 mg/kg,the serum tumor necrosis factor-α(TNF-α)content was significantly higher than that of the control group(P<0.05).When the Vc level exceeded 1133.79 mg/kg,the serum nterleukin-2(IL-2)content was significantly higher than that of the control group(P<0.05).There was no significant difference of serum transforming growth factor-β(TGF-β),interleukin-4(IL-4),interleukin-8(IL-8)and interleukin-10(IL-10)among groups.When the Vc level exceeded 573.79 mg/kg,the serum malondialdehyde(MDA)content was significantly lower than that of the control group(P<0.05),on the contrary,the serum glutathione peroxidase(GSH-Px)content was significantly higher than that of the control group(P<0.05).When the Vc level exceeded 285.39mg/kg,the serum catalase(CAT)content was significantly higher than that of the control group(P<0.05).Compared with the control group,the serum total superoxide dismutase(T-SOD)content in the 573.79～1133.79mg/kg Vc groups was significantly higher(P<0.05).Compared with the control group,the intestinal MDA content in the 1133.79mg/kg Vc group,the intestinal copper zinc superoxide dismutase(CuZn-SOD)activity in the 573.79～2231.98mg/kg Vc group,the intestinal manganese superoxide dismutase(Mn-SOD)activities in the 150.85mg/kg Vc group and the intestinal CAT activity in the 285.79～1133.79mg/kg Vc group were significantly increased(P<0.05).However,there was no significant difference in the intestinal GSH-Px activities among the groups(P>0.05).Compared with the control group,the intestinal TNF-α content in the 150.65～573.79mg/kg Vc group,the intestinal IL-2 content in the 285.39～573.79mg/kg Vc group and the intestinal IL-8 content in the 573.79mg/kg Vc group were significantly increased(P<0.05).However,there was no significant difference in the intestinal TGF-β,IL-4 and IL-10 content among the groups(P>0.05).In the control group,the intestinal tissue showed slight pathological injury,such as loss of microvilli,enlargement of mucosal epithelial cell space,loosening of lamina propria and decrease or disappearance of goblet cells.On the other hand,the intestinal tissue of the other Vc groups was almost undamaged,the villi developed relatively well,the mucosal structure was relatively intact,and the epithelial cells were arranged neatly.3.Effects of Vc on percentage death,intestinal antioxidant activity,cytokine content and intestinal morphological structure after infection with Aeromonas hydrophila of juvenile blunt snout bream.The results showed that:At 12 h and 24 h after challenge,573.79 mg/kg Vc group had the best anti-disease effect,and its relative percent survival(RPS)is up to 57.15%.With the prolongation of the time of challenge,the content of MDA increased firstly and then decreased,finally tended to be stable,and the appropriate level of Vc could significantly reduce the content of MDA in the intestine(0h～7d)(P<0.05).With the prolongation of challenge time,the activity of CuZn-SOD increased firstly and then decreased,finally tended to be stable,and the appropriate level of Vc could significantly increase the activity of CuZn-SOD in intestine(0h～4d)(P<0.05).With the prolongation of challenge time,the activity of Mn-SOD decreased firstly and then increased,finally tended to be stable,and the appropriate level of Vc could significantly increase the activity of CuZn-SOD in intestine(0h,12h,3d)(P<0.05).With the prolongation of challenge time,the activity of CAT decreased firstly and then tended to be stable,and the appropriate level of Vc could significantly increase the activity of CAT in the intestine(0h～7d)(P<0.05).With the prolongation of challenge time,the activity of GSH-Px decreased firstly and then increased,finally tended to be stable,and the appropriate level of Vc could significantly increase the activity of GSH-Px in the intestine(12h,2d,72 h,4d,7d)(P<0.05).With the prolongation of challenge time,the content of TNF-α increased firstly and then tended to be stable,and the appropriate level of Vc could significantly increase the content of TNF-α in intestine(0h～7d)(P<0.05).With the prolongation of challenge time,the content of TGF-βdecreased firstly and then increased,finally tended to be stable,and the appropriate level of Vc could significantly increase the content of TGF-β in the intestine(2d～4d)(P<0.05).With the prolongation of challenge time,the content of IL-2 increased firstly and then decreased,finally tended to stable,and the appropriate level of Vc could significantly increase the content of IL-2 in intestine(12h～1d)(P<0.05).With the prolongation of challenge time,the content of IL-4 increased firstly and then decreased,finally tended to be stable,and the appropriate level of Vc could significantly increase the content of IL-4 in intestine(3d)(P<0.05).With the prolongation of challenge time,the content of IL-8 increased firstly and then tended to stable,and the appropriate level of Vc could significantly increase the content of IL-8 in intestine(12h,2d)(P<0.05).With the prolongation of challenge time,the content of IL-10 decreased firstly and then increased,finally tended to be stable,and the appropriate level of Vc could significantly increase the content of IL-10 in intestine(ld)(P<0.05).After infection with Aeromonas hydrophila,different degrees of inflammatory symptoms appeared in each test group,and the most obvious symptom was in 12h～3d.In the control group,the symptoms of enteritis were the most serious,the main manifestations were large area damage of villi,serious exfoliation of epithelial cells,enlargement of mucosal epithelial cell space,disorder of nuclear arrangement and mucosal layer.A large number of inflammatory cells such as neutrophils infiltrated in the submucosa and the lamina propria and even appeared adhesion or exfoliation of villi.Compared with the control group,the symptoms of enteritis in the experimental group with different levels of Vc were relatively alleviated.On the 7th day,it was found that the goblet cells in the mucosal layer of each experimental group increased,the intestinal villi began to be repaired slowly,the symptoms of enteritis basically disappeared,and the fish basically returned to normal.In conclusion,the optimal Vc requirements for growth and non-specific immunity of juvenile blunt snout bream were 250.18 mg/kg and 573.79 mg/kg,respectively. |
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