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An90days feeding trial was conducted to estimate the adequate dietary vitamin C (Vc) requirement of juvenile Wuchang bream (Megalobrama amblycephala) and its effect on growth,, hematology and muscle physiochemical indexes.450healthy fish were selected and randomly divided into six groups that Vc were supplemented at0mg/kg (the control group),30,60,120,240and480mg/kg (the treated groups)(the analyzed Vc levels were0.2,33.4,65.8,133.7,251.5and501.5mg/kg diet).1. Effects of dietary vitamin C on growth performance, hematology and muscle physiochemical indexes of juvenile Wuchang bream (Megalobrama amblycephala)After90days feeding, the growth performance, hematology and muscle physiochemical indexes were investigated. The results showed that the weight gains (WG) and specific growth rate (SGR) had an increasing trend with the increase of dietary vitamin C, the fish in251.5mg/kg and501.5mg/kg Vc group had significantly higher SGR then those in the control group (P<0.05); while the feed coefficient reduced with increasing of dietary Vc, but no significant difference were observed among dietary treatments (P>0.05). The fish body indices were not significantly different among groups, except the viscerosomatic index in fish with133.7mg/kg Vc was significantly higher then the control group (P<0.05). Compared with the control, the contents of total leukocytes in fish with65.8-501.5mg/kg Vc, serum glutamic-oxaloacetic transaminase in fish with65.8mg/kg Vc and serum glutamic-pyruvic transaminase in fish with133.7mg/kg and501.5mg/kg Vc were significantly reduced (P<0.05). The contents of serum triglycerides in fish with501.5mg/kg Vc, superoxide dismutase activities in the muscle in fish with133.7mg/kg Vc were significantly improved (P<0.05). Other indices, such as erythrocyte, hemoglobin, platelets and cooking loss of muscle were not significantly different among groups (P>0.05). Based on broken-line model analyses at this study, the minimum dietary vitamin C level is150mg/kg to get optimal WG.2. Effects of dietary vitamin C on the non-specific immunity and three HSPs mRNA expression of juvenile Wuchang bream (Megalobrama amblycephala) After90days feeding, the serum immunological, liver antioxidant indices and the the basal expression level of three heat shock peoteins (HSPs) mRNA were measured. The results showed, compared with the control,501.5mg/kg dose of Vc group increased the contents of serum C3,133.7mg/kg doses of Vc groups improved the contents of serum C4;65.8-251.5mg/kg doses of Vc groups enhanced the hepatic SOD activity and all treated groups enhanced the hepatic ASAFR activity;133.7and251.5mg/kg dosed of Vc groups improved the expression level of HSP60mRNA,251.5mg/kg dose of Vc group improved the expression levels of HSP70and HSP90mNA significantly (P<0.05). Other indices, such as total protein, albumin, serum cortisol and MDA were not significantly different among groups (P>0.05). Therefore, the basal diet supplemented with vitamin C could improve the immunity of juvenile Wuchang bream effectively.3. Effects of dietary vitamin C on the non-specific immunity and three HSPs mRNA expression of juvenile Wuchang bream(Megalobrama amblycephala) under pH stressAfter90days feeding,20fish per tank were exposed to pH stress (pH≈9.5) for15days, the serum immunological and liver antioxidant indices, and the basal expression level of three heat shock peoteins (HSPs) mRNA were investigated at1d,5d,10d and15d, respectively. The results showed that compared with the fish before pH stress, the serum ALP, TP, C3, C4and liver SOD, ASAFR levels were significantly reduced (P<0.05); the serum ALT, cortisol, liver MDA and HSP60, HSP70mRNA levels were significantly increased (P<0.05); among them, the serum cortisol, SOD, ASAFR, MDA and HSP60, HSP70mRNA levels increased at first and then decreased. After pH stress, compared with the control group, the serum ALP, TP, C3and C4levels, liver SOD and ASAFR activities, and expression levels of three HSPs mRNA increased in vitamin C groups in varying degrees and serum ALT, cortisol and liver MDA levels decreased to some extent (P<0.05). Thus, the basal diet supplemented with vitamin C could improve non-specific immunity of fish, antioxidant capacity and three HSPs mRNA expression levels, and enhance resistance to pH stress.4. Effects of dietary vitamin C on the disease resistance of juvenile Wuchang bream (Megalobrama amblycephala) against Aeromonas hydrophila infection under pH stressAfter pH stress,6fish taken from each tank were intraperitoneally infected with A. hydrophila, the cumulative mortality of each group was counted at12hours and24hours after infection, and the biochemical parameters of serum and liver, the basal expression level of three heat shock peoteins (HSPs) mRNA in liver before and after infection were determined. The results showed that compared with the fish before infection, the WBC and HGB levels, liver SOD and ASAFR activity were significantly reduced (P<0.05); the serum ALT, AST, liver MDA and HSP60, HSP70mRNA levels were significantly increased (P<0.05). After infection, compared with the lowest level of vitamin C group (the0.2mg/kg Vc group), the serum TP level were increased in65.8and133.7mg/kg groups; the expression levels of HSP60and HSP70mRNA were inceased and the serum ALT and cortisol levels were reduced in251.5mg/kg group (P<0.05). Statistic data also showed that after infection with A. hydrophila, the cumulative mortalities of vitamin C groups were significantly lower than the control group (0.2mg/kg Vc group) in12h and24h (P<0.05). Thus, the basal diet supplemented with vitamin C could enhance disease resistance of fish against pathogenic infection.
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