Effects Of Hypoxia On Antioxidant Defense And Lysozyme In Carassius Auratus

Carassius auratus is a natural triploid gynogenesis crucian carp that is distributed in Qihe River.In recent years,with the increasing of breeding density and the amount of bait casting,water eutrophication is serious,hypoxia phenomena and disease occur frequently.C.auratus,which thrives in substratum water,is subjected to hypoxic and eutrophic conditions.However,the response of C.auratusto hypoxic stress remains unclear.In this study,the activities of antioxidant enzymes and lysozyme,as well as the content of malondialdehyde were measured in different organs and tissues of C.auratus.Compared the activities of antioxidant enzymes and lysozyme,and the malondialdehyde content in C.auratus between hypoxia and normoxia.Analyzed the response of antioxidant enzymes and lysozymein C.auratus hepatopancreas to poly I:C injection after hypoxia.Understanding the effects of antioxidant level and immune to hypoxic stress and pathogenic analogues poly I:C stimulation under hypoxia condition,comprehending the response and the mechanism of C.auratus to hypoxia,ascertaining defenses capability against pathogens under hypoxia condition,are helpful to provide basic data for C.auratus farming and guide the aquaculture and improve the immunity of fish.The results showed that the antioxidant enzyme activities in hepatopancreas were highest among the different tissues and organs of C.auratus and the activities in spleen were lowest.The values of malondialdehyde contents in different tissues and organs were in hepatopancreas > kidney > intestine > brain > gill > muscle > spleen,and there was no significant difference in malondialdehyde comparison between muscle and spleen,as well as intestine and brain(P>0.05),but significant difference occurred among the other tissues and organs(P<0.05).The order of lysozyme activities in different tissues and organs was in hepatopancreas > kidney> gill > brain > intestine > muscle > spleen,and the lysozyme activities in hepatopancreas and kidney were higher than other tissues and organs(P<0.05).It was demonstrated that all of the tested tissues exist the activities of antioxidant enzymes and lysozymeand indicated tissue specificity,which is due to the differences of physiological function and metabolism level in different tissues and organs of C.auratus.The activities of lysozyme and antioxidant enzymes and the content of malondialdehyde were measured in C.auratus at different time points(7 d,14 d and 21 d)after different dissolved oxygen levels exposure(1,2,4,and 6 mg/L,the DO level of 6 mg/L was used as the control).The DO level of 4 mg/L increased the activities of antioxidant enzymes and lysozyme in the hepatopancreas,kidney and spleen(P <0.05).This result implies that slight hypoxic stress can enhance antioxidant defenses in the hepatopancreas,kidney and spleen to alleviate the damage of oxidative stress.At the DO concentrations of 1 and 2 mg/L,the activities of antioxidant enzymes significantly decreased(P<0.05).The content of malondialdehyde significantly increased in hypoxia groups than control group(P <0.05).It was suggested that the capacities of antioxidant defense decrease under hypoxia and shown that oxidative stress occurs when the generation rate of ROS exceeds the removal rate,and lead to the accumulation of MDA,a lipid peroxidation product.Therefore,the content of MDA increased with the decreasing of dissolved oxygen concentration.Lysozyme activity significantly decreased in the hepatopancreas,gill,kidney and spleen at DO levels of 1 and 2 mg/L(P<0.05),indicating reduced antibacterial ability.In conclusion,hypoxia can influencethe content of malondialdehyde and the activities of lysozyme and antioxidant enzymes in a concentration-dependent manner.The reduced activities of antioxidant enzymes and the accumulation of malondialdehyde at the DO level of ? 2 mg/L implied the decrease in antioxidative ability and the occurrence of oxidative stress.The decrement in lysozyme activity indicated that the antibacterial ability was weakened to some degree.Therefore,hypoxic stress at DO levels ? 2 mg/L should be removed by aeration to avoid the oxidative damage resulting from the reduced antioxidative ability and prevent the outbreak of diseases caused by weakened antibacterial effects.The experimental fish were cultivated under different dissolved oxygen levels(6 mg/L,4 mg/L,2 mg/L and 1 mg/L)respectively for 7 days.Then,each fish was injected 2 mg/mL poly I: C 100 ?l in the treated group and was injected 0.75% NaCl 100?l in the control group.The activities of SOD,CAT,GPx,lysozyme and the mRNA relative expression of Cu/Zn-SOD,Mn-SOD,CAT,GPx,c-Lysozyme and g-Lysozyme,as well as the content of malondialdehyde,were measured after injection at 0 h,12 h,24 h,48 h,96 h and 168 h.After poly I: C stimulation in hypoxia,antioxidant enzyme activities and mRNA relative expression levels were significantly reduced,compared to the corresponding control,at 48 h,96 h and 168 h(P<0.05).It is indicated that antioxidant defense ability is reduced in hepatopancreas of C.auratus after poly I: C infection in hypoxia and then caused serious oxidative stress.MDA content was significantly higher in the treated group than in the control group.It was implied that oxidative stress was intensified in hepatopancreasof C.auratus after poly I: C injection in hypoxia.The lysozyme activity at different time points were significantly reduced in the treated group than in the control group,except the 12 h increased significantly(P<0.05).This shown that the virus resistance and immune ability enormously weaken after poly I: C injection in hypoxia.In this study,it is demonstrated that the oxidation resistance and antibacterial ability decrease under hypoxia and cause oxidative stress.Pathogenic analogues poly I: C stimulation under hypoxia condition can further weaken antioxidant defense and innate immunity ability and aggravate the degree of oxygen stress.This indicated that low oxygen conditions may increase the susceptibility to pathogenic bacteria of C.auratus.In the cultivation of C.auratus,when DO decrease to less than 4 mg/L,dissolve oxygen should be supplemented and increased to maintain the normal level of antioxidant defense in fish.