| Environment stress could cause a stress response in euryhaline penaeids.Fistly,the mortality,growth performance,osmoregulation gene expression,digestive enzyme activity,and resistance against Vibrio parahemolyticus of white shrimp Litopenaeus vannamei reared under conditions of gradual changes to a low-pH(6.65–8.20)and gradual changes to a high-pH(8.20–9.81)versus a normal pH environment(8.14–8.31)during a 28-d experiment were evaluated.Consequently,under gradual-high pH,the cumulative mortality rate(CMR)rose with time until 39.9%on days 28,the weight gain percentage(WGP)and length gain percentage(LGP)decreased continuously.However,under gradual-low pH,the CMR of shrimp stabilized at 6.67%during 7-28d;the WGP and LGP decreased first and then returned to normal.These results indicated that L.vannamei displayed a moderate tolerance to gradual-low pH.Under gradual-low pH,the osmoregulation gene Na~+/K~+-ATPase,CAc,and CAg transcripts of shrimp increased continuously or then back to normal,the amylase,lipase,and trypsin activities decreased first and then returned to normal or increased,the MR with V.parahaemolyticus immersion decreased continuously.Thus,the major adaptation mechanism of shrimp to gradual-low pH might be its high osmoregulation ability,which made shrimp achieve a new,balanced steady-state with enhanced digestive enzyme activities to meet energy requirement after long-term exposure.Meanwhile,the gradual-low pH environment would probably inhibit disease outbreak in the shrimp farming.Then,the oxidative stress,antioxidant responses and oxidative damage in the hepatopancreas and midgut of shrimp were investigated.Consequently,shrimp enhanced antioxidant enzyme gene MnSOD,GPx,and stress protein gene Hsp70 transcripts in the hepatopancreas and midgut as early defense mechanism to scavenge excessive ROS during short-term(?7 d)gradual-low and high pH stress.Meanwhile,the hepatopancreas was more sensitive to pH variation and its oxidative stress than midgut because of earlier ROS production increase,antioxidant response and oxidative damage.Then,suppressed antioxidant response in the hepatopancreas and midgut of shrimp suggested a loss of antioxidant regulatory capacity caused by aggravated oxidative damage after long-term(?14 d)gradual-high pH stress,leading to continuous death.However,enhanced antioxidant enzyme GPx,GST,and Hsp70transcripts in the hepatopancreas and midgut might be long-term(?14 d)antioxidant adaptation mechanism of shrimp to gradual-low pH stress,which could prevent further ROS perturbation and weaken oxidative damage to achieve a new immune homeostasis,contributing to stable survival rate.Therefore,it is vital to protect hepatopancreas for controlling shrimp death under gradual-high p H stress.Secondly,the survival,growth performance,osmoregulation gene expression,digestive enzyme activity,and resistance against Vibrio parahemolyticus of the L.vannamei reared under conditions of cyclic serious/medium hypoxia(0.8-3.5 mg/L)versus normoxia(6.4-7.5 mg/L)during a 28-d experiment were investigated.Consequently,under cyclic serious/medium hypoxia,the CMR of shrimp increased continuously.The WGP and LGP of shrimp decreased continuously,the osmoregulation gene Na~+/K~+-ATPase,CAc,and CAg transcripts in the gill of shrimp increased first and then returned to normal or decreased.The amylase,lipase,and trypsin activities in the hepatopancreas of CSMH shrimp decreased continuously,the MR with V.parahaemolyticus immersion increased continuously.Therefore,cyclic serious/medium hypoxia could reduce survival and growth performance of shrimp during long-term exposure,the major reason was broken osmoregulation mechanism,which made shrimp lose balanced steady-state and reduce digestive enzyme activities.Meanwhile,cyclic serious/medium hypoxia would probably lead to outbreak of infectious diseases in the shrimp farming.Then,the hypoxia inducible factors 1a(HIF-1a),antioxidant responses and oxidative damage in the hepatopancreas and midgut of shrimp were investigated.Results showed enhanced HIF-1a,antioxidant enzyme gene MnSOD,Gpx,GST,MT,and stress protein gene Hsp70 transcript in the hepatopancreas and midgut during short-term cyclic serious/medium hypoxia(?7 d),which suggested early adaptive mechanism of shrimp to scavenge excessive ROS.Meanwhile,HIF-1a transcript and oxidative damage were induced earlier in the hepatopancreas than the midgut.Thus,the hepatopancreas could be more sensitive to hypoxia and its oxidative stress than the midgut.However,long-term(?14 d)cyclic serious/medium hypoxia could disrupt cellular antioxidant mechanism with depressed antioxidant responses,and then aggravate oxidative damage in the hepatopancreas and midgut,particularly the hepatopancreas would lose antioxidant ability earlier than the midgut.Therefore,it is vital to protect hepatopancreas for controlling shrimp death and growth inhibition under cyclic serious/medium hypoxia.Finally,effect of glutathione(GSH)on shrimp under cyclic serious/medium hypoxia was studied.The survival,growth performance,and oxidative index and transcriptome in the hepatopancreas of L.vannamei reared under conditions of normoxia,cyclic serious/medium hypoxia,cyclic serious/medium hypoxia and supplement of 75 mg/kg GSH,and cyclic serious/medium hypoxia and supplement of150 mg/kg GSH.Consequently,supplementation of 75 mg/kg GSH significantly improved GSH content and hepatopancreas index,and maintained integrity of histological structure in the hepatopancreas of shrimp under cyclic serious/medium hypoxia,promoting significantly enhanced shrimp survival and growth.Especially,shrimp supplemented with 75 mg/kg GSH under cyclic serious/medium hypoxia even have the basically same growth performance with the shrimp under normoxia.However,supplementation of 150 mg/kg GSH made no sense.Obviously,supplementation of 75 mg/kg GSH could relieve shrimp death and growth inhibition under cyclic serious/medium hypoxia.Then,oxidative indexes and transcriptome in the hepatopancreas were analyzed in this experiment.Supplementation of 75 mg/kg and 150 mg/kg GSH significantly enhanced HIF-1 signal pathway,GSH metabolism pathway,antioxidant enzyme(SOD,CAT,GPx,POD,GST)and stress protein HSP gene transcripts,and reduced ROS and malondialdehyde(MDA)content in the hepatopancreas of shrimp under cyclic serious/medium hypoxia,and shrimp supplemented with 75 mg/kg GSH have a higher basal level antioxidant defense system,lower ROS and MDA content and more integrated histological structure than shrimp supplemented with 150 mg/kg GSH,even the basically same ROS production and MDA content and histological structure with the shrimp under normoxia.Supplementation of 75 mg/kg rather than 150 mg/kg GSH significantly enhanced immune recognition gene LRR,TIR and humoral immunity gene Serpin transcripts in the hepatopancreas of shrimp under cyclic serious/medium hypoxia.Supplementation of 75 mg/kg and 150 mg/kg GSH significantly enhanced cellular immunity gene Rab,Ran and integrin gene transcripts in the hepatopancreas of shrimp under cyclic serious/medium hypoxia,and shrimp supplemented with 75 mg/kg GSH have a higher basal level of cellular immunity level than shrimp supplemented with 150 mg/kg GSH.Therefore,supplementation of 75 mg/kg GSH could relieve shrimp death and growth inhibition by enhancing antioxidant defense system and innate immune response,and maintaining histological structure under cyclic serious/medium hypoxia. |
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