Oxidative stress during stressful heat exposure and recovery in the North Sea eelpoutZoarces viviparusL.

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Abstract
The interplay between antioxidants, heat shock proteins and hypoxic signaling is supposed to be important for passive survival of critical temperature stress, e.g. during unfavorable conditions in hot summers. We investigated the effect of mild (18 degrees C), critical (22 degrees C) and severe (26 degrees C) experimental heat stress, assumed to induce different degrees of functional hypoxia, as well as the effect of recovery following heat stress on these parameters in liver samples of the common eelpout Zoarces viviparus. Upon heat exposure to critical and higher temperatures we found an increase in oxidative damage markers such as TBARS (thiobarbituric reactive substances) and a more oxidized cellular redox potential, combined with reduced activities of the antioxidant enzyme superoxide dismutase at 26 degrees C. Together, these point to higher oxidative stress levels during hyperthermia. In a recovery-time series, heat-induced hypoxia and subsequent reoxygenation upon return of the fishes to 12 degrees C led to increased protein oxidation and chemiluminescence rates within the first 12 h of recovery, therein resembling ischemia/reperfusion injury in mammals. HSP70 levels were found to be only slightly elevated after recovery from sub-lethal heat stress, indicating minor importance of the heat shock response in this species. The DNA binding activity of the hypoxia-inducible transcription factor (HIF-1) was elevated only during mild heat exposure (18 degrees C), but appeared impaired at more severe heat stress. We suppose that the more oxidized redox state during extreme heat may interfere with the hypoxic signaling response.

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