Response of oxidative stress parameters and activation of hypoxia inducible factor to changing environmental temperature in polar and temperate zoarcid fishKatja Heise1, Mikko Nikinmaa2, Hans O. Pörtner3,Doris Abele4We are interested in the cellular mechanisms that support wide ranging latitudinal distribution of marine eurythermal fish and have studied a new facet of thermal tolerance in zoarcids from different climatic regions. The common eelpout, <I>Zoarces viviparus</i>, from the North Sea was compared to the Antarctic eelpout, <I>Pachycara brachycephalum</i>, with respect to cellular oxidative stress parameters and the potential to induce metabolic reorganization during functional hypoxic stress. Functional hypoxia sets on at the limits of thermal tolerance and we hypothesize that survival of thermal stress in ectotherms involves hypoxic signalling to induce metabolic reorganisation.It turned out very clearly that polar eelpout maintain a stronger antioxidant defence system (antioxidant enzyme activities and 3 fold higher levels of liver glutathione) to protect their susceptible membranes from oxidative damage and in so doing decreased the levels of lipid peroxidation damage and the formation of lipid radical chain reactions.EMSA-binding activity of the hypoxia inducible transcription factor (HIF-alpha) was found above habitat temperature at 5°C in polar eelpout and at low temperature of 6°C in North Sea eelpout. In both cases, accumulation of ROS mediated damage correlated with HIF stabilization and a decrease in enzymatic antioxidant defence. We see here that metabolic ROS production becomes unbalanced at the thermal limit of the tolerance range of an ectotherm and in some way or other this is involved in metabolic reorganization during onset of functional hypoxia.