Hsp70 is not a sensitive indicator of thermal limitation in Gadus morhua

The levels of heat-shock proteins of the 70 kDa family (Hsp70s) were measured in different soft tissues of Atlantic cod Gadus morhua from different locations and after exposure to various thermal conditions: acute temperature increments (1 degrees C day(-1)), mid-term (73 days at 4-15 degrees C) and long-term thermal acclimation (278 days at 8-15 degrees C), and seasonal and latitudinal temperature variations (field samples). Tissue specific distribution patterns of Hsp70s were observed: liver > gills > red blood cells > brain > white muscle. Thus, different tissues may have required different levels of protection by Hsp70s, and possibly this was related to the rate of protein synthesis. There were no differences in tissue Hsp70s between Arctic cod populations (Arctic, i.e. Barents and White Seas, Norwegian coast, and North or Baltic Seas). No changes in Hsp70s levels were observed in response to temperature variation of any intensity (acute fluctuation or seasonal and latitudinal) within the range of physiological temperatures (4-15 degrees C) in wild and laboratory Atlantic cod. This confirms previous observations that changes in Hsp70 caused by such temperature variation are often small in fishes. Probably, the constitutive level of Hsp70s in Atlantic cod was high enough to overcome potentially harmful effects of temperature variations within the physiological range. A suppressing effect of high temperature (15 degrees C) has already been observed at a systematic level (as reduced rate of somatic growth), whereas it is not reflected in modified Hsp70s. Therefore, Hsp70s apparently played a secondary role in defining thermal tolerance limits in Atlantic cod. These conclusions are in line with a recent concept of thermal tolerance which indicated that the first line of thermal limitation in the cold and warm is a loss in aerobic scope. (c) 2005 The Fisheries Society of the British Isles.