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Heat shock proteins (particularly Hsp70) and other components of the stress-response system in the nucleated erythrocytes of a cold resistant (Bufo viridis) and cold sensitive (B. regularía) toads were investigated. Toads are terrestrial and poikilothermic, they are vulnerable and constantly exposed to diverse ambient temperatures, both in short-term (daily changes in temperature) and in long-term (seasonal changes). It was found that Hsp70 in red blood cells of the two species of toads is expressed variably and in relatively high levels at controlled room temperature. Comparable levels of Hsp70 were observed in the two species under a variety of ambient conditions. The formation of heat shock factor 1 -heat shock element binding complex, considered a first step in the stress-response system was unconnected to thermal conditions. Although toads prove to possess the components involved in the cellular stress-response, the relationships and regulation of this system are substantially different from the regular response of the system in mammals. The high levels of resting Hsp70 in toad erythrocytes could be beneficial in facilitating survival in variable thermal and stress conditions. This hypothesis is supported by the different responses observed to cold shock in two exemplary species, a homeotherm (chicken, Gallus domesticus) and a poikilotherm (lizard, Agama stelio). It is suggested that a cellular strategy of maintenance at a high level of Hsp70 is predominant in vulnerable species, and makes them prepared for temperature changes and unpredictable stress stimuli that may interfere with appropriate cellular functions.

Expression of Hsp70 in heart and skeletal muscle in two species of toads (Bufo viridis and B. regularis) that reside in different climatic regions and differ profoundly in their temperature sensitivity was studied at room temperature and upon various acclimation conditions. The effects of hypoxia, phenylephrine and clonidine on heart rate, ECG and Hsp70 expression were also investigated. High and variable levels of Hsp70 were detected in heart tissue of both species, compared with Hsp70 levels detected in skeletal muscle. Although there seems to be somewhat higher expression in B. regularis heart, low temperature did not influence the Hsp70 level appreciably and did not differ much from the control. Expression of HSF1 in heart extract of B. viridis and Xenopus laevis in relation to various thermal procedures falls within the same range, and if at all, it was reduced by the thermal treatments. HSF1-HSE binding occurred randomly in the two species, and was unrelated to the thermal treatments. Specific HSF1-HSE binding occurred only when thermal treatment was combined with phenylephrine injection. Hsp70 expression was highest at control conditions, unrelated to HSF1-HSE binding, and remained almost unchanged after phenylephrine injections and cold shock. Heart rate was greatly reduced and QRS amplitude was diminished in response to both hypoxia and clonidine injection. Hsp70 expression, on the other hand, was maximal in hypoxic tissue at room temperature, and was paralleled by a modest binding of HSF1 to HSE. In all other conditions, the prominent radioactive complex was unrelated to the expression of Hsp70. Hsp level both in the heart and skeletal muscle do not seem then to be directly involved in the response to thermal changes and pharmacological stimuli. The results are discussed in the framework of a strategy, frequent among poikilotherms, that involves maintenance of a high resting level of Hsp.

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