Heat acclimation increases the basal HSP72 level and alters its productiondynamics during heat stress

It has been previously shown that heat acclimation leads to an elevated bas al level of 72-kDa heat shock protein (HSP72). Augmented expression of HSP7 2 is considered as a cytoprotective response. This led us to hypothesize th at alterations in the heat shock protein (HSP) defense pathway are an integ ral part of the heat acclimation repertoire. To investigate this, we studie d the temporal profile of basal HSP expression upon acclimation and the dyn amics of their accumulation subsequent to acute heat stress (HS). In parall el, HSP72 mRNA level before and after HS was measured. For comparison, HSC mRNA [the constitutive member of 70-kDa HSP (HSP70) family] was measured in similar conditions. Heat acclimation was attained by continuous exposure o f rats to 34 degrees C for 0, 1, 2, and 30 days. HS was attained by exposur e to 41 or 43 degrees C for 2 h. Thermoregulatory capacity of the rats was defined by rectal temperature, heating rate, and the cumulative heat strain invoked during HS. HSP72 and HSP70 gene transcripts were measured in the l eft ventricle of the heart by means of Western immunoblotting and semiquant itative RT-PCR, respectively. The resultant acclimatory change comprised a higher resting level of the encoded 72-kDa protein (Delta 175%, P < 0.0001) . After HS, peak HSP72 mRNA level was attained, 40 and 20 min post-HS at 41 and 43 degrees C, respectively, vs. 60 and 4 0 min in the nonacclimated gr oup. The subsequent HSP synthesis, however, was dependent on the severity o f the cumulative heat strain. At the initial phase of heat acclimation, aug mented HSP72 transcription unaccompanied by HSP synthesis was observed. It is concluded that; upon heat acclimation, the HSP defense pathway is predis posed to a faster response. At the initial phases of heat acclimation, inab ility to elevate the HSP cytosolic level rules out their direct cytoprotect ive role.