HSP25 IN ISOLATED-PERFUSED RAT HEARTS - LOCALIZATION AND RESPONSE TO HYPERTHERMIA

Recent investigations concentrate on the correlation between the myoca rdial expression of the inducible 70-kDa heat shock protein (HSP70i) b y different stress conditions and its possible protective effects. Onl y few studies have focused on the involvement of small heat shock prot eins in this process. We analyzed the location of the small heat shock protein HSP25 in isolated cardiomyocytes as well as its location and induction in isolated perfused hearts of rats. By immunofluorescence m icroscopy HSP25 was found to colocalize with actin in the I-band of my ofibrils in cardiomyocytes of isolated perfused hearts as well as in i solated neonatal and adult cardiomyocytes. Hyperthermic perfusion of i solated hearts for 45 min resulted in modulation of different paramete rs of heart function and in induction of HSP25 and HSP70i. Temperature s higher than 43 degrees C (44-46 degrees C) were lethal with respect to the contractile function of the hearts. Compared to control hearts perfused at 37 degrees C, significant increases during hyperthermic pe rfusion at 42 degrees C and 43 degrees C were obtained for heart rate, contraction velocity and relaxation velocity. In response to hyperthe rmia at 43 degrees C and after subsequent normothermic perfusion for 1 35 min at 37 degrees C, left ventricular pressure, contraction velocit y and relaxation velocity remained significantly elevated. However, he art rate returned to control values immediately after the period of he at treatment. HSP25 is constitutively expressed even in normothermic p erfused hearts as shown by Western blotting. Hyperthermia increased th e content of HSP25 only in the left ventricular tissue. In contrast, H SP70i was strongly induced in all analyzed parts of the myocardium (le ft ventricle, right ventricle, septum). Our findings suggest a differe ntial regulation of HSP25 and HSP70i expression in response to hyperth ermia in isolated perfused hearts. The constitutively expressed HSP25 seems to be located adjacent to the myofibrils which implies a specifi c role of this protein even under unstressed conditions for the contra ctile function of the myocardium.