Moderate hypothermia during cardiopulmonary bypass increases intramyocardial synthesis of heat shock protein 72.

Objectives: This study was undertaken to test the hypothesis that the myocardial protective effect of moderate hypothermia during cardiopulmonary bypass involves upward regulation of heat shock protein 72.
Methods: Sixteen young pigs were randomly assigned to a temperature regimen during standardized cardiopulmonary bypass of normothermia or moderate hypothermia (temperatures 37 degrees C and 28 degrees C, respectively, n = 8 per group). Myocardial probes were sequentially sampled from the right ventricle before and during bypass and 6 hours after bypass. Messenger RNA encoding for heat shock protein 72 was assessed by competitive reverse transcriptase-polymerase chain reaction, and heat shock protein 72 synthesis was assessed by Western blot and immunohistochemical methods. Induction of apoptosis was assessed by gene expression of apoptosis-regulating proteins (Bcl-xL, Bak, and Fas) according to competitive reverse transcriptase polymerase chain reaction. Apoptotic cells were identified with an in situ apoptosis-detection kit (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling) in combination with morphologic criteria. Necrotic cells were detected by standard histologic methods.
Results: Moderate hypothermia rather than normothermia was associated with earlier and higher gene expression and synthesis of heat shock protein 72 in the myocardium during and after cardiac surgery. In the hypothermia group both heat shock protein 72 and the messenger RNA encoding it were detected as soon as 30 minutes after initiation of bypass and before aortic clamping, whereas in the normothermia group they were not detected before aortic clamping. Immunohistochemical methods showed localization of heat shock protein 72 in the cardiomyocytes, endothelial cells, and macrophages. Although the percentage of necrotic cells in the myocardium was lower in the hypothermic group, the induction of apoptosis regulatory proteins and the percentage of apoptotic cells did not differ between the groups.
Conclusions: These results suggest that the myocardial protective effect of moderate hypothermia during cardiopulmonary bypass involves upward regulation of heat shock protein 72 and inhibition of necrosis but not of apoptosis.