Objective: It has been reported that rapid cooling of the heart during normothermic coronary circulation at reperfusion after ischemia promotes early recovery of cardiac function due to the positive inotropic effects on the myocardium produced by cooling. The aim of the present study was to investigate the myocardial protective effect of rapid cooling by measuring heat shock protein (HSP) levels and examining the relationship between cardiac function, intracellular Ca concentration, and intracellular pH after rapid cooling.
Methods: Isolated perfused rat hearts were subjected to ischemia for 60 minutes at a myocardial temperature of 37 degrees C. One group of hearts (group R) was subjected to 3 minutes of rapid cooling (</=4 degrees C) 3 minutes after reperfusion was started. The other hearts (group C) were not cooled. Myocardial protection was then compared in the two groups by measuring heart rate, coronary flow, left ventricular contraction, intracellular Ca concentration (fura-2 fluorescence), and intracellular pH (BCECF-AM fluorescence) and examining HSP70 by western blot.
Results: (1) Left ventricular contraction was 105+/-0.05% of the pre-ischemia value in group R after rapid cooling, whereas recovery was poor in group C (63+/-2%). (2) The intracellular Ca concentration in group R rose to 141+/-8% of the pre-ischemia value during ischemia, but decreased to 101+/-3% at reperfusion after rapid cooling. In group C, the Ca concentration was 144+/-4% of the pre-ischemia value during ischemia and rose further to 156+/-6% at reperfusion. (3) In group R, the intracellular pH was 6.85+/-0.05 during ischemia and rose after reperfusion was started, but after rapid cooling, decreased again to 7.43+/-0.02 after 60 minutes reperfusion. In group C, the pH was 6.88+/-0.06 during ischemia and rose to 7.90+/-0.09 60 minutes after reperfusion was started. (4) HSP70 was detected in both groups, but the concentration (luminescence) was lower in group R than in group C.
Conclusion: Rapid cooling reduces myocardial injury by preventing intracellular Ca overload at reperfusion via inhibition of the Na(+)/Ca(2+) exchanger and also decreases the appearance of HSPs, the so-called homeostasis proteins. (Ann Thorac Cardiovasc Surg 2003; 9: 301-6)