Single or multiple episodes of brief period of ischemia and reperfusion(ischemic preconditioning, IP) have been shown to limit infarct size after a subsequent longer period of ischemia. A considerable number of possible mechanisms has been proposed, however, controversies still remain. Accordingly, we evaluated the effect of four cycles of 5 minutes ischemia and 5 minutes reperfusion(IP) followed by subsequent 30 minutes ischemia(ISCH) and 60 minutes reperfusion using isolated Langendorff-Perfused rabbit hearts.

After a 50-minute recovery phase, parameters of the left ventricular function(LVF) including left ventricular developed pressure(LVDP), contractility and the heart rate were recorded, and ultrastructure was examined. Myosin ATPase activity was determined by measurement of inorganic phosphorus and isozymes of the myosin heavy chain were examined by polyacrylamide gel electrophoresis containing pyrophosphate buffer. The ISCH hearts showed severe to irreversible change of the cardiac myocytes homogenously in contrast to the IP hearts in which changes were not homogenous and irreversible injury was only focal. However, parameters of the LVF were not significantly different between the IP and the ISHC hearts during reperfusion. Myosin ATPase activities were also not significantly different(0.67±0.123 µmol/mg protein/h in the IP hearts, 0.56±0.172 µmol/mg protein/h in the ISCH hearts, and 0.76±0.239 µmol/mg protein/h in the control hearts). Band patterns of the myofibrillar proteins, separated by sodium ddodecyl sulfate-polyacrylamide gel electrophoresis, revealed no differences between the IP, ISCH and the control hearts. Myosin heavy chains in the IP and the ISCH hearts were separated into 3 isozymes, V₁,V₂and V₃in pyrophosphate gel electrophoresis in contrast that the control hearts revealed two isozymes, V₁and V₂. However, there were no differences in the protein composition and electrophoretic motility between the IP and the ISCH hearts.

These results indicate that IP could not attenuate the changes in LVF, myosin ATPase activity and myosin isozymes on reperfusion, however, it could attenuate the ultrastructural changes of the cardiac myocytes.