It has been known that a brief ischemic insult followed by reperfusion ('preconditioning') shows a protective effect on subsequent ischemic insult. The mechanism of this preconditioning was not fully understood, but oxygen free radicals, such as superoxide, hydrogen peroxide, which are released at the time of cardiac ischemia and reperfusion, were thought to have an important role in cardiac damage. We investigated the mechanism of preconditioning by examining the effects of oxidants on contractile function of isolated heart and the expression of cytokines and antioxidative genes.

Isolated perfused rat hearts were exposed to a 5 minutes ischemia (1 × PC) and to four times of 5 minutes ischemia, each seperated by 10 min (4 × PC). Both 1 × PC and 4 × PC groups were reperfused for 60 minutes. Control experiments were performed by perfusiong the hearts with buffer using the same time frame without ischemia-reperfusion. The effects of oxidants in contractile function of isolated rat heart were observed. The induction of the expression of two cytokines (TNF-α and interleukin-1) and two antioxidative enzyme genes (Mn-superoxide dismutase and catalase) were examined in control, 1 × PC and 4 × PC hearts by RT-PCR.

In the control group, hydrogen peroxide or t-BHP inhibited contractile function of isolated heart as a dose-dependent manner, and IC50 were 36.3 µM or 0.34 mM, respectively. In 4 × PC hearts, inhibitory action of hydrogen peroxide on contractile function was diminished significantly, but the t-BHP action was not. In 4 × PC hearts TNF-α, interleukin-1α and catalase gene expression was significantly increased compared to both 1 × PC and control values.

Repeated ischemia and reperfusion caused the induction of inflammatory and catalase genes, which may contribute to relative resistance of hydrogen peroxide in 4 × PC hearts.