Delayed protection of the ischemic heart--from pathophysiology to therapeutic applications

Preconditioning the heart with brief episodes of ischemia paradoxically increases its resistance to subsequent ischemic episodes, and markedly limits infarct size. Although preconditioning is now considered as the most powerful antiischemic intervention known, its beneficial effects are short-lived since they are lost if the reperfusion period after preconditioning is extended past 2-3 h. There is, however, some evidence of a delayed phase of protection, manifest 24 h after the initial preconditioning stimulus, associated with a decrease in infarct size, a prevention of postischemic contractile dysfunction (stunning) and a reduction in endothelial injury. The delayed beneficial effects of preconditioning resemble those induced by prior heat stress, and might be related to the expression of stress proteins (heat shock proteins or HSP). Evidence for a role of HSP derives from observations showing that brief ischemia is a potent stimulus for HSP expression. Moreover, transfection of isolated cells with HSP or overexpression of HSP in transgenic mice renders the myocytes more resistant to ischemia. Once produced, HSP are believed to facilitate protein synthesis, stabilize newly formed proteins and repair denatured ones. Alternatively, delayed preconditioning may be mediated by antioxidant enzymes such as superoxide dismutase or catalase, which are also upregulated by ischemia and this could lead to a lesser production of oxygen-derived free radicals during reperfusion. Indeed, in isolated myocytes, prevention of hypoxia-induced expression of superoxide dismutase (using an antisense oligonucleotide) abolished the delayed protective effect of preconditioning. Importantly, recent in vivo evidence suggests that the delayed protection may be mediated by adenosine, through activation of A1-receptors, and by stimulation of protein kinase C. Finally, although the exact mechanisms by which preconditioning induces delayed protection are still mostly unknown, the fact that the expression of protective proteins such as HSP can be induced by many other means than ischemia suggests that it is possible to pharmacologically stimulate this expression and thus possibly mimic the endogenous protective pathway. This could lead to the development of new pharmacological interventions which induce delayed myocardial protection in clinical situations such as angioplasty, coronary bypass surgery or even in patients at high risk of infarction.