THE 1990 MERCK-FROSST AWARD - ISCHEMIC AND REPERFUSION INJURY IN THE HEART - CELLULAR MECHANISMS AND PHARMACOLOGICAL INTERVENTIONS

Reperfusion in the heart represents an important form of tissue injury, particularly in view of the emerging importance of reperfusion protocols aimed at salvaging the ischemic myocardium. Both the manifestations and the causes of reperfusion injury are multifold. With respect to the former, reperfusion injury can be characterized by various abnormalities including development of arrhythmias, contractile dysfunction, ultrastructural damage as well as various defects in intracellular biochemical homeostasis. The mechanisms underlying myocardial reperfusion injury are equally complex, but most likely involve numerous processes acting in concert resulting in eventual cell death. In this review, a description of various such potential mechanisms, which represent primary interests of the author, are presented. An understanding of these mechanisms has led to novel pharmacological approaches towards the protection of the reperfused myocardium. For instance, several lines of evidence implicate enhanced eicosanoid, and in particular prostaglandin, synthesis in reperfusion injury, since (1) such injury is involved with enhanced prostaglandin biosynthesis, (2) inhibition of prostaglandin synthesis with various nonsteroidal anti-inflammatory drugs attenuates injury, and (3) exogenous prostaglandins increase injury. Another intracellular process that is emerging as an important contributor to reperfusion injury in the heart is the Na+/H+ exchanger, which is most likely activated upon reperfusion. Such activation would lead to numerous intracellular disturbances including the increased synthesis of prostaglandins and elevated intracellular Ca2+ concentrations. Indeed, inhibitors of Na+/H+ exchange such as amiloride have been shown to effectively inhibit reperfusion injury. Reperfusion is also associated with depressed mitochondrial function, particularly in subsarcolemmal mitochondria which are rapidly injured as a result of both ischemic and reperfusion conditions. Preservation of mitochondrial function with dissimilar approaches such as carnitine or phosphatidylcholine administration markedly reduces reperfusion injury. A nonpharmacological novel approach towards the protection of the reperfused myocardium represents the induction of so-called stress or heart shock proteins in the heart prior to initiation of ischemia and reperfusion. The salutary effect of the heat shock response may be dependent not on the heat shock proteins themselves, but through the concomitant elevation of tissue catalase content resulting in enhanced detoxification of intracellular hydrogen peroxide. Thus reperfusion injury represents numerous complex events such that manipulations aimed at limiting such injury can be initiated to prevent specific defects with the ultimate goal of an overall reduction in cell damage.