Response to: Effect of inhibition of Na+/Ca+ exchanger at the time of myocardial reperfusion on hypercontracture and cell death

See article by Inserte et al. [12] (pages 739–748) in this issue. The functional and biochemical characterization of the sarcolemmal Na/Ca exchanger (NCX) testifies to the success of novel approaches by numerous investigators [1,2] and, in particular, development of new techniques for measuring sarcolemmal ion fluxes and cytosolic Na and Ca concentrations ([Na]i and [Ca]i) [3,4]. Our current understanding of the role NCX plays in ischemia-induced changes in cytosolic calcium concentration ([Ca]i) is, to a great extent, derived from studies which used recently developed technology to measure intracellular Na and Ca in the intact heart [5,6]. Much of this work supports the general hypothesis that ischemia-induced myocardial injury is largely the result of the following chain of events: (1) increased anerobic metabolism increases cytosolic proton concentration ([H]i); (2) protons stimulate Na-dependent pH regulatory transporters such as Na/H exchange and Na-HCO3 cotransport; (3) increased Na uptake increases [Na]i; (4) decreased driving force for Ca extrusion via NCX increases [Ca]i; (5) a cascade of Ca-dependent events leads to cell injury and/or death [7–9]. Although this series of events has been articulated by numerous investigators and supported by numerous studies, a number of its ‘finer points’ are commonly overlooked. For instance, the hypothesis does not invoke any alteration in Na efflux to cause the increase in [Na]i which is linked to increases in [Ca]i. While many investigators continue using the historical argument that ischemia-induced increases in [Na]i are the result of decreased Na/K ATPase activity, there is, in fact, little evidence for diminished Na efflux via the Na/K pump early during ischemia when [Na]i has been demonstrated to increase significantly. Rather, there is good evidence that Na efflux via Na/K ATPase is increased early during ischemia in response to the increase in [Na]i which results from increased uptake via such pathways as Na/H exchange [10]. That is, increases in [Na]i are more important in causing the depletion of ATP, than vice versa. The argument is more than semantic, because appropriate therapies can only be developed if one correctly identifies the cause of the pathology. In this context one must know whether increases in the relevant intracellular ions (Na, Ca, and H) are the result of increased influx (or production) or decreased efflux or both. With respect to Na, ischemia starts a vicious cycle where increased protons stimulate Na uptake which stimulates further ATP hydrolysis (e.g. by the Na/K pump) to release more protons [11]. Regarding the role of NCX, a number of points raised by Inserte et al. [12] in their paper in this issue of Cardiovascular Research, are worthy of reiteration. Because NCX in the heart exchanges three Na per Ca it carries positive charge in the direction of net Na flux. Thus the negative resting membrane potential favors Na entry and depolarization favors Na efflux such that flux through the exchanger normally changes direction during each cardiac cycle. Nevertheless, it is well accepted that on a time averaged basis the exchanger acts as the major net efflux pathway for Ca that enters via sarcolemmal channels during each action potential [4]. During ischemia, however, the cell membrane is depolarized and intracellular Na rises significantly such that the time averaged driving force for NCX is likely to be directed to drive Ca into the cell within a few minutes after ischemia begins. A number of studies have now presented evidence that the driving force for NCX remains near equilibrium while [Na]i rises under conditions such as acidification and ischemia [13–15]. This lends further support to the hypothesis that NCX is the most important pathway determining [Ca]i during ischemia. Additionally, a number of investigators have presented results suggesting that early during reperfusion, there is a ‘surge’ of Na entry into the cell [16]. This is thought to be partly due to a rapid increase in extracellular pH which alters the kinetics of Na/H exchange so as to increase the rate of Na entry into the cell [17]. As predicted by the hypothesis described above, this surge in Na entry is associated with a further rapid increase in [Ca]i early during reperfusion [16]. However, reports are conflicting on whether [Na]i increases or decreases immediately after reperfusion, and there is some evidence that Na/K ATPase activity recovers rapidly enough during reperfusion to prevent a reperfusion-induced surge in Na uptake [18]. It is likely that the response to reperfusion is model dependent, nevertheless, the general hypothesis predicts that any therapeutic intervention which limits Ca uptake will limit ischemia/reperfusion injury. Logical targets are major Na and Ca entry pathways, such as Na/H exchange and NCX, respectively. Inhibition of Na/H exchange has proven exceedingly effective in limiting myocardial ischemia/reperfusion injury in laboratory models [19], but clinical protocols have not yet been optimized [20]. On the other hand, NCX was not initially considered a good therapeutic target, in part because a specific inhibitor was not available and, perhaps even more importantly, because of the exchanger's normal role as the major efflux pathway for Ca. With the introduction of a more specific NCX inhibitor however, Inserte and Piper and their coworkers have begun studies and provide evidence in the present paper [12] that NCX inhibition during reperfusion will limit ischemia/reperfusion injury in rat and pig hearts if the correct concentration and time of exposure is used. Although they do not provide data addressing the effect of the inhibitor on [Ca]i in the intact heart, their results are consistent with the hypothesis that NCX inhibition decreases calcium overload during ischemia/reperfusion. These findings [12] should encourage additional basic and clinical studies aimed at confirming the mechanism of action of NCX inhibition as well as optimizing NCX inhibition therapies to limit myocardial ischemia/reperfusion injury in humans.