Effect of inhibition of Na+/Ca2+ exchanger at the time of myocardial reperfusion on hypercontracture and cell death

Objective: There is recent evidence that Ca2+ influx via reverse mode Na+/Ca2+ exchange (NCX) at the time of reperfusion can contribute to cardiomyocyte hypercontracture. However, forward NCX is essential for normalization of [Ca2+], during reperfusion, and its inhibition may be detrimental. This study investigates the effect of NCX inhibition with KB-R7943 at the time of reperfusion on cell viability. Methods: The effect of several concentrations of KB-R7943 added at reperfusion was studied in Fura-2 loaded quiescent cardiomyocytes submitted to 40 min of simulated ischemia (NaCN 2 mM, pH 6.4), and in rat hearts submitted to 60 min of ischemia. [Ca2+](i) and cell length were monitored in myocytes, and functional recovery and LDH release in isolated hearts. From these experiments an optimal concentration of KB-R7943 was identified and tested in pigs submitted to 48 min of coronary occlusion and 2 It of reperfusion. Results: In myocytes, KB-R7943 at concentrations up to 15 muM reduced [Ca2+'](i) rise and the probability of hypercontracture during re-energization (P < 0.01). Nevertheless, in rat hearts, the effects of KB-R7943 applied during reperfusion after 60 min of ischemia depended on concentration and timing of administration. During the first 5 min of reperfusion, KB-R7943 (0.3-30 μM) induced a dose-dependent reduction in LDH release (half-response concentration 0.29 μM). Beyond 6 min of re-flow, KB-R7943 had no effect on LDH release, except at concentrations ≥15 μM, which increased LDH. KB-R7943 at 5 μM given during the first 10 min of reflow reduced contractile dysfunction (P = 0.011), LDH release (P = 0.019) and contraction band necrosis (P = 0.014) during reperfusion. Intracoronary administration of this concentration during the first 10 min of reperfusion reduced infarct size by 34% (P = 0.033) in pigs submitted to 48 min of coronary occlusion. Conclusions: These results are consistent with the hypothesis that during initial reperfusion NCX activity results in net reverse mode operation contributing to Ca2+ overload, hypercontracture and cell death, and that NCX inhibition during this phase is beneficial. Beyond this phase, NCX inhibition may impair forward mode-dependent Ca2+ extrusion and be detrimental. These findings may help in the design of therapeutic strategies against lethal reperfusion injury, with NCX as the target. (C) 2002 Elsevier Science B.V. All rights reserved.