NHE-1 and NHE-6 activities: ischemic and reperfusion injury.

The study 1 published in this issue of Circulation Research showing that a null mutation of NHE-1 improves the tolerance of the heart to ischemia and reperfusion (I/R) is an important contribution for the following reasons: (1) In the animals with null mutation, contracture during the ischemic period was less and ATP levels were preserved compared with wild-type animals. This observation, on the one hand, provides evidence that protection by downregulation of NHE-1 during the ischemic period itself is indeed possible and, on the other hand, it argues against the suggestion that the exchanger is inactive during this same period.2 (2) In contrast with chronic blockade of the NHE-1 by pharmacological interventions,3 the long-term absence of the exchanger does not elicit major compensatory changes that, in turn, might negate the cardioprotective effect of blocking its activity for a relative short term. This point is related to a recent publication3 showing that long-term treatment with the NHE-1 blocker cariporide is followed by an upregulation of the functional units of the exchanger in a similar way to the well-known tolerance phenomenon following -adrenergic receptor blockade. The absence of such upregulation negates possible hypersensitivity to ischemia upon withdrawal of the medication. The risk is evident in hearts with upregulation of NHE-1, which gain Na i more rapidly during ischemia, and show impaired recovery after reperfusion.4 (3) No additional protection was obtained by adding the NHE-1 blocker eniporide to the NHE-1 null mice, suggesting that there is not another NHE isoform that can be blocked with this compound to add additional protection; the findings additionally hint that the attenuation of the injury obtained by the absence of the sarcolemmal NHE-1 is maximal and, therefore, no further beneficial effect will be detected by blocking the mitochondrial NHE (MNHE). The classical explanation of the mechanism by which the NHE-1 blockade protects against I/R is as follows. During ischemia, a cytosolic acidosis of approximately 1 pH unit occurs in about 10 minutes. This cytosolic acidosis stimulates the NHE-1, increasing its activity and augmenting cytosolic Na and Ca levels.5,6 Although other mechanisms could contribute to the increase in intracellular Na (Na i) during ischemia, it has been shown that blockade of the NHE-1 before ischemia abolishes the increase in Na i during this period5,8 and diminishes the increase in cytosolic Na and Ca during reperfusion.5,6 The increase in Ca secondary to the increase in Na seems to be caused by the Na -Ca exchanger (NCX) working in reverse mode,9 and cytosolic Ca overload is a necrotic and apoptotic signal. Although with some contradictory results, it has been possible to obtain protection from I/R by blocking the NHE-1 or the NCX only after the onset of reperfusion,10–12 suggesting that there is protection against the reperfusion injury induced by these mechanisms in addition to protection from the ischemic injury. Although other mechanisms can be operative,13 the activation of NHE-1 at the onset of reperfusion has been linked to the increase in reactive oxygen species (ROS).14–16 This exchanger reaches its maximal activity early after reperfusion. It has been proposed that the increase in ROS leads to activation of the mitogen-activated protein kinase pathway, phosphorylating ERK1/2 and the cytosolic tail of the NHE-1, increasing the exchanger activity.17 On the other hand, it has been also reported that ERK1/2 mediates activation of the Na -HCO3 cotransport and that blockade of this mechanism protects against I/R injury.18 The available evidence, therefore, establishes that the cytosolic Ca overload induced by the NCX working in its reverse mode can be prevented by blocking the NHE-1, decreasing Na i. A recent publication19 makes things more difficult to interpret. In this study, the authors showed that cariporide preserves mitochondrial proton gradient and delays ATP depletion in mouse-derived myocytes (HL-1) after simulated ischemia and suggested that these mitochondrial changes are not secondary to changes in the cytosol. They concluded that, during ischemia, cariporide acts at the mitochondrial level, delaying mitochondrial matrix acidification and preserving ATP levels. They also suggested that the prevention of mitochondrial Ca overload was not cariporide’s mechanism of protection. Mitochondrial NHE (MNHE) is apparently encoded by NHE-6, with a molecular structure similar to that of NHE1.20–22 However, there are still some concerns about the identification of the NHE-6 with the MNHE.21,22 The MNHE can be blocked by several NHE blockers including cariporide19,23,24; therefore, this drug blocks both the NHE-1 (sarcolemmal) and the NHE-6 (mitochondrial) gene products. In a more recent study,25 the effect of cariporide on cell death induced by oxidative stress was examined in cultured neonatal cardiomyocytes. The inhibitor suppressed cytosolic Na and Ca accumulation and the loss of mitochondrial The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Centro de Investigaciones Cardiovasculares, La Plata, Argentina. Correspondence to Horacio E. Cingolani, MD, Centro De Investigaciones Cardiovasculares, Facultad de Ciencias Medicas, Universidad Nacional de Le Plata-60 y 120, La Plata 1900, Argentina. E-mail [email protected] (Circ Res. 2003;93:694-696.) © 2003 American Heart Association, Inc.