Na+/H+ exchange in ischemia, reperfusion and preconditioning

We read with interest the recent paper in Cardiovascular Research by Xiao and Allen [1], on the role of the cardiac sarcolemmal Na+/H+ exchanger (NHE) in ‘reperfusion damage’ and ischemic preconditioning, and the accompanying editorial by Fiolet [2]. The former concludes that (1) NHE is inhibited during ischemia, (2) NHE is activated rapidly during reperfusion and contributes to reperfusion damage and (3) the protective mechanism of preconditioning is NHE inhibition during early reperfusion. However, the validity of these conclusions beyond the specific conditions of the relevant study is debatable. The first two conclusions are based on the comparable protective efficacy of the specific NHE inhibitor HOE-642 when given from shortly before reperfusion as opposed to from before the onset of ischemia [1], as well as the authors' earlier observation [3,4] that there is only a small rise in [Na+]i during ischemia, which is unaffected by NHE inhibition. Contrary to this, accumulated evidence (see recent reviews [5,6]) indicates that in intact hearts subjected to ischemia and reperfusion in vitro [7–10] and in vivo [11–15], NHE inhibitors are dramatically more effective when given prior to ischemia as opposed to during or shortly before reperfusion, an observation which is consistent with the original report of Karmazyn [16] on the cardioprotective potential of these agents. Furthermore, studies utilising nuclear magnetic resonance spectroscopy (which enables estimation of global [Na+]i) have shown that a substantial rise in [Na+]i occurs during ischemia and that this rise is markedly attenuated by amiloride analogues [17,18] as well as by HOE-642 [19]. Much of this evidence, which suggests that NHE activity is sustained during ischemia and that this activity contributes significantly to the myocardial injury that arises from ischemia and reperfusion, has not been acknowledged by Xiao and Allen [1]. It is noteworthy also that these investigators have themselves noted [3] a significant increase in [Na+]i during ischemia, when they paced their isolated rat heart model at the more physiological rate (for the rat) of 5 Hz (compared with 2 Hz in their recent study [1]); however, they have not reported the effect of NHE inhibition on [Na+]i under this condition. The third conclusion of the recent paper by Xiao and Allen [1] is based on the observed reduction in NHE activity (as assessed by the [Na+]i and pHi changes that occurred during exposure to Na+ lactate) in preconditioned hearts, and the absence of an additive effect of NHE inhibition with HOE-642 when this was combined with ischemic preconditioning. Whether the [Na+]i and pHi changes noted during exposure to Na+ lactate after 5 min of reperfusion provide an accurate estimation of NHE activity is unclear, because (1) under the conditions employed, the Na+/HCO3− symporter can also contribute to [Na+]i and pHi changes, and (2) the NHE is relatively inactive at non-acidic pHi and exposure to Na+ lactate produced little intracellular acidosis (e.g. in Fig. 5, pHi values after ‘initial acidosis’ are ≥7.2 in all three examples [1]). Furthermore, as noted by Fiolet [2], any reduction in NHE activity during reperfusion in preconditioned hearts may not necessarily be the underlying mechanism of the observed protection but may instead reflect the improved bioenergetic and ionic preservation of preconditioned myocardium. It is notable also that several earlier studies [8,20,21] have shown that NHE inhibition and ischemic preconditioning do indeed afford additive benefit, which is contrary to the findings of Xiao and Allen [1]. Only one of these studies (by Bugge and Ytrehus [8]) was acknowledged in the recent paper [1], and this was qualified with the statement that “by manipulating the conditions they could observe additive effects of preconditioning and NHE blockade”. The phrase “manipulating the conditions” presumably refers to the extension by Bugge and Ytrehus [8] of the ischemic period from 30 min (which produced infarct sizes of 37% in the control group and <7% with NHE inhibition or preconditioning alone, thus leaving no scope for detecting any additive benefit with their combination) to 45 min (which produced infarct sizes of 64% in the control group, 24% with preconditioning, 15% with NHE inhibition, and 4% with the combination of NHE inhibition and preconditioning). The balance of evidence that is available in this important and clinically relevant field therefore allows alternative conclusions to be drawn, which contradict those of Xiao and Allen [1]. These conclusions are that (1) NHE is active during ischemia and this activity is an important contributor to myocardial injury during ischemia and reperfusion, (2) NHE activity during reperfusion contributes to a much lesser extent to such injury, and (3) preconditioning is unlikely to afford protection by inhibiting NHE activity during reperfusion.