Impact of extracellular buffer composition on cardioprotective efficacy of Na+/H+ exchanger inhibitors

Abstract

There is controversy over whether the cardioprotective effects of Na+/H+ exchanger inhibitors are exerted primarily during ischemia or during subsequent reperfusion, possibly because of interstudy differences in experimental conditions. We studied the impact of perfusate buffer composition on the relative degree of protection afforded by Na+/H+ exchanger inhibition during ischemia vs. reperfusion. Isolated rat hearts (n = 8/group) were perfused (37 degrees C, 75 mmHg) with bicarbonate- or N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered medium and subjected to 20 min of global zero-flow ischemia and 45 min of reperfusion. One of two structurally distinct Na+/H+ exchanger inhibitors [5-(N,N-dimethyl)amiloride (DMA) or (3-methylsulfonyl-4-piperidinobenzoyl)guanidine methanesulfonate (HOE-694), 10 mumol/l] was transiently (5 min) infused 1) immediately before ischemia, 2) during initial reperfusion, or 3) during both of these periods. With bicarbonate-buffered medium, neither drug improved the postischemic recovery of left ventricular developed pressure (LVDP) when given only during reperfusion. In contrast, HOE-694 improved the postischemic recovery of LVDP from 39 +/- 5% in control to 66 +/- 6% (P < 0.05) when given before ischemia and from 33 +/- 4% in control to 65 +/- 4% (P < 0.05) when given before ischemia plus during reperfusion. With the latter protocol, the cardioprotective effect of HOE-694 occurred in a dose-dependent manner at 0.1-10 mumol/l. In contrast to the results with bicarbonate-buffered medium, in the presence of N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered medium, DMA and HOE-694 significantly improved recovery of LVDP (from 34 +/- 5% in controls to 56 +/- 3 and 71 +/- 8%, both P < 0.05) when given only during reperfusion. They also provided significant protection when given before ischemia or before ischemia plus during reperfusion; with the latter protocol, HOE-694 produced an almost complete recovery of LVDP (88 +/- 9 vs. 30 +/- 7% in controls, P < 0.05). In conclusion, our results suggest that the influence of Na+/H+ exchanger activity during reperfusion on the extent of functional recovery is modulated significantly by perfusate buffer composition. As a consequence, the cardioprotective efficacy of Na+/H+ exchanger inhibitors may be overestimated under bicarbonate-free conditions.

A unique pathway of cardiac myocyte death caused by hypoxia–acidosis

“…Increased activity of the Na + /H + exchanger can cause Ca 2+ overload because the elevated intracellular Na + is subsequently exchanged for Ca 2+ via the Na + /Ca 2+ exchanger (Pierce and Czubryt, 1995). Inhibition of Na + /H + exchange has been shown to protect against ischemic injury, possibly by preventing this increase in Ca 2+ (Bond et al, 1993;Shimada et al, 1996). Conversely, inhibition of the vacuolar ATPase promotes apoptosis, in part by shifting the proton load towards the Na + /H + transporter, thereby increasing Ca 2+ uptake, and in part by reducing the myocyte capacity to control intracellular pH (Gottlieb et al, 1996;Karwatowska-Prokopczuk et al, 1998;Long et al, 1998).…”

Na + /H + Exchange Inhibition With HOE642 Improves Postischemic Recovery due to Attenuation of Ca 2+ Overload and Prolonged Acidosis on Reperfusion

“…This finding is consistent with previous reports that provide evidence that Na ϩ /H ϩ exchange inhibitors are more effective when given before ischemia rather than at the time of reperfusion. 27,28 …”

KR-32570, a novel Na+/H+ exchanger-1 inhibitor, attenuates hypoxia-induced cell death through inhibition of intracellular Ca2+ overload and mitochondrial death pathway in H9c2 cells

“…On the basis of previous reports that the acylguanidine moiety is a pharmacophore of NHE-1 inhibitors (Shimada et al, 1996), we have focused on finding novel NHE-1 inhibitory guanidine analogues. In such attempts, we found that a new guanidine derivative, [5-(2-methoxy-5-chloro-5-phenyl)furan-2-ylcarbonyl]guanidine (KR-32570) showed a greater potency against NHE-1 activity compared to cariporide, and markedly improved cardiac contractile function in isolated rat heart ischemic model (Lee et al, 2005a,b).…”