Na/H exchange inhibition protects newborn heart from ischemia/reperfusion injury by limiting Na+-dependent Ca2+ overload

Abstract

The results of the Guardian/Expedition trials demonstrate the need for more precisely controlled studies to inhibit Na/H exchange (NHE1) during ischemia/reperfusion. This is because overwhelming evidence is consistent with the hypothesis that myocardial ischemic injury results in part from increases in intracellular Na (Nai) mediated by NHE1 that in turn promote Na/Ca exchanger-mediated increases in intracellular Ca ([Ca]i) and Ca-dependent cell damage. We used a more potent and specific NHE1 inhibitor HOE 694 (HOE) to test whether inhibition of NHE1 during ischemia limits increases in Nai and [Ca]i in newborns. NMR was used to measure pHi, Nai, [Ca]i, and ATP in isolated newborn rabbit hearts. Perfusion pressure, left ventricular developed pressure, and creatine kinase were measured. HOE was added before global ischemia. Results are reported as mean +/- SE. Nai (mEq/kg dry weight) rose from 11.6 +/- 0.9 before ischemia to 114.0 +/- 16.1 at the end of ischemia and recovered to 55.2 +/- 11.8 in the control group. During ischemia and reperfusion, the corresponding values for Nai in the HOE group (63.1 +/- 8.4 and 15.9 +/- 2.5, respectively, P < 0.05) were lower than control. In the control group [Ca]i (nM/L) rose from 331 +/- 41 to 1069 +/- 71 and recovered to 814 +/- 51, whereas in the HOE group [Ca]i rose less (P < 0.05): 359 +/- 50, 607 +/- 85, and 413 +/- 40, respectively. Total creatine kinase release was significantly reduced in the HOE group. Perfusion pressure and left ventricular developed pressure also recovered significantly better in the HOE group than in the control. In conclusion, NHE1 inhibition diminishes ischemia-induced increases in Nai and therefore [Ca], and thus diminishes myocardial injury in neonatal hearts.

Figure 1

In newborn rabbit hearts, the NHE inhibitor HOE694 (5 μM) decreases accumulated intracellular Na during ischemia and reperfusion. Intracellular Na content (mEq/kg dry weight) is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles), and without any treatment (open squares). * p<0.05.

Figure 2

The cytosolic Ca accumulation was significantly decreased in the HOE694 (5 μM) treated hearts during ischemia and reperfusion. Cytosolic Ca concentration (nM) is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles) and without HOE694 (open squares). * p<0.05.

Figure 3

The NHE inhibitor HOE694 (5 μM) decreases intracellular proton concentration at the end of reperfusion. Intracellular pH is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles) and without HOE694 (open squares).

Figure 4

In newborn rabbit hearts, the NHE inhibitor HOE694 (5 μM) improves ATP recovery during reperfusion. Myocardial ATP (% of control) is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles) and without HOE694 (open squares). * p<0.05.

Figure 5

In newborn rabbit hearts, the NHE inhibitor HOE694 (5 μM) limits the increase in perfusion pressure (a linear function of coronary resistance in this model) during reperfusion. Perfusion pressure (% of control) is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles) and without HOE694 (open squares). * p<0.05.

Figure 6

During reperfusion LVDP was significantly greater in the HOE treated hearts than the control hearts. LVDP (% of control) is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles) and without HOE694 (open squares). * p<0.05.

Figure 7

Myocardial CK release was significantly reduced during reperfusion in HOE treated hearts compared to untreated hearts. CK (IU/g dry weight) is plotted vs. time before, during, and after ischemia with HOE694 (closed triangles) and without HOE694 (open squares). *p<0.05