Blockade of the Na+-Ca2+ exchanger is more efficient than blockade of the Na+-H+ exchanger for protection of the myocardium from lethal reperfusion injury.

Since the Na(+)-H(+) exchanger (NHE) is not the only pathway of Na(+) influx into cardiomyocytes during ischemia/reperfusion, we hypothesized that blockade of Na(+)-Ca(2+) exchanger (NCX) may be a more efficient strategy than is NHE inhibition for protecting the myocardium from infarction. To test this hypothesis, we compared KB-R7943 (KBR), a novel selective NCX blocker, with cariporide, a selective NHE blocker, with regard to their protective effects against infarction. In isolated rabbit hearts, infarction was induced by 30-min global ischemia/2-h reperfusion, and infarct size was determined by tetrazolium staining and expressed as a percentage of area at risk (%IS/AR). Hearts received no drugs, or were infused with cariporide (1 microM) for 10 min or KBR (0.3 or 10 microM) for 5 min before ischemia or after the onset of reperfusion. Protein level of NCX was assessed by Western blotting. Cariporide infusion before ischemia significantly reduced %IS/AR from 63.9 +/- 2.9% to 20.2 +/- 3.0%, but its infusion upon reperfusion failed to achieve a significant protection (%IS/AR = 53.8 +/- 4.6%). In contrast, KBR infusion similarly reduced infarct size both when infused before ischemia (%IS/AR = 33.3 +/- 6.3% and 21.9 +/- 4.7% by 0.3 and 10 microM KBR, respectively) and when infused for only 5 min after reperfusion (%IS/AR = 35.3 +/- 7.1% and 31.5 +/- 2.1% by 0.3 and 10 microM KBR, respectively). Protein levels of NCX after 30-min ischemia and 30-min ischemia/30-min reperfusion were similar to baseline values in both untreated controls and hearts treated with 0.3 microM KBR upon reperfusion. These results suggest that lethal reperfusion injury is more efficiently suppressed by blockade of the NCX than by blockade of the NHE.