title: Overexpression of the Na+/H+ exchanger and ischemia-reperfusion injury in the myocardium.
abstract: In the myocardium, the Na(+)/H(+) exchanger isoform-1 (NHE1) activity is detrimental during ischemia-reperfusion (I/R) injury, causing increased intracellular Na(+) (Na(i)(+)) accumulation that results in subsequent Ca(2+) overload. We tested the hypothesis that increased expression of NHE1 would accentuate myocardial I/R injury. Transgenic mice were created that increased the Na(+)/H(+) exchanger activity specifically in the myocardium. Intact hearts from transgenic mice at 10-15 wk of age showed no change in heart performance, resting intracellular pH (pH(i)) or phosphocreatine/ATP levels. Transgenic and wild-type (WT) hearts were subjected to 20 min of ischemia followed by 40 min of reperfusion. Surprisingly, the percent recovery of rate-pressure product (%RPP) after I/R improved in NHE1-overexpressing hearts (64 +/- 5% vs. 41 +/- 5% in WT; P < 0.05). In addition, NMR spectroscopy revealed that NHE1 overexpressor hearts contained higher ATP during early reperfusion (levels P < 0.05), and there was no difference in Na(+) accumulation during I/R between transgenic and WT hearts. HOE642 (cariporide), an NHE1 inhibitor, equivalently protected both WT and NHE1-overexpressing hearts. When hearts were perfused with bicarbonate-free HEPES buffer to eliminate the contribution of HCO(3)(-) transporters to pH(i) regulation, there was no difference in contractile recovery after reperfusion between controls and transgenics, but NHE1-overexpressing hearts showed a greater decrease in ATP during ischemia. These results indicate that the basal activity of NHE1 is not rate limiting in causing damage during I/R, therefore, increasing the level of NHE1 does not enhance injury and can have some small protective effects.
authors.affiliation: Laboratory of Signal Transduction, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina, USA.
authors.forename: Kenichi
authors.initials: K
authors.lastname: Imahashi
authors.forename: Fatima
authors.initials: F
authors.lastname: Mraiche
authors.forename: Charles
authors.initials: C
authors.lastname: Steenbergen
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Murphy
authors.forename: Larry
authors.initials: L
authors.lastname: Fliegel
mesh_terms: D000818:Animals; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008822:Mice, Transgenic; D015428:Myocardial Reperfusion Injury; D009206:Myocardium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation; D014883:Water-Electrolyte Imbalance / Q000150:complications / Q000378:metabolism*
chemical_list: D017923:Sodium-Hydrogen Exchangers; D012964:Sodium