Inhibition of NHE protects reoxygenated cardiomyocytes independently of anoxic Ca(2+) overload and acidosis

We investigated the question of whether inhibition of the Na(+)/H(+) exchanger (NHE) during ischemia is protective due to reduction of cytosolic Ca(2+) accumulation or enhanced acidosis in cardiomyocytes. Additionally, the role of the Na(+)-HCO(3)(-) symporter (NBS) was investigated. Adult rat cardiomyocytes were exposed to simulated ischemia and reoxygenation. Cytosolic pH 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF)], Ca(2+) (fura 2), Na(+) sodium-binding benzolfuran isophthatlate (SBFI)], and cell length were measured. NHE was inhibited with 3 micromol/l HOE 642 or 1 micromol/l 5-(N-ethyl-N-isopropyl)-amiloride (EIPA), and NBS was inhibited with HEPES buffer. During anoxia in bicarbonate buffer, cells developed acidosis and intracellular Na and Ca (Na(i) and Ca(i), respectively) overload. During reoxygenation cells underwent hypercontracture (44.0 +/- 4.1% of the preanoxic length). During anoxia in bicarbonate buffer, inhibition of NHE had no effect on changes in intracellular pH (pH(i)), Na(i), and Ca(i), but it significantly reduced the reoxygenation-induced hypercontracture (HOE: 61.0 +/- 1.4%, EIPA: 68.2 +/- 1.8%). The sole inhibition of NBS during anoxia was not protective. We conclude that inhibition of NHE during anoxia protects cardiomyocytes against reoxygenation injury independently of cytosolic acidification and Ca(i) overload.