Inhibition of NHE protects reoxygenated cardiomyocytes independently of anoxic Ca2+ 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 Ca2+ accu mulation or enhanced acidosis in cardiomyocytes. Additionally, the role of the Na+-HCO3- symporter (NBS) was investigated. Adult rat cardiomyocytes we re exposed to simulated ischemia and reoxygenation. Cytosolic pH [2', 7'- b is(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF)], Ca2+ (fura 2), Na+ [so dium-binding benzolfuran isophthatlate (SBFI)], and cell length were measur ed. NHE was inhibited with 3 mu mol/l HOE 642 or 1 mu mol/l 5-(N-ethyl-N-is opropyl)-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 con clude that inhibition of NHE during anoxia protects cardiomyocytes against reoxygenation injury independently of cytosolic acidification and Ca-i over load.