Protection of reoxygenated cardiomyocytes against hypercontracture by inhibition of Na+/H+ exchange

Effects of Na+/H+ exchange inhibition and cytosolic acidosis on reoxygenated adult rat ventricular cardiomyocytes were investigated. Cells were incubated in anoxic media at pH 6.4 until pCa of < or = 5, intracellular pH (pHi) of 6.5, and cytosolic Na+ of 50 mM were reached. On reoxygenation, medium pH was changed to 7.4 to activate Na+/H+ exchange. In one group, 20 microM HOE-694, an inhibitor of Na+/H+ exchange, was added. With or without HOE-694, cytosolic Ca2+ and Na+ returned to control levels within 10 min of reoxygenation. In the absence of HOE-694, the pHi renormalized (to 7.2) within 8 min, but irreversible hypercontracture and transient Ca2+ oscillations were observed. In the presence of HOE-694, pHi stayed acidotic (at 6.5), hypercontracture was prevented, and Ca2+ oscillations were attenuated. When the Na+ pump was inhibited with 0.1 mM ouabain, even partial recovery of Ca2+ control became impossible unless HOE-694 was added. Our conclusions are 1) activation of Na+/H+ exchange does not impair recovery of cytosolic Na+ and Ca2+ control unless activity of the sarcolemmal Na+ pump is critically reduced, and 2) due to prolongation of cytosolic acidosis, inhibition of Na+/H+ exchange protects against reoxygenation-induced hypercontracture and cytosolic Ca2+ oscillations.