Oxygen Radical Scavenger, Protects Isolated Cardiac Myocytes From Hypoxic Injury by Inhibition of Na+-Ca21 Exchange and Not by Its Antioxidant Effects

mm Hg) glucose-free buffer alone (n=50) or with the addition of the oxygen radical scavengers 1,3-dimethyl-2-thiourea (DMTU, 25 mM, n=46), human recombinant superoxide dismutase (SOD, 1,000 units/ml, n=40), or the combination of these agents (n=41). All cells responded by undergoing contracture to a rigor form. Hypoxia was then continued for a second period (T2), the duration of which correlates inversely with survival. After reoxygenation, cells either retained their rectangular shape (survival) or hypercontracted to a rounded form (death). For the group of cells with a T2 period >30 minutes, no cell exposed to buffer alone (n=20) or to SOD (n =16) survived, in contrast to 15 of 24 (63%) cells exposed to DMTU. The addition of SOD to DMTU offered no advantage to DMTU alone. The protective effect of DMTU was not observed when it was added at reoxygenation, suggesting that this agent has an important effect during the hypoxic period when intracellular Ca2' is known to rise, most likely because of the reversal of Na+-Ca2' exchange. Therefore, the effects ofDMTU on Ca2' regulation (indexed by indo-1 fluorescence) during hypoxia were studied. DMTU significantly blunted the [Ca2'] rise during the hypoxic period. When normoxic, electrically stimulated cells were exposed to this agent, they displayed