Nitroxides are known to exert superoxide dismutasemimetic properties and to decrease O 2 . -and H 2 O 2 -mediated cytotoxicity. However, the effect of nitroxides on ⅐ NO homeostasis has not been studied yet. The present study investigates the effect of nitroxides on the detectable amount of ⅐ NO released by 3-morpholinosydnonimine (SIN-1) and cultured endothelial cells. Cultured bovine aortic and atrial endothelial cells stimulated with 10 M A23187 released a stable flux of ⅐ NO, as detected by

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... ⅐ NO chemiluminescence. Addition of 100 units/ml SOD or 10 M of the nitroxides 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL), 3-carboxy-proxyl, and 3-ethoxycarbonyl-proxyl, increased the chemiluminescence signal. The effect of these nitroxides on the amount of ⅐ NO released from cell monolayers was dose-dependent, with the highest efficacy between 30 and 100 M. EPR spin trapping in SIN-1 solutions revealed the formation of ⅐ OH adducts from spontaneous dismutation of O 2 . and concomitant reaction with H 2 O 2 . Both SOD and TEMPOL increased the signal intensity of the ⅐ OH adduct by accelerating the dismutation of O 2 . . The results of this study demonstrate that the SOD-mimetic activity of nitroxides increases the amount of bioavailable ⅐ NO in vitro.