Enhanced release of nitric oxide causes increased cytotoxicity of S-nitroso-N-acetyl-dl-penicillamine and sodium nitroprusside under hypoxic conditions

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Abstract
S-Nitroso-N-acetyl-d l-penicillamine (SNAP) and sodium nitroprusside (SNP), both of which are known to release nitric oxide (•NO), exhibited cytotoxicity against cultivated endothelial cells. Under hypoxic conditions 5 mM SNAP and 20 mM SNP induced a loss in cell viability of about 90% and 80% respectively, after an 8 h incubation. Under normoxic conditions, cell death was only 45% and 42% respectively within the same time period. Concentrations of •NO liberated from SNAP and SNP were measured by the oxyhaemoglobin method and by two of the recently developed nitric oxide cheletropic traps (NOCTs). The •NO concentrations from SNAP and SNP increased from 74 µM and 28 µM to 136 µM and 66 µM respectively within 15 min of hypoxic incubation, and then decreased to 36 µM and 28 µM. In the respective normoxic incubations the •NO levels from SNAP and SNP remained in the region of about 30 µM and 20 µM respectively. In contrast, spermine/NO adduct (spermineNONOate) was shown to be more toxic under normoxic than under hypoxic conditions. Under either of these conditions, the concentration of •NO liberated from 2 mM spermineNONOate was about 20 µM. The results demonstrate that the cytotoxicity of SNAP and SNP, but not of spermineNONOate, is significantly enhanced under hypoxic compared with normoxic incubations. Studies on the •NO-releasing behaviour of these compounds indicate that the increased toxicity of SNAP and SNP under hypoxic conditions is related to the influence of O2 on the chemical processes by which •NO is produced from the precursors, rather than to an increased sensitivity of the hypoxic cells towards •NO.