NITROBLUE TETRAZOLIUM INHIBITS OXIDATION OF GLYCERYL TRINITRATE TO NITRIC-OXIDE IN BOVINE AORTIC SMOOTH-MUSCLE CELLS

The effects of nitroblue tetrazolium (NBT), a well-known scavenger of superoxide anions and an inhibitor of nicotinamide adenine dinucleotid e (NADPH)-dependent oxidations, were assessed on the metabolism of gly ceryl trinitrate (GTN) to nitric oxide (NO) by bovine aortic smooth mu scle cells (SMC). The extent of this metabolism was determined by meas uring NO formed, using the inhibition of thrombin-induced platelet agg regation and relaxation of rabbit aortic strips as bioassay systems. I n addition, NO produced from GTN by SMC was measured as nitrite (NO2-) , one of its breakdown products. The antiplatelet effect of GTN (44 mu M) was potentiated by SMC (0.12-0.46 x 10(5) cells) treated with indo methacin (10 mu M) and this was inhibited in a concentration-dependent manner when the cells were pretreated with NBT (100 mu M). NBT (3-100 mu M) also reduced the formation of NO2- from GTN (600 mu M) by SMC ( 3 x 10(5) cells). Furthermore, relaxations of endothelium-denuded stri ps of the rabbit aorta by GTN (10(-9)-10(-6) M) were attenuated when t he strips were pretreated with NBT (100 or 500 mu M). The formation of NO from L-arginine (L-Arg) by SMC was not affected by NBT. The hypote nsive responses to GTN (0.25-1 mg/kg, i.v.) in anaesthetized rats were inhibited by pretreatment with NBT (1.25 mg/kg, i.v.) but NBT did not alter the hypotensive responses induced by SIN-1. Thus, NBT inhibited the bioconversion of GTN to NO both in vitro and in vivo. NBT may be a useful pharmacological tool to investigate the enzymic pathway(s) in volved in the conversion of GTN to NO by smooth muscle cells or other cells.