Production of free NO from nitrite in tissue homogenates from rat and sheep arteries. A: injection at time zero of 100 μl of tissue homogenate (4 μg protein/μl, n = 8 for each tissue type) into 20 ml of PBS (pH 6.2) containing 100 μM nitrite resulted in release of detectable amounts of NO into argon gas being continuously purged through the buffer. B: cumulative NO production in rat aorta (RA) homogenates was greater than that in sheep femoral artery (SF) homogenates. Heme oxidizer [1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one] (ODQ; 10 μM) significantly inhibited NO production in both homogenates (n = 4 for rat and 6 for sheep), whereas xanthine oxidase inhibitor oxypurinol (100 μM, n = 4 for rat and 7 for sheep) and aldehyde dehydrogenase inhibitor daidzin (0.5 μM, n = 3 for rat and 7 for sheep) did not. *Significant difference between rat and sheep (P < 0.05, 1-way ANOVA); **significant effect of ODQ (P < 0.05, 1-way ANOVA).