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Figure 3 Effects of three-day NTG treatment of rabbits (continuous or 12 h patch on/patch off) on vascular O₂^•– production of rabbit aortic tissue with an intact endothelium. Superoxide production was measured under control conditions (C) and after the administration of L-NMA (10^–3 mol/liter, 30-min incubation time). In vessels from control animals, L-NMA increased vascular steady-state O₂^•– levels, indicating that NO produced in endothelial cells quenches baseline O₂^•–-induced LDCL. In nitrate-tolerant tissue, basal O₂^•– production was markedly elevated. Here, L-NMA paradoxically decreased LDCL, indicating that NOS III represents a significant O₂^•– source. A nitrate-free interval reduced O₂^•– levels and restored the LDCL-increasing effect of L-NMA, indicating that intermittent therapy can prevent the uncoupling of NOS III. Data are presented as the mean values ± SEM of five to 10 experiments. * Indicates p < 0.05 versus C; indicates p < 0.05 versus NTG continuous.