The effect of oxidative stress on endothelium-dependent and nitric oxide donor-induced relaxation: implications for nitrate tolerance. 
Increased inactivation of nitric oxide (NO) by superoxide has been implicated in nitrate tolerance.  Here, we set out to compare the inhibitory effect of superoxide on endothelium-dependent, acetylcholine (ACh)-mediated vascular relaxation with that on the endothelium-independent effects of glyceryl trinitrate (GTN) and another NO donor drug, S-nitrosoglutathione (GSNO).  Rings of thoracic aorta from adult male Wistar rats (350-450 g) were precontracted with phenylephrine (approximately EC(90)) prior to cumulative additions (10 nM/L-10 microM/L) of GTN, GSNO, or ACh.  Rings were then treated with the superoxide generator pyrogallol (300 micromol/L) alone or following pretreatment with the Cu/Zn superoxide dismutase inhibitor diethyldithiocarbamate (DETCA; 100 micromol/L), and cumulative additions of the vasodilators were repeated.  All experiments were conducted in the presence of catalase (3000 U/ml) to prevent accumulation of hydrogen peroxide.  Relaxation to ACh was abolished by pyrogallol-derived superoxide.  Relaxation to GSNO was significantly inhibited by superoxide (P < 0.05, n = 8) and was more pronounced at lower GSNO concentrations.  However, GTN was relatively resistant to inhibition by superoxide with modest inhibition only occurring in rings pretreated with DETCA prior to pyrogallol (P < 0.05; n = 8).  In contrast to GSNO, the inhibitory effect was more pronounced with high concentrations of GTN, suggesting that the mechanism underlying superoxide-mediated inhibition is different for the two NO donor drugs.  Further experiments showed that vascular responses to ACh were not inhibited (P > 0.05, n = 6) in aortic rings made tolerant to GTN (10 micromol/L, 2-h incubation) and that treatment of vessels with the antioxidant vitamin C (1 mmol/L) successfully prevented the development of tolerance.  Taken together, these results suggest that superoxide is not a major factor in tolerance in vitro and imply that the protective actions of vitamin C are unrelated to its antioxidant activity in this setting.