Background—The basis for progressive attenuation of the effects of organic nitrates during long-term therapy (nitrate tolerance) remains controversial; proposed mechanisms include impaired nitrate bioconversion resulting in decreased release of nitric oxide (NO) from nitrates and/or increased NO clearance through a reaction with incrementally generated superoxide (O₂^–).

Methods and Results—Patients undergoing elective coronary artery bypass were randomized to receive 24 hours of intravenously infused nitroglycerin (NTG; nitrate group) or no nitrate therapy (control group). Discarded segments of the internal mammary artery and saphenous vein were used to examine (1) vascular responsiveness to NTG, sodium nitroprusside, and the calcium ionophore A23187; (2) bioconversion of NTG to 1,2- and 1,3-glyceryl dinitrate; and (3) the generation of O₂^–. Responses to NTG were reduced 3- to 5-fold in vessels from the nitrate group compared with control vessels (P<0.01 for both types of segments), whereas responses to sodium nitroprusside and A23187 were unchanged. Tissue content of 1,2-glyceryl dinitrate was lower (P=0.012) in the saphenous veins from the nitrate group than in those from the control group. O₂^– generation was greater (P<0.01) in internal mammary artery samples from the nitrate group than in those from the control group. However, incremental O₂^– generation induced by an inhibitor of superoxide dismutase did not affect NTG responses.

Conclusions—NTG tolerance in patients with coronary artery disease is nitrate-specific and is associated with evidence of impaired NTG bioconversion. Tolerance was associated with incremental O₂^– generation, but short-term elevation of O₂^– did not affect NTG responsiveness, suggesting increased NO clearance by O₂^– has a minimal contribution to tolerance.