Nitric oxide-dependent relaxation of explanted pig vessel segments and vascular myeloperoxidase deposition. (A) Relaxation of explanted segments of the internal mammary artery in response to acetylcholine. There was a significant attenuation of acetylcholine-dependent relaxation in vessel segments from animals treated with myeloperoxidase (56.9 ± 3% in myeloperoxidase-treated pigs vs. 89.4 ± 4% in rings from human serum albumin-treated animals, P < 0.01). Addition of H₂O₂ further decreased acetylcholine-dependent vasorelaxation in rings explanted from myeloperoxidase-treated animals (45.4 ± 4%, *P = 0.021), whereas vessel segments from human serum albumin-treated animals showed no H₂O₂-dependent impairment of vessel relaxation (89.5 ± 5%). (B) Nitroglycerine-induced relaxation was significantly reduced in explanted segments of the internal mammary artery derived from myeloperoxidase-treated animals (P < 0.001). (C) Heparin-dependent liberation of myeloperoxidase after ex vivo perfusion of explanted pig internal mammary artery. Vessels from myeloperoxidase-treated animals revealed increased levels of myeloperoxidase in the eluate when compared with human serum albumin-treated pigs. (D) Immunofluorescent imaging displayed vascular myeloperoxidase deposition in explanted internal mammary artery segments of myeloperoxidase-treated pigs when compared with human serum albumin-treated animals (red, PECAM; green, MPO; blue, DAPI; magnification ×200).