Growth-related oncogene alpha (GRO-) is a member of the CXC chemokine family, which is involved in the inflammatory process including atherosclerosis. We hypothesized that GRO- may affect endothelial functions in both porcine coronary arteries and human coronary artery endothelial cells (HCAECs). Vasomotor function was analyzed in response to thromboxane A2 analogue U46619 for contraction, bradykinin for endothelium-dependent vasorelaxation, and sodium nitroprusside (SNP) for endothelium-independent vasorelaxation. In response to 10^-6 M bradykinin, GRO- (50 and 100 ng/ml) significantly reduced endothelium-dependent vasorelaxation by 34.73% and 48.8%, respectively, compared with controls (P < 0.05). There were no changes in response to U46619 or SNP between treated and control groups. With the lucigenin-enhanced chemiluminescence assay, superoxide anion production in GRO--treated vessels (50 and 100 ng/ml) was significantly increased by 50% and 86%, respectively, compared with controls (P < 0.05). With real time PCR analysis, endothelial nitric oxide synthase (eNOS) mRNA levels in porcine coronary arteries and HCAECs after GRO- treatment were significantly decreased compared with controls (P < 0.05). The eNOS protein levels by both immunohistochemistry and western blot analyses were also decreased in GRO--treated vessels. Antioxidant seleno-L-methionine and anti-GRO- antibody effectively blocked these effects of GRO- on both porcine coronary arteries and HCAECs. In addition, GRO- immunoreactivity was substantially increased in the atherosclerotic regions compared to non-atherosclerotic regions in human coronary arteries. Thus, GRO- impairs endothelium-dependent vasorelaxation in porcine coronary arteries through a mechanism of over-production of superoxide anion and downregulation of eNOS. GRO- may contribute to human coronary artery disease.