Lactosylceramide (LacCer) is a member of the glycosphingolipid family, which has been implicated in the atherogenic process. The goal of this study was to determine the effects and molecular mechanisms of LacCer on endothelial functions in porcine coronary arteries and human coronary endothelial cells (HCAECs). The vessel rings and HCAECs were treated with different concentrations of LacCer for 24 hours. Vasomotor function was studied using a myograph tension system in response to thromboxane A2 analog U46619, bradykinin and sodium nitroprusside (SNP). Superoxide anion production was determined using lucigenin-enhanced chemiluminescence. The expression of endothelial nitric oxide synthase (eNOS), NADPH oxidase subunit NOX4 and catalase was determined by real-time PCR. LacCer (0.1, 1 and 10 microM) significantly decreased endothelium-dependent vasorelaxation (bradykinin) in porcine coronary artery rings in a concentration-dependent manner compared with untreated controls (P<0.05). High concentration of LacCer (10 microM) also reduced endothelium-independent vasorelaxation (SNP). However, LacCer did not affect vessel contraction (U46619). Antioxidant selenomethionine (SeMet) effectively reversed LacCer-induced endothelial dysfunction in the vessel rings. Furthermore, LacCer significantly increased superoxide anion production in the vessel rings in a concentration-dependent manner compared with untreated controls (P<0.05). In response to LacCer treatment, NOX4 mRNA levels were significantly increased, while the expression of catalase and eNOS was significantly decreased in HCAECs compared with controls (P<0.05). LacCer causes endothelial dysfunction with potential mechanisms of the down-regulation of eNOS and increase of oxidative stress due to the activation of NADPH oxidase and inhibition of internal antioxidant catalase. This study suggests that LacCer may represent a risk factor to the vascular system and antioxidant SeMet may have clinical applications for prevention of vascular disease.