EFFECTS OF NATIVE AND OXIDIZED LOW-DENSITY LIPOPROTEINS ON FORMATION AND INACTIVATION OF ENDOTHELIUM-DERIVED RELAXING FACTOR

The influence of native (N-) and oxidized (Ox-) low density lipoproteins (LDLs) on endothelium-dependent vasomotion is still controversial. We investigated the short-term effects of N-LDL and Ox-LDL on the formation of endothelium-derived relaxing factor (EDRF) in native and cultured endothelial cells and on its inactivation after release from the cells. N-LDL was isolated from fresh human plasma via sequential ultracentrifugation and oxidized by incubation with Cu2+. EDRF released from cultured endothelial cells was inactivated by both N-LDL and Ox-LDL (1 mg/ml) as detected in a bioassay system. N-LDL reduced the EDRF-mediated vasodilations of the detector segments by 38.5 +/- 5.3%, and Ox-LDL, by 55.5 +/- 4.6%. The effects of lipoproteins on EDRF formation were studied in cultured endothelial cells preincubated with either N-LDL or Ox-LDL (1 mg/ml for 1 hour) and stimulated for EDRF release with bradykinin after washout of the lipoproteins. EDRF was assessed by measuring its stimulatory effect on the activity of a purified, soluble guanylate cyclase. Both N-LDL and Ox-LDL did not reduce the bradykinin-induced EDRF formation. Consistent with this finding, acetylcholine-induced, EDRF-mediated dilations of intact rabbit femoral artery segments were not impaired by luminal exposure to N-LDL or Ox-LDL (1 mg/ml for 1 hour). However, these relaxations were significantly reduced by preincubation of aortic ring preparations with the same concentrations of the same charges of N-LDL and Ox-LDL. In conclusion, neither N-LDL nor Ox-LDL acutely impairs the formation of EDRF but does inactivate EDRF after its release from endothelial cells. This mechanism may be important in regions with accumulation of lipoproteins in the arterial wall and may favor the development of inappropriate vasoconstruction.