The Inhibition of Endothelium-Dependent Vasorelaxation by Oxidized Low Density Lipoprotein and Lysophosphatidylcholine
The vascular endothelium produces endothelium-derived relaxing factor(s) which regulate(s) vascular tone. In atherosclerotic arteries, endothelium-dependent relaxation is markedly reduced; this impairment is thought to play an important role in the pathogenesis of coronary spasm. The purpose of this study was to evaluate the effects of native and modified low density lipoprotein (LDL) on endothelium-dependent relaxation and to clarify the mechanism underlying the inhibitory effects of oxidized LDL. LDL(density, 1.020–1.060) was isolated by ultracentrifugation from freshly-harvested normal human plasma. Phospholipase A2-treated LDL and copper-oxidized LDL were prepared by the method of Quinn et al. For recording isometric force, rabbit aortic strips equilibrated with oxygenated Kreb’s buffer were precontracted with phenylephrine. We measured phosphoinositide hydrolysis by a 3H inositol labelling method and cytosolic Ca2+ level by using fura-2 fluorescence spectroscopy in cultured bovine aortic endothelial cells. Preincubation of the strips with native LDL(0.5–2 mg protein/ml) had no effect on acetylcholine(Ach)- induced endothelium-dependent relaxation. Oxidized LDL(0.1–0.5 mg protein/ml) and phospholipase A2-treated LDL (0.01–0.1 mg protein/ml) inhibited this relaxation. Native and modified LDL did not inhibit nitroglycerin-induced relaxation. Lysophosphatidylcholine(LPC) extracted from modified LDL by thin layer chromatography inhibited Ach-induced relaxation.