Although it is established that highly oxidized LDL modify both vasodilator and vasoconstrictor responses in normal and atherosclerotic arterial tissue, there is a paucity of data on the relationship between the degree of the oxidative modification of LDL and vasomotor response. We therefore compared the impact of native LDL (Nat-LDL), and of partially (P-oxLDL), of moderately (M-oxLDL) and of highly oxidized LDL (H-oxLDL) on the vasomotor response of isolated human internal mammary artery and of rat thoracic aorta. Copper-mediated oxidative modification for up to 24 h at 37 degrees C was characterised by a progressive increase in the net negative electrical charge of LDL, and in the content of oxysterols; by contrast, lipid hydroperoxide and TBARS content peaked in M-oxLDL at 6 h. Neither basal vascular tone nor vasoconstriction induced by KCl (100 mmol/l) were modified significantly in arterial segments in relation to the degree of LDL oxidation. While Nat-LDL did not modify the contractile response of rat aorta to norepinephrine, increase in the degree of oxidative modification of LDL progressively and significantly shifted the norepinephrine response curve to the right (EC50 values for Nat-LDL, M-oxLDL and H-oxLDL: 1.2+/-0.5x10(-8), 3.5+/-1x10(-7), 1.3+/-0.4x10(-6) mol/l respectively) with reduction in the maximal effect (74.5+/-12.2 and 100.1+/-6.2% for H-oxLDL and M-oxLDL respectively, P < 0.05 versus controls). Similar findings were made in human arteries treated with H-oxLDL (P < 0.05 for EC50 and maximal response versus controls). The acetylcholine-induced, endothelial-dependent relaxation of rat aortic segments was significantly and progressively impaired with increase in the degree of LDL oxidation, maximal relaxation with H-oxLDL being 3-fold less (P < 0.05) than Nat-LDL at the same protein concentration (100 microg/ml). Acetylated LDL was without effect. Our data indicate that the increase in the degree of copper-mediated, oxidative modification of LDL parallels progressive reduction in the vasomotor response of the arterial wall to norepinephrine-induced contraction and to acetylcholine-induced relaxation subsequent to precontraction. Our data are consistent with the hypothesis that the major oxysterols (7-ketocholesterol, 7beta-hydroxycholesterol) present in Ox-LDL underlie such effects.