Oxidative modification of low-density lipoproteins and the inhibition of relaxations mediated by endothelium-derived nitric oxide in rabbit aorta.

1. The mechanism by which Cu(2+)-oxidized low-density lipoproteins (oxLDL) inhibit acetylcholine (ACh)-evoked relaxations mediated by endothelium-derived nitric oxide (EDRF) in rabbit aortic rings was investigated. The proposed role of lysophosphatidylcholine (LPC) in the inhibition was also studied. 2. The kinetics of lipid peroxidation of native low-density lipoproteins (LDL) from individual donors, as measured by changes in conjugated diene concentration, were related to the inhibitory effects of the resultant oxLDL. It was found that the more susceptible LDL was to oxidation, the greater the inhibition. 3. No correlation was found between the inhibitory effects of oxLDL and LPC content. 4. Synthetic 1-palmitoyl LPC produced an inhibition of ACh-induced relaxations and when added to precontracted rings evoked nitric oxide-mediated endothelium-dependent relaxation. This latter effect was not elicited by oxLDL. 5. Synthetic 1-palmitoyl (10 microM) had no effect on relaxations evoked by glyceryl trinitrate in endothelium-denuded aortic rings in contrast to the inhibition found previously for oxLDL. 6. Concentrations of oxLDL and phospholipase A2-treated LDL which inhibited relaxation contained very different LPC concentrations. Unlike oxLDL, the inhibitory effects of phospholipase A2-treated LDL preparations were independent of the donors and showed no lag period. 7. We suggest that there are differences in the mechanisms by which oxLDL and 1-palmitoyl LPC exert their inhibitory effects on relaxation. 8. The inhibition of relaxation by oxLDL (1-2 mg protein ml-1) was prevented by the presence of high-density lipoproteins (HDL; 1-2 mg protein ml-1).9. It is proposed that prevention of the inhibition of relaxation by HDL is consistent with the inhibitory factor(s) being lipophilic constituents of oxLDL. However, variations in the inhibitory effects of oxLDL preparations are not due to differences in their LPC content and factors other than LPC must contribute to the inhibition.