IMPAIRMENT OF ENDOTHELIAL FUNCTION INDUCED BY GLYC-OXIDIZED LIPOPROTEIN A [LP(A)]

Diabetic patients develop endothelial dysfunction early in the course of the disease. Atherogenic lipoproteins such as LDL and Lp(a) are imp ortant risk factors for endothelial dysfunction and undergo nonenzymat ic glycation in hyperglycaemia. Here we assessed whether glycation of Lp(a) potentiates its damaging influence on endothelial function. Huma n Lp(a) was glycated by dialyzation for 7 days against buffer containi ng 200 mmol/l glucose, or sham-treated without glucose and oxidized by incubation with Cu++. The degree of glycation accounted to 32 +/- 4%, and glycation rendered Lp(a) more susceptible to oxidative modificati on when exposed to Cu++. Isolated rings of rabbit aorta were superfuse d with physiological salt solution, and isometric tension was recorded . Incubation of the aortic rings with sham-treated or with 30 mu g/ml glycated Lp(a), not oxidized, had no influence on acetylcholine-induce d, endothelium-dependent relaxation. Exposure of the aortic rings to 3 0 mu g/ml oxidized non-glycated (ox) Lp(a) caused a significant inhibi tion (19% at 1 mu M acetylcholine) of the endothelium-dependent relaxa tion. Incubation of aortic rings with 30 mu g/ml oxidized glycated (gl yc-ox) Lp(a) attenuated endothelium-dependent relaxation more potently than oxLp(a) (by 34% at 1 mu M acetylcholine). The presence of diethy l-dithio-carbamate (DDC), an inhibitor of the endogenous superoxide di smutase (SOD), potentiated the inhibition of relaxation induced by oxL p(a) and by glyc-oxLp(a) [38% inhibition at I mu M acetylcholine for o xLp(a), and 49% inhibition at 1 mu M acetylcholine for glyc-oxLp(a)]. Go-incubation with the Oi scavenger 4,5-dihydroxy-1,3-benzene disulfon ic acid disodium salt (TIRON) prevented the inhibition of relaxation b y the oxidized lipoproteins, suggesting that enhanced NO-inactivation by O-2(-) could be the underlying mechanism for the impairment of endo thelium-dependent dilations by ox- and glyc-oxLp(a). The concentration of lysophosphatidycholine, a lipoprotein oxidation product and stimul us for O-2(-) formation, was significantly enhanced in oxLp(a) and in glyc-oxLp(a) compared to native lipoproteins. Conclusion: Glycation en hances the endothelium-damaging influence of oxLp(a), presumably by en hancing oxidative stress. The likely mechanism for attenuation of endo thelium-dependent dilations is increased formation of Oi, resulting in inactivation of nitric oxide. This mechanism may play an important ro le in diabetic patients and may contribute to disturbed organ perfusio n.