Oxidized low density lipoprotein (LDL) and platelet intracellular calcium: Interaction with nitric oxide

The present study tested the effects of ox-low density lipoprotein (LDL) on nitric oxide (NO)-dependent decrease in agonist-stimulated [Casup 2sup +](i). The effects of ox-LDL on platelet aggregation were also evaluated. Platelets loaded with FURA 2 AM (2 mumol/litre) were incubated with NO-donors for 2-10 min to obtain a 40-50% reduction in [Casup 2sup +](i) and with NO-donors plus ox-LDL (100 mug of protein/ml). Thrombin (0.03 U/ml) was used as an agonist. In some experiments 8-Br-cGMP (0.5-1 mmol/l) was used to investigate the NO-dependent intraplatelet signalling system. Slightly oxidized LDL was obtained by leaving native LDL in the light at room temperature for at least 7 days. Ox-LDL did not cause any increase in thrombin-induced [Casup 2sup +](i) (control: 215.4 +/- 44.3 nmol/l, ox-LDL 223.4 +/- 35.3 nmol/l, M +/- SEM; n = 8) and platelet aggregation (control: 78.7 +/- 4.9%, ox-LDL: 78.9 +/- 4.2%, n = 12). Ox-LDL antagonized the effects of NO-donors on platelet [Casup 2sup +](i) (NO-donor: 137.4 +/- 22.1 nmol/l, NO + ox-LDL: 177.3 +/- 27.6 nmol/l, n = 11; P < 0.001) and platelet aggregation (NO-donor: 15.4 +/- 3.4%, NO + ox-LDL: 28.9 +/- 3.8%, n = 24; P < 0.001). Ox-LDL did not affect the inhibitory activities of 8-Br-cGMP on platelet aggregation (8-Br-cGMP: 22.0 +/- 8.5%, 8-Br-cGMP + ox-LDL: 19.3 +/- 7.8%, n = 5) and platelet [Casup 2sup +](i). In conclusion, slightly oxidized LDL does not directly activate platelets and does not affect the intracellular NO-dependent signalling system. The present results suggest that LDL reduces the antiplatelet activity of NO mainly by preventing its biological effects.