Abstract
The cellular mechanisms by which LDL inhibits thrombin-induced endothelium-dependent relaxation in porcine coronary arteries were investigated using rabbit-washed platelets and cultured porcine endothelial cells. Thrombin-induced platelet aggregation was dose-dependently inhibited by sodium nitroprusside (10-7-10-3M), which activates guanylate cyclase. When platelets were coincubated, either with suspended endothelial cells or endothelial cells cultured on microcarrier beads, and then stimulated with thrombin, platelet aggregation was reduced and the lag time increased as the number of endothelial cells increased. However, the presence of LDL (1mg prot./ml) greatly reversed the inhibition by the endothelial cells, while the presence of hemoglobin (10-6M) which is known to degrade the endothelium-derived relaxing factor (EDRF), completely blocked such inhibition. Likewise, thrombin-induced 14C-serotonin release from the platelets was inhibited by coincubation of the platelets and endothelial cells, but such inhibition was not observed when either LDL or hemoglobin were present. In addition, platelets aggregated as usual when they were exposed to an EC-thrombin mixture 5min after stimulation of the endothelial cells with thrombin. In contrast to native LDL, heat (60°C, 10min)-treated LDL and acid (pH 2, 30min)-treated LDL showed no such effects.
These results suggest that thrombin-induced plate-let aggregation and secretion were simultaneously inhibited by EDRF release from the thrombin-stimulated endothelial cells, and that by interacting with EC, LDL blocked EDRF release and thereby reversed the inhibitory effects of the endothelial cells on platelet aggregation.
