Relationship between cyclic AMP and thromboxane formation in platelet-endothelial cell interactions.

Human platelet aggregation was triggered in the presence of various numbers of cultured endothelial cells. Thromboxane B2 formation and platelet cyclic AMP were also measured. Using a low concentration of thrombin (0.025U/ml) as aggregating agent, the inhibition of platelet aggregation correlated with that of thromboxane formation and was directly related to both the number of endothelial cells and platelet cyclic AMP. In contrast, using arachidonic acid (10(-5)M) instead of thrombin, platelet aggregation could be abolished although thromboxane formation was not affected. These results suggest that platelet aggregation induced by low concentrations of thrombin might be dependent on prostaglandin endoperoxides/thromboxane A2 production which could be inhibited by cyclic AMP. The normal synthesis of thromboxane B2 from exogenous arachidonate indicates that cyclic AMP is only active upon the liberation of endogenous arachidonate from platelet phospholipids.

Lipoxygenase activity of intact human platelets.

The oxygenation by lipoxygenase of different icosaenoic and docosaenoic acids by intact human platelets was studied. The HPLC analysis of the hydroxy compound (s) derived from icosaenoic acids showed that the 12-derivatives predominate. The increase of the fatty acid concentration markedly enhanced their oxygenation except for icosapentaenoic acid. The conversion of this acid into its hydroxy derivative rose in the presence of arachidonic acid, probably through both its cyclo-oxygenase and lipoxygenase product formation. Since 12-hydroxy-icosaenoic acids are modulators of PGH2-induced platelet aggregation, we conclude that the interactions between polyunsaturated fatty acids during their oxygenation by platelet lipoxygenase could be relevant to the regulating activity of dietary fatty acids.

Interactions between prostaglandin precursors during their oxygenation by human platelets.

The oxygenation through the prostaglandin synthase complex and the lipoxygenase pathways of the three prostaglandin precursors was investigated in human platelets. These precursors (dihomogammalinolenic (8,11,14-eicosatrienoic; 20:3 (8,11,14)), arachidonic (5,8,11,14-eicosatetraenoic; 20:4 (5,8,11,14)) and 5,8,11,14,17-eicosapentaenoic (20:5 (5,8,11,14,17)) acids) were used alone or simultaneously. We have found that 20:4 (5,8,11,14) increases the oxygenation of 20:3 (8,11,14) by the prostaglandin synthase complex while 20:5 (5,8,11,14,17) decreases the oxygenation of 20:4 (5,8,11,14) by the same enzyme complex. On the other hand, the utilization of 20:5 (5,8,11,14,17) by the prostaglandin synthase complex and the lipoxygenase was markedly enhanced in the presence of 20:3 (8,11,14), 20:4 (5,8,11,14) or both. Besides, the increased concentrations of 20:5 (5,8,11,14,17) failed to enhance its oxygenation to such an extent while the addition of 20:3 (8,11,14) or 20:4 (5,8,11,14) allows the marked potentiation of the 20:5 (5,8,11,14,17) oxygenation at any concentration. This indicates that, to be efficient, the utilization of 20:5 (5,8,11,14,17) by platelet oxygenases needs the presence of 20:3 (8,11,14), 20:4 (5,8,11,14) or their derivatives. In addition, using small concentrations of each prostaglandin precursor close to concentrations presumably released from platelet phospholipids during aggregation, all show the same tendencies. We conclude that the interactions we have observed between prostaglandin precursors during their oxygenation by human platelets could be of primary importance to explain the modifications of platelet reactivity reported after dietary manipulations.