title: Differential effect of external calcium on the oxygenated metabolism of endogenous and exogenous arachidonic acid in platelets.
abstract: The oxygenation of arachidonic acid into thromboxane B2 (TXB2), 12-hydroxy-heptadecatrienoic (HHT) and 12-hydroxy-eicosatetraenoic (12-HETE) acids has been examined in human platelets in the absence or presence of 1mM calcium. From endogenous arachidonic acid, external calcium did not affect the formation of cyclo-oxygenase products (TXB2 and HHT) but enhanced that of 12-HETE when thrombin at high concentrations was the agonist. Dose-response curves performed with thrombin and collagen revealed that increased stimulation resulted in higher ratios of 12-HETE/HHT. On the other hand external calcium did not alter significantly the synthesis of either products from exogenous arachidonic acid and the total conversion of the substrate was unchanged. We conclude that extracellular calcium may facilitate the liberation of arachidonic acid from platelet phospholipids when induced by high thrombin concentrations. The excess of arachidonic acid liberated would then be diverted towards the lipoxygenase pathway.
mesh_terms: D019377:12-Hydroxy-5,8,10,14-eicosatetraenoic Acid; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000097:blood*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D002851:Chromatography, High Pressure Liquid; D005231:Fatty Acids, Unsaturated / Q000096:biosynthesis / Q000097:blood; D006801:Humans; D006893:Hydroxyeicosatetraenoic Acids / Q000096:biosynthesis / Q000097:blood; D007700:Kinetics; D010084:Oxidation-Reduction; D013929:Thromboxane B2 / Q000096:biosynthesis / Q000097:blood
chemical_list: D001095:Arachidonic Acids; D005231:Fatty Acids, Unsaturated; D006893:Hydroxyeicosatetraenoic Acids; C025560:12-hydroxy-8,10-heptadecadienoic acid; D016718:Arachidonic Acid; D013929:Thromboxane B2; D019377:12-Hydroxy-5,8,10,14-eicosatetraenoic Acid; D002118:Calcium