Enhancement of eicosaenoic acid lipoxygenation in human platelets by 12-hydroperoxy derivative of arachidonic acid

Oxygenation of polyunsaturated fatty acids and oxidative stress within blood platelets

The oxygenation metabolism of arachidonic acid (ArA) has been early described in blood platelets, in particular with its conversion into the potent labile thromboxane A₂ that induces platelet aggregation and vascular smooth muscle cells contraction. In addition, the primary prostaglandins D₂ and E₂ have been mainly reported as inhibitors of platelet function. The platelet 12-lipoxygenase (12-LOX) product, i.e. the hydroperoxide 12-HpETE, appears to stimulate platelet ArA metabolism at the level of its release from membrane phospholipids through phospholipase A₂ (cPLA₂) and cyclooxygenase (COX-1) activities, the first enzymes in prostanoid production cascade. Also, 12-HpETE may regulate the oxygenation of other polyunsaturated fatty acids (PUFA) by platelets, especially that of eicosapentaenoic acid (EPA). On the other hand, the reduced product of 12-HpETE, 12-HETE, is able to antagonize TxA₂ action. This is even more obvious for the 12-LOX end-products from docosahexaenoic acid (DHA), 11- and 14-HDoHE. In addition, 12-HpETE plays a key role in platelet oxidative stress as observed in pathophysiological conditions, but may be regulated by DHA with a bimodal way according to its concentration. Other oxygenated products of PUFA, especially omega-3 PUFA, produced outside platelets may affect platelet functions as well.

Pathophysiologic role of redox status in blood platelet activation. Influence of docosahexaenoic acid

Decrease of platelet glutathione peroxidase activity results in increased life span of lipid hydroperoxides, especially the 12-lipoxygenase product of arachidonic acid, 12-HpETE. Phospholipase A2 activity is subsequently enhanced with the release of arachidonic acid, which results in higher thromboxane formation and platelet function. Docosahexaenoic acid may either potentiate platelet lipid peroxidation or lower it when used at high or low concentrations, respectively. In the case of slowing down lipid peroxidation, docosahexaenoic acid was specifically incorporated in plasmalogen ethanolamine phospholipids. This could have a relevant pathophysiologic role in atherothrombosis.

Aldehydes from n-6 fatty acid peroxidation. Effects on aminophospholipids

4-Hydroxy-nonenal (4-HNE) is a major by-product of n-6 fatty acid peroxidation. It has been described to covalently bind biomolecules expressing primary amine, especially the Lys residues in proteins. Low-density lipoproteins (LDL) are well-described macromolecules to be modified by 4-HNE, making them available to scavenger receptors on macrophages. Those macrophages then become foam cells and play an active role in atherogenesis. This paper reports on the covalent binding of 4-HNE to phosphatidylethanolamine (PE), a major aminophospholipid in biological membranes. In contrast, phosphatidylserine (PS) is virtually not modified by 4-HNE. One stable adduct, the Michael adduct PE/4-HNE is a poor substrate of secreted phospholipase A(2) and is not cleaved by phospholipase D. Plasmalogen PE, an important subclass of PE, is covalently modified by 4-HNE as well, but appears to be further degraded on its sn-1 position, the alkenyl chain, which might alter the antioxidant potential of the molecule. An aldehyde homologous to 4-HNE has been characterized as a breakdown product of 12-hydroperoxyeicosatetraenoic acid (12-HpETE) and named 4-hydroxy-2E,6Z-dodecadienal (4-HDDE). This compound as well as 4-HNE was detected in human plasma. Finally, 4-HDDE appears almost 3-fold more active than 4-HNE to make covalent adducts with PE. We conclude that 4-HNE and 4-HDDE are two biologically relevant markers of n-6 fatty acid peroxidation that may alter the phospholipid-dependent cell signaling.

Inhibition of prostaglandin H synthase and activation of 12-lipoxygenase by 8,11,14,17-eicosatetraenoic acid in human endothelial cells and platelets

The effects of the marine fatty acid 20:4n-3, an isomer of arachidonic acid (20:4n-6), have been compared to that of 20:5n-3 on 20:4n-6 oxygenation in human platelets and endothelial cells. In platelets, 20:4n-3 added along with 20:4n-6 was as potent as 20:5n-3 in inhibiting prostaglandin H synthase (PGH synthase) activity. From 2.5- to 10 microM of 20:4n-6, the synthesis of thromboxane B2 and 12-hydroxy-5,8,10-heptadecatrienoic acid, reflecting the PGH/thromboxane synthase activity, was lowered by 5 and 10 microM of both fatty acids. In contrast, 20:4n-3, but not 20:5n-3, strongly stimulated the lipoxygenase activity at each concentration of 20:4n-6 used whatever the amount of 20:4n-3 added. The effects of both n-3 polyunsaturated fatty acids on endothelial cell PGH/prostacyclin synthases were compared after 2- and 24-hr incubation with the cells, leading to moderate (2 hr) and high (24 hr) concentrations of these fatty acids in membrane phospholipids. The incorporation of 20:4n-3 and 20:5n-3 occurred mostly in phosphatidylcholine and phosphatidylethanolamine and did not alter the 20:4n-6 level of phospholipid classes after 2-hr supplementation, whereas it was drastically decreased after 24 hr. The synthesis of prostacyclin obtained after cell stimulation by 0.1 U/mL thrombin was unaffected by the fatty acid modifications induced after 2-hr supplementation, whereas it was strongly depressed after 24 hr. It was concluded that 20:4n-3 is not an agonist for platelet activation, despite its close structural analogy with 20:4n-6, and is as potent as 20:5n-3 in inhibiting PGH synthase activities, showing that the double bond at C5 is not necessary for inhibition. In contrast, the oxygenation of 20:4n-6 by 12-lipoxygenase was stimulated by 20:4n-3 but not by 20:5n-3, which might be related to the efficient oxygenation of 20:4n-3 by this enzyme compared with 20:5n-3.

Involvement of lipid peroxidation in platelet signalling

A well-known signalling pathway in blood platelets consists in the release of arachidonic acid (AA) from membrane phospholipids and its specific oxygenation into bioactive derivatives. In particular, cyclic prostaglandin endoperoxides and thromboxane A2 are potent inducers of platelet functions and are produced in greater amounts when the level of lipid hydroperoxides is higher than normal, as 'physiological concentrations' of such peroxides activate the cyclooxygenation of AA. In this context, a lower activity of platelet glutathione peroxidase (GPx), the key-enzyme for the degradation of lipid hydroperoxides, has been reported in aging, which will ensure a longer life span to those peroxides. Accordingly, the biosynthesis of pro-aggregatory prostanoids is elevated in platelets from the elderly. On the other hand, fatty acids from marine origin have been recognized as inhibitors of platelet functions, and they may alter the redox status of cells. They may for instance increase the platelet GPx activity, an effect that can be prevented by antioxidants. Overall, these data point out the relevance of the redox status in platelet functions.

Low concentrations of lipid hydroperoxides prime human platelet aggregation specifically via cyclo-oxygenase activation

There is mounting evidence that lipid peroxides contribute to pathophysiological processes and can modulate cellular functions. The aim of the present study was to investigate the effects of lipid hydroperoxides on platelet aggregation and arachidonic acid (AA) metabolism. Human platelets, isolated from plasma, were incubated with subthreshold (i.e. non-aggregating) concentrations of AA in the absence or presence of hydroperoxyeicosatetraenoic acids (HPETEs). Although HPETEs alone had no effect on platelet function, HPETEs induced the aggregation of platelets co-incubated with non-aggregating concentrations of AA, HPETEs being more potent than non-eicosanoid peroxides. The priming effect of HPETEs on platelet aggregation was associated with an increased formation of cyclo-oxygenase metabolites, in particular thromboxane A2, and was abolished by aspirin, suggesting an activation of cyclo-oxygenase by HPETEs. It was not receptor-mediated because the 12-HPETE-induced enhancement of AA metabolism was sustained in the presence of SQ29, 548 or RGDS, which blocked the aggregation. These results indicate that physiologically relevant concentrations of HPETEs potentiate platelet aggregation, which appears to be mediated via a stimulation of cyclo-oxygenase activity.

Interactions between arachidonic and eicosapentaenoic acids during their dioxygenase-dependent peroxidation

Eicosapentaenoic acid (EPA), a major polyunsaturated fatty acid of fish has been widely proposed as a potential nutrient for decreasing platelet-endothelial cell interactions and the subsequent atherogenesis and thrombogenesis. This is mainly based upon the decrease of arachidonic acid (AA) oxygenation into bioactive molecules like thromboxane A2. In addition, EPA may be oxygenated into its own active derivatives via cell dioxygenases. We report evidence for the requirement of specific peroxides, adequately provided by AA, to allow EPA to be oxygenated into its bioactive products like prostaglandin I3, a prostacyclin mimetic. On the other hand, we present some data that argue for a decreased basal AA dioxygenation (specific peroxidation) by small concentrations of EPA. The interactions between AA and EPA are then dual, EPA being able to counteract AA oxygenation whereas EPA requires AA to be efficiently oxygenated.

Lower levels of lipid peroxidation in human platelets incubated with eicosapentaenoic acid

The modulatory effects of eicosapentaenoic acid (EPA) on platelet arachidonic acid (AA) metabolism were applied to an in vitro model of oxidant stress. Unstimulated normal human blood platelets were first treated with a thiol-oxidizing agent, azodicarboxylic acid bis(dimethylamide) (diamide) (1 microM), and then incubated with a low concentration of EPA (100 nM). Diamide treatment led to a lower alpha-tocopherol content compared to control. Formation of MDA, a marker of the overall lipid peroxidation, as well as formation of 12-hydroxyeicosatetraenoic acid (12-HETE), the 12-lipoxygenase end-product of AA, were both higher in diamide-treated platelets. Subsequent incubation of diamide-treated platelets with EPA counteracted the effects of oxidant stress induced pharmacologically by diamide. Interestingly, EPA prevented the alpha-tocopherol level from falling and the overall lipid peroxidation from increasing as it did during diamide treatment. In particular, incubation of diamide-treated platelets with EPA led to significantly lower amounts of 12-HETE. Conversely, preincubation of platelets with 100 nM EPA protected cells from oxidizing effects induced by diamide treatment, either on the level of lipid peroxides or on the antioxidant status. These results indicate that, in this particular model, EPA permitted platelets to have control levels of tocopherol, MDA and 12-HETE despite diamide treatment. Low concentrations of EPA might have prevented the increase of lipid hydroperoxides and especially the transient accumulation of 12-hydroperoxyeicosatetraenoic acid (12-HPETE).

Decrease in platelet reduced glutathione increases lipoxygenase activity and decreases vitamin E

Unstimulated normal human blood platelets were treated with azodicarboxylic acid bis(dimethylamide) (diamide), a thiol-oxidizing agent. Oxygenated arachidonic acid (AA) metabolites, malondialdehyde (MDA), and tocopherols were then quantified by high-performance liquid chromatography (HPLC). Diamide treatment partially decreased the amount of reduced glutathione (GSH) content and induced a subsequent decrease in peroxidase activity. However, formation of 12-hydroxy-eicosatetraenoic acid (12-HETE), the end-product of lipoxygenation of AA, increased. Formation of MDA, a marker of overall lipid peroxidation, was also enhanced. Furthermore, platelet alpha-tocopherol, but not gamma-tocopherol, significantly decreased. These results indicate that enhanced "basal" lipoxygenase activity, as a marker of specific AA oxygenation, may be linked to decreased platelet antioxidant status.

Hemin and hemeprotein bleaching during linoleic acid oxidation by lipoxygenases

Hemin and hemoglobin are bleached by lipoxygenases, type 1 (from soybean) or type 2 (from platelets), during linoleic acid oxidation. This process has been found to be related to the inhibition of the lipoxygenase activity, measured as hydroperoxide generation and to produce oxodienes as well. All these parameters have been determined simultaneously from measurements of the absorbance at 234, 285, 375 and 410 nm to detect hydroperoxides, oxodienes, hemin and hemoglobin, respectively, using a diode array spectrophotometer. The inhibition of lipoxygenase activity by these pigments has been found to be competitive with linoleic acid, showing an increase of 4-7-fold of the Km value of linoleic acid in the presence of concentrations of hemin and hemoglobin as low as 0.2 and 0.02 microM, respectively, for the case of platelet lipoxygenase activity. The concentrations of hemin and of hemoglobin producing the inhibition of 50% of lipoxygenase activity are: 0.25 and 0.02 microM for the platelet isoenzyme, and 1.4 and 0.18 microM for the soybean isoenzyme, respectively. From the quenching of the intrinsic fluorescence of soybean lipoxygenase activity by hemin, we have obtained a dissociation constant of hemin-soybean lipoxygenase of 0.5 microM. The results obtained in this paper for the cooxidation process of hemin and hemoglobin by lipoxygenase can be rationalized in terms of hemin binding at or near to the catalytic center, resulting in a lesser binding of linoleic acid and an enhanced release of radicals, and pigment bleaching by radicals and lipid hydroperoxides.

Lowered platelet glutathione peroxidase activity in patients with intrinsic asthma

Platelet glutathione peroxidase (GSH-Px) activity and serum selenium (Se) levels were determined in 20 patients with intrinsic asthma. Nine of the patients had NSAID-intolerance. The mean value of GSH-Px activity in the patients was 47.0 +/- 7.1 U/10(11) platelets, which is significantly lower than that of 56.4 +/- 12 U/10(11) platelets in the controls (P less than 0.01). There was also a tendency towards lowered Se levels in the patients compared with controls. The results are discussed in view of the protective role of GSH-Px against oxidative stress and the tentative regulatory function of GSH-Px in arachidonic acid metabolism.

Functions and tocopherol content of blood platelets from elderly people after low intake of purified eicosapentaenoic acid

Elderly people present an increased incidence of atherosclerosis and vascular cerebral damages, associated with blood platelet hyperactivity and a stimulation of arachidonic acid metabolism in vivo. The effects of a low intake of purified eicosapentaenoic acid (EPA) on platelet hyperactivity in old human subjects has been investigated. In a randomized, double blind study, 8 people took during 2 months a daily intake of 100 mg of eicosapentaenoic acid (EPA) given as a triglyceride (1,3-dioctanoyl,2-eicosapentaenoyl-glycerol), and 8 other subjects ingested a placebo. A slight, but significant reduction of platelet-rich plasma aggregation in response to epinephrine and arachidonic acid occurred after EPA intake, as well as a decreased aggregation of washed platelets induced by thrombin, although collagen- and U-46619-induced aggregations were not significantly modified. EPA intake failed to affect arachidonic acid metabolism in thrombin-stimulated platelets or in clotted venous blood. The urinary excretion of thromboxane, 6-keto-PGF1 alpha and their 2,3-dinor-metabolites was also not modified. Similarly, no change in the plasma and platelet lipid fatty acid compositions could be observed. Platelet, but not plasma, alpha- and gamma-tocopherol were enhanced by EPA intake. An increase of platelet vitamin E has been associated with a decrease of aggregation, especially in vitamin E-deficient subjects, like elderly people. Therefore, low intake of EPA might have contributed to inhibit platelet aggregation by increasing cellular vitamin E.

Liberation and oxygenation of polyenoic acids in stimulated platelets

Radiolabeled polyenoic acids were incorporated into human platelet lipids using albumin as vector. Platelets were then triggered with 0.1 or 1 U/ml thrombin, and 0.5 or 2 x 10(-6) M calcium ionophore A23187. Lipid extracts were analyzed for neutral lipids, free fatty acids, monohydroxylated acids, prostanoids and glycocerophospholipid subclasses. During platelet activation induced by thrombin or by ionophore, arachidonic and eicosapentaenoic acids were liberated from phospholipids in large amounts and were subsequently oxygenated via platelet oxygenases. Substantial amounts of lipoxygenase products and thromboxanes were produced from these acids. Liberation and oxygenation of linoleic, alpha-linolenic, and docosahexaenoic acids were much less pronounced. Polyenoic acid liberation from phospholipid subclasses also behaved quite differently. Apart from alpha-linolenic and adrenic acids, which were poorly liberated, all the others were freed from phosphatidylinositol. In addition, arachidonic, eicosapentaenoic, and 5, 8, 11-eicosatrienoic acids were liberated from phosphatidylcholine at high concentrations of agonists and partially reincorporated into phosphatidylethanolamine. Finally, linoleic acid was deacylated from phosphatidylinositol and phosphatidylserine and almost entirely reacylated into phosphatidylcholine, whereas docosahexaenoic acid was deacylated from phosphatidylcholine and phosphatidylinositol reacylated into phosphatidylethanolamine, respectively. It is concluded that these polyenoic acids, all for which modulate platelet functions, exhibit very different metabolisms. They may act via their oxygenated derivatives and/or at the membrane phospholipid level.

Vitamin E fails to alter the aggregation and the oxygenated metabolism of arachidonic acid in normal human platelets

Using low doses of vitamin E, either in vitro or in vivo, we have succeeded in almost doubling plasma and platelet alpha-tocopherol in healthy humans. Despite such an enrichment, platelet aggregation induced by collagen and thromboxane A2 minetic U46619 was not much affected, although that induced by exogenous arachidonic acid was significantly decreased. Similarly, the oxygenation of exogenous arachidonic acid was not modified. When incubated with thrombin some variations in the formation of endogenous cyclooxygenase and lipoxygenase products could be observed, although rarely significantly. The tendency was a decrease after in vivo enrichment and an increase when enrichment occurred in vivo. Serum oxygenated metabolites of arachidonic acid as well as urinary metabolites of thromboxane and prostacyclin were also not affected after vitamin E supplementation. Since the lipoxygenation of eicosapentaenoic acid was very strongly peroxide-dependent, the effect of alpha-tocopherol enrichment was tested and the 12-hydroperoxide derivative of arachidonic acid was used as a physiological peroxide. No modification could be observed, confirming that vitamin E does not alter the specific peroxidation of polyunsaturated fatty acids in normal platelets. We conclude that vitamin E supplementation neither affects arachidonic acid-dependent aggregation nor the oxygenated metabolism of arachidonic acid in normal human platelets.

Occurrence of the 15-hydroxy derivative of dihomogammalinolenic acid in human platelets and its biological effect

Various polyunsaturated fatty acids are oxygenated by platelet lipoxygenase at the n - 9 position. The present paper reports that platelets may also oxygenate dihomogammalinolenic acid (20:3(n - 6)) at the n - 6 position, leading to the formation of substantial amounts of 15-OH-8,11,13-20:3 characterized by its ultraviolet spectrum, HPLC and GC-MS analysis. Its formation was inhibited by aspirin and eicosatetraynoic acid, but not by heneicosatetraynoic acid, a specific inhibitor of platelet lipoxygenase. The time-course of its synthesis was very close to that of 12-OH-8,10-17:2 (HHD), the non-cyclic cyclooxygenase side-product, but different from that of 12-OH-8,10,14-20:3, the platelet lipoxygenase end-product of 20:3 (n - 6). Overall, these results indicate that 15-OH-20:3 could be a cyclooxygenase metabolite generated in an aborted process. Like other monohydroxy derivatives of polyenoic fatty acids, 15-OH-20:3 was able to modulate thromboxane-induced platelet aggregation. The derivative exhibited a biphasic effect on the aggregation. It potentiated at concentrations below 2.10(-7) M and inhibited at higher doses. It is concluded that the potentiating activity might explain at least part of the transient enhancement of the platelet activation observed in adding exogenous 20:3(n - 6).

Different metabolic behavior of long-chain n-3 polyunsaturated fatty acids in human platelets

Whereas numerous studies deal with the effects and metabolism of eicosapentaenoic acid (20:5(n - 3)) in platelets, very few concern docosahexaenoic acid (22:6(n - 3)), although both acids are consumed in equal amounts from most fish fat. The present paper reports the modulation of 22:6(n - 3) oxygenation as well as that of endogenous arachidonic acid (20:4(n - 6)) in 22:6(n - 3)-rich platelets. Like the oxygenation of 20:5(n - 3), the lipoxygenation of 22:6(n - 3) occurred at a low level when incubated alone, but was markedly increased in the presence of 20:4(n - 6), suggesting a similar peroxide tone dependency. 20:5(n - 3) could not replace 20:4(n - 6) in the increasing 22:6(n - 3) lipoxygenation, whereas 22:6(n - 3) shared the potentiating effect of 20:4(n - 6) on both the cyclooxygenation and the lipoxygenation of 20:5(n - 3). On the other hand, 20:5(n - 3), 22:6(n - 3) or 20:5(n - 3) + 22:6(n - 3) enrichment of platelet phospholipids inhibited the formation of cyclooxygenase but not lipoxygenase products from endogenous 20:4(n - 6) in thrombin-stimulated platelets. In doing so, 22:6(n - 3) appeared even more potent than 20:5(n - 3), although it was not liberated after acylation in phospholipids, the opposite of what was observed with 20:5(n - 3). Therefore, it seems that, in contrast to 20:5(n - 3), which may compete with endogenous 20:4(n - 6) at the cyclooxygenase level, 22:6(n - 3) would affect the latter enzyme activity in a different way. We conclude that 20:5(n - 3) and 22:6(n - 3) behave differently and might act synergistically on the inhibition of platelet functions after fish fat intake.

Hydroperoxides produced by n-6 lipoxygenation of arachidonic and linoleic acids potentiate synthesis of prostacyclin related compounds

In a previous paper we reported that arachidonic acid (20:4(n-6] strongly enhances the endothelial cell synthesis of prostaglandin I3 (PGI3) from eicosapentaenoic acid (20:5(n-3], in stimulating the cyclooxygenase rather than the prostacyclin synthase (Bordet et al. (1986) Biochem. Biophys. Res. Commun. 135, 403-410). In the present study, endothelial cell monolayers were co-incubated with exogenous 20:5(n-3) or docosatetraenoic acid (22:4(n-6], and n-6 lipoxygenase products of 20:4(n-6) or linoleic acid (18:2(n-6], namely 15-HPETE and 13-HPOD, respectively. Prostaglandins or dihomoprostaglandins were then measured by gas chromatography-mass spectrometry. Both hydroperoxides, up to 20 microM, stimulated the cyclooxygenation of 20:5(n-3) and 22:4(n-6), in particular the formation of PGI3 and dihomo-PGI2, respectively. Higher concentrations inhibited prostacyclin synthetase. In contrast, the reduced products of hydroperoxides, 15-HETE and 13-HOD, failed to stimulate these cyclooxygenations, 13-HPOD appeared more potent than 15-HPETE and the cyclooxygenation of 22:4(n-6) seemed to require higher amounts of hydroperoxides to be efficiently metabolized than 20:5(n-3). These data suggest that prostacyclin potential of endothelium might be enhanced by raising the peroxide tone.

Differential effects of 15-HPETE on arachidonic acid metabolism in collagen-stimulated human platelets

The 15-hydroperoxyeicosatetraenoic acid (15-HPETE) has been shown to affect platelet aggregation induced by collagen, arachidonic acid (AA), and PGH2-analogue. Furthermore, it also inhibits the platelet cyclooxygenase and lipoxygenase enzymes, and prostacyclin synthase. The present study was designed to test the effect of 15-HPETE on the mobilization of endogenous AA in collagen-stimulated human platelets. For this purpose, human platelets pretreated with BW755C (a dual inhibitor of cyclooxygenase and lipoxygenase) were stimulated with collagen in the presence of varied concentrations of 15-HPETE. We observed a significant inhibition of oxygenases at all concentrations of 15-HPETE. In contrast, our results indicate that 15-HPETE at lower concentrations (10 microM and 30 microM) significantly stimulated the collagen-induced release of AA from phospholipid sources. Although higher concentrations of 15-HPETE (50 microM and 100 microM) caused some inhibition of AA accumulation in the free fatty acid fraction (25% and 60%), the degree of inhibition was significantly lower than the inhibition observed for the oxygenases (65% and 88% for cyclooxygenase and 77% and 94% for lipoxygenase respectively). These results provide support that hydroperoxides also regulate phospholipases presumably by a different mechanism, which may be important in the detoxification of phospholipid peroxides.

Arachidonic acid strongly stimulates prostaglandin I3 (PGI3) production from eicosapentaenoic acid in human endothelial cells

Eicosapentaenoic acid (EPA) is a prominent polyunsaturated fatty acid in fish oil which inhibits blood platelet aggregation and thromboxane A2 formation but not prostacyclin-like material generation from vascular endothelium. In this study we investigated interaction between EPA and arachidonic acid (AA) during their oxygenation by cultured endothelial cells. As measured by gas chromatography-mass spectrometry (GC-MS), AA increased markedly prostaglandin I3 (PGI3) production from EPA while that of PGI2 from AA was decreased by EPA. However, increasing the ratio AA/EPA over one almost suppressed the inhibition of PGI2 formation by EPA, and the stimulation of PGI3 production by AA was even higher. The effect of AA on EPA conversion to minor prostaglandins like PGE3 and PGF3 alpha was similar then confirming the stimulating effect and suggesting it is occurring at the cyclooxygenase instead of the prostacyclin synthase level. Altogether these data indicate that, in certain nutritional states where the liberation of EPA from endothelial cells will be accompanied with that of endogenous AA, substantial amounts of PGI3 could contribute to the prostacyclin-like activity of the vessel wall in addition to PGI2.