Arachidonic acid uptake by human platelets is mediated by CD36

The involvement of glycoprotein (GP) IV (CD36) in arachidonic acid uptake by human platelets was investigated using an anti-CD36 monoclonal antibody (MAB). The binding of [¹⁴C]arachidonic acid to MAB-treated platelets was significantly reduced compared with untreated platelets. The MAB also inhibited arachidonic acid-induced platelet aggregation and thromboxane A₂ synthesis in a dose-dependent manner. Pre-incubation of gel-filtered platelets with the MAB (10mg/I) inhibited arachidonic acid-induced platelet aggregation by 50% and collagen-induced platelet aggregation by 7-8% and the lag time was increased by 200%. Although the mechanism of platelet aggregation is not fully understood yet, the inhibition of arachidonic acid-induced platelet aggregation by the MAB could be the result of a reduced uptake of exogeneously added arachidonic acid by the MAB-treated platelets. Our data clearly indicate that arachidonic acid uptake by platelets is mediated, at least in part, by CD36.     

Arachidonic acid uptake by human platelets is mediated by CD36

The involvement of glycoprotein (GP) IV (CD36) in arachidonic acid uptake by human platelets was investigated using an anti-CD36 monoclonal antibody (MAB). The binding of [(14)C]arachidonic acid to MAB-treated platelets was significantly reduced compared with untreated platelets. The MAB also inhibited arachidonic acid-induced platelet aggregation and thromboxane A(2) synthesis in a dose-dependent manner. Pre-incubation of gel-filtered platelets with the MAB (10mg/I) inhibited arachidonic acid-induced platelet aggregation by 50% and collagen-induced platelet aggregation by 7-8% and the lag time was increased by 200%. Although the mechanism of platelet aggregation is not fully understood yet, the inhibition of arachidonic acid-induced platelet aggregation by the MAB could be the result of a reduced uptake of exogeneously added arachidonic acid by the MAB-treated platelets. Our data clearly indicate that arachidonic acid uptake by platelets is mediated, at least in part, by CD36.     

Investigations on platelet function in diabetes mellitus

Investigations on the platelet function in diabetes mellitus were performed on 28 patients with insulin-dependent diabetes and in 33 healthy controls of similar age. In the diabetic patients it was possible to induce 50% of maximal aggregation by lower concentrations of adenosine diphosphate or arachidonic acid than in the controls. In the presence of N-ethyl maleimide, platelets from diabetic patients produced significantly more malondialdehyde than those from normal controls. After addition of arachidonic acid the platelets from the diabetic patients also synthesized more thromboxane B2. This synthesis of thromboxane was inversely correlated to the minimal concentration of arachidonic acid necessary to induce 50% platelet aggregation. Circulating platelet aggregates were more common in the diabetic patients than in the controls. Plasma levels of beta-thromboglobulin and platelet factor 4 were raised in parallel in the diabetic patients and correlated with the increased production of thromboxane B2 by the platelets from the same patients. Platelets from patients with diabetes thus demonstrated signs of hyperreactivity both in vivo and in vitro. This may be of clinical importance for the development of vascular complications in this disease.     

Platelet malondialdehyde production and aggregation responses induced by arachidonate, prostaglandin-G2, collagen, and epinephrine in 12 patients with storage pool deficiency

We assessed the integrity of the prostaglandin synthetic pathway by measuring malondialdehyde (MDA) production and studied platelet aggregation responses to arachidonic acid and PGG2 in 12 patients with storage pool deficiency (SPD). Eight patients were deficient only in dense granules (delta-SPD) and four were deficient in both dense and alpha-granules (alpha delta-SPD). Production of MDA in response to arachidonic acid (AA), epinephrine, and collagen suggested that the transformation of AA to prostaglandin metabolites was normal in delta- SPD but abnormal in alpha delta-SPD and that the liberation of AA from phospholipids were abnormal in the majority of patients with SPD. Since the content of secretable adenosine diphosphate (ADP) is diminished in SPD platelets, the aggregation responses of these platelets to AA and PGG2 were studied to help answer the question whether these agents aggregate platelets directly or through release of endogenous ADP. Among patients with delta-SPD, aggregation by both AA and PGG2 was decreased in four albinos whose platelets were markedly deficient in ADP. In contrast, normal, or less strikingly abnormal, responses were observed in patients whose platelets either contained higher levels of platelet ADP or showed increased sensitivity to ADP. The more marked impaired responses to AA and PGG2 in patients with alpha delta-SPD suggest that substances derived from alpha-granules may also play a role in platelet aggregation by these agents. The aggregation responses in these patients with various types of SPD is consistent with a theory that granule-derived ADP mediates platelet aggregation by AA and PGG2.     

Effect of latex-stimulated granulocytes on platelet aggregation in man

The effect of granulocytes on human platelet aggregation was investigated in vitro. Platelet function was assayed by photometric technique. Incubation of platelets with latex-stimulated granulocytes for 1 h at room temperature resulted in total inhibition of arachidonic acid-induced platelet aggregation. ADP-induced platelet aggregation was suppressed, lacked secondary wave and was pursued by swift disaggregation. Platelet aggregates induced by collagen dispersed faster under the influence of stimulated granulocytes. The present results indicate that granulocytes may play a role in the hemostatic mechanism in man.     

Characterization of platelet function in cyclic hematopoietic dogs

Platelet aggregation to incremental doses of eight different platelet agonists (collagen, thrombin, platelet-activating factor [PAF], arachidonic acid [AA] plus epinephrine, the calcium ionophore A23187, ADP, phospholipase C [PLC], and 12-O-tetradecanoyl phorbol-13-acetate [TPA]) was compared in normal (N) and cyclic hematopoietic (CH) dogs. Platelet aggregation was defective with collagen, PAF, TPA, and possibly thrombin as agonists but normal when ADP, PLC, arachidonic acid plus epinephrine, and A23187 were used as agonists with CH platelets. In heterozygous CH dogs, platelet aggregation was intermediately defective when tested with collagen and PAF as agonists. Thromboxane B2 (TXB2) concentrations (mean +/- SD; pg/10(6) platelets), as measured by RIA, were similar in CH and normal dogs both prior to (CH: 7.6 +/- 7.0; N: 5.5 +/- 3.9) and after collagen stimulation (collagen: 141.3 +/- 42.5; 123.1 +/- 38.4). Granule storage pools of serotonin and platelet adenine nucleotides were markedly decreased in homozygous CH but not heterozygous CH dogs. Thrombin stimulated phosphorylation of 40- and 20-kd proteins in platelets from CH and normal dogs to an equal extent. However, collagen-stimulated phosphorylation of the 40- but not the 20-kd protein was significantly decreased in platelets from CH dogs. These data suggest that there is a biochemical defect in platelets from CH dogs that results in storage pool disease and decreased phosphorylation of a 40-kd protein.     

Inhibition of platelet aggregation by protease inhibitors. Possible involvement of proteases in platelet aggregation

The possible participation of proteases in human platelet aggregation was explored using various protease inhibitors and substrates. Protease inhibitors used included naturally occurring inhibitors of serine proteases and synthetic inhibitors that modify the active site of protease. Substrates used were synthetic substrates for the trypsin type as well as for the chymotrypsin type of protease. All these inhibitors and substrates inhibited platelet aggregation and serotonin release induced by ADP, collagen, epinephrine, or thrombin. In ADP- and epinephrine-induced platelet aggregation the second phase of aggregation was most efficiently inhibited. The inhibitors suppressed the formation of malondialdehyde during platelet aggregation. Release by aggregating agents of arachidonate and its metabolites from indomethacin-treated platelets as well as nontreated platelets was also inhibited. The inhibitors apperar to interact with stimulated platelets but not with unstimulated platelets. These observations suggest that the interaction of an aggregating agent with its platelet receptor activates a unique precursor serine protease that in turn activates platelet phospholipase to liberate arachidonic acid (the precursor of the potent platelet aggregating agent thromboxane A2) from platelet phospholipids.     

Release of arachidonic acid from human platelets. A key role for the potentiation of platelet aggregability in normal subjects as well as in those with nephrotic syndrome

Low (nonaggregating) concentrations of collagen that potentiate platelet aggregation did not induce the formation of measurable amount of malondialdehyde (MDA) but released small but significant amounts of radioactivity from 14C-arachidonic acid-labeled platelets. A major portion of the radioactive compounds released by nonaggregating concentrations of collagen existed as arachidonic acid and a minor part as thromboxane B2. The nephrotic syndrome enhances platelet aggregability, and this effect is abolished by correcting hypoalbuminemia in vitro and in vivo by the addition of albumin, which is the main carrier for free fatty acids, including arachidonic acid. Human albumin (fatty acid free) inhibited collagen-induced aggregation, MDA formation, and release of the radioactivity from 14C-arachidonic acid-labeled platelets in normals as well as in those with nephrotic syndrome. These data support our hypothesis that the main mechanism responsible for the potentiation of platelet aggregation is the release of arachidonic acid from platelet membrane phospholipids via the activation of phospholipase A2. Furthermore, enhanced platelet aggregation in the nephrotic syndrome was at least partly attributable to an increased availability of arachidonic acid released secondary to hypoalbuminemia. Albumin inhibits aggregation probably by binding to released arachidonic acid preventing arachidonic acid from being metabolized to potent aggregating substances, endoperoxides and thromboxane A2. The mechanism of release of arachidonic acid may play a key role in the potentiation of platelet aggregability in normals as well as in pathologic conditions such as the nephrotic syndrome.     

Platelet dysfunction and alteration of prostaglandin metabolism after chronic alcohol consumption

To study the effects of chronic alcohol consumption on platelet functions, the rate of arachidonate-induced platelet aggregation, the production of malondialdehyde in platelets, and plasma levels of prostaglandin endoperoxide metabolites were examined in 88 chronic alcoholics and 24 healthy controls. The rate of platelet aggregation and the production of malondialdehyde in platelets were greater in chronic alcoholics both on admission and 1 week after. However, these alterations returned to the level of healthy controls within 4 weeks of abstinence from alcohol and were independent of the number of circulating platelets. Furthermore, on admission, plasma levels of thromboxane B2 were significantly increased in chronic alcoholics when compared with those of healthy controls (400.8 +/- 36.5 versus 241.7 +/- 28.9 pg/ml plasma; p less than 0.025) and were also significantly correlated with malondialdehyde production in washed platelet debris (r = 0.6049; p less than 0.001). In contrast, plasma levels of 6-keto prostaglandin F1 alpha and prostaglandin E were not altered after chronic alcohol consumption. As a result, the ratio of 6-keto prostaglandin F1 alpha to thromboxane B2 was markedly decreased in chronic alcoholics (0.31 +/- 0.03 versus 0.62 +/- 0.13; p less than 0.001). These results strongly suggest that the imbalance in prostaglandin endoperoxide metabolites is produced by chronic alcohol ingestion. Moreover, a significant correlation was observed between platelet aggregation rate and malondialdehyde production during platelet aggregation (r = 0.559; p less than 0.005). Thus, we conclude that chronic alcohol consumption alters platelet thromboxane metabolism, which is likely associated with the increased ability of platelets to aggregate.     

Hydrogen peroxide as trigger of platelet aggregation.

In order to verify if H2O2 affects platelet function, platelet-rich plasma and human washed platelets were incubated with subthreshold concentrations (STC) of collagen or arachidonic acid or ADP and/or with 75-150 microM H2O2. While H2O2 alone did not affect platelet aggregation, it amplified platelet aggregation response in samples stimulated with STC of arachidonic acid and collagen but not in samples stimulated with STC of ADP. When platelets were preventively treated with aspirin, a cyclooxygenase inhibitor, the platelet activation by H2O2 was not observed. Thromboxane A2 (TxA2) was not produced by human washed platelets stimulated with STC of arachidonic acid, collagen or by H2O2 alone. On the contrary, when STC of agonists were tested on platelets supplemented with H2O2 an evident TxA2 production was seen. This effect was prevented by aspirin pretreatment or by the addition of catalase, an enzyme which destroys H2O2. This study suggests that H2O2 triggers the activation of platelets exposed to STC of collagen and arachidonic acid, via the cyclooxygenase pathway.     

Familial Bleeding Tendency with Partial Platelet Thromboxane Synthetase Deficiency: Reorientation of Cyclic Endoperoxide Metabolism

Three family members from three successive generations presented with a moderate bleeding tendency and a functional platelet defect. They had absent aggregation with arachidonic acid (0.6--3 microM), reversible aggregation with ADP (4 microgram) and cyclic endoperoxide analogues, single wave aggregation only with adrenaline (5.4 microgram) and a prolonged template bleeding time (> min). Malondialdehyde formation was reduced after N-ethylmaleimide stimulation (2--6 nmol/10(9) platelets; control values 8--12 nmol) and serum thromboxane B2 values were reduced (33--101 ng/ml; control values 200--700 ng/ml). When the platelets were incubated with [3H]arachidonic acid the final metabolite of the lipoxygenase pathway (HETE) was produced in normal amounts but the production of thromboxane B2 and HHT was decreased whereas prostaglandin F2a, and E2 and probably D2 were increased. Evidence for enhanced production of prostaglandin D2 was also provided by the rise in the patient's platelet cyclic AMP levels following stimulation with arachidonic acid. The patient's washed platelets stimulated the production of 6-keto PGF 1a by aspirin-pretreated cultured bovine endothelial cells. The plasma levels of 6-keto PGF1a (439--703 pg/ml; normal 181 +/- 46 pg/ml) were raised. The decreased production of thromboxane B2, HHT and malondialdehyde and increased formation of prostaglandin F2a, E2, D2 and of 6-keto PGF1a are compatible with a partial platelet thromboxane synthetase deficiency and reorientation of cyclic endoperoxide metabolism. The markedly prolonged bleeding time would result not only from reduced formation of thromboxane A2 but also from increased production of the aggregation inhibiting prostaglandins PGI2 and PGD2.     

Altered platelet function in cirrhosis of the liver: impairment of inositol lipid and arachidonic acid metabolism in response to agonists

Hemorrhagic disorders are common in patients with liver cirrhosis and result from several factors including impaired platelet function. We evaluated platelet aggregation and arachidonic acid metabolism in response to standard agonists in platelet-rich plasma from 12 cirrhotic patients with mild impairment of liver function (Child A), 12 patients with severe liver dysfunction (Child B and C) and 12 healthy subjects. Platelet aggregation and thromboxane A2 production were consistently reduced in patients with severe liver impairment. To determine whether the platelet dysfunction is due to an intrinsic platelet defect or a circulating inhibitor, we measured platelet aggregation and thromboxane A2 synthesis on washed platelets in healthy subjects and in Child B and C patients. The aggregating response of washed platelets in response to thrombin, collagen and arachidonic acid was markedly reduced, suggesting an intrinsic platelet defect. The biochemical events underlying platelet aggregation were investigated by prelabeling platelets with [1-14C]arachidonic acid. Thrombin-induced activation of phospholipase C (measured as the release of [1-14C]phosphatidic acid) and phospholipase A2 (measured as the release of [1-14C]arachidonic acid and its metabolites) was greatly impaired in platelets from patients with severe liver impairment. We conclude that in advanced cirrhosis there is a severe reduction in platelet aggregatory response to physiologic agonists due to an intrinsic platelet defect which is related to an impairment of the platelet transmembrane signaling mechanism induced by receptor stimulation.     

Short-term oral ingestion of Ginkgo biloba extract (EGb 761) reduces malondialdehyde levels in washed platelets of type 2 diabetic subjects

We have recently reported that ingestion of Ginkgo biloba extract (EGb 761) (a) significantly reduced collagen-induced platelet aggregation and thromboxane B2 (TXB2) production in both non-diabetic individuals as well as those with type 2 diabetes mellitus (T2DM), (b) significantly reduced platelet malondialdehyde (MDA), an index of lipid peroxidation, in non-diabetic subjects. In the present study we report that ingestion of EGb 761 (120 mg daily for 3 months), significantly decreased platelet MDA-thiobarbituric acid reacting substances (TBARS) (41 +/- 9 pmol/10(7) platelets versus 30 +/- 11 pmol/10(7) platelets) (p < 0.005) in T2DM subjects with normal cholesterol levels (total cholesterol, 164 +/- 22 mg/dl; age, 54 +/- 9 years; BMI, 35.0 +/- 8.8 kg/m2, n = 12). In T2DM subjects with high cholesterol (total cholesterol, 218 +/- 15 mg/dl; age, 52 +/- 5 years; BMI, 36.2 +/- 6.6 kg/m2, n = 7), EGb 761 ingestion reduced the platelet TBARS from 29 +/- 9 to 22 +/- 9 pmol/10(7) platelets (p < 0.04). Because ingestion of EGb 761 did not alter platelet counts it is concluded that EGb 761, probably due to the flavonoid fraction, reduced the TBARS by inhibiting cyclooxygenase (COX)-1-mediated arachidonic acid oxygenation or by reducing the arachidonic acid pool. This is likely to lead to a reduction of platelet hyperactivity, a significant contributor to the development of cardiovascular disease in T2DM patients. Because of other reported beneficial properties of EGb 761, such as stimulation of pancreatic beta-cell function in T2DM subjects with pancreatic exhaustion, it appears that T2DM subjects might benefit from ingesting EGb 761 as a dietary supplement.     

Platelet aggregation and dense granule secretion in a colony of dogs with spontaneous hypertension.

AIM: Since canine hereditary essential hypertension has been previously reported in a colony of Siberian husky dogs, we tested the suitability of this model for use in studies on the platelet defect identified in humans with essential hypertension. METHODS: Platelet aggregation and dense granule ATP secretion were measured in dogs with essential hypertension and normotensive control dogs. RESULTS: The platelets from the hypertensive group showed significantly increased aggregation and secretion in response to stimulation with ADP. There was no significant increase in aggregation or secretion when platelets from hypertensive dogs were stimulated by platelet-activating factor, thrombin, calcium ionophore A23187 or phorbol myristate acetate. CONCLUSION: The increased aggregation and secretion responses in platelets from the hypertensive dogs suggest that a defect similar to that reported in humans with essential hypertension exists and that further investigation is warranted.     

Influence of hydroxyl radical scavengers on platelet function

The influence of four hydroxil radical (OH.) scavengers on platelet function was investigated. OH. scavengers inhibited ADP, collagen, arachidonic acid, PAF-induced platelet aggregation, and platelet cyclooxygenase pathway activation, which was studied by evaluating platelet malondialdehyde and serum thromboxane A2 formation. The latter was not affected by superoxide dismutase, catalase, or metal ion chelants such as desferioxamine or DETAPAC. The detection of deoxyribose degradation by stimulated platelets suggested that platelets produce OH.. This study shows that activated platelets produce free radicals and that antioxidant agents such as OH. scavengers inhibit platelet function.     

Effects of copper-aspirin complex on plasma 6-keto-prostaglandin F(1alpha) level and platelet cytosolic calcium in rabbits

Effects of copper-aspirin complex on washed platelet aggregation, thromboxane B(2) formation and 6-ketoprostaglandin F(1alpha) level were monitored by Born's and Terashita's methods, respectively. The influence of copper-aspirin complex on cytosolic free calcium was examined using the fluorescent indicator, Fura 2-AM. Copper-aspirin complex significantly inhibited arachidonic acid-induced aggregation in washed platelets. The IC(50) value was 9.6 micromol L(-1). Copper-aspirin complex significantly decreased arachidonic acid-induced thromboxane B(2) formation by 87.1% in washed platelets. Ten mg kg(-1) of copper-aspirin complex given intragastrically markedly increased the plasma level of 6-keto-prostaglandin F(1alpha). Aspirin, however, reduced both thromboxane B(2) formation and 6-keto-prostaglandin F(1alpha) level. In the presence of CaCl2 1 mmol L(-1), copper-aspirin complex (20, 40 and 80 micromol L(-1)) markedly lowered arachidonic acid-induced increase in platelet calcium from the resting level (270+/-36 nmol L(-1)) to 213+/-14, 170+/-20 and 135+/-17 nmol L(-1), respectively. In the presence of ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid 1 mmol L(-1), copper-aspirin complex (20, 40 and 80 micromol l L(-1)) significantly suppressed the release of intracellular calcium induced by arachidonic acid from 127+/-23 nmol L-1 to 108+/-17, 93+/-12 and 70+/-13 nmol L(-1).     

Abnormal function and thromboxane release from platelets of dogs with cyclic hematopoiesis

Investigation of platelet function in dogs with cyclic hematopoiesis (CH) revealed a platelet aggregation disorder. Collagen-induced aggregation of CH dog platelets was significantly abnormal, although normal aggregation in response to ADP was observed. Aggregation was particularly defective on days 2-4 and 14 of the 14-day neutrophil cycle that is typical of CH dogs. The lack of response to collagen suggested a defect in the arachidonic acid pathway of platelet metabolism, since platelet-generated thromboxane-B2 levels were about 30% (p less than 0.0005) of control values. Platelets from dogs heterozygous for CH demonstrated moderately depressed responses to collagen that were intermediate between the values found for platelets from CH dogs and platelets from normal, mixed-breed dogs. Not only does this work indicate a platelet defect in CH dogs, but this phenomenon may be useful as a genetic marker for identification of dogs heterozygous for the CH gene.     

Indications for the presence of an atypical protease-activated receptor on rat platelets

 Activation of the protease-activated receptor (PAR)-1, one of four known PARs (PAR-1 to PAR-4), can be mimicked by thrombin receptor activating peptides (TRAPs) based on the PAR-1 tethered ligand. Interestingly, despite being activatable by thrombin, rodent platelets do not express PAR-1 and thus do not respond to PAR-1-derived TRAPs, indicating different activation mechanisms between human and rodent platelets. Using a rat platelet aggregation model, we determined that TRAPs based on the tethered ligand of PAR-1 fail to activate rat platelet aggregation at concentrations up to 1 mmol/l. In addition, TRAPs inhibit thrombin-mediated rat platelet aggregation, indicating the presence of a modified PAR-1 in this species. In order to determine characteristics of this putative receptor, we tested a panel of synthesized TRAPs based on the rat sequence (R) and human sequence (H) of the PAR-1 tethered ligand for their ability to inhibit thrombin-induced rat platelet aggregation. Peptides R1–9, R4–9, R4–10, and H4–10 inhibited rat platelet aggregation in response to α-thrombin [inhibitory concentration (IC) 50% 0.25–1.5 mmol/l]. None of these peptides blocked epinephrine-, collagen-, or arachidonic acid-induced platelet aggregation. Alanine substitution mapping of H4–10 indicated that both Leu⁴ and Arg⁵ are essential for inhibition. Inhibition of thrombin's catalytic activity required peptide concentrations tenfold higher than inhibition of platelet aggregation (IC50% 3–5 mmol/l). No prolongation of thrombin clotting time in response to TRAPs was detected at peptide concentrations up to 5 mmol/l. Our data suggest that (1) rat platelets express a PAR-1 subtype, (2) residues Leu⁴ and Arg⁵ of the tethered ligand peptide are required for binding to this new receptor, and (3) further analysis of peptide sequences might reveal a novel PAR-1 subtype. Received: 10 December 1999 / Accepted: 25 April 2000     

Interaction between platelets and lupus anticoagulant

10 consecutive patients fulfilled the diagnostic criteria for lupus anticoagulant. 4 had concomitant systemic lupus erythematosus, 1 Waldenstrom's disease and 5 had no apparent underlying disease. Only the case with Waldenstrom's disease presented a bleeding tendency, with bleeding time greater than 20 min; the others had a history of thrombotic complications. A defect of platelet aggregation induced by ADP, epinephrine, collagen and arachidonic acid was documented in the Waldenstrom's disease case whose lupus anticoagulant was an IgM. In the others, lupus anticoagulant, identified as IgG immunoglobulins, produced no aggregation abnormalities. However, beta-thromboglobulin levels in platelets, plasma and urine were consistent with a pattern of platelet activation in all cases. IgG immunoglobulins separated from sera of 6 patients showed lupus anticoagulant activity, with no effects on platelet aggregation of normal platelet-rich plasma, but they induced secretion of beta-thromboglobulin from normal platelets.     

Platelet function studies in coronary heart disease. IX. Increased platelet prostaglandin generation and abnormal platelet sensitivity to prostacyclin and endoperoxide analog in angina pectoris

Platelet prostaglandin generation (malondialdehyde production) and platelet sensitivity to prostacyclin (a vasodilator and platelet aggregation inhibitor) and to epoxymethanodienoic acid (EMA) (a vasoconstrictor and platelet aggregation stimulant endoperoxide analog) were studied in patients with angina pectoris and in control subjects. Platelet malondialdehyde production was higher in patients than in control subjects (mean ± standard error of the mean 2.50 ± 0.30 versus 1.70 ± 0.13 nmol/10⁹ platelets, p < 0.02). Platelets from patients were significantly less sensitive to prostacyclin's antiaggregatory effects than were those from control subjects (amount of prostacyclin required for 50 percent platelet aggregation inhibition 1.90 ± 0.35 versus 0.68 ± 0.05 ng, p < 0.02). Furthermore, less EMA was required to induce 50 percent platelet aggregation in patients with angina pectoris than in the normal subjects (133 ± 8 versus 194 ± 16 ng, p < 0.001). These observations suggest that increased platelet prostaglandin generation and abnormal platelet sensitivity to prostacyclin and endoperoxide analog in certain patients with coronary artery disease are important potential mechanisms in the pathogenesis of myocardlal ischemia.