Platelet Thromboxane A2 Generation Research Articles

Acetyl-CoA carboxylase 1 deletion reduces platelet phospholipid and alters thrombus formation

Acetyl-CoA carboxylase (ACC) is the key enzyme regulating de novo lipogenesis (DNL). In recent years, ACC inhibitors has been of great interest to handle a wide range of diseases, especially cancers and cardiovascular diseases. However, the impact of its inhibition on hemostasis and thrombosis remains under-investigated. We previously demonstrated that ACC1 activation promotes thrombus formation by increasing platelet phospholipid content and thromboxane A2 generation. Here, we sought to evaluate whether ACC1 platelet-specific deletion may affect platelet functions and by decreasing phospholipid content. To test this hypothesis we generated a new Cre transgenic mouse strain in which ACC1 is specifically deleted in megakaryocyte and platelet (GpIbCre±x ACC1 flx/flx mouse). Lipidomics analysis was carried out on the commercial Lipidyzer platform. Thromboxane A2 secretion was evaluated by ELISA. Platelet functions were assessed in vitro by aggregometry and ex vivo by flow chamber assay. Thrombus growth was analyzed in vivo with intravital microscopy. As expected, ACC1 deletion was restricted to megakaryocytic lineage. Hematological parameters show a platelet count decrease (30%) and a platelet volume increase (31%) in ACC1 knockout mice (ACC1 pKO) compared to the control mice. In agreement with our hypothesis, lipidomics analyses highlight a significant reduction in arachidonic acid containing phosphotidylethanolamine plasmalogen in ACC1 KO platelets. Logically, thromboxane A2 generation (upon thrombin or CRP stimulation) is also altered in ACC1 pKO mice in comparison to controls. This decrease in phospholipid content is associated with a default in thrombin and CRP-induced platelet aggregation and with impaired dense granules secretion. In line with these results, platelet-specific ACC1 deletion reduces thrombus growth and fibrinogen receptor activation. Platelet-specific ACC1 deletion leads to a decrease in phospholipid content which, in turn, decreases platelet thromboxane A2 generation, dense granules secretion and platelet aggregation. Thrombus formation is also reduced in platelet-specific ACC1 knockout mice, highlighting the importance of de novo lipogenesis and ACC-produced lipids in the regulation of platelet functions.

Antiplatelet Effect of Daphnetin Is Regulated by cPLA2-Mediated Thromboxane A2 Generation in Mice.

Coumarin derivatives have been recognized for their antithrombotic, anti-inflammatory, and antioxidant properties, and daphnetin is one of the natural coumarin derivatives isolated from Daphne Koreana Nakai. Although the pharmacological value of daphnetin is well documented in diverse biological activities, its antithrombotic effect has not been studied to date. Here, we characterized the role and underlying mechanism of daphnetin in the regulation of platelet activation using murine platelets. In order to check the effect of daphnetin on platelet function, we first measured the effect of daphnetin on platelet aggregation and secretion. Collagen-induced platelet aggregation and dense granule secretion were partially inhibited by daphnetin. Interestingly, 2-MeSADP-induced secondary waves of aggregation and secretion were completely inhibited by daphnetin. It is known that 2-MeSADP-induced secretion and the resultant secondary wave of aggregation are mediated by the positive feedback effect of thromboxane A2 (TxA2) generation, suggesting the important role of daphnetin on TxA2 generation in platelets. Consistently, daphnetin did not affect the 2-MeSADP-induced platelet aggregation in aspirinated platelets where the contribution of TxA2 generation was blocked. Additionally, platelet aggregation and secretion induced by a low concentration of thrombin, which is affected by the positive feedback effect of TxA2 generation, were partially inhibited in the presence of daphnetin. Importantly, 2-MeSADP- and thrombin-induced TxA2 generation was significantly inhibited in the presence of daphnetin, confirming the role of daphnetin on TxA2 generation. Finally, daphnetin significantly inhibited 2-MeSADP-induced cytosolic phospholipase A2 (cPLA2) and ERK phosphorylation in non-aspirinated platelets. Only cPLA2 phosphorylation, not ERK phosphorylation, was significantly inhibited by daphnetin in aspirinated platelets. In conclusion, daphnetin plays a critical role in platelet function by inhibiting TxA2 generation through the regulation of cPLA2 phosphorylation.

Anti‐platelet effect of prednisolone is regulated by ERK‐mediated thromboxane generation

Endogenous hypercortisolism is associated with increased risk of blood‐related diseases such as high blood pressure and hyperglycemia that result in fatal cardiovascular disorders. Platelet plays a major role in hemostasis and thrombosis. Although it is well known that glucocorticoids exert a significant effect on hemostasis, the effect of endogenous cortisol on platelet function is much less clear.We aimed to characterize the in vitro effect of prednisolone and its molecular basis on regulation of platelet function using murine platelets.We first evaluated the concentration‐dependent effect of prednisolone on 2‐MeSADP‐induced platelet function and found that 2‐MeSADP‐induced secondary wave of aggregation and dense granule secretion were completely inhibited from 500 nM prednisolone. Since 2‐MeSADP‐induced secretion and the resultant secondary wave of aggregation is mediated by thromboxane A2 (TxA2) generation, this result suggested a role of prednisolone on TxA2generation in platelets. Consistently, prednisolone did not affect the 2‐MeSADP‐induced aggregation in aspirin‐treated platelets, where the secondary wave of aggregation and secretion were blocked by eliminating the contribution of TxA2 generation by aspirin. In addition, AYPGKF‐ and thrombin‐induced platelet aggregation and secretion were inhibited in the presence of prednisolone by inhibiting the positive‐feedback effect of TxA2 generation on platelet function. Furthermore, prednisolone completely inhibited 2‐MeSADP‐induced TxA2 generation confirming the role of prednisolone on TxA2 generation. Finally, Western blot analysis revealed that prednisolone inhibited the 2‐MeSADP‐induced ERK phosphorylation.In conclusion, prednisolone affects platelet function by the inhibition of TxA2 generation through the regulation of ERK phosphorylation, thereby shedding light on the clinical characterization and its treatment efficacy in patients with cardiovascular disorders in the future.

Characterization of molecular basis of anti‐platelet effect of daphnetin

Coumarin derivates have been known for their antithrombotic, anti‐inflammatory, and antioxidant activities and daphnetin is one of the natural coumarin derivatives isolated from Daphne Korean Nakai. Although the pharmacological value of daphnetin is well documented in diverse biological activities, its antithrombotic effect is not studied till date.We investigated the functional role of daphnetin and its molecular basis on the regulation of platelet activation using murine platelets.In order to check the effect of daphnetin on GPVI‐mediated platelet response, we first measured the effect of daphnetin on collagen‐induced platelet aggregation and secretion. Collagen‐induced platelet aggregation and dense granule secretion were partially inhibited in the presence of daphnetin. Interestingly, 2‐MeSADP‐induced secondary wave of aggregation and secretion were completely inhibited in the presence of daphnetin. It has been known that 2‐MeSADP‐induced secretion and the resultant secondary wave of aggregation are mediated by positive‐feedback effect of thromboxane A2 (TxA2) generation, suggesting the important role of daphnetin on TxA2 generation in platelets. Consistently, daphnetin did not affect the 2‐MeSADP‐induced platelet aggregation in aspirin‐treated platelets where the contribution of TxA2 generation is blocked. Additionally, AYPGKF‐induced platelet aggregation and secretion were inhibited in the presence of daphnetin by blocking the positive‐feedback effect of TxA2 generation. Moreover, 2‐MeSADP‐induced TxA2 generation was completely inhibited in the presence of daphnetin confirming the role of daphnetin on TxA2 generation. Finally, 2‐MeSADP‐induced ERK phosphorylation was inhibited in the presence of daphnetin.We conclude that daphnetin plays a role in platelet function by inhibiting the TxA2 generation through the regulation of ERK phosphorylation.

Platelet-specific deletion of acetyl-CoA carboxylase 1 decreases phospholipid content and impairs platelet functions

Abstract Background Acetyl-CoA carboxylase (ACC), the first enzyme regulating lipid synthesis, promotes thrombus formation by increasing platelet phospholipid content and thromboxane A2 generation. Purpose Our study sought to evaluate whether ACC1 platelet-specific deletion may affect platelet functions by decreasing phospholipid content. Methods We generated a new Cre transgenic mouse strain that allows megakaryocyte/platelet specific ACC1 deletion (GpIbCre+/− x ACC1 flx/flx mouse). In vitro, platelet functions were assessed by aggregometry and flow cytometry. In vivo, hemostasis was assessed via the measurement of bleeding time. Lipidomics analysis was carried out on the commercial Lipidyzer platform. Thromboxane A2 secretion was evaluated by ELISA. Results As expected, ACC1 deletion was restricted to the megakaryocytic lineage. Hematological parameters in platelet-specific ACC1 knockout mice showed a decrease in platelet count by 30% and an increase in platelet volume by 31%, compared to ACC1 flx/flx platelets. In vitro, platelets from platelet-specific ACC1 knockout mice displayed a decrease in thrombin and CRP-induced platelet aggregation, associated with impaired dense granules secretion. In contrast, ADP-induced platelet aggregation was higher in the absence of ACC1. In vivo, platelet-specific ACC1 knockout mice showed a normal bleeding time. In agreement with our hypothesis, lipidomics analyses showed that ACC1 deletion in platelets was associated with a significant decrease in arachidonic acid-contaning phosphatidylethanolamine plasmalogen, and subsequently with a reduced production of thromboxane A2 upon thrombin or CRP stimulation. Conclusion Platelet-specific ACC1 deletion led to a decrease in phospholipid content which, in turn, decreased platelet thromboxane A2 generation, dense granules secretion and aggregation upon thrombin and CRP, but not ADP stimulation. Further studies are needed to elucidate the impact of ADP on platelet functions Funding Acknowledgement Type of funding sources: Other. Main funding source(s): Fonds pour la formation à la Recherche dans l'Industrie et l'Agriculture (FRIA)

Tetrahydrocurcumin Downregulates MAPKs/cPLA2 Signaling and Attenuates Platelet Thromboxane A2 Generation, Granule Secretion, and Thrombus Growth.

Platelet granule secretion plays a key role in atherothrombosis. Curcumin, a natural polyphenol compound derived from turmeric, exerts multiple biological activities. The current study sought to investigate the efficacy of tetrahydrocurcumin (THC, the major active metabolite of curcumin) on platelet granule secretion in vitro and thrombus formation in vivo. We found that THC significantly attenuated agonist-induced granule secretion in human gel-filtered platelets in vitro, including CD62P and CD63 expression and platelet factor 4, CCL5, and adenosine triphosphate release. These inhibitory effects of THC were partially mediated by the attenuation of cytosolic phospholipase A2 (cPLA2) phosphorylation, leading to a decrease in thromboxane A2 (TxA2) generation. Moreover, the MAPK (Erk1/2, JNK1/2, and p38 MAPK) signaling pathways were downregulated by THC treatment, resulting in reduced cPLA2 activation, TxA2 generation, and granule secretion. Additionally, THC and curcumin attenuated murine thrombus growth in a FeCl3-induced mesenteric arteriole thrombosis model in C57BL/6J mice without prolonging the tail bleeding time. THC exerted more potent inhibitory effects on thrombosis formation than curcumin. Through blocking cyclooxygenase-1 activity and thus inhibiting platelet TxA2 synthesis and granule secretion with aspirin, we found that THC did not further decrease the inhibitory effects of aspirin on thrombosis formation. Thus, through inhibiting MAPKs/cPLA2 signaling, and attenuating platelet TxA2 generation, granule secretion, and thrombus formation, THC may be a potent cardioprotective agent.

Role of CalDAG-GEFI in Calcium-Dependent Thromboxane A2 Generation

The second messenger Ca2+ is essential for various platelet functions, including integrin activation, thromboxane A2 (TxA2) generation and granule release. In previous studies, we have identified Ca2+- and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI) as a key molecule regulating Ca2+-dependent activation of Rap1 and integrins in platelets. Here we demonstrate that CalDAG-GEFI/Rap1 signaling is required for Ca2+-dependent ERK 1/2 activation and TxA2 generation in platelets. Impaired TxA2 formation was observed in collagen- and convulxin-activated CalDAG-GEFI deficient platelets, both at high and low doses of agonists. In contrast, no defect in TxA2 generation was observed in mutant platelets stimulated with high doses of PAR4-activating peptide, suggesting that CalDAG-GEFI may be particularly important for TxA2 generation in platelets activated through the major collagen receptor, GPVI. Our data further demonstrate a strong correlation between reduced TxA2 generation and impaired activation of ERK1/2 and Rap1 in mutant platelets. Pretreatment of CalDAG-GEFI−'/− platelets with 2-MesAMP, an inhibitor of P2Y12/Gi signaling, blocked the residual TxA2 production. These data suggest that CalDAG-GEFI synergizes with the Gi signaling pathway to control TxA2 generation. Finally, addition of thromboxane A2 analog, U46619, restored dense granule release in CalDAG-GEFI-deficient platelets stimulated with low doses of convulxin, suggesting that impaired TxA2 generation contributes to the defect in granule release previously described for CalDAG-GEFI−/− platelets. In conclusion, our studies strongly support that CalDAG-GEFI is the elusive Ca2+ sensor that links increases in intracellular Ca2+ to the signaling pathways regulating integrin activation and TxA2 formation in stimulated platelets.

Risks and Benefits of Antiplatelet Therapy in Uremic Patients

Patients with renal insufficiency are at an increased risk for cardiovascular morbidity and mortality. Despite being at a substantial risk for thrombotic events, patients with renal insufficiency also experience a greater number of hemorrhagic complications associated with antiplatelet therapy than individuals with normal renal function. This review focuses on the benefits and risks of antiplatelet therapy in patients with impaired kidney function suffering from an acute coronary syndrome.

Regulation and functional consequences of ADP receptor-mediated ERK2 activation in platelets

We have previously shown that ADP-induced thromboxane generation in platelets requires signalling events from the G(q)-coupled P2Y1 receptor (platelet ADP receptor coupled to stimulation of phospholipase C) and the G(i)-coupled P2Y12 receptor (platelet ADP receptor coupled to inhibition of adenylate cyclase) in addition to outside-in signalling. While it is also known that extracellular calcium negatively regulates ADP-induced thromboxane A2 generation, the underlying mechanism remains unclear. In the present study we sought to elucidate the signalling mechanisms and regulation by extracellular calcium of ADP-induced thromboxane A2 generation in platelets. ERK (extracllular-signal-regulated kinase) 2 activation occurred when outside-in signalling was blocked, indicating that it is a downstream event from the P2Y receptors. However, blockade of either P2Y1 or the P2Y12 receptors with corresponding antagonists completely abolished ERK phosphorylation, indicating that both P2Y receptors are required for ADP-induced ERK activation. Inhibitors of Src family kinases or the ERK upstream kinase MEK [MAPK (mitogen-activated protein kinase)/ERK kinase] abrogated ADP-induced ERK phosphorylation and thromboxane A2 generation. Finally ADP- or G(i)+G(z)-induced ERK phosphorylation was blocked in the presence of extracellular calcium. The present studies show that ERK2 is activated downstream of P2Y receptors through a complex mechanism involving Src kinases and this plays an important role in ADP-induced thromboxane A2 generation. We also conclude that extracellular calcium blocks ADP-induced thromboxane A2 generation through the inhibition of ERK activation.

Coxibs and Cancer Prevention

Preventive medicine is becoming a cornerstone in our concept of health. This is especially significant in regard to cancer, as cancer is predicted to become the leading cause of death, surpassing heart disease, by the end of this decade. The prevention of colorectal cancer (CRC) has become an important public health goal because of the high incidence of CRC, with more than 945,000 new cases expected worldwide in 2006, and the considerable mortality and morbidity associated, with more than 492,000 deaths expected worldwide in the same year. The past 2 decades have seen the emergence of chemopreventive agents that have 1 of 3 effects: inhibiting, delaying, or reversing carcinogenesis. Notwithstanding a substantial body of evidence suggesting an inverse relationship between aspirin or nonsteroidal anti-inflammatory drug use and CRC incidence and mortality, the use of traditional nonsteroidal anti-inflammatory drugs in the chemoprevention of CRC is limited by their gastrointestinal toxicity. The favorable gastrointestinal safety profile of selective cyclooxygenase-2 inhibitors has therefore made them particularly attractive for this purpose. There has been concern, however, that selective cyclooxygenase-2 inhibitors may increase the risk of cardiovascular events, possibly by reducing endothelial prostacyclin production while leaving platelet thromboxane A2 generation unopposed. In the intriguing jigsaw puzzle of cancer prevention, we now have a definite positive answer for the basic question "if," but several other parts of the equation (proper patient selection, ultimate drug, optimal dosage, and duration) are missing. The most challenging task is to find the proper place for these interventions in the entire effort of cancer prevention, in subjects at risk for colorectal neoplasia, and in those at risk for other tumors. The achievement of this important goal may contribute to the conversion of CRC into a truly preventable disease.

Mechanism of Activation and Role of Extracellular Signal Regulated Kinase in ADP-Induced Thromboxane A2 Generation in Platelets.

We have previously shown that ADP-induced thromboxane A2 generation in platelets requires co-ordinated signaling events from the Gq-coupled P2Y1 receptor and the Gi-coupled P2Y12 receptor in addition to outside-in signaling. It is also known that ADP-induced thromboxane A2 generation is completely abolished in the presence of extracellular calcium, but the mechanism of this negative regulation is not known. In this study we sought to identify the important signaling molecules in ADP-induced thromboxane A2 generation in platelets and characterize the regulation of these molecules by extracellular calcium. Erk2 activation occurred when outside-in signaling was blocked, indicating that it is a downstream event from the P2Y receptors. However, blockade of either P2Y1 or the P2Y12 receptors with corresponding antagonists completely abolished Erk phosphorylation, indicating that both P2Y receptors are required for ADP-induced Erk activation. However, blockade of Erk upstream kinase MEK had not effect on ADP-induced aggregation in aspirin-treated platelets, but dramatically inhibited aggregation as well as secretion in non-aspirinated platelets, suggesting that Erk might be important for thromboxane A2 generation. Finally, PP1 and PP2, inhibitors of Src family kinases, but not PP3, an inactive analog, abolished ADP-induced Erk phosphorylation and thromboxane A2 generation. Interestingly, ADP-induced Erk phosphorylation was completely inhibited in the presence of extracellular calcium, indicating that Erk is a key signaling molecule regulated by extracellular calcium in the negative regulation of thromboxane A2 generation. We conclude that Erk2 is activated downstream of P2Y receptors through a complex mechanism involving Src kinases and plays an important role in ADP-induced thromboxane A2 generation. We also conclude that extracellular calcium blocks ADP-induced thromboxane A2 generation through the inhibition of Erk activation.

Platelet activation is increased in cyclosporin A-induced hypertensive rats.

One of the most severe side effects of the immunosuppressive agent, cyclosporin A (CsA), is increased risk of thromboembolic complications and drug-related hypertension. Because platelets might be involved in these processes, we tested the possibility of CsA affecting platelet activation, which might contribute to these adverse drug reactions. The experiments were done using Wistar rats, treated or not (control) with CsA (Sandimmun Neoral), 5 and 30 mg/kg/day, for 7 weeks. Systolic, diastolic, and mean blood pressures, intracellular free calcium concentration ([Ca2+]i), platelet serotonin (5-HT) contents, and aggregation were determined, at weeks 0, 2, and 7 of treatment. Inositol phosphates (InsP) production, platelet thromboxane A2 (TXA2) generation, and morphology of platelets, through electron microscopy studies, also were compared. It was demonstrated that blood pressures increased in the CsA-treated groups, when compared with the control group, after 2 and 7 weeks of administration. CsA at both "attack" and "maintenance" doses increased basal, 5-HT, and thrombin-evoked [Ca2+]i after 2 and 7 weeks versus the control group. However, basal and evoked InsP production was stimulated by 5 mg/kg of CsA, but inhibited by 30 mg/kg, when compared with the control. Platelet 5-HT contents decreased significantly after 2 and 7 weeks in the CsA-treated groups, when compared with the control group. Collagen-induced whole blood platelet aggregation increased drastically in the "attack" CsA-treated group, whereas adenosine diphosphate (ADP)-induced platelet aggregation did not reach statistical significance. Finally, in vitro basal, collagen-, and ADP-evoked platelet TXA2 generation increased in both CsA concentrations, versus the control. In conclusion, our study demonstrates that both CsA doses alter platelet calcium homeostasis (even affecting the calcium fluxes differently), 5-HT and TXA2 contents and aggregation, which might contribute to the development and/or maintenance of high blood pressures and increased risk of thromboembolic complications.

The Peripheral Serotonergic System and Platelet Aggregation in Cyclosporin A-Induced Hypertensive Rats

Cyclosporin A plays an important role in preventing rejection in allograft transplant recipients. However, the therapeutic use of cyclosporin A is associated with increased incidence of thromboembolic complications and drug-related hypertension. In order to study the mechanisms by which cyclosporin A induces these abnormal pathophysiological situations, we have assessed the platelet serotonin contents and whole blood platelet aggregation in control rats as well as in rats treated (orally) with 30 and 5 mg/kg/day of cyclosporin A, after 2 and 7 weeks of treatment. These doses correspond respectively to CsA “peak” and “trough” concentrations achieved in human blood in clinical practice (immediately following an intake of a daily dose of CsA and when the blood concentration stabilizes, respectively). Both trough and peak doses caused an increase in blood pressure after 2 and 7 weeks. Platelet serotonin content decreased in the cyclosporin-treated groups, in contrast with the control. Collagen-induced whole blood platelet aggregation increased drastically for the peak concentration-treated group, while adenosine 5′-diphosphate-induced platelet aggregation did not reach statistical significance. Finally, in vitro platelet thromboxane A2 generation increased in cyclosporin A concentrations when platelets were stimulated with either collagen or adenosine 5′-diphosphate. In conclusion, both tested cyclosporin A concentrations induced important changes in platelet serotonin and thromboxane content and aggregation, factors which may play a decisive role in the development and/or maintenance of hypertension and thrombotic complications.

Activation of Ca(2+)-sensitive cytosolic phospholipase A2 (cPLA2) in human platelets.

Blood platelets respond to the physiological agonist thrombin with shape change, aggregation and release of granular contents (1). Thrombin also evokes the rapid release of arachidonic acid esterified to platelet membrane phospholipids and thereby initiates the synthesis of prostaglandin endoperoxides, thromboxane A2 (TXA2) and other metabolites. The release of arachidonic acid from thrombin-stimulated platelets can be attributed largely to the action of PLA2 that specifically hydrolyzes phospholipids containing arachidonic acid (2,3). Platelet TXA2 generation represents a major pathway mediating both physiological and pathological platelet activities (4,5) and therefore the modulation of the involved PLA2 in platelets may play an important role in both haemostasis and thrombosis.

Defective Platelet Aggregation in Response to Platelet-Activating Factor in Uremia Associated With Low Platelet Thromboxane A2 Generation

The bleeding tendency associated with uremia is likely due to a qualitative platelet dysfunction. So far the data available on platelet aggregation are conflicting. Since platelet-activating factor (PAF) plays a role in primary hemostasis, we studied platelet aggregation in response to PAF in 40 patients with chronic uremia on regular hemodialysis and 12 control subjects. Our results showed that in 28 of 40 uremics, platelet aggregation response to PAF was normal, whereas in the remaining 12 it was defective in that no second wave of aggregation was elicited even if the PAF concentrations were increased by a factor of 10,000. This abnormal response was peculiar to PAF and only partially related to factor(s) of plasma origin. The number of platelet PAF receptors and their affinity for the agonist were comparable in controls and "PAF-unresponsive" patients. The defective platelet aggregation in response to PAF was associated with a statistically significant reduction (P less than 0.01) in thromboxane A2 (TxA2) generation in platelet-rich plasma (PRP) challenged with PAF (10 and 100 nmol/L). When PRPs from PAF-unresponsive patients were preincubated with a stable analogue of prostaglandin endoperoxides/TxA2 U-46619, an irreversible platelet aggregation in response to PAF was obtained. Thus in a subpopulation of uremics, platelet aggregation in response to PAF is selectively abnormal as a consequence of a reduced TxA2 generation.

Pharmacokinetics of enteric-coated aspirin and inhibition of platelet thromboxane A2 and vascular prostacyclin generation in humans.

We evaluated whether an enteric-coated aspirin formulation showed a "presystemic" component in its antiplatelet effect and if so would spare vascular cyclooxygenase. In six healthy volunteers, 30 to 45 minutes after ingestion of 325 mg enteric-coated aspirin, platelet thromboxane A2 generation was inhibited by about 20% before any drug could be detected in the peripheral venous blood. A further decline in thromboxane A2 generation occurred with appearance of aspirin in blood between 60 and 240 minutes. No presystemic component could be detected after 325 mg aspirin tablets. Ten patients undergoing saphenectomy received 325 mg of either aspirin tablet or enteric-coated aspirin; 12 hours later platelet thromboxane A2 and peripheral vascular prostacyclin generation were significantly reduced by 98% and 58%, respectively. The effects of the two aspirin formulations were not different. Aspirin formulations with "presystemic" component in their antiplatelet effect may not necessarily result in sparing of peripheral vascular cyclooxygenase.