Effect of prolonged treatment with acetylsalicylic acid and dipyridamole on platelet thromboxane A2 production in atherosclerotic subjects

Primary stimuli of icosanoid release inhibit arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. Mechanism of action of hydrogen peroxide and methyl mercury in platelets

Icosanoid formation in platelets depends on the concentration of free arachidonate that is mainly liberated from membrane phospholipids by phospholipase A2. The concentration of free arachidonate is also controlled by the activities of the reacylating enzymes arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. In human platelet microsomes we determined the high enzyme activities of 5.9 nmol.min-1.(10(9) platelets)-1 for the arachidonoyl-CoA synthetase and 37 nmol.min-1.(10(9) platelets)-1 for the lysophospholipid acyltransferase. The activities of these reacylating enzymes were strongly reduced by hydrogen peroxide (H2O2) and methyl mercury that are primary stimuli of arachidonate release in intact platelets. H2O2 inhibited the arachidonoyl-CoA synthetase with an IC50 of 3.3 mmol/l without affecting the lysophospholipid acyltransferase. Sulfhydryl group protection by 3-mercapto-1,2-propanediol did not overcome the inhibition but glutathione prevented the inhibition of the arachidonoyl-CoA synthetase by H2O2. This suggests that glutathione by virtue of the glutathione peroxidase reduces H2O2 rather than that it protects free sulfhydryl groups of the arachidonoyl-CoA synthetase. Methyl mercury left the arachidonoyl-CoA synthetase activity unaffected but inhibited the lysophospholipid acyltransferase activity with an IC50 of 3.4 mumol/l. The inhibition is probably evoked by the blockade of sulfhydryl groups of the lysophospholipid acyltransferase because it disappeared when 3-mercapto-1,2-propanediol was added at a concentration higher than that of methyl mercury. Thrombin as a physiological full agonist, Ca2+ less than or equal to 1 mmol/l, the calcium ionophore A23187 and phorbol 12-myristate 13-acetate (TPA) and 1-oleoyl-2-acetylglycerol as model stimuli of protein kinase C neither influenced arachidonoyl-CoA synthetase nor lysophospholipid acyltransferase. It is concluded that the inhibitory effect of H2O2 and methyl mercury on the arachidonate-reacylating enzymes arachidonoyl-CoA synthetase or lysophospholipid acyltransferase, respectively, are responsible for their capacity to stimulate icosanoid release in intact cells. Thrombin and its intracellular messengers Ca2+ and diacylglycerol do not directly affect arachidonoyl-CoA synthetase and lysophospholipid acyltransferase.

Acetylsalicylic acid and the balance between prostacyclin and thromboxane A2

Arachidonic acid is metabolized in endothelial cells to antiaggregatory, vasodilatory prostacyclin (PGI2), and in platelets to aggregatory, vasoconstrictory thromboxane A2 (TxA2). The balance of these two prostanoids is supposed to be involved with thrombogenesis and atherogenesis. Acetylsalicylic acid (ASA) inhibits irreversibly the key enzyme of the synthesis of these prostanoids, i.e. cyclo-oxygenase. Platelets do not synthetize new protein, but endothelial cells do. Because of this, and certain pharmacokinetic characteristics of ASA, it should be possible to shift the balance between PGI2 and TxA2 to the dominance of the former with the proper dose of this drug. Altogether more than 50,000 subjects have volunteered for studies on the effect of ASA in the primary or secondary prevention of myocardial infarction or ischemic stroke. The results show that it is possible to reduce vascular attacks by ASA. Furthermore, ASA has also found to prevent pre-eclampsia. Conclusions on the effect of ASA on the PGI2/TxA2-balance are hampered by uncertainties concerning the measurement PGI2 and TxA2 productions in vivo. It is, however, evident that the doses of ASA used in most trials have been high enough to inhibit partly also the production of PGI2. Whether smaller doses or less frequent administration would be more efficient, remains to be studied.

Effects of salicylic and acetylsalicylic acid alone and in combination on platelet aggregation and prostanoid synthesis in man

The present study was designed to investigate the effects of salicylate on the antiplatelet action of acetylsalicylic acid as well as on in vivo prostanoid formation and platelet function in healthy volunteers. In the first study six female volunteers received 350 mg acetylsalicylic acid intravenously, with and without previous oral administration of sodium salicylate (1200 mg daily for 3 days). Urinary prostanoid excretion as well as platelet aggregation and thromboxane formation were measured before and during salicylate and after acetylsalicylic acid. In the second study seven female volunteers received sodium salicylate (52.6 mg kg-1) or acetylsalicylic acid (60.7 mg kg-1) for 8 days in a randomized cross-over protocol. Urinary prostanoid excretion, platelet aggregation and thromboxane formation as well as salicylate plasma concentrations were determined before, during and after administration of each drug. Sodium salicylate did not impair the complete suppression of arachidonic acid-induced platelet thromboxane formation and aggregation obtained by the single intravenous dose of acetylsalicylic acid in the first study. Sodium salicylate in the second study did not affect urinary excretion of prostaglandin E2, its major urinary metabolite (7 alpha-hydroxy-5,11-diketo-tetranor-prostane-1,16-dioic acid), and 2,3-dinor-6-keto-prostaglandin F1 alpha, the main urinary metabolite of epoprostenol (prostacyclin, PGI2). In contrast, acetylsalicylic acid significantly decreased excretion rates of these prostanoids by 64, 59 and 61%, respectively. In both studies platelet aggregation and thromboxane formation induced by collagen, thrombin or arachidonic acid were not significantly affected by salicylate administration, whereas acetylsalicylic acid inhibited platelet aggregation induced by all three agents as well as thrombin- and arachidonic acid induced thromboxane formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential inhibition of platelet thromboxane and lung prostacyclin production by sulphinpyrazone, acetylsalicylic acid and indomethacin by human tissues in vitro

To compare the inhibition of human platelet and lung cyclo-oxygenases by sulphinpyrazone (SP), acetylsalicylic acid (ASA) and indomethacin, we investigated their effects on platelet thromboxane A2 (TxA2) production during spontaneous clotting and on prostacyclin (PGI2) and TxA2 productions of superfused minced human lung. The synthesis of proaggregatory, vasoconstricting TxA2 and antiaggregatory, vasodilating PGI2 were evaluated by measuring the concentration of their stable metabolites thromboxane B2 (TxB2) and 6-keto-prostaglandin F1 alpha respectively, by radioimmunoassays. The basal platelet TxB2 production was 241.0 +/- 56.3 ng/ml (mean +/- SEM, n = 12). The concentrations needed for 50% inhibition of this production (IC50) were 41.3 mumol/l for sulphinpyrazone, 6.3 mumol/1 for ASA and 0.094 mumol/l for indomethacin. The lung generated 23.8 +/- 5.5 ng/g/min (mean +/- SEM, n = 6) of 6-keto-PGF1 alpha and 8.5 +/- 1.8 ng/g/min of TxB2. The IC50 values for pulmonary 6-keto-PGF1 alpha and TxB2 productions were 530.0 mumol/l for SP, 370.0 mumol/l for ASA and 50.0 mumol/l for indomethacin. Thus pulmonary cyclo-oxygenase, presumably originating from endothelial cells, was 13, 59, and 532 times more resistant to these prostaglandin synthesis inhibitors (PGI's) than platelet cyclo-oxygenase. These data suggest that there are considerable differences in the concentration ranges of various PGI's by which the PGI2/TxA2 balance can be shifted to a dominance of PGI2.

One milligramme of acetylsalicylic acid daily inhibits platelet thromboxane A2 production

To seek the lowest dose of acetylsalicylic acid (ASA) capable of inhibiting platelet thromboxane A2 (TxA)2 production, 18 healthy volunteers ingested 9 mg, 3 mg or 1 mg of ASA/day for twenty days and the release of TxB2 (a metabolite of TxA2) during the spontaneous clotting of blood was measured by radioimmunoassay. In addition, the production of prostacyclin (epoprostenol, PGI2) was investigated by measuring the urinary excretion of its break-down product, 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) by radioimmunoassay. Significant inhibition of platelet TxA2 production was seen from the 15th day of treatment onwards with 1 mg of ASA (maximally 15%), from the 4th day of treatment onwards with 3 mg of ASA (maximally 40%), from the 1st day of treatment onwards with 9 mg of ASA (maximally 67%). No ASA dose changed platelet counts or urinary 6-keto-PGF1 alpha excretion. One mg of ASA daily, is the lowest dose ever shown to inhibit platelet TxA2 production.