Thromboxane A2 Synthetase Research Articles

Inhibitory effects of trimetazidine dihydrochloride on aggregation, serotonin release and malondialdehyde production in rabbit platelets.

Aggregation, serotonin release and malondialdehyde (MDA) production via cyclooxygenase and thromboxane A2 synthetase were investigated in rabbit platelets. Trimetazidine dihydrochloride (TMZ) attenuated the collagen-induced aggregation more strongly than the arachidonic acid (AA)-, thromboxane A2 agonist (U-46619)-, Ca2+-ionophore (A-23187)- and ADP-induced aggregation: IC50 values were 1.0 +/- 0.1, 4.4 +/- 0.3, 4.3 +/- 0.4, 4.1 +/- 0.7 and 3.3 +/- 0.2 mM, respectively. TMZ decreased dose-dependently the serotonin release induced by collagen and A-23187, but did not decrease that induced by AA. TMZ also decreased the MDA production induced by collagen and A-23187 (IC50: 0.3 +/- 0.03 and 1.0 +/- 0.1 mM, respectively), but did not decrease the production induced by AA. Furthermore, TMZ decreased dose-dependently the MDA production induced by exogenous phospholipase A2. On the other hand, indomethacin (10 microM) attenuated the aggregation induced by collagen and AA, but not by the other agents, and decreased the serotonin release and the MDA production induced by collagen, A-23187 and AA. The present results suggest that TMZ may inhibit the process preceding the cyclooxygenase pathway in the AA cascade, and subsequently may attenuate the aggregation and the serotonin release via thromboxane A2 production from endogenous AA.

O-30 - Effect of OKY-046, a selective thromboxane (TX)A2 synthetase inhibitor, on PAF-induced airway hyperresponsiveness in guinea pigs

O-30 - Effect of OKY-046, a selective thromboxane (TX)A2 synthetase inhibitor, on PAF-induced airway hyperresponsiveness in guinea pigs

Thromboxane A2 synthetase inhibitors. I. Syntheses and activities of various N-heteroaromatic derivatives.

Basic N-heteroaromatic derivatives (1, 2, 4-triazole, thiazole and pyrimidine derivatives) having a 2-[4-(carboxy)phenoxy]ethyl moiety or a 4-[2-(carboxy)vinyl]benzyl moiety were prepared, and evaluated for ability to inhibit thromboxane A2 (TXA2) synthesis. Among the compounds prepared in this study, the 5-substituted thiazole derivatives 14d, 27 and 28 were more potent inhibitors of TXA2 production than the corresponding imidazole derivatives, and the 1-substituted 1H-1, 2, 4-triazole derivatives 14a and 15a were almost as potent as the corresponding imidazole derivatives.

Effects of prostaglandins on perilymphatic oxygenation. Enhancement of cochlear autoregulation by prostacyclin.

Effects of prostaglandin E1, prostacyclin, a prostacyclin analogue (OP-2507), and an inhibitor of thromboxane A2 synthetase (OKY-046) on the perilymphatic oxygen tension were measured in the feline tympanic perilymph under various respiratory conditions and blood pressure ranges. Factors influencing the capacity of the autoregulative properties of the inner ear vessels to accommodate changes in systemic blood pressure were also examined. Intravenous administration of prostacyclin, OP-2507, and OKY-046 prevented a decrease in perilymphatic oxygen tension when the systemic blood pressure decreased. Even a negative correlation of the differential coefficients between the perilymphatic oxygen tension and the systemic blood pressure was noticed during the administration of prostacyclin. Thus, prostacyclin enhanced the autoregulative property of the inner ear vessels. Furthermore, correlation studies between perilymphatic oxygen tension and cochlear blood flow demonstrated a partial dissociation, indicating the presence of autoregulation in the mean systemic blood pressure range under 80 mmHg.

Effects of OKY-046, a selective thromboxane A2 synthetase inhibitor, on ventricular arrhythmias and prostaglandins during coronary artery ligation and reperfusion in anesthetized dogs.

Effects of OKY-046, a selective thromboxane synthetase inhibitor, on ischemia-induced ventricular arrhythmias were investigated in anesthetized dogs. OKY-046 (30 mg/kg, intravenously) decreased ventricular arrhythmias significantly both during 30 min of coronary artery ligation and subsequent reperfusion, which corresponded with a significant reduction of thromboxane concentration, suggesting that thromboxane released during ligation is an important factor in the development of ventricular arrhythmias. Inhibition of thromboxane synthesis with OKY-046 might be useful for the prevention of ischemia-induced ventricular arrhythmias.

Effects of OKY-046, a Selective Thromboxane A2 Synthetase Inhibitor, on Ventricular Arrhythmias and Prostaglandins during Coronary Artery Ligation and Reperfusion in Anesthetized Dogs

Effects of OKY-046, a selective thromboxane synthetase inhibitor, on ischemia-induced ventricular arrhythmias were investigated in anesthetized dogs. OKY-046 (30 mg/kg, intravenously) decreased ventricular arrhythmias significantly both during 30 min of coronary artery ligation and subsequent reperfusion, which corresponded with a significant reduction of thromboxane concentration, suggesting that thromboxane released during ligation is an important factor in the development of ventricular arrhythmias. Inhibition of thromboxane synthesis with OKY-046 might be useful for the prevention of ischemia-induced ventricular arrhythmias.

Thromboxane A2 production in allergen-induced immediate and late asthmatic responses. Its possible role in inducing the late response.

To determine whether thromboxane A2 (TXA2) participates in allergen-induced asthmatic responses, we measured plasma TXB2 levels during allergen-induced immediate and late asthmatic responses (IAR and LAR) in atopic asthmatics. We also examined the effect of a TXA2 synthetase inhibitor, OKY-046, on the responses. Plasma TXB2 levels increased in both IAR and LAR, being 2.2 times higher in LAR than in IAR. Plasma TXB2 levels in LAR reached the peak 4 hours after allergen challenge when FEV1 had just begun to decrease in LAR. OKY-046 inhibited both IAR and LAR with decreased plasma TXB2 levels. We conclude that TXA2 is produced in allergen-induced asthmatic responses and participates in the responses, especially in inducing LAR.

Effects of inhibition of thromboxane A 2 synthesis in aspirin-induced asthma

Since inhibition of cyclooxygenase precipitates asthmatic attacks in patients with aspirin idiosyncrasy, we have evaluated the effects of pharmacologic inhibition of thromboxane A2 (TXA2) synthetase, next to cyclooxygenase enzyme in arachidonic acid cascade. Sixteen patients with aspirin-induced asthma received increasing doses on 3 days (25 to 400 mg) of an imidazole derivative, OKY-046, which specifically blocks TXA2 synthetase. Twenty-three healthy control subjects received a single dose of 400 mg of OKY-046. In both patients and control subjects, the inhibitor at a dose of 400 mg produced (1) a pronounced fall in thromboxane B2 serum levels, (2) a rise in serum 6-keto-prostaglandin F1 alpha, and (3) a depression in platelet aggregability to arachidonic acid and adenosine diphosphate. The drug, however, neither precipitated attacks of asthma nor impaired pulmonary function tests throughout a 24-hour observation period. Five patients, but none of the control subjects, developed transient nasal congestion about 1 hour after taking the drug. Thus, inhibition of TXA2 synthetase, contrary to inhibition of cyclooxygenase, does not affect bronchopulmonary function in patients with asthma and aspirin intolerance.

Possible Role of Endothelial Thromboxane A2 in the Resting Tone and Contractile Responses to Acetylcholine and Arachidonic Acid in Canine Cerebral Arteries

The amount of immunoreactive thromboxane B2 (iTXB2) released from isolated canine arteries was determined by radioimmunoassay. The amount of iTXB2 released from the cerebral, coronary, mesenteric, and saphenous arteries was 47.0 +/- 7.2, 4.0 +/- 0.6, 4.9 +/- 0.5, and 2.7 +/- 0.4 pg/mg wet weight tissue/30 min, respectively. The release of iTXB2 from the cerebral artery was decreased to less than 50% by the administration of indomethacin (10(-5) M) or OKY-046 (10(-4) M), and by intimal rubbing. The release of iTXB2 was enhanced nearly twofold by the addition of arachidonic acid (AA) (10(-5) M) to the medium, but not by the addition of acetylcholine (ACh) (10(-6) M). The cerebral arterial strips maintained the resting tone, which was reduced maximally by papaverine (10(-4) M). The resting tone was also reduced dose dependently by a cyclooxygenase inhibitor (indomethacin), a thromboxane A2 (TXA2) synthetase inhibitor (OKY-046), and a TXA2 antagonist (ONO-3708). The resting tone of rubbed strips was about half that of intact strips. ACh and AA induced similar transient contractions in the cerebral artery. Contractions produced by these agents were attenuated by indomethacin (10(-7) M), aspirin (5 X 10(-5) M), OKY-046 (10(-6) M), and ONO-3708 (10(-8) M), and abolished by intimal rubbing. From these results, it is concluded that TXA2 is produced in the endothelial cells and may be involved in maintaining the resting tone and contractile response to AA in the canine cerebral artery.

Taurine and icosanoids in the heart

Experiments were carried out on non-working isolated rabbit hearts perfused by Tyrode solution: the effects of Taurine introduced into the coronary circulation were studied • - on the biosynthesis of the anti-thromboxane synthetase factor (“FATS”) • - and on the TXA2 and PGI2 synthetase activities of cardiac tissue. The effects of Taurine were simultaneously studied on the biosynthesis of TXA2 and PGI2 in vitro . Experiments performed under the adopted conditions have shown that • - in vitro Taurine did not significantly modify the biosynthesis of TXA2 and PGI2 • - ex vivo Taurine did not change the biosynthesis of “FATS” but inhibited both TXA2 and PGI2 synthetase activities of the cardiac tissue: Taurine was more active on the TXA2 synthetase activity than on the PGI2 one. Thus Taurine promoted the formation of vasodilator and antiaggregating PGI2 at the expenses of vasoconstrictor and proaggregating TXA2. This could at least partly explain the beneficial effects of Taurine in the physiopethology of the heart.

The role of prostanoids in the feline intestinal vascular and central haemodynamic responses to i.v. infusion of live E. coli

Bacterial infusion in the cat, causing experimental septic shock, induces an early vascular response mainly characterized by pulmonary hypertension and intestinal vasoconstriction. Prostanoids are held to be important mediators of the pulmonary vascular reaction. This study was performed to explore the involvement of prostanoids in the central haemodynamics and the small intestinal vascular reactions in experimental septic shock. Aortic blood pressure was continuously monitored, as were aortic blood flow, the pressure in a. pulmonalis and the small intestinal venous outflow. All cats (n = 24) were given live E. coli (10(10) ml-1) as a continuous intravenous infusion. One series was pretreated with indomethacin, another with UK-38,485, a specific thromboxane A2 synthetase inhibitor, and a third series served as untreated control. The pulmonary hypertensive response was clearly attenuated in the two pretreated series, in fact abolished in the one given UK-38,485. The early intestinal vasoconstriction was eliminated in the two pretreated series. Later during bacteraemia, when untreated and indomethacin-pretreated cats showed intestinal vasoconstriction, UK-38-485-pretreated animals kept intestinal blood flow within the preseptic range. These data suggest that in the cat, thromboxane A2 is the prostanoid mediating the vascular reactions, not only in the lung but also in the small intestine.

Effect of Thromboxane A<sub>2</sub> Synthetase Inhibitor on Immediate-Type Hypersensitivity Reactions

The effect of the thromboxane (TX) A2 synthetase inhibitor, OKY-046, on human leukocyte histamine release and bronchial hypersensitivity in asthmatic subjects was evaluated. It was found that OKY-046 inhibited IgE- and Ca2+ ionophore A23187-mediated leukocyte histamine release in a dose-dependent fashion (IC50: 1.0 and 3.0 X 10(-3) M, respectively) and that OKY-046 could diminish bronchial hypersensitivity, determined by leukotriene D4 inhalation, following a 2-week oral medication. These data suggest that the TXA2 synthetase inhibitor can produce favorable effects upon the course of immediate-type hypersensitivity reactions.

Role of the prostaglandin-thromboxane system in the development and maintenance of spontaneous hypertension in the rat.

In spontaneously hypertensive rats (SHR) between the ages of 6 and 8 weeks before the development of established hypertension, repeated daily subcutaneous administration of indomethacin, an inhibitor of cyclo-oxygenase, at a dose of 5 mg/kg/day enhanced significantly the development of spontaneous hypertension, but repeated daily subcutaneous administration of OKY 046, an inhibitor of thromboxane (TX)A2 synthetase, at a dose of 12 mg/kg/day did not alter the development of spontaneous hypertension. In SHR between the ages of 15 and 18 weeks with established hypertension, indomethacin or OKY 046 did not alter the high blood pressure as compared with the injection of vehicle. In both young and adult SHR, indomethacin decreased significantly urinary prostaglandin (PG)E2 and TXB2 excretion but not PGE2. These results indicate that cyclo-oxygenase products other than TXA2 may play a protecting role in the development of spontaneous hypertension in the rat whereas their contribution to the maintenance of hypertension may be unlikely. In addition, it is suggested that TXA2 may not be involved in the development and maintenance of spontaneous hypertension in the rat.

選択的Ｔｈｒｏｍｂｏｘａｎｅ Ａ２ Ｓｙｎｔｈｅｔａｓｅ Ｉｎｈｉｂｉｔｏｒ（ＯＫＹ‐０４６）のＯｚｏｎｅ吸入後の気道過敏性高進に対する抑制効果

選択的Ｔｈｒｏｍｂｏｘａｎｅ Ａ２ Ｓｙｎｔｈｅｔａｓｅ Ｉｎｈｉｂｉｔｏｒ（ＯＫＹ‐０４６）のＯｚｏｎｅ吸入後の気道過敏性高進に対する抑制効果

Mechanism of the renin-angiotensin system and thromboxane A2 on the maintenance of GFR during reduced renal perfusion pressure in dogs

The interaction between the renin-angiotensin (RA) system and thromboxane A2 (TXA2) was examined in dogs, before and during the renal artery pressure (RAP) was decreased. Intrarenal arterial administration of a small dose of angiotensin II (AII) reduced renal blood flow (RBF) and glomerular filtration rate (GFR) in non-treated dogs when RAP was maintained at the normal level. When mean RAP was decreased to 60 mmHg by means of an aortic clamp, AII reduced RBF, but increased GFR up to 122% of the control value. In dogs pretreated by captopril, GFR decreased when RAP was reduced to 60 mmHg, while RBF was well maintained. However, with the administration of AII, RBF decreased markedly. By indomethacin pretreatment, GFR and RBF decreased during reduced RAP, and their reduction rates were accelerated with the administration of AII into the renal artery. By pretreatment with the thromboxane A2 synthetase inhibitor UK38485, the changes of RBF and GFR following AII infusion were similar to nontreated dogs at normal RAP, but during reduced RAP, AII infusion into the renal artery decreased GFR to 80% of the control value. The thromboxane B2 (TXB2) production by the renal cortex during reduced RAP increased to 2.7-fold that at normal RAP, but TXB2 production did not increase during reduced RAP in captopril pretreated dogs. These results suggest that the RA system and prostanoids, especially TXA2, are important factors for maintaining GFR at low RAP.

P-243 - Effects of Y-20811, a novel thromboxane A2 synthetase inhibitor, on electrically induced thrombosis in the coronary artery of dogs

P-243 - Effects of Y-20811, a novel thromboxane A2 synthetase inhibitor, on electrically induced thrombosis in the coronary artery of dogs

A possible role of thromboxane A2 in endothelium in maintaining resting tone and producing contractile response to acetylcholine and arachidonic acid in canine cerebral arteries.

Endothelial thromboxane A2 (TXA2) in maintaining the resting tone and producing the contractile response to acetylcholine (ACh) and arachidonic acid was studied in canine cerebral artery. The spontaneous release of TXB2 from cerebral artery was about tenfold higher than that of coronary, mesenteric and saphenous arteries. The resting tone, the release of TXB2 and the contraction produced by arachidonic acid were decreased by the presence of cyclooxygenase inhibitor, TXA2 synthetase inhibitor, TXA2 antagonist and rubbing of the luminal side of preparations. The contraction produced by ACh was inhibited by the presence of the above inhibitors and rubbing of the preparations without decreasing the release of TXB2. These results suggest that the resting tone of canine cerebral artery and the contractile response to arachidonic acid are related to activation of TXA2 synthesis in the endothelium.

Thromboxane A2 as an enhancing factor of coronary vasospasticity in variant angina.

To clarify the role of thromboxane A2 (TXA2) in evoking coronary spasm, we compared coronary arterial spasticity induced by ergonovine maleate (EM) with coronary sinus thromboxane B2 (TXB2: a stable catabolite of TXA2) in 34 patients with documented variant angina and 11 patients with chest pain syndrome (CPS). We also examined the effect of OKY-1581 (8 mg/kg, i.v.), a TXA2 synthetase inhibitor, on the coronary arterial spasticity of these patients. When blood samples were taken from coronary sinus just before EM test, all patients with variant angina exhibiting markedly augmented TXB2 levels (424 +/- 138 pg/ml), had positive EM test results, while CPS exhibiting lower TXB2 levels (223 +/- 38 pg/ml), had negative EM test. We found that the amounts of EM needed to induce coronary spasm were inversely correlated with TXB2 levels in coronary sinus. In 7 out of these 8 patients, OKY-1581 was found to attenuate the increased spasticity with reduction of coronary sinus TXB2 levels. In 3 patients, an EM rechallenge at symptomatically quiescent stage resulted in negative test with augmented TXB2 levels being markedly decreased. These findings indicate that increased TXA2 in circulating plasma is closely correlated with the hypersensitivity of coronary arteries to EM in patients with variant angina, suggesting a possible role of augmented TXA2 production in the enhancement of coronary vascular spasticity.

全血中でのＰＧＩ２生成に関する検討

The factors affecting PGI2 synthesis following the platelet aggregation induced by collagen (1μg/ml) in whole blood were examined.Trapidil, possessing the anti-platelet aggregating activity and the enhancing effect of PGI2 synthesis in endothelial cells, markedly promoted PGI2 synthesis in whole blood, while aspirin showed no effect. The increment of PGI2 synthesis ran parallel to the concentration of trapidil, whereas TxA2 synthesis was decreased. OKY-046, the inhibitor of TxA2 synthetase, also increased PGI2 level and decreased TxA2 level in whole blood in the same manner of trapidil. The level of PGI2 in the suspension of leukocytes added trapidil (1mM), arachidonic acid (0.3M) and collagen (1μg/ml) was signficantly higher than that in the suspension added 1mM of aspirin in place of trapidil.These results suggest that trapidil has the inhibitory activity of TxA2 synthetase, resulting in the increment of PGI2 in whole blood. This synthesis of PGI2 was found in the leukocyte suspension in the presence of arachidonic acid.

ウサギ血小板Ｃａ｀２＋´動態におけるＴＸＡ２の役割

Calcium appears to play a main role in number of important platelets functions. In this study we examined the role of TXA2 in the Ca2+ mobilization in rabbit platelets using OKY-046, a specific TXA2 synthetase inhibitor, and STA2, aTXA2 analogue.OKY-046 (10-6-10-4M) dose-dependently inhibited 0.5mM arachidonic acid (AA)-induced 45Ca2+ uptake in platelets. Aspirin (10-3M) also significantly inhibited the 45Ca2+ uptake. OKY-046 and aspirin also inhibited the 45Ca2+ uptake stimulated by collagen (20μg/ml) but not by ADP (20μM). On the other hand, STA2 (1-10μM) dose-dependently stimulated 45Ca2+ uptake in platelets.STA2 (10μM) elevated the intracellular free Ca2+ concentration in the presence of medium Ca2+ in Quin 2-loaded platelets. This elevation was inhibited by nifedipine (0.1μM), a calcium-channel blocker, but not by TMB-8 (30μM), an intracellular calcium antagonist.STA2 (10μM) also stimulated the formation of 3H-phosphatidic acid (PA) in the presence of EGTA in 3H-AA-labeled PRP.These findings suggest that TXA2 stimulates the Ca2+ influx via phospholipid metabolism including the PA formation in rabbit platelets.