Effect of OKY-046, a thromboxane A2 synthetase inhibitor, on arachidonate-induced platelet aggregation: possible role of "prostaglandin H2 steal" mechanism

To clarify the mode of action of a selective thromboxane A2 (TXA2) blockade in platelet reactivity, we examined the effect of (E)-3-[4-(1-imidazolylmethyl) phenyl]-2-propenoic acid hydrochloride (OKY-046), a potent TXA2 synthetase inhibitor, on human platelet aggregation induced by arachidonic acid (1 mM) in the absence and presence of aspirin-treated aortic microsomes containing prostacyclin (PGI2) synthetase activity ex vivo. The production of TXA2 and PGI2 in platelet rich plasma was determined by the amounts of their stable catabolites, TXB2 and 6-keto-PGF1 alpha respectively, measured by radioimmunoassay. In the absence of aortic microsomes, OKY-046 (greater than 10(-5) M) produced more than 90% inhibition of TXA2 production, whereas platelet aggregation was less inhibited, about 40% inhibition over control, by OKY-046 in that concentration. In the presence of aortic microsomes, the inhibitory effect of OKY-046 on platelet aggregation was markedly augmented in a dose-dependent manner in proportion to the increment of PGI2 production, which paralleled the OKY-046-induced inhibition of TXA2. These results suggest that a selective TXA2 blockade produces effects on platelet aggregation mainly in dual fashion in the presence of PGI2 synthetase: one is due to mere inhibition of TXA2 synthetase and the other is due to the enhancement of PGI2 production probably involving "prostaglandin H2 (PGH2) steal" mechanism, in which PGH2 accumulated in platelets is partly converted to a substrate of PGI2 synthetase in aortic microsomes to produce PGI2.     

Renal prostaglandins and phospholipase A2 in spontaneously hypertensive rats

Phospholipase A2 activity and prostaglandin synthesis were studied in the renal cortex and medulla of stroke-prone spontaneously hypertensive rats (SHRSP) and age-matched normotensive Wistar-Kyoto rats (WKY) aged 10-50 weeks. Enhanced phospholipase A2 activity was found in both the cortical and the medullary microsomes of SHRSP kidneys. Phospholipase A2 activity progressively increased with age in SHRSP, but not in WKY. Prostaglandin E2 (PGE2) and thromboxane A2 (TXA2) were the major prostaglandins found in the cortex, and PGE2 was the major prostaglandin found in the medulla. Prostaglandin l2 (PGI2) was synthesized in both the cortex and medulla, but cortical PGI2 synthesis was much lower than medullary synthesis. Enzymatic activity for all prostaglandin syntheses analysed here were higher in SHRSP. There was a greater age-related increase in prostaglandin synthesis in SHRSP kidneys than in WKY kidneys. In addition, the ratios of PGE2/TXB2 and 6-keto-prostaglandin F1 alpha (PGF1 alpha)/thromboxane B2 (TXB2) decreased in SHRSP. This may produce vasoconstriction and increase vascular resistance in SHRSP. These data suggest that increased prostaglandin synthesis and phospholipase A2 activity have an important role in the development and maintenance of hypertension in SHRSP.     

Increased prostacyclin and thromboxane A2 biosynthesis in atherosclerosis.

It has been proposed that atherosclerotic arteries produce less prostacyclin (PGI2) than nonatherosclerotic arteries do, thereby predisposing arteries to vasospasm and thrombosis in vivo. We reexamined this concept by measuring spontaneous as well as arachidonate-induced PGI2 biosynthesis in aortic segments from nonatherosclerotic and cholesterol-fed atherosclerotic New Zealand White rabbits. Thromboxane A2 (TXA2) generation was also measured. Formation of PGI2, as well as TXA2, as measured by radioimmunoassay (RIA) of their metabolites, was increased in atherosclerotic aortic segments relative to nonatherosclerotic segments (P less than or equal to 0.05) at 0, 5, 10, 15, and 30 min of incubation with arachidonate. Pretreatment of arterial segments with indomethacin inhibited PGI2 as well as TXA2 formation, whereas pretreatment with the selective TXA2 inhibitor OKY-046 inhibited only TXA2 release, thus confirming the identity of icosanoids. To confirm the RIA data, aortic segments were incubated with [14C]arachidonate prior to stimulation with unlabeled arachidonate. The uptake of arachidonate was similar, but the release of incorporated [14C]arachidonate was significantly (P less than or equal to 0.05) greater in atherosclerotic segments than in nonatherosclerotic ones. Conversions of released [14C]arachidonate to 6-keto[14C]prostaglandin F1 alpha and [14C]thromboxane B2 were similar in the two types of aortic segments. Thus, synthesis of PGI2 as well as TXA2 is increased in atherosclerosis, and this alteration in arachidonate metabolism is related to increased release of arachidonate.     

Differential effects of oxidative stress on hepatic endothelial and Kupffer cell eicosanoid release in response to endothelin-1

OBJECTIVE: The vasoconstrictor endothelin-1 can induce vasomodulators release like nitric oxide in the liver. Here the authors explored whether endothelin-1 can stimulate endothelial and Kupffer cells release of other vasomodulators under normal and stress conditions. METHODS: Cells were cultured for 24 h and treated with H2O2 (25 microM) for 6 h and subsequently with endothelin-1 (10 nM) for 10 min. Eicosanoid release was assessed in the media by enzyme immunoassay. RESULTS: Endothelin-1 mediated cPLA2 phosphorylation and increased prostaglandin (PG) I2, PGE2 and thromboxane A2 (TXA2) release in endothelial cells while it only increased TXA2 in Kupffer cells. H2O2 significantly increased PGI2, PGE2 and TXA2 in endothelial cells through an upregulation of cyclooxygenase-2, thromboxane synthase A2, and phosphorylation of cPLA2. Endothelin-1-induced PGI2, PGE2, and TXA2 release in endothelial cells were inhibited by H2O2 correlating with the absence of further cPLA2 phosphorylation. In Kupffer cells, H2O2 only increased TXA2 synthesis and further endothelin-1 stimulation of TXA2 was possible through a higher increase in cPLA2. CONCLUSION: These results indicate that under normal conditions endothelial cells play a pivotal role in liver microcirculation regulation. Oxidative stress not only disrupts the basal balance of vasomodulators in the liver but also affects endothelin-1-induced effects in both Kupffer cells and endothelial cells.     

Altered renal and platelet arachidonic acid metabolism in cirrhosis

Urinary excretion rates of prostaglandin (PG) E2, PGF2 alpha, 6-keto-PGF1 alpha, and thromboxane (TX) B2 were evaluated in three groups of cirrhotic patients [without ascites (group 1, 13 cases), with ascites and normal renal function (group 2, 15 cases), and with ascites and renal failure (group 3, 5 cases)] and in 14 healthy controls. All urinary arachidonate metabolites were significantly increased in group 2 patients. Patients with renal failure showed lower PGE2, PGF2 alpha, and TXB2 values than those from group 2; PGF2 alpha values were also lower than controls. Platelet TXA2 production during whole blood clotting was significantly reduced in all groups of patients. Administration of low-dose aspirin and sulindac, two cyclooxygenase inhibitors selectively sparing renal cyclooxygenase activity, effectively inhibited platelet TXA2 production without affecting urinary TXB2 excretion, thus ruling out platelets as a possible source of urinary TXB2. We conclude that patients with ascites and normal renal function show an overall activation of the renal PG system. Renal production of vasodilating PGE2 and PGI2 may be involved in supporting renal function in these patients. A reduced platelet synthesis of proaggregatory TXA2 also occurs in cirrhotic patients. This may play a role in the bleeding tendency of cirrhosis.     

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.     

Metabolism of endogenous arachidonic acid by isolated lungs of neonatal and adult rabbits stimulated with calcium ionophore: effect of hypoxia.

We have determined the effect of hypoxia on arachidonic acid metabolism by rabbit lungs stimulated with calcium ionophore A23187. Isolated lungs of neonatal and adult rabbits were perfused during normoxia (pO2 greater than 100 torr) or hypoxia (pO2 less than 40 torr) and arachidonic acid metabolism stimulated by the addition of A23187 (5 microM) to the perfusate. Cyclooxygenase metabolites PGE2, TxA2, and PGI2 were measured by radioimmunoassay and lipoxygenase metabolites LTB4, LTC4, LTD4, and LTE4 by HPLC. During normoxia, neonatal lungs synthesized the three cyclooxygenase metabolites that we measured in similar amounts. LTC4 constituted 56% of the leukotrienes produced. Hypoxia caused a 100% increase in the amount of PGI2 synthesized by neonatal lungs; however, total TxB2 and PGE2 production was not altered significantly. LTC4 production decreased significantly during hypoxia, whereas LTE4 production increased. Adult lungs synthesized significantly lower amounts of both cyclooxygenase and 5-lipoxygenase products than neonatal lungs. During normoxia, PGE2 was measured in highest amount in adult lungs. Similar to the neonatal lungs, LTC4 was the predominant leukotriene (56%) measured. During hypoxia, there was a 100% increase in PGI2 production by adult lungs, as was observed in neonatal lungs. There was also a small but significant increase in PGE2 production, with no change in TxA2 production. LTC4 production also decreased, but there was a marked increase in synthesis of LTB4 by adult lungs. Our data demonstrate that hypoxia alters the profile of arachidonic acid metabolites produced by rabbit lungs stimulated with A23187. Also, the age of the rabbit significantly affects both the amount and profile of metabolites synthesized by stimulated lungs during normoxia and hypoxia.     

Orchidectomy increases the formation of prostanoids and modulates their role in the acetylcholine-induced relaxation in the rat aorta

AIMS: This study examines the effect of endogenous male sex hormones on thromboxane A2 (TXA2), prostaglandin (PG) I2, PGF(2 alpha), and PGE I2 release, as well as their role in acetylcholine (ACh)-mediated relaxation in the aorta. METHODS AND RESULTS: Aortic segments from orchidectomized and control male Sprague-Dawley rats were used to measure COX-2 protein expression. ACh-induced relaxation of these segments was also determined in the absence and presence of the COX-2 inhibitor NS-398, the TXA2 synthesis inhibitor furegrelate, the PGI2 synthesis inhibitor tranylcypromine (TCP), or the thromboxane-prostanoid (TP) receptor antagonist SQ-29 548. Furthermore, TXA2, PGI2, PGF(2 alpha), and PGE2 release as well as the vasomotor effect of exogenous TXA2, PGI2, PGF(2 alpha), and PGE2 were measured. COX-2 expression was increased in aortas from orchidectomized rats. NS-398 did not modify the ACh-induced relaxation in arteries from both control or orchidectomized rats. Furegrelate did not modify the ACh-induced relaxation in aortas from control animals but, in aortas from orchidectomized rats, it increased that response. TCP decreased the ACh-induced relaxation in both groups. The TP receptor antagonist, SQ29 548 failed to modify ACh-induced relaxation in aortas from either rat group. Pre-incubating arteries from orchidectomized rats with TCP plus furegrelate did not modify the decrease in the ACh response induced by TCP alone, but this response was restored by co-incubation of TCP plus SQ29 548. ACh-induced TXA2, PGI2, PGF(2 alpha), and PGE2 release were increased by orchidectomy. The presence of furegrelate plus TCP increased the ACh-induced PGE2 release more in arteries from orchidectomized than in those from control rats. The contractile responses induced by the TXA2 mimetic U-46619 or by exogenous PGF(2 alpha) were similar in arteries from control and orchidectomized rats, while those induced by exogenous PGE2 were increased in arteries from orchidectomized rats; the vasodilator response induced by exogenous PGI2 was decreased in arteries from orchidectomized rats. CONCLUSION: These data show that endogenous male sex hormone deprivation increases COX-2 expression, the release of TXA2, PGI2, PGF(2 alpha), and PGE2 and the contractile response induced by exogenous PGE2 and TXA2, while it decreases the relaxation induced by exogenous PGI2. Despite the predominance of vasoconstrictor prostanoids derived from COX-2 in aortas from orchidectomized rats, the ACh-induced relaxation remains increased.     

L-Arginine improves endothelial function in renal artery of hypertensive Dahl rats

OBJECTIVES: To clarify whether endothelium-derived contracting factor (EDCF) is developed in renal artery of hypertensive Dahl rats and whether prolonged oral L-arginine treatments prevent development of EDCF and hypertension. DESIGN: The effect of prolonged salt treatment with or without L-arginine on the renal artery was examined. METHODS AND RESULTS: Dahl salt-sensitive and -resistant rats were fed a 0.4 or an 8% NaCl diet for 4 weeks. High sodium intake increased arterial pressure in Dahl salt-sensitive rats. The rings of renal arteries were suspended for isometric tension recording. Only in the hypertensive rats, more than 1 micromol/l acetylcholine induced an endothelium-dependent contraction response. The contraction was completely inhibited by indomethacin or ONO-3708 [prostaglandin H2 (PGH2)/thromboxane A2 (TXA2) receptor antagonist], and partially inhibited by OKY-046 (TXA2 synthetase inhibitor). Acetylcholine-induced relaxation was significantly depressed in hypertensive rats, which was partially improved by SQ29548 (PGH2/TXA2 receptor antagonist). Oral L-arginine, but not ONO-8809 (orally active PGH2/TXA2 receptor antagonist) treatment, inhibited the contraction and amended the relaxation. The endothelium-independent contraction to TXA2 receptor agonist U46619 and relaxation to nitroprusside were not altered by L-arginine treatment The L-Arginine treatment reduced blood pressure and sodium retention with increases in urinary NO2-/NO3- and cGMP excretion. Hydralazine treatment also inhibited development of EDCF. CONCLUSIONS: The present results suggest that impaired endothelium-dependent relaxation to acetylcholine is caused in part by induction of EDCF synthesis/release in renal arteries of hypertensive Dahl rats. L-arginine can attenuate sodium retention and development of hypertension, which lead to a decrease in EDCF synthesis in renal arteries.     

一氧化氮对实验性肾炎中肾小球花生四烯酸产物的影响

目的：探讨一氧化氮（ＮＯ）在实验性肾小球肾为中对肾小球花生四烯酸（ＡＡ）产物合成的调节作用。方法：制备大鼠加速性肾毒性肾炎（ＮＳＮ）模型,应用市郊和液相层析及放免法测定肾小球合成的前列腺素（ＰＧ）Ｅ２、６－酮－ＰＧＦ１α、血栓素（ＴＸ）Ｂ２、白三烯（ＬＴ）Ｂ４、ＬＴＣ４、５－羟二十碳四烯酸（５－ＨＥＴＥ）、１２－ＨＥＴＥ、１５－ＨＥＴＥ。对ＮＳＮ大鼠应用诱生型ＮＯ合成酶（ｉＮＯＳ）抑制剂Ｌ－ＮＩＬ     

Vascular responses to leukocyte products in atherosclerotic primates

Little is known about the possible role of leukocytes in the pathogenesis of vasospasm. We hypothesized that vasoactive products released by leukocytes might produce constriction of atherosclerotic arteries. To test this hypothesis, we infused fmet-leu-phe (fMLP), a peptide that activates leukocytes to release their vasoactive products, into the perfused hind limb of normal and atherosclerotic cynomolgus monkeys. Infusion of fMLP did not change resistance of large arteries in normal monkeys. In contrast, fMLP produced pronounced constriction of large arteries in atherosclerotic monkeys. To determine whether leukotrienes, platelet-activating factor, or prostaglandin E2 (PGE2), which are released by leukocytes, may contribute to leukocyte-induced vasoconstriction in atherosclerotic monkeys, we injected leukotriene D4, platelet-activating factor, and PGE2 intra-arterially into the perfused hind limb. Leukotriene D4 and platelet-activating factor had minimal effects on large arteries in both normal and atherosclerotic monkeys. PGE2 produced marked constriction of large arteries in atherosclerotic, but not normal, monkeys. Thus, pronounced constriction in atherosclerotic, but not normal, arteries during infusion of fMLP suggests that products released by leukocytes may mediate vasoconstriction in atherosclerotic vessels. Vasoconstrictor responses to PGE2 are profoundly potentiated by atherosclerosis, which suggests that PGE2 may contribute to leukocyte-induced vasoconstriction.     

Intravenous infusion of a selective inhibitor of thromboxane A2 synthetase in man: influence on thromboxane B2 and 6-keto-prostaglandin F1 alpha levels and platelet aggregation

The effect of the selective thromboxane A2 synthetase inhibitor OKY-1581, a pyridine derivative [sodium (E)-3-(4-(3-pyridylmethyl)phenyl)-2-methyl-2-propenoate], on thromboxane B2 and 6-keto-prostaglandin F1 alpha levels and platelet aggregation was studied in human volunteers. To clarify its effectiveness as an enzyme inhibitor, OKY-1581, at doses of 17, 83, 167, 417, 833, and 1667 micrograms/kg (n = 5 for each group), was injected intravenously, or was infused (10 micrograms/kg/min; n = 5) over 3 hr on 3 successive days. OKY-1580 (OKY-1581 free acid) was rapidly converted to its main beta-oxidized product, OKY-1565, and its reduced form, OKY-1558. During the study, plasma thromboxane B2 levels, inhibition of thromboxane B2 production in serum, and inhibition of rabbit platelet thromboxane A2 synthetase were monitored continuously. Twenty-five minutes after the injection of the above doses, plasma thromboxane B2 levels decreased by 4 +/- 7%, 40 +/- 14%, 57 +/- 7%, 68 +/- 6%, 93 +/- 5%, and 96 +/- 5% (mean +/- SD), respectively. Thromboxane B2 production in serum was decreased by 2 +/- 8%, 70 +/- 10%, 75 +/- 8%, 81 +/- 10%, and 96 +/- 8%, respectively, and rabbit platelet thromboxane A2 synthetase by 2 +/- 7%, 52 +/- 8%, 79 +/- 10%, 80 +/- 9%, 96 +/- 8%, and 95 +/- 7%. These parameters returned to the control levels 24 hr after the injection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prostacyclin synthase gene transfer modulates cyclooxygenase-2-derived prostanoid synthesis and inhibits neointimal formation in rat balloon-injured arteries

Previous studies have shown that prostacyclin (PGI(2)) synthase (PCS) gene transfer inhibits neointimal formation in balloon-injured arteries. However, the role of each cyclooxygenase (COX) isoform in this healing mechanism remains unknown. We hypothesized that overexpression of PCS may modulate COX-2-mediated prostaglandin (PG) metabolism. That is to say, excessive PGH(2) derived from COX-2 after balloon injury may be converted into PGI(2) rather than PGE(2) or thromboxane (TX) A(2) by overexpressed PCS. We examined the expression of COX isoforms and evaluated the role of COX-2 with regard to the effects of PCS gene transfer by using 4-(4-cyclohexyl-2-methyloxazol-5-yl)-2-fluorobenzenesulfonamide (JTE-522), a selective COX-2 inhibitor. Rats were divided into 4 groups in conjunction with PCS gene transfer and JTE-522 treatment. The PCS gene (30 microg) was transfected into rat balloon-injured arteries by a lipotransfection method. JTE-522 (30 mg/kg per day) was administered for 14 days after balloon injury. Immunohistochemical analysis demonstrated marked COX-2 expression on the neointima. PCS gene transfer markedly inhibited neointimal formation, but JTE-522 reversed this beneficial effect. PCS gene transfer augmented PGI(2) production and decreased PGE(2) production without affecting TXA(2) production, but JTE-522 inhibited this increase in PGI(2) production. In conclusion, PCS gene transfer modulated COX-2-mediated prostanoid synthesis and inhibited neointimal formation after balloon injury.     

The synthesis, release and action of leukotrienes in the isolated, unstimulated, buffer-perfused rat heart

When the perfusion medium of an isolated, non-recirculating, Langendorff rat heart is changed from Krebs buffer to coronary effluent, a significant vasoconstriction (23%, P less than 0.005) is observed. In this study we have investigated the involvement of leukotrienes in this phenomenon. We have extracted and quantified leukotrienes C4, D4 and E4 in samples of coronary effluent taken at different times during the first 2 h of perfusion; the total amounts released during this time were 9, 5 and 32 pmol of LTC4, LTD4 and LTE4 respectively. We have used two different methods to prevent the action of the effluent leukotrienes on the heart. Firstly, we have blocked the leukotriene receptors in the heart, with FPL 55712 (3.8 microM), during perfusion with effluent and, secondly, we have perfused with coronary effluent which was collected in the presence of a leukotriene synthesis inhibitor, AA861 (1 microM). The addition of FPL 55712 to the effluent decreased the normally observed vasoconstriction such that after 30 min the coronary flow rate (CFR) was 114 +/- 3% (n = 6) compared with 66 +/- 1% (n = 7) with effluent alone (P less than 0.005). Effluent collected in the presence of AA861 also caused a decrease in the normally observed vasoconstriction such that by 30 min the CFR was still 88 +/- 2% (n = 6, P less than 0.005 compared to controls). We have confirmed the proposed involvement of leukotrienes in the effluent-induced vasoconstriction by investigating the effect of a mixture of the synthetic leukotrienes C4, D4 and E4, when each of them was present at the same concentration as measured in the coronary effluent; the vasoconstriction observed was superimposable upon that seen with effluent. This vasoactive effect of the leukotriene mixture was not secondary to a change in contractility, since this only decreased to 97 +/- 5% (n = 9) during the 30 min of the leukotriene infusion. Finally, we have studied the effects of the same two leukotriene blockers in normal, buffer-perfused hearts after an initial perfusion of either 30 or 120 min. Application of either AA861 or FPL 55712 resulted in a dramatic vasodilatation (25 to 45% increase), a larger effect always being observed after the shorter initial period of perfusion. Our conclusions are two-fold. Firstly, isolated, buffer-perfused rat hearts synthesize leukotrienes C4, D4 and E4 in considerable amounts and release them into the coronary effluent and secondly, the coronary flow rates of isolated, buffer-perfused rat hearts are partly controlled by the action of internally produced leukotrienes.     

RU486 antagonizes the inhibitory action of progesterone on prostacyclin and thromboxane A2 synthesis in cultured rat myometrial explants

A myometrial explant culture system was developed to investigate the effect of progesterone and the antiprogestagen RU486 on prostacyclin (PGI2) and thromboxane A2 (TXA2) synthesis by the rat myometrium. After culture, eicosanoid synthesis was stimulated with arachidonic acid (AA) and the calcium ionophore A23187 (A23187). Spontaneous release of eicosanoids was also studied. Progesterone inhibited the spontaneous release of PGI2 and TXA2 release by myometrial explants in a concentration- and time-dependent manner. Adequate inhibition of myometrial eicosanoid synthesis by physiological concentrations was achieved at 18 h of culture: all subsequent studies were carried out after an 18-h culture of explants. A23187- and AA-stimulated PGI2 and TXA2 synthesis were inhibited equipotently by progesterone. 17 beta-Estradiol alone was without effect on spontaneous AA- or A23187-stimulated PGI2 or TXA2 synthesis and was without effect on progesterone-elicited inhibition of eicosanoid synthesis in the myometrial explants. The protein synthesis inhibitors, actinomycin D and cycloheximide, did not block the inhibitory action of progesterone on A23187- or AA-stimulated eicosanoid synthesis by the myometrial explants and alone mimicked the inhibitory action of progesterone. The inhibitory action of progesterone on AA- and A23187-stimulated PGI2 and TXA2 synthesis was antagonized in a concentration-dependent manner by RU486. These data indicate that within this ex vivo system, progesterone probably inhibits myometrial cycloxygenase, that progesterone may exert this action through inhibition of a modulating or permissive protein, and that the antiprogestagen RU486 is an effective in vitro antagonist or progesterone.     

The lung as a metabolic organ

The fundamental importance of the lung in providing oxygen and eliminating carbon dioxide is well known. However the lung has another critical role as the site of numerous and important metabolic events. Some of these metabolic activities are essential to the normal performance of pulmonary gas exchange. Impairment of pulmonary metabolic activities can, therefore, have far-reaching repercussions on many organ systems. Special features of pulmonary vascular beds which are helpful for the metabolism of various vasoactive substances through pulmonary circulation are described. The lung can uptake or metabolize various vasoactive substances including acetylcholine, serotonin, bradykinin, prostaglandin E2 (PGE2), PGF2 alpha, leukotriene C4 (LTC4), LTD4 and LTE4. On the other hand the lung can synthetize and release various vasoactive substances including histamine, serotonin, LTC4, LTD4, LTE4, thromboxane A4 (TXA4), PGD2, PGE2, PGF2 alpha, PGI2, LTB4, and PAF (platelet activating factor). We also investigated the metabolism of LTC4 and LTD4 through isolated perfused guinea pig lung lobes. And it is clarified that the infused LTC4 was converted to LTD4 and LTE4, while the infused LTD4 was converted to LTE4. Seventeen years ago, we demonstrated the release of prostaglandins from the lung during mechanical ventilation at large tidal volumes in anesthetized mongrel dogs. We thought this PG is PGE2 which dilates pulmonary vasculatures. In the present study we investigated the release of PG from healthy volunteers by spontaneous hyperventilation. We demonstrated that serum levels of 6-keto PGF1 alpha and PGE2 showed a marked increase following the spontaneous hyperventilation. Various humoral factors related to the pulmonary vascular responses and various humoral factors which related to the tracheobronchial responses were also presented.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thromboxane synthetase inhibition and pulmonary response to hypoxia in conscious adult sheep

This study investigated the effects of a thromboxane synthetase inhibitor (OKY-046) and a cyclooxygenase inhibitor (ketoprofen) on hypoxic pulmonary vasoconstriction in conscious adult sheep in order to evaluate the physiological role of thromboxane and other cyclooxygenase products. In addition, we studied the effects of histamine H1 (chlorpheniramine) and H2 antagonists (cimetidine) on hypoxic pulmonary vascular tone. Hypoxia caused a 37% rise in pulmonary arterial pressure (p less than 0.05) and a 36% increase in pulmonary vascular resistance (p less than 0.05). Pretreatment with intravenous OKY-046 10 mg/kg or ketoprofen 2 mg/kg had no effect on normoxic pulmonary vascular tone and inhibited the increase in plasma TXB2 concentration during hypoxia without affecting the pulmonary pressor response to hypoxia. Cimetidine produced an increase in hypoxic pulmonary vascular tone when individual members of the group were compared, but there was no statistically significant difference when the group was compared to the control study. Chlorpheniramine had no effect on hypoxic pulmonary tone. These data suggest that hypoxic pulmonary vasoconstriction is not mediated by release of TXA2, that hypoxic vascular tone is not modulated by cyclooxygenase products, and that the histamine H2 receptor may play a modulating role in hypoxic pulmonary vasoconstriction in conscious adult sheep.     

Characterization of prostaglandin and thromboxane receptors expressed on a megakaryoblastic leukemia cell line, MEG-01s.

MEG-01s, an established human megakaryoblastic leukemia cell line, exhibited specific high-affinity binding sites for [3H]iloprost, a stable prostaglandin (PG) I2 analogue, for [3H]SQ-29548, a stable thromboxane (TX) A2 antagonist and, for [3H]PGE2/PGE1, but not for [3H]PGD2. In the MEG-01s cells, iloprost/PGI2, or PGE1 stimulated cAMP production with ED50 values practically identical to the IC50 values for the [3H] iloprost binding. STA2 and U46619, TXA2/PGH2 agonists, PGE2/PGE1, iloprost/PGI2, and thrombin elevated the intracellular concentrations of Ca2+ ([Ca2+]i), as determined by Fura-2 fluorescence signals. Elevation of [Ca2+]i by PGE2/PGE1 and iloprost, but not that by TX-agonists or thrombin, was totally dependent on the presence of extracellular Ca2+. This effect by PGE2/PGE1 was partially inhibited by prior treatment of the cells with islet-activating protein (IAP), while that by TX-agonists or by PGI2/iloprost was not affected. We tentatively conclude from these results that: (1) MEG-01s cells express (a) PGI2/PGE1 receptor(s) coupled to adenylate cyclase and Ca2+ influx, a TXA2/PGH2 receptor coupled to the phosphatidylinositol-turnover-Ca2+ system, and the PGE2/PGE1 receptor coupled to Ca2+ influx; (2) the receptors for TXA2/PGH2 and iloprost and those for PGE2/PGE1 and thrombin are coupled to IAP-insensitive and IAP-sensitive GTP-binding proteins, respectively, and function in a different manner to elevate [Ca2+]i. Thus, the MEG-01s cell line is a pertinent model for studying eicosanoid receptor-mediated signal transduction in platelet/megakaryocyte systems.     

Role of prostaglandin and thromboxane biosynthesis in gastric necrosis produced by taurocholate and ethanol

The effects of a new selective inhibitor of thromboxane biosynthesis, OKY-1581, and a potent inhibitor of cyclooxygenase, indomethacin, on gastric mucosal lesions induced by taurocholate or ethanol and mucosal generation of prostaglandins have been studied in rats. OKY-1581 prevented, dose dependently, the formation of taurocholate- but not ethanol-induced gastric necrosis, and this effect was accompanied by an increase in gastric mucosal generation of prostaglandin E2 and I2-like activity and a reduction in the thromboxane generation during platelet aggregation. OKY-1581 enhanced the cytoprotective action of "mild" irritants such as 5 mM taurocholate against gastric damage by 100 mM taurocholate, whereas indomethacin produced opposite effects. This study indicates: (1) the inhibition of thromboxane biosynthesis results in increased generation of prostaglandins which seems to contribute to the gastric mucosal integrity and, (2) thromboxanes may be involved in the pathogenesis of taurocholate-induced gastric mucosal lesions.     

Prostanoid production in varicose veins: evidence for decreased prostacyclin with increased thromboxane A2 and prostaglandin E2 formation

In this study, formation of arachidonic acid-derived prostanoids was investigated in saphenous veins of varicosed and nonvaricosed patients, all undergoing saphenectomy respectively for varicosis or in preparation for coronary bypass operation. Venous production of prostacyclin (PGI2), thromboxane A2 (TXA2) and prostaglandin E2 (PGE2) was assessed by bioassay and/or radioimmunologic assays as appropriate. Fragments of saphenous veins from varicosed patients produced significantly less PGI2 and more TXA2 and PGE2 than those from the control patients. Addition of arachidonic acid to incubation mixtures dose dependently increased release of these prostanoids, but the levels of PGI2 produced were consistently lower in veins from varicosed patients. No differences were found in varicosed patients between various segments of the same vein, no matter whether macroscopically affected or unaffected. These results demonstrate that the cyclooxygenase pathway in the venous wall of subjects with varicosis is shifted toward lesser formation of PGI2 and higher production of proaggregatory and proinflammatory prostanoids such as PGE2 and TXA2. These biochemical changes may be relevant to inflammation and thrombogenesis in varicosis.