Effects of a thromboxane synthetase inhibitor and a thromboaxane antagonist on release and activity of thromboxane A2 and prostaglandin in vitro

The TxA₂ synthetase inhibitor, dazoxiben, and the TxA₂ antagonist, ±SQ 29, 548, were examined for effects on release and vasoactivity of TxA₂ and prostacyclin. Isolated perfused guinea pig lungs were used as the enzyme source from which TxA₂ and prostacyclin were released in response to injections of arachidonic acid or bradykinin. Both dazoxiben and ±SQ 29, 548 inhibited contraction of the superfused rat aorta and bovine coronary artery after arachidonic acid injection through the lung. ±SQ 29, 548 abolished contractions of the rat aorta, but significant aorta contracting activity persisted during dazoxiben treatment. Dazoxiben significantly inhibited arachidonate-induced release of TxA₂ (immunoreactive TxB₂)iinto the superfusate, but TxA₂ release was significantly potentiated by ±SQ 29, 548. Thus, in the presence of enhanced TxA₂ concentrations, ±SQ 29, 548 effectively antagonized the vasospastic effect of TxA₂. Dazoxiben diverted a significantly greater amount of arachidonic acid into prostacyclin synthesis (immunoreactive 6-keto-PGF_1α), changing original coronary vasoconstriction into relaxation. ±SQ 29, 548 did not significantly modify lung prostacyclin synthesis. Moreover, with ±SQ 29, 548, the absence of TxA₂-mediated coronary contraction unmasked active relaxation of the superfused bovine coronary artery, coincident with thromboxane and prostacyclin release. Dazoxiben consistently inhibited TxA₂ synthesis and enhanced prostacyclin synthesis. ±SQ 29, 548 augmented TxB₂ release in response to arachidonate, but not bradykinin, and did not significantly alter 6-keto-PGF_1α release in response to either arachidonate or bradykinin. In terms of vasoactivity measured , ±SQ 29, 548 and dazoxiben produced similar anti-vasospastic effects, although this was accomplished by completely different mechanisms.     

Prostaglandins and thromboxane outflow from the perfused mesenteric vascular bed in spontaneously hypertensive rats

To clarify the metabolism of PGE2, prostacyclin (PGI2) and thromboxane A2 (TxA2) in small vessels in spontaneously hypertensive rats (SHR), we removed superior mesenteric vascular beds from 10 week old SHR and age matched normotensive controls (WKY). The mesenteric artery was perfused with Krebs-Henseleit buffer and samples of effluent collected every 15 minutes during 3 hours perfusion for analysis of PGE2, 6-keto-PGF1 alpha (a stable metabolite of PGI2) and TxB2 (a stable metabolite of TxA2) by specific radioimmunoassays. Levels of all three arachidonic acid (AA) metabolites, PGE2, 6-keto-PGF1 alpha and TxB2, in the mesenteric effluent were significantly reduced in SHR as compared to WKY. TxB2 was detected in all samples throughout the perfusion. 6-keto-PGF1 alpha/PGE2 ratios and TxB2/PGE2 ratios were significantly increased in SHR. 6-keto-PGF1 alpha/TxB2 ratios in the first four samples were significantly decreased in SHR as compared to WKY. These data suggest that there may be reduced availability of PG precusor AA and unbalanced synthesis of PGs in small vessels in SHR. Both may have relevance to the development of hypertension in the animals.     

Metabolic protection of the reperfused canine endocardium by the thromboxane synthetase inhibitor, dazoxiben

This study investigated whether dazoxiben, a thromboxane synthesis inhibitor, could reverse regional contractile dysfunction and protect against adenine nucleotide loss in the "stunned myocardium". Hearts from anesthetized dogs were "stunned" by 15 min of left anterior descending coronary artery occlusion followed by 3 hr of reperfusion. Left ventricular segment shortening (%SS) and regional myocardial blood flow (RMBF) were measured by sonomicrometry and the radioactive microsphere technique, respectively. Local coronary venous blood was withdrawn and thromboxane A2 and prostacyclin measured by radioimmunoassay. Transmural biopsies from the reperfused and nonischemic areas were taken at 3 hr following reperfusion for tissue metabolite analysis. During ischemia, %SS, RMBF and area at risk were decreased to similar levels in both control and dazoxiben-treated hearts indicating equivalent degrees of flow deprivation. During reperfusion, %SS recovered only partially and was not significantly improved by dazoxiben. Dazoxiben augmented peak prostacyclin production (123 +/- 31% vs. 292 +/- 49% of preocclusion values) following reperfusion, while it completely blocked thromboxane A2 production. Dazoxiben attenuated the decline in endocardial ATP (69 +/- 5% vs. 92 +/- 9% normalized to the nonischemic zone) and total adenine nucleotides. The results indicate that dazoxiben may elicit a cardioprotective effect on energy metabolism in the reperfused heart, but this is dissociated from any improvement in regional contractile function.     

Prostacyclin and thromboxane synthesis by endometrial cancer and leiomyomas

To study the role of prostacyclin (PGI2) and thromboxane A2 (TxA2) in uterine tumors, pieces of endometrial cancer (n = 12) and leiomyomas (n = 12) were incubated in vitro, and the productions of 6-keto-prostaglandin F1a (6-keto-PGF1a, a hydration product of PGI2) and thromboxane B2 (TxB2, a hydration product of TxA2), measured by radioimmunoassay, were compared to those of corresponding healthy tissues. The production of 6-keto-PGF1a by endometrial cancer (20.8; 15.1-85.0 ng/mg protein/min, median and interquartile range), by healthy endometrium (25.5; 10.0-55.0), by healthy myometrium (34.9; 25.0-59.9) and by leiomyoma (20.3; 10.2-45.1) was similar. The production of TxB2 was increased by endometrial cancer (55.5; 10.5-155.2, p less than 0.02) in comparison with endometrium (9.8; 4.3-35.1), myometrium (3.8; 2.1-8.0) and leiomyoma (1.9; 1.0-3.8). The 6-keto-PGF1a/TxB2 ratio in endometrial cancer (0.9; 0.3-1.5) was smaller (p less than 0.02) than that in healthy endometrium (3.3; 1.9-4.8). Thus, TxA2 may be a factor in endometrial cancer.     

The relation between thromboxane and prostaglandin synthesis in human decidua tissue: a comparison of eicosanoid synthesis in minced tissue with that in a cell-free preparation

Radiotracer studies and radioimmunoassay measurements demonstrate that minced tissues of human decidua produce chiefly thromboxane B2 (TxB2) (70% of total eicosanoids) and small amounts of prostaglandin F2 alpha (PGF2 alpha) (13%) PGD2 (8%), 6-keto-PGF1 alpha (5%) and PGE2 (4%). Inhibition of thromboxane synthesis with a specific inhibitor (OKY-1581: sodium (E)-3-[4(-3-pyridylmethyl)-phenyl]-2-methyl propenoate) increased prostaglandin formation in general, with the main product being PGF2 alpha (38%), a nonenzymic derivative of PGH2. Crude particulate fractions prepared from the same tissue synthesized two major products from [3H]arachidonate, TxB2 and 6-keto-PGF1 alpha (54 and 30%, respectively) and some PGF2 alpha and PGE2 (8-8%). However, in the presence of reduced glutathione (GSH), PGE2 became the main product (81%) (TxB2, 15%; PGF2 alpha, 2%; and 6-keto-PGF1 alpha, 2%). Half-maximal stimulation of PGE2 synthesis occurred at 46 microM GSH. The GSH concentration of tissue samples was found to be 110 +/- 30 microM. We conclude that human first trimester decidua cells possess the key enzymes of prostaglandin and thromboxane synthesis. Apparently, the production of these compounds is controlled by a specific mechanism in the tissue, which keeps PGE and prostacyclin synthesis in a reversibly suppressed state, whereas the formation of thromboxane is relatively stimulated.     

The role of thromboxane A2 in reperfusion injury

Thromboxane A2 (TXA2) receptor antagonists can limit infarct size in models of coronary occlusion and reperfusion, but it was unknown if these compounds can mitigate reperfusion injury. Anesthetized open chest dogs were subjected to left circumflex coronary (LCX) occlusion for 90 min. Two minutes before reperfusion, the dogs were given iv saline (0.9% NaCl) or the TXA2 antagonist SQ 29,548 (0.2 mg/kg + 0.2 mg/kg/hr). Reperfusion was instituted for 5 hr at which time infarct size was determined. Regional myocardial blood flow was determined before, during, and after occlusion. SQ 29,548 treatment resulted in a significant reduction in infarct size (57 +/- 7 and 34 +/- 8% of the left ventricular area at risk infarcted in the saline and SQ 29,548 groups, respectively). No differences in collateral flow during occlusion were observed between groups, but SQ 29,548 treatment resulted in a significantly higher subendocardial reperfusion flow (54 +/- 10 and 93 +/- 14 ml/min/100g for the saline and SQ 29,548 groups, respectively). Thus, TXA2 seems to play a role in exacerbating reperfusion injury and TXA2 receptor blockade may have potential as a mode of therapy for ischemia-reperfusion damage.     

Role of endothelium, oxygen and ionic milieu in the prostacyclin and thromboxane production of rat aortic tissue slices.

The present study was executed in order to get further data on the role of vessel wall constituents in prostanoid synthesis and on the effect of anorganic constituents on it. Prostacyclin and tromboxane production of rat aortic tissue slices with intact endothelium and after mechanical as well as chemical endothelium removal were studied. The effects of hypoxia and changes in the ionic milieu on the release of these prostanoids were also examined. The tissue slices were incubated in normal or in modified Krebs-Ringer solution, bubbled with 95% O2 and 5% CO2 (with the exception of the studies in hypoxic conditions). Prostacyclin and thromboxane release was determined by specific radioimmunoassay of the stable metabolites, 6-keto-PGF1 alpha and TxB2, from the incubation medium. 174 tissue samples obtained from 164 rats were studied. Mechanical removal of the endothelium increased prostacyclin production of the aortic segments about fivefolds from a basal rate of 52.9 +/- 19.4 ng/gr/min, while it had no significant effect on thromboxane release (basal rate 0.83 +/- 0.13 ng/gr/min). Treating the endothelium with 1.0 M HCl almost totally suppressed prostacyclin release. Lowering the partial oxygen tension of the incubation medium significantly decreased the production of prostacyclin, while release of TxB2 somewhat increased. Increasing the Ca2+ concentration of the medium between 0-5 mM the release of prostacyclin was augmented and the release of thromboxane was diminished. Potassium free medium caused a very large increase in prostacyclin release of the tissue slices. The results show that release of vasoactive prostanoids from isolated rat aortic wall is dependent not exclusively on the endothelium and that various methods of endothelium removal may have distinct influences on prostacyclin and thromboxane productions. The changes in anorganic constituents of the surrounding medium could massively affect prostacyclin and thromboxane production of rat aortic tissue. The alternative effects of the above listed treatments on the release of prostacyclin and thromboxane from the rat aortic wall suggest the existence of different mechanisms in the control of the production of the two major prostanoids possessing opposite physiological effects.     

Pharmacological modulation of prostacyclin and thromboxane production of rat and cat venous tissue slices.

To reveal a potential modulating effect of vasoactive pharmacological agents on the prostanoid production of the venous wall, prostacyclin and thromboxane release from venous tissue slices was studied. Aortic and caval vein samples from 20 rats as well as from 21 cats were studied. Prostacyclin and thromboxane productions were determined by radioimmunoassay as 6-keto-PGF1 alpha and TxB2 released into the incubation medium. Venous tissue produced significantly less prostacyclin per unit weight than arterial tissue in rats (30.7 +/- 4.6 vs. 52.1 +/- 8.2 pg/mg/min), while in cats an opposite situation was found (16.6 +/- 3.2 vs. 7.06 +/- 1.9 pg/mg/min). Thromboxane production of venous tissue was consequently higher than corresponding values for aortic tissue (3.72 +/- 0.46 vs. 1.54 +/- 0.14 in rats and 3.4 +/- 0.6 vs. 1.33 +/- 0.19 in cats, all values in pg/mg/min). Norepinephrine and dopamine significantly increased both the prostacyclin and the thromboxane release from venous tissue, while isoproterenol had no effect. Vasopressin significantly increased thromboxane release and decreased the ratio of prostacyclin vs. thromboxane production (from 10.4 +/- 1.6 to 7.5 +/- 1.6, in acetylsalicylic acid pretreated cats). Angiotensin and thrombin had no significant effects. Bradykinin (0.5 microgram/ml) significantly augmented prostacyclin release from venous tissue (14.4 +/- 2.6 from 10.9 +/- 2.4 pg/mg/min) and decreased thromboxane release (0.65 +/- 0.18 from 1.35 +/- 0.22 pg/mg/min). Methionine-enkephalin (5 micrograms/ml) significantly reduced the thromboxane release from venous tissue slices. The presented material demonstrates that several vasoactive agents modulate the vasoactive prostanoid release of the venous wall. In some cases, the prostacyclin and the thromboxane productions are influenced separately, which in turn will have its impact on smooth muscle activity and thrombocyte aggregation.     

Estrogen potentiates constrictor prostanoid function in female rat aorta by upregulation of cyclooxygenase-2 and thromboxane pathway expression

Estrogen potentiates vascular reactivity to vasopressin (VP) by enhancing constrictor prostanoid function. To determine the cellular and molecular mechanisms, the effects of estrogen on arachidonic acid metabolism and on the expression of constrictor prostanoid pathway enzymes and endoperoxide/thromboxane receptor (TP) were determined in the female rat aorta. The release of thromboxane A2 (TxA2) and prostacyclin (PGI2) was measured in male (M), intact-female (Int-F), ovariectomized-female (OvX-F), and OvX + 17beta-estradiol-replaced female (OvX + ER-F) rats. The expression of mRNA for cyclooxygenase (COX)-1, COX-2, thromboxane synthase (TxS), and TP by aortic endothelium (Endo) and vascular smooth muscle (VSM) of these four experimental groups was measured by RT-PCR. The expression of COX-1, COX-2, and TxS proteins by Endo and VSM was also estimated by immunohistochemistry (IHC). Basal release of TxA2 and PGI2 was similar in M (18.8 +/- 1.9 and 1,723 +/- 153 pg/mg ring wt/45 min, respectively) and Int-F (20.2 +/- 4.2 and 1,488 +/- 123 pg, respectively) rat aortas. VP stimulated the dose-dependent release of TxA2 and PGI2 from both male and female rat aorta. OvX markedly attenuated and ER therapy restored VP-stimulated release of TxA2 and PGI2 in female rats. No differences in COX-1 mRNA levels were detected in either Endo or VSM of the four experimental groups (P > 0.1). The expression of both COX-2 and TxS mRNA were significantly higher (P < 0.05) in both Endo and VSM of Int-F and OvX + ER-F, compared with M or OvX-F. Expression of TP mRNA was significantly higher in VSM of Int-F and OvX + ER-F compared with M or OvX-F. IHC revealed the uniform staining of COX-1 in VSM of the four experimental groups, whereas staining of COX-2 and TxS was greater in Endo and VSM of Int-F and OvX + ER-F than in OvX-F or M rats. These data reveal that estrogen enhances constrictor prostanoid function in female rat aorta by upregulating the expression of COX-2 and TxS in both Endo and VSM and by upregulating the expression of TP in VSM.     

Arachidonic acid metabolism in isolated rat aorta. Dependence of prostacyclin biosynthesis on extracellular potassium concentration

Slices of rat aorta were incubated in Krebs-Ringer bicarbonate buffer for measurements of immunoreactive 6-ketoprostaglandin F1 alpha, thromboxane (TX) B2, prostaglandin (PG)E2, and PGF2 alpha, and in Tris buffer (pH 9.3) for determination of prostacyclin (PGI2)-like activity. No significant generation of TXB2, PGE2, or PGF2 alpha by rat aortic tissue could be detected. The time-dependent release of 6-keto-PGF1 alpha Krebs-Ringer bicarbonate buffer closely correlated with PGI2 generation in alkaline Tris buffer. During a 30-min incubation period, 6-keto-PGF1 alpha, release was 79.8 +/- 3.3 pmol/mg at a buffer potassium concentration of 3.9 mmol/liter and significantly increased by 23% to 98.3 +/- 8.5 pmol/mg (P less than 0.025) in the absence of potassium in the incubation medium. A smaller decrease in buffer potassium concentration to 2.1 mmol/liter and an increase to 8.8 mmol/liter did not significantly alter aortic 6-keto-PGF1 alpha release. Changes in the incubation buffer sodium concentration from 144 mmol/liter to either 138 or 150 mmol/liter at a constant potassium concentration of 3.9 mmol/liter did not alter the recovery of 6-keto-PGF1 alpha. Our results support the concept that PGI2 is the predominant product of arachidonic acid metabolism in rat aorta. They further show that PGI2 can be recovered quantitatively as 6-keto-PGF1 alpha under the present in vitro conditions. In addition, this in vitro study points to the potassium ion as a modulator of vascular PGI2 synthesis with a stimulation at low potassium concentrations.     

Enrichment of endothelial cell arachidonate by lipid transfer from high density lipoproteins: relationship to prostaglandin I2 synthesis

We have previously shown that plasma high density lipoproteins (HDL) stimulate release of prostacyclin, measured as its stable metabolite, 6-keto-PGF1 alpha, by cultured porcine aortic endothelial cells. The present experiments were designed to elucidate the contribution of HDL lipids to endothelial cellular phospholipid pools and to prostacyclin synthesis. In experiments with reconstituted HDL, both the lipid and protein moieties were required to stimulate prostacyclin release in amounts equivalent to the native HDL particle. Endothelial cells incorporated label from reconstituted HDL containing cholesteryl [1-14C]arachidonate into the cellular neutral and phospholipid pools as well as into 6-keto-PGF1 alpha and PGE2. Labeled arachidonate incorporated into endothelial cell lipids from reconstituted HDL containing cholesteryl [1-14C]arachidonate was also metabolized to prostaglandins after the cells were exposed to the calcium ionophore, A-23187. Both rat and human HDL which stimulated 6-keto-PGF1 alpha release (rat greater than human) increased the weight percentage of arachidonate in endothelial cell phospholipids; phospholipid arachidonate in the enriched cells fell after exposure to the phospholipase activator, A-23187, with release of 6-keto-PGF1 alpha which was greater than in control cells. Rat HDL that was depleted of cholesteryl arachidonate (achieved by incubation with human low density lipoproteins (LDL) in the presence of cholesteryl ester transfer protein) stimulated 6-keto-PGF1 alpha release less than native rat HDL. LDL enriched in cholesteryl arachidonate stimulated 6-keto-PGF1 alpha release more than native LDL. ApoE-depleted HDL also stimulated 6-keto-PGF1 alpha release more than apoE-rich HDL suggesting the apoE receptor was not involved in the response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Selective thromboxane synthetase inhibition by picotamide and effects on endotoxin-induced lethality

The efficacy of N,N'-bis-(3-picolyl)-methoxyisophthalamide (picotamide) as an in vitro thromboxane synthetase inhibitor and its effect on endotoxin (LPS)-induced lethality in rats were assessed. Picotamide at 0.5 and 1.0 mM concentrations significantly (P less than 0.05) inhibited basal and LPS-stimulated synthesis of TxA2 measured by its stable immunoreactive (i) metabolite TxB2 in rat peritoneal macrophages. This compound did not inhibit synthesis of i6-keto-PGF1 alpha, the stable metabolite of PGI2, and produced significant shunting to i6-keto-PGF1 alpha. For lethality studies rats were pretreated, by gavage with picotamide, at either 75, 150, 300, or 600 mg/kg 2 hr prior to iv S. enteritidis (LPS, 20 mg/kg). Both 150 and 300 mg/kg doses of picotamide significantly (P less than 0.05) improved survival in endotoxin shock at 48 hr. These studies demonstrate that picotamide is a selective thromboxane synthetase inhibitor, and that it may be useful during disease states characterized by increased TxA2 synthesis.     

Renal synthesis of prostacyclin and thromboxane in healthy women: differential effects of a short-term saline loading

It is accepted that the urinary excretions of the stable metabolites of prostaglandin (PG)I2 and thromboxane(Tx) A2, 6-keto-PGF1alpha (6KPGF) and TxB2 respectively, provide an accurate estimate of both basal and stimulated renal synthesis of their precursors. The excretory profile of these metabolites has been evaluated in healthy women submitted to a short-term expansion in extracellular fluid volume. Salt retention (SR group, n=6) was induced by physiological saline (0.9% NaCl) i.v. infusions (2 L per day) over a period of 2 days. On the third day the increase in body weight was 0.92 +/- 0.27 kg (P<0.05). The results of the study have been compared to those previously obtained in normal balance of sodium and potassium (N group, n=20) and in induced salt depletion (SD group, n=14). A common study protocol was used. Basal values of plasma renin activity (PRA) and urinary aldosterone excretion were determined. Renal functional exploration [clearance (cl.) method] was performed during hypotonic polyuria (induced by oral water load) and subsequent moderate antidiuresis (induced by low-dose infusion of an antidiuretic hormone analogue). Urinary 6KPGF and TxB2 concentrations were estimated by RIA method and their urinary excretions were determined at both high and low urinary flow rates. The linear regressions of the urinary metabolite excretions vs. urinary flow rate were estimated by using the data obtained in both hypotonic polyuria and antidiuresis. Salt retention (SR vs. N group) was effective in decreasing the basal values of plasma renin activity and urinary aldosterone excretion. Moreover, during hypotonic polyuria it was effective in increasing the absolute and fractional excretions of sodium and chloride, in the absence of significant variations in mean arterial pressure and creatinine cl. Regarding urinary prostanoid excretions the following results were obtained. 1. Comparative data for hypotonic polyuria. In the SR vs. N group, the urinary excretion of 6KPGF was significantly higher, whereas that of TxB2 was not significantly different. In the SR vs. SD group, the urinary excretion of 6KPGF was not significantly different, whereas that of TxB2 was significantly lower. 2. Comparative data for the regression lines of the urinary prostanoid excretions vs. diuresis. In the SR vs. N group, the regression line slope for 6KPGF excretion was significantly higher, whereas that for TxB2 excretion was not significantly different. In the SR vs. SD group, the regression line slope for 6KPGF excretion was not significantly different, whereas that for TxB2 excretion was significantly lower. 3. Correlative data in the SR group during hypotonic polyuria. The plasma chloride concentration was positively correlated with urinary flow rate, absolute and fractional chloride excretions, and 6KPGF excretion but not with TxB2 excretion. In conclusion, functionally effective salt retention in healthy women induces a selective stimulation of renal synthesis of prostacyclin, unlike salt depletion, in which the synthesis of both PGI2 and TxA2 is upregulated.     

OKY-046 inhibits anaphylactic bronchoconstriction and reduces histamine level in bronchoalveolar lavage fluid in sensitized guinea pigs.

We investigated the effects of OKY-046, a potent and selective thromboxane A2 (TxA2) synthetase inhibitor, on anaphylactic bronchoconstriction and release of chemical mediators into airway lumen in sensitized guinea pigs in vivo. OKY-046 dose-dependently inhibited antigen-induced anaphylactic bronchoconstriction with or without mepyramine, a histamine H1 antagonist. In the presence of mepyramine, OKY-046 (300 mg/kg, p.o.) elicited significant reductions in histamine (1 min) and TxB2 increases (1-15 min) in bronchoalveolar lavage (BAL) fluid but significantly increased the plasma level of 6-keto-PGF1 alpha, a stable PGI2 metabolite, after antigen challenge. On the contrary, indomethacin only significantly reduced increases in TxB2 levels. These results suggest that the antiasthmatic effect of OKY-046 is probably due to inhibition of TxA2 synthesis and suppression of histamine release via a PGI2 shunting mechanism.     

Characterization of thromboxane receptor blocking effects of SQ 29548 in the feline pulmonary vascular bed.

The effects of SQ 29548, a thromboxane (Tx) A2 receptor blocking agent, on responses to the TxA2 mimic U46619 were investigated in the pulmonary vascular bed of the intact-chest cat under constant-flow conditions. The administration of SQ 29548 in doses of 0.25-1 mg/kg iv reduced vasoconstrictor responses to U-46619; however, responses to prostaglandins (PG) F2 alpha and D2 and to serotonin were also decreased. After administration of SQ 29548 in doses of 0.05-0.1 mg/kg iv, responses to U-46619 and U-44069 were reduced significantly, and the dose-response curves for these TxA2 mimics were shifted to the right in a parallel manner at a time when responses to PGF2 alpha and PGD2 were not altered. The low doses of the TxA2 receptor blocking agent significantly reduced responses to the PG and TxA2 precursor arachidonic acid but were without significant effect on vasoconstrictor responses to serotonin; histamine; norepinephrine; angiotensin II; the major PGD2 metabolite 9 alpha,11 beta-PGF2; BAY K 8644, an agent that enhances calcium entry; and endothelin-1. The present data show that at low doses SQ 29548 selectively blocks TxA2 receptor-mediated responses in a competitive and reversible manner in the pulmonary vascular bed. These data suggest that responses to arachidonic acid are mediated in large part by the formation of TxA2 and provide evidence in support of the hypothesis that a discrete TxA2 receptor unrelated to PGF2 alpha or PGD2 receptors is present in undefined resistance vessel elements in the feline pulmonary vascular bed.     

Modulation of Platelet thromboxane A2 and arterial prostacyclin by dietary vitamin E

Platelets from vitamin E-deficient and vitamin E-supplemented rats generate the same amount of thromboxane A2 (TxA2) when they are incubated with unesterified arachidonic acid. Platelets from vitamin E-deficient rats produced more TxA2 than platelets from vitamin E-supplemented rats when the platelets are challenged with collagen. Arterial tissue from vitamin E-deficient rats generates less prostacyclin (PGI2) than arterial tissue from vitamin E- supplemented rats. The vitamin E effect with arterial tissue is observed when the tissue is incubated with and without added unesterified arachidonic acid. These data show that arterial prostacyclin synthesis is diminished in vitamin E-deficient rats. Vitamin E, in vivo, inhibits platelet aggregation both by lowering platelet TxA2 and by raising arterial PGI2.     

Release of arachidonic acid metabolites from isolated human alveolar type II cells.

Human alveolar type II cells are thought to play a role in the pathogenesis of lung injury. Patterns of mediator release of arachidonic acid metabolism by type II cells were therefore studied after challenge with calcium ionophore A23187, opsonized zymosan and hydrogen peroxide. A time- and concentration dependent release of cyclooxygenase products was observed, with release of PGE2 greater than 6-keto-PGF1 alpha greater than TxB2. Addition of glutathione or bicarbonate further increased the production of PGE2. N-ethylmaleimide, a sulfhydryl (SH) reactant, induced a dose-dependent increase in the release of TxB2 and 6-keto-PGF1 alpha, but not of PGE2. This relates most likely to the SH-dependency and glutathione requirement of the PGE2 isomerase and SH-independence of thromboxane and prostacyclin isomerase.     

Influence of SQ 30741 on thromboxane receptor-mediated responses in the feline pulmonary vascular bed.

The effects of SQ 30741, a thromboxane A2 (TxA2) receptor blocking agent, on responses to the TxA2 mimic, U-46619, were investigated in the pulmonary vascular bed of the intact-chest cat under constant-flow conditions. The administration of SQ 30741 in doses of 1-2 mg/kg iv markedly reduced vasoconstrictor responses to U-46619 without altering responses to prostaglandin (PG) F2 alpha or PGD2 and serotonin. SQ 30741 had no significant effect on mean vascular pressures in the cat, and the dose-response curve for U-46619 was shifted to the right in a parallel manner with a similar apparent maximal response. In addition to not altering responses to PGF2 alpha, PGD2 alpha, or serotonin, SQ 30741 (2 mg/kg iv) was without significant effect on pulmonary vasoconstrictor responses to the PGD2 metabolite 9 alpha, 11 beta-PGF2, norepinephrine, angiotensin II, BAY K 8644, endothelin 1, or endothelin 2. Although responses to vasoconstrictor agents, which act through a variety of mechanisms, were not altered, responses to the PG and TxA2 precursor, arachidonic acid, were reduced significantly. The duration of the TxA2 receptor blockade was approximately 30 and 75 min at the 1- and 2-mg/kg iv doses of the antagonist, respectively. The present data show that SQ 30741 selectively blocks TxA2 receptor-mediated responses in a competitive and reversible manner in the pulmonary vascular bed. These data suggest that responses to arachidonic acid are due in large part to the formation of TxA2 and that discrete TxA2 receptors unrelated to receptors activated by PGD2 or PGF2 alpha are most likely located in resistance vessel elements in the feline pulmonary vascular bed.     

Effects of thromboxane synthase inhibition on air emboli lung injury in sheep

We tested the effects of OKY-046, a thromboxane synthase inhibitor, on lung injury induced by 2 h of pulmonary air infusion (1.23 ml/min) in the pulmonary artery of unanesthetized sheep with chronic lung lymph fistula so as to assess the role of thromboxane A2 (TxA2) in the lung injury. We measured pulmonary hemodynamic parameters and the lung fluid balance. The concentrations of thromboxane B2 (TxB2) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) in plasma and lung lymph were determined by radioimmunoassay. Air infusion caused sustained pulmonary hypertension and an increase in pulmonary vascular permeability. The levels of TxB2 and 6-keto-PGF1 alpha in both plasma and lung lymph were significantly elevated during the air infusion. TxB2 concentration in plasma obtained from the left atrium was higher than that from the pulmonary artery at 15 min of air infusion. When sheep were pretreated with OKY-046 (10 mg/kg iv) prior to the air infusion, increases in TxB2 were prevented. The pulmonary arterial pressure, however, increased similarly to that of untreated sheep (1.8 X base line). The increase in lung lymph flow was significantly suppressed during the air infusion. Our data suggest that the pulmonary hypertension observed during air embolism is not caused by TxA2.     

Sexual dimorphism in prostanoid-potentiated vascular contraction: roles of endothelium and ovarian steroids

The effects of constrictor prostanoid (CP) pathway inhibitors on vascular reactivity to vasopressin (VP) and phenylephrine (PE) were examined in thoracic aortas of male, female, and ovariectomized (OVX) female Sprague-Dawley rats. Maximal contractile response of control (Cont) aortas to VP was markedly higher in females (3,885 +/- 332 mg/mg ring wt) than in males (810 +/- 148 mg). Indomethacin (Indo; 10 microM) attenuated maximal response to VP in females (3,043 +/- 277 mg) but not in males. SQ-29,548 (SQ; 1 microM) attenuated maximal response to VP in females (3,042 +/- 290 mg) to a similar extent as Indo. Dazoxiben (Daz; 10 microM) alone had no effect, but Daz + SQ attenuated maximal contractile response to VP to a similar extent as SQ alone. Removal of the endothelium in female aortas attenuated contractile responses to VP in Cont aortas. OVX attenuated maximal contractile response to VP in Cont aortas (2,093 +/- 329 mg) and abolished the attenuating effects of Indo. Indo, SQ, and Daz exerted identical effects on contractile responses of male, female, and OVX female aortas to PE. These findings establish the following in the rat aorta: 1) CP, probably thromboxane and/or endoperoxide, is responsible for approximately 25-30% of contractile responses of females, but not males, to VP and PE; 2) CP production by the female aorta is primarily endothelial in origin; and 3) ovarian steroids modulate production and/or actions of CP in female aortas.