Increased thromboxane A2 production but normal prostacyclin by the placenta in hypertensive pregnancies

Increased phospholipase A2 and thromboxane but not prostacyclin production by placental trophoblast cells from normal and preeclamptic pregnancies cultured under hypoxia condition

In this study we determined whether hypoxia could promote vasoactivator thromboxane (TX) and prostacyclin (PGI2) as well as phospholipase A2 (PLA2) production by placental trophoblast cells (TCs) from normal and preeclamptic (PE) pregnancies. Placentas were obtained immediately after delivery from normal (n=9) and preeclamptic (n=9) pregnancies. TCs were isolated by dispase digestion of villous tissue and purified by Percoll gradient centrifugation. TCs (5x10(6) cells/well) were cultured with Dulbecco's Modified Eagles Medium (DMEM) under hypoxia condition (2% O2/5% CO2/93% N2) for 48 h. TCs cultured under normoxia condition (5% CO2/air) were used as control. Culture medium was collected at the end of incubation. Productions for TX, PGI2 and PLA2 were measured by ACE competitive enzyme immunoassay (EIA). Comparisons were made using the Mann-Whitney U test or paired t-test and the data are expressed as mean+/-SE (pg/microg cellular protein). Significance was set at a p-value of <0.05. We found: (1) PE-TCs produced more TXB2 and PLA2 than normal-TCs under normoxia conditions, TXB2: 4.33+/-1.03 vs. 1.84+/-0.29 pg/microg protein, p<0.05; PLA2: 0.38+/-0.08 vs. 0.21+/-0.03 pg/microg protein, p<0.05, respectively. (2) Hypoxia promoted both PE- and normal-TCs to generate more TXB2 and PLA2, TXB2: 6.36+/-1.72 vs. 3.05+/-0.45 pg/microg; PLA2: 0.52+/-0.10 vs. 0.30+/-0.04 pg/microg, respectively. (3) No change in 6-keto PGF1alpha production was observed for normal-TCs or PE-TCs when compared under normoxia vs. hypoxia condition, normal-TCs: 0.20+/-0.05 vs. 0.21+/-0.05 pg/microg; PE-TCs: 0.38+/-0.05 vs. 0.36+/-0.04 pg/microg, respectively. We concluded that hypoxia promotes both PLA2 and TX, but not PGI2, production by placental trophoblast cells cultured under hypoxia condition. These results suggest that increased PLA2 release may alter the arachidonic acid cascade and promote TX synthesis. Relative hypoxia could contribute to the increase in TX production and result in vasoconstriction in placental vasculature in preeclampsia.

Eicosanoids in preeclampsia

Preeclampsia is characterized by an imbalance between two cyclooxygenase metabolites of arachidonic acid, thromboxane and prostacyclin, that favors thromboxane. Because of the biologic actions of these two eicosanoids, this imbalance might explain major clinical symptoms of preeclampsia, such as hypertension, platelet aggregation and reduced uteroplacental blood flow. In the maternal circulation, this imbalance is primarily manifested by decreased production of prostacyclin by endothelial cells. Platelet thromboxane synthesis is only increased in severe preeclampsia. In the placenta and in leukocytes, the imbalance is exacerbated by increased production of thromboxane coupled with decreased production of prostacyclin in both mild and severe preeclampsia. Longitudinal measurements of urinary metabolites of thromboxane and prostacyclin reveal that the thromboxane/prostacyclin imbalance predates the onset of clinical symptoms of preeclampsia. The imbalance between thromboxane and prostacyclin is most likely caused by oxidative stress, which is manifest in preeclampsia by increased lipid peroxidation and decreased antioxidant protection. Oxidative stress may drive this imbalance because lipid peroxides activate the cyclooxygenase enzyme to increase thromboxane synthesis, but at the same time they inhibit prostacyclin synthase to decrease prostacyclin synthesis. Low-dose aspirin therapy (50-150 mg/day) has been considered for the prevention of preeclampsia because it selectively inhibits thromboxane synthesis. Several studies reported dramatic decreases in the incidence of preeclampsia with low-dose aspirin therapy. However, two large multicenter studies reported only modest decreases, which dampened enthusiasm. The two large studies were "intent to treat" studies which included patients who were noncompliant and who discontinued the use of aspirin. In one of the studies for which compliance statistics were available only 53% of the aspirin group had a compliance rate greater than 75%, which raises a question as to whether the effectiveness of aspirin was being tested. Low-dose aspirin therapy should not yet be dismissed for the prevention of preeclampsia, but be reconsidered with emphasis on compliance using doses of aspirin in the range of 100-150 mg/day combined with antioxidants.

The fetoplacental pressor effects of low-dose acetylsalicylic acid and angiotensin II in the ex vivo cotyledon model

OBJECTIVE

Our purpose was to investigate perfusion pressure changes ex vivo induced by angiotensin II on fetoplacental vasculature pretreated with low-dose acetylsalicylic acid.

STUDY DESIGN

Two cotyledons from each of 12 placentas were perfused. The intervillous space of one cotyledon was infused with acetylsalicylic acid (5 x 10(-5) mol/L) similar to the serum concentration of women receiving daily low-dose aspirin therapy (60 to 81 mg). The control cotyledon was infused with an equivalent amount of normal saline solution. Two doses of angiotensin II, 1 x 10(-11.5) and 1 x 10(-10) moles, were injected as boluses into the chorionic arteries of each cotyledon. A 3 x 10(-7) mole dose of angiotensin II was also injected into the intervillous space. Statistical analysis was performed with analysis of variance, and results are expressed as mean pressure change in millimeters of mercury +/- SEM.

RESULTS

Perfusion pressure response did not vary between cotyledons pretreated with acetylsalicylic acid and control cotyledons when 3 x 10(-7) moles of angiotensin II was injected into the intervillous space (8.0 +/- 1.9 mm Hg vs 9.8 +/- 1.6 mm Hg, p = 0.59). There were no differences between cotyledons in pressure response to 1 x 10(-11.5) moles of angiotensin II injected into the fetal circuit (5.9 +/- 0.8 mm Hg vs 6.7 +/- 0.9 mm Hg, p = 0.51). However, in the cotyledons pretreated with acetylsalicylic acid there was a decrease in the pressor response to 1 x 10(-10) moles of angiotensin II (14.1 +/- 1.4 mm Hg vs 21.5 +/- 3.3 mm Hg, p = 0.05).

CONCLUSIONS

Low-dose aspirin infused into the intervillous space decreases vasoconstriction elicited by angiotensin II in the fetoplacental compartment. This suggests that maternal low-dose aspirin therapy has effects in the fetoplacental circulation in addition to its effects in the maternal circulation.

GnRH in pregnancy

Human trophoblast produce GnRH and its precursor, immunologically and chemically identical to those of hypothalamic origin. Placental GnRH stimulates human chorionic gonadotropin secretion by the syncytiotrophoblast. It is known that GnRH analogue has negative effect on early rat pregnancy and may cause abortion through its action on the corpus luteum. A significant reduction of progesterone production was found in pregnant rats treated with GnRH agonist. GnRH caused a significant decrease in the maximal contraction intensity of non-pregnant and pregnant uterine muscle strip, following the action of oxytocin and acetylcholine. It was observed that treatment of pregnant rat with pharmacological doses of GnRH was able to delay parturition. Experimentally, GnRH significantly inhibited the release of placental prostaglandins E and F and thromboxane B 2 in a dose dependent fashion. Maternal circulating GnRH levels at 25-35 weeks of gestation were significantly higher in women who later had post-term pregnancies. In an other study maternal circulating GnRH concentration was found to be significantly lower in four patients who developed preterm labor and delivery. Low doses of GnRH in pregnant rats produced inhibition of postpartum lordosis behavior.

Dose-related action of estradiol on placental prostanoid prediction

Prostanoids play an important role throughout all of pregnancy and during the initiation and progress of labor. The human placenta at term produces large quantities of prostanoids, yet little is known of the factors that regulate their biosynthesis. Herein, we report the effect of estradiol or estradiol and progesterone on the basal release of placental prostanoids from fresh human term placental explants using a perifusion system. The basal release of prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha), thromboxane (TxB2) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) increased about 50% from the fifth to the ninth hour in culture, while the release of 13, 14-dihydro-15-keto-PGF (PGFM) remained constant and hCG release decreased. The dose-related effect of estradiol (20-2,000 ng/ml) in the perifusing medium starting at the fifth hour of perifusion (i.e., the zero treatment time) effected no change in the release of TxB2, PGF2 alpha, PGFM or hCG. A biphasic action on the release of 6-keto-PGF1 alpha was observed, i.e. it was significantly decreased when incubated with 20 ng/ml of estradiol, but effected an increase after exposure to 200 ng/ml. The concomitant addition of progesterone (2,000 ng/ml) with estradiol (200 ng/ml) significantly inhibited the stimulatory action of estradiol at this dose. The release of PGF2 was inhibited in a dose-related fashion with increasing dose of estradiol. The addition of progesterone with estradiol (2,000 and 200 ng/ml, respectively) reversed the inhibition of PGE2 by estradiol alone. These data demonstrate that physiologic levels of estradiol affect 6-keto-PGF1 alpha and PGE2 release from the human term placenta, but do not significantly alter production of TxB2, PGFM or hCG under these conditions.

Absence of relaxation to lactate in human placental vessels of pregnancies with severe preeclampsia

OBJECTIVE

Our objective was to determine whether the observed relaxation to lactate and other agents in placental vessels of normal pregnancies is altered in severe preeclampsia.

STUDY DESIGN

Isolated placental arteries and veins from women with severe preeclampsia and uncomplicated term pregnancies were precontracted with prostaglandin F2 alpha under 5% oxygen and 5% carbon dioxide with the balance nitrogen (Po2 35 to 38 torr) and then exposed to lactate (1 to 10 mmol/L, pH 7.4, n = 8 to 15), arachidonic acid (0.01 to 10 mumol/L, n = 6 to 13), nitroglycerin (1 nmol to 1 mumol/L, n = 4 to 12), or forskolin (0.01 to 10 mumol/L, n = 6 to 9). The response to lactate was also examined in placental vessels from appropriate-for-gestational-age preterm deliveries (n = 8) for comparison with a similar group with severe preeclampsia (n = 8). The t test and analysis of variance statistics were used.

RESULTS

Relaxation to lactate was markedly inhibited in both placental arteries and veins of women with severe preeclampsia compared with vessels from uncomplicated term or preterm pregnancies. Responses to the other relaxing agents were not altered in the severely preeclamptic vessels.

CONCLUSIONS

In severe preeclampsia absence of lactate-induced dilatation of placental vessels may contribute to the fetal complications associated with impaired blood flow and vasospasm.

Placental prostanoid release in severe intrauterine growth retardation

Our objective was to evaluate prostanoid release from the placentae of pregnancies complicated by severe intrauterine growth retardation (IUGR) and without hypertension, compared with placentae from normal, uncomplicated term pregnancies. A perifusion system was utilized to study the release of prostanoids 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), thromboxane B2(TxB2), prostaglandin E2 (PGE2) and prostaglandin F2 alpha (PGF2 alpha) from human placentae from pregnancies complicated by normotensive severe IUGR (n = 9, five at term and four preterm) and normal control pregnancies (n = 6). For each placenta, triplicate chambers of tissue were perifused at a rate of 6 ml/h, and samples were collected from hours 5-10. Prostanoids were measured using specific and sensitive radioimmunoassays. In the IUGR group, the basal placental production of the vasoconstrictor thromboxane was not increased, nor was the ratio of cumulative TxB2 to 6-keto-PGF1 alpha elevated compared with normal term controls. In three term IUGR placentae, the ratio was significantly decreased compared with controls. The basal placental production of the vasoconstrictor PGF2 alpha was likewise not increased compared with controls, nor was the ratio of PGF2 alpha to PGE2 elevated. Two of the placentae in the term IUGR group demonstrated significant elevations of PGE2 and 6-keto-PGF1 alpha. Overall, the IUGR placentae released normal or low normal levels of the prostanoids studied. The pattern of placental prostanoid release over time was similar to that of the normal term placentae. The term and preterm placentae of pregnancies complicated by severe IUGR did not exhibit an excess production of vasoconstrictor prostanoids. Therefore, strategies designed to reduce thromboxane production in severe IUGR without hypertension may be unjustified.

Differentiation and growth on a fibrin matrix modulate the cyclooxygenase expression and thromboxane production by cultured human placental trophoblasts

Preeclampsia is associated with altered placental production of several end-products of cyclooxygenase activity. Thromboxane (TX) is one of these end-products, and trophoblast is a source of villous thromboxane. We cultured term trophoblast in the presence or absence of fibrin to study how differentiation and epithelial-matrix interactions regulate cyclooxygenase expression. The cellular trophoblast present during the first 24 h of culture on uncoated plastic produced TXB2, but little or no TX was produced in cultures grown longer than 24 h when differentiation into syncytial trophoblast occurred. Growth of cells on a fibrin matrix enhanced cellular trophoblast TX production five-fold. Medium containing 10 mumol/l arachidonic acid maximized thromboxane production in cells cultured for less than 24 h, regardless of growth surface, but this medium had little or no effect on TX production by cultures grown for more than 24 h. In contrast, exogenous arachidonic acid enhanced prostaglandin E2 (PGE2) production by both cellular and syncytial trophoblast. Cytochemical staining indicated that changes in cyclooxygenase content occurred with trophoblast differentiation. Western immunoblot analysis of cells cultured in the presence or absence of a fibrin matrix showed cyclooxygenase was induced under both growth conditions. Detectable cyclooxygenase protein disappeared beyond 24 h in cells grown on uncoated plastic. In contrast, cells grown beyond 24 h on fibrin showed sustained expression of cyclooxygenase by Western immunoblotting, and this enzyme protein expression correlated with increased PGE2 production by the differentiated trophoblast.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical localization of thromboxane synthase in human intrauterine tissues

Uterine tissues are known to be able to synthesize thromboxane A2 (TXA2), but there is little information about the nature of cells actually responsible for its production. In this study human placenta, fetal membranes, umbilical cord and pregnant myometrium were investigated immunohistochemically. The avidin-biotin method for a monoclonal antibody against human thromboxane synthase (Tü 300) was applied on frozen tissue sections. In placenta, fetal membranes and umbilical cord, staining was positive for Hofbauer cells and fibroblasts. Further, in sections of placenta, capillary endothelium showed antigenicity for TX synthase. Leiomyocytes in the umbilical cord vessels contained the enzyme as well. Preparations of pregnant myometrium were shown to express TX synthase in leiomyocytes, endothelial cells and connective tissue cells. Amnion, trophoblast and decidua did not possess antigenicity for this enzyme. Since TXA2 plays an important role for the regulation of vascular tone and aggregation of platelets and may stimulate myometrial contractions during parturition, the abundance of TX synthase in pregnancy-specific tissues confirms previous in vivo and in vitro observations. Further, TXA2 synthesized by Hofbauer cells may be involved in immunological reactions during pregnancy, and the number and level of activation of Hofbauer cells may be closely related to the initiation of labour. Thromboxane production by the endothelium lining the fetal vessels points to its regulatory role for the blood flow in the fetoplacental unit.

Eicosanoid production by frozen tissue in vitro is markedly changed

The influence of storing deep frozen human tissue specimens on their eicosanoid production in tissue culture was evaluated. Pieces of placenta and fetal membranes (n = 6) were divided and part processed immediately after obtaining the tissues, part after being kept at -20 degrees C for some days. Eicosanoid production was studied in tissue culture with incubation in oxygenated Hank's balanced salt solution (HBSS) at 37 degrees C for 1 h. Prostaglandin E2 (PGE2), prostacyclin (PGI2), thromboxane A2 (TXA2) and leukotriene B4 (LTB4) production were significantly different in the tissues frozen prior to the incubation. This should be considered when interpreting eicosanoid synthesis in tissue cultures for their significance in vivo.

Cyclooxygenase gene expression in human endometrium and decidua

The objective of this study was to determine if genes for cyclooxygenase (COX) and 5-lipoxygenase, key enzymes in the synthesis of eicosanoids, are expressed in human endometrium and to determine if the level of expression is affected by decidualization or preeclampsia, a form of pregnancy-induced hypertension. RT-PCR was used to detect mRNA in tissues obtained from various patients. Results demonstrated that COX and 5-lipoxygenase genes are expressed in endometrium and decidua. COX gene expression in endometrium is 2-3-fold greater than in decidua while 5-lipoxygenase gene expression is similar. Neither COX nor 5-lipoxygenase gene expression in decidua is altered by changes resulting from preeclampsia. Thus, genes for key enzymes in the synthesis of eicosanoids are expressed in human endometrium and decidua. Selective down-regulation is evident as decidualization results in a significant reduction in the gene expression of COX, while 5-lipoxygenase gene expression remains unchanged.

Eicosanoid production by intrauterine tissues before and after labor in short-term tissue culture

Prostanoid production by intrauterine tissues from pregnant and non-pregnant women has been studied intensively over the last decade. Little is known about the lipoxygenase metabolites of arachidonic acid (AA). The production of prostaglandins and HETEs by pregnancy specific human tissues was investigated in a short-term culture system. Tissue samples were obtained after uncomplicated pregnancies from placenta, fetal membranes and decidua of deliveries before (n = 6) and after the onset of labor (n = 8) and incubated for 1 hour in oxygenated HBSS. In the supernatant, PGE2, PGF2 alpha, 6-keto-PGF1 alpha and TXB2 were measured with RIA and 15-, 12- and 5-HETE with HPLC and UV-detection. The main AA-metabolite in all tissue incubations was 12-HETE. Decidua produced 12 to 28 times more prostaglandins than placenta and fetal membranes with 6-keto-PGF1 alpha as the main metabolite. The main cyclooxygenase derivative measured from placenta and fetal membrane incubations was TXB2. After labor, fetal membranes showed an increase in total prostaglandin (significant for PGE2) and a decrease in HETE synthesis. The physiologic significance of 12-HETE in reproduction is still poorly understood, but a shift in AA metabolism from HETEs to prostaglandins may be involved in the initiation of labor. Furthermore, these results point to different roles of the tissue compartments within the pregnant uterus for the parturition process.

Effects of ridogrel on the prostacyclin-thromboxane ratio in nulliparous third trimester-pregnant rhesus monkeys

The effects of ridogrel, a thromboxane synthetase inhibitor and endoperoxide receptor antagonist, were studied in twelve pregnant, nulliparous Rhesus monkeys (Macaca mulatta) during the last trimester of pregnancy. Ridogrel was administered intravenously in two groups of animals (n = 6), at either 0.1 mg/kg or 1.0 mg/kg. Ultrasonic assessment of the fetuses during and after the infusion period revealed no obvious changes in fetal condition. Both dosages reduced serum thromboxane levels 30 minutes after administration, 96.6% and 99.6% suppression, 0.1 mg/kg and 1.0 mg/kg, respectively (P < 0.0001), while the 1.0 mg/kg dose produced continued suppression for 24 hours (78% suppression, P < 0.0001) and was lower than the 0.1 mg/kg 24 hour value (P < 0.008). Prostacyclin levels increased to 340% and 472% of baseline values, 0.1 mg/kg and 1.0 mg/kg, respectively at 30 minutes and to 928% and 255% of baseline at 24 and 48 hours after treatment in the 1.0 mg/kg group (P < 0.0003). Ridogrel caused no changes in maternal or neonatal outcome. The potential for the use of this compound for the treatment of preeclampsia is discussed.

Thromboxane synthetase inhibition as a new therapy for preeclampsia: animal and human studies minireview

The role of the eicosanoids in the pathophysiology of preeclampsia is reviewed, and the results of animal model and human studies with thromboxane synthetase inhibitors in preeclampsia are described. Potential benefits and limitations of therapy are discussed.

The role of prostaglandins in obstetrical disorders

All pregnancy-associated tissues are capable of producing prostaglandins including PGI2 and TXA2. In normal pregnancy there is a dominance of PGI2 over TXA2 which may contribute to the maternal circulatory adaptation to pregnancy. Furthermore, both fetoplacental PGI2 and TXA2 production are important regulators of the fetal blood supply. It has been clearly established that in pre-eclampsia PGI2 production decreases in the fetoplacental tissues and quite probably also in the maternal tissues. The effect of this change may be further exaggerated by the simultaneous stimulation in pre-eclampsia of TXA2 production. The reason for PGI2 deficiency is not known. Other vasoactive agents, such as endothelin, may act in concert with prostaglandins. Relative PGI2 deficiency is likely to exist also in IUGR and lupus anticoagulant syndrome of pregnancy. In the latter, lupus anticoagulant may directly inhibit the synthesis of PGI2. One study suggests PGI2 deficiency also in early pregnancies of women with a history of repeated abortions. Prostaglandin production increases during full-term labour, and similar but smaller changes also occur in preterm labour. A silent bacterial infection may trigger the onset of preterm labour through cytokine-stimulated increase of prostaglandin production. No data were found on prostaglandin production in post-term pregnancies. That oligo-polyhydramnios is possibly prostaglandin mediated is suggested by the control of polyhydramnios by indomethacin treatment. Smoking decreases the production of PGI2 and possibly increases that of TXA2, which may lead to decreased blood flow and IUGR. Which constituent of cigarette smoke exerts this effect is not known. Ethanol consumption causes aberrations in prostaglandin metabolism which cannot be directly connected with fetal alcohol effects.

Study of thromboxane and prostacyclin metabolism in an in vitro model of first-trimester human trophoblast

OBJECTIVES

The purpose of our study was to establish an in vitro tissue culture system to study eicosanoid metabolism in first-trimester trophoblastic tissue. Thromboxane A2, a potent vasoconstrictor, and prostacyclin, a potent vasodilator, were analyzed to evaluate their production in early pregnancy.

STUDY DESIGN

Trophoblastic tissue was obtained via transabdominal chorionic villous sampling from 33 pregnancies at 9 to 12 weeks' gestation for cytogenetic diagnosis. Initially, tissue obtained from the cytogenetics lab was morphologically consistent with villous core cells. Through altering cell density and passage, the cells became morphologically consistent with cytotrophoblasts. The cell lines were exposed to arachidonic acid (50 mumol/L) and aspirin (1 to 100 mumol/L) for 24 hours. Thromboxane B2 and 6-keto prostaglandin F2 alpha were measured by radioimmunoassay.

RESULTS

Villous core cells and cytotrophoblasts increased production of thromboxane A2 and prostacyclin in the presence of arachidonic acid (p < 0.002). The villous core cells produced more thromboxane A2 and prostacyclin than cytotrophoblasts (p < 0.02). A significant inhibition of both thromboxane A2 and prostacyclin production was seen in the presence of 100 mumol/L aspirin in both cell types (p < 0.05).

CONCLUSIONS

This model may be useful for studying placental function in the first trimester because individual placental compartments can be evaluated in tissue culture. At the cellular level we were not able to detect a preferential decrease in thromboxane A2 production in the presence of aspirin (1 to 100 mumol/L).

Placental lipid peroxides and thromboxane are increased and prostacyclin is decreased in women with preeclampsia

OBJECTIVE

There is an imbalance of increased thromboxane and decreased prostacyclin in placentas of women with preeclampsia, but this may not be the only imbalance. There is also an abnormal increase in serum lipid peroxides in preeclamptic women. Lipid peroxides are toxic compounds that damage cells and inhibit prostacyclin synthesis. The following study examined lipid peroxides to determine if they were also increased in placentas of preeclamptic women.

STUDY DESIGN

Placental tissue for nine normal and eight preeclamptic women were frozen in liquid nitrogen immediately after delivery. Frozen tissue samples (1 gm) were homogenized and analyzed for lipid peroxides by malondialdehyde and hydrogen peroxide equivalents and for thromboxane and prostacyclin by radioimmunoassay of their stable metabolites, thromboxane B2 and 6-keto prostaglandin F1 alpha.

RESULTS

Lipid peroxides were significantly higher in preeclamptic placentas than in normal placentas by both analytic methods (49 +/- 5 vs 31 +/- 1 nmol/gm for malondialdehyde and 5.3 +/- 0.3 vs. 3.2 +/- 0.3 mumol/gm for hydrogen peroxide equivalent; mean +/- SE; p < 0.01, respectively). Thromboxane was significantly higher and prostacyclin significantly lower in preeclamptic placentas than in normal placentas (213 +/- 23 vs 158 +/- 14 ng/gm for thromboxane and 24 +/- 3 vs 53 +/- 7 ng/gm for prostacyclin, p < 0.05). The thromboxane/prostacyclin and lipid peroxides/prostacyclin ratios were threefold higher in preeclamptic placentas than in normal placentas.

CONCLUSION

Placental levels of both lipid peroxides and thromboxane are increased and prostacyclin decreased in preeclampsia. We speculate that abnormally increased levels of lipid peroxides in preeclamptic placentas may be a cause of decreased prostacyclin.

Dose-related action of gonadotropin-releasing hormone on basal prostanoid production from the human term placenta

The dose-related effect of gonadotropin-releasing hormone on placental prostanoids was studied with a perifusion system. Villous tissues were perifused with medium 199 (1 ml/hr) and at the beginning of the fifth hour, either 0, 10(-10), 10(-9), 10(-8), 10(-7), or 10(-6) mol/L gonadotropin-releasing hormone was added to the medium of triplicate chambers. The concentration of prostaglandin E, prostaglandin F, 13,14-dihydro-15-keto-prostaglandin F2 alpha, 6-keto-prostaglandin F1 alpha, and thromboxane B2 in the effluent medium, collected every hour, was determined by specific radioimmunoassay. The cumulative release after gonadotropin-releasing hormone treatment for each chamber was calculated, and replicate chambers were averaged. Linear regression analysis of the average for each dose from three different placentas was used to determine the dose-response relationship. Gonadotropin-releasing hormone significantly inhibited the release of placental prostaglandin E, prostaglandin F, and thromboxane B2 in a dose-dependent fashion. Gonadotropin-releasing hormone had no significant effect on 13,14-dihydro-15-keto-prostaglandin F2 alpha and 6-keto-prostaglandin F1 alpha, although there was an apparent increase in 13,14-dihydro-15-keto-prostaglandin F2 alpha. These data support the hypothesis that chorionic gonadotropin-releasing hormone inhibits prostanoid production from the placenta, which in turn may regulate various functions of prostanoids during pregnancy.

Transfer of aspirin across the perfused human placental cotyledon

Pregnancy-induced hypertension is associated with a reduction in prostacyclin synthesis that is relative to normotensive pregnancy, whereas thromboxane A2 synthesis is unchanged or increased. The net effect is a decreased prostacyclin/thromboxane ratio that may result in the reduced fetal-placental blood flow seen in pregnancy-induced hypertension because thromboxane is known to constrict this circulation. Low-dose aspirin (acetylsalicylic acid), which is used to treat pregnancy-induced hypertension, selectively inhibits thromboxane synthesis and therefore may alter fetal-placental blood flow. We have investigated the transfer of acetylsalicylic acid in the perfused human placental cotyledon and its effects on fetal-placental perfusion pressure. Human placental cotyledons were perfused with tissue culture medium 199 plus 5% polyvinylpyrrolidone that was gassed with 95% oxygen/5% carbon dioxide at flow rates of 10 ml/min (maternal) and 4 ml/min (fetal). Acetylsalicylic acid (10(-5) mol/L) was added to the maternal circuit, and cotyledons were perfused for 1 hour with aliquots taken from a closed fetal circuit every 5 minutes. Acetylsalicylic acid was assayed by spectrofluorometry at 306/412 nm. Our data indicate an initial rapid transfer of acetylsalicylic acid during the first 5 minutes into the fetal-placental circulation, the concentration then decreased to a steady state at 0.4 x 10(-5) mol/L. Resting perfusion pressure of both maternal and fetal circulation did not change after the addition of acetylsalicylic acid to maternal perfusate and transfer to the fetal circulation.

Increased placental production of leukotriene B4 in gestational hypertension

Production of some lipoxygenase and cyclooxygenase derivatives of arachidonic acid was measured in placental tissue obtained from women with gestational hypertension and with normal pregnancies. The levels of leukotriene B4 were about five times higher in placentas from hypertensive women and also raised thromboxane A2 and reduced prostaglandin E2 levels were observed. Prostacyclin production was lowered only in women with more severe hypertension, in association with the highest measured levels of leukotriene B4 and thromboxane A2. It is suggested that increased placental levels of leukotriene B4 and thromboxane A2 appear already in mild gestational hypertension, while depression of prostacyclin may occur only at more severe stages of gestational hypertensive disease.

Effects of magnesium and isradipine on contractile activation induced by the thromboxane A2 analog U46619 in human uteroplacental arteries in term pregnancy

The effects of Mg++ and the dihydropyridine calcium channel blocker isradipine on contractions induced by the thromboxane A2 mimetic U46619 were studied in isolated human uteroplacental arteries. In all preparations investigated U46619 10(-10) to 10(-6) mol/L induced concentration-related contractile responses. In maternal vessels U46619 produced a biphasic concentration-response curve, whereas the compound in fetal vessels produced a sigmoid concentration-response curve parallel to that of prostaglandin F2 alpha. Mg++ (1.0 to 6.0 mmol/L) and isradipine (10(-10) to 10(-5) mol/L) both relaxed U46619-induced contractions by up to about 40%. In preparations precontracted by U46619 and exposed to isradipine 10(-6) mol/L, addition of Mg++ 6.0 mmol/L consistently produced a further small relaxation. Removal of the endothelium or pretreatment with indomethacin 10(-6) mol/L or digoxin 10(-6) mol/L did not influence the inhibition produced by Mg++ or isradipine. In vessels pretreated in Ca(++)-free medium Mg++ 6.0 mmol/L depressed and isradipine abolished responses to Ca++ (0.01 to 4.0 mmol/L) after depolarization with K+ 124 mmol/L. In maternal and fetal arteries stimulated with U46619, however, Mg++ 6.0 mmol/L and isradipine 10(-6) mol/L produced a similar, partial inhibition of responses to Ca++ (0.01 to 4.0 mmol/L). Mg++ seems to inhibit transmembrane Ca++ influx, although less efficiently than the dihydropyridine calcium antagonist isradipine. In addition, Mg++ may interfere with Ca++ at intracellular binding sites. Provided that preeclampsia comprises enhanced vascular actions of thromboxane A2, the present results support the established use of Mg++ in the treatment of this condition and suggest that calcium antagonists are of potential benefit.

Thromboxane and prostacyclin levels in fetal rabbit brain and placenta after intrauterine partial ischemic episodes

The appearance of arachidonic acid (AA) oxidation products in fetal rabbit brain and placenta under normal or partial short-term ischemic episodes induced by placental blood vessel restriction was examined. Intracerebral administration of [3H]AA into close-to-term rabbit fetuses gave rise to radioactively labeled prostaglandin (PG) E2, thromboxane B2, and 6-keto-PGF1 alpha metabolites as detected by HPLC analysis. A significant increase of 20-30% of [3H]AA precursor into eicosanoids was detected in brain of fetuses after 2-h restriction. The thromboxane B2 and 6-keto-PGF1 alpha levels were determined by radioimmunoassay technique over a period of 48 h following ischemic episodes. Thromboxane B2 content in affected animals was higher by five- and twofold at 3 h over control fetal brain and placental tissue values, respectively, and remained significantly higher for 24 h. 6-Keto-PGF1 alpha levels reached a peak value that was greater by 2.5- and 1.5-fold at 6 h for the ischemic brain and placental tissue, respectively, compared with control fetuses. PGE2 levels were less affected, attaining a maximum of 1.9- and 1.1-fold in brain and placenta correspondingly. The thromboxane/prostacyclin ratio reached a maximum in the brain after approximately 3 h, while that in the placenta continued to rise even after 20 h. Persisting high levels of thromboxane are indicative of cerebral vasoconstriction and may suggest possible damaging effects.

[Preeclampsia: physiopathology and prospects for early detection]

Preeclampsia is a complication of pregnancy characterized by hypertension, edema and proteinuria, beginning after 20 weeks of gestation. Six percent of the pregnant women in North America develop this disease, which is associated with increased morbidity and mortality for the mother and her baby. The physiopathology remains uncertain despite many research efforts. Actual hypotheses seek to explain the vasospasm that characterizes the disease. Among the many factors influencing vascular reactivity and possibly implicated are: the renin-angiotensin system, prostaglandins, progesterone and its metabolites, calcium, magnesium, digoxin-like immunoreactive substance(s), auricular natriuretic factor, substances secreted by platelets and leukotrienes. Prevention of the disease is limited by the absence of a biological or clinical marker with good sensitivity and appropriate specificity. Many biochemical or hematological parameters have been reported: uric acid, calcium, magnesium, proteinuria, blood iron, hematocrit, platelet count, antithrombin III, estrogen and progesterone. The combination of several tests could be superior to the use of each test individually, providing a better sensitivity and improving the positive predictive value. With early detection, new therapies for the prevention of the disease could be experimented on the higher risk women before the apparition of clinical symptoms or signs. Furthermore, those tests could be used in the study of the pathophysiology and in the choice of the best therapy.

Altered excretion of prostaglandin and thromboxane metabolites in pregnancy-induced hypertension

The renal and systemic metabolites (the latter as 2,3-dinor derivatives) of prostacyclin and thromboxane A2 were measured, along with renal prostaglandin E2 and kallikrein, in the urine of 15 patients with pregnancy-induced hypertension, 15 normotensive pregnant women matched for both age and gestational age, and 15 normotensive nonpregnant control women. Urinary excretion of all prostaglandin and thromboxane metabolites studied proved significantly higher in normotensive pregnant women than in controls. Prostaglandin E2, 6-keto-prostaglandin F1 alpha, and 2,3-dinor-6-keto-prostaglandin F1 alpha were significantly lower in pregnancy-induced hypertensive women than in normotensive pregnant women, whereas thromboxane B2 and 2,3-dinor-thromboxane B2 showed no significant differences in the two groups. A significant negative correlation (r = -0.636, p less than 0.01) was found between urinary 2,3-dinor-6-keto-prostaglandin F1 alpha and mean blood pressure in the two groups of pregnant women taken as a whole. These data indicate that, in pregnancy-induced hypertension, there is an imbalance between vasodilator and vasoconstrictor factors, not only in the kidneys, but also at the systemic vascular level. This imbalance, which may in itself produce vasoconstriction, may also potentiate the hypertensive effect of catecholamines and angiotensin II.

Inhibition of the human placental NAD- and NADP-linked 15-hydroxyprostaglandin dehydrogenases by nonsteroidal anti-inflammatory drugs

A number of nonsteroidal anti-inflammatory drugs are non-competitive or mixed inhibitors of human placental NAD- and NADP-linked 15-hydroxyprostaglandin dehydrogenases. Cis- and trans-sulindac sulfide and cis- and trans-sulindac inhibit the NAD-linked enzyme as well or better than they inhibit various cyclooxygenases in vitro. The remainder of the compounds tested are at least one order of magnitude less effective as inhibitors of the 15-hydroxyprostaglandin dehydrogenases than they are as inhibitors of cyclooxygenases. Cis- and trans-sulindac sulfide are sufficiently strong inhibitors of the NAD-linked enzyme (Kis of 7.8 microM and 6.8 microM respectively) to raise the possibility that they might also inhibit this enzyme in vivo.

In vitro activity of nicotinamide adenine dinucleotide- and nicotinamide adenine dinucleotide phosphate-linked 15-hydroxyprostaglandin dehydrogenases in placentas from normotensive and preeclamptic/eclamptic pregnancies

Concentrations of prostaglandins E2 and I2 may be decreased in preeclamptic and eclamptic pregnancies. Because these prostaglandins produce vasodilation and inhibit platelet aggregation it has been suggested that a reduction in their biosynthesis might play an important role in the pathogenesis of the hypertension and coagulation abnormalities associated with preeclampsia. Placental tissue is an extremely rich source of several enzymes that catalyze the catabolism of prostaglandins. The present study was initiated to determine whether one of these catabolic enzymes might be increased in preeclamptic/eclamptic pregnancies. The activities of the NAD- and the NADP-linked 15-hydroxyprostaglandin dehydrogenases were measured in 16 preeclamptics (mean diastolic pressure, 108 +/- 13 mmHg) and compared with 16 normotensive controls matched for age (20.8 +/- 5.43 vs. 20.6 +/- 5.16) and gestational week of delivery (34.6 +/- 5.40 vs. 35.0 +/- 5.06). These results indicate that the activity of the placental NAD-linked 15-hydroxyprostaglandin dehydrogenase is elevated in preeclampsia (40.1 +/- 31.3 vs. 14.9 +/- 8.30 mU/g tissue, P less than 0.01). If this increase were also expressed in vivo, its effect on prostaglandin metabolism could be mistaken for impaired prostacyclin biosynthesis unless both the 6-keto- and 6,15-diketo-metabolites of prostacyclin were measured.

Beneficial effects of U-63,557A, a thromboxane synthetase inhibitor, in an ovine model of pregnancy-induced hypertension

Pregnancy-induced hypertension was induced in five ewes (gestational day 135; term 150 days) by 72 hours of food deprivation. Maternal arterial pressure, uterine blood flow, platelet function, renal function, and plasma levels of 6-ketoprostaglandin F1 alpha and thromboxane B2 were measured before and during hypertension and after three intravenous injections of U-63,557A; sodium 5-(3'-pyridinylmethyl) benzofuran-2-carboxylate, monohydrate (30 mg/kg every 8 hours). Blood pressure increased (p less than 0.03), and returned to normal after U-63,557A. Left uterine artery blood flow increased after U-63,557A (p less than 0.03). Creatinine clearance decreased during hypertension (p less than 0.03) and increased after U-63,557A. Urine protein increased during hypertension (p less than 0.03) and decreased after treatment. Platelet count dropped during hypertension (p less than 0.03) and was elevated after treatment. Collagen lag phase decreased during hypertension (p less than 0.03) and increased after treatment. After U-63,557A, 6-ketoprostaglandin F1 alpha levels were higher (p less than 0.04) than baseline or hypertensive values. Administration of a thromboxane synthetase inhibitor caused resolution of hemodynamic, renal, and coagulation dysfunctions that occurred in ovine pregnancy-induced hypertension.

Pregnancy-induced hypertension: development of a model in the pregnant sheep

Selected hemodynamic, renal, and biochemical parameters were assessed in chronically instrumented third-trimester pregnant ewes and in the same ewes after induction of pregnancy toxemia. Ewes with induced pregnancy toxemia developed hypertension, proteinuria, ketonuria, decreased glomerular filtration rate, decreased cardiac output, and decreased left uterine artery blood flow. Histological and transmission electron microscopy revealed the development of renal morphologic changes consistent with those observed in human pregnancy-induced hypertension. These studies have elucidated that pregnancy-induced hypertension can be produced experimentally in the pregnant ewe. Furthermore, the pathophysiologic features of ovine pregnancy toxemia are similar to those of human preeclampsia, and therefore the sheep provides a suitable animal model to study the human condition, which still remains a major complication of pregnancy, jeopardizing both mother and fetus.

Pressor and depressor actions of prostanoids in the intact human fetoplacental vascular bed

Effects of the prostanoids PGE2, PGF2 alpha, PGI2, PGE1, 6-keto-PGE1 and the PGH2 analogue U46619, on fetoplacental perfusion pressure were measured in human term placental cotyledons in which both fetal and maternal circuits were perfused at constant flow. Fetal arterial injections of bolus doses of PGE2, PGF2 alpha and U46619 caused reversible increases in fetal perfusion pressure. Pressor responses to PGF2 alpha and U46619 were dose-related, with threshold doses of less than or equal to 2 nmoles and 14 pmoles, respectively. Reversible dose-related decreases in fetoplacental perfusion pressure were elicited by fetal arterial injection of PGI2, 6-keto-PGE1 and PGE1; threshold concentrations were: PGI2 less than or equal to 10 pmoles, PGE1 less than or equal to 141 pmoles and 6-keto-PGE1 less than or equal to 270 pmoles. The potent depressor effect of PGI2 and the potent pressor effect of U46619, a TxA2 mimetic, in the intact human fetoplacental vascular bed suggest that endogenous PGI2 and TxA2 could play a role in modulating fetoplacental blood flow in the term placenta.

Prostacyclin and thromboxane in gynecology and obstetrics

The gynecologic and obstetric implications of the smooth muscle-relaxing, antiaggregatory prostacyclin and its endogenous antagonist, thromboxane A2, are reviewed. In addition to the vascular wall and circulating platelets, which are primary sources for prostacyclin and thromboxane A2, respectively, reproductive tissues produce great amounts of these prostanoids, evidently for the regulation of the vascular tone and/or vascular platelet interaction. Several gynecologic and obstetric disorders are characterized by abnormalities in prostacyclin and/or thromboxane A2. In primary menorrhagia the uterine release of prostacyclin is increased, and consequently menstrual blood loss can be reduced with various prostaglandin synthesis inhibitors. Prostacyclin relaxes the nonpregnant myometrium in vitro and may also do so in vivo, although intravenous infusion of prostacyclin has no effect upon the uterine contractility in nonpregnant or pregnant subjects. Patients with pelvic endometriosis may have increased levels of prostacyclin and thromboxane A2 metabolites in the peritoneal fluid. The prostacyclin/thromboxane A2 balance shifts to thromboxane A2 dominance in patients with gynecologic cancer. During pregnancy the production of prostacyclin and thromboxane A2 increases in the mother and fetoplacental tissue. Preeclampsia and other chronic placental insufficiency syndromes are accompanied by prostacyclin deficiency in the mother and in fetomaternal tissues and by an overproduction of thromboxane A2, at least in the placenta. These changes may account for the vasoconstriction and platelet hyperactivity, which are pathognomonic for hypertensive pregnancies. By directing the prostacyclin/thromboxane A2 balance to prostacyclin dominance (by dietary manipulation, administration of prostacyclin and/or its analogues, drugs with prostacyclin-stimulating and/or thromboxane A2-inhibiting action), it may be possible to prevent and/or treat hypertensive pregnancy complications in the future.