Maternal prostacyclin, thromboxane, and placental blood flow

Prostaglandin I2 mediates weak vasodilatation in human placental microvessels

Human placental vessels (HPVs) play important roles in the exchange of metabolites and oxygen in maternal-fetal circulation. Endothelial-derived prostacyclin (prostaglandin I2, PGI2) is a critical endothelial vasodilator in the body. However, the physiological and pharmacological functions of endothelial PGI2 in the human placenta are still unclear. Human, sheep, and rat blood vessels were used in this study. Unlike non-placental vessels (non-PVs), the PGI2 synthesis inhibitor tranylcypromine (TCP) did not modify 5-hydroxytryptamine (5-HT)-induced vascular contraction, indicating that endothelial-derived PGI2 was weak in PVs. Vascular responses to exogenous PGI2 showed slight relaxation followed by a significant contraction at a higher concentration in HPV, which was inhibited by the thromboxane-prostanoid (TP) receptors antagonist SQ-29,548. Testing PVs and non-PVs from sheep also showed similar functional results. More TP receptors than PGI2 (IP) receptors were observed in HPVs. The whole-cell K+ current density of HPVs was significantly weaker than that of non-PVs. This study demonstrated the specific characteristics of the placental endogenous endothelial PGI2 system and the patterns of placental vascular physiological/pharmacological response to exogenous PGI2, showing that placental endothelial PGI2 does not markedly contribute to vascular dilation in the human placenta, in notable contrast to non-PVs. The results provide crucial information for understanding the endothelial roles of HPVs, which may be helpful for further investigations of potential targets in the treatment of diseases such as preeclampsia.

Markers of oxidative stress and systemic vasoconstriction in pregnant women drinking > or =48 g of alcohol per day

BACKGROUND

The precise pathway by which alcohol causes the characteristic features of fetal alcohol spectrum disorders is unknown. Proposed mechanisms for fetal injury from maternal alcohol use include cellular damage from oxidative stress and impaired fetal oxygenation related to maternal systemic vasoconstriction. Our objective was to compare the levels of urinary markers of oxidative stress and systemic vasoconstriction between women consuming large amounts of alcohol during pregnancy and women who did not drink alcohol during pregnancy.

METHODS

Pregnant women consuming > or =48 g alcohol per day (n = 29) on average and pregnant women who abstained from alcohol use (n = 39) were identified using detailed interviews and home visits. Random maternal urine specimens were collected. Urinary levels of the oxidative stress marker, 8-isoprostane F2alpha, and of the vasoactive prostaglandin metabolites, 2,3-dinor-6-keto-prostaglandin F1alpha (a vasodilator) and 11-dehydro-thromboxane B2 (a vasoconstrictor), were measured using mass spectrometric methods. All analyte levels were corrected for urinary creatinine.

RESULTS

In crude analyses, there was no significant difference in 8-isoprostane F2alpha between pregnant drinkers and nondrinkers (2.16 vs. 2.08 ng/mg creatinine, respectively, p = 0.87). There were no significant differences between the drinking and nondrinking groups in levels of 2,3-dinor-6-keto-prostaglandin F1alpha (1.03 vs. 1.17 ng/mg creatinine, respectively, p = 0.50), 11-dehydro-thromboxane B2 (0.72 vs. 0.59 ng/mg creatinine, respectively, p = 0.21), or the ratio of vasodilatory metabolite to vasoconstrictive metabolite (1.73 vs. 2.72, respectively, p = 0.14). Adjusting for maternal age, marital status, smoking, and gestational age at sampling did not substantially alter the results.

CONCLUSION

Our results show no difference in levels of urinary eicosanoid markers of oxidative stress and systemic vasoconstriction between pregnant women who drink heavily and pregnant women who abstain. These findings speak against a role for maternal oxidative stress or systemic vasoconstriction in the pathogenesis of alcohol damage to the fetus.

The production of prostanoids and the identification of macrophages in human decidua vera tissue

In decidual tissues of first trimester therapeutic abortions we have identified a large macrophage population that is stained by a specific antibody (Ki-M8). Also, we evaluated the capacity of decidual cells, obtained before and after labor, to synthesize prostanoids from radiolabeled arachidonic acid. Decidua collected after vaginal delivery produced significantly more prostaglandin (PG) F2 alpha, PGE2 and thromboxane (TX) B2 (7.4 +/- 1.3, 7.2 +/- 1.3, 11.7 +/- 1.1 pmol/mg protein/4 h, mean +/- SD, respectively) than did tissues from women who had caesarean sections (3.3 +/- 0.8, 1.4 +/- 0.5, 6.3 +/- 1.8 pmol/mg protein/4 h, respectively). Moreover, PGF2 alpha and PGE2 were rapidly converted to their 13,14-dihydro-15-keto-metabolites particularly in tissues obtained after vaginal delivery. All this lends further support to the proposition that decidual activation may be important in the onset of labor.

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.

Cyclooxygenase inhibition reduces placental transfer: reversal by carbacyclin

The effect of the cyclooxygenase inhibitors, indomethacin and ibuprofen, on diffusional transfer in the human placenta was assessed with the dual-perfused isolated placental lobe. Antipyrine, a freely diffusible substance, was used as an indicator of placental transfer efficiency. Each inhibitor (100 mumol/L) was perfused for 30 minutes after a baseline period, resulting in a significant reduction in antipyrine clearance. During a subsequent washout period, ibuprofen-inhibited antipyrine clearance returned to baseline values, whereas indomethacin-inhibited clearance remained reduced. An additional 30 minutes perfusion of 500 mumol/L of ibuprofen resulted in a further reduction in antipyrine clearance compared with 100 mumol/L of ibuprofen, suggesting a dosage-related effect. The perfusion of each inhibitor caused a reduced production of 6-keto-prostaglandin F1 alpha (the stable metabolite of prostacyclin) in the fetal circulation. The simultaneous perfusion of carbacyclin, a prostacyclin analogue, at 100 nmol/L and 1 mumol/L resulted in a dosage-dependent reversal of the effects of ibuprofen (500 mumol/L) on antipyrine clearance. The results indicate that the inhibition of cyclooxygenase activity reduces placental transfer and that the effects of these inhibitors are reversed by carbacyclin. This study suggests that the use of cyclooxygenase inhibitors during pregnancy could compromise the developing fetus by reducing placental transfer.

The effect of oxytocin and betamimetic stimulation on prostaglandin release in perfused human fetal placenta

The existing data on prostaglandins indicate that they are involved in human parturition and regulation of fetoplacental blood flow. The interference of endogenous and exogenous oxytocin and prostaglandins and, on the other hand, betamimetics, which are commonly used during pregnancy, in the regulation of these phenomena is poorly understood. The production of prostaglandins by fetal placental cotyledons was studied using an in vitro perfusion technique. Isolated cotyledons were perfused without (control) or with oxytocin (200 pg/ml, 2000 pg/ml) or the betasympathomimetic drug ritodrine (10 micrograms/ml, 50 micrograms/ml). The release of prostaglandin F2 alpha (PGF2 alpha), prostaglandin E2 (PGE2), 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TxB2) was measured by radioimmunoassay. The release of prostaglandins was also studied during a recovery period after the drug infusion. Oxytocin at a concentration of 200 pg/ml significantly decreased the release of PGF2 alpha. A higher concentration of oxytocin did not cause any changes in prostaglandin production. During ritodrine infusion the perfusion pressure was decreased, but the addition of ritodrine to the perfusion medium had no effect on prostaglandin release. During the recovery period, after ritodrine infusion, the release of PGF2 alpha was significantly decreased. It is suggested that oxytocin at the physiological concentration may protect the fetus from adverse effects of PGF2 alpha before labor in decreasing the release of PGF2 alpha, yet this small decrease in formation of PGF2 alpha is obviously of minor clinical importance because the perfusion pressure remained constant. Ritodrine had no effect on prostaglandin release and so the decrease in the perfusion pressure is probably the result of beta 2-receptor stimulation.

The effects of alpha- and beta-adrenergic stimulation on contractility and prostaglandin (prostaglandins E2 and F2 alpha and 6-keto-prostaglandin F1 alpha) production of pregnant human myometrial strips

The effects of catecholamines and alpha- and beta-adrenergic agonists and antagonists on the spontaneous contractility of superfused pregnant human myometrial strips are reported. Prostaglandins (prostaglandins E2 and F2 alpha and 6-keto-prostaglandin F1 alpha) were analyzed in the effluent of the superfusion medium by specific radioimmunoassays. Both epinephrine and norepinephrine dose-dependently (10 ng/ml to 1 microgram/ml) stimulated the contractility of the myometrial strips and significantly increased the synthesis of all prostaglandins assayed. alpha-Adrenergic blockers inhibited the catecholamine-induced increase in contractility. This was associated with decreased prostaglandins F2 alpha and E2 concentrations and a further increase in 6-keto-prostaglandin F1 alpha levels. Exclusive beta-adrenergic stimulation with beta-mimetic drugs had the same effect. Conversely, epinephrine stimulation together with beta-blockers resulted in a further increase in the prostaglandins F2 alpha and E2 release of the myometrial strips. This effect was even more pronounced with specific alpha-adrenergic stimulant drugs. Our results demonstrate the interrelationship of alpha- and beta-adrenergic stimulation and the prostaglandin system. alpha-Adrenergic stimulation increases myometrial contractility and the synthesis of prostaglandins F2 alpha and E2. beta-Adrenergic stimulation reduces contractility by further enhancing 6-keto-prostaglandin F1 alpha production.

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.

Ritodrine infusion during late pregnancy: effects on fetal and placental blood flow, prostacyclin, and thromboxane

To study the effects of ritodrine on fetal and placental blood flow and maternal prostacyclin and thromboxane A2, 14 women with premature uterine contractions between the thirty-first and thirty-sixth weeks of pregnancy were treated with intravenous infusions of ritodrine, with incremental doses up to 200 micrograms per minute. The intervillous and the umbilical vein blood flows were measured before and after 1 hour of infusion of ritodrine, with the xenon 133 method and with a combination of real-time and Doppler ultrasonic equipment, respectively. Ritodrine decreased maternal diastolic and mean arterial pressures, as well as placental vascular resistance, but caused no significant changes in intervillous and umbilical vein blood flows. Ritodrine stimulated the synthesis of vasodilatory prostacyclin, as seen from a rise in maternal plasma 6-keto-prostaglandin F1 alpha, but inhibited the platelets' capacity to generate the vasoconstrictor thromboxane A2. Thus, apart from maternal hemodynamic changes, the intervillous and umbilical circulations are maintained during short-term administration of ritodrine in normotensive pregnancies.

Failure of exogenous prostacyclin to change placental and fetal blood flow in preeclampsia

Seven patients with acute preeclampsia and six with superimposed preeclampsia were infused intravenously with incremental doses of prostacyclin (up to 8 ng/min/kg during 80 minutes). Prostacyclin infusion was accompanied by significant decreases in maternal blood pressure and consistent rises in maternal plasma or urinary 6-keto-prostaglandin F1 alpha, but it caused no changes in maternal or fetal pulse rate or uterine contractility. Moreover, prostacyclin did not change the placental and umbilical blood flow, which were measured before and at the end of infusion. All women experienced facial flushing and two complained of headache during infusion. There was no difference in prostacyclin effects between women with acute or superimposed preeclampsia. These results may be taken as evidence that intravenous prostacyclin is not a specific therapy to increase placental or umbilical blood flow in preeclampsia.

The effects of betamimetics and glucocorticoids on fetal vascular prostacyclin and platelet thromboxane synthesis in humans

Since the vasodilatory, antiaggregatory prostacyclin (PGI2) and its antagonist thromboxane A2 (TxA2) evidently take part in the regulation of the fetoplacental blood flow, the influence of drugs commonly used during pregnancy, such as betamimetics (ritodrine and buphenine) and glucocorticoids (hydrocortisone and dexamethasone), on fetal PGI2 and TxA2 productions was investigated. Specimens of umbilical artery were superfused in vitro without (control) or with the drugs studied (10(-8) - 10(-3) mol/l) and the release of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), a stable breakdown product of PGI2, was measured by radioimmunoassay. When studying TxA2 production, umbilical vein blood samples were allowed to clot in the absence and presence of the drugs studied, and the formation of thromboxane B2 (TxB2) a stable breakdown product of TxA2, was measured by radioimmunoassay. Ritodrine and buphenine had no influence on the production of either PGI2 or TxA2, but hydrocortisone and dexamethasone at concentrations of 10(-3) M/l inhibited umbilical PGI2 production to 5.5 +/- 1.4% (mean +/- SEM) and 69.9 +/- 5.0% of the control level, respectively, although they had no effect on TxA2 synthesis. It is concluded that if betamimetics alter the fetoplacental circulation, as suggested by some authors, they do not exert this effect through the fetal PGI2 or TxA2. In contrast, maternal glucocorticoid treatment may suppress fetal PGI2 generation, and thus perhaps reduce the fetoplacental blood flow.

The effects of ritodrine on prostaglandin metabolite concentrations in the blood of pregnant baboons

To investigate the relationship between beta sympathomimetic drugs and prostaglandins, we measured prostaglandin metabolites in the plasma of pregnant baboons that were given the drug ritodrine. Animals were at a mean gestation of 120 days, which is equivalent to 27 weeks in women. Ritodrine was infused i.v. at a rate of 23 micrograms/min or 80 micrograms/min for 4 h. Plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM), 11-deoxy-13,14-dihydro-15-keto-11,16-cyclo-prostaglandin E2 (PGEM-11), and 6-keto-prostaglandin-F1 alpha (6-keto-PGF1 alpha) were determined as indicators of the precursors, prostaglandin F2 alpha, prostaglandin E2, and prostacyclin (PGI2), respectively. At the 23 micrograms/min ritodrine infusion rate plasma levels of 6-keto-PGF1 alpha were decreased by 51% (+/- 15%) in the animals receiving the drug (p less than 0.005) whereas corresponding levels in control animals did not differ significantly from pre-infusion levels. At the 80 micrograms/min ritodrine infusion rate, the plasma levels of 6-keto-PGF1 alpha decreased by 66% (+/- 18%) compared with pre-infusion levels (p less than 0.001) but again the levels in control animals were not changed significantly. Plasma levels of PGFM and PGEM-11 in animals during ritodrine treatment did not differ significantly from baseline values.