Effects of shear stress on eicosanoid gene expression and metabolite production in vascular endothelium as studied in a novel biomechanical perfusion model

This study investigated the effects of shear stress on gene expression of prostacyclin synthesis-related enzymes cyclooxygenases (COX-1 and COX-2), prostacyclin synthase (PGS), and thromboxane synthase (TXS) and their metabolites prostaglandin (PGI(2)) and thromboxane A(2) (TXA(2)) in endothelium of intact conduit vessels. Paired human umbilical veins were perfused at high/low shear stress (25/<4 dyn/cm(2)) at identical intraluminal pressure (20 mmHg) for 1.5, 3, or 6 hours in a computerized vascular model. High shear perfusion induced a significant, monophasic upregulation of PGS and TXS gene expressions after 6 hours. COX-1 and COX-2 mRNA showed a biphasic response with peaks at 1.5 and 6 hours, with a nadir level at 3 hours. Shear-induced gene expression was associated with a significantly greater accumulation of 6-keto prostaglandin F(1alpha) and TXA(2) in the perfusion medium. Thus, shear stress independently of perfusion pressure alters the expression of prostacyclin synthesis-related enzymes and the biosynthesis of their vasoactive metabolites.     

Metabolism of the endocannabinoids, 2-arachidonylglycerol and anandamide,into prostaglandin,thromboxane, and prostacyclin glycerol esters and ethanolamides

Cyclooxygenase-2 (COX-2) action on the endocannabinoids, 2-arachidonylglycerol (2-AG) and anandamide (AEA), generates prostaglandin glycerol esters (PG-G) and ethanolamides (PG-EA), respectively. The diversity of PG-Gs and PG-EAs that can be formed enzymatically following COX-2 oxygenation of endocannabinoids was examined in cellular and subcellular systems. In cellular systems, glycerol esters and ethanolamides of PGE(2), PGD(2), and PGF(2alpha) were major products of the endocannabinoid-derived COX-2 products, PGH(2)-G and PGH(2)-EA. The sequential action of purified COX-2 and thromboxane synthase on AEA and 2-AG provided thromboxane A(2) ethanolamide and glycerol ester, respectively. Similarly, bovine prostacyclin synthase catalyzed the isomerization of the intermediate endoperoxides, PGH(2)-G and PGH(2)-EA, to the corresponding prostacyclin derivatives. Quantification of the efficiency of prostaglandin and thromboxane synthase-directed endoperoxide isomerization demonstrated that PGE, PGD, and PGI synthases catalyze the isomerization of PGH(2)-G at rates approaching those observed with PGH(2). In contrast, thromboxane synthase was far more efficient at catalyzing PGH(2) isomerization than at catalyzing the isomerization of PGH(2)-G. These results define the in vitro diversity of endocannabinoid-derived prostanoids and will permit focused investigations into their production and potential biological actions in vivo.     

Conversions of prostaglandin endoperoxides by prostacyclin synthase from pig aorta

Partially purified prostacyclin synthase from pig aorta converted the prostaglandin (PG) endoperoxide PGH2 to prostacyclin (PGI2), and PGH1 to 12-hydroxy-8,10-heptadecadienoic acid (HHD). Both reactions were inhibited by 15-hydroperoxy-5,8,11,13-eicosatetraenoic acid (15-HP) in a dose-dependent rashion. However, the reactions PGH2 leads to PGI2 and PGH1 leads to HHD appeared to differ: substrate availability was rate limiting in the latter reaction, while the enzyme became rapidly saturated witth PGH2 and a steady rate of prostacyclin formation was observed at higher substrate levels.     

In SHR aorta, calcium ionophore A-23187 releases prostacyclin and thromboxane A2 as endothelium-derived contracting factors

In mature spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY), acetylcholine and the calcium ionophore A-23187 release endothelium-derived contracting factors (EDCFs), cyclooxygenase derivatives that activate thromboxane-endoperoxide (TP) receptors on vascular smooth muscle. The EDCFs released by acetylcholine are most likely prostacyclin and prostaglandin (PG)H(2), whereas those released by A-23187 remain to be identified. Isometric tension and the release of PGs were measured in rings of isolated aortas of WKY and SHR. A-23187 evoked the endothelium-dependent release of prostacyclin, thromboxane A(2), PGF(2alpha), PGE(2), and possibly PGH(2) (PGI(2) > thromboxane A(2) = PGF(2alpha) = PGE(2)). In SHR aortas, the release of prostacyclin and thromboxane A(2) was significantly larger in response to A-23187 than to acetylcholine. In response to the calcium ionophore, the release of thromboxane A(2) was significantly larger in aortas of SHR than in those of WKY. In both strains of rat, the inhibition of cyclooxygenase-1 prevented the release of PGs and the occurrence of endothelium-dependent contractions. Dazoxiben, the thromboxane synthase inhibitor, abolished the A-23187-dependent production of thromboxane A(2) and inhibited by approximately one-half the endothelium-dependent contractions. U-51605, an inhibitor of PGI synthase, reduced the release of prostacyclin elicited by A-23187 but induced a parallel increase in the production of PGE(2) and PGF(2alpha), suggestive of a PGH(2) spillover, which was associated with the enhancement of the endothelium-dependent contractions. These results indicate that in the aorta of SHR and WKY, the endothelium-dependent contractions elicited by A-23187 involve the release of thromboxane A(2) and prostacyclin with a most likely concomitant contribution of PGH(2).     

The effects of prostaglandins on secretion of glucagon and insulin by the perfused rat pancreas.

The secretion of both glucagon and insulin by the isolated perfused rat pancreas was significantly stimulated by 10(-7) M PGH2. Experiments to show that the stimulated secretion was mediated by conversion of PGH2 to TXA2 or TXB2 revealed no correlation between the amount of secretion and the amount of thromboxane formed. Conversion of PGH2 with a crude platelet thromboxane synthase preparation caused a progressive loss of ability to secret insulin, whereas the capacity to stimulate release of glucagon remained at about one-half the maximal level. This relatively stable and selective secretagogue action on the alpha-cells appeared to be due to the formation of PGD2 by the platelet preparation. Direct administration of PGD2 confirmed this interpretation and showed clearly that this prostaglandin is a potent secretagogue for glucagon with little activity in stimulating the release of insulin. Our results have shown high and relatively equal stimulation of secretion by alpha- and beta-cells with exogenous PGE2, PGF2 alpha, and PGH2, little or no secretion by either cell type with TXA2, TXB2, or PGI2, and a unique selective stimulatory action of PGD2 upon the alpha-cell.     

Novel action of estrone on vascular tissue: regulation of NOS and COX activity

The hypothesis tested in the present work is that estrone non-genomically regulates aortic nitric oxide synthase (NOS) and cyclooxygenase (COX) activities in female rats, and that such regulation depends on ovarian function. We found that physiological concentrations of estrone (E(1)) (0.1-10nM) significantly increased nitric oxide (NO) production (133 and 163% above control). The stimulatory action of E(1) on NOS activity was independent of calcium influx since the increase in NO elicited by the hormone was not affected by EGTA or verapamil. When COX activity was measured, we observed that estrone enhanced thromboxane (TXB(2)) production and prostacyclin (PGI(2)) release, but not prostaglandin (PGF(2), PGD(2), and PGE(2)) synthesis. Finally we demonstrated that the hormonal effect on NOS activity was not detected in rat aortic strips (RAS) isolated from animals deprived of ovarian activity (FR(-)) or ovariectomized rats (OVX). These results suggest that estrone exerts a direct, non-genomic action on rat aortic metabolism, which involves NOS and COX activation and depends on ovarian activity.     

On the mechanism of prostacyclin and thromboxane A2 biosynthesis

The present research describes studies which address the mechanism of prostacyclin (PGI2) and thromboxane A2 (TXA2) biosynthesis. In addition to prostaglandin H1 (PGH1), PGG2, PGH2, and PGH3, also 8-iso-PGH2, 13(S)-hydroxy-PGH2, and 15-keto-PGH2 were applied to determine the substrate specificities and kinetics of prostacyclin and thromboxane synthase in more detail. Human platelet thromboxane synthase converted PGH1, 8-iso-PGH2, 13(S)-hydroxy-PGH2 and 15-keto-PGH2 into the corresponding heptadecanoic acid (C17) plus malondialdehyde, whereas the thromboxane derivative was formed only from PGG2, PGH2, and PGH3 together with the corresponding C17 metabolite and malondialdehyde in a 1:1:1 ratio. In contrast, PGG2, PGH2, 13(S)-hydroxy-PGH2, 15-keto-PGH2 and PGH3 were almost completely isomerized to the corresponding prostacyclin derivative by bovine aortic prostacyclin synthase, whereas PGH1 and 8-iso-PGH2 only produced the corresponding C17 hydroxy acid plus malondialdehyde. Isotope-labeling experiments with [5,6,8,9,11,12,14,15-2H]PGH2 revealed complete retention of label and no isotope effect in the course of thromboxane biosynthesis, but the loss of one 2H atom at C-6 with an isotope effect of 1.20 during PGI2 formation. Prostacyclin and thromboxane synthase bind both 9,11-epoxymethano-PGF2 alpha and 11,9-epoxymethano-PGF2 alpha at the heme iron, but according to their difference spectra in opposite ways with respect to the 9- and 11-position. In agreement with published model studies, a cage radical mechanism is proposed for both enzymes according to which the initial radical process is terminated through oxidation of carbon-centered radicals by the iron-sulfur catalytic site, followed by ionic rearrangement to PGI2 or TXA2. Various Fe(III) model compounds as well as liver microsomes or cytochrome P-450CAM can also form small amounts of PGI2 and TXA2, but mainly yield 12(S)-hydroxy-5,8,10-heptadecatrienoic acid plus malondialdehyde probably by a radical fragmentation pathway.     

The adenylate cyclase of rheumatoid synovial fluid macrophages is more sensitive for dl-5E, 19,14-di dehydro-carbo-prostacyclin, a stable prostacyclin analogue, than for prostaglandin E2

dl-5E, 19,14-di dehydro-carbo-prostacyclin (DDH-carbo PGI2), a stable prostacyclin (PGI2) derivative, but not prostaglandin (PG) E2, stimulated the adenylate cyclase of synovial fluid macrophages, isolated from rheumatoid patients with an active synovitis, in a dose dependent manner (10-1000 ng/ml). DDH-carbo PGI2 also stimulated synovial macrophage cAMP synthesis when injected into the knee joint. Exogenous arachidonic acid (AA) had little effect on cyclic-AMP (cAMP) formation or PGI2 release (assayed as 6ketoPGF1 alpha). It stimulated, however, the release of PGE2 and, to a lesser extent, thromboxane (Tx) A2 (measured as TxB2).     

Coupling between cyclooxygenases and terminal prostanoid synthases

Biosynthesis of prostanoids is regulated by three sequential enzymatic steps, namely phospholipase A2, cyclooxygenase (COX), and terminal prostanoid synthase. Recent evidence suggests that lineage-specific terminal prostanoid synthases, including prostaglandin (PG) E2, PGD2, PGF2alpha, PGI2, and thromboxane synthases, show distinct functional coupling with upstream COX isozymes, COX-1 and COX-2. This can account, at least in part, for segregated utilization of the two COX isozymes in distinct phases of PG-biosynthetic responses. In terms of their localization and COX preference, terminal prostanoid synthases are classified into three categories: (i) the perinuclear enzymes that prefer COX-2, (ii) the cytosolic enzyme that prefers COX-1, and (iii) the translocating enzyme that utilizes both COXs depending on the stimulus. Additionally, altered supply of arachidonic acid by phospholipase A2s significantly affects the efficiency of COX-terminal prostanoid synthase coupling. In this review, we summarize our recent understanding of the coupling profiles between the two COXs and various terminal prostanoid synthases.     

Prostaglandin D synthase in microvessels from the rat cerebral cortex

Microvessels, a mixture composed predominantly of small arterioles and capillaries (7-80 micro diameter), were isolated from the rat cerebral cortex by selective nylon sieving and glass bead elutriation. The morphology and purity of the microvessel and cerebral cortex filtrate (virtually free of vascular contamination) were monitored by light microscopy and by the activity of several enzymes: gamma -glutamyl transpeptidase, GSH-S-transferase, prostacyclin synthase and PGD synthase. Prostacyclin and PGD synthesizing activities as well as gamma-glutamyl transpeptidase activity were localized to the microvessels of the rat cerebral cortex whereas GSH-S-Transferase was restricted to the non-vascular filtrate fraction. The characteristics of the PGD synthase were similar to those of the purified enzyme previously described for the rat brain. The microvessel (MV) PGD synthase was localized to the cytosol fraction of the microvessels and did not require reduced glutathione for activity. The enzyme was inhibited by pre-incubation with p-hydroxymercuribenzoate (ImM) or N-ethylmaleimide (ImM). The MV RGD synthase saturated at 15-20 microM PGH2, exhibited an apparent Km of 9.6 microM, and a pH optimum of 8.0-8.1. These findings suggest roles for both prostacyclin and PGD synthesis by the rat cerebral vasculature in the autoregulation of cerebral blood flow and/or neural function. These studies also indicate that the major source of PGI2 and PGD2 synthesis by rat brain homogenates is the microvasculature.     

On the multiplicity of platelet prostaglandin receptors. II. The use of N-0164 for distinguishing the loci of action for PGI2, PGD2, PGE2 and hydantoin analogs

The omega-chain variant analogs of prostacyclin (PGI2) and PGD2 in which n-amyl side-chain has been replaced by a cyclohexyl group have been prepared and their cardiovascular activities have been compared to those of BW-245C(Fig. 1) a potent anti-aggregatory vasodilator bearing a cyclohexyl-terminated side-chain on a hydantoin skeleton. The cyclohexyl group has little effect on PGI2, but converts PGD2 to a long lasting hypotensive agent and increases the platelet anti-aggregatory potency of PGD2 by a factor of 8. The prostaglandin antagonist N-0164 selectively blocks the anti-aggregatory actions of PGD2, cyclohexyl-PGD2, and BW-245C; with essentially no effect on PGI2, cyclohexyl-PGI2 and PGE2 at comparably effective doses. The latter observation is contrary to an earlier report by MacIntyre, but supports the view that the anti-aggregatory effect of high doses of PGE2 (EC50=50 microM) is mediated by the PGI2 receptor. The hydantoin acts at the platelet PGD2 receptor.     

Testing the "redirection hypothesis" of prostaglandin metabolism in the kidney

Furosemide increases the synthesis of two major renal eicosanoids, prostacyclin (PGI2) and thromboxane A2 (TXA2), by stimulating the release of arachidonic acid which in turn is metabolized to PGG2/PGH2, then to PGI2 and TXA2. PGI2 may mediate, in part, the early increment in plasma renin activity (PRA) after furosemide. We hypothesized that thromboxane synthetase inhibition should direct prostaglandin endoperoxide metabolism toward PGI2, thereby enhancing the effects of furosemide on renin release. Furosemide (2.0 mg . kg-1 i.v.) was injected into Sprague-Dawley rats pretreated either with vehicle or with U-63,557A (a thromboxane synthetase inhibitor, 2 mg/kg-1 followed by 2 mg/kg-1 X hr-1). Urinary 6ketoPGF1 alpha and thromboxane B2 (TXB2), reflecting renal synthesis of PGI2 and TXA2, as well as PRA and serum TXB2, were measured. Serum TXB2 was reduced by 96% after U-63,557A. U-63,557A did not affect the basal PRA. Furosemide increased PRA in both vehicle and U63,557A treated rats. However, the PRA-increment at 10, 20 and 40 min following furosemide administration was greater in U-63,557A-treated rats than in vehicle-treated rats and urine 6ketoPGF1 alpha excretion rates were increased. These effects of thromboxane synthesis inhibition are consistent with a redirection of renal PG synthesis toward PGI2 and further suggest that such redirection can be physiologically relevant.     

Effects of OKY 1581 on bronchoconstrictor responses to arachidonic acid and PGH2

The influence of OKY 1581, a thromboxane synthase inhibitor, on airway responses to arachidonic acid and endoperoxide, [prostaglandin (PG) H2], were investigated in anesthetized, paralyzed, mechanically ventilated cats. Intravenous injections of arachidonic acid and PGH2 caused dose-related increases in transpulmonary pressure and lung resistance and decreases in dynamic and static compliance. OKY 1581 significantly decreased airway responses to arachidonic acid but not to PGH2. Sodium meclofenamate, a cyclooxygenase inhibitor, abolished airway responses to arachidonic acid but had no effect on airway responses to PGH2. OKY 1581 or meclofenamate has no effect on airway responses to PGF2 alpha, PGD2, or U 46619, a thromboxane mimic. In microsomal fractions from the lung, OKY 1581 inhibited thromboxane formation without decreasing prostacyclin synthesis or cyclooxygenase activity. These studies show that OKY 1581 is a selective thromboxane synthesis inhibitor in the cat lung and suggest that a substantial part of the bronchoconstrictor response to arachidonic acid is due to thromboxane A2 formation. Moreover, the present data suggest that airway responses to endogenously released and exogenous PGH2 are mediated differently and that a significant part of the response to exogenous PGH2 may be due to activation of an endoperoxide/thromboxane receptor, since responses to PGH2 are blocked by the thromboxane receptor antagonist SQ 29548.     

Arachidonic acid strongly stimulates prostaglandin I3 (PGI3) production from eicosapentaenoic acid in human endothelial cells

Eicosapentaenoic acid (EPA) is a prominent polyunsaturated fatty acid in fish oil which inhibits blood platelet aggregation and thromboxane A2 formation but not prostacyclin-like material generation from vascular endothelium. In this study we investigated interaction between EPA and arachidonic acid (AA) during their oxygenation by cultured endothelial cells. As measured by gas chromatography-mass spectrometry (GC-MS), AA increased markedly prostaglandin I3 (PGI3) production from EPA while that of PGI2 from AA was decreased by EPA. However, increasing the ratio AA/EPA over one almost suppressed the inhibition of PGI2 formation by EPA, and the stimulation of PGI3 production by AA was even higher. The effect of AA on EPA conversion to minor prostaglandins like PGE3 and PGF3 alpha was similar then confirming the stimulating effect and suggesting it is occurring at the cyclooxygenase instead of the prostacyclin synthase level. Altogether these data indicate that, in certain nutritional states where the liberation of EPA from endothelial cells will be accompanied with that of endogenous AA, substantial amounts of PGI3 could contribute to the prostacyclin-like activity of the vessel wall in addition to PGI2.     

Coupling between cyclooxygenase, terminal prostanoid synthase, and phospholipase A2.

We have recently shown that two distinct prostaglandin (PG) E(2) synthases show preferential functional coupling with upstream cyclooxygenase (COX)-1 and COX-2 in PGE(2) biosynthesis. To investigate whether other lineage-specific PG synthases also show preferential coupling with either COX isozyme, we introduced these enzymes alone or in combination into 293 cells to reconstitute their functional interrelationship. As did the membrane-bound PGE(2) synthase, the perinuclear enzymes thromboxane synthase and PGI(2) synthase generated their respective products via COX-2 in preference to COX-1 in both the -induced immediate and interleukin-1-induced delayed responses. Hematopoietic PGD(2) synthase preferentially used COX-1 and COX-2 in the -induced immediate and interleukin-1-induced delayed PGD(2)-biosynthetic responses, respectively. This enzyme underwent stimulus-dependent translocation from the cytosol to perinuclear compartments, where COX-1 or COX-2 exists. COX selectivity of these lineage-specific PG synthases was also significantly affected by the concentrations of arachidonate, which was added exogenously to the cells or supplied endogenously by the action of cytosolic or secretory phospholipase A(2). Collectively, the efficiency of coupling between COXs and specific PG synthases may be crucially influenced by their spatial and temporal compartmentalization and by the amount of arachidonate supplied by PLA(2)s at a moment when PG production takes place.     

Production of prostacyclin and prostaglandin E2 in resting and IL-1beta-stimulated A549, HUVEC and hybrid EA.HY 926 cells.

Production of arachidonic acid (AA) metabolites - prostacyclin (PGI(2)) in large vessels and prostaglandin E(2) (PGE(2)) in microcirculation is intrinsically involved in maintenance of vascular wall homeostasis. EA.hy 926 is a hybrid cell line, is derived by fusion of HUVEC with A549 cells. The aim of this study was to examine the production of prostacyclin and PGE2 in resting and IL-1beta-stimulated EA.ha 926 cells, in comparison with its progenitor cells. Non-stimulated EA.hy 926 cells has been found to produce much lower amounts of prostacyclin than resting HUVEC. Resting hybrid cells produced more PGE(2) than prostacyclin, despite they expressed high levels of COX-1 and PGI(2) synthase. On the contrary to HUVEC and A549, EA.hy 926 cells did not respond to IL-1beta with COX-2 induction and increase of prostaglandin production, however they did it in response to lysophosphatidylcholine (LPC). The characteristics of EA.hy 926 cells in terms of the pattern of prostanoid formation could facilitate studies on endothelial metabolism and role of these important lipid mediators.     

Endogenous prostaglandin formation in the field-stimulated guinea-pig vas deferens: comparison of the inhibitory effects of indomethacin and NS-398

Prostaglandin (PG) E2 inhibited both phases of contraction produced by electrical field stimulation of the guinea-pig vas deferens. PGF2alpha and PGD2 were without effect on this preparation. Carbacyclin (a PGI2) analogue inhibited the first phase of contraction at higher concentrations, whereas U46619 (a thromboxane mimetic) potentiated both phases of contraction. As exogenous arachidonic acid inhibits both phases of contraction of the electrically field-stimulated guinea-pig vas deferens, it is likely that the arachidonic acid is converted to PGE2 in the vas deferens. Indomethacin, a non-specific inhibitor of prostaglandin H synthase (PGHS), attenuated the inhibitory actions of exogenous arachidonic acid when examined on the first phase of contraction. NS-398, a relatively specific inhibitor of PGHS-2, also prevented the inhibitory action of exogenous arachidonic acid. However, NS-398 was much less effective than indomethacin in this respect even though NS-398 and indomethacin inhibit PGHS-2 with similar potencies. Consequently, the findings suggest that exogenous arachidonic acid is converted to PGE2 in the guinea-pig vas deferens by the actions of PGHS-1 and, to a lesser extent, by PGHS-2.     

Up-regulation of endothelial cyclooxygenase-2 and prostanoid synthesis by platelets. Role of thromboxane A2

Platelet-vascular endothelial cell interactions are central to the maintenance of vascular homeostasis. Thromboxane A2 (TXA2) and prostacyclin (prostaglandin (PG)I2) are the major products of cyclooxygenase (COX) metabolism by platelets and the vascular endothelium, respectively. Here we report the effects of platelet-endothelial interactions on human umbilical vein endothelial cells (HUVECs) COX-2 expression and prostanoid synthesis. Co-incubation of platelets with HUVECs resulted in a dose-dependent induction in COX-2 expression. This was accompanied by a relatively small increase in thromboxane B2 synthesis (2 ng) by comparison to the production of 6-keto-PGF1alpha and PGE2, which increased by approximately 14 and 12 ng, respectively. Abrogation of platelet-HUVEC interactions excluded direct cell-cell contact as a required event. Preincubation of HUVECs with SQ29548, a TXA2 receptor antagonist, dose-dependently inhibited platelet-induced COX-2 expression and prostanoid synthesis. Similarly, if platelet TXA2 synthesis was inhibited no induction of COX-2 was observed. Furthermore, a TXA2 analog, carbocyclic TXA2, induced HUVEC COX-2 expression and the synthesis of 6-keto-PGF1alpha and PGE2. This was also associated with an increase in the expression and activity of PGI synthase and PGE synthase but not TX synthase. Platelet co-incubation (or TXA2) also selectively activated the p44/42 mitogen-activated protein kinase pathway to regulate HUVEC COX-2 expression. Thus it seems that platelet-derived TXA2 can act in a paracrine manner to up-regulate endothelial COX-2 expression and PGI2 synthesis. These observations are of particular importance given the recent observations regarding selective COX-2 inhibitors and the suppression of PGI2 synthesis.     

Control of prostacyclin synthesis in pregnancy-induced hypertension

Uterine prostacyclin synthase (PGI synthase) and prostaglandin endoperoxide synthase (PGH synthase) concentrations, measured by specific immunoradiometric assays, did not differ between patients with severe pregnancy-induced hypertension (syn. pre-eclampsia; n = 5) and normotensive gravidae (n = 6) with comparable gestational ages (34 - 38 weeks). Myometrial microsomes from pre-eclamptic women contained ten times more PGI synthase than PGH synthase and the ratio of PGI synthase to PGH synthase (mean +/- SD; 10.1 +/- 3.9) was not different from that in normotensive pregnancies. None of the pre-eclamptic patients had myometrial enzyme levels that were more than one standard deviation below the mean established previously for pregnancies ranging from 32 to 42 weeks of gestation. These findings indicate that the commonly observed association between deficient PGI2 production and pregnancy-induced hypertension cannot be explained in terms of a generalized lack of immunoassayable prostacyclin or prostaglandin endoperoxide synthases.