Increase In Prostacyclin Synthesis Research Articles

Enhanced prostacyclin formation and Wnt signaling in sclerostin deficient osteocytes and bone

We show that prostacyclin production is increased in bone and osteocytes from sclerostin (Sost) knockout mice which have greatly increased bone mass. The addition of prostacyclin or a prostacyclin analog to bone forming osteoblasts enhances differentiation and matrix mineralization of osteoblasts. The increase in prostacyclin synthesis is linked to increases in β-catenin concentrations and activity as shown by enhanced binding of lymphoid enhancer factor, Lef1, to promoter elements within the prostacyclin synthase promoter. Blockade of Wnt signaling reduces prostacyclin production in osteocytes. Increased prostacyclin production by osteocytes from sclerostin deficient mice could potentially contribute to the increased bone formation seen in this condition.

Defective alterations in the collagen network to prostacyclin in COPD lung fibroblasts

BackgroundProstacyclin analogs are potent vasodilators and possess anti-inflammatory properties. However, the effect of prostacyclin on extracellular matrix (ECM) in COPD is not well known. Collagen fibrils and proteoglycans are essential ECM components in the lung and fibroblasts are key players in regulating the homeostasis of ECM proteins. The aim was to study the synthesis of prostacyclin and its effect on fibroblast activity and ECM production, and in particular collagen I and the collagen-associated proteoglycans biglycan and decorin.MethodsParenchymal lung fibroblasts were isolated from lungs from COPD patients (GOLD stage IV) and from lungs and transbronchial biopsies from control subjects. The prostacyclin analog iloprost was used to study the effect of prostacyclin on ECM protein synthesis, migration, proliferation and contractile capacity of fibroblasts.ResultsTGF-β1 stimulation significantly increased prostacyclin synthesis in fibroblasts from COPD patients (p < 0.01), but showed no effect on fibroblasts from control subjects. Collagen I synthesis was decreased by iloprost in both control and COPD fibroblasts (p < 0.05). Conversely, iloprost significantly altered biglycan and decorin synthesis in control fibroblasts, but iloprost displayed no effect on these proteoglycans in COPD fibroblasts. Proliferation rate was reduced (p < 0.05) and contractile capacity was increased in COPD fibroblasts (p < 0.05) compared to control fibroblasts. Iloprost decreased proliferative rate in control fibroblasts (p < 0.05), whereas iloprost attenuated contraction capacity in both COPD (p < 0.01) and control fibroblasts (p < 0.05).ConclusionsIloprost reduced collagen I synthesis and fibroblast contractility but did not affect the collagen-associated proteoglycans or proliferation rate in fibroblasts from COPD patients. Enhanced prostacyclin production could lead to improper collagen network fibrillogenesis and a more emphysematous lung structure in severe COPD patients.

Multisystem Organ Failure

Multisystem Organ Failure

Chylomicron-remnant-like particles modify production of vasoactive mediators by endothelial cells.

Endothelial-cell dysfunction is a critical initiating event in the pathogenesis of atherosclerosis. Although there is evidence to suggest that chylomicron remnants (CMRs), lipoproteins derived from the diet, influence endothelial-cell function to generate a pro-atherogenic phenotype, the mechanisms involved remain undefined. We have examined the effects of CMR-like particles (CMR-LPs) on human endothelial-cell function, focusing on the cyclo-oxygenase (COX) and nitric oxide synthase (NOS) pathways. CMR-LPs strongly enhanced the expression of the inducible cyclo-oxygenase COX-2 and increased prostacyclin synthesis in a biphasic manner. Studies with the COX-2-selective inhibitor NS-398 confirmed the COX-2 dependency of the later increase in prostanoid production. Pre-incubation with CMR-LPs reduced basal and thrombin-stimulated cGMP generation, whereas expression of endothelial NOS was not modified by remnant treatment.

Effects of green tea catechin on prostaglandin synthesis of renal glomerular and renal dysfunction in streptozotocin-induced diabetic rats.

The purpose of the present study was to investigate the effects of green tea catechin on prostaglandin synthesis of renal glomerular and renal dysfunction in rats with streptozotocin-induced diabetes. Sprague-Dawley rats weighing 100 +/- 10 g were randomly assigned to one normal group and three groups with streptozotocin-induced diabetes. The diabetic groups were classified to a catechin-free diet (DM group), a 0.25% catechin diet (DM-0.25C group) and a 0.5% catechin diet (DM-0.5C group) according to the levels of catechin supplement in their diet. The animals were maintained on an experimental diet for 4 weeks. At this point, they were injected with streptozotocin to induce diabetes. They were killed on the sixth day. The catechin supplementation groups (DM-0.25C, DM-0.SC groups) showed a decrease in thromboxane A2 synthesis but an increase in prostacyclin synthesis, compared to the DM group. The ratio of prostacyclin/thromboxane A2 was 53.3% and 38.1% lower in the DM and DM-0.25C groups, respectively, than in the normal group. The ratio in the DM-0.5C group did not differ from that in the normal group. The glomerular filtration rate in catechin feeding groups (DM-0.25C and DM-0.5C groups) was maintained at the normal level. The urinary beta2-microglobulin content in the DM-0.5C group was significantly lower than that in the normal group. On the sixth day after induction of diabetes, the urinary microalbumin content in the DM, DM-0.25C and DM-0.5C groups had increased 5.40, 4.02, 3.87 times, respectively, compared with the normal group. In conclusion, kidney function appears to be improved by green tea catechin supplementation due to its antithrombotic action, which in turn controls the arachidonic acid cascade system.

Peroxisome Proliferator-activated Receptor δ Activation Promotes Cell Survival following Hypertonic Stress

COX2-selective non-steroidal anti-inflammatory drugs (NSAIDs) cause selective apoptosis of renal medullary interstitial cells (RMIC) in vivo and reduce their ability to tolerate hypertonic stress in vitro. To determine the mechanism by which COX2 activity promotes RMIC viability, we examined the capacity of COX2-derived prostanoids to promote RMIC survival. Although RMICs synthesize prostaglandin E2 (PGE2) PGI2 > PGF2a > TxA2, only PGI2 enhanced RMIC viability following hypertonic stress. RMICs do not express the prostacyclin receptor, but they do express the prostacyclin responsive nuclear transcription factor peroxisome proliferator-activated receptor delta (PPARdelta). Hypertonic stress increased PGI2 synthesis 330% above base line and also activated a PPARdelta specific reporter (delta response element (DRE)) by 90% above base line. Conversely DRE activity was only inhibited by the COX2-selective inhibitor SC236 but not by a COX1-selective NSAID (SC560). Overexpression of PPARdelta using an adenovirus not only drove DRE activity but also prevented RMIC death due to COX2 inhibition. These studies are consistent with a model whereby hypertonicity activates COX2-derived prostaglandin production, which promotes RMIC viability through PPARdelta. Inhibition of PPARdelta activity may contribute to the renal papillary necrosis associated with analgesic and/or NSAID use.

Calcium-dependent synthesis of prostacyclin in ATP-stimulated venous endothelial cells.

Prostacyclin is a powerful vasodilator that is released from vascular endothelial cells. Previous studies in our laboratory have indicated that arachidonic acid metabolites from venous endothelium play an important role in the dilation of adjacent arterioles during muscle stimulation. Furthermore, recent studies have suggested that ATP released from red blood cells during hypoxia stimulates dilation of arterioles. We tested the hypothesis that an ATP-induced increase in intracellular Ca(2+) in venous endothelium promotes prostacyclin synthesis. Small branches of femoral veins were isolated from male golden hamsters, placed in a 1 mL bath, and cannulated for perfusion with 3-(N-morpholino) propanesulfonic acid (MOPS)-buffered physiological salt solution at 37 degrees C. Prostacyclin synthesis was determined by enzyme immunoassay of bath solution. Perfusion of veins with ATP increased prostacyclin synthesis from 50 +/- 5 to 627 +/- 46 pg/mL (n=49). ATP-induced prostacyclin synthesis was inhibited by removal of extracellular Ca(2+), chelation of intracellular Ca(2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) (10 micromol/L for 10 minutes), and preincubation with cytosolic phospholipase A(2) (PLA(2)) inhibitors, AACOCF(3), and bromoenol lactone. Changes in intracellular Ca(2+) in cultured human venous endothelial cells were assessed by fura-2 spectrofluorometry. ATP induced a transient Ca(2+) peak within seconds, and the subsequent Ca(2+) plateau was abolished by removal of extracellular Ca(2+). An increase in prostacyclin synthesis was detected in these cells 2 minutes after application of ATP. These findings suggest that the ATP-induced increase in intracellular Ca(2+) stimulates prostacyclin synthesis in venous endothelial cells.

Increased prostacyclin synthesis by atherosclerotic arteries from estrogen-treated monkeys

We hypothesized that the atheroinhibitory and cardioprotective effects of estrogen may be mediated in part through increased prostacyclin formation by the artery wall. Atherosclerotic abdominal aorta was collected at necropsy from ovariectomized female monkeys fed an atherogenic diet alone or with added Premarin ®. Basal and arachidonate-stimulated prostacyclin and thromboxane synthesis by artery segments was measured by radioimmunoassay. In contrast to no observed differences in basal release of prostacyclin by the control and estrogen-treated arteries, there was a marked increase (∼165%) in arachidonate-stimulated formation of prostacyclin by estrogen-treated arteries, and prostacyclin synthesis was inversely correlated with plaque size. No differences were observed in basal or arachidonate-stimulated thromboxane synthesis by the control and estrogen-treated arteries. In light of known antiatherogenic and vasodilatory effects of estrogen, increased prostacyclin synthesis by estrogen-treated arteries may, in part, explain estrogen's beneficial effects on the artery wall.

Saudi Experience with Classic Homocystinuria

Classic homocystinuria is an autosomal recessive disorder due to cystathionine ss-synthase deficiency. The clinical, radiological and neurophysiological findings of classic homocystinuria diagnosed at King Faisal Specialist Hospital and Research Centre (KFSH&RC) are presented in this report. Twenty-four patients (15 females and 9 males) were referred to KFSH&RC for work-up of mental retardation, seizures, thrombo-embolic episodes and dislocation of the ocular lenses. The common clinical findings included ectopia lentis (20 patients), skeletal system involvement (18 patients), vascular system involvement (9 patients), and mental retardation (all patients to varying degrees). Unusual findings consisted of a patient who developed severe lower gastrointestinal bleeding, a patient with insulin-dependent diabetes mellitus, probably due to vasculopathy, and another having severe bronchiectasis, which may have been due to fibrillin disruption, and required the resection of a lobe of the lung. The parents of 21 patients were first-degree relatives, and 19 patients had one or more family members affected by the same disease. All patients had markedly elevated plasma levels of methionine. Cystathionine synthase activity in the fibroblast was measured in 25% of the patients and was deficient. Only four patients responded to pyridoxine and their methionine level decreased to almost normal range. The aim of this study was to increase the awareness of this disease in the scientific and medical community, in particular in the general pediatrician working in Saudi Arabia who first encounters the clinical manifestations of the disease. Early detection through tandem mass spectrometry of blood spot screening and treatment are important, and may prevent the major complications of this disease.

Effect of camonagrel, a selective thromboxane synthase inhibitor, on retinal vascularization in experimental diabetes

Platelet hyperactivity accompanied by an increased synthesis of thromboxane and/or a decreased prostacyclin production are important factors in ischemic diabetic retinopathy. We studied the effect of camonagrel and dazoxiben, two thromboxane synthase inhibitors, on retinal vascularization in a model of streptozotocin-induced diabetes in rats. Ten nondiabetic rats, 10 diabetic animals treated with saline (i.e., not treated), and 60 diabetic animals treated with dazoxiben or camonagrel (10, 50 or 100 mg kg −1 day −1 p.o.) were studied. All treatments lasted for 90 days. Dazoxiben and camonagrel produced a dose-dependent reduction in platelet aggregation and thromboxane synthesis. Dazoxiben increased prostacylin synthesis by 78% at 100 mg kg −1 day −1, and camonagrel by 154%. Dazoxiben increased retinal vascularity by 74%, and by 183% after camonagrel treatment. Prostacyclin synthesis showed a direct linear correlation with the degree of retinal vascularization ( r 2=0.6733, P<0.00001). We conclude that an increased prostacyclin synthesis may have a greater influence than the inhibition of thromboxane synthesis in preventing ischemic diabetic retinopathy in experimental diabetes. Camonagrel may be an alternative treatment in the prevention of these lesions.

Effect of corticotropin-releasing factor on prostaglandin synthesis in endothelial cells and fibroblasts

Recent evidence suggests that not only the end product of the hypothalamic-pituitary-adrenal axis, but also other hormones in the axis may be involved in regulation of the inflammatory response. We investigated the role of CRF in the regulation of prostaglandin (PG) synthesis in fibroblasts and endothelial cells. Recombinant human interleukin-1 alpha (IL-1 alpha) increased prostacyclin synthesis in endothelial cells by 66% and prostaglandin E2 (PGE2) synthesis in fibroblasts by 91%. The PG response to IL-1 alpha was suppressed to about 50% by simultaneous addition of CRF in endothelial cells (75.6 +/- 6.2 vs. 159.7 +/- 14.9 ng 6-keto-PGF1 alpha/mg protein) and fibroblasts (115.5 +/- 23 vs. 233.6 +/- 42 ng PGE2/mg protein). IL-1 alpha enhanced phospholipase A2 activity by 30% and prostaglandin H synthase activity by 60%, and the two effects were completely blocked by CRF. It is concluded that CRF suppresses IL-1 alpha-induced PG synthesis through actions on both phospholipase A2 and cyclooxygenase. In view of the essential role of central PGE2 in IL-1 alpha-induced CRF/ACTH release, these findings suggest a novel regulatory cascade in immune-neuroendocrine interactions.

Effect of corticotropin-releasing factor on prostaglandin synthesis in endothelial cells and fibroblasts.

Recent evidence suggests that not only the end product of the hypothalamic-pituitary-adrenal axis, but also other hormones in the axis may be involved in regulation of the inflammatory response. We investigated the role of CRF in the regulation of prostaglandin (PG) synthesis in fibroblasts and endothelial cells. Recombinant human interleukin-1 alpha (IL-1 alpha) increased prostacyclin synthesis in endothelial cells by 66% and prostaglandin E2 (PGE2) synthesis in fibroblasts by 91%. The PG response to IL-1 alpha was suppressed to about 50% by simultaneous addition of CRF in endothelial cells (75.6 +/- 6.2 vs. 159.7 +/- 14.9 ng 6-keto-PGF1 alpha/mg protein) and fibroblasts (115.5 +/- 23 vs. 233.6 +/- 42 ng PGE2/mg protein). IL-1 alpha enhanced phospholipase A2 activity by 30% and prostaglandin H synthase activity by 60%, and the two effects were completely blocked by CRF. It is concluded that CRF suppresses IL-1 alpha-induced PG synthesis through actions on both phospholipase A2 and cyclooxygenase. In view of the essential role of central PGE2 in IL-1 alpha-induced CRF/ACTH release, these findings suggest a novel regulatory cascade in immune-neuroendocrine interactions.

In vitro effect of mopidamol on platelet-subendothelium interaction

Chemical substitutions in the structure of the pyrimido-pyrimidine nucleus give rise to pyrimidopyrimidine compounds with platelet antiaggregatory effects, such as RA-8 (dipyridamole), RA233 (mopidamol) and RA-642. All these compounds inhibit platelet aggregation-induced by ADP through an increase in platelet CAMP levels (1,2). Mopidamol, however, is 3-10 times more potent than dipyridamole and RA-642 both in whole blood and platelet-rich plasma (3) and determines an increase in CAMP levels which is almost 22 times greater than that caused by equimolar concentrations of dipyridamole and RA-642 (2). In contrast to dipyridamole which causes an increase in prostacyclin synthesis, the effect of mopidamol on arachidonic acid metabolism is negligible. On the other hand, platelet thromboxane synthesis is inhibited by mopidamol but unaffected by dipyridamole (4).

Platelet activation and prostacyclin release in essential hypertension

To evaluate platelet activation thromboxane A 2 (TxA 2) and beta-thromboglobulin ( βTG) were used as markers and in addition we studied the biosynthesis of prostacyclin. Synthesis of TxA 2 and prostacyclin was assessed by measurement of urinary metabolites. Fifteen untreated hypertensive patients (HT) and 15 age-matched normotensive controls (NT) were investigated at rest, during and after exercise. HT patients were re-examined after 3 months on enalapril. During basal conditions there was no difference in the excretion of Tx-M, PGI-M or βTG between the groups. During strenous exercise HT exhibit a significantly higher increase in prostacyclin synthesis (162%) compared to NT (76%). The levels of βTG increased with 82% in the HT and 24% in the NT group, Tx-M increased with 27% and 23% respectively. Treatment with the ACE-inhibitor enalapril did not significantly alter these findings. These results indicate that there is no evidence of basal platelet activation in early essential hypertension. Strenous exercise leads to some increase in Tx-M in both groups, with no pronounced differences between the groups. Hypertensive patients exhibit a significantly increased prostacyclin response to exercise which could be due to differences in vessel-wall reactivity. Enalapril seems to exert no effect on platelet activation or on prostacyclin biosynthesis.

Correlation between oxidation of low density lipoproteins and prostacyclin synthesis in cultured smooth muscle cells

The effect of antioxidants on the oxidation of low density lipoproteins in relation to prosta-cyclin synthesis was investigated in the prescence of rabbit smooth muscle cells (SMC) and Fe-containing culture medium. The lipid peroxidation of low density lipoproteins (LDL) assayed as thiobarbituric acid reactive substances was increased from 0.5 to 1.4 nmol malondialdehyde mL by the presence of smooth muscle cells. Two potent antioxidants, nordihydroguairetic acid (NDGA) and butylated hydroxytoluene (BHT), inhibited lipoprotein oxidation by ic 50 values of 0.2 and 0.8 μM, respectively. Inhibition of lipoprotein oxidation was associated with an increased prostacyclin synthesis by the SMC, the effect being more pronounced with nordihydroguairetic acid than with butylated hydroxytoluene. The stable metabolite of the lipid hydroperoxide, 15-hydroxyeicosatetraenoic acid (15-HETE), formed in the 15-lipoxygenase pathway was measured following antioxidant treatment and found to be eliminated or greatly reduced by both antioxidants. The results presented show that lipid hydroperoxides, formed as a consequence of lipoprotein oxidation and promoted by the smooth muscle cells through a lipoxygenase reaction, may regulate prostacyclin synthase, a process which may be influenced by the addition of antioxidants.

Cyclooxygenase is an immediate-early gene induced by interleukin-1 in human endothelial cells.

The monokine interleukin-1 (IL-1) inhibits endothelial cell growth and induces prostacyclin production in human endothelial cells. Since cyclooxygenase (Cox) is the rate-limiting enzyme in the synthesis of prostanoids, we evaluated the ability of IL-1 to stimulate Cox expression by human umbilical vein endothelial cells (HUVEC) in vitro. Our data demonstrate that 1) the Cox mRNA is expressed at low levels in untreated cells; 2) IL-1 alpha induces the Cox mRNA within 2 h, and this induction is sustained for more than 24 h; 3) IL-1 alpha induction is dose-dependent; 4) cycloheximide potentiates the induction of the Cox mRNA by IL-1 alpha while actinomycin D prevents the induction, and 5) IL-1 alpha also stimulates Cox production in a time-dependent fashion which correlates with the increase in prostacyclin synthesis. These data suggest that Cox is an immediate-early gene induced by IL-1 in HUVEC and may contribute to the regulation of the endothelial cell differentiation pathway in vitro.

Effect of hyperlipidemic serum on lipid peroxidation, synthesis of prostacyclin and thromboxane by cultured endothelial cells: Protective effect of antioxidants

An increased lipid peroxides and a decreased production of prostacyclin have been shown in advanced atherosclerotic lesions and plasma. Our purpose was to determine whether the similar findings could be observed in cultured endothelial cells, and whether antioxidants could protect the cell against peroxide injury. In these experiments we have used bovine aortic endothelial cells in culture to address the issue of hyperlipidemia-induced arterial damage. Results of the present study showed that different concentration of hyperlipidemic sera from atherogenic rabbits induced a time- and dose-dependent alteration in the production of prostacyclin and levels of lipid peroxides in endothelial cells. Endothelial cells incubated with hyperlipidemic serum increased prostacyclin generation significantly during the initial stages and then continuously decreased. When endothelial cells were incubated for 36 h, TXA 2 generation was also impaired and at the same time the cellular lipid peroxides content increased. There was a positive correlation between the concentration of hyperlipidemic serum and lipid peroxides and an inverse correlation with prostacyclin synthesis. The medium supplemented with antioxidant selenium or vitamin E showed a significant decrease in lipid peroxides and an increase in prostacyclin synthesis. These results suggest that both hyperlipidemic serum and lipid peroxides injury endothelial cells and inactivate prostacyclin synthetase, resulting in a decrease of prostacyclin production, while antioxidants have a protective effect. We conclude that the increase in lipid peroxides in association with hyperlipidemia results in alteration of prostacyclin synthesis that may play an important role in the pathogenesis of atherosclerosis.

Evidence for coordinate, selective regulation of eicosanoid synthesis in platelet-derived growth factor-stimulated 3T3 fibroblasts and in HL-60 cells induced to differentiate into macrophages or neutrophils.

We used Swiss 3T3 fibroblasts stimulated with platelet-derived growth factor and HL-60 cells induced to differentiate into macrophages or neutrophils to study the regulation of prostaglandin and leukotriene synthesis. Addition of platelet-derived growth factor to quiescent 3T3 fibroblasts led within 4 h to a dramatic and preferential increase in prostacyclin synthesis from endoperoxide prostaglandin H2, and microsomal assays showed a strong platelet-derived growth factor-dependent increase in the maximal velocities (Vmax) of both prostaglandin H synthase and prostacyclin synthase. In contrast, addition of phorbol ester to HL-60 cells to induce differentiation into macrophages led within 4 h to a strong and preferential increase in thromboxane synthesis from prostaglandin H2, and microsomal assays disclosed a major rise in Vmax for both prostaglandin H synthase and thromboxane synthase. No comparable changes occurred in HL-60 cells that were differentiating into neutrophils, though upregulation of 5-lipoxygenase pathway enzymes occurred in both differentiation systems. Actinomycin D and cycloheximide prevented the appearance of all of these enzymes of eicosanoid synthesis in all three model systems. Thus, the distinctive patterns of eicosanoid synthesis that are seen in replicating fibroblasts and in differentiating macrophages and neutrophils appear to depend on a coordinate, selective upregulation of several enzymes of eicosanoid biosynthesis that is specific for each cell system.

Effect of various plasminogen activators on prostacyclin synthesis in cultured vascular cells.

In this study, we examined the effects of various plasminogen activators on arachidonic acid release and prostacyclin biosynthesis in cultured rat aortic smooth muscle cells and bovine pulmonary artery endothelial cells. Prostacyclin was the major product formed from arachidonic acid in aortic smooth muscle cells and endothelial cells. When intact cells were incubated with streptokinase, one of the plasminogen activators, a significant stimulatory effect on prostacyclin biosynthetic activity in cells was evident without any cellular damage at all concentrations used (1-5,000 units/ml). Streptokinase also caused a marked release of arachidonic acid. However, when it was incubated with cell-free homogenates and [3H]arachidonic acid, it did not show any effects on prostacyclin biosynthesis. The addition of urokinase and tissue-type plasminogen activator had no effect on prostacyclin biosynthesis. Urokinase stimulated the release of arachidonic acid from cells, but it did not show any effect on prostacyclin release at any concentration of urokinase (1-5,000 units/ml). The release of arachidonic acid and the increased prostacyclin synthesis were not observed when tissue-type plasminogen activator was added. These results indicate that, among various plasminogen activators investigated, only streptokinase causes the release of arachidonic acid and prostacyclin. This could be a beneficial effect in thrombolytic therapy.

Mechanism of hyperoxia-induced pulmonary vascular paralysis: effect of antioxidant pretreatment.

We designed experiments using isolated rabbit lungs to determine the effect of hyperoxia on the pulmonary vasoconstriction caused by the infusion of the lipid peroxide tert-butyl hydroperoxide (t-bu-OOH), which produces vasoconstriction by stimulating the pulmonary synthesis of thromboxane. Exposure to 48-60 h of 100% O2 at 1 ATA markedly reduced the increase in pulmonary artery pressure caused by t-bu-OOH infusion. We also investigated whether the mechanism for the attenuated vasoconstriction was due to altered production of arachidonate mediators or oxidant-induced damage to the contractile mechanism. In addition to infusing t-bu-OOH, which selectively stimulates thromboxane production, we also infused Intralipid, an esterified fatty acid emulsion that stimulates production of both thromboxane and prostacyclin. These experiments were done to study the effect of hyperoxia on prostacyclin synthesis. To determine if antioxidant therapy would prevent the changes in mediator production and vascular reactivity caused by hyperoxia, we pretreated animals with the antioxidants butylated hydroxyanisole (BHA) or vitamin E. The lack of vascular reactivity to t-bu-OOH was not due to a decrease in thromboxane synthesis or an increase in prostacyclin synthesis. Hyperoxia did not affect thromboxane synthesis during basal conditions or after stimulation of synthesis by t-bu-OOH. 100% O2 also did not effect the basal synthesis of prostacyclin by the lung. Hyperoxia did, however, markedly reduce prostacyclin synthesis when it was stimulated by Intralipid infusion. Antioxidant pretreatment did not reverse the inhibition of prostacyclin synthesis but did prevent the loss of vascular reactivity caused by hyperoxia. Thus hyperoxia causes vascular paralysis through oxidant-induced injury to the pulmonary vasculature.