Effects of nitrogen dioxide exposure on prostacyclin synthesis in lung and thromboxane A2 synthesis in platelets in rats

Effects of nitrogen dioxide (NO2) exposure on prostacyclin (PGI2) synthesis in the rat lung and thromboxane A2 (TXA2) synthesis in the platelets were studied. Male Wistar rats were exposed to 10 ppm NO2 for 1, 3, 5, 7 and 14 days. PGI2 synthesizing activity of homogenized lung decreased. The damage of PGI2 synthesizing activity reaches its maximum at 3 days. At 14 days, PGI2 synthesizing activity returned to the normal level. The activity of PGI2 synthetase decreased significantly. The formation of lipid peroxides due to NO2 exposure may cause the depression of PGI2 synthesizing activity of lung. On the other hand, platelet TXA2 synthesizing activity increased. This increased TXA2 synthesizing activity lasted at least till 3 days. Then, it returned to the normal level. The counts of platelet were decreased significantly by 1, 3, 5 and 7 days NO2 exposure. Then the decreased counts of platelet returned to the normal level at 14 days NO2 exposure. These results indicate that the depression of PGI2 synthesizing activity of lung by NO2 exposure cause an increase in TXA2 synthesizing activity of platelets. It may contribute to induce platelet aggregation and to the observed decrease in the number of platelets during NO2 exposure.     

Prostacyclin and thromboxane A2 release in isolated rat lungs

The influence of platelets and platelet membranes on the generation of prostacyclin (PGI₂) and thromboxane A₂(TXA₂) by isolated rat lung and porcine aortic endothelial cell, as measured by RIA of their stable end-producs, 6-oxo-PGF_1α and TXB₂ respectively, was studied. After introduction of either aspirin-treated platelets or membranes from aspirin-treated platelets to the perfusate, 1 5-fold increase in the amount of 6-oxo-PGF_1α and TXB₂ in the perfusate was observed. Treatment of the lung with aspirin produced a 50% reduction in the platelet-stimulated release of PGI₂ and TXA₂. Treatment of the lung with the phospholipase inhibitor, mepacrine, significantly reduced the platelet-stimulated release of PGI₂ and TXA₂. Incubation of endothelial cells with untreated platelet membranes did not alter the generation of PGI₂. These results suggest that platelet-stimulated release of PGI₂ and TXA₂ occurs via mechanical stimulation of phospholipase A₂, liberating arachidonic acid.     

Inhibition of pulmonary thromboxane A2 synthase activity and airway responses by CGS 13080

The effects of CGS 13080, a thromboxane (TXA₂) synthase inhibitor, on airway responses to arachidonic acid (AA) were investigated in the anesthetized cat. Feline and human lung microsomal fraction exhibited prostaglandin I₂ (PGI₂, prostacyclin), and TXA₂ synthase activities, and human platelet microsomal fractions exhibited TXA₂ synthase activity. Cat and human lung microsomal fractions, but not human platelets, exhibited the presence of GSH-dependent PGE₂ isomerase activity. CGS 13080 inhibited TXA₂ synthase activity in all three microsomal fractions in a concentration-dependent manner. The increases in transpulmonary pressure and lung resistance and decreases in dynamic compliance in response to AA were decreased significantly by CGS 13080. These data suggest that the bronchoconstrictor actions of AA are mediated in large part by the formation of TXA₂. The data further indicate that cyclooxygenase products other than TXA₂ are involved in the bronchoconstrictor response to AA since meclofenamate had greater inhibitory activity than did CGS 13080. Moreover, the effects of CGS 13080 were due to inhibition of TXA₂ synthase rather than an effect on TXA₂ receptors, since airway responses to the TXA₂ mimic, U46619, were not altered. The present data show that CGS 13080 inhibits TXA₂ synthase activity without altering cyclooxygenase, PGI₂ synthase, or GSH-dependent PGE₂ isomerase activities. The data further indicate that in vivo administration of CGS 13080 may selectively increase PGI₂ synthase activity.     

Enhancement of PGI2 formation by a new vasodilator, 2-nicotinamidoethyl nitrate in the coupled system of platelets and aortic microsomes

Exogenous arachidonate addition to the coupled system of platelets and aortic microsomes resulted in production of TXA₂ and PGI₂ (detected as the stable degradation products, TXB₂ and 6-keto PGF_1α, respectively). Imidazole, papaverine and dipyridamole increased PGI₂ and decreased TXA₂ in the coupled system. All of these agents inhibited TXA₂ formation by platelets from arachidonate. Nitroglycerin did not show any effect on PGI₂ and TXA₂ formation in the coupled system and on TXA₂ formation by platelets. In contrast with these compounds, in spite of showing no inhibitory effect on TXA₂ formation by platelets alone, 2-nicotinamidoethyl nitrate (SG-75) increased PGI₂ and decreased TXA₂ in the coupled system. It is suggested that SG-75 accelerated the conversion of PGH₂ to PGI₂ so that smaller amounts of TXA₂ was produced in the coupled system.     

Influence of chronic antigen exposure on the metabolism of PGH2 by microsomal preparations of airway and parenchymal tissues in the sheep

The effects of repeated antigen exposure on the synthesis of mediators by lung tissues are not well understood. To investigate the influence of antigen challenge on the synthesis of prostaglandins by central airway and peripheral lung tissues, fourteen sensitive sheep underwent biweekly exposure to aerosolized $\text{Ascaris suu}$ antigen (7) or saline (7). Following the fifth exposure, microsomal and high speed supernatant fractions were prepared from trachealis muscle and lung parenchyma. Synthesis of thromboxane (TX) A₂, prostaglandin (PG) D₂ and PGI₂ from the PG endoperoxide intermediate, PGH₂, was assayed over a range of substrate concentrations from 3–200 uM. Synthesis of PGI₂ by trachealis microsomes was approximately 5-fold greater than that of TXA₂. PGI₂ and TXA₂ production was identical in tracheal preparations from Ascaris- and saline-exposed animals. In parenchymal tissues, where TXA₂ production predominated over PGI₂ by 9-fold, preparations from Ascaris- exposed animals synthesized 50% more TXA₂ than controls at PGH₂ concentrations of 25 uM and above, whereas synthesis of PGI₂ and PGD₂ were similar in preparations from both groups of animals. The density of pulmonary mast cells was decreased by 21% in the Ascaris group, whereas polymorphonuclear leukocyte density was unchanged. These results demonstrate the differential synthesis of TXA₂ and PGI₂ in central airways and peripheral lung regions of the sheep. They further indicate that repeated exposure of the airways to antigen selectively enhances TXA₂ synthesis in the lung periphery of sensitized animals. The site of this increased enzymatic activity, whether in resident cells or newly-infiltrated cells, has not been determined.     

Effect of dipyridamole of prostaglandin generation by human platelets and vessel walls

These experiments were conducted to determine the effects of dipyridemole on human platelet aggregation, platelet thromboxane A₂ (TXA₂) and human vessel wall prostacyclin (PGI₂) generation. Dipyridamole in varying concentrations (5 to 50 μg/ml) had no direct effect on ADP-induced platelet aggregation in vitro, but it potentiated PGI₂-induced platelet aggregation inhibition at these concentrations. Dipyridamole also inhibited arachidonic acid-induced platelet TXA₂ generation at these concentrations. In continuously perfused umbilical vein segments, dipyridamole treatment resulted in stimulation of PGI₂ release determined by bioassay and by measurement of its stable metabolite 6-keto-PGF_1α. Minimum concentration of dipyridamole causing PGI₂ release was 50 μg/ml. These in vitro studies suggest that anti-thrombotic effects of dipyridamole in man are mediated mainly by potentiation of PGI₂ activity and to some extent by TXA₂ suppression. Stimulation of PGI₂ release by human vessels may not be seen in usual therapeutic concentrations.     

Regulation of microvascular prostacyclin and thromboxane with inhibitors or arachidonic acid metabolism

The levels of the stable degradation products of prostacyclin (PGI₂) and thromboxane A₂ (TXA₂): 6-oxo-prostaglandin F_1α(6-oxo-PGE_1α) and thromboxane B₂ (TXB₂) respectively were determined in the effluent of the rabbit epigastric skin flap after infusion of exogenous arachidonic acid. The blood to the flap passes through the microcirculation and thus the changes in eicosanoid biosynthesis in this part of the vasculature were recorded. The aim was to use inhibitors of arachidonic acid metabolism to increase the PGI₂/TXA₂ ratio. This may be potentially beneficial to ischaemic skin flaps by reducing platelet aggregation associated with damaged microvascular endothelium, overcoming vasospasm and increasing microvascular blood flow. Increased PGI₂/TXA₂ ratios (up to 5-fold) were best achieved using TXA₂ synthetase inhibitors such as dazoxiben hydrochloride. These were significantly more potent than the phosphodiesterase inhibitor dipyridamole, and the lipoxygenase inhibitor Bay g6575. No increase in blood flow was achieved. The cyclooxygenase inhibitor indomethacin did slow the blood flow at high concentrations (above 10⁻⁵ M), and inhibited both PGI₂ and TXA₂ synthesis. Approximately 2-fold higher concentrations of dazoxiben hydrochloride and dipyridamole were required to produce the same TXA₂ synthetase inhibition in the flap microvasculature compared with platelets .     

Modulation of the platelet thromboxane A2 and aortic prostacyclin synthesis by dietary selenium and vitamin E

Vitamin E and selenium (Se) interact synergistically as an important antioxidant defense mechanism. Se, an essential component of glutathione peroxidase (GSH-Px) and vitamin E decompose fatty acid hydroperoxides and hydrogen peroxides generated by free radical reactions. Vitamin E and GSH-Px may modulate arachidonic acid metabolism and the activity of cyclooxygenase enzymes by affecting peroxide concentration. The balance between arterial wall prostacyclin (PGI₂) production and platelet thromboxane (TX)A₂ directly influences platelet activity. In order to elucidate the differential role of dietary vitamin E and Se in aortic PGI₂ and platelet TXA₂ synthesis, 1-mo-old F344 rats were fed semipurified diets containing different levels of vitamin E (0, 30, 200 ppm) and Se (0, 0.1, 0.2 ppm) for 2 mo. Thromboxane B₂ (TXB₂) and 6-keto-PGF₁α, were measured by radioimmunoassay (RIA) after incubation of whole blood and aortic rings at 37°C for 10 and 30 min, respectively. Vitamin E deficiency reduced plasma vitamin E to 5–17% of control-fed rats, and supplementation increased it to 53% of the control-fed rats. Se supplementation in vitamin E-supplemented animals increased plasma GSH-Px by 17%, compared to vitamin E-deficient rats. Se and vitamin E supplementation did not have a similar effect on TXB₂ and PGI₂ synthesis. Se deficiency did not alter platelet TXB₂ synthesis, but significantly decreased aortic PGI₂ synthesis. It was necessary to supplement with both antioxidants in order to increase, PGI₂ synthesis. Se and vitamin E deficient groups had a higher TXB₂/PGI₂ ratio (0.17±0.08) compared to Se- and vitamin E-supplemented groups (0.03±0.01). These results confirm previous reports in humans and animals and are in accordance with epidemiological data indicating an inverse relationship between plasma Se and platelet aggregation. Thus, further suggesting that vitamin E and Se may have a specific role in controlling TXA₂ and PGI₂ synthesis.     

Effects of nitrogen dioxide exposure on the contents of prostaglandins and thromboxane B2 in broncho alveolar lavage

Effects of 10 ppm nitrogen dioxide (NO₂) exposure on the contents of prostaglandins (PGs) and thromboxane (TX) B₂ in broncho-alveolar lavage (BAL) of rats were studied. In the BAL of normal rats, the amounts of PGs and TXB₂ in the whole lavage were 6-keto-PGF_1α (38.0 ± 6.4 ng) > TXB₂ (11.8 ± 4.0 ng) > PGF_2α (5.7 ± 1.6 ng) PGE (0.5 ± 0.3 ng). Rats were exposed to NO₂ for 1, 3, 5, 7 and 14 days. The NO₂ exposure decreased in the level of 6-keto-PGF_1α by about 35% throughout the exposure. The level of TXB₂ was higher in the day 5 exposure group (155%). The contents of PGF_2α and PGE first, decreased and then transiently increased on days 3 and 5. PG 15-hydroxy-dehydrogenase activity of lung homogenate decreased correspondingly on day 3 and 5. Then the contents PGF_2α and PGE decreased on day 7 and 14.6-keto-PGF_1α and TXB₂ are stable metabolites of PGI₂, a strong bronchorelaxant and TXA₂, a strong bronchoconstrictor respectively. Therefore the results suggested that the decrease in 6-keto-PGF_1α, a major prostanoid in the BAL and the increase in TXB₂ may correlate with broncho constriction by NO₂ exposure.     

Aggregation and thromboxane B2 formation in platelets and vascular prostacyclin production from genetically obese rats

Obesity, diabetes, hyperlipidaemia and age are conditions predisposing to atheroscleorosis and arterial occlusion. Recently it has been claimed that increased synthesis of thromboxane A₂ by platelets and decreased synthesis of prostacyclin (PGI₂) by blood vessels play an important role. The “Zucker” rat, a genetically obese animal with hyperlipidaemia, hyperinsulinaemia and normoglycaemia was used to study platelet aggregation, thromboxane (TXB₂) production and aortic PGI₂ synthesis. Two age groups (6–8 months and 14–16 months old) and their homozygote lean controls were used. In the obese rats no increased aggregation was found with ADP, arachidonic acid and collagen. On the contrary platelets from young fatty rats were less sensitive to ADP than platelets from lean young animals. An increase in platelet sensitivity to aggregating agents with age was observed, especially in the obese rats. TXB₂ measured in platelet rich plasma after exposure to ADP, arachidonic acid, arachidonic acid plus ADP and collagen was similar in the fatty and lean animals.Production of PGI₂ from incubated aortic rings was lowest in young lean animals. No differences existed between the other groups of rats studied. Insulin added to aortic rings had no influence on PGI₂ production. It is concluded that age rather than obesity, hyperlipidaemia or hyperinsulinaemia may cause platelet hyperresponsiveness to aggregating agents. Thromboxane and plateletaggregation do not closely correlate. PGI₂ production is not reduced by metabolic alterations, thought to predispose to atherosclerosis.     

Preliminary Study of Pulsed-Electromagnetic Fields Effects on Endothelial Cells Line Secretions: Evidence of a Potential Increased Thrombotic Risk

We investigated the role of a 1 Hz low-strength magnetic pulse superimposed on the environmental electromagnetic field (emf) on the secretion of anti-aggregant (prostacyclin or PGI₂) and pro-aggregant (thromboxane A₂ or TXA₂) agents in the EaHy-926 endothelial cell line. We established that magnetic pulse exposure has opposite effects on the two secretions: PGI₂ is decreased, whereas TXA₂ is increased, with a PGI₂/TXA₂ ratio shifted toward thrombosis. We also show that the effect of the magnetic field depends on its orientation, normal or parallel, to the cell monolayer. Finally, we show that the amplitude of the effect does not increase with the magnitude of the magnetic pulse, particularly with PGI₂ secretion, which is increased as the field magnitude is decreased, suggesting a new concept for defining a threshold for health hazards.     

Myocardial prostacyclin and thromboxane A2 synthetase activities during ischemia and reperfusion in isolated rabbit heart

The aim of the study was to determine the prostacyclin (PGI₂) and thromboxane A₂ (TXA₂) synthetase activities of myocardial tissue and their variation during ischemia and reperfusion. Regional ischemia was induced by 10 min occlusion of the left anterior descending coronary artery in isolated Langendorff rabbit hearts. Biosynthesis of PGI₂ and TXA₂ were carried out by using arachidonic acid as substrate and left ventricle microsomes (LVM) from ischemic and non-ischemic areas as sources of PGI₂ and TXA₂ synthetase. 6-keto-PGF_1α and TXB₂, stable metabolites of PGI₂ and TXA₂ respectively, were determined by radioimmunoassay. Experiments carried out under the adopted conditions showed that LVM were able to synthetise PGI₂ as well as TXA₂ from arachidonic acid. On the other hand, ischemia depressed both PGI₂ and TXA₂ synthetase activities of cardiac tissue: the depression was more pronounced on TXA₂ synthetase than on PGI₂ synthetase with no significant difference between ischemic and non-ischemic regions. Moreover, ischemia increased the ratio indicating therefore that it can facilitate the formation of PGI₂. The post ischemic reperfusion of the heart counteracted the decrease in PGI₂ synthetase induced by ischemia which returned to the normal level: reperfusion also slightly reversed the decrease in TXA₂ synthetase. However, the diminution in TXA₂ synthetase of non-ischemic myocardium was attenuated but it remained lower than the normal level. These results suggested that the whole left ventricle is affected by regional ischemia. Furthermore it appears that myocardial TXA₂ synthetase is more vulnerable than PGI₂ synthetase to a lack of oxygen and nutrients.     

The association of thromboxane A2 receptor with lipid rafts is a determinant for platelet functional responses

We have investigated the presence of thromboxane A₂ (TXA₂) receptor associated with lipid rafts in human platelets and the regulation of platelet function in response to TXA₂ receptor agonists when lipid rafts are disrupted by cholesterol extraction. Platelet aggregation with TXA₂ analogs U46619 and IBOP was almost blunted in cholesterol-depleted platelets, as well as α_IIbβ₃ integrin activation and P-selectin exposure. Raft disruption also inhibited TXA₂-induced cytosolic calcium increase and nucleotide release, ruling out an implication of P2Y₁₂ receptor. An important proportion of TXA₂ receptor (40%) was colocalized at lipid rafts. The presence of the TXA₂ receptor associated with lipid rafts in platelets is important for functional platelet responses to TXA₂.     

Sex-related difference in the effect of aspirin on prostaglandin metabolism in rat platelet and aorta

The effect of aspirin on the production of the arterial prostacyclin (PGI₂)-like substance and platelet malondialdehyde (MDA) was investigated in rats of both sexes. No significant sex difference observed with the arterial PGI₂-like substance. But, following the aspirin treatment, the production of the PGI₂-like substance was significantly decreased in male rats. There was significant sex difference in the production of platelet MDA before the aspirin treatment. And after the aspirin treatment, platelets of both sexes produced significantly less MDA. It is possible that sex difference in the effect of aspirin is related to the quantitative difference of cyclooxygenase activity between platelets and vasal wall.     

Increased platelet activity after termination of prostacyclin infusion into man

Infusion of PGI₂ at a dose of 5 or 10 ng/kg/min during 72 hours into patients with peripheral vascular disease was followed by increased susceptibility of platelets to proaggregatory action of ADP and collagen but not that of arachidonate. The above effects were observed 24 hours after termination of infusion of PGI₂. A tendency to an increased formation of TXA₂ in PRP aggregated by arachidonate was also noticed. Infusion of PGI₂ at a dose of 2 mg/kg/min during 72 hours into the patients caused the decreased platelt aggregability to ADP and arachidonate but not to collagen, and a decreased tendency of production of TXA₂ in PRP aggregated by arachidonate. The existence of a “rebound effect” in platelets after a long term PGI₂ therapy is suggested.     

Prostacyclin inhibits 5-hydroxytryptamine release but stimulates thromboxane synthesis during cardiopulmonary bypass

The antiaggregating agent prostacyclin (PGI₂) was infused into ten dogs during cardiopulmonary bypass (CPB) to minimize thrombocytopenia and platelet dysfunction. The animals were anesthetized, placed on mechanical ventilation and underwent thoracotomy. After heparinization with 300 u/kg, animals were assigned to control (n=5) or PGI₂ treated groups (n=5). Thoracotomy and then CPB decreased platelet numbers to below 30, 000/mm³ (p < 0.05) and fibrinogen to less than 150 mg/dl (p < 0.05). PGI₂ at 100 ng/kg·min was infused for the 2 h period of CPB. PGI₂ infusion did not prevent these changes, but did prevent platelet serotonin release. In the control group after CPB, platelet serotonin fell from the baseline value of 1.11 μg/10⁹ to 0.35 μg/10⁹ platelets (p < 0.05). In contrast, PGI₂ treatment resulted in a serotonin increase to 2.27 μg/10⁹ platelets (p < 0.05). Thromboxane B₂ concentrations of platelets and plasma rose during CPB (p < 0.05). Surprisingly, PGI₂ infusion accentuated this rise in platelet and plasma thromboxane B₂ (p < 0.05). These data indicate that during CPB, an infusion of PGI₂: 1) does not prevent thrombocytopenia; 2) increases platelet serotonin uptake despite, 3) an associated rise in platelet and plasma thromboxane B₂.     

Consequences of long-term selenium-deficient diet on the prostacyclin and thromboxane release from rat aorta

It is known that peroxides, which are increased during Se deficiency because of reduced glutathione peroxidase (GSH-Px) activity, can influence the prostacyclin I₂/thromboxane A₂ (PGI₂/TXA₂) ratio. In this study we analyzed the PGI₂ and TXA₂ formation of aortas of long-term Se-deficient rats. Despite low GSH-Px activity in the Se-deficient group, the basal PGI₂ and TXA₂ formation was not different versus control animals (PGI₂: 2295 ± 1134 pg/mg vs 2940 ± 1134 pg/mg; TXA₂: 3.83 ± 1.06 pg/mg vs 5.67 ± 2.99 pg/mg). However, we checked the capacity of the aortas of Se-deficient rats to compensate for a suddenly increased peroxide concentration. After peroxide stimulation, the PGI₂ release was significantly lower in the Se-deficient group compared to the control group (PGI₂: 3507 ± 1829 pg/mg vs 7986 ± 2636 pg/mg). Again, the TXA₂ release did not show any differences. The release ratio of PGI₂/TXA₂ decreased under peroxide stress in Se-deficient animals. Although long-term Se deficiency showed a relatively well-balanced metabolism under resting conditions, sudden stress, accompanied by an excessive radical production, cannot be compensated.     

Effect of taurine on arterial, uterine and cardiac PGI2 and TXA2 synthesis in the rat

The influence of taurine (in drinking water for 6 weeks) on PGI₂ and TXA₂ synthesis by some female rat organs was investigated using radioimmunoassay and platelet antiaggregatory bioassay. Taurine 100 and 200 mg/kg/day increased aortic PGI₂ release from 0.59 ± 0.04 (control) to 0.85 ± 0.05 and 1.01 ± 0.06 ng/mg, respectively and that by the myometrium from 0.24 ± 0.02 (control) to 0.38 ± 0.01 and 0.50 ± 0.04 ng/mg wet tissue, respectively (P < 0.05, n = 6). It did not affect PGI₂ and TXA₂ production in the heart or TXA₂ in the aorta. Taurine 200 mg/kg depressed uterine TXA₂ synthesis from 148.6 ± 9.8 (control) to 85.4 ± 6.8 pg/mg (P < 0.05, n = 6). Furthermore taurine 0.4 and 0.8 mM in vitro stimulated PGI₂ released by the myometrial and aortic tissues from pregnant rats. The stimulant effect of taurine on PGI₂ may be related to its antioxidant effect whereas its inhibitory effect on uterine TXA₂ may result from direction of synthesis towards PGI₂. It is concluded that endogenous taurine may participate in regulation of PGs synthesis and that prostanoids may contribute to its known actions. On broad basis, taurine-induced release of PGI₂ may prove of potential value in those ailments characterised by deficiency in PGI₂ release.     

Prostacyclin and thromboxane formation in human umbilical arteries following stimulation with vasoactive autacoids

The formation of prostacyclin (PGI₂) and thromboxane A₂ (TXA₂) (measured as the stable metabolites 6-keto-PGF_1α and TXB₂) during stimulation with vasoactive autocoids was registered in human umbilical arteries perfused . Responses were registered within 3–4 minutes after addition of the subtances. Both angiostensin I and II were found to increase the formation of PGI₂ while depressing that of TXA₂. Serotonin increased the formation of TXA₂ but not that of PGI₂. Both PGE₂ and PGF_2α stimulated the PGI₂ formation. The TXA₂ mimetic U46619, increased PGI₂ production, whereas PGI₂ slighlty increased the formation of TXA₂. All responses were found to be completely inhibited by indomethacin.     

Characterization of prostacyclin synthesis in cultured human arterial smooth muscle cells, venous endothelial cells and skin fibroblasts.

The interaction of human platelets with one another and with the blood vessel wall is thought to be regulated in part by a balance between two arachidonic acid metabolites: thromboxane A₂, synthesized by platelets, and prostacyclin (PGI₂), synthesized by the vessel wall. We have studied the ability of cultured human vascular cells to synthesize PGI₂ from arachidonic acid. Four strains of human arterial smooth muscle cells synthesized a mean of 1.36 ng PGI₂ per 10⁵ cells, with a range of 0.2–5.3 ng PGI₂ per 10⁵ cells among the different strains. Human umbilical vein endothelial cells synthesized a mean of 7.16 ng PGI₂ per 10⁵ cells with a range of 2.3–14.0 ng per 10⁵ cells. In contrast, cultured human diploid skin fibroblasts synthesized only 0.27 ng PGI₂ per 10⁵ cells with a range of 0.05–0.6 ng per 10⁵ cells. When cultured cells were mixed with platelets, PGI₂ synthesis from added arachidonate was reduced rather than stimulated. Thus the major precursor cyclic endoperoxides utilized for PGI₂ synthesis are formed within the cells and not from endoperoxides synthesized by platelet cyclooxygenase. Aspirin has been proposed as an anti-thrombotic agent. Aspirin could be ineffective, however, if it inhibited not only platelet cyclooxygenase but that of vessel wall cells as well. Measurement of the rate constant or potency for aspirin inhibition of PGI₂ synthesis in cultured cells indicates that the cyclooxygenase in both cell types of the blood vessel wall is 14–44 fold less sensitive to aspirin inactivation than that in platelets, and appropriate levels of aspirin can selectively block human platelet thromboxane A₂ synthesis without compromising the capacity of the vasculature to produce PGI₂.