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.     

Aspirin inhibits rat megakaryocyte thromboxane synthesis

This study examines the question of whether the aspirin-induced delay in the recovery of platelet cyclooxygenase pathway activity, as measured by RIA of thromboxane B₂, results from a direct effect on megakaryocyte cyclooxygenase. From our measurement of recovery of TXB₂ and information on megakaryocyte transit time in rats, we propose that thromboxane synthesis may represent a relatively late step in the differentiation of megakaryocytes. Megakaryocyte thromboxane production was depressed by 70% and that of platelets by 85% at two hr after 20 mg/kg oral aspirin dissolved in DMSO. Full megakaryocyte thromboxane recovery occurred by 72 hr and preceded complete platelet thromboxane recovery by 24 hr. Whereas megakaryocyte thromboxane synthesis showed substantial recovery by 36 hr after aspirin, platelet recovery did not begin for 24 hr and achieved a maximal recovery rate over the following 12 hr. This finding is consistent with predictions based upon human data for both megakaryocyte labeling studies and post-aspirin platelet recovery. We conclude from our data and from estimates of megakaryocyte maturation times in marrow, that thromboxane synthesis develops in rat megakaryocytes after approximately 48 hr of cytoplasmic differentiation toward platelet shedding. This metabolic capacity therefore serves as a marker of megakaryocyte differentiation.     

A regimen for low-dose aspirin?

The effects of different regimens of 40 mg aspirin on platelet thromboxane A2 synthesis and vascular prostacyclin synthesis were determined in patients who were undergoing elective surgery for removal of varicose veins. Aspirin 40 mg taken at intervals of 48 hours consistently reduced platelet thromboxane A2 synthesis to a level at which it failed to support platelet aggregation and the associated release reaction. This effect lasted for at least 36 hours. In contrast, aspirin 40 mg every 72 hours did not have the same consistent effect. Both dose regimens led to a reduction in vascular prostacyclin synthesis 12 hours after the last dose, but 36 or 72 hours after the last dose prostacyclin synthesis was not reduced; thus the inhibition of prostacyclin synthesis was short lived. If the balance between platelet thromboxane A2 and vascular prostacyclin synthesis is important in thrombosis 40 mg aspirin every 48 hours may have the maximum antithrombotic effect.     

Effects of ozone on lung mechanics and cyclooxygenase metabolites in dogs.

To determine if acute exposure to ozone can cause changes in the production of cyclooxygenase metabolites of arachidonic acid (AA) in the lung which are associated with changes in lung mechanics, we exposed mongrel dogs to 0.5 ppm ozone for two hours. We measured pulmonary resistance (RL) and dynamic compliance (Cdyn) and obtained methacholine dose response curves and bronchoalveolar lavagate (BAL) before and after the exposures. We calculated the provocative dose of methacholine necessary to increase RL 50% (PD50) and analyzed the BAL for four cyclooxygenase metabolites of AA: a stable hydrolysis product of prostacyclin, 6-keto-prostaglandin F1 alpha (6-keto-PgF1 alpha); prostaglandin E2 (PgE2); a stable hydrolysis product of thromboxane A2, thromboxane B2 (TxB2); and prostaglandin F2 alpha (PgF2 alpha). Following ozone exposure, RL increased from 4.75 +/- 1.06 to 6.08 +/- 1.3 cm H2O/L/sec (SEM) (p less than 0.05), Cdyn decreased from 0.0348 +/- 0.0109 TO .0217 +/- .0101 L/cm H2O (p less than 0.05), and PD50 decreased from 4.32 +/- 2.41 to 0.81 +/- 0.49 mg/cc (p less than 0.05). The baseline metabolite levels were as follows: 6-keto PgF1 alpha: 96.1 +/- 28.8 pg/ml; PgE2: 395.8 +/- 67.1 pg/ml; TxB2: 48.5 +/- 11.1 pg/ml; PgF2 alpha: 101.5 +/- 22.6 pg/ml. Ozone had no effect on any of these prostanoids. These studies quantify the magnitude of cyclooxygenase products of AA metabolism in BAL from dog lungs and demonstrate that changes in their levels are not prerequisites for ozone-induced changes in lung mechanics or airway reactivity.     

Arachidonic acid metabolites and an early stage of pulmonary hypertension in chronically hypoxic newborn pigs

Our purpose was to determine whether production of arachidonic acid metabolites, particularly cyclooxygenase (COX) metabolites, is altered in 100-400-microm-diameter pulmonary arteries of piglets at an early stage of pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. A cannulated artery technique was used to measure responses of 100-400-microm-diameter pulmonary arteries to arachidonic acid, a prostacyclin analog, or the thromboxane mimetic. Radioimmunoassay was used to determine pulmonary artery production of thromboxane B(2) (TxB(2)) and 6-keto-prostaglandin F(1alpha) (6-keto-PGF(1alpha)), the stable metabolites of thromboxane and prostacyclin, respectively. Assessment of abundances of COX pathway enzymes in pulmonary arteries was determined by immunoblot technique. Arachidonic acid induced less dilation in pulmonary arteries from hypoxic than in pulmonary arteries from control piglets. Pulmonary artery responses to prostacyclin and were similar for both groups. 6-Keto-PGF(1alpha) production was reduced, whereas TxB(2) production was increased in pulmonary arteries from hypoxic piglets. Abundances of both COX-1 and prostacyclin synthase were reduced, whereas abundances of both COX-2 and thromboxane synthase were unaltered in pulmonary arteries from hypoxic piglets. At least partly due to altered abundances of COX pathway enzymes, a shift in production of arachidonic acid metabolites, away from dilators toward constrictors, may contribute to the early phase of chronic hypoxia-induced pulmonary hypertension in newborn piglets.     

Arachidonic acid is preferentially metabolized by cyclooxygenase-2 to prostacyclin and prostaglandin E2

The two cyclooxygenase isoforms, cyclooxygenase-1 and cyclooxygenase-2, both metabolize arachidonic acid to prostaglandin H2, which is subsequently processed by downstream enzymes to the various prostanoids. In the present study, we asked if the two isoforms differ in the profile of prostanoids that ultimately arise from their action on arachidonic acid. Resident peritoneal macrophages contained only cyclooxygenase-1 and synthesized (from either endogenous or exogenous arachidonic acid) a balance of four major prostanoids: prostacyclin, thromboxane A2, prostaglandin D2, and 12-hydroxyheptadecatrienoic acid. Prostaglandin E2 was a minor fifth product, although these cells efficiently converted exogenous prostaglandin H2 to prostaglandin E2. By contrast, induction of cyclooxygenase-2 with lipopol- ysaccharide resulted in the preferential production of prostacyclin and prostaglandin E2. This shift in product profile was accentuated if cyclooxygenase-1 was permanently inactivated with aspirin before cyclooxygenase-2 induction. The conversion of exogenous prostaglandin H2 to prostaglandin E2 was only modestly increased by lipopolysaccharide treatment. Thus, cyclooxygenase-2 induction leads to a shift in arachidonic acid metabolism from the production of several prostanoids with diverse effects as mediated by cyclooxygenase-1 to the preferential synthesis of two prostanoids, prostacyclin and prostaglandin E2, which evoke common effects at the cellular level.     

Measurements of 6-keto-prostaglandin F1 alpha and thromboxane B2 in bleeding time blood: relation to bleeding and vascular disorders?

The body's ability to produce prostacyclin and thromboxane by blood vessels and platelets may be important in hemostatic and thrombotic disorders and in blood pressure regulation. There are limitations to the information that can be derived from measurement of the active substances or metabolites in plasma and urine. Assays for thromboxane and prostacyclin in bleeding time blood reflect production in response to a single standardized vascular injury, and show considerable promise in furthering our understanding of the production of these chemicals in vivo. These assays may improve the assessment of risk of developing thrombotic disorders and improve the ability to monitor treatment. Studies to date have focused largely on the influences of various doses of aspirin on the production of prostacyclin and thromboxane in bleeding time blood, but also suggest that smokers are high thromboxane producers. In addition, individuals who exhibit type A behavior, a behavior pattern characterized by a relatively high level of ambitiousness, hostility, and competitive drive and a chronic sense of urgency appear to be low prostacyclin producers. Diets enriched in sunflower oil were found to diminish thromboxane production, while diets high in canola oil enhanced prostacyclin formation.     

Slow intravenous administration of low dose aspirin inhibits both vascular and platelet cyclooxygenase activity: an experimental study in the rat

Aspirin irreversibly inhibits cyclooxygenase, thus preventing thromboxane (Tx)A2 production in platelets and prostacyclin in vascular cells. While it is generally accepted that the inhibitory effect of low dose aspirin is cumulative on platelet cyclooxygenase, it is still a matter of debate whether a similar phenomenon also occurs on vascular cyclooxygenase. We have measured in anesthetized rats the inhibitory effect of two doses of aspirin (2.5 and 5.0 mg/kg), given intravenously either as a bolus or as a continuous infusion (for 30 min), on platelet TxB2 and 6-ketoprostaglandin F1 alpha generation by different vascular segments. Aspirin significantly inhibited both platelet and vascular cyclooxygenase independently of the rate of drug administration. The aspirin peak plasma levels at the end of bolus injection was about 170 times higher than the average level measured during the slow infusion (1.21 +/- 0.15 micrograms/ml). At this concentration aspirin did not affect in vitro either platelet or vascular cyclooxygenase activity. Thus the inhibitory effect of aspirin on both platelet and vascular cyclooxygenase seems to be related to total exposure of the enzyme to the drug rather than to the maximal drug concentration attainable in the systemic circulation. These findings may be relevant to the current debate on optimal conditions for the biochemical selectivity of aspirin as an antithrombotic drug.     

Effects of estradiol on the metabolism of arachidonic acid by aortas and platelets in rats

We have previously reported that estradiol treatment stimulates prostacyclin production by cultured rat aortic smooth muscle cells, through the stimulation of fatty acid cyclooxygenase and prostacyclin synthetase activities. In order to see whether estradiol stimulates the fatty acid cyclooxygenase activity in platelets, intact rats were treated with estradiol, and thromboxane biosynthesis in platelets and prostacyclin production by aortas were investigated. Estradiol significantly stimulates prostacyclin production by aortas. However, no significant effect on thromboxane biosynthesis in platelets is observed. Our present results support the idea that estradiol would be a protective hormone in atherosclerotic heart disease.     

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.     

Role of endothelium, oxygen and ionic milieu in the prostacyclin and thromboxane production of rat aortic tissue slices.

The present study was executed in order to get further data on the role of vessel wall constituents in prostanoid synthesis and on the effect of anorganic constituents on it. Prostacyclin and tromboxane production of rat aortic tissue slices with intact endothelium and after mechanical as well as chemical endothelium removal were studied. The effects of hypoxia and changes in the ionic milieu on the release of these prostanoids were also examined. The tissue slices were incubated in normal or in modified Krebs-Ringer solution, bubbled with 95% O2 and 5% CO2 (with the exception of the studies in hypoxic conditions). Prostacyclin and thromboxane release was determined by specific radioimmunoassay of the stable metabolites, 6-keto-PGF1 alpha and TxB2, from the incubation medium. 174 tissue samples obtained from 164 rats were studied. Mechanical removal of the endothelium increased prostacyclin production of the aortic segments about fivefolds from a basal rate of 52.9 +/- 19.4 ng/gr/min, while it had no significant effect on thromboxane release (basal rate 0.83 +/- 0.13 ng/gr/min). Treating the endothelium with 1.0 M HCl almost totally suppressed prostacyclin release. Lowering the partial oxygen tension of the incubation medium significantly decreased the production of prostacyclin, while release of TxB2 somewhat increased. Increasing the Ca2+ concentration of the medium between 0-5 mM the release of prostacyclin was augmented and the release of thromboxane was diminished. Potassium free medium caused a very large increase in prostacyclin release of the tissue slices. The results show that release of vasoactive prostanoids from isolated rat aortic wall is dependent not exclusively on the endothelium and that various methods of endothelium removal may have distinct influences on prostacyclin and thromboxane productions. The changes in anorganic constituents of the surrounding medium could massively affect prostacyclin and thromboxane production of rat aortic tissue. The alternative effects of the above listed treatments on the release of prostacyclin and thromboxane from the rat aortic wall suggest the existence of different mechanisms in the control of the production of the two major prostanoids possessing opposite physiological effects.     

Synergistic effect of D-003 and aspirin on experimental thrombosis models

D-003 is a mixture of higher primary aliphatic saturated acids purified from sugarcane wax, with antiplatelet and antithrombotic effects experimentally demonstrated. Octacosanoic acid is the main component of D-003, followed by triacontanoic, dotriacontanoic, and tetracontanoic acids, while other acids are minor components. This work investigates the effects of combination therapy D-003+aspirin (ASA) on arachidonic acid (AA)-induced sudden death in mice and bleeding time in rats. In addition, the effects of D-003 on serum levels of two metabolites of AA: thromboxane A(2) and prostacyclin, assessed through the measurement of their stable metabolites: thromboxane B(2) (TxB(2)) and 6 keto PgF1alpha by radioimmunoassay kits, were also investigated. Combination therapy of D-003 (50mg/kg) and ASA (3mg/kg) significantly increased bleeding time in rats in a synergistic manner compared with D-003 or ASA alone. Moreover, the combined treatment of D-003 (200mg/kg) and ASA (5mg/kg) in mice protected against AA-induced sudden death (83% survivors) in a synergistic manner which was compared with each treatment alone (33% survivors). These results indicate that antiplatelet effects of D-003 are not mediated by a cyclooxygenase inhibition. D-003 and ASA monotherapies reduced serum TxB(2) levels, whereas D-003, but not ASA, significantly increased 6 keto PgF1alpha levels.     

Kinetic analyses of the effects of hyperoxia and hypoxia on vascular cyclooxygenase activity in vitro

Kinetic analyses were performed to understand the mechanism of hyperoxic induced inhibition of prostacyclin synthesis by human umbilical arteries. Brief exposure of arterial segments to oxygen resulted in over 30% decrease in Vmax of cyclooxygenase in treated vessels. In contrast, cyclooxygenase from hypoxic arterial segments showed approximately a 49% increase in Vmax. There were no significant differences in apparent Km values. These studies suggest that the decreased production of prostacyclin by hyperoxic tissue is due to cyclooxygenase inactivation.     

Mechanism of phosgene-induced lung toxicity: role of arachidonate mediators

We have previously shown that phosgene markedly increases lung weight gain and pulmonary vascular permeability in rabbits. The current experiments were designed to determine whether cyclooxygenase- and lipoxygenase-derived mediators contribute to the phosgene induced lung injury. We exposed rabbits to phosgene (1,500 ppm/min), killed the animals 30 min later, and then perfused the lungs with a saline buffer for 90 min. Phosgene markedly increased lung weight gain, did not appear to increase the synthesis of cyclooxygenase metabolites, but increased 10-fold the synthesis of lipoxygenase products. Pre- or posttreatment with indomethacin decreased thromboxane and prostacyclin levels without affecting leukotriene synthesis and partially reduced the lung weight gain caused by phosgene. Methylprednisolone pretreatment completely blocked the increase in leukotriene synthesis and lung weight gain. Posttreatment with 5,8,11,14-eicosatetraynoic acid (ETYA), a nonmetabolized competitive inhibitor of arachidonic acid metabolism, or the leukotriene receptor blockers, FPL 55712 and LY 171883, also dramatically reduced the lung weight gain caused by phosgene. These results suggest that lipoxygenase products contribute to the phosgene-induced lung damage. Because phosgene exposure did not increase cyclooxygenase synthesis or pulmonary arterial pressure, we tested whether phosgene affects the lung's ability to generate or to react to thromboxane. Infusing arachidonic acid increased thromboxane synthesis to the same extent in phosgene-exposed lungs as in control lungs; however, phosgene exposure significantly reduced pulmonary vascular reactivity to thromboxane but not to angiotension II and KCl.     

Cyclooxygenase-2 and an early stage of chronic hypoxia-induced pulmonary hypertension in newborn pigs.

Our objective was to determine whether cyclooxygenase (COX)-2-dependent metabolites contribute to the altered pulmonary vascular responses that manifest in piglets with chronic hypoxia-induced pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. The effect of the COX-2 selective inhibitor NS-398 on responses to arachidonic acid or acetylcholine (ACh) was measured in endothelium-intact and denuded pulmonary arteries (100- to 400-microm diameter). Pulmonary arterial production of the stable metabolites of thromboxane and prostacyclin was assessed in the presence and absence of NS-398. Dilation to arachidonic acid was greater for intact control than for intact hypoxic arteries, was unchanged by NS-398 in intact arteries of either group, and was augmented by NS-398 in denuded hypoxic arteries. ACh responses, which were dilation in intact control arteries but constriction in intact and denuded hypoxic arteries, were diminished by NS-398 treatment of all arteries. NS-398 reduced prostacyclin production by control pulmonary arteries and reduced thromboxane production by hypoxic pulmonary arteries. COX-2-dependent contracting factors, such as thromboxane, contribute to aberrant pulmonary arterial responses in piglets exposed to 3 days of hypoxia.     

In vivo biosynthesis of thromboxane and prostacyclin during exposure to physiological levels of epinephrine

The effects of 20-min epinephrine infusion (0.025 and 0.3 nmol/kg/min) on the in vivo synthesis of thromboxane A2 and prostacyclin were studied in ten healthy male volunteers. We assessed the in vivo biosynthesis of thromboxane A2 and prostacyclin by measurement of the urinary metabolites 2,3-dinor-TxB2 and 2,3-dinor-6-keto-PGF1 alpha, respectively. Epinephrine infusion did not cause any significant changes in the urinary excretion of the two metabolites. Thus, we conclude that physiological levels of epinephrine do not affect the in vivo biosynthesis of thromboxane A2 and prostacyclin.     

Altered glomerular eicosanoid biosynthesis in uranyl nitrate-induced acute renal failure

The present studies were designed (1) to examine the pattern of changes in eicosanoid biosynthesis in isolated rat glomeruli, and (2) to correlate these changes with the previously observed alterations in renal perfusion and glomerular filtration rate which occur after uranyl nitrate administration, a model of toxin-induced acute renal failure. In the first part of this study, the in vitro and the in vivo effects of two cyclooxygenase inhibitors were examined for their ability to inhibit rat glomerular eicosanoid biosynthesis. Inhibition of prostaglandin E2 and prostaglandin F2 alpha generation by 1 mM aspirin in vitro was 76 and 82%, respectively. Similar inhibitions of 85 and 72% of biosynthesis of the above-mentioned lipids by 0.1 mM indomethacin were also noted. Intraperitoneal administration of aspirin (150 mg/kg) resulted in a significant inhibition of 88% or greater of prostaglandin E2, prostaglandin F2 alpha, 6-keto-prostaglandin F2 alpha, and thromboxane B2 biosynthesis. These results indicated that the expected alterations produced under in vivo conditions were detectable by in vitro techniques used in this study. 24 h after the administration of uranyl nitrate (25 mg/kg), significant increases in the biosynthesis of prostaglandin E2 (124%) and prostaglandin F2 alpha (88%) were observed when compared to the control values. No significant changes in prostacyclin or thromboxane formation were noted at this time. A further increase in the biosynthesis of prostaglandin E2 (248%), prostaglandin F2 alpha (262%), and a significant increase in prostacyclin (120%), measured as 6-keto-prostaglandin F1 alpha, were noted at 48 h. No changes in thromboxane B2 biosynthesis were noted. It is concluded that these data are consistent with the hypothesis that the increased glomerular biosynthesis of vasodilator eicosanoids (i.e., prostaglandin E2 and prostacyclin) may play a significant role in the homeostatic regulation of renal perfusion and glomerular filtration after acute toxic injury to the kidney.     

Changes in prostacyclin and thromboxane biosynthesis and their catabolic enzyme activity in kidneys of aging rats

The effects of aging on the prostacyclin and thromboxane biosynthesis and prostaglandin catabolic enzyme activity in rat kidney were investigated. The prostacyclin biosynthesis, using arachidonic acid as substrate, was the greatest in young kidneys (2 months old) and then progressively decreased in mature (12 months old) and old (24 months old) kidneys, while thromboxane biosynthetic activity showed no significant change as a function of age. When prostaglandin H2 was used as substrate, the prostacyclin and thromboxane biosynthesis showed similar results as when arachidonic acid was used as substrate; the prostacyclin biosynthesis progressively decreased and thromboxane biosynthesis showed no significant change as a function of age. The fatty acid cyclooxygenase in kidney was measured by a specific radioimmunoassay. No significant change in renal fatty acid cyclooxygenase as a function of age was found. Thus, we concluded that the progressive decrease in renal prostacyclin biosynthesis as a function of age is due to a defect in prostacyclin synthetase in aged kidneys. The prostaglandin catabolic enzyme, NAD+-dependent 15-hydroxyprostaglandin dehydrogenase, in kidneys was also investigated. The enzyme activity progressively decreased as a function of age, which suggested a decrease in the metabolism of thromboxane A2 in aged kidneys. The present results, indicating a decrease in renal prostacyclin biosynthesis and renal 15-hydroxyprostaglandin dehydrogenase activity with aging, might contribute to a plausible explanation of the progressive decrease in renal functions in the elderly.     

Differential salicylate-aspirin interaction on vascular prostacyclin and platelet thromboxane synthesis in patients undergoing saphenectomy

We studied the ability of salicylate to counteract the effect of aspirin on platelet thromboxane synthesis and prostacyclin formation in venous tissue in patients undergoing saphenectomy. A single intravenous dose of 40 mg aspirin completely blocked thromboxane formation and reduced prostacyclin to about 43% of the control values. When salicylate (1000 mg po) corresponding in anesthetized subjects to blood levels of 25.9 +/- 5 micrograms/ml was administered before aspirin, vascular prostacyclin was no longer inhibited, whereas platelet thromboxane was still significantly blocked. These results suggest that the combination of salicylate with aspirin at an appropriate dose ratio may result in almost complete dissociation of the drug's effect on platelets and vessels in man.     

Possible inhibitory function of endogenous 15-hydroperoxyeicosatetraenoic acid on prostacyclin formation in bovine aortic endothelial cells

Arachidonic acid is metabolized via the cyclooxygenase pathway to several potent compounds that regulate important physiological functions in the cardiovascular system. The proaggregatory and vasoconstrictive thromboxane A2 produced by platelets is opposed in vivo by the antiaggregatory and vasodilating activity of prostacyclin (prostaglandin I2) synthesized by blood vessels. Furthermore, arachidonic acid is metabolized by lipoxygenase enzymes to different isomeric hydroxyeicosatetraenoic acids (HETE's). This metabolic pathway of arachidonic acid was studied in detail in endothelial cells obtained from bovine aortae. It was found that this tissue produced 6-ketoprostaglandin F1 alpha as a major cyclooxygenase metabolite of arachidonic acid, whereas prostaglandins F2 alpha and E2 were synthesized only in small amounts. The monohydroxy fatty acids formed were identified as 15-HETE, 5-HETE, 11-HETE and 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT). The latter two compounds were produced by cyclooxygenase activity. Nordihydroguaiaretic acid (NDGA), a rather selective lipoxygenase inhibitor and antioxidant blocked the synthesis of 15- and 5-HETE. It also strongly stimulated the cyclooxygenase pathway, and particularly the formation of prostacyclin. This could indicate that NDGA might exert its effect on prostacyclin levels by preventing the synthesis of 15-hydroperoxyeicosatetraenoic acid (15-HPETE), a potent inhibitor of prostacyclin synthetase. 15-HPETE could therefore act as an endogenous inhibitor of prostacyclin production in the vessel wall.