Effects of a thromboxane synthetase inhibitor and a thromboxane antagonist on release and activity of thromboxane A2 and prostacyclin in vitro

The TxA2 synthetase inhibitor, dazoxiben, and the TxA2 antagonist, +/- SQ 29,548, were examined for effects on release and vasoactivity of TxA2 and prostacyclin. Isolated perfused guinea pig lungs were used as the enzyme source from which TxA2 and prostacyclin were released in response to injections of arachidonic acid or bradykinin. Both dazoxiben and +/- SQ 29, 548 inhibited contraction of the superfused rat aorta and bovine coronary artery after arachidonic acid injection through the lung. +/- SQ 29,548 abolished contractions of the rat aorta, but significant aorta contracting activity persisted during dazoxiben treatment. Dazoxiben significantly inhibited arachidonate-induced release of TxA2 (immunoreactive TxB2) into the superfusate, but TxA2 release was significantly potentiated by +/- SQ 29,548. Thus, in the presence of enhanced TxA2 concentrations, +/- SQ 29,548 effectively antagonized the vasospastic effect of TxA2. Dazoxiben diverted a significantly greater amount of arachidonic acid into prostacyclin synthesis (immunoreactive 6-keto-PGF1 alpha), changing original coronary vasoconstriction into relaxation. +/- SQ 29,548 did not significantly modify lung prostacyclin synthesis. Moreover, with +/- SQ 29,548, the absence of TxA2-mediated coronary contraction unmasked active relaxation of the superfused bovine coronary artery, coincident with thromboxane and prostacyclin release. Dazoxiben consistently inhibited TxA2 synthesis and enhanced prostacyclin synthesis. +/- SQ 29,548 augmented TxB2 release in response to arachidonate, but not bradykinin, and did not significantly alter 6-keto-PGF1 alpha release in response to either arachidonate or bradykinin. In terms of vasoactivity measured in vitro, +/- SQ 29,548 and dazoxiben produced similar anti-vasospastic effects, although this was accomplished by completely different mechanisms.     

Increased thromboxane A2 production but normal prostacyclin by the placenta in hypertensive pregnancies

The production of vasodilatory, antiaggregatory prostacyclin (PGI₂) and vasoconstrictory, proaggregatory thromboxane A₂ (TxA₂) by the placenta was studied in the cases of hypertensive pregnancy complications by superfusing pieces from maternal and fetal sides of placentae of 9 pre-eclamptic, 6 hypertensive and 11 healthy women and measuring the release of 6-keto-prostaglandin F_1α (6-keto-PGF_1α) and thromboxane B₂ (TxB₂), the breakdown products of PGI₂ and TxA₂ respectively, from the superfusate. Both sides of the placentae from the controls produced 6-keto-PGF_1α (maternal side 0.5±0.1 ng/g/min dry weight of tissue, mean±SEM; fetal side 0.7±0.2 ng/g/min) and TxB₂ (maternal side 2.5±0.4 ng/g/min; fetal side 2.7±0.5 ng/g/min with no correlation between the two. The 6-keto-PGF_1α production was normal in hypertensive complications whereas the TxB₂ production was increased on the fetal side of the placentae obtained from the pre-eclamptic (3.7±0.3 ng/g/min: p<0.05) and hypertensive women (4.1±0.4 ng/g/min; p<0.025). This may explain the occurrence of microthrombi and infarctions in placentae of hypertensive women.     

Prostacyclin and thromboxane synthesis by endometrial cancer and leiomyomas

To study the role of prostacyclin (PGI₂) and thromboxane A₂ (TxA₂) in uterine tumors, pieces of endometrial cancer (n=12) and leiomyomas (n=12)_were incubated in vitro, and the productions of 6-keto-prostaglandin F_1a (6-keto-PGF_1a, a hydration product of PGI₂) and thromboxane B₂ (TxB₂, a hydration product of TxA₂), measured by radioimmunoassay, were compared to those of corresponding healthy tissues. The production of 6-keto-PGF_1a by endometrial cancer (20.8; 1.5–85 ng/mg protein/min, median and interquartile range), by healthy endometrium (25.5; 10.0–55.0), by healthy myometrium (34.9; 25.0–59.9) and by leiomyoma (20.3; 10.2–45.1) was similar. The production of TxB₂ was increased by endometrial cancer (55.5; 10.5–155.2, p < 0.02) in comparison with endometrium (9.8; 4.3–35.1), myometrium (3.8; 2.1–8.0) and leiomyoma (1.9; 1.0–3.8). The 6-keto-PGF_1a/TxB₂ ratio in endometrial cancer (0.9; 0.3–1.5) was smaller (p < 0.02) than that in healthy endometrium (3.3; 1.9–4.8). Thus, TxA₂ may be a factor in endometrial cancer.     

Effect of acute ethanol intake on thromboxane and prostacyclin in human

To investigate the effects of acute ethanol administration on the production of proaggregatory thromboxane A₂ (TxA₂) and anti-aggregatory prostacyclin (PGI₂), ethanol (1.5 g/kilogram body weight) was given to eight healthy nonsmoking men, and the stable metabolites thromboxane B₂ (TxB₂) and 6-keto-prostaglandin F_1α (6-keto-PGF_1α), respectively, measured by radioimmunoassay from serial blood samples before drinking and during the ensuing 18 hours. Each subject was studied as his own control on another occasion when only an equivalent volume of water was given. Serum TxB₂ level decreased (p < 0.01) from 206 + 31 ng/ml (mean) ± S.E. to 167² ± 24 and 161 ± 23 ng/ml (two and four hours after beginning of the drinking, respectively) concomitantly with the attainment of maximal blood ethanol concentrations (about 120 mg/100 ml), whereas no changes occurred in plasma 6-keto-PGF_1α concentrations. Our results may provide an explanation for known effects of ethanol on platelet aggregation. They also raise speculation whether TxA₂-inhibition and the antiatherogenic effect of alcohol intake are somehow related.     

Thromboxane and pulmonary hypertension following E. coli endotoxin infusion in sheep: Effect of an imidazole derivative

We assessed the effect of a specific thromboxane synthetase inhibitor (an imidazole derivative) on pulmonary hemodynamics and the concentrations of TxB₂ (TxA₂), 6-keto-PGF_1α (PGI₂), and PGF_2α in pulmonary lymph and transpulmonary blood samples following intravenous administration of E. coli endotoxin (1 μg/kg) in sheep. In control animals the rise in pulmonary artery pressure correlated with increases in plasma and lymph TxB₂ concentrations and large transpulmonary concentration gradients of this metabolite were measured. In imidazle treated animals both pulmonary hypertension as well as increases in plasma and lymph TxB₂ concentrations were substantially reduced. In contrast, peak concentrations of 6-keto-PGF_1α (PGI₂) and PGF_2α were severalfold higher than those measured in control animals. This suggests a shunting of endoperoxide metabolism towards prostacyclin and primary prostaglandins and documents the specificity of the thromboxane synthetase inhibitor. Out study provides evidence that endotoxin-induced pulmonary hypertension is mediated by pulmonary synthesis of TxA₂.     

Differential inhibition of fetal vascular prostacyclin and platelet thromboxane synthesis by monsteroidal anti-inflammatory drugs in humans

To study the synthesis of proaggregatory, vasoconstricting thrombone A₂ (TxA₂) by human fetal platelets we evaluated the formation of its stable metabolite thromboxane B₂ (TxB₂) during thrombin-induced spontaneous clotting of blood from the umbilical vein of 13 healthy infants. We further compared the effects of acetylsalicyclic acid, indomethacin, naproxen sodium and diclofenac sodium on platelet TxA₂ production in response to thrombin-induced aggregation during spontaneous clotting, and on prostacyclic (PGI₂) production by umbilical arteries in a superfusion system by measuring the 6-keto-prostaglandin F_1α (6-keto-PGF_1α) concentration in the superfusate. For every drug four concentrations covering the clinically significant range were studied. The basal production of TxB₂ by fetal platelets (181.5±22.5 ng/ml, mean±SEM) was comparable with that of adults (216.1±11.5 ng/ml). The concentrations of the drugs needed for 50 % inhibition of TxB₂ generation were 19.0 umol/1 for acetylsalicylic acid, 0.09 umol/1 for indomethacin, 0.06 umol/1 for diclofenac sodium and 4.2 umol/1 for naproxen sodium. The basal production of 6-keto-PGF_1α by umbilical arteries was 24.5±3.2 ng/min/g. The concentrations of the drugs needed for 50 % inhibition of 6-keto-PGF_1α production were 360.0 umol/1 for acetylsalicylic acid, 4.0 umol/1 for indomethacin, 2.3 umol/1 for diclofenac sodium and 15.0 umol/1 for naproxen sodium. Thus fetal platelet cyclo-oxygenase was 4–44 times more sensitive to these prostaglandin synthesis inhibitors than umbilical artery cyclo-oxygenase.     

Microsomal preparations of normal bovine iris-ciliary body generate prostacyclin-like but not thromboxane-A2-like activity

Indomethacin-treated bovine iris-ciliary body microsomes (IBIM) have been studied for their ability to convert PG endoperoxides into either thromboxance-A₂ (TxA₂)-like or prostacyclin (PGI₂)-like activity. The biological activity of the ocular tissue microsomes were compared with either indomethacin-treated human platelet microsomes (for TxA₂-like activity) or rabbit aorta microsomes (for PGI₂-like activity) under appropriate incubation conditions. No evidence could be found for the formation of TxA₂-like activity from PG endoperoxides by the IBIM. In contrasts, when the IBIM were incubated with PGH₂ for 1 min at 22°C without cofactors, PGI₂-like activity was produced, causing profound relaxation of the isolated dog coronary artery preparation without contracting the rabbit aorta and inhibiting arachidonic acid-induced platelet aggregation. Equivalent quantities of boiled IBIM failed to aleter the biological activity of PGH₂ under identical conditions. Tranylcypromine (500 μg/ml) completely abolished the appearance of PGI₂-like activity. Furthermore, the PGI₂-like activity found was stable for 10 min at 22°C at pH 8.5 but completely lost under similar conditions at pH 5.5. It is concluded than microsomal preparations of normal bovine iris-ciliary body can synthesize PGI₂-like activity in substantial amounts but not TxA₂-like activity.     

The pericardium as a source of prostacyclin in the dog, ox and rat

Cyclo-oxygenase products of arachidonic acid metabolism formed by the pericardium and epicardial surface of dog heart were identified and quantitated by radioimmunoassay after separation by high-pressure liquid chromatography. Pieces of pariental pericardium, of dog, ox and rat, when incubated produced mainly 6-keto-PGF_1α, with lesser amounts of PGE₂, PGF_2α and thromboxane B₂. Biosynthesis of all prostanoids increased during incubation of the pariental pericardium of each species with arachidonic acid, but 6-keto-PGF_1α was still the major metabolite. When slices of dog heart were incubated with arachidonic acid (1 μg/ml) the rates of 6-keto-PGF_1α formation by the pariental pericardium was much greater than that of the myocardium and endocardium. Epicardial slices appeared to be intermediate in 6-keto-PGF_1α formation. The hearts of anesthetized dogs were also irrigated with Krebs' solution, and during the first 5 min of epicardial irrigation the pericardial fluid leaving the heart again contained high levels of 6-keto-PGF_1α, with lesser amounts of the other prostanoids. Addition of arachidonic acid (3 μg/ml) to the irrigating fluid caused an increase in all measured prostanoid levels, although 6-keto-PGF_1α remained the predominant metabolite. In contrast, intravenous infusion of isoproterenol selectively increased the release of 6-keto-PGF_1α from the irrigated heart. It is concluded that the pericardium and epicardium continuously release prostacyclin into the pericardial fluid, and that the increased release of this substance observed when cardiac workload increases derives mainly from these membranous sources. This raises the interesting possibility that pericardial prostacyclin might influence coronary vascular tone and chemoreflexes which arise from the epicardium during myocardial ischemia.     

Production rates of prostaglandin F, 6-keto-PGF1α and thromboxane B2 by perifused human endometrium

Human endometrium obtained from fresh hysterectomy specimens was perifused for 7 hr in 95% O₂/5% CO₂ at 37°C. The phase of the menstrual cycle was determined by histological examination. The concentrations of PGF, 6-keto-PGF_1α and TxB₂ in 20 min fractions of the perifusion medium were measured by radioimmunoassay and production rates were calculated in terms of dry weight of tissue. Biphasic patterns of production were observed; high initial values fell to about 20% at 2 hr and then increased to relatively stable values at about 4 hr which were maintained for the next 2 hr. During this latter period, production rates in endometria taken at different phases of the cycle differed markedly from each other; the production rates of PGF in secretory and early proliferative endometria were low (15.8 ± 2.6, mean ± SEM and 67.2 ± 8.3 ng/min/g respectively) whereas they were high in late proliferative and premenstrual endometria (188.0 ± 16.7 and 196.4 ± 16.9 ng/min/g respectively). The patterns of production of 6-keto-PGF_1α and TxB₂ were similar to those of PGF but the absolute values were much lower (<10%). We conclude that the observed rates of production of prostaglandins by perifused human endometrium are consistent with synthesis being stimulated either by estrogen or withdrawal of hormonal support and being inhibited by progesterone.     

Prostacyclin,thromboxane and prostaglandin F2αin suction blister fluid of human skin: Effect of systemic aspirin and local glucocorticosteroid treatment

Changes in prostaglandin (PG) metabolism are known to be involved in various skin diseases. To elucidate the behavior of hree vasoactive PGs in human skin, namely prostacyclin (PG1₂), thromboxane A₂ (TxA₂) and PGF_2α, their stable metabolites, 6-keto-PGF_1α. TxB₂, and 13, 14 dihydro-15-keto PGF_2α (MPGF_2α), respectively, were measured by radioimmunoassays in suction blister fluids of 29 healthy male subjects. Nine of them were treated with acetylsalicylic acid (0.5 g × 4/day for one day beforehand), eight with local glucocorticoid (clobetasol-17-propionate, Dermovat^R Cream, twice a day for seven days) and 12 served as controls. All three PGs were detected in blister fluid. In controls the mean (±SD) concentration of 6-keto-PGF_1α was 1160 ± 470 pg/ml (n=12) that of TxB₂ 1590 ± 610 pg/ml (n=12) and that of MPGF_2α 1800 ± 710 pg/ml (n=12), levels which are higher than the respective concentrations in human plasma. The preceding aspirin treatment decreased the 6-keto-PGF_1α levels by 40 % (P<0.005), the TxB₂ levels by 80 % (P<0.001) and MPGF_2α levels by 35 % (P<0.05), whereas the preceding local glucocorticoid caused no changes in these PG levels. The results show that 1) PG1₂, TxA₂, and PGF_2α are locally released in the suction blister fluid of healthy human skin, 2) systemic treatment with a PG synthesis inhibiting drug, acetysalicylic acid, reduces this release, and 3) locally applied clobetasol-17-propionate does not affect the levels of prostaglandins and thromboxane as measured by our methods.     

Renal arachidonic acid metabolism and cellular changes in the rabbit renal vein constricted kidney: Inflammation as a common process in renal injury models

The process of renal inflammation was examined using the partial renal vein constricted rabbit kidney (RVC) as a model. Forty eight hours of partial renal vein constriction in the rabbit was associated with an increase in prostaglandin (PG) and thromboxane (Tx) production. The perfused RVC kidney showed an enhanced time-dependent increase in PG and Tx production in response to bradykinin stimulation when compared with the unlatered contralateral (CLK) or normal kidney. At 6 hrs of perfusion bradykinin stimulation lateral (CLK) or normal kidney. At 6 hrs of perfusion bradykinin stimulation released 2950±350 ng PGE₂, 61±15 ng TxB₂ from the RVC, and 225±85 ng PGE₂ and undetectable TxB₂ from the CLK. Histological examination of the RVC cortex showed an increase in fibroblast-like cells, a modest increase in the interstitial space and an appearance of macrophages and lymphocytes not seen in the normal of CLK. Endotoxin has been reported to stimulate macrophages in culture to produce PGE₂ and TxB₂. Endotoxin (100 ng)_stimulation of the perfused RVC kidney caused an immediate, followed by a chronically increasing, release of PGs and Tx. Two hours after endotoxin injection 50 ml of effluent fromt the RVC contained 1450±107 ng PGE₂ and 15.0±4.5 ng TxB₂. Other models of renal inflammation (e.g., the hydronephrotic kidney, chronic glomerulonephritis) also show the histological appearance of macrophages. In addition, hydronephrotic kidneys undergo fibroblast proliferation and changes in arachidonic acid metabolism similar to what we observed in the RVC. This work suggests that the inflammatory process (mononuclear cell infiltration), fibroblast-like cell proliferation, and accompanying changes in arachidonate metabolism) is common among different forms of renal injury.     

Effect of inhibition of thromboxane production on the leukotriene D4-mediated bronchoconstriction in the guinea pig

Leukotriene D₄ (LTD₄) administered intravenously to anesthetized, spontaneously breathing guinea pigs elicited decreases in dynamic lung compliance (C_dyn) and airway conductance (G_AW) with a maximal response achieved at 0.5 min. Simultaneously, plasma levels of thromboxane metabolite, TxB₂, and the prostacyclin metabolite, 6-keto-PGF_1α, increased 10-fold over pre-LTD₄ levels. Pretreatment of the guinea pigs with meclofenamic acid delayed the onset of the LTD₄-induced bronchoconstriction, antagonized the magnitude of the decreases in C_dyn and G_AW, and blocked the increase in plasma TxB₂ and 6-keto-PGF_1α levels. The thromboxane synthetase inhibitor, UK 37,248, suppressed the LTD₄-induced bronchoconstriction, while it completely blocked TxB₂ production without significantly affecting 6-keto-PGF_1α. The SRS-A end organ antagonist, FPL 55712, blocked both the LTD₄-induced bronchoconstriction and the production of the arachidonic acid metabolites. These results suggest that thromboxane A₂ plays an important role in mediating part of the bronchoconstriction elicited by intravenously administered LTD₄ in the guinea pig.     

Excretion of primary prostanoids and their metabolites during acute volume expansion

Simultaneous determination of urinary excretion rates of primary unmetabolized prostanoids and their enzymatic metabolites were performed by gas chromatography-mass spectrometry (GC/MS) or tandem mass spectrometry (GC/MS/MS). Changes in kidney function were induced by acute (4 h) volume expansion. Despite marked changes in urine flow, GFR, urinary pH, osmolality, sodium and potassium excretion, only a insignificant or transient rise in the enzymatic prostanoid metabolites (2,3-dinor-6-keto-PGF_1α, PGE-M, 2,3-dinor-TxB₂ and 11-dehydro-TxB₂) was observed. The excretion rates of the primary prostanoids were elevated in parallel with the rise in urine flow: PGE₂ rose (p < 0.05) from 14.2 ± 4.0 to 86.2 ± 20.7, PGF_2α from 60.0 ± 4.9 to 119.8 ± 24.0, 6-keto-PGF_2α from 7.2 ± 1.3 to 51.5 ± 17.0, and txB₂ from 11.2 ± 3.3 to 13.6 ± 3.6 ng/h/1.73 m² ( ) at the maximal urine flow. Except for 6-keto-PGF_1α and TxB₂, this rise in urinary prostanoid levels was only transient despite a sustained fourfold elevated urine flow. We conclude that urine flow rate acutely affect urine prostanoid excretion rates, however, over a prolonged peroid of time these effects are not maintained. The present data support the concept that urinary levels of primary prostanoids mainly reflect renal concentrations whereas those of enzymatic metabolites reflect systemic prostanoid activity. From the excretion pattern of TxB₂ one can assume that this prostanoid represents renal as well as systemic TxA₂ activity.     

Modulation of platelet thromboxane A2 and arterial prostacyclin by dietary vitamin E

Platelets from vitamin E-deficient and vitamin E-supplemented rats generate the same amount fo thromboxane A₂ (TxA₂) when they are incubated with unesterified arachidonic acid. Platelets from vitamin E-deficient rats produce more TxA₂ than platelets from vitamin E-supplemented rats when the platelets are challenged with collagen. Arterial tissue from vitamin E-deficient rats generates less prostacyclin (PGI₂) than arterial tissue from vitamin E-supplemented rats. The vitamin E effect with arterial tissue is observed when the tissue is incubated with and without added unesterified arachidonic acid. These data show that arterial prostacyclin synthesis is diminished in vitamin E-deficient rats. Vitamin E, $\text{in}$$\text{vivo}$, inhibits platelet aggregation both by lowering platelet TxA₂ and by raising arterial PGI₂.     

Some interrelationships between glucose levels thromboxane production and prostacyclin production in normal and diabetic mice

We investigated the relationship between glucose levels and platelet thromboxane production or aortic prostacyclin production, using radioimmunoassay (RIA) to measure thromboxane B₂ and 6-keto-PGF_2α. We found a direct relationship (p<.05) between plasma glucose levels and thromboxane A₂ production by arachidonate stimulated platelets in platelet rich plasma of normal mice. However, when mice were deprived of food overnight, the glucose level fell but the TxB₂ production rose significantly. Moreover, mice with streptozotocin diabetes had significantly elevated glucose levels. but normal TxB₂ production, which also rose significantly after fasting. Thus in our laboratory both fasting and diabetes nullify or reverse the direct relationship between glucose levels and TxB₂ production seen in normal fed mice. This makes it diffisult to ascribe the correlation between glucose and TxB₂ levels in normal fed animals to cause and effect. RIA revealed an inverse correlation between glucose levels and 6-keto-PGF_1α production which was highly significant in aortas taken from fasted mice and stimulated for 10 minutes with 0.1mM arachidonate. This inverse correlation was present with either normal or diabetic aortas. Moreover, fasting increased the production of 6-keto-PGF_1α. However there was a significant elevation of 6-keto production by aortas of mice with diabetes of 5–6 weeks duration, compared to aortas of normal mice. Therefore either diabetes in these mice reversed a normal inhibitory effect of glucose on 6-keto production, or else the inverse correlation between glucose levels and 6-keto production does not represent a cause and effect relationship between the two variables.     

Effects of acute and chronic ethanol administration on thromboxane and prostacyclin levels and release in rat brain cortex

The effects of acute (3 g/kg i.p. two jours before sacrifice) and chronic (6% in drinking water and libitum for 15 days) ethanol administration to male rats (200 g body weight) on basal levels and release of TxB_2n2 and 6-keto-PGF_1α in brain cortex were studied. Also the effects of chronic ethanol (30 days) on the fatty acid composition of brain cortical tissue and liver phospholipids were investigated. Acute treatment reduced basal levels of 6-keto-PGF_1α in brain cortical tissue (rats sacrificed by microwave radiation) and decreased the accumulation of 6-keto-PGF_1α in brain cortex after post-decapitation ischemia (PDI). Basal TxB₂ levels were also reduced in brain cortex, but TxB₂ release during PDI was enhanced. Chronic treatment (15 days) induced changes of TxB₂ and 6-ketoPGF_1α levels and release during PDI in brain cortex less pronounced than those observed after acute treatment. The reduced effectiveness of chronic ethanol on brain vasoactive eicosanoids suggest adaptation processes. After chronic treatment (30 days), the fatty acid composition of brain cortex total phospholipids were not significantly modified. Changes of eicosanoid production after ethanol were thus independent from modifications of the fatty acid precursor pool(s). Ethanol-induced changes in the production of vascular eicosanoids in the CNS may be of relevance to the action of the compound on the CNS and may also have implications for the clinic.     

Inhibition of prostaglandin biosynthesis by SQ 28,852, A 7-oxabicyclo-[2.2.1]heptane analog

7-Oxabicyclo[2.2.1]heptane analogs of prostaglandin (PG) H₂ can act as thromboxane (Tx) A₂ receptor antagonists or agonists, PGI₂ and/rr PGD₂ receptor agonists, or exhibit a mixture of the above activities. SQ 28,852, a new analog with a hexyloxymethyl omega side chain, is a potent inhibitor of PG synthesis. SQ 28,852 inhibited collagen and arachidonic acid (AA)-induced platelet aggregation and TxB₂ and PGE₂ formation, but did not block platelet aggregation induced by ADP or the TxA₂ mimics, 9,11-azoPGH₂, SQ 26,655, and U-46,619. It also blocked conversion of AA to TxB₂, PGE₂, and 6-ketoPGF₁α by microsomal preparations of human platelets, bovine seminal vesicles, and bovine aortas, respectively, but did not inhibit the conversion of PGH₂ to TxA₂ by the platelet microsomal preparation. SQ 28,852 (p.o.) protected mice against the lethal effects of AA (75 mg/kg, i.v.). The I₅₀ values for SQ 28,852, indomethacin and aspirin were 0.025, 0.05 and 15 mg/kg, respectively. Neither SQ 28.852 nor indomethacin protected mice from death caused by 9,11-azoPGH₂. SQ 28,852 (0.01 to 1 mg/kg, i.v.) inhibited AA-induced bronchoconstriction in anesthetized guinea pigs for at least 60 min. As an inhibitor of AA-induced bronchoconstriction, SQ 28,852 was 16- and 45-times more potent than indomethacin at 3 and 60 min after i.v. administration, respectively. SQ 28,852 did not inhibit brochoconstriction induced by histamine or 9.11-azoPGH₂, indicating its specificity of action . SQ 28,852 is the first example of a new class of cyclooxygenase inhibitors whose structure is similar to that of the naturally occurring endoperoxide, PGH₂.     

Fish oil administration as a supplement to a corn oil containing diet affects arterial prostacyclin production more than platelet thromboxane formation in the rat

The administration to male rats of 5 en % fish oil (FO) as supplement to a diet containing 5 en % corn oil (CO), selectively and markedly decreased arterial parameters (6-keto-PGF_1α formation and platelet antiaggregatory activity) assessed in isolated aortic segments perfused with autologous platelet rich plasma (PRP). Platelet parameters (ADP-induced aggregation, TxB₂ formation in thrombin-stimulated PRP and sensitivity to exogenous PGI₂) were instead minimally affected. Eicosapentaenoic acid (EPA, 20:5 n-3) did not accumulate in plasma, platelet and aorta lipids and arachidonic acid (AA, 20:4 n-6) levels declined markedly only in the plasma compartment. When FO was given alone at the same 5 en % level, both arterial and platelet parameters were similarly affected. EPA accumulated in plasma cholesterol esters and was present in appreciable concentrations also in platelets and aortic walls. AA levels declined markedly in plasma lipids and appreciably also in platelet and aorta lipids. It is concluded that a) arterial and platelet parameters are differentially affected by FO administration depending upon the presence of n-6 polyunsaturated fatty acids in the diet, b) 6-keto-PGF_1α production by arterial tissues does not seem to be related to changes of PG precursor fatty acid levels in the phospholipid fraction.     

Stimulation of vascular prostacyclin synthesis by extracellular ADP and ATP

Extracellular ADP and ATP stimulated the synthesis of prostacyclin — as reflected by the release of 6-keto-PGF_1α — in the rabbit aorta, the rabbit pulmonary artery and the rat aorta. A doubling of 6-keto-PGF_1α output was produced by 3 μM ADP. Adenosine had no effect and the stimulation by ADP was blocked by quinidine, but not by theophylline. This stimulation was abolished by indomethacin and lost after mechanical removal of the endothelium. Stimulation of vascular prostacyclin synthesis by ADP released from aggregating platelets could help localize thrombus formation to areas of vascular damage.     

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 .