Inhibitory effect of a selective thromboxane A2 receptor antagonist, EP 092, on platelet aggregation in whole blood ex vivo and in vivo.
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1. The inhibitory effect of a selective prostaglandin H2 (PGH2)/thromboxane A2 receptor antagonist, EP 092, on platelet aggregatory responses in whole blood ex vivo (guinea-pig: Rhesus monkey) and intravascular aggregation in vivo (rabbit) has been investigated. 2. Collagen (0.1-10.0 micrograms ml-1) caused a concentration-dependent decrease in single platelet count in samples of both guinea-pig and Rhesus monkey citrated whole blood incubated ex vivo. EP 092 administered to guinea-pigs by intravenous (0.1-3.0 mg kg-1) or oral (1.0-10.0 mg kg-1) routes significantly inhibited the platelet responses to collagen (ED50 values 1.3 +/- 0.2 and 1.4 +/- 0.2 mg kg-1 respectively). Similar potency against collagen-induced whole blood aggregation was observed in Rhesus monkey blood samples following EP 092 given orally (ED50 0.9 +/- 0.3 mg kg-1). 3. The duration of action of EP 092 against collagen aggregatory responses ex vivo in both guinea-pigs and Rhesus monkeys was between 3 and 6 h following oral administration at 3.0 mg kg-1. 4. The inhibitory activity demonstrated by EP 092 against collagen-induced aggregation of Rhesus monkey whole blood ex vivo was not accompanied by any significant reduction in thromboxane A2 formation except at the highest dose tested (10 mg kg-1). 5. The intravascular aggregatory response induced by collagen or thrombin in the anaesthetized rabbit was significantly inhibited by an intravenous infusion of EP 092 (10 mg kg-1). EP 092 appeared less potent and its effect was of shorter duration in this preparation compared with its inhibitory effect on ex vivo aggregation, being evident immediately after infusion of drug but not after a further 30 min. 6. It is concluded that collagen-induced platelet aggregatory response in guinea-pig and Rhesus monkey whole blood ex vivo and rabbit in vivo exhibit a thromboxane-dependent component which can be inhibited in a dose-related fashion by pretreatment with the thromboxane antagonist EP 092. In the rabbit, moreover, the data support the possibility of a role for thromboxane in the intravascular aggregatory response to thrombin. (+info)
Prostacyclin as a potent effector of adipose-cell differentiation.
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The terminal differentiation of Ob1771 pre-adipose cells induced by arachidonic acid in serum-free hormone-supplemented medium containing insulin, transferrin, growth hormone, tri-iodothyronine and fetuin (5F medium) was strongly diminished in the presence of inhibitors of prostaglandin synthesis, namely aspirin or indomethacin. Carbaprostacyclin, a stable analogue of prostacyclin (prostaglandin I2) known to be synthesized by pre-adipocytes and adipocytes, behaved as an efficient activator of cyclic AMP production and was able, when added to 5F medium, to mimic the adipogenic effect of arachidonic acid. Prostaglandins E2, F2 alpha and D2, unable to affect the cyclic AMP production, failed to substitute for carbaprostacyclin. However, prostaglandin F2 alpha, which is another metabolite of arachidonic acid in pre-adipose and adipose cells, able to promote inositol phospholipid breakdown and protein kinase C activation, potentiated the adipogenic effect of carbaprostacyclin. In addition, carbaprostacyclin enhanced both a limited proliferation and terminal differentiation of adipose precursor cells isolated from rodent and human adipose tissues maintained in primary culture. These results demonstrate the critical role of prostacyclin and prostaglandin F2 alpha on adipose conversion in vitro and suggest a paracrine/autocrine role of both prostanoids in the development of adipose tissue in vivo. (+info)
Competitive antagonism at thromboxane receptors in human platelets.
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The inhibitory effects of three prostanoid analogues, EP 045, EP 092 and pinane thromboxane A2 (PTA2), on the aggregation of human platelets in vitro have been investigated. In diluted platelet-rich plasma (PRP), EP 045 (20 microM) and EP 092 (1 microM) completely inhibited irreversible aggregation responses to thromboxane A2 (TXA2), prostaglandin H2 (PGH2) and five chemically stable thromboxane mimetics, including 11,9-epoxymethano-PGH2 and 9,11-azo-PGH2. Reversible aggregation produced by the prostanoid analogue, CTA2, was also inhibited. The block of the stable agonist action was surmountable. In plasma-free platelet suspensions EP 045 and EP 092 were more potent antagonists. Schild analysis indicated a competitive type of antagonism for EP 045 (affinity constant of 1.1 X 10(7) M-1); the nature of the EP 092 block is not clear. Primary aggregation waves induced by ADP, platelet activating factor (Paf) and adrenaline were unaffected by EP 045 and EP 092, whereas the corresponding second phases of aggregation were suppressed. Aggregation and 5-hydroxytryptamine (5-HT) release induced by either PGH2 or 11,9-epoxymethano-PGH2 were inhibited in a parallel manner by EP 045. Inhibition of thromboxane biosynthesis is not involved in these effects. EP 045 and EP 092 did not raise adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels in the platelet suspensions. In plasma-free platelet suspensions PTA2 produced a shape change response which could be blocked by EP 045. PTA2, therefore, has a thromboxane-like agonist action. The block of the aggregatory action of 11,9-epoxymethano-PGH2 by PTA2 appears to be mainly due to competition at the thromboxane receptor. However, PTA2 produced a slight rise in cyclic AMP levels; this could be due to a very weak stimulant action on either PGI2 or PGD2 receptors present in the human platelet. Functional antagonism by PTA2 may therefore augment its thromboxane receptor blocking activity. The results are discussed in terms of (a) the specificity of antagonism produced by EP 045, EP 092 and PTA2, (b) the validity of affinity constant determinations for receptor antagonists when aggregation is the biological response, and (c) the characteristics of the human platelet thromboxane receptor in comparison with those of thromboxane receptors in smooth muscle. (+info)
Synthesis of 13,14-dehydroprostacyclin methyl ester: a potent inhibitor of platelet aggregation.
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The structure of the most recently discovered, biologically highly active prostaglandin, PGI2 or prostacyclin, is correctly predicted on biogenetic grounds, and a general synthesis starting with prostaglandins of the F2alpha series is reported. Starting with the biologically active 13,14-dehydro-PGF2alpha, the synthesis involves formation of a 5-bromo-6,9alpha-epoxy derivative, followed by esterification and dehydrobromination of the methyl ester to form the prostacyclin structure. The stereochemistry at C-5 and C-6 of all reported products is assigned on the basis of experimental findings and mechanistic reasoning. 13,14-Dehydroprostacyclin methyl ester is considerably more stable at pH 7.5 than prostacyclin. It inhibits platelet aggregation induced by a variety of agents and causes an increase in renal blood flow in the dog at nanomolar levels. (+info)
Prostaglandin E inhibition of T-lymphocyte colony formation: a possible mechanism of monocyte modulation of clonal expansion.
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Prostaglandin and monocyte modulation of a T-lymphocyte cell capable of undergoing clonal expansion was studied. Circulating human mononuclear cells were isolated by density centrifugation. After 24 h in culture with phytohemagglutinin present, the cells were mixed with 0.3% agar and overlayed onto a 0.5% agar layer that contained media and phytohemagglutinin. At day 6, colonies that contained greater than 50 cells were counted. These colonies represented clonal prolifertion of a phytohemagglutinin-responsive T-lymphocyte precursor. This responder cell accounted for less than 0.3% of the starting cell population. Colonies were comprised of cells which, when isolated, formed E rosettes. These colony cells could be shown to have helper or suppressor function as measured by their ability to promote or inhibit immunoglobulin synthesis. By these latter criteria the colony cells were considered to be mature T lymphocytes. The addition of prostaglandin E to the cultures demonstrated a linear, r = 0.82, dose-dependent inhibition of colony formation with a 50% point of inhibition (I50) = 0.18 muM. Low numbers of normal monocytes when added to the cultures mimicked the effect of synthetic prostaglandin E2. A highly significant correlation could be shown for endogenous prostaglandin E levels and colony counts. It appears that monocytes through their synthesis of prostaglandin E2 can restrict the clonal expansion of a circulating T-lymphocyte precursor. (+info)
Prevention of toxic effects of cyclosporin on pancreatic B-cells of rats by Rioprostil, a new prostaglandin analogue.
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Cyclosporin 5, 10, and 20 mg/kg bw was given to rats once daily intragastrically and caused a dose dependent, significant decrease of glucose dependent insulin release from the arterially perfused isolated pancreas, without affecting animal behaviour, weight gain, microscopic appearances of the pancreas, or kidney function. Subcutaneous injection of a new synthetic prostaglandin analogue Rioprostil 7.5 micrograms/kg bw twice daily completely prevented the effect of cyclosporin 5 mg/kg bw and protected significantly against the effects of cyclosporin 10 and 20 mg/kg bw. (+info)
A comparative study of the involvement of the prostaglandin H2/thromboxane A2 pathway in intravascular platelet aggregation in guinea-pigs and rats.
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The effects of indomethacin, dazoxiben and EPO45 on collagen-induced platelet aggregation in vivo were studied in guinea-pigs and rats to determine the involvement of the prostaglandin endoperoxide/thromboxane A2 pathway in the aggregatory response. Indomethacin and EPO45 (a thromboxane receptor antagonist) partially inhibited platelet aggregation in rats. It was concluded that only one third of the aggregatory response to collagen was mediated by the products of cyclo-oxygenase conversion of arachidonic acid. In rats, dazoxiben was inactive although the conversion of the prostaglandin endoperoxides to thromboxane A2 was inhibited (measured as thromboxane B2). 6-keto PGF1 alpha was detected in plasma after collagen was injected into dazoxiben-treated rats. In this species therefore, the endoperoxides have significant aggregatory activity whilst the apparent increase in the level of prostacyclin was not sufficient to have any anti-aggregatory effect. All three drugs were active in the guinea-pig. About 60% of the aggregatory response to collagen was due to the products of the cyclo-oxygenase pathway, the main mediator being thromboxane A2. In guinea-pigs, dazoxiben also elevated 6-keto PGF1 alpha in the plasma after an injection of collagen. However, this apparent increase in prostacyclin production did not contribute to the anti-aggregatory effect. (+info)
Double-blind clinical trial on gastroduodenal ulcer healing with prostaglandin E2 analogues.
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Seventy-seven patients with gastroduodenal ulcer were treated with two methyl-prostaglandin E2 analogues, m-PGE2, in a double-blind clinical trial. Each of three groups was given 15 S-15 methyl PGE2 methyl ester, 15 R-15 methyl PGE2 methyl ester, and placebo, respectively. Both forms of m-PGE2 analogues appeared to reduce gastric acid secretion, to shorten ulcer healing, and also to produce some side-effects, form 'S' being the more potent. Prompt healing of the ulcer with these agents did not prevent the recurrence of the disease. As the serum gastrin response to a meal after m-PGE2 administration was not reduced, this agent seems directly to affect oxynthic cells. (+info)