thromboxane-a2 and 1-benzylimidazole

It has been postulated that metabolites of the arachidonic acid pathway exert an important influence on hemostasis and thrombosis. This notion is based on in vitro experiments. We have utilized two experimental models to elucidate the physiologic roles of thromboxane A2 (TxA2) and prostacyclin (PGI2) in the modulation of thrombus formation. The role of TxA2 in promoting thrombus formation was evaluated in a rabbit model where the aorta was deendothelialized by a balloon catheter technique and indium-111-labeled platelets were used as a marker for quantifying platelet deposition. Both 1-benzylimidazole, a thromboxane synthase inhibitor, and 13-azaprostanoic acid, an antagonist of thromboxane/endoperoxide receptors significantly reduced the platelet deposition onto the damaged vessel wall. The data indicate the TxA2 plays an important role in thrombosis and hemostasis. The influence of PGI2 insufficiency due to accelerated PGI2 degradation on microvascular thrombosis was evaluated in a unique clinical disease, i.e. thrombotic thrombocytopenic purpura (TTP). Accelerated PGI2 degradation was observed in several patients with chronic TTP. The degradation abnormalities were corrected by plasma infusion in vivo or serum supplement in vitro. To test the hypothesis that PGI2 must be bound to serum macromolecules to prevent rapid hydrolysis, serum binding capacity for PGI2 was measured by Sephadex G-25 gel filtration. The binding capacity was significantly reduced in the patients and was corrected by serum supplement. Abnormalities of PGI2 binding were also noted in a group of patients with ischemic stroke. Our findings suggest that there exist in the serum certain constituents which bind and stabilize PGI2.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Arachidonic Acid; Arachidonic Acids; Blood Platelets; Chromatography, Gel; Epoprostenol; Humans; Imidazoles; In Vitro Techniques; Prostanoic Acids; Serum Albumin; Thrombophlebitis; Thromboxane A2; Thromboxane-A Synthase

Thromboxane A2 (TXA2) is a proaggregatory vasoconstrictor that is synthesized and released during reperfusion of ischaemic brain. We administered a TXA2 receptor antagonist, SQ29,548, and a thromboxane A synthase inhibitor, 1-benzylimidazole (1-BI), to rats subjected to 30 min of reversible forebrain ischaemia. Cerebral thromboxane B2 (TXB2), the stable metabolite of TXA2, measured after 60 min of reperfusion was 0.37 +/- 0.08 ng/mg brain protein in animals treated with SQ29,548/1-BI compared with 1.20 +/- 0.16 in ischaemic controls (p < 0.05). Cerebral pH determined by 31P magnetic resonance spectroscopy was higher in treated animals, 7.06 +/- 0.04, than in ischaemic controls, 6.5 +/- 0.01, after 20 min of reperfusion (p < or = 0.01). The significant elevation of cerebral pH in treated animals persisted at 30 (7.17 +/- 0.05 vs. 6.5 +/- 0.01; p < or = 0.01), 35 (7.17 +/- 0.05 vs. 6.44 +/- 0.04; p < or = 0.01), and 40 min of reperfusion (7.06 +/- 0.06 vs. 6.37 +/- 0.01; p < or = 0.05). We conclude that SQ29,548/1-BI reduces thromboxane levels and promotes resolution of tissue acidosis in ischaemic brain. The combination of a TXA2 receptor antagonist with a thromboxane A synthase inhibitor deserves further study as a potential treatment for acute cerebral infarction.

Topics: Acidosis; Animals; Brain Ischemia; Bridged Bicyclo Compounds, Heterocyclic; Drug Therapy, Combination; Fatty Acids, Unsaturated; Hydrazines; Hydrogen-Ion Concentration; Imidazoles; Magnetic Resonance Imaging; Male; Prosencephalon; Rats; Rats, Wistar; Receptors, Thromboxane; Reperfusion Injury; Thromboxane A2; Thromboxane-A Synthase

Bovine lung thromboxane synthase was immobilized on phenyl-Sepharose beads by adsorption. The immobilized enzyme was catalytically active and synthesized both TXA2 and HHT. The production of both products was inhibited by 1-benzylimidazole and furegrelate. Multiple additions of PGH2 dramatically reduced the ability of the enzyme to synthesize TXA2, but did not effect the synthesis of HHT. In addition, 1-benzylimidazole did not protect thromboxane synthase from inactivation with multiple additions of PGH2. When the enzyme was incubated with PGH2 in the presence of 1-benzylimidazole, the synthesis of TXA2 was inhibited. When the inhibitor was removed the enzyme had still been inactivated by PGH2 in the presence of 1-benzylimidazole. Thus the substrate inactivation of the enzyme does not require the production of TXA2. Our data suggests that the synthesis of TXA2 and HHT can be differentially inactivated and may occur at different sites on the enzyme.

Topics: Animals; Cattle; Enzyme Activation; Imidazoles; Lung; Microsomes; Prostaglandin Endoperoxides, Synthetic; Prostaglandin H2; Prostaglandins H; Substrate Specificity; Thromboxane A2; Thromboxane-A Synthase

PAF-receptor antagonists are known to inhibit gastrointestinal damage induced by endotoxin. In the present study, the interaction between the biosynthesis of PAF and thromboxane (TX) A2, as putative mediators of the acute intestinal damage induced by endotoxin, has been investigated in the anaesthetised rat. Bolus intravenous administration of lipopolysaccharide from E. coli (5-50 mg/kg) induced dose-related jejunal damage, assessed using both macroscopic and histological techniques. This damage was accompanied by significant increases in the jejunal formation of PAF determined by bioassay, and of TXB2, determined by radioimmunoassay. Pretreatment with the structurally-unrelated thromboxane synthase inhibitors, 1-benzyl imidazole (10-50 mg/kg) or OKY 1581 (25 mg/kg) substantially reduced both jejunal damage and TXB2 formation, but did not inhibit PAF formation. Likewise, pretreatment with indomethacin (5 mg/kg) or BW 755C (50 mg/kg) reduced jejunal damage and TXB2 formation but did not affect PAF formation. Pretreatment (2h) with dexamethasone (4 mg/kg) reduced jejunal damage and the formation of both TXB2 and PAF. Intravenous infusion of PAF (100 ng/kg/min for 10 min) induced jejunal damage and significantly increased the formation of TXB2, whereas non-specific jejunal damage induced by oral administration of ethanol did not augment PAF formation. The present findings that inhibition of jejunal thromboxane formation is associated with a substantial reduction in jejunal damage, with no corresponding inhibition in PAF formation, therefore suggests a complex interaction or sequential release of these tissue destructive mediators underlying the intestinal damage induced by endotoxin.

Topics: 4,5-Dihydro-1-(3-(trifluoromethyl)phenyl)-1H-pyrazol-3-amine; Animals; Anti-Inflammatory Agents, Non-Steroidal; Escherichia coli; Ethanol; Imidazoles; Indomethacin; Jejunum; Lipopolysaccharides; Male; Methacrylates; Platelet Activating Factor; Prostaglandin-Endoperoxide Synthases; Pyrazoles; Rats; Rats, Inbred Strains; Shock, Septic; Thromboxane A2; Thromboxane-A Synthase

The aim of this study was to evaluate the response to water deprivation in rats during development in the presence and in the absence of either benzylimidazole or acetaminophen (10 mg/kg of each), inhibitors of the synthesis of thromboxanes and other arachidonic acid metabolites. Whereas water deprivation induced an increment in urine osmolality both in control and vehicle-treated animals, this response was blocked by benzylimidazole and acetaminophen in the 5-day-old rats. No effect was observed in the older rats. Benzylimidazole also increased sodium excretion in the newborn rat. To assess the effect of benzylimidazole on epithelial cells deprived of the influence of hemodynamic factors, sodium transport and water flow were studied in the frog skin and in the toad bladder, respectively. In the frog skin epithelium, benzylimidazole (10(-4) M) inhibited the vasopressin-stimulated (100 mU/ml) sodium transport and in the toad bladder it decreased the vasopressin-stimulated (5 mU/ml) hydroosmotic flow. Our results suggest that thromboxanes are necessary for a full development of the response to vasopressin in sensitive epithelia. In the rat, thromboxanes and other arachidonic acid metabolites appear to play a role in the neonate but not in the older animal.

Topics: Acetaminophen; Age Factors; Animals; Body Water; Dinoprostone; Female; Imidazoles; Rana pipiens; Rats; Rats, Inbred Strains; Sodium; Thromboxane A2; Vasopressins

The contractile responses of spiral strips of human digital arteries to samples of a suspension of human platelets aggregated by thrombin have been studied at different time intervals after aggregation. Platelets added to the arterial strips 1 min after aggregation of the platelets produced contractile responses which were significantly greater than those produced by corresponding platelets added 20 min after aggregation. When platelets were aggregated in the presence of indomethacin or the thromboxane synthetase antagonist 1-benzylimidazole, contractile responses produced by the platelets 1 min after aggregation were significantly reduced. They were then not significantly different from those produced by addition of the aliquots 20 min after aggregation, which were unaffected. Pretreatment of the arteries with the serotonin antagonist ketanserin nearly abolished the contractile responses produced by addition of the platelets 20 min after aggregation, and significantly reduced those produced by addition of the platelets 1 min after aggregation. Ketanserin did not affect the contractile responses of the arteries to potassium chloride, prostaglandin F2 alpha, or the endoperoxide analogue U-46619, but antagonized the contractile effects of exogenous serotonin. Combination of pretreatment of the arteries with ketanserin and aggregation of the platelets in indomethacin or 1-benzylimidazole virtually abolished contractile responses to platelets added both 1 min and 20 min after aggregation. Tensions developed to different dilutions of platelets added 1 min after aggregation to arteries pretreated with ketanserin were not significantly different from those obtained to the same dilutions added 20 min after aggregation to arteries not pretreated with ketanserin.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Arteries; Blood Platelets; Humans; Imidazoles; In Vitro Techniques; Indomethacin; Ketanserin; Muscle Contraction; Piperidines; Platelet Aggregation; Serotonin Antagonists; Thromboxane A2; Time Factors

There is accumulating evidence that the presence of the imidazole ring is essential for the inhibition of the thromboxane synthetase activity of several compounds synthetized so far. Inhibition of arachidonic acid (AA) induced platelet aggregation was observed by us with a series of tripeptides in which the imidazole ring belongs to histidine, included in a proper aminoacid sequence. Among these compounds the N-acetyl-L-phenylalanyl-L-phenylalanyl-L-histidine methylester (ZAMI-420) was the most effective. The bioassay of the supernatant of an AA-added platelet-rich plasma (PRP) preparation, preincubated with increasing concentrations of ZAMI-420, showed a dose-related reduction in thromboxane-A2 (TXA2)-like activity, an increase of PG-like response and disappearance of the TXB2 peak in the radiochromatogram; RIA experiments led to the same results. ZAMI-420 does not influence cyclooxygenase activity as the AA-induced increase in O2-consumption by the microsomal fraction of bovine seminal vesicles was unaltered by this compound. Administered orally to the rat, ZAMI-420 was able to prevent gastric damage provoked by a variety of pharmacological agents, including ASA, 5HT and alcohol. The protective effect on experimentally-induced gastric damage is not mediated through the antisecretory properties of this compound but is more likely to be due to a cytoprotective mechanism based on blockade of TXA2 synthesis.

Topics: Animals; Arachidonic Acids; Biological Assay; Carbenoxolone; Chromatography, Thin Layer; Cimetidine; Ethanol; Female; Guinea Pigs; Imidazoles; Oligopeptides; Platelet Aggregation; Prostaglandin-Endoperoxide Synthases; Rabbits; Radioimmunoassay; Rats; Rats, Inbred Strains; Serotonin; Stomach; Stomach Ulcer; Thromboxane A2; Thromboxanes; Vasoconstriction

Topics: Animals; Dogs; Imidazoles; Platelet Aggregation; Regional Blood Flow; Stomach; Stomach Ulcer; Thromboxane A2; Thromboxane-A Synthase; Thromboxanes; Vasoconstriction