thromboxane-b2 and nictindole

Human platelets pre-exposed to arachidonic acid (AA) (0.1-1 mM) or to the endoperoxide analogue U46619 (1-3 microM) and then washed and resuspended, failed to respond with aggregation or secretion to a second challenge by either agonist. The response to thrombin at low (0.04-0.1 u ml-1) but not at high (2.5 u ml-1) concentrations was also inhibited by pre-exposure to AA and U46619. The ability of platelets to synthesize thromboxane (Tx) B2 from AA or upon challenge with thrombin persisted despite platelet desensitization. In the presence of the reversible cyclo-oxygenase (CO) inhibitors methyl salicylate (MS) or L8027, pre-exposure to AA had no effect on subsequent challenge by the same agonist or by U46619, whereas platelet desensitization by pre-exposure to U46619 persisted. However, platelet activation by, and desensitization to AA and U46619, was prevented by trimetoquinol and compound L636499, two thromboxane/endoperoxide receptor antagonists. In contrast to the CO inhibitors, the thromboxane synthetase inhibitor dazoxiben, which in 3 'responders' out of 5 subjects suppressed aggregation, secretion, and Tx formation induced by AA, failed to prevent AA-induced desensitization. Compared to quiescent cells the distances between platelets desensitized after re-exposure to AA were reduced in electron microscopy, but the tight connections associated with aggregated cells were not observed. Degranulation was also not observed and cell morphology resembled that of normal quiescent platelets. In conclusion, (a) AA and U46619 desensitize human platelets at a similar site sensitive to prostaglandin/thromboxane receptor antagonists, and show cross-desensitization; (b) desensitization by AA appears to be mediated by a CO-dependent metabolite, as CO inhibitors prevent desensitization by AA but not to U46619; (c) the failure of dazoxiben to prevent desensitization by AA suggests that a metabolite other than TxA2, possibly the endoperoxides, mediates the phenomenon; (d) desensitization does not involve inactivation of CO or thromboxane synthetase enzymes.

Topics: Adenosine Triphosphate; Arachidonic Acid; Arachidonic Acids; Blood Platelets; Cyclooxygenase Inhibitors; Humans; Imidazoles; Indoles; Microscopy, Electron; Platelet Aggregation; Prostaglandin Endoperoxides, Synthetic; Receptors, Cell Surface; Receptors, Prostaglandin; Receptors, Thromboxane; Salicylates; Thrombin; Thromboxane B2

N(7-Carboxyheptyl) imidazole, 4-[2-(1H-imidazol-1-yl) ethoxy] benzoic acid (dazoxiben) and imidazo [1,5-alpha] pyridine-5-hexanoic acid (CGS 13080) are potent selective inhibitors of platelet thromboxane synthetase that have little or no effect on the cyclooxygenase activity. Oral doses of the substances given to rats inhibited platelet thromboxane B2 production induced by intra-venous administration of collagen (100 micrograms/kg). Plasma concentrations of immunoreactive 6-keto PGF1 alpha in treated animals were increased above corresponding concentrations in untreated animals. There were small effects on the thrombocytopenia with CGS 13080 and carboxyheptylimidazole but not with dazoxiben. However these results did not always achieve statistical significance. Confirmation that the immunoreactive prostaglandin measured was actually 6-keto PGF1 alpha was obtained by the facts that indomethacin abolished its appearance in plasma and that the other prostaglandins were not present in sufficient quantities to cross-react with the antiserum to 6-keto PGF1 alpha. Two different antisera to 6-keto PGF1 alpha detected the same changes. Administration of thromboxane synthetase inhibitors to rats causes redirection of prostaglandin production from thromboxane to prostacyclin when platelets are stimulated with collagen in vivo.

Topics: 6-Ketoprostaglandin F1 alpha; Animals; Blood Platelets; Collagen; Imidazoles; Indoles; Indomethacin; Male; Oxidoreductases; Pyridines; Radioimmunoassay; Rats; Rats, Inbred Strains; Thrombocytopenia; Thromboxane B2; Thromboxane-A Synthase; Thromboxanes

The mechanism of action of LTB4 has been investigated on the guinea-pig lung parenchymal strip. Mepacrine (20 microgram/ml), an inhibitor of phospholipase A2, abolished the action of LTB4 on parenchymal strips. Eicosatetraynoic acid (10 microgram/ml) and BW755C (40 microgram/ml) which are inhibitors of cyclooxygenase and lipoxygenase pathways, produced a marked inhibition of the lung strip contraction to LTB4. Similarly, aspirin (30 micrograms/ml) and flufenamate (1 microgram/ml) showed a strong inhibition of the contraction of parenchymal strips to LTB4; these results suggested that cyclooxygenase products mediate the action of LTB4. The response to LTB4 was unaffected by 15-hydroperoxyeicosatatraenoic acid (15-HPETE; 1 microgram/ml) while L8027 (25 ng/ml) reduced the contraction by 50%, suggesting that thromboxane A2 rather than prostacyclin was involved. Since parenchymal strips do not appear to be very sensitive to PGF2 alpha, PGE2 and the endoperoxides, and since effluents from LTB4-treated lungs produced contractions of lung strip and rabbit aorta which were reduced after 5 min. at 25 degrees, thromboxane A2 was postulated to mediate the lung effect of LTB4. The release of thromboxane B2 (TxB2) from lungs stimulated with LTB4 was confirmed by gas-chromatography-mass spectrometric (GC-MS) analyses.

Topics: Animals; Arachidonic Acids; Aspirin; Female; Flufenamic Acid; Guinea Pigs; Imidazoles; Indoles; Leukotriene B4; Leukotrienes; Lipid Peroxides; Lung; Male; Muscle Contraction; Peroxides; Prostaglandins; Pyridines; Quinacrine; Thromboxane A2; Thromboxane B2; Thromboxanes

Topics: Adenosine Triphosphate; Animals; Arachidonic Acids; Bronchi; Female; Guinea Pigs; Imidazoles; In Vitro Techniques; Indoles; Male; Platelet Aggregation; Prostaglandins E; Pyridines; Thromboxane A2; Thromboxane B2; Thromboxanes