COMPETITIVE-INHIBITION OF HYDROGEN PEROXIDE-INDUCED AGGREGATION OF CALF PLATELETS BY PROSTAGLANDIN H-2 THROMBOXANE A2 RECEPTOR LIGANDS
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Date
1992
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JOURNAL OF BIOSCIENCES
Abstract
Hydrogen peroxide (H2O2)-induced aggregation of calf platelets and its modification by agents with specific properties were characterized employing a spectrophotometric assay. An Arrhenius activation energy of 20 +/- 1 kcal/mol was found in the temperature range of 25-degrees-36-degrees-C. Rate inhibition occurred on either side of this temperature range, and under anaerobic conditions. Exogenous Ca2+ ions were not required but Ca2+ ions, at 1 mM-concentration, optimally increased rates and extent of aggregation at suboptimal H2O2 concentrations but only extent of aggregation at optimal H2O2 concentrations. Ba2+, Sr2+, Cd2+, Mn2+ and Ni2+ ions (1 mM) and Zn2+, Pb2+ and Hg2+ ions (10-mu-M) were inhibitory. The cyclo-oxygenase inhibitor, indomethacin (10-30-mu-M) exerted only mild inhibition by a competitive mechanism. Another cyclooxygenase inhibitor, aspirin, functioned to increase aggregation. Ligands acting directly at the prostaglandin H2/thromboxane A2 receptor (5Z, 9, 11, 13E, 15(S) 15-hydroxy 9(11) epoxy methano prosta 5, 13-dien-1-oic acid, pinane thromboxane A2, arachidonic acid, eicosapentaenoic acid, and N-ethylmaleimide) functioned as competitive inhibitors. Another platelet-activating sulphydryl reagent, thimerosal, also inhibited competitively while the protein kinase C inhibitor, sphingosine, and the protein kinase C modulator, Zn2+ ions, inhibited by different mechanisms. The results indicate direct action of H2O2 at the prostaglandin H2/thromboxane A2 receptor, possibly its sulphydryls, to activate the protein kinase C pathway, independently of cyclo-oxygenase products. The results underscored the power of the kinetic approach for investigating mechanisms of platelet activation.
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Life Sciences & Biomedicine - Other Topics
Citation
17 ,2;129-140