Competitive inhibition of hydrogen peroxide-induced aggregation of calf platelets by prostaglandin H2/thromboxane A2 receptor ligands.
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Date
1992-06
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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°-36°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 mM) were inhibitory. The cyclo-oxygenase inhibitor, indomethacin (10-
30 mM) exerted only mild inhibition by a competitive mechanism. Another cyclooxygenase
inhibitor, aspirin, functioned to increase aggregation. Ligands acting directly at
the prostaglandin H2/thromboxane A, 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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Keywords
Platelet aggregation, H2O2; Kinetics, prostaglandin H2/thromboxane A2 recptor ligands
Citation
Jamaluddin M, Thomas A. Competitive inhibition of hydrogen peroxide-induced aggregation of calf platelets by prostaglandin H2/thromboxane A2 receptor ligands. Journal of Biosciences. 1992 Jun; 17(2): 129-140.