Journal of Biosciences

, Volume 17, Issue 2, pp 141–149 | Cite as

Platelet activating factor-induced aggregation of calf platelets: Apparent positive cooperativity in the kinetics and non competitive inhibition by diltiazem

  • M. Jamaluddin
  • A. Thomas


Aggregation of calf platelets by platelet activating factor was characterized by a spectrophotometric method. The aggregation kinetics of both platelet-rich plasma and purified platelets showed concave up double-reciprocal plots and linear Hill plots withh > 1 (1.7 ± 02) consistent with positive cooperativity. Comparable values of maximum rates of aggregation(R) were obtained with platelet-rich plasma (0.25 ± 0.08) and purified platelets (0.28 ± 0.18) but the half-maximal saturation concentration (S0.5) differed greatly between platelet-rich plasma (6 ± 3 nM) and purified platelets (0.28 ± 0.18 nM). An Arrhenius activation energy of 21 ±2 kcal/mol was found for aggregation of purified platelets. Diltiazem was inhibitory with half-maximal inhibitory concentration (I0.5) of 4 M but the inhibition was not competitive. Diltiazem inhibited rates but not the extent of shape-change. The receptor-antagonist and sulphydryl reagent N-ethylmaleimide and the platelet antagonistic omega-3-fatty acid, 5,8,11,14,17-eicosa pentaenoic acid, inhibited both rates and extent of shape-change reactions and inhibited aggregation competitively (I0.5 ∼ 5 M). Eicosa pentaenoic acid at > 25 M could abolish shape-change reactions and at 50 M served as an activator of platelets and the activation was enhanced by aspirin (1 mM). Although N-ethylmaleimide at > 20 M could also induce platelet activation it failed to induce aggregation and aspirin had no effect on the shape-change reactions induced by it.


Platelet aggregation Paf kinetics cooperativity diltiazem omega-3-fatty acid N-ethylmaleimide aspirin 

Abbrevations used


Platelet activating factor








17-eicosapentaenoic acid


arachidonic acid


prostaglandin H2


thromboxane A2


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Copyright information

© Indian Academy of Sciences 1992

Authors and Affiliations

  • M. Jamaluddin
    • 1
  • A. Thomas
    • 1
  1. 1.Thrombosis Research UnitSree Chitra Tirunal Institute for Medical Sciences and TechnologyThiruvananthapuramIndia

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