Role of arachidonic acid metabolism in human platelet activation and irreversible aggregation

Previous studies from our laboratory have demonstrated that the aggregation response of platelets inhibited by agents blocking cyclooxygenase activity could be restored to a normal state of sensitivity by prior stimulation of alpha-adrenergic receptors. Since cyclooxygenase activity and thromboxane synthesis are not absolutely required for irreversible platelet aggregation, it is important to define precisely what role this pathway serves in platelet physiology. The present study has evaluated the influence of agents that selectively block arachidonic acid conversion at different steps of synthesis. Inhibition of peroxidase, cyclooxygenase, lipoxygenase, and thromboxane synthetase blocked the second wave response of platelets to several agonists, but did not cause dissociation of aggregates preformed by prior exposure to arachidonate (AA) or adenosine diphosphate. Phospholipase (A2/C) inhibitors, similar to prostaglandin inhibitors, blocked the second wave response of platelets to the action of agonists and, in addition, caused dissociation of aggregates induced by aggregating agents. Results of our study demonstrate that when single agonists are tested at threshold concentrations, products of arachidonate metabolism may play a role in the activation process. However, continued generation of these metabolites does not appear to be essential for the maintenance of irreversible aggregation. When a combination of agents or high concentration of physiological agonists are used, both activation and irreversible aggregation can be secured independent of prostaglandin synthesis or the release reaction.