Biphasic effects of phospholipase A2 on platelet aggregation. Effect of prostaglandin synthesis inhibitors and essential fatty acid deficiency.

Phospholipase A2 has a biphasic action upon the aggregation of rat platelets. In the first phase, occurring after shorter incubation periods with the enzyme, aggregation is enhanced. Longer incubation periods lead to an inhibition of the aggregation. The first phase disappears after the addition of indomethacin whereas the second phase persists. Incubation of platelets with phospholipase A2 leads to serotonin release. Prostaglandins are formed without platelet aggregation. Whereas the same effects occurred at the high dose of phospholipase A2 when platelets of essential fatty acid deficient rats were used, a difference was seen at the lower dose. It is concluded that in the first phase, arachidonic acid is liberated and transformed into aggregation inducing intermediates which are formed in the prostaglandin synthesis. In the second phase, changes may occur in the outer membrane which lead to diminished sensitivity to aggregating agents.