STUDIES ON THE MECHANISM OF THE INHIBITION OF PLATELET-AGGREGATION AND RELEASE INDUCED BY HIGH-LEVELS OF ARACHIDONATE

Abstract

Arachidonic acid induced a biphasic pattern of [human] platelet aggregation and the release of both dense and .alpha.-granule components. Low levels of arachidonate (0.025-0.1 mM) specifically induced aggregation and release, while high concentrations (0.15-0.35 mM) caused a progressive inhibition of these platelet responses in human gel-filtered platelets (GFP). The mechanism(s) responsible for this arachidonate-induced turn-off of platelet function was studied. EM studies demonstrated that there was no gross damage to the platelets during the turn-off. Active synthesis of malondialdehyde and thromboxane A2 was seen at the high arachidonate levels, despite the inhibition of aggregation. GFP inhibited by 0.25 mM arachidonate were capable of undergoing aggregation and serotonin release in response to other stimuli, such as collagen or thrombin. GFP appeared to be metabolically intact and functional during the inhibition by high arachidonate levels. TLC studies revealed that prostaglandin metabolism was not changed at the high arachidonate levels. Indomethacin (20 .mu.M) did not abolish the arachidonate-induced inhibition of platelet function. The inhibitory effect of high arachidonate did not depend on its conversion to other prostaglandin products. Platelet cAMP levels increased 2-fold at the high arachidonate concentrations (1.3 .+-. 0.3 pmol/108 platelets at peak aggregation, vs. 2.9 .+-. 0.4 pmol/108 platelets at inhibition by 0.25 mM arachidonate, P < 0.001). Prostaglandin D2, a platelet inhibitor known to increase cAMP, generated a similar rise (to 2.4 .+-. 0.2 pmol/108 platelets). The magnitude of the arachidonate-induced increase in platelet cAMP levels can account for the inhibition of aggregation and release.