Regulation of human platelet activation--analysis of cyclooxygenase and cyclic AMP-dependent pathways.

We have studied the regulation of human platelet activation by cyclic AMP (cAMP), and the cyclooxygenase products by examining the effect of prostacyclin (PGI2) and indomethacin on platelet aggregation, release reaction and thromboxane B2 (TxB2) generation induced by the full dose range of common platelet agonists in both platelet-rich plasma and washed platelets. Platelet aggregation, [14C]-5HT and TxB2 release induced by "threshold" and "supramaximal" concentrations of ADP, adrenaline, platelet-activating factor (PAF) and U46619 were totally abolished by low concentrations of PGI2 (3-6 nM). In contrast, platelet activation induced by submaximal concentrations of collagen, thrombin and the calcium ionophore A23187 was only partially inhibited by PGI2 (3-3000 nM). PAF-induced release reaction like that induced by ADP and adrenaline was totally dependent on the cyclooxygenase products and aggregation, while U46619-induced release reaction was only partially dependent on aggregation and the cyclooxygenase products. While both PGI2 (18-3000 nM) and indomethacin (10 microM) abolished collagen-induced aggregation and the aggregation-mediated release reaction, neither inhibitor significantly inhibited platelet adhesion or the adhesion-mediated release reaction. Maximal thrombin-induced aggregation and release reaction was also not significantly inhibited by PGI2 (300 nM) or indomethacin (10 microM). Thromboxane (TxB2) generation induced by sub-maximal to maximal concentrations of collagen, thrombin and A23187 was, although significantly inhibited, not abolished by PGI2. These results demonstrate that PAF is a "weak" agonist similar to ADP and adrenaline, U46619 is an agonist intermediate between weak and strong which induces a release reaction that is only partially dependent on aggregation, but unlike the strong agonists, is totally susceptible to inhibition by PGI2, PGI2 is an indirect inhibitor of phospholipase activation, which does not significantly inhibit non-aggregation-mediated arachidonate mobilization, induced by the strong agonists, and the so-called third pathway in the collagen and thrombin-induced release reaction, which is insensitive to indomethacin, is also insensitive to elevators of cAMP such as PGI2.