ANTIAGGREGATORY ACTIVITY OF 8-EPI-PROSTAGLANDIN F(2-ALPHA) AND OTHER F-SERIES PROSTANOIDS AND THEIR BINDING TO THROMBOXANE A(2) PROSTAGLANDIN H-2 RECEPTORS IN HUMAN PLATELETS

8-Epi-prostaglandin F-2 alpha (8-epi-PGF2(alpha)) is a nonenzymatic, f ree radical-catalyzed peroxidation product of arachidonic acid that ha s potent biological activity, including contraction of vasculature and inhibition of aggregation induced by thromboxane (TX) A(2) mimetics. In the present study, we demonstrate that 8-epi-PGF(2 alpha) could inh ibit platelet aggregation induced by the TX mimetics U46619 and I-BOP as well as low-dose collagen but not thrombin or the primary wave of a ggregation caused by high-dose ADP. The secondary (TX-dependent) wave of aggregation induced by high-dose ADP, however, was not affected. Th is suppression was dose dependent where 3.6 and 3.3 mu M of 8-epi-PGF( 2 alpha) caused 50% inhibition of platelet aggregation induced by U466 19 and I-BOP, respectively, whereas 10 mu M caused approximately 72% i nhibition of collagen-induced aggregation. In contrast, 8-epi-PGF(2 al pha) significantly potentiated reversible platelet aggregation in resp onse to low-dose ADP. These results indicate that 8-epi-PGF(2 alpha) h as partial agonist activity. 9 alpha, 11 beta-PGF(2), a structural iso mer of 8-epi-PGF(2 alpha), inhibited platelet aggregation induced by c ollagen, high- and low-dose ADP and thrombin, demonstrating marked dif ferences between structural isomers where 9 alpha, 11 beta-PGF(2) inhi bited platelet aggregation induced by TX-dependent as well as TX-indep endent stimuli. In addition to platelet aggregation, we performed comp etition-binding assays on washed human platelets using [I-125]BOP to f urther investigate the interaction of 8-epi-PGF(2 alpha) and 9 alpha, 11 beta-PGF(2) with TXA(2)/PGH(2) receptors. We found that 8-epi-PG(2 alpha) was a relatively poor competitor of TX receptor binding sites c ompared with unlabeled I-BOP, SQ 29548 or U46619. The IC50 for 8-epi-P GF(2 alpha) and 9 alpha, 11 beta-PGF(2) to displace 0.05 nM [I-125]BOP was 3.3 and 14 mu M, respectively, which was approximately 1000 times less effective than unlabeled I-BOP (IC50 = 2.2 nM) and SQ 29548 (IC5 0 = 2.9 nM). A similar result was observed using a platelet membrane p reparation. Our results suggest that 8-epi-PGF(2 alpha) is a partial a gonist of TXA(2)/PGH(2) platelet receptors. Its structural isomer, 9 a lpha, 11 beta-PGF(2), is an antiaggregatory compound that works throug h a mechanism other than antagonism of TXA(2)/PGH(2) receptors.