Antiaggregatory activity of 8-epi-prostaglandin F(2α) and other F-series prostanoids and their binding to thromboxane A₂/prostaglandin H₂ receptors in human platelets

8-Epi-prostaglandin F(2α) (8-epi-PGF(2α)) is a nonenzymatic, free radical-catalyzed peroxidation product of arachidonic acid that has potent biological activity, including contraction of vasculature and inhibition of aggregation induced by thromboxane (TX) A₂ mimetics. In the present study, we demonstrate that 8-epi-PGF(2α) could inhibit 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 aggregation caused by high-dose ADP. The secondary (TX-dependent) wave of aggregation induced by high-dose ADP, however, was not affected. This suppression was dose dependent where 3.6 and 3.3 μM of 8-epi-PGF(2α) caused 50% inhibition of platelet aggregation induced by U46619 and I-BOP, respectively, whereas 10 μM caused approximately 72% inhibition of collagen-induced aggregation. In contrast, 8- epi-PGF(2α) significantly potentiated reversible platelet aggregation in response to low-dose ADP. These results indicate that 8-epi-PGF(2α) has partial agonist activity. 9α, 11β-PGF₂, a structural isomer of 8-epi- PGF(2α), inhibited platelet aggregation induced by collagen, high- and low- dose ADP and thrombin, demonstrating marked differences between structural isomers where 9α, 11β-PGF₂ inhibited platelet aggregation induced by TX- dependent as well as TX-independent stimuli. In addition to platelet aggregation, we performed competition-binding assays on washed human platelets using [¹²⁵I]BOP to further investigate the interaction of 8- epi-PGF(2α) and 9α,11β-PGF₂ with TXA₂/PGH₂ receptors. We found that 8- epi-PGF(2α) was a relatively poor competitor of TX receptor binding sites compared with unlabeled I-BOP, SQ 29548 or U46619. The IC₅₀ for 8-epi- PGF(2α) and 9α,11β-PGF₂ to displace 0.05 nM [¹²⁵I]BOP was 3.3 and 14 μM, respectively, which was approximately 1000 times less effective than unlabeled I-BOP (IC₅₀ = 2.2 nM) and SQ 29548 (IC₅₀ = 2.9 nM). A similar result was observed using a platelet membrane preparation. Our results suggest that 8-epi-PGF(2α) is a partial agonist of TXA₂/PGH₂ platelet receptors. Its structural isomer, 9α,11β-PGF₂, is an antiaggregatory compound that works through a mechanism other than antagonism of TXA₂/PGH₂ receptors.