Effects of thromboxane synthetase inhibitors on aggregation of rabbit platelets

Thromboxane (TX) synthetase activity was selectively inhibited by (E)-3-[4-(1-imidazolylmethyl)phenyl]-2-propenoic acid hydrochloride monohydrate (OKY-046) and sodium (E)-3-[4-(3-pyridylmethyl)phenyl]-2-methyl-propenoate (OKY-1581) (OKYs). Their IC50 for the rabbit platelet enzyme were found to be 11nM and 3nM respectively. Arachidonic acid (AA) or collagen induced platelet aggregation, and generated TXA 2 and prostaglandins (PGs) in rabbit platelets. OKYs inhibited platelet aggregation and TXA2 generation without affecting PGs generation, while aspirin inhibited platelet aggregation, and TXA2 and PGs generation. There was a parallel relation between the degree of inhibition of platelet aggregation and TXA2 generation by OKYs, but the inhibitory effects of aspirin on platelet aggregation was related to that on both TXA2 and PGs generation. However, OKYs and aspirin did not inhibit ADP-induced platelet aggregation which did not involve the generation of TXA2 and PGs. These results suggested that TXA2 generation is related to platelet aggregation induced by AA or collagen, and that the inhibitory effect of OKYs on platelet aggregation is due to the inhibition of TX synthetase.