Abstract
Antiplatelet and antithrombotic effects of a thromboxane (TX) A2 receptor antagonist, ONO-3708, which does not alter prostaglandin generation, were studied to investigate the role of TXA2 in thrombogenesis. TXA2 synthesis of washed platelets stimulated with 1mM arachidonate was perfectly inhibited by 10μM dazoxiben, a TXA2 synthetase inhibitor, but never by 10μM ONO-3708 when quantitated with high-performance liquid chromatography. Ten μM ONO-3708 perfectly inhibited platelet aggregation (PA) to 1mM arachidonate and 1μM STA2, a stable analogue of TXA2, in not only 5 responders but also 5 non-responders to dazoxiben on PA to arachidonate out of 10 normal volunteers. These results indicate that the difference between responders and non-responders to TXA2 synthetase inhibitors is not attributable to the difference in TXA2 receptor among them.
Threshold concentration of STA2 to induce irreversible PA (0.52±0.40μM) was significantly (p<0.01) lower and IC50 of ONO-3708 on PA to 1μM STA2 (1.78±1.03μM)was significantly (p<0.01) higher in 24 patients with cerebral ischemia than in 10 normal controls (0.96±0.12μM and 0.40±0.21μM, respectively) or 10 patients with neurological disorders except for stroke (patient controls; 0.90±0.18μM and 0.45±0.30μM, respectively). It is postulated from these results that there is an abnormality in TXA2 receptors of platelets in patients with cerebral ischemia.
Secondary PA to ADP, epinephrine and PAF, which was not influenced by 10μM dazoxiben, was inhibited by 10μM ONO-3708. This might imply the role of TXA2 receptor on secondary PA to these agonists. Synergistic effect of ONO-3708 on PA to these agonists when added with dazoxiben was greater in whole blood, where prostacyclin is generated by leukocytes, than in platelet rich plasma. PA to collagen was perfectly inhibited by 10μM ONO-3708 alone. Filter bleeding time (Uchiyama and Didisheim), an in vitro test of shear-induced PA, remained unchanged by 10μM ONO-3708 or dazoxiben alone, but significantly (p<0.05) prolonged from 210±158sec to 325±178sec by ONO-3708 when added with dazoxiben. These results indicate that shear-induced PA is reduced when multiple pathways leading to PA are inhibited but not when only arachidonate-TXA2 pathway is inhibited.
