Aspirin does not inhibit human megakaryocyte thromboxane synthesis in vivo

Thromboxane antagonism in experimental canine carotid artery thrombosis

BACKGROUND AND PURPOSE

The two objectives of this study were to assess the potential of BAY U 3405 to prevent arterial thrombosis in response to vessel wall injury and to determine the ability of BAY U 3405 to prevent thrombotic reocclusion after thrombolysis with anisoylated plasminogen streptokinase activator complex.

METHODS

Dogs were instrumented with a carotid flow probe, stimulating electrode, and a stenosis. Current (150 microA) was applied to the intimal surface of the right carotid artery, and time to occlusive thrombus formation was noted. BAY U 3405 was administered, and the procedure for thrombus formation was repeated for the left carotid artery.

RESULTS

BAY U 3405 administration prevented occlusive arterial thrombosis formation. Ex vivo platelet aggregation was inhibited, bleeding time increased, and thrombus weight reduced after BAY U 3405 treatment. In a second group, thrombi were formed initially in both carotid arteries, BAY U 3405 was administered as before, and anisoylated plasminogen streptokinase activator complex was infused in the right carotid artery proximal to the occlusive thrombus. BAY U 3405 did not alter the incidence of rethrombosis compared with the lytic agent alone.

CONCLUSIONS

BAY U 3405 prevented primary arterial thrombosis, corresponding to inhibition of platelet aggregation, and increased bleeding times. BAY U 3405, however, did not prevent rethrombosis after successful thrombolysis with anisoylated plasminogen streptokinase activator complex, despite the fact that platelet reactivity was inhibited. The data are consistent with the concept that the residual thrombus represents a more effective thrombogenic stimulus as compared with arterial wall injury alone and that the mechanisms associated with primary versus secondary thrombus formation may require separate therapeutic approaches.

Influence of aspirin on human megakaryocyte prostaglandin synthesis

To evaluate whether currently popular aspirin regimens have an effect on the prostaglandin synthesis in human megakaryocytes we measured thromboxane B2 (TXB2) synthesis in response to thrombin stimulation in human megakaryocytes ex vivo. Human megakaryocytes were purified by Counterflow Centrifugal Elutriation from bone marrow punctures, taken from volunteers before and 2 hours after ingestion of one dose of 500 mg (n = 4), 80 mg (n = 4) or 40 mg (n = 2) aspirin. Subsequently, megakaryocytes were purified before and 12 h after ingestion of 80 mg (n = 3) aspirin twice daily for 1 week and 12 h after 500 mg (n = 3) aspirin. On average, 140 +/- 102 x 10(3) (mean +/- 1 SD) megakaryocytes were recovered. We found that aspirin inhibits megakaryocyte cyclooxygenase in a dose-dependent manner. Two hours after 500 mg of aspirin, TXB2 synthesis in megakaryocytes was inhibited by 96.8 +/- 2%, whereas one dose of 80 and 40 mg aspirin showed an inhibition of 79.4 +/- 13.7% and 80 +/- 6.2% respectively. However, the inhibition of TXB2 synthesis seems not to be long-lasting since, 12 h after the ingestion of aspirin, an increase of megakaryocyte TXB2 production could be observed which reached significance after the 500 mg aspirin dosage (p < 0.048). We conclude that human megakaryocyte cyclooxygenase is sensitive to aspirin inhibition and that low doses of aspirin (40 and 80 mg) enter the systemic circulation and are able to inhibit megakaryocyte cyclooxygenase, but this inhibition is incomplete and megakaryocyte cyclooxygenase seems to recover within 12 h after ingestion of aspirin.

In vitro bleeding test--a simple method for the detection of aspirin effects on platelet function

We investigated platelet function of 21 healthy blood donors before, 4 hrs and 1 to 7 days after a single oral dose of 0.02 (n = 3), 0.05 (n = 3), 0.1 (n = 3), 0.5 (n = 2) and 1.0 g (n = 10) aspirin. Three additional donors received 0.02 g aspirin/day for 5 days. A new and simple in vitro bleeding test (Thrombostat) using whole blood was far more sensitive than all other tests for platelet function (subaquatic bleeding time, thrombelastrogram, resonance-thrombogram, platelet adherence, spreading and aggregation). With this method using 2mM CaCl2 as additional agent all donors showed significantly increased in vitro bleeding volumes, for at least 2 days after ingestion of 0.1 to 1 g aspirin. In the majority of cases the aspirin effect could be detected even after 5-6 days. In 2 of 3 donors even 0.05 g aspirin was detectable. There was already a definite effect seen after 2 days in all 3 cases when 0.02 g was ingested daily. The new in vitro bleeding test should be suitable for the control of low dose aspirin prophylaxis of arterial thromboembolic disorders.