13-Azaprostanoic acid (13-APA), a specific thromboxane/endoperoxide receptor antagonist, reverses platelet aggregation stimulated by the endoperoxide analog U46619. The present report demonstrates that 13-APA also potentiates prostacyclin (PGI2) reversal of U46619-induced aggregation. Human platelet rich plasma was aggregated with 3 × 106M U46619. Deaggregation was induced 2 min. sitosequent to the addition of aggregating agent and was measured over a 3 min. period. Concentrations of 13-APA (4 × 10-4M) and PGI2(4 × 10-9M) were chosen such that each agent individually induced approximately 20% deaggregation. Addition of half of the above concentrations of these agents i.e. 2 × 10-4M 13-APA plus 2 × 10-9M PGI2resulted in 62% deaggregation, demonstrating that the observed response was supraadditive. Only 8% deaggregation was induced by 2 × 10-4M 13-APA alone and 0% by 2 × 10-9M PGI2 alone. PGI2 causes platelet deaggregation presumably through elevation of cAMP. 13-APA, however, did not increase cAMP levels even at concentrations of 13-APA as high as 1.2 × 10-3M i.e. 9.8 ± 1.3 pmoles/ml for control and 10.8 ± 1.2 for 13-APA. Nevertheless it is possible that the observed potentiation of deaggregation was the result of 13-APA facilitating PGI2 stimulation of adenylate cyclase. Measurement of cAMP during deaggregation, however, showed no significant difference between treatment with PGI2 alone and treatment with PGI2 plus 13-APA i.e. 11.3 ± 0.4 pmoles/ml for control, 11.4 ± 0.3 pmoles/ml for 13-APA, 16.1 ± 0.5 pmoles/ml for PGI2 and 16.5 ± 0.8 pmoles/ml for PGI2 plus 13-APA. These results clearly establish that 13-APA and PGI2deaggregate platelets by distinctly separate mechanisms. In this regard we propose that PGI2 causes platelet deaggregation by stimulating intraplatelet calcium sequestration through a cAMP dependent process. 13-APA, on the other hand, blocks the ability of U46619 to mobilize intraplatelet calcium. The combination of these two mechanisms presumably results in the observed potentiation of deaggregation.
Cangrelor (ARC69931MX) produces global inhibition of human platelet function by increasing cAMP levels through a Gi‐independent mechanism.