ROLE OF ADENOSINE UPTAKE AND METABOLISM BY BLOOD-CELLS IN THE ANTIPLATELET ACTIONS OF DIPYRIDAMOLE, DILAZEP AND NITROBENZYLTHIOINOSINE

Adenosine (Ado, 10 .mu.M) did not inhibit ADP-induced human platelet aggregation in whole blood. However, if the blood was preincubated with dipyridamole (10 .mu.M), a potent inhibitor of the erythrocytic nucleoside transport system (NTS), Ado acted as a strong inhibitor of platelet aggregation. Similarly, Ado inhibited platelet aggregation in whole blood in the presence of other potent NTS inhibitors, dilazep (1 .mu.M) and p-nitrobenzylthioinosine (NBMPR, 1 .mu.M). RA 233 (10 .mu.M), an analog of dipyridamole which is a potent inhibitor of platelet cAMP phosphodiesterase (PDE), did not evoke the Ado effect in whole blood. However, in platelet-rich plasma (PRP), RA 233 potentiated strongly Ado-mediated inhibition, whereas dipyridamole, dilazep, dilazep and NBMPR were without activity. 5''-Methyl-thioadenosine (MTA), an Ado receptor antagonist, reversed the inhibition produced by a nucleoside transport system inhibitor plus Ado in whole blood. Dipyridamole (10 .mu.M), dilazep (1 .mu.M) or NBMPR (1 .mu.M) blocked [14C]Ado (10 .mu.M) uptake by blood cells in whole blood, whereas RA 233 (10 .mu.M) was not effective. The combination of 2''-deoxycoformycin (dCF, 5 .mu.M), a tight-binding inhibitor of adenosine deaminase (ADA), plus 5-iodotubercidin (ITu, 10 .mu.M), a potent inhibitor of adenosine kinase (Ado kinase), gave comparable Ado-mediated inhibition of platelet aggregation in whole blood as was obtained when the blood was pretreated with dilazep. These studies suggest that the in vivo antiplatelet actions of drugs such as dipyridamole and dilazep results from their abilities to block erythrocytic Ado uptake and subsequent metabolism, thus elevating the extracellular steady-state contentration of the physiologically occurring, antiplatelet agent, Ado.