Synergistic inhibition of thrombin-induced platelet aggregation by the novel nitric oxide donor GEA 3175 and adenosine

1 The influence of the novel nitric oxide-donor GEA 3175 on thrombin- and ionomycin-stimulated human platelets was investigated. The effect of GEA 3175 was compared with that of adenosine, an activator of platelet adenylyl cyclase. 2 GEA 3175 inhibited thrombin-induced secretion of ATP but did not affect aggregation; similar results were obtained with adenosine. 3 Thrombin-stimulated rises in the cytosolic free Ca2+ concentration, [Ca2+](i), were dose-dependently inhibited by GEA 3175 and adenosine. GEA 3175 and adenosine maximally reduced the initial rise in [Ca2+](i) by 41% and 35%, respectively. 4 Simultaneous exposure to GEA 3175 and adenosine nearly abolished both the functional responses (i.e. aggregation and degranulation) and the rises in [Ca2+](i) in thrombin-stimulated platelets. 5 Aggregation and increases in [Ca2+](i) triggered in platelets by the Ca2+-ionophore ionomycin were only marginally affected by a combination of GEA 3175 and adenosine. 6 GEA 3175 potently increased the guanosine 3':5'-cyclic monophosphate (cyclic GMP) content in platelets but did not affect adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. Adenosine did not increase either the cyclic AMP or the cyclic GMP levels in platelets. However, adenosine and GEA 3175 combined significantly elevated the platelet cyclic AMP content. 7 The results show that simultaneous exposure to GEA 3175 and adenosine promotes potent antiaggregatory properties in platelets in vitro. The findings suggest that blockage of the cytosolic Ca2+-signal, which is probably mediated by an amplified cyclic nucleotide response, is an important event during the synergistic inhibition of thrombin-induced aggregation.