Further insights into the anti-aggregating activity of NMDA in human platelets.

1. In the present study the effect of N-methyl-D-aspartate (NMDA) on thromboxane B2 synthesis and on [Ca2+]i was studied in human platelets. 2. NMDA (10(-7) M) completely inhibited the synthesis of thromboxane B2 from exogenous arachidonic acid (AA), while it did not interfere with the aggregating effect of the thromboxane A2 receptor agonist U-46619. 3. NMDA (0.1 microM - 10 microM) dose-dependently increased intracellular calcium in washed platelets preloaded with fura 2 AM, and this effect was not additive with that of AA. 4. NMDA shifted the dose-response curve of AA to the right. At the highest AA concentrations platelet aggregation was not inhibited. 5. The antiaggregating effect of NMDA was not antagonized by N(G)-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase (NOS) inhibitor. 6. Finally, NMDA (0.01 nM - 100 nM) associated with either aspirin or indomethacin significantly potentiated the antiaggregating activity of both cyclo-oxygenase inhibitors. 7. It was concluded that NMDA is a potent inhibitor of platelet aggregation and thromboxane B2 synthesis in human platelet rich plasma (PRP).