Studies on the importance of the proposed release of nitric oxide from platelets

The importance of the presence of the L-arginine (L-Arg)-nitric oxide (NO) pathway in platelet function was investigated. This was achieved by studying the interactions between anti-platelet agents such as glyceryl trinitrate (GTN) or iloprost (Ilo) and the NO precursor L-Arg. Thrombin or collagen-induced aggregation of human washed platelets was inhibited in a concentration-dependent fashion by a 3 min incubation with GTN (40-200 microM). This effect was reversed by oxyhaemoglobin (oxyHb, 10 microM). GTN at the lowest concentration tested (40 microM) potentiated the anti-aggregatory activity of subthreshold concentrations of Ilo (0.2 nM). GTN did not potentiate the action of L-arginine (100 microM) and L-Arg by itself failed to influence thrombin or collagen-induced platelet aggregation. The platelet responses to thrombin or collagen were not affected by a 3 min or 1 h treatment with the NO synthesis inhibitor NG-monomethyl-L-arginine (MeArg, 300 microM). This treatment did not alter the platelet inhibitory effects of GTN and in the presence of MeArg, exogenously applied L-Arg did not potentiate the anti-platelet activity of GTN. These results indicate that under our experimental conditions human washed platelets do not generate sufficient amounts of NO to modify (1) platelet aggregation or (2) the anti-platelet activity of GTN or iloprost.