Characterization of the l‐arginine: nitric oxide pathway in human platelets

Abstract

1 The activation of the l-arginine:nitric oxide (NO) pathway during aggregation of human platelets by adenosine 5′-diphosphate (ADP), arachidonic acid, thrombin and the calcium ionophore A23187 and its inhibition by N^G-monomethyl-l-arginine (l-NMMA), N^G-nitro-l-arginine methyl ester (l-NAME) and N-iminoethyl-l-ornithine (l-NIO) were studied. The inhibition of the cytosolic platelet NO synthase by these compounds was also examined. 2 Platelet aggregation induced by ADP (1–10 μm) and arachidonic acid (0.1–10 μm), but not that induced by thrombin (1–30 mu ml⁻¹) or A23187 (1–10 nm), was inhibited by l-, but not d-arginine (1–30 μm). However, in the presence of a subthreshold concentration of prostacyclin (0.1 nm) or of M & B 22948 (1 μm), a selective inhibitor of guanosine 3′:5′-cyclic monophosphate (cyclic GMP) phosphodiesterase, l-arginine caused concentration-dependent inhibition of aggregation induced by all of these aggregating agents. 3 l-NMMA, l-NAME and l-NIO (all at 1–30 μm), but not their d-enantiomers, enhanced to the same extent platelet aggregation induced by ADP, arachidonic acid and thrombin without affecting that induced by A23187. 4 In the presence of 300 μm l-arginine, the NO synthase in platelet cytosol was inhibited by l-NMMA, l-NAME and l-NIO with IC₅₀s of 74 ± 9, 79 ± 8 and 8.5 ± 1.5 μm (n = 3), respectively. 5 These results indicate that the l-arginine: NO pathway in human platelets plays a role in the modulation of platelet aggregation.