Rapid reversibility of nitric oxide induced platelet inhibition.

Nitric oxide is known to attenuate human platelet activation. Mechanistically this is achieved by stimulation of soluble guanylyl cyclase, followed by cGMP production, and concomitant protein phosphorylation. Although inhibitory actions of nitric oxide on various platelet parameters are well documented, considerably less information is available on the reversibility of this effect. In order to study the onset of proaggregatory signaling pathways after ceasing nitric oxide generation we used the nitric oxide-donor sodium nitroprusside in combination with cyanide. For sodium nitroprusside the generation of nitric oxide requires the release of cyanide prior to nitric oxide. Furthermore, the addition of exogenous cyanide blocks nitric oxide liberation from the nitric oxide-donor. Our data indicate, that the inhibitory potency of sodium nitroprusside on platelet aggregation, calcium mobilization, and a cGMP increase is reversed by cyanide addition. We put special attention to nitric oxide-mediated cGMP increase, followed by the extreme rapid cGMP degradation after cyanide administration followed by the onset of major proaggregatory signaling pathways. Our study aims at the physiological importance of a permanently active, probably shear stress induced nitric oxide-synthase in endothelial cells that functions as a negative thromboregulatory mechanism.