The effect of endothelium-derived nitric oxide on ex vivo whole blood platelet aggregation in man.

The effects of changes in endogenous endothelium-derived nitric oxide (NO) on forearm blood flow and ex vivo platelet aggregation have been studied in 7 healthy volunteers. Measurements were made of forearm blood flow and ex vivo collagen-stimulated platelet aggregation during unilateral brachial artery infusions of saline, acetylcholine (ACh), NG monomethyl-L-arginine (L-NMMA), and prostacyclin (PGI2). The uninfused arm acted as a control. Forearm blood flow was increased by ACh, an agent which stimulates NO release, and decreased by L-NMMA, an agent which stereospecifically inhibits NO synthesis. Collagen-stimulated platelet aggregation measured ex vivo in whole blood draining the infused arm was unaltered by either ACh or L-NMMA. Conversely, PGI2, an agent which acts independently of NO, caused an increase in forearm blood flow which was accompanied by significant inhibition of platelet aggregation. The results suggest that release of endothelium-derived NO in quantities sufficient to cause substantial changes in blood vessel tone does not lead to changes in platelet aggregation in the blood flowing through the vessels. It is, however, still possible that endothelium-derived NO modulates platelet activity at the level of the endothelium.