Synergism between Nitric Oxide and Hydrogen Peroxide in the Inhibition of Platelet Function: The Roles of Soluble Guanylyl Cyclase and Vasodilator-Stimulated Phosphoprotein

Abstract In previous studies, a strong synergism between low concentrations of hydrogen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of cyclic GMP. In this investigation, hydrogen peroxide and NO had no effect on the activity of pure soluble guanylyl cyclase or its activity in platelet lysates and cytosol. H 2 O 2 was found to increase the phosphorylation of vasodilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-kDa form that results from phosphorylation at serine 157 . This occurs both in the presence and in the absence of low concentrations of NO, even at submicromolar concentrations of the peroxide, which alone was not inhibitory to platelets. These actions of H 2 O 2 were inhibited to a large extent by an inhibitor of cyclic AMP-dependent protein kinase, even though H 2 O 2 did not increase cyclic AMP. This inhibitor reversed the inhibition of platelets induced by combinations of NO and H 2 O 2 at low concentrations. The results suggest that the action on VASP may be one site of action of H 2 O 2 but that this event alone does not lead to inhibition of platelets; another unspecified action of NO is required to complete the events required for inhibition.