Synergism between nitric oxide and hydrogen peroxide in the inhibition of platelet function: The roles of soluble guanylyl cyclase and vasodilator-stimulated phosphoprotein

In previous studies, a strong synergism between low concentrations of hydro gen peroxide and nitric oxide in the inhibition of agonist-induced platelet aggregation has been established and may be due to enhanced formation of c yclic 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. H2O2 was found to increase the phosphorylation of vaso dilator-stimulated phosphoprotein (VASP), increasing the amount of the 50-k Da 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 sub micromolar concentrations of the peroxide, which alone was not inhibitory t o platelets. These actions of H2O2 were inhibited to a large extent by an i nhibitor of cyclic AMP-dependent protein kinase, even though H2O2 did not i ncrease cyclic AMP. This inhibitor reversed the inhibition of platelets ind uced by combinations of NO and H2O2 at low concentrations. The results sugg est that the action on VASP may be one site of action of H2O2 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. st ool Academic Press.