Inhibition of ADP-induced platelet adhesion to immobilised fibrinogen by nitric oxide: Evidence for cGMP-independent mechanisms

Nitric oxide (NO) is an established regulator of platelet function, although the processes by which NO modulates platelet adhesion are unclear. We studied the importance of Ca2+ and phosphoinositol-3-kinase (PI3kinase) as targets for NO signalling, in the physiological context of platelet adhesion using adenosine diphosphate (ADP)-stimulated adhesion to immobilised fibrinogen. DPTA-NONOate induced a time and concentration-dependent inhibition of adhesion, and reduced protein tyrosine phosphorylation. The action of NO was cGMP-independent despite activation of the cGMP-signalling cascade, as evidenced by VASP phosphorylation. Furthermore, the cGMP-independent mechanism did not involve PKA. Platelet activation by ADP requires Ca2+ and PI3kinase-dependent signalling pathways. We examined the effect of NO on these pathways using two approaches. Firstly, we dissected the signalling pathways using the P2Y1-receptor antagonist A3P5P, and secondly, directly inhibited Ca2+ mobilisation and PI3kinase activity. ADP-induced adhesion was reduced but not abolished by A3P5P, suggesting signalling from P2Y12 can induce adhesion. NO further reduced adhesion in the presence of A3P5P, indicating that NO inhibited adhesion independently of any effects on Ca2+ mobilisation. Dimethyl bis-(o-aminophenoxy) ethane-tetraacetic acid (BAPTA) and wortmannin both partially inhibited ADP-induced adhesion, but completely abolished adhesion when used in combination, demonstrating that ADP-induced adhesion requires Ca2+ and PI3kinase-regulated pathways. Combination of either dimethyl-BAPTA or wortmannin with DPTA-NONOate enhanced inhibition of both the Ca2+ and PI3kinase-dependent pathways when compared to the levels of inhibition with either agent alone. Thus, we demonstrate that NO inhibits αIIbβ3-mediated adhesion, by targeting both Ca2+ and PI3kinase pathways in a cGMP-independent manner.