Cyclic nucleotides inhibit MAP kinase activity in low-dose collagen-stimulated platelets

Abstract Collagen-induced platelet activation is a complex process involving multiple signaling pathways. The role(s) of MAP kinases (ERKs and p38MAPK ) are unclear, although at high, but not low, collagen concentrations p38MAPK is involved in cPLA2 -mediated arachidonic acid release, prior to thromboxane generation. Cyclic nucleotides are conventionally regarded as mediators of platelet inhibition. However recent studies suggested a role for cGMP early in a MAP kinase pathway in platelet activation. In the current study the roles and relationships of MAP kinases, cyclic nucleotides and cPLA2 in platelet activation by low-dose collagen and a thromboxane analogue (U46619) have been evaluated. Stimulants of neither adenylate cyclase (PGI2 ) nor guanylate cyclase (NaNP) alone had any effect on the basal phosphorylation of either MAP kinase. PGI2 inhibited ERK/p38MAPK phosphorylation in response to both agonists which was unaffected by a cPLA2 inhibitor (AACOCF3 ). NaNP inhibited collagen-induced ERK/p38MAPK phosphorylation, which was enhanced by AACOCF3 and reversed by a guanylate cyclase inhibitor (ODQ). However NaNP had no effect on U46619-induced p38MAPK phosphorylation. Thus adenylate cyclase activation inhibits low-dose collagen-induced MAP kinase phosphorylation both prior, and distal, to thromboxane release. The study also supports an inhibitory, rather than stimulatory, role for guanylate cyclase in platelet signaling.