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

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.