Synergistic phosphorylation of platelet rap1B by SIN-1 and iloprost

Human platelets suspended in plasma or buffer were incubated with low concentrations of the nitric oxide (NO)-donor 3-morpholino-syndnonime (SIN-1; 100 nM to 1 microM) and the stable prostacyclin analogue iloprost (50 or 100 pM) and analyzed for cyclic nucleotide levels and protein phosphorylation. SIN-1 and iloprost synergistically stimulated the phosphorylation of rap1B and the 50 kDa vasodilator-stimulated phosphoprotein. SIN-1 stimulated platelet cyclic GMP and cAMP-levels and enhanced the increase in cyclic AMP elicited by iloprost. It was found that the mechanism underlying the synergistic phosphorylation of the 50 kDa protein and rap1B was different: synergistic phosphorylation of the 50 kDa protein seemed to be mediated by activation of both protein kinases A and G, whereas the synergistic rap1B phosphorylation could be attributed entirely to activation of protein kinase A. Measurement of rap1B phosphorylation might be a useful tool to monitor the action of systemically applied prostacyclin-analogues and nitrovasodilators in pharmacological studies.