Phosphorylation of rap1B by protein kinase A is not involved in platelet inhibition by cyclic AMP

The functional consequence of cyclic AMP-dependent phosphorylation of rap1B for stimulus-induced platelet activation is not known. Platelets were pretreated with the stable prostacyclin-analogue iloprost and resuspended in plasma without iloprost. Western blot analysis showed that rap1B was completely converted into its phosphorylated form in the iloprost-pretreated platelets. Surprisingly, the platelets that contained phosphorylated rap1B were found to respond fully to activation by a wide variety of stimuli: aggregation upon stimulation by collagen, phorbol ester, vasopressin, ADP, epinephrine, and ATP-secretion from dense granules induced by collagen, thrombin-receptor activating peptide, vasopressin and phorbol ester were unchanged as compared to control. The results indicate that cyclic AMP-dependent phosphorylation of rap1B does not play a role in the inhibition of the various signal transduction pathways that lead to platelet aggregation and dense granule secretion.