Cyclic AMP Raises Intracellular Ca2+ in Human Megakaryocytes Independent of Protein Kinase A

&NA; The immature megakaryoblastic cell line MEG‐01 responds to iloprost with an increase in cytosolic Ca2+ and cAMP. The Ca2+ response is almost absent in CHRF‐288–11 cells, but cAMP formation is preserved in this more mature megakaryoblastic cell line. Also, in human hematopoietic stem cells, iloprost induces a Ca2+ response and cAMP formation. The Ca2+ response is downregulated during megakaryocytopoiesis, but cAMP formation remains unchanged. The Ca2+ increase may be caused by cAMP‐mediated inhibition of Ca2+ sequestration, because it is (1) independent of Ca2+ entry; (2) mimicked by forskolin, an activator of adenylyl cyclase, and isobutylmethylxanthine, an inhibitor of phosphodiesterases; and (3) preserved in the presence of inhibitors of protein kinase A and inositol‐1,4,5‐triphosphate receptors. The small GTPase Rap1 has been implicated in the control of Ca2+ sequestration. Indeed, Rap1 activation parallels the iloprost‐ and forskolin‐induced Ca2+ increase and is unaffected by the calcium chelator 1,2‐bis(2‐aminophenoxy)ethane‐N,N,N′,N′,‐tetraacetic acid‐AM. These findings reveal a novel mechanism for elevating cytosolic Ca2+ by cAMP, possibly via GTP‐Rap1.