PF4 and pulmonary arterial hypertension: Platelet alphagranules store a number of important molecules that are typicallyelevated in PAH, including platelet factor-4 (PF4), PDGF, vascularendothelium growth factor (VEGF), TGF-β, p-selectin and von Willebrandfactor (vWF).21,22 Serotoninis stored in platelet-dense granules and is also implicated in thepathobiology of PAH.