Proposed mechanism for the regulation of cAMP-mediated endothelial WPB exocytosis by myosin IIa. (A) In quiescent condition, basal myosin IIa with an activity of phosphorylated-S1943 maintains the tension of the FA-anchored actin cytoskeleton, resulting in a dynamic balance of WPB movement and the distribution of WPBs along peripheral stress fibers. (B) In contrast to A, inhibition of the basal activity of myosin IIa by blebbistatin leads to relaxation of the actin network, and subsequently an inward movement of WPBs to the perinuclear region. (C) On stimulation by cAMP agonist, myosin IIa with high activity of phosphorylation at S1916 promotes the movement of WPBs toward the plasma membrane and subsequent cargo release via an interaction with zyxin in the formation of an actin framework around exocytosing granules.