Myosin II activity is required for cAMP-mediated formation of functional actin framework around WPBs before fusion and subsequent exocytosis. (A) Representative images showing VWF secretion with acute DMSO or blebbistatin treatment in the presence of forskolin in HUVECs coexpressing PSL-lum-mCherry (PSL-lum) and Lifeact-GFP (Lifeact). (B) Representative time-courses of actin framework formation in cells with acute DMSO or blebbistatin treatment. (C) Normalized fluorescence intensity (FI) of Lifeact-GFP around exocytosing WPBs (Ci) and of PSL-lum-mCherry (Cii) in DMSO- and blebbistatin-treated cells. (D) Assembly times for the actin framework as in B. ***P < .001; scale bars, 5 μm in A, 1 μm in E (mean ± standard deviation; Student’s t-test).