cAMP derivates influence formation and dissociation of tight junctions in a calcium switch model. HUVEC were treated for 24 h with cAMP or its derivates. Calcium was either chelated by addition of 3 mM EDTA or medium was exchanged with calcium-free PBS. After 2 h cells were washed and cultivated for additional 4 h in EGM medium supplemented with the appropriate cAMP derivates. Cells were fixed and stained for CLDN5 (red) and ZO-1 (green). White arrows indicate specific loss of tricellular junctions. Shown are representative micrographs of three independent experiments. Scale bar represents 25 μm.