The barrier between blood and the surrounding interstitium, limiting the exchange of fluid and solutes, is provided by a single layer of endothelial cells that are coupled by intercellular junctions. Various pathological conditions in humans, such as edema, hemorrhagic stroke, and vascular malformations, have been, at least in part, caused by impaired endothelial barrier function and disruption of cell junctions.

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cAMP stabilizes the endothelial barrier, in part by activation of Rac1. However, the effects on cell junctions on the ultrastructural level are largely unknown. Here, in human dermal microvascular endothelial cells, we used forskolin/rolipram (F/R) to increase cAMP as well as O‐Me‐cAMP to stimulate Epac/Rap 1 signaling. Under both conditions, endothelial barrier properties were enhanced as detected by increased transendothelial electrical resistance (TER) which was paralleled by activation of Rac 1. Compared to controls, electron microscopy revealed a significantly higher frequency of complex intercellular junctions containing both adherens and tight junctions which may reflect the increase in VE‐cadherin and claudin 5 immunostaining intensity at cell borders. In contrast, cleft length and intercellular overlap were not significantly reduced on the ultrastructural level but adherens and tight junction immunostaining appeared to be lineralized indicating recruitment of adhesion molecules to and formation of junction strands within the intercellular cleft. Barrier stabilization and junction reorganization in response to O‐Me‐cAMP were completely abolished in the presence of NSC‐23766 which specifically interferes with Rac1 activation. These results indicate that Rac1‐mediated junction reorganization was required for Epac/Rap1‐mediated cAMP‐induced barrier stabilization. DFG SFB688, A4