Regulation of endothelial permeability by Rac1

Increases in endothelial (EC) permeability are induced by stress fibers that are formed by cell‐spanning actin filaments that facilitate EC contraction. Of potential importance in this context, is the stress fiber‐inducing small GTPase, Rac1. However the permeability regulatory role of Rac1 is confusing, since in HUVEC monolayers expression of the constitutively active, or the inactive forms of Rac1, each increases permeability (Wojciak‐Stothard, JCS 114:1343‐55). To test this hypothesis in microvascular EC, we established monolayers of rat lung microvascular EC (RLMEC) expressing either vector alone, or the inactive form of Rac1, N17Rac1. A 10‐min exposure to the permeability agonist, H2O2 (100 μM) increased stress fiber formation in vector‐, but not in N17Rac1‐expressing EC monolayers, confirming the stress fiber‐enhancing role of Rac1. To determine barrier responses, we grew RLMEC monolayers in inserts and we quantified EC barrier properties in terms of the trans‐monolayer electrical resistance (TER). In control monolayers, a 10‐min exposure to H2O2 decreased TER (ohms.cm 2 ) from baseline of 30±3, by 5±2 (mean±SE; n=4; P<0.05). However in monolayers expressing N17Rac1, TER was similar to control at baseline, and it failed to decrease following H2O2 exposure. We conclude that in lung microvascular EC, Rac1 causes H2O2‐induced increase of EC permeability (support: HL36024).