Phallacidin prevents thrombin-induced increases in endothelial permeability to albumin

Calf pulmonary artery endothelial monolayers cultured on polycarbonate filters were utilized to study 125I-labeled albumin permeability and actin filament distribution in response to thrombin challenge. Thirty-minute exposure to alpha-thrombin (10(-7) M) significantly increased albumin clearance rates. These changes were associated with marked alterations in actin filament distribution, resulting in loss of peripheral actin bands and an increase in the number of cytoplasmic stress fibers. Because the actin peripheral filaments are thought to play an important role in junctional stability, we postulated that stabilization of actin filaments should protect against thrombin-induced barrier disruptions. Pretreatment of cells with 0.3 microM 7-nitrobenz-2-oxa-1,3-diazole (NBD)-phallacidin, a specific actin-stabilizing agent, prevented the changes in actin filament distribution and markedly attenuated the increase in albumin permeability. Because of the potential toxicity of phallatoxins, we evaluated the effects of pretreatment on cell viability and growth parameters. There were no differences in viability, seeding efficiency, or doubling times in cells treated with 0.3 microM NBD-phallacidin in comparison to controls. Our data support the hypothesis that actin filaments, particularly peripheral bands, contribute significantly to the maintenance of barrier function in cultured endothelial cells.