The ability of the beta-adrenergic agonist, isoproterenol, to attenuate the thrombin-induced increase in endothelial permeability was examined by measuring 125I-labeled albumin clearance across endothelial cell monolayers. Bovine pulmonary artery endothelial cells (CCL-209) were grown to confluence on gelatinized, polycarbonate micropore filters and mounted on modified Boyden chambers with Dulbecco's modified Eagle's medium (DMEM) and 0.5% bovine serum albumin. alpha-Thrombin at 0.2 nM to 2 microM produced a dose-related increase (P less than 0.01) in 125I-labeled albumin clearance from the DMEM control value. Light and electron microscopy revealed that the thrombin-induced increase in permeability correlated with changes in cell shape and rearrangement of filamentous actin. Coincubation of 2 microM isoproterenol with 2 microM alpha-thrombin reduced (P less than 0.01) the thrombin-induced increase in albumin clearance and the observed morphological changes. This attenuation was not caused by inhibition of thrombin's enzymatically active site, since isoproterenol did not impair thrombin's fibrinogen clotting activity nor its amidolytic cleavage of an artificial substrate (Spectrozyme-TH). Coincubation of 20 microM propranolol, a beta-adrenergic antagonist, with 2 microM isoproterenol and thrombin blocked the permeability-decreasing effect of isoproterenol. Both 2 microM isoproterenol and 2 pM alpha-thrombin alone decreased (P less than 0.01) albumin clearance below the DMEM control value. These results suggest that isoproterenol can reduce the thrombin-induced increase in endothelial permeability in vitro by directly maintaining actin filaments and the shape of endothelial cells.