Mechanisms of α‐thrombin, histamine, and bradykinin induced endothelial permeability

α‐Thrombin, bradykinin, and histamine are endogenous mediators that increase endothelial permeability. We examined the mechanism by which these three vasoactive mediators could alter permeability to albumin of human umbilical vein endothelial cells (HUVEC). HUVEC were grown to confluence on Transwell membranes and we monitored the flux of fluorescein isothiocyanate‐labeled human serum albumin across the membrane from the upper to lower chamber of the Transwell. Addition of α‐thrombin, bradykinin, or histamine increased the permeability coefficient of the HUVEC monolayer. At 30 min the permeability coefficient for α‐thrombin was 4.92 × 10−6 cm/sec while histamine was 4.47 × 10−6 cm/sec. Maximum changes in the permeability coefficient were about three‐fold control baseline values (1.59 × 10−6 cm/sec). There was also a temporal difference in the magnitude of the permeability coefficient. α‐Thrombin and bradykinin induced HUVEC permeability was increased for the first 90 min after which it returned to control levels. In contrast, histamine increased the permeability of the HUVEC monolayer throughout the 2 h experiment. To determine a possible intracellular mechanism of the altered permeability coefficients, HUVEC were labeled with FURA‐2 and intracellular calcium was monitored by digital fluorescence ratio imaging. Maximum intracellular calcium in HUVEC was increased by α‐thrombin (245 ± 20 nM) and histamine (210 ± 22 nM), but not by bradykinin (70 ± 7 nM) as compared to control (69 ± 10). Fluorescent photomicrographs of HUVEC stimulated with the three agonists indicated that α‐thrombin and histamine substantially altered HUVEC f‐actin arrangement, while bradykinin had no effect on HUVEC f‐actin distribution. These data support previous in vitro and in vivo studies demonstrating increased permeability by all three agonists. These data also show, for the first time, that histamine and α‐thrombin increased permeability by calcium‐dependent intracellular pathways, but bradykinin operates through a calcium‐independent mechanism. © 1996 Wiley‐Liss, Inc.