Role of the Actin Cytoskeleton in Regulating Endothelial Permeability in Venules

ABSTRACT Objective: This study was performed to determine the effect of myosin light chain kinase (MLCK) inhibition on histamine‐ and thrombin‐induced venular permeability in the rat mesentery, coincidental with actin cytoskeleton changes. Methods: The mesenteric microvasculature of rats was perfused with a fluorescent tracer plus thrombin, histamine, or buffered saline, and the preparation was suffused with the MLCK inhibitor ML‐7. The microvasculature then was stained for actin. Results: The average (±SE) number of leaks per micrometer of venule length of the thrombin plus 5 µM ML‐7 treatment (35.3 ± 5.9 × 10−4; n = 224) was significantly lower than that for the thrombin‐only treatment (61.7 ± 5.6 × 10−4; n = 385; p < 0.001). The histamine preparations required higher concentrations of ML‐7 to significantly reduce the number of leaks. A concentration of 100 µM reduced the average leak number from 20.8 ± 3.9 × 10 4 (n = 140) to 2.5 ± 0.8 × 10−4 (n = 383; p < 0.001), but 20 µM ML‐7 had no effect. Although leaky areas of both the thrombin‐ and histamine‐treated preparations showed disruptions of the peripheral actin rim coincident with fluorescein isothiocyanate‐bovine serum albumin leaks, qualitative and quantitative differences were identified. Conclusions: The results suggest both similar and dissimilar mechanisms for thrombin and histamine regarding in situ endothelial gap formation.