Role of the actin cytoskeleton in regulating endothelial permeability in venules

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 muM ML-7 treatment (35.3 +/- 5.9 x 10(-4); n = 224) was significantly lower than that for the thrombin-only treatment (61.7 +/- 5.6 x 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 x 10(-4) (n = 140) to 2.5 +/- 0.8 x 10(-4) (n = 383; p < 0.001), but 20 muM 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.