A site of action of dexamethasone on leukocyte extravasation in microcirculation

When leukotriene (LT) B4 (15 pmol/50 microliters) was applied topically over the microvasculature of the hamster cheek pouch, polymorphonuclear (PMN) leukocytes started rolling and adhering on the inner wall of the venule. Over 90% of the PMN leukocytes adhered disappeared from the vascular lumen, taking 7.3 +/- 0.6 min for disappearance. Then, the PMN leukocytes stayed in the space between the endothelial cells and the pericyte basement membrane, as ascertained by electron micrographs. The number, which could be counted in the space, increased to 2.7, 12.2 and 19.2 cells on the TV screen for 30, 60 and 90 min, respectively. Administration of dexamethasone (0.1 or 0.3 mg/kg, i.p.) two hours before the LTB4 application did not cause the reduction of the numbers of adhered leukocytes nor the rate of disappearance, but reduced the numbers of leukocytes present in the interstitial space. Thus, dexamethasone inhibited the step of the penetration of the pericyte basement membrane.