ENDOTHELIAL GAPS AND ADHERENT LEUKOCYTES IN ALLERGEN-INDUCED EARLY-PHASE AND LATE-PHASE PLASMA LEAKAGE IN RAT AIRWAYS

wExposure of sensitized individuals to antigen can induce allergic res ponses in the respiratory tract, manifested by early and late phases o f vasodilatation, plasma leakage, leukocyte influx, and bronchoconstri ction. Similar responses can occur in the skin, eye, and gastrointesti nal tract. The early-phase response involves mast cell mediators and t he late-phase response is leukocyte dependent, but the mechanism of le akage is not understood. We sought to identify the leaky blood vessels , to determine whether these vessels contained endothelial gaps, and t o analyze the relationship of the gaps to adherent leukocytes, using b iotinylated lectins or silver nitrate to stain the cells in situ and M onastral blue as a tracer to quantify plasma leakage. Most of the leak age occurred in postcapillary venules (<40-mu m diameter), whereas mos t of the leukocyte migration (predominantly neutrophils) occurred in c ollecting venules. Capillaries and arterioles did not leak. Endothelia l gaps were found in the leaky venules, both by silver nitrate stainin g and by scanning electron microscopy, and 94% of the gaps were distin ct from sites of leukocyte adhesion or migration. We conclude that end othelial gaps contribute to both early and late phases of plasma leaka ge induced by antigen, but most leakage occurs upstream to sites of le ukocyte adhesion.