- In isolated perfused rat lungs we have examined the effect of inhibition of receptor-mediated endocytosis and of intracellular acidic compartments activity on adsorptive and fluid-phase uptake and transport of macromolecules by capillary endothelium. Cationized and native ferritin were used as electron-dense tracers of adsorptive and fluid-phase transport, respectively. Luminal endothelial vesicles were labeled with tracers at 4 degrees C for 2 minutes and the density of ferritin particles in endothelial multivesicular bodies and in the capillary basement membrane, after a 60-minute chase with tracer-free perfusate at 37 degrees C, was determined by electron microscopic morphometry. Fluid-phase transport of native ferritin in 60-minute perfusions at 37 degrees C was also investigated. Adsorptive endocytosis was inhibited in chase experiments with hypertonic (800 mOsm) perfusate. Endothelial acidic compartments activity was decreased by adding either dextran sulfate or chloroquine to the chase media. We found that hypertonic solutions, dextran sulfate, and chloroquine inhibit cationized ferritin accumulation in multivesicular bodies and in the extravascular space. Hypertonic buffer or dextran sulfate have no effect on native ferritin uptake and transport in either pulse-chase or continuous perfusions. Since adsorptive or receptor-mediated mechanisms play a role in the transendothelial movement of plasma proteins, and other plasma constituents, our findings suggest a general role of the endothelial acidic compartments in modulating microvascular permeability to macromolecules.