Capillary permeability to interstitial microinjections of macromolecules and influence of capillary hydrostatic pressure on endothelial ultrastructure.

The transcapillary flux of horseradish peroxidase (HRP, mol. diam. 50--60 A) and ferritin (mol. diam. 110--120 A), microinjected interstitially into the biceps femoris muscle of rats, was analyzed in the electron microscope. From 1 min postinjection HRP was observed to occupy endothelial plasmalemmal vesicles in all positions to be expected if a vesicular transendothelial transport of material occurs in the interstitium-to-lumen direction. Permeation of HRP through the intercellular clefts of the endothelium could not be demonstrated unequivocally. The periendothelial basement membrane appeared to constitute a significant permeability barrier against ferritin. The endothelial ultrastructure in muscle capillaries was analysed morphometrically after artificial perfusions of rat hindquarters at venous outflow pressures of -4 cm H2O and +10 cm H2O. In the "high pressure" material the endothelium was thinner and the frequency of vesicles open towards the interstitium was lower than after perfusion at low outflow pressure. The findings may indicate that net transendothelial transport via vesicles varies with hemodynamics.