Ultrastructural observations on endothelial cell vesicles after horseradish peroxidase microinjection into blood and dextran perfused microvessels of rat mesentery.

Horseradish peroxidase (HRP) was injected for 5 sec into the arteriole that supplies the mesocecum in rats. This microperfusion was followed by fixation (with arrest of blood flow in 203 sec) either immediately or after an interval of 10 sec, 20 sec, 60 sec, or 120 sec and a diaminobenzidine cytochemical processing of the tissue was performed. The distribution of HRP was determined in the electron microscope. After immediate fixation HRP labelled 93% of luminal endothelial vesicles, 45% of cytoplasmic, closed vesicles and 4% of the abluminal ones but from 10 sec later virtually no labelling was seen. This finding seems best explained by a vesicular organization where most vesicles communicate with the extracellular space, directly or via other vesicles. However, a few observations of isolated labelled abluminal vesicles, even opposite endothelial cell nuclei already after 5 sec HRP exposure, indicate that classical vesicular transport in quanta also occurs. The same microvasculature was observed after 2 min artificial perfusion with 3% dextran 70 in Tyrode solution with or without 0.2% albumin followed by 5 sec HRP and fixation. In the absence of albumin HRP labelled vacuole-like vesicles and deep-penetrating clusters of vesicles in the endothelium. Moreover, isolated large vesicles and probably also vesicular conglomerates formed transendothelial channels. It is suggested that vesicular conformational changes might contribute to the permeability increasing effect of protein-free perfusion.