A quantitative study of vascular permeability to horseradish peroxidase, and the subsequent fate of the tracer, in rat brains after portocaval anastomosis.

Labelled blood vessel segments, exudation patches, and pericytic granular cells were counted in sections of rat brain which had been fixed and tested with DAB from 3 min to 24 h after intravenous injection of horseradish peroxidase (HRP). In normal rats, 3 min after injection, only 1% of the total blood vessel segments had walls penetrated by HRP, but in rats 14 days after portocaval anastomosis (PCA) 44% of the total segments had labelled walls, a frequency of 3033 permeable segments/mm3 of brain. In normal rats no exudation patches were found, but after PCA there were seven patches/mm3 of brain. Maximum vascular wall labelling and maximum exudation occurred in the cerebellum and medulla of the operated rats. Hardly any pericytic cells were labelled 3 min after injection. In operated rats 2 h after injection of HRP, labelled vascular walls were fewer and exudation had disappeared, but 204 pericytic cells/mm3 of brain contained HRP-labelled granules, i.e. 60% of the total pericytes. Highest numbers were found in the most anterior section of the brain sample. Later after injection these numbers declined. It is suggested that most of the protein which leaked into the brain after PCA returned to the blood whence it came, but that some was captured and hydrolysed by the granular pericytes.