The mechanism of escape of plasma protein from small blood vessels in the mucosa of the small intestine of the rat.

A study has been made in rats of the relative rates of escape of plasma protein, measured by accumulation of Evans Blue, and of the large marker particle, HgS, into uninjured small bowel and into an area of intradermal injection of histamine. The rate of escape of Evans Blue, per unit mass of tissue, into small bowel and into an area of histamine-stimulated skin were found to be almost the same, but leakage of HgS into the bowel was only about 1/10 of leakage of the same tracer into a site of intradermal histamine injection. If it be assumed, as is generally accepted, that all the protein that leaks from histamine-stimulated vessels does so via gaps in vascular endothelium that are large enough to allow escape of large marker particles like HgS, these findings show that only a small fraction of the protein that leaks into normal intestinal mucosa can escape via gaps, and that most of the leakage must occur by a route not permeable to particles of HgS. The findings give no indication of the nature of the alternative route for escape of protein.