Vascular permeability of the renal medullary vessels in the mouse and rat.

The vascular permeability of the vessels of the renal medulla of normal rats and mice and potassium-deficient rats was studied by using ferritin, horseradish peroxidase and carbon particles as markers. Fenestrated endothelia are more permeable to ferritin and horseradish peroxidase than nonfenestrated endothelia. The ease with which the tracers are able to pass from the vascular lumina through the endothelia into the interstitium is directly related to the size of the tracers (horseradish peroxidase permeates most readily, followed by ferritin and carbon particles, in this order), suggesting the presence of a molecular sieving mechanism for the passage of substances through the capillary walls of the renal medulla. At the initial stage after injection of horseradish peroxidase and ferritin, these tracers pass from the medullary vascular lumina into the interstitium, not from the tubular lumina into the interstitium. It appears that the passage of ferritin and horseradish peroxidase through the vascular wall is enhanced in potassium-deficient rats.