Variations in capillary permeability from apex and crypt in the villus of the ileo-jejunum

The permeability of fenestrated capillaries in an organ is believed to be homogeneous. However, the permeability of fenestrated capillaries in different organs and to various exogenous tracers varies from a complete restriction, as found in the eye (Pino and Essner 1980, 1981; Pino 1985a) to the freely permeable peritubular capillaries of the kidney (Venkatachalam and Karnovsky 1972). In the present report we demonstrate that within any single intestinal villus from the ileo-jejunum of the rat, the permeability of fenestrated capillaries is not uniform. Exogenous hemoglobin (Einstein-Stokes radius [ESR] = 3.2 nm) exists all capillaries at any villar level in less than 5 min. In contrast, all villar capillaries restrict catalase (ESR = 5.2 nm) at 5 min, but by 60 min the tracer is present extravascularly in crypt and lower villar regions. Apical capillaries are slightly permeable to catalase at 2 h, but the bulk of the tracer remains in the lumina. The particulate tracer ferritin (ESR = 6.1 nm) is restricted 3-10 times more by apical capillaries than basal ones and is found in increasing concentration extravascularly at lower villar and crypt levels after 20 min. Following an 18-h circulation, a second dosage of ferritin is restricted by the endothelium at all villar levels. Immunocytochemical localizations of the plasma proteins albumin (ESR = 3.5 nm) and IgG (ESR = 5.5 nm) revealed an apparent lack of restriction at all villar levels. These results demonstrate that apical villar capillaries in the ileo-jejunum are more restrictive to exogenous molecules with ESR greater than or equal to 5.2 nm. Also, the passage of tracer molecules out of an endothelium alters the subsequent permeability of that vessel.