Permeability characteristics of the epithelium in the rat caput epididymidis.

Micropuncture techniques were used to study the in-vivo transfer of radioactive compounds of different molecular weights across the epithelium of the rat caput epididymidis. Small molecular weight compounds such as 3H2O and [14C]urea equilibrated in less than 20 min and 2 h respectively after intravenous injection. Compounds of larger molecular weight ([3H]polyethylene glycol, [3H]inulin and 125I-labelled bovine serum albumin) entered the lumen more slowly and reached less than 5% of blood plasma concentrations 3 h after injection. The luminal concentration of [3H]inulin remained below 7% of the blood level for 18 h. These results demonstrate that a permeability barrier exists in the epithelium of the rat caput epididymidis.