Measurement of albumin permeability across endothelial monolayers in vitro

We have developed an experimental system to measure the permeability of the cultured endothelial monolayer. The luminal-to-abluminal flux of ¹²⁵I-albumin across cultured pulmonary endothelium was expressed as a clearance rate equal to the permeability-surface area product. After clearance rate measurement for a 30-min base-line period, a test agent was added to the luminal side, and the clearance rate was remeasured during a 30-min experimental period. In control studies the base-line clearance rate was 0.343 ± 0.017 μl/min. After correction for the diffusional resistances of the filter and unstirred layers, the calculated permeability of the endothelial monolayer was 1.2 x 10^-5 cm/s. When culture medium was the test agent, the experimental clearance rate was unchanged from the base-line value. After addition of 4 mM oleic acid to the luminal chamber, the clearance rate was 0.528 ± 0.017 μl/min compared with a base-line value of 0.330 ± 0.008 μl/min (P < 0.005). This method allows the calculation of endothelial permeability with correction for unstirred layers and the use of each monolayer as its own control.