An experimental study of the penetration of polycyclic aromatic hydrocarbons through a model of the bronchial lining layer

Abstract

The penetration of benzo[a]pyrene (BaP) through a nonbiological experimental model of the bronchial lining layer (BLL) was studied. The purpose was to investigate how the lipid-aqueous structure of the BLL might influence the rate of penetration of polycyclic aromatic hydrocarbons (PAHs) from the ambient air to the bronchial epithelium. The experimental model was built up in a petri dish by (A) a thin layer of paraffin at the bottom, simulating the lipophilic membranes of the epithelial cells; (B) an aqueous starch gel on top of the paraffin, simulating the viscous aqueous region of the BLL; and (C) a thin layer of phosphatidylcholine, simulating the surfactant lipid layer at the air interface. BaP was administered on top of the barrier either diffusely or from a point source, and the penetration was studied by measuring the concentration of BaP as a function of time both in the liquid phase and in the paraffin. Comparisons were made with a purely aqueous barrier without the thin phospholipid layer. The results show that the rate of penetration of BaP through the purely aqueous barrier is orders of magnitude higher than that of the lipid-aqueous barrier. A thin layer of phospholipids at the air interface thus has a tremendous influence on the rate of penetration of lipophilic substances and probably this, rather than the release rate of PAHs from their carrier particles, is the rate-determining step in the overall transport of PAHs from such particles to the bronchial epithelium.