COMPARISON OF THE UPTAKE OF FINE AND ULTRAFINE TIO2 IN A TRACHEAL EXPLANT SYSTEM

To examine the relationship between particle uptake by pulmonary epith elial cells and particle size, we exposed rat tracheal explants to fin e particles (FPs; 0.12 mu m) or ultrafine particles (UFPs; 0.021 mu m) of titanium dioxide for 3 or 7 days. By electron microscopy, particle s were found in the epithelium at both time points, but in the subepit helial tissues, they were found only at 7 days. The volume proportion of both FPs and UFPs in the epithelium increased from 3 to 7 days; it was greater for UFPs at 3 days but was greater for FPs at 7 days. The volume proportion of particles in the subepithelium at 7 days was equa l for both dusts, but the ratio of epithelial to subepithelial volume proportion was similar to 2:1 for FPs and 1:1 for UFPs. Mean volume of individual particle aggregates was similar for both dusts at 3 days b ut was markedly smaller for FPs at 7 days. These observations suggest that the behavior of particles of different size is complex: UFPs pers ist in the tissues as relatively large aggregates, whereas the size of FP aggregates becomes smaller over time. UFPs appear to enter the epi thelium faster, and once in the epithelium, a greater proportion of th em is translocated to the subepithelial space compared with FPs. Howev er, if it is assumed that the volume proportion is representative of p article number, the number of particles reaching the interstitial spac e is directly proportional to the number applied; i.e., overall, there is no preferential transport from lumen to interstitium by size.