Riboflavin-enhanced transport of serum albumin across the distal pulmonary epithelium

Purpose. Conjugation of bovine serum albumin (BSA) with riboflavin (BSA-riboflavin) increases its uptake into cultured epithelial cells. Our purpose was to determine whether transport of BSA-riboflavin across the intact distal pulmonary epithelium is also increased, and whether transcytosis plays a role. Methods. In anesthetized rats, we instilled ³H-BSA-riboflavin or ³H-BSA into the trachea and measured their appearance in blood. In isolated, perfused rat lungs we measured the distal pulmonary epithe lium permeability-surface area product (PS) for FITC-BSA or FITC-BSA-riboflavin. Results. In intact rats we found 2.1 times more ³H-BSA-riboflavin than ³H-BSA appeared in blood 60 min after intratracheal instillation of the protein. In isolated, perfused rat lungs we found that BSA-riboflavin had double the PS of BSA (2.63 vs. 1.46 x 10^-5 cm³/sec). The addition of transcytosis inhibitors monensin or nocodazole (both 3 x 10^-5 M) reduced the BSA-riboflavin PS to that of BSA and had no effect on the PS of unconjugated BSA. Simultaneous measurements of ³H-sucrose PS showed no differences in paracellular transport among any of the experimental groups. Conclusions. Conjugation with riboflavin increases the flux of BSA across the distal pulmonary epithelium. The increased transport appears to be due to transcytosis, which apparently does not play a significant role in the movement of unconjugated BSA across the distal pulmonary epithelium.