EFFECTS OF PROTEASE INHIBITORS ON VASOPRESSIN TRANSPORT ACROSS RAT ALVEOLAR EPITHELIAL-CELL MONOLAYERS

The transepithelial transport of arginine vasopressin (AVP) across cultured rat alveolar epithelial cell monolayers was studied. At 0.1 nM donor [I-125]AVP, the radiolabel flux measured in the apical-tobasoiateral (AB) direction was about 10 times greater than that in the reverse (BA) direction. HPLC analyses of the basolateral receiver fluid collected at the end of these flux measurements showed that about 97% of total [I-125]label represented subspecies of AVP, whereas the apical receiver fluid contained largely intact AVP (similar to 85% of total [I-125]label). Both donor fluids contained virtually no degradation products of AVP (>99%). In the presence of an excess 0.1 mM unlabeled AVP in the apical donor fluid, the P-app for radiolabeled AVP in the AB direction was decreased by similar to 68%, while the fraction of intact AVP in the basolateral receiver fluid was increased six-fold as compared to that observed at 0.1 nM [I-125]AVP alone. Under this condition, the flux of intact AVP was approximately the same in both directions. When the concentration of apical camostat mesylate, an aminopeptidase inhibitor, was varied from 0 to 2 mM, the radiolabeled flux in the AB direction (with 0.1 nM [I-125]AVP in the donor fluid) was significantly decreased in a dose-dependent manner, yielding commensurably elevated concentrations of intact AVP in the basolateral receiver fluid. In contrast, leupeptin (0.5 mM), a serine protease inhibitor, was without effect. These data, taken together, suggest that apically-presented AVP undergoes proteolysis (most likely by peptidases localized at apical cell membranes of alveolar epithelium). It does not appear that intact AVP traverses the alveolar epithelium by saturable processes but primarily via passive diffusional pathways. Thus, the high bioavailability reported in previous studies on the pulmonary instillation and/or delivery via aerosolization of AVP is likely due to passive diffusion of the peptide utilizing the large surface area available in the distal respiratory tract of the mammalian lung. Furthermore, inclusion of appropriate protease inhibitor may increase the overall transport of intact AVP across the alveolar epithelial barrier.