Kinetics and mechanisms of macromolecular disposition in the rat lung.

摘要

The kinetics and mechanisms of macromolecular absorption from the airways of the rat lung were studied in vivo and in vitro, following identical solute administration methods. The isolated perfused rat lung (IPRL) was used as an in vitro model, and the disposition of 7.4 and 4.3 kDa fluorophore-labeled polyhydroxyethylaspartamide (F-PHEA) and other small and macromolecular reference solutes was investigated across doses and in the presence of a variety of biochemical inhibitors.;Absorption profiles of F-PHEA, and the solute's lung distribution at different times after administration, were statistically identical in vivo and in vitro, showing that the IPRL possessed a viable mucociliary escalator and that macromolecular solute absorption rates from the IPRL were predictive of those in vivo. A kinetic model of the IPRL incorporating mucociliary clearance alongside active and passive absorption was developed and validated. This model was employed successfully to analyze each solute's absorption components from the rat lung following simultaneous, across-dose, nonlinear regression analysis of airway-to-perfusate absorption data. 7.4 and 4.3 kDa F-PHEA absorption from the pulmonary lung compartment occurred actively via the polyaspartamide transporter with values for Vmax,P and Km,P of 4.37/3.60 mug/min and 56.6/76.8 mug, respectively. In contrast, the magnitude of F-PHEA's passive absorption component, ka,P, was inversely related to its molecular weight, consistent with the absorption of a small proportion of each solute dose passing by restricted diffusive transport through tight junctions in the pulmonary epithelium.;The active component of 7.4 kDa F-PHEA absorption, which enhanced airway-to-perfusate transfer at low doses, was significantly inhibited in the IPRL at lowered temperature (25°C; 68.4%) and in the presence of 1.0 mM 2,4-dinitrophenol (53.3%), 100 muM ouabain (75.8%), 30.0 muM monensin (66.0%) and 30.0 muM nocodazole (68.4%). This suggested that polyaspartamide-transporter was dependent on ATP-derived energy, and probably employed an intracellular vesicular transcytotic mechanism. The airway-to-perfusate transfer of 376 Da fluorescein and 4.4 kDa fluorophore-labeled dextran had no active (dose-dependent) absorption component. However, each of these solutes showed values for solute's molecular weight-dependent passive absorption and solute-independent mucociliary clearance, when compared to those for F-PHEA.