Drug transport across Xenopus alveolar epithelium in vitro

Prostaglandin E2 induces upregulation of Na+ transport across Xenopus lung epithelium

The apical mucus on pulmonary epithelia is not only critical for physiological functions such as gas exchange or inflammatory processes, but also contains surfactants and multiple molecules that mediate cellular responses. A tight control of transepithelial ion transport maintains viscosity of this layer and, e.g., the amiloride-sensitive sodium channels (ENaCs) in lung epithelia of vertebrates are the most important regulatory sites for transcellular sodium uptake. Dysfunction of this sodium transport results in reduced liquid absorption and causes massive problems with gas exchange. We used dissected lungs of Xenopus laevis in Ussing chambers to investigate the influence of prostaglandin E2 (PGE2) on the regulation of short-circuit current (ISC) and amiloride-sensitive sodium absorption (Iami). Apical application of PGE2 (1 microM) increased ISC by 38% and Iami by approximately 60%. In contrast, a different prostaglandin, PGI2, neither affected ISC nor Iami. Forskolin increased current to a similar magnitude and preincubation of the lung with an RP-isomer of cyclic AMP, an inhibitor of protein kinase A (PKA), abolished the effects of both PGE2 and forskolin. Transepithelial Na+ uptake was also upregulated by the prostaglandin receptor agonists misoprostol and sulprostone. The Iami in Xenopus oocytes that heterologously expressed ENaCs was not affected by PGE2.

An in vitro pulmonary permeation system with simulation of respiratory dynamics

To study the effect of respiration on transpulmonary permeation kinetics of drugs, an in vitro pulmonary permeation system, which consists of a setup for the simulation of respiratory dynamics, was developed. The system is composed offour major components: a pair of horizontal-type half-cells, a model air-blood barrier, an instrument for the application and regulation of respiratory pressure, and a pressure monitoring system. Calibration studies were performed and results showed that the primary respiration parameters (the peak inspiration pressure, respiratory frequency, and the percent inspiration time) can be controlled at a reproducible manner. This system appears to simulate very well the respiratory dynamics observed normally under physiologic conditions. After calibration, the system was utilized to characterize and quantitate the effect of respiration on the transpulmonary permeation of drugs using progesterone as the model drug. The results showed that progesterone permeability is increased as much as 1.8-5.6 folds by application of a respiratory pressure, depending on the combination of respiration parameters. Further studies demonstrated that the enhancement in pulmonary permeation triggered by respiratory pressure is resulted from the stretching of the lung tissue, not by the pressure gradient itself. The observations lead to the conclusion that the system developed in this investigation is a useful in vitro tool for studying the kinetics of pulmonary drug permeation under a physiologically simulating respiratory dynamics. The studies have provided scientific evidence for demonstrating that respiration is an important factor in determining the kinetics of transpulmonary drug permeation through possible alteration in the properties of the air-blood barrier.