Differential role of actin in lung endothelial and epithelial barrier properties in perfused rabbit lungs

Lung fluid balance is critically dependent on capillary endothelial and alveolar epithelial barrier properties, and cytoskeletal components have been implicated in these barrier functions. In an earlier study, we perfused Clostridium botulinum C2 toxin, which effects selective loss of non-muscle F-actin, through isolated rabbit lungs: a severalfold increase in the capillary filtration coefficient (Kfc) was noted, together with attenuations and disruptions of endothelial cells upon electron microscopic examination. In this model we have investigated the influence of the C2 toxin on alveolar epithelial barrier properties. Epithelial permeability was assessed by continuous monitoring of the transepithelial passage of technetium-labelled diethylenetriamine penta-acetic acid (99mTc-DTPA), offered to the alveolar surface by aerosol technique. Intravascular administration of hydrogen peroxide, used as control agent, was shown to provoke a four- to fivefold increase in the clearance rate of 99mTc-DTPA under conditions of severe fluid leakage into the lung interstitial and alveolar space. Intravascular administration of C2 toxin caused a dose- and time-dependent increase in Kfc values (8-15 fold), but the Tc-DTPA clearance rate was entirely unaffected. Moreover, transbronchial application of C2 toxin again reproduced the manifold increase in Kfc data (about six fold), but the rate of transepithelial passage of the hydrophilic Tc-DTPA complex remained unchanged. We conclude that the barrier properties of the lung microvascular endothelial and epithelial layer are differentially regulated. It is suggested that the actin microfilament system plays a decisive role in the structural and functional integrity of the endothelial but not the epithelial barrier.