Alveolar microvessels in isolated perfused dog lungs: structural and functional studies after production of moderate and severe hydrodynamic edema.

We have reported earlier that increased endothelial vesiculation follows the development of septal edema and alveolar flooding in isolated dog lung preparations. In this report, established ultrastructural morphometric analyses are coupled with data from physiologic and indicator-dilution studies to evaluate the stage of edema development at which de novo formation of alveolar microvessel plasmalemmal vesicles occurs. The interpretation that alveolar microvessel plasmalemmal vesicles increase prior to alveolar flooding, the final stage of edema formation, is consistent with the results reported here. Moderate pulmonary edema, characterized by substantial fluid cuffing around extra-alveolar arteries and veins and by fluid accumulation restricted to the thick sides of the alveolar septa, is associated with increased vesiculation in alveolar vessel endothelium. Further, a larger percentage of the vesicle population seen is directly attached to the endothelial luminal or abluminal surfaces. The functional significance of an increased population and an altered intracellular distribution of vesicles remains undetermined. The vesicles may provide a minor defense against excessive septal interstitial fluid accumulation, and subsequent alveolar flooding, by contributing to retrograde transport to the blood. Increased vesiculation, on the other hand, may represent an adaptive cellular response to interstitial fluid accumulation.