Mechanisms Regulating Lung Epithelial Barrier Function in Sepsis
Funding:
Department of Veterans Affairs, Merit Review
Principal Investigator:
Project Summary:
Septic lung disease is an often fatal complication of numerous clinical conditions, and it affects a significant percentage of the VA patient population. Septic lung disease is characterized by a gross cellular damage to the lung epithelium. This degrades the barrier functions of the alveolus, which allows blood components, including red cells and plasma, to enter into the alveolar air space. The presence of these materials in the air space is associated with numerous ARDS-like phenomena, including interstitial fibrosis, type-II cell proliferation, and hyaline membrane disease. It is not known why these components remain trapped within the air space, but it appears to be a consequence of alterations in the barrier properties of the alveolar septum. It is also a consequence of inhomogenous perfusion of edematous alveoli; blood components cannot be removed from alveoli that are not perfused.
The goals of our application are to determine how sepsis affects these mechanisms, specifically, how sepsis alters both the barrier properties of the alveolar epithelium and interalveolar perfusion distribution. These are subjects about which nothing is currently known, and we will address these gaps in our knowledge by using new methods developed in our laboratory. We recently quantified the barrier properties of the uninjured lung by using hetastarch as a macromolecular tracer. Hetastarch has a broad molecular weight range (1E3 - 1E7 daltons) that makes it ideal for this purpose. We inflated isolated lungs with hetastarch (Het), then measured the subset of Het molecules that crossed the epithelial barrier and entered the lung perfusate. From these data, we were able to calculate the permeability of the lung epithelial barrier for the first time. We found this permeability to be 1/1000 that of systemic capillaries. This explains why the epithelial barrier is normally impermeable to molecules such as albumin that become trapped in the alveolar air space in conditions such as non-septic (cardiogenic) pulmonary edema. However, we want to know how this permeability is affected by sepsis. We also discovered that the cells that compose the epithelial barrier are capable of removing macromolecules from the air space by endocytosis. We found that particles of colloidal gold instilled into the air space (6 nm diameter) were present in vesicles in both the type I and type II alveolar epithelial cells. This suggests that albumin in edema fluid (7.5 nm diameter) may be removed by this mechanism, although the rate at which this occurs is not known. It is also not known how this process is affected by sepsis. Our results are the first to demonstrate vesicular uptake by type I cells, which compose 90% of the alveolar epithelial surface area. We recently developed a method that allows us to measure perfusion distribution among alveoli. Our method is to infuse small diameter fluorescent latex particle (4 µm diameter) into the pulmonary circulation, then use confocal microscopy to map the trapping patterns of these particles. Statistical analysis of these patterns allows us to quantify perfusion distribution. Using this method, we recently showed that perfusion distribution among alveoli is altered by vasoactive substances such as prostaglandins. This suggests that vasoactive substances present during sepsis may have a similar effect.
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General Surgery
University of Wisconsin Department of Surgery
First published: 07/15/02 Last updated: 07/05/08 webmaster@surgery.wisc.edu
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