Lung disease in cystic fibrosis (CF) arises from obstruction and destruction of small airways (bronchioles). A fluid layer over the inner bronchiolar epithelial surface is normally the first line of airway defense against inhaled pathogens. Since fluid transport in CF is defective and bronchioles invariably become infected in CF, it is evident that proper fluid transport is essential to respiratory health. Despite the importance of this conclusion, small airways have largely evaded studies of their native fluid transport components because it is very difficult to obtain intact bronchiolar epithelium without destroying it. Recently, we successfully applied techniques to small airways that we developed much earlier for microperfusing single sweat duct tubules. We propose here to determine the properties of fluid transport in freshly isolated native, intact epithelia of bronchioles. In contrast to the common notion that the same cells both secrete and absorb, we now have evidence that this epithelium is made up of distinct secretory and absorptive cells. This proposal will define the previously undetermined basic fluid transport properties of small airway epithelia and test the novel hypothesis that in small airways some cells secrete while others absorb fluid. Three specific aims will use a combination of electrophysiological, RT-PCR gene expression assays, and immunocytochemistry to: 1.) optimize conditions and systems to preserve and examine bronchioles, 2.) determine basic absorptive and secretory properties of small airway epithelia, and 3.) show that there are functionally and structurally separate zones of absorptive and secretory cells in small airway epithelia. Lay Abstract: The genetic disease of cystic fibrosis (CF) is a rare form of obstructive lung disease in which the very small airways or bronchioles become so damaged and plugged with mucus that the patient dies. Normal bronchioles are hollow with a thin layer of fluid that coats their inner surfaces and helps remove inhaled debris, bacteria, and viruses. The basic problem in CF is abnormal fluid formation, which tells us that problems in the CF lung start in this fluid layer. Studies of small airways in their natural state are difficult, and very few have been attempted. We have applied a micro-method we used on small sweat gland tubes to study small intact bronchiole tubes. We propose to test a new finding that cells that secrete fluid are different from those that absorb fluid as opposed to the common idea that the same cells do both. This work may fundamentally change how we think about how normal bronchioles handle fluid and how bronchioles in CF and other forms of inflammatory lung diseases are destroyed. Correctly understanding this fundamental structure/function is critical to developing effective therapies.