Defining the role of cyclic nucleotide-gated cation channels in adult lung fluid homeostasis.

In the womb, the baby?s lungs are filled with fluid. At birth, this fluid is removed so that the baby can begin to breathe. The process of removing this fluid involves the opening of tiny, specialised molecules known as channels. As the child grows to adulthood, these channels must stay open in order to prevent the lung filling up totally with fluid. If they do not, the lung begins to fill with fluid again and breathing becomes very difficult, finally resulting in serious danger. This is what often happens in: 1) diseases such as asthma, chronic bronchitis, cystic fibrosis, smoking-related illness and infections where normal liquid movement is dramatically altered and; 2) heart failure, when excess fluid collects in the lung very frequently. Although we know a lot about the channels which open at birth, we know very little about how they work in the adult lung. In fact, we are still not certain which ones are important or where exactly they are situated within the lung. We will answer these important questions using our new methods of studying these channels and their effects in small pieces of lung and in whole lung. For the first time, these studies will allow us to find out which channels are present, how they behave and which parts of the lung are most important. Why do we need to know this? This new knowledge will tell us how the adult lung normally works and will help us to understand what happens when it goes wrong. Increasing our detailed knowledge of how the lung normally deals with fluid has the potential to be applied to all the conditions mentioned above and our results will help doctors design ways of helping people to recover more quickly. This might be by the new use of medicines already available or by stimulating manufacture of new drugs which will target the mechanism discovered by our research work.

The transition from placental to atmospheric oxygen delivery within the lung at birth is contingent upon active transepithelial sodium transport and is critically dependent upon the amiloride-sensitive sodium channel. After birth, the amiloride-sensitivity of the reabsorptive driving force wanes with postnatal age, suggesting that amiloride-independent cation channels become increasingly important to efficient postnatal gaseous exchange. Classically, alveolar type II cells were considered the sites of active transport whilst alveolar type I cells were thought to play a passive role in the process. However, evidence is emerging which challenges this view and it is now clear that type I cells have the potential to contribute to the generation of the transepithelial driving force. The goal of the application is to use an integrated strategy (from single molecule to physiological system) to test the hypothesis that alveolar ion and fluid transport is driven by a conductive cation transport via a cell-specific combination of amiloride-sensitive and amiloride-resistant pathways and to answer four outstanding questions in the field:
1) What is the differential cellular expression profile of amiloride-resistant cation channel subunits within the adult alveolar epithelium?
2) What is the molecular identity of the adult alveolar amiloride-insensitive cationic current of alveolar type II cells?
3) What is the differential functional expression within each alveolar epithelial cell type of the key ion channels in lung?
4) What is the contribution of each of these cell-specific ion channels to adult alveolar fluid homeostasis?
We are in the unique position to answer these key questions since we can use molecular and cellular physiology to study isolated cells and cells in slices and to correlate the findings with direct assay of whole lung fluid dynamics. As such, this research will provide the exciting opportunity for a group of UK investigators to determine categorically the role of a specific channel family in important physiological and pathological situations. In so doing, it will ultimately lead to the further refinement of strategies aimed at alleviating life-threatening conditions characterised by perinatal and post natal lung fluid overload.