Cystic Fibrosis - Summary of Article
It has been known for quite some time that CFTR is a cAMP regulated chloride ion channel whose mutation is intimately linked to cystic fibrosis (CF). However, the abnormality in the chloride ion channels is not the only biochemical peculiarity associated with the disease. Of particular interest to researchers is the finding that sodium ion (Na+) absorption in the cells of CF patients is both elevated and positively related to the cAMP concentration, whereas, in normal cells, cAMP concentration is inversely related to the rate of Na+ absorption. The malfunction of the Na+ pump becomes immediately apparent, for the primary symptom of CF is a chronic bacterial infection of the lungs, due to the abnormally low viscosity of the fluid which bathes the cellular epithelium; by pumping ions into the airway, rather than into the cell, a thermodynamically unfavorable concentration gradient of sodium ions is is established, in which the airway is hypotonic and osmosis is kinetically favored. In the case of CF, the local chloride pump is inoperative, and the sodium pumps are incredibly overactive, importing Na+ from the airway, and taking water out of the fluid inside the lungs in the process. Therefore, is is inappropriate to place the sole responsibility for the low fluid viscosity directly upon the mutation of the CFTR protein. Studies have demonstrated that the elevated Na+ absorption is not due to an overexpression of the Na+ transport protein, ENaC, in the membrane, nor can cAMP sensitivity be linked to a genetically mutated ENaC. The possibility that the altered CFTR indirectly led to the Na+ channel phenomena by altering the functionality of normal ENaC formed the basis of a hypothesis which was tested by M. Stutts, C. Canessa, J.Olsen, M. Hamrick, J. Cohn, B. Rossler, and R. Boucher (Science) in the experiment explained here.
In order to formulate a system of experiments which would be comprehensive to the study, the researchers first pooled all the knowledge available which was relevant to their topic. In addition to what was outlined above, they also noted that the Na+ absorption, via ENaC, was inhibited by the the presence of amiloride and its analogs, and that previous studies demonstrated that the same process was stimulated by the presence of forskolin in the CF cells. Furthermore, the three subunits which compose ENaC, alpha, beta, and gamma, function almost identically to the subunits of a similar transport protein found in rats, rENaC. Armed with this knowledge and considering the available techniques of experimentation, the scientists devised a stratagem which would determine whether or not the presence of CFTR in epithelial cells was responsible for regulating ENaC's function in the cell. Therefore, the researchers' ultimate goal was to study the behavior of ENaC in both the absence and presence of CFTR.
The first task at hand was to define a standard of measurement with which to...