Cystic fibrosis is the most common, fatal genetic disease in the United States. It causes the body to produce thick, sticky mucus that builds up in the lungs and blocks the airways. This makes it easy for bacteria to grow and leads to repeated serious lung infections. The thick, sticky mucus can also block tubes in the pancreas, preventing digestive enzymes from reaching the small intestine.
The disorder results from mutations in the gene for the cystic fibrosis transmembrane conductance regulator (CFTR), a membrane channel regulator essential for proper salt and water movement across some epithelia. Currently, there are two essentially opposite explanations for the inability of the body to clear mucus from the airways in cystic fibrosis. The first is that the defective CFTR is unable to aid in fluid secretion in cystic fibrosis airway glands. The second explanation is that the glands still secrete fluid via non-CFTR pathways, but the fluid is reabsorbed by other channels. In fact, it has been proposed that one of CFTR's functions is to inhibit the activity of a channel called the epithelial Na+ channel (ENaC).
Nam Soo Joo and colleagues at Stanford University attempted to determine which hypothesis was correct by measuring the secretion from glands from patients with cystic fibrosis and from normal pigs. They added ENaC inhibitors to the glands to determine if the channel plays a role in mucus clearance. The researchers found no evidence that the inhibitors altered secretion rates in either normal or cystic fibrosis glands. This suggested that loss of CFTR-mediated fluid secretion is the culprit in cystic fibrosis.
"We previously showed that cystic fibrosis airway glands have defective gland secretion in response to certain drugs," explains Joo. "The results of our present study provide evidence that the defective cystic fibrosis gland secretion is not due to a potentially excessive fluid reabsorption pathway within glands but is due to most likely to a lack of fluid secretion from cystic fibrosis glands."