Epithelial Sodium Channel - Clinical Significance

Clinical Significance

ENaC interaction with CFTR is of important pathophysiological relevance in cystic fibrosis. CFTR is a transmembrane channel responsible for chloride transport and defects in this protein cause cystic fibrosis, partly through upregulation of the ENaC channel in the absence of functional CFTR.

In the airways, CFTR allows for the secretion of chloride, and sodium ions and water follow passively. However, in the absence of functional CFTR, the ENaC channel is upregulated, and further decreases salt and water secretion by reabsorbing sodium ions. As such, the respiratory complications in cystic fibrosis are not solely caused by the lack of chloride secretion, but instead by the increase in sodium and water reabsorption. This results in the deposition of thick, dehydrated mucus, which collects in the respiratory tract, interfering with gas exchange and allowing for the collection of bacteria. Nevertheless, an upregulation of CFTR does not correct the influence of high-activity ENaC. Probably other interacting proteins are neccessary to maintain a functional ion homiostasis in epithelial tissue of the lung, like potassium channels, aquaporins or Na/K-ATPase.

In sweat glands, CFTR is responsible for the reabsorption of chloride in the sweat duct. Sodium ions follow passively through ENaC as a result of the electrochemical gradient caused by chloride flow. This reduces salt and water loss. In the absence of chloride flow in cystic fibrosis, sodium ions do not flow through ENaC, leading to greater salt and water loss. (This is true despite upregulation of the ENaC channel, as flow in the sweat ducts is limited by the electrochemical gradient set up by chloride flow through CFTR.) As such, patients' skin tastes salty, and this is commonly used to help diagnose the disease, both in the past and today by modern electrical tests.

The β and γ subunits are associated with Liddle's syndrome.

Amiloride and triamterene are potassium-sparing diuretics which act as epithelial sodium channel blockers.