How the Chloride Sweat Test in Cystic Fibrosis Works

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Mutations in the CFTR gene are responsible for cystic fibrosis.  CFTR encodes a chloride transporter which in addition to its being expressed in the lungs and pancreas, is also found in the kidney.  As I have blogged about previously, the CFTR chloride channel is responsible for chloride secretion in the human kidney, and may play a role in cyst expansion in individuals with polycystic kidney disease.  
It turns out the CFTR chloride channel also has some interesting interactions with ENaC:  the epithelial sodium transporter expressed in the collecting ducts of the kidney we all know and love.  Like CFTR, ENaC is expressed in a variety of tissues other than just the kidney.  It is also expressed in the skin and lungs, for example.  
In the skin, coupled ionic transport via CFTR and ENaC are responsible for normal sweat gland function; CFTR secretes chloride and the ENaC reabsorbs sodium from the skin.  In the sweat gland, CFTR has a stimulatory effect on ENaC.  Thus, in cystic fibrosis, when CFTR isn’t working, ENaC activity is also depressed, and sodium reabsorption from the skin is suboptimal.  This explains why patients with cystic fibrosis have sweat which tastes extremely salty, and provides the rationale for the chloride sweat test used in the diagnosis of cystic fibrosis.  
The CFTR/ENaC relationship is just the opposite in tissues such as the lung:   CFTR exerts an inhibitory effect on ENaC, and therefore in patients with cystic fibrosis, ENaC activity is significantly upregulated.  The abnormally high sodium uptake from airway mucus results in a thick, viscous, dehydrated substance which is retained in the airways and predisposes to pulmonary infections, the hallmark of cystic fibrosis.  

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