Salty sweat

Question

Dear,

I would like to know why the sweat of a CF patient is saltier. I do understand the function of the CFTR protein. But that is just my problem. A CF patient has no chloride transport out of the cell. How comes it the sweat is saltier?

Thank you in advance!
Best regards,

Answer

Dear B.,
This is a very good question.
As you know, the CFTR channel is a channel through which salt (chloride ions) flows. In the lung, the CFTR protein allows flow of chloride ions from within the cell to the outside. As a consequence of that also sodium and water flow to the outside. That way the mucosal membranes are kept moist. When the CFTR channel is dysfunctional or absent, less chloride is transported and also less sodium and water goes to the outside. Thereby the mucosal membranes are dryer and the secretions are thicker and stickier.
In the sweat gland the opposite occurs. Here the CFTR channel (still transporter for chloride) is routed the other way around. First sweat is being produced in the sweat coil and then most of the salt content is being removed out of the sweat. This happens in the sweat duct through the CFTR channel. When the CFTR channel is not functional more chloride stays behind in the sweat and also more sodium gets lost. That is why the sweat of CF patients tastes saltier. Therefore it is important that patients with CF take extra salt on hot days or when they exercise intensely.

The sweat test still remains the most important test to make the diagnosis of CF. Sweat is collected after the stimulation of the sweat gland. The sweat of patients with CF contains a higher amount of chloride and sodium than the sweat of normals. Sweat chloride value higher than 60 mmol/L are a very reliable indicator of cystic fibrosis.

M Boon, K De Boeck

04.08.2011