"Mild" mutation of CFTR gene and sodium reabsorption? [UWorld q]

[worldbeater]

UWorld: QID 804

Why is the answer sodium reabsorption? It's not that clear to me from the explanation.

-6 y/o Caucasian patient presents with sinusitis since birth
-Negative sweat chloride test
-Nasal transepithelial potential difference taken - more negative then normal

Answers pts:
-CFTR gene should usually secrete Cl-, in this case its mutated, so it remains in a higher concentration in the tissue. This accounts for the "more negative than normal" test I am guessing.

-It indirectly increases Na+ reabsorption by "releasing inhibitory effect on the apical sodium channel" (this line I don't get)

Any help would be appreciated, thanks.

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[SBR249]

http://en.wikipedia.org/wiki/Epithelial_sodium_channel#Clinical_significance

CFTR directly interacts with ENaC to downregulate the latter's activity.

 

[worldbeater]

Awesome, makes sense now, thanks SBR249.

-CFTR gene should normally be secreting Cl (into the lumen of the airway)- ->>
-CFTR gene is not working properly, so no Cl- secretion -->
-activates epithelial sodium channel, located on top of the cell,
which increases Na+ reabsorption (from the lumen into the serum)

Didn't know there was a separate Wiki page for this type of channel.

 

[SBR249]

I wouldn't say activates ENaC. ENaC activity is repressed by CFTR.