hgheith
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The defect is the transport of Cl- ions across epithelia, there are 2 mechanisms, but the one defective in cystic fibrosis is the cAMP-dependent mechanism (cAMP-dependent Cl- channels also called cysytic fibrosis transmembrane conductance regulator ie CFTR)
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| s_braga
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Ok, thanks. But, do you know what are the two mechanisms physiologicaly?
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| ssrpk
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different mechanisms at diferent locations
in sweat glands it absorbs Cl-
in respiratory epithelia it secrtes!
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| s_braga
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So, what happens in CF? Since CFTR is mutated, in sweat glands the patient LOSES Cl- and in epithelia, the patient RETAINS Cl-? And why do the mucus becomes viscous? And what has Na to do with the whole process?
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| hgheith
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This is what I got from robbins:
Normal epithelia, Cl- ions are transported across cell membrane by transmembrane proteins that form Cl- channels. (they are like gates that Cl- enters and leaves the cell by)
There are 2 types of Channels: 1- Ones that open by cAMP dependent pathway and 2- ones that are regulated by Ca2+ ions.
In CF, the CFTR channels are defective (cAMP mech) and so it's impermeable to Cl-, and the effect on transport function is tissue specific:
In sweat glands: Na+ and Cl- secreted into the lumen of the sweat gland are normal (Na+ is part of sweat) the problem is the epithelium that lines the sweat ducts has that mutation with the CFTR channels and so it's impermeable to Cl-, so reabsorption of Cl- and Na+ does not happen and that's how you get high concentrations of NaCl in sweat, called "salty babies"
In respiratory tract: Normally Cl- is secreted into airways through the cAMP channels, in CF the CFTR channels are mutated and so Cl- cannot be transported from the epithelium into the airway lumen. This indirectly causes increased absorption of Na+ and water from airway (and out of the mucus) into blood, the mucus then has lower water content and becomes dehydrated and viscus which obstructs the air passages, causes defective mucociliar acition, and recurrent pulm infections.
There is a nice diagram in robbins that helps understand the concept in ch 7 under genetic dz's
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