CFTR and salt secretion Flashcards
What is a good model system for salt secretion
The shark rectal gland because it secretes a hige amount of chloride and is very robust so lasts a long time.
What channel blockers were used to show the first observations of Cl secretion
If NaKATPase is blocked by ouabain, chloride secretion is prevented.
If barium is used to block K channels then chloride secretion is lost (Both channels set up a driving force for chloride secretion)
Furosemide blocks NKCC and reduces chloride secretion.
What is the model for chloride secretion
NaKATPase maintains a low intracellular conc of Na
The basolateral K channel sets up a negative Vm
The low Na and -ve Vm mean there is a driving force for the uptake of Na via NKCC1
Na brings Cl and K into the cell with it. The K is recycled back but the Cl accumulates in the cell.
Chloride channels in the apical membrane secrete chloride from the cell.
How many TMDs has CFTR got
12
How many nucleotide binding domains has CFTR got and what mutation is associated with the first nucleotide binding domain
2 and DeltaF508 - the most common CFTR mutant
What are the 6 classes of CFTR mutant
- Null production
- Trafficking - channels are made but not trafficked, often sent for degredation (DeltaF508)
- Regulation - Protein is made and trafficked to the membrane but isn’t regulated properly when there. Stimulatory pathways don’t work
- Conduction - Issue with conduction through the pore, no ions are able to move through
- Partial reduction - there is loss in mRNA - some CFTR makes it but not enough to support normal cellular function
- High turnover - Resides in the membrane but has a short half life.
What is the model for Cl secretion in the lower to midcrypt cells of the colon
Basolateral NaKATPase, K and NKCC1 bring chloride into the cell and are able to be secreted by CFTR, stimulated by cAMP.
Also an apical K channel which provides a strong drivinf force for Cl secretion (Stimulated by Ca2+)
To activate the pathways for CFTR and K, prostaglandin E2 can be used. PGE2 acts on the PG receptor which induces a rise in cAMP, this stims CFTR and NKCC1 increasing Cl secretion.
OR.. Ach added binds its receptor and increases calcium intracellular conc. This stimulates K channels again increasing Cl secretion.
What is carbachol
An Ach receptor agononist - used experimentally to raise intracellular calcium
What is Indomethacin
A PG production inhibitor, used experimentally to reduce cAMP production
What are forskolin and IBMX
cAMP stimulators (via AC and phosphodiesterase respectively)
What experiments were done on cf and non cf rectal tissue and what did they show
In non cf tissue - carbachol causes a negative shift in the Vm due to Cl secretion
Addition of indomethacin means that CFTR function is prevented to a point where addition of carbachol and increased calcium cannot induce Cl secretion (cAMP is an absolute requirement of CFTR)
This can be bypassed however with the addiction of IBMX and forskolin
In CF rectal tissue, none of these approaches have any effect on Cl secretion, Cl secretion in the human colon is mediated by CFTR.
What is the effect of CF on the colonic mucosa
Newborns have increased risk of meconium ileus (a blockage of the ileum)
MI equivelent in adults who frequently suffer from distal intestinal obstruction syndrome.
What causes alveolar oedema in CF patients
CFTR is required for Cl reabsorption in the alveolar cell. With no CFTR function, Cl not reabsorbed, reducing water reabsorption, so water remains on the surface of the apical membrane, causing oedema
How is CF responsible for salty sweat
CFTR required for Cl reabsorption as well as water. No CFTR function there is a problem with NaCl reabsorption so sweat is salty.
