Lecture 04 Salt Secretion Flashcards
Describe the epidemiology and pathophysiology of cystic fibrosis (CF)
most common lethal genetic disease in Caucasians
genetic mutation in CFTR channel
~95% of males and ~5% of females are infertile
CFTR important in development of vas deferent
CFTR inactivity linked to thickening of mucous at cervix
biggest killer of cystic fibrosis is airway diseases
symptoms include exocrine pancreatic insufficiency and increased chloride in sweat
What is the basis of epithelia secretion?
Give examples of secreting epithelia
intermediate-to-leaky epithelia are capable of secreting sodium-chloride rich fluid
exocrine gland acini, sweat gland coil, small intestine, upper airway, choroid plexus, shark rectal gland
Why is shark rectal gland a good model?
large tissue size
easy access to the tissue (lab location at the time 70s/80s)
tissue very robust and sensitive
What channels are present in the basolateral membrane of the shark rectal gland cells?
What compounds can be used to inhibit these channels?
What does inhibiting these channels cause?
sodium-potassium ATP-ase - ouabain - decreases chloride secretion by decreasing the sodium gradient
potassium-selective channels - barium - depolarises the cell and decreases chloride secretion by decreasing the potassium gradient
NKCC1 - furosemide - directly prevents chloride secretion
What can be said about chloride secretion?
the amount of chloride secretion can be calculated if there is no active compound
intracellular chloride concentration without an active compound is calculated at 17mM
the active transport provided by sodium-potassium ATPase makes the actual intracellular chloride concentration 70mM
can be said that chloride is above electrochemical equilibrium
Are there any mediators of chloride secretion? How do they work?
cAMP opens the chloride channel in the apical membrane acting as a regulator
increasing cAMP increases the open probability of the chloride channel
What affect does Furosemide have on the shark rectal gland cells?
What experimental evidence is there?
inhibits NKCC1
decreases chloride accumulation
decreases secretion
use microelectrodes to measure the open probability of the chloride channel
as the concentration of chloride increases in the ECF, the Nernst potential moves with it
Describe the features and pathophysiology of CFTR
12 transmembrane spanning domains
one subunit to make a functional unit
different protein interactions will affects the function
protein has two nucleotide binding domains - 1 and 2
has a PKA phosphorylation site
over 1200 mutations linked to the CFTR protein
most common delta-F508 mutation in NBD1
What are the six classes of CFTR mutations? What do these mutations affect?
I - null production - unstable mRNA, no protein formed
II - trafficking - protein misfolding not trafficked to the membrane
III - regulation - incorrect regulation at the membrane
IV - conduction or gating - incorrect regulation in the open probability
V - partial reduction in mRNA - not normal amount, often too little
VI - high turnover of CFTR - decreased time for the channel at the membrane, endocytosis or degradation
How much functional protein is required for normal function? Are carriers affected?
15-20%
carriers have one functional copy giving 50% functional protein so will not have any symptoms
Which are the most severely affected classes (ranked)? Why?
classes I-III
struggle with pancreatic insufficiency - struggle to secrete digestive enzymes
classes IV-V
pancreatic sufficiency
What is the function of CFTR in the colon? Describe the mechanism in the model isolated rat colonic crypt cells
controls the water content of the faeces
CFTR in the apical membrane of lower-to-mid crypt cells secretes salt into the GI tract
increasing chloride secretion causes increased water in the faecal content leading to diarrhoea
decreasing chloride secretion causes decreased water in the faecal content leading to GI tract blockages
What mediators affect chloride secretion in the colon?
What channels do these mediators affect?
PGE2 (prostaglandin E2) - increases levels of cAMP which stimulates CFTR to increase chloride secretion
acetylcholine - increases intracellular calcium levels causing increased potassium movement through potassium-selective channels leading to hyperpolarisation and increased chloride secretion
What drugs can be given to increase chloride secretion in colonic mucosa? What is their mode of action?
carbachol (CCH) - increases ACh receptor activation leading to increased intracellular calcium
IBMX - inhibits phosphodiesterase which increases cAMP
forskolin - activates adenylyl cyclase which increases cAMP
What drug can be given to decrease chloride secretion in colonic mucosa? What is its mode of action?
indomethacin - inhibits production of prostaglandins including PGE2
