Salt secretion and cftr Flashcards
Cystic fibrosis - what is it? Incidence? Symptoms?
Most common lethal genetic disease in caucasians - airway disease!!
- Exocrine pancreatic insufficiency, increased sweat Cl- concentration, male infertility (Cl- channel needed in development –> absence of vas deferens!)
Model for basis of secretion - why??
Where are leaky epithelia found?
Shark rectal gland! Large, robust and lasts for a long time
Model for leaky epithelia –> secretion of Nacl rich fluid such as what is seen in the choroid plexus, upper airway and proximal tubule
What are Ouabain and Barium and what are their impacts in normal cells?
OUABAIN - Na+/K+/ATPase pump blocker –> when blocked secretion is at 0 as this sets up the driving force for the secretion of Cl-!!
BARIUM - K+ channel (basolateral membrane) blocker –> inhibits Cl- secretion !! Causes membrane to depolarise and membrane potential shifts +vely; ions not recycled again
Relevance of K+ and membrane potential in Cl- secretion?
- Low intracell Na+ concentration
- -ve membrane potential - both critical for secretion of Cl-!!!
What is FUROSEMIDE and what is it used for?
Na+/K+/2Cl- transport blocker –> causes secretion to shift to 0 –> no Cl- being brought in from basolateral side!!
Role of Na+/K+/ATPase in Cl- secretion?
Sets up the driving force for the influx of Na+ through NKCC1. Blocking this therefore causes a reduction in the function of NKCC1
Role of NKCC1 in Cl- secretion?
Brings Cl- into cell depending on the functioning of ATPase pump (sets up the driving force for influx of Na+ via NKCC1)
What does it mean if Cl- is ‘above electrochemical equilibrium?’
An active component is involved and causing accumulation of Cl- – this is seen when the ic [Cl-] > calculated value!!
Did this to work out what the pathway of Cl- would be on the apical membrane
How can you use the nerst potential to work out IC [Cl-]??
Take the membrane potential and ec [Cl-], rearrange the nerst equation to find out the ic [Cl-] IF CL- IS PASSIVELY DISTRIBUTED (no active componant involved in Cl- being brought into the cell). If they are the same, no active componant involved. If higher, active componant involved
Outline completed model for Cl- secretion?
- NKCC1 –> brings in Cl- under the influence of the driving force set up by the pump in the basolateral membrane
- PUMP - sets up the driving force for the activity of NKCC1 by maintaining a low ic [Na+]
- K+ channel - recycles K+ back out of the cell via the basolateral membrane, which causes hyperpolarisation of the membrane and stimulates secretion of Cl- via CFTR
What are the 6 classes of CFTR mutations
1 - null production
2 - trafficking (dF508!!)
3 - regulatory mutation
4 - conduction - gating mutation - channel doesnt open and close properly
5 - partial reduction - less mRNA is produced
6 - high turnover CFTR - reduced time at membrane so less protein there at any given time point
Properties of the CFTR channel?
12 tmds, 1 sub unit, 1 regul. domain, 2 nucleotide binding domains (VERY IMPORTANT –> commonly mutated!!)
Diagnostic cut off point and mutation severity –> what are they are what classes of mutations are normally associated with what?
> 60mmol/L [Cl-] in the sweat = diagnostic cut off!
Class 1-3 of mutations –> pancreatic insufficiency , individuals most ill
Class 4 and 5 –> less severe
How is the isolated rat colonic crypt a good model for Cl- secretion?
Determines water content of rat faesces –> lower 2/3 of colon secretes Cl- –> H2o follows. Too much secretion = diarrhoea.
Role of K+ and Na+ in the colon?
K+ channels = hyperpolarisation of the membrane, increasing the driving force for the secretion of Cl-
Na+ ions undergo paracellular secretion
Ach receptors and Prostoglandins on epithelial cells - what are their roles?
Increase in Ca2+ inside cell stimulates Ach receptors, which activates K+ channels to stimulate Cl- secretion! Prostoglandin receptor (PGE2) are activated by cAMP --> stimulates pKA --> Cl- secretion
Effects of stimulation with ; Forskolin, Carbachol, Indomethacin, IBMX?
Forskolin –> increase in cAMP so increase in secretion?
Carbachol –> Achr activator by increasing i.c Ca2+
IBMX –> increase cAMP by inhibiting Phosphodiesterase enzyme
Indomethacin –> inhib PG production and decrease cAMP levels
Impact of phosphodiesterases?
Cause break down of prostoglandins –> less stimulation of PGE2 –> break down cAMP, decreasing Cl- secretion!!
Impact of adenylate cyclase?
Increase cAMP levels to increase Cl- secretion!
CFTR not full story- other Cl- channels?
CLCs - voltage gated Cl- channels
Caccs - Ca2+ activated Cl- channels
Pathology of CF newborns colonic mucosa
Lack of Cl- channel is seen –> results in blockages caused by thick mucus –> no Cl - secretion so water doesn’t follow and thick mucus. Causes blockages in digestive system and death
UPPER AIRWAY - MODEL role of CFTR?
CFTR —| Enac –> loss of function in CFTR therefore thick mucous produced!
ALVEOLAR MODEL - role of CFTR?
Enac and CFTR active together!!
No NKCC2 in basolateral membrane, instead K+/Cl- contrasporter moves Na+/Cl- out which creates driving force for ABSORPTION of Cl- via CFTR!!
When not working can cause alveolar oedema (fluid in alveoli)
DISTAL SWEAT GLANDS MODEL - role of CFTR
CFTR stimulates Enac allowing Na+ to be reabsorbed
Faulty CFTR means salty sweat!!
In the sweat glands, fluid moves down sweat duct and ions are secreted, then they are reabsorbed at more distal point (via CFTR —> Enac)
