lecture 5 Flashcards
what are the key steps in epithelial NaCl and water absorption?
- passive facilitative entry of Na+ across apical membrane down electrochemical gradient
- active exit of Na+ across basolateral membrane
- paracellular diffusion of Cl- through tight junctions - forms NaCl through 1 molecule of each
- osmotically driven water absorption - as osmotic gradient is created
what is the pump-leak model?
- Na+ influx channel - ENaC
- Na+ pumped out cells across basolateral membrane via Na+K+ ATPase pump
- causes paracellular transport of Cl- via tight junctions to maintain electroneutrality
- increases NaCl concentration (lowers water potential) on basolateral side of epithelium, drives osmotic movement of water
- K+ channels on basolateral membrane maintain membrane potential and recycle K+
what is ENaC?
epithelial sodium channel
what is the role of ENaC in epithelial tissues?
kidney -> Na+ retention, control of body Na+and water balance, blood pressure control
lung -> Na+ and water absorption, control of amount of ASL and alveolar lining fluid
colon -> Na+ and water reabsorption from diet
sweat gland -> Na+ retention, reabsorption of Na+ by sweat ducts (not followed by water), produces hypotonic sweat secretion
what is the structure of ENaC?
-heterotrimer of 3 alpha, beta, gamma subunits coded by 3 genes
-each subunit has 2 transmembrane domains forming pore of channel
-highly selective for Na+
-constantly active but can be regulated
-long extracellular loops are site for regulation by CAPs and SPLUNC1
what is an inhibitor of ENaC?
amiloride
-potent, specific inhibitor
-used clinically
what is the role of ENaC in the kidneys?
-vital in determining final urinary salt composition through regulatory action of natriueretic hormones (aldosterone) and other paracrines (RAS system)
-aldosterone is important in long term regulation of whole body Na+ and water balance - involves change in ENaC function in aldosterone-sensitive distal nephron
what is the aldosterone-sensitive distal nephron (ASDN)?
-last third of the distal convoluted tubule, connecting tubule and the cortical collecting duct
what is the function of aldosterone?
-stimulates Na+ reabsorption through ENaC by principle cells in ASDN
-overall effect increased NaCl and H2O absorption which causes blood volume and BP to rise
how does aldosterone act on principal cells of the distal nephron?
- aldosterone enters cells by diffusion and binds to cytosolic receptors
- once bound, transported to nucleus where aldosterone receptors regulate transcription of certain genes
what does aldosterone increase?
- surface ENaC levels by 2-5 fold
- Na+/K+ ATPase density
- ATP supply to support increased Na+/K+ ATPase activity
- K+ excretion across apical membrane via ROMK
how does aldosterone stimulate Na reabsorption?
-increases number of ENaC channels
-by decreasing rate of retrieval/degradation
how is ENaC degraded?
- made in ER
- goes to trans Golgi
- stays in membrane for few hours
- removed to be degraded by proteosomes
what is degradation of ENaC regulated by?
ubiquitin ligase (NEDD4-2)
how does ubiquitin ligase regulate degradation?
-NEDD4-2 binds t oPY motif and adds ubiquitin group to a lysine residue in N terminal
-ubiquitinisation signals internalisation of ENaC, followed by degradation of ENaC
-alodsterone stimulates production of serum glucocorticoid regulated kinase 1 (SGK1)
-SGK1 phosphorylates NEDD4-2 which prevents binding to ENaC
-inhibits ENaC ubiquitination so ENaC channels increase
how is hypertension caused?
-by disregulated ENaC
1. excess aldosterone or mineralocorticoid action
2. liddles syndrome - gain in function of ENaC
what is liddles syndrome?
-rare, autosomal dominant genetic disease
-caused by mutations in genes encoding 3 subunits SCNN1a,b,g
-leads to change in cytoplasmic region of ENaC preventing NEDD4-2 binding
-leads to hypokalaemia - low blood K+ levels
how is K+ excretion enhanced in Liddles syndrome?
-apical pm is more depolarised, brings in more sodium
-electrical gradient for K+ excretion is increased
-excessive loss of K+ in urine
what does blocking ENaC with amiloride do?
-hyper polarises the apical membrane
-reduces K+ loss
-helps prevent hypokalaemia
-lowers BP
what is the role of ENaC in the lungs?
-ENaC regulates ASL
-regulates ALF volume for gas transfer
-expressed in apical pm of surface epithelial and alveolar type II cells
what regulates ENaC in the conducting airways?
-channel activating proteases eg. CAPS
-anti-proteases
-regulating peptides eg. SPLUNC1
-other local acting factors
-CFTR
how do channel activating proteases regulate ENaC?
-by CAPs
-increase ENaC activity via proteolytic cleavage of external loops of alpha and gamma ENaC
how do anti-proteases regulate ENaC?
reduce ENaC activity by inhibiting proteases
how do regulatory peptides regulate ENaC?
eg. SPLUNC1
reduces ENaC function
secreted into ASL and binds ENaC
protects from internalisation and CAP activation
