Epithelial transport Flashcards
What term is used to describe movement of ions across epithelia?
Unidirectional transport
What is the fundamental feature of epithelia that underpins unidirectional transport?
Asymmetry, the Na-KATPase is limited to the basolateral membrane
What gradients does the basolateral Na-KATPase generate across the cell?
Generates an inward sodium electrochemical gradient and an outward potassium electrochemical gradient - this exists across both membranes.
What defines the difference between tight and leaky epithelia?
How tight the tight junctions are (how much do they prevent movement of water and solutes between cells)
Where are tight epithelia characteristically found and why?
Distal end of the system (i.e collecting duct and colon)
More selective in the way they handle the load (no paracellular transport) gives more fine-tuned regulation
Where are leaky epithelia characteristically found and why?
In the proximal part of the system (i.e jejunum/ileum and PCT)
Allow bulk absorption of isosmotic solutions
What is the transcellular pathway of absorption and what underpins it?
Through the cells membrane, depends on active transport processes.
What is the paracellular pathway of absorption and what underpins it?
Between cells, occurs passively by diffusion and convection.
Direction depends on electrical and chemical gradients for ions
Osmotic and hydrostatic pressure gradients for water.
What determines the direction and magnitude of a p.d across the epithelium?
Orientation of p.d depends on which ions move and in which direction
Magnitude of p.d depends on whether the epithelium is leaky (so that the charge ‘shunts’) i.e ions move in the opposite direction paracellularly to disperse the electrochemical gradient.
What is the asymmetric permeability described in the Ussing model (in simple terms)?
Basolateral membrane is more permeable to K+ (Em close to Ek) and the apical membrane is more permeable to Na+ (Em close to ENa).
Why is there asymmetric permeability, describe asymmetry and ussing model.
Basolateral: Na/KATPase, K+ leak channels (hence high K+ permeability),
High sodium permeability, due to sodium channels and co-transporters (driven by gradient from pump)
Additionally (leaky only): passive movement via the paracellular route down electrochemical gradients established by Na+ coupled movement.
What is secondary active transport?
Where other substances harness the sodium gradient generated by its translocation to move across the apical membrane via a co-transporter
Explain the difference in water permeability between the tight and leaky epithelia
Tight: very low but can be upregulated via the introduction of aquaporin II channels induced by ADH.
Leaky: high and unregulated, largely by transcellular pathway through aquaporin I and III channels.
In leaky epithelia is most of the water movement through the transcellular or paracellular pathway
Transcellular, via AQP I/III channels
What underpins secretory epithelia?
Na-KATPase creates inward Na+ gradient
Cl- enters cell via basolateral Na/K/2Cl
Cl- moves out passively apically on CFTR
Lumen negative p.d causes Na+ to move into lumen paracellularly, water follows
