L34. Epithelia (2)

Question 1

What drives absorptive epithelia?

Answer 1

Absorption driven by Na+ transport

Question 2

What drives secretory epithelia?

Answer 2

Secretion driven by Cl- transport

Question 3

What are the mechanisms of ion/molecular movement?

Answer 3

Simple diffusion –> Ions and molecules can move through a membrane, if it is permeable for these ions or molecules

Facilitated diffusion –> Requires a specific membrane protein e.g. aquaporins, GLUTs

Active transport –> Requires a specific membrane protein and energy
* Primary active: ATP e.g. Na+/K+-ATPase
* Secondary active: Na+ e.g. SGLT1, SGLT2

Question 4

Tell me about saturation of diffusion and mediated transport?

Answer 4

Diffusion occurs at a constant rate and cannot be saturated

Mediated transport can be saturated, dependent on the capacity of the transporter

Question 5

Tell me about sodium transport in absorptive epithelia?

Answer 5

• Transport from mucosal to serosal solution or apical to basolateral, driven by Na+ transport
• Na+/K+-ATPase transports 3Na+ ions out of the cell and 2K+ ions into the cell and generates a low sodium concentration inside the cell (15mM) and a higher Na+ concentration (145mM) in the interstitium
• Na+/K+-ATPase requires energy in the form of ATP, because the Na+ is moved uphill against its gradient (15mM inside the cell compared to 145mM outside cell)
• Sodium channel in apical membrane is permeable to Na+ and Na+ flows down its chemical gradient through the sodium channel and into the cell, with no energy required
• Typical in tight epithelia where only Na+ is transported through apical membrane, water often follows, hormone regulated
• Na+/K+-ATPase transports only two positive ions (K+) into the cell and three positive ions (Na+) out of the cell; this generates a negative charge inside the cell; K+ channels make the basolateral membrane permeability for K+ and the K+ gradient drives more K+ out of the cell making the cell more negative (-70mV); this negative charge of -70mV inside the cell can attract the positive ions such as Na+ (like a magnet); therefore, the electrical driving force together with the chemical gradient generates the electro-chemical gradient

Question 6

Tell me about sodium and solute X (glucose) transport in absorptive epithelia?

Answer 6

• Transport from mucosal to serosal solution or apical to basolateral, driven by Na+ and solute e.g. glucose transport
• Na+/K+-ATPase in basolateral membrane transports 3Na+ ions out of the cell in exchange for 2K+ ions into the cell; sets up gradient for Na+ to move down its concentration gradient through an apical transporter that moves both Na+ and the solute together into the cell
• Requires energy in the form of ATP, because the Na+ is moved uphill against its gradient (15mM inside the cell compared to 145mM outside cell)
• Na+ from mucosal/lumen solution moved by active transport to the blood side through basolateral Na+/K+-ATPase up its concentration gradient. Solute X is cotransported with Na+ into the cell up its concentration gradient
• Solute X moved by facilitated diffusion/transporter through basolateral membrane to blood side down its concentration gradient
• Typical in leaky epithelia where Na+ and a solute are transported through apical membrane together
• With the sodium gradient as the driving force, other substances such as glucose can be absorbed, if there is a sodium dependent glucose transporter (SGLT1 or SGLT2) present; once the glucose inside the cell has reached a certain concentration, glucose is released into the interstitium via facilitated diffusion = sodium-independent glucose transport by glucose transporters (GLUTs)

Question 7

Tell me about Na+, Cl-, water movement in absorptive epithelia - LEAKY?

Answer 7

• In leaky epithelium increase in positively charged sodium absorption through channels and/or transporters cause the lumen to become negatively charged, and the interstitium becomes positively charged
• This drives paracellular absorption of negatively charged chloride (magnetic effect)

Question 8

Tell me about the absorption of sodium and water?

Answer 8

• In leaky epithelium sodium absorption increases the water gradient over the epithelium
• This drives both trans and paracellular absorption of water

Question 9

What is Na+/water absorption in the PCT of the kidney?

Answer 9

• Leaky epithelium
• High water permeability, both trans-cellular through AQP1, and paracellular through the tight junction

Question 10

What is Na+/water absorption in the CCT of the kidney?

Answer 10

• Tight epithelium
• Low water permeability, only transcellular through AQP2