2. Penetrating the Membrane

Question 1

Describe the structure and function of Na+K+ ATPase.

Answer 1

STRUCTURE: alpha subunit: binds K+, Na+, ATP and Ouabain
beta subunit: glycoprotein - directs pump to surface

FUNCTION: (ATP hydrolysis: 3Na+ out and 2K+ in)

• Forms Na+ and K+ gradients (for excitability)
• Drives secondary active transport

Question 2

Describe the function and behaviour of NCX.

Answer 2

Sodium Calcium Exchanger (NCX) is a SECONDARY active transporter.

Uses Na+ gradient to transport 3Na+ in: 1Ca2+ in therefore it can be described as ELECTROGENIC.

AT RESTING POTENTIAL: removes large quantities of Ca2+.

REVERSED IN DEPOLARISATION: allows Ca2+ influx. This occurs in:

• Cardiac action potential
• Ischaemia (when NaK ATPase inhibited due to no ATP)

Question 3

What mechanisms control cellular pH?

Answer 3

NaK ATPase maintains inward Na+ gradient for other transporters.

ACIDIC CELL:

1. NaH Exchanger (NHE) - extrudes H+ (for each Na+ imported)
2. Na-Cl-HCO3-H+ Exchanger - extrudes H+ and imports HCO3
3. Na-HCO3 Cotransporter - imports 3HCO3 (with each Na+ imported)

ALKALI CELL:
1. Anion Exchanger (‘band 3’): extrudes HCO3 (for each Cl imported)

Question 4

Cells use conductive systems (channels) and cotransport systems to maintain volume. What other method is there?

Answer 4

CARBONIC ANHYDRASE: catalase the reaction:

H2CO3 CO2 + H2O

CO2 and H2O can traverse the bilayer therefore when the cell requires ions carbonic anhydrase generates H2CO3 which dissociates into H+ and HCO3-. The reverse occurs when the cell needs to remove ions. CO2 and H2O continually enters or leaves the cell when carbonic anhydrase uses or generates them.

Generating or removing ions affects the osmotic potential of the cell allowing it to resist swelling or shrinking. These ions can also be exchanged for others such as Cl- or Na+.

Question 5

How is bicarbonate (HCO3-) reabsorbed in the proximal tubule?

Answer 5

1. NaK ATPase maintains inward Na+ gradient.
2. NHE exports H+ into the lumen using this gradient. The H+ collects HCO3- and forms H2CO3.
3. Carbonic Anhydrase (membrane) converts H2CO3 into CO2 and H2O which traverse the membrane and enter the cell.
4. Carbonic Anhydrase (intracellular) converts CO2 and H2O back to H2CO3, which splits into H+ and HCO3-.
5. Anion Exchanger exports HCO3- into capillary for a Cl-.
6. The H+ ions is re-exported into the lumen by NHE to sequester another bicarbonate ion.

Question 6

How does a loop diuretic work?

Answer 6

Loop diuretic act on the thick ascending limb.

They inhibit the Na+K+2Cl- Cotransporter therefore less of these ions are taken up from the lumen.

Question 7

How do thiazides work?

Answer 7

Thiazides act on the Distal Convoluted Tubule.

They inhibit the Na+Cl- Cotransporter this fewer of these two ions are taken up from the lumen.

Question 8

Both spironolactone and amiloride act on the cortical collecting duct. Which transporters does each inhibit?

Answer 8

Spironolactone: inhibits ROMK at both luminal and capillary surfaces.
It also inhibits NaK ATPase on the capillary surface (reduces uptake if Na from the lumen).

Amiloride: inhibits ENaC and NHE thus reduced Na+ uptake from lumen.

Question 9

Give the FOUR mechanisms of transport across the membrane and when they are used.

Answer 9

1. Simple Diffusion (non-polar or small molecules)
2. Facilitated Diffusion (ions or large polar molecules)
   a) Channels - when open, these allow transport without conformational change.
   b) Carriers - conformational change allows transport of substrates
3. Primary Active Transport (transport against concentration or electrochemical gradients)
4. Secondary Active Transport (a favourable gradient of one molecule is used to cotransport another molecule)