M&R Session 2 (Lecture 2.2)

Question 1

What are the functions of the Na+ pump?

Answer 1

1) Forms Na+ and K+ gradients (contributes small amount to membrane potential)
2) Drives secondary active transport (control of pHi, regulation of cell volume and Ca2+i, absorption of Na+ in epithelia, nutrient uptake e.g. glucose).

Question 2

List and describe the primary active transport control of calcium.

Answer 2

1) PMCA (Ca2+ out with H+ in using ATP) high affinity low capacity.
2) SERCA (sarcoendoplasmic reticulum Ca2+ ATPase). removes Ca2+ for H+ into the SER, high capacity low affinity.

Question 3

List and describe the secondary active transport/facilitated transport control of calcium.

Answer 3

1) NCX (3Na+ in for Ca2+ out) low affinity but high capacity

2) Mitochondrial Ca2+ uniports operate at high Ca2+

Question 4

Describe the NCX transporter.

Answer 4

Secondary active transport control of calcium. Expels one Ca2+ for 3Na+ influx.
Electrogenic - current flows in direction of Na+.
Expels calcium during cell recovery.
Possible role in cell toxicity during ischaemia/reperfusion (
Activity can be reversed if cell becomes depolarised so influx of calcium during action potential.

Question 5

What happens to the Na + pump during ischaemia?

Answer 5

ATP in the cell depletes so the pump is inhibited. This causes the cell to depolarise and reverses the action of NCX. Calcium influx causes toxicity.

Question 6

What are the main acid extruders? What effect does this have on the cell?

Answer 6

1) NHE (Na+ influx for H+ efflux)
2) NBC (Na+-HCO3- cotransporter), Na+ and HCO3- influx for H+ and Cl- efflux.

Causes the cell pH to increase.

Question 7

What are the main base extruders? What effect does this have on the cell?

Answer 7

AE (anion exchanger) Cl- influx for HCO3- efflux. e.g. Band 3

Causes the cell pH to decrease.

Question 8

Describe the NHE transporter.

Answer 8

Exchanges Na+ influx for H+ efflux
Electroneutral
Regulates pHi
Regulates cell volume
Activated by GFs

Question 9

Which two transporters are involved in cell volume regulation?

Answer 9

1) NBC

2) AE

Question 10

What needs to be done to i) reduce cell volume and ii) increase cell volume? Which ions are involved?

Answer 10

i) Extrude ions
ii) influx ions
iii) Na+, K+, Cl- (+AAs)

Question 11

Why is electroneutrality important in cell volume regulation?

Answer 11

Needs to be electroneutral transport so membrane potential and pH is not affected.

Question 12

Which transported are involved in cell volume regulation? (All extrude water)

Answer 12

1) K+ channel with Cl- channel efflux
2) K+ efflux/Cl- efflux symport + AA efflux
3) AE + KHE (K+ out for H+ in) forms HCO3- + H+ = H2CO3. H2CO3 acted on by carbonic anhydrase to form CO2 which diffuses out of the cell.

Question 13

Which transported are involved in cell volume regulation? (All influx water)

Answer 13

1) Influx of Na+/K+/2Cl-
2) Na+/Cl- symporter influx
3) Reversal so CO2 diffuses forms H2CO3 which dissocaites into H+ and HCO3- and leaves via AE and NHE (Na+ influx for H+ efflux)

Question 14

What do i) loop diuretics, ii) thiazides, iii) amiloride, iv) spironolactone inhibit? Give examples where prossible

Answer 14

i) NKCC2 cotransporter (influx of Na+, K+ and 2Cl-) e.g. furosemide
ii) NCCT symporter (Na+/Cl- influx) e.g. benzothiazidine
iii) ENaC (epithelial Na+ uniport)
iv) ENac (“) aka postassium sparing diuretic