Lecture 6

Question 1

What are the two classes of glucose carriers that mediate the transfer of glucose across the plasma membrane?

Answer 1

1) Sodium Coupled glucose transporters
2) Facilitative glucose transporters
Functionally and structurally distinct

Question 2

Where is Na-dependent glucose contrasporter expressed?

Answer 2

In absorptive/reabsorptive epithelia such as intestine and kidney.

Question 3

Describe secondary active transports:

Answer 3

Usually Na+ coupled in mammalian cells, they use energy of Na+ gradient to move co-substrates aganist their electrochemical gradient.

Question 4

Define stochiometry:

Answer 4

The Na+/Substrate coupling ratio.

Question 5

Why is equilibrium [S]i/[S]o not achieved in vivo?

Answer 5

Because of energy losses, transporter slippages and dissipation of [S] gradient by other mechanisms.

Question 6

Describe SGLT1:

Answer 6

High affinity for glucose (Km=0.15mM)
Stoichiometry: 2Na:1 sugar
High concentrative capacity: x10,000 but needs energy from 2Na

Question 7

Describe SGLT2:

Answer 7

Low-affinity for glucose (Km=2mM)
Stoichiometry: 1Na+:1 sugar
Lower concentrative capacity: x100 but only needs energy from 1 Na+

Question 8

What are ion-coupled transporters of amino acids selective for?

Answer 8

A particular TYPE of amino acid substrate.

Question 9

What is the selectivity of ion-cupled transporters of amino acids based on?

Answer 9

1) Charge (anionic, cationic, zwitterionic or neutral)
2) size (small or large neutral)
3) structure (ex. N in glutamine, asparagine, histidine)

Question 10

What are the four functions of NKCC?

Answer 10

Cotransport in epithelial NaCl absorption
TALH: countercurrent multiplier
Cell volume regulation
Modulation of neurotransmission

Question 11

What are the three classes of symporters discussed in the lecture?

Answer 11

1) Sodium-Glucose Transporters
2) Ion-coupled transporters of amino acid
3) Na+K+Cl- cotransporters

Question 12

What are the three important transport related functions of the Na+/H+ exchanger?

Answer 12

Epithelial absorption and secretion (2,3,4)
Cell Volume Regulation (1)
pHi regulation (1,5)

Question 13

What are the two NHE1 functions?

Answer 13

1) Ion translocation by ubiquitous NHE1 changes pHi and cell volume, in turn regulating cell proliferation and migration
2) NHE1 binds directly to epiretinal membrane (actin binding) proteins and acts as a membrane anchor, critical for cytoskeletal assembly and cell shape determination.

Question 14

How is NHE1 activity regulated?

Answer 14

Through phosphorylation

Question 15

Describe Na+/Ca2+ exchanger:

Answer 15

Na+ inwards/Ca2+ outwards usual, notably in cardiac cycle
Contribute to keep internal [Ca2+] concentration low
Stoichiometry: 3 Na+ in for 1 Ca2+

Question 16

Why is ouabain used in case of heart failure?

Answer 16

Ouabain is used because it reduces Na+ gradient and therefore internal Ca2+ concentration increases as a result of reduced activity of NCX1 which in turn results in increased contractile force.

Question 17

List the five properties of Facilitative Glucose Transporters:

Answer 17

1) Integral membrane proteins
2) Present on the surface of a cell membrane
3) Transport glucose down a concentration gradient by facilitative diffusion (accellerate a form of passive transport)
4) Energy independent process
5) Can operate bi-directionally and mediate trans-cellular glucose transport, but always in the direction of the chemical gradient.

Question 18

What are the two classes of antiporters discussed in the lecture?

Answer 18

1) Na+/H+ exchangers

2) Na+/Ca2+ exchangers

Question 19

List the components of I of GLUTs:

Answer 19

High-affinity binding protein: GLUT1, 3 and 4

Low affinity bindign protein: GLUT2

Question 20

List the components of II of GLUTs:

Answer 20

Very low affinity for glucose, transport fructose
GLUT 5, 7, 9, 11
Myoinositol transporter (HMT1)

Question 21

List the components of class III of GLUTs:

Answer 21

GLUT6, 8, 10, 12

Question 22

Describe GLUT1

Answer 22

Km for glucose: 2-5
Widely expressed; high concentrations in brain, RBC and endothelial cells
Constitutive glucose transporter

Question 23

Describe GLUT2:

Answer 23

Km for glucose: 25
Kidney, small intestine, liver, pancreatic beta cells.
Low affinity glucose transporter, has a role in sensing glucose concentrations in islets

Question 24

Describe GLUT3:

Answer 24

Km for glucose: lowest
Neurons, placenta
High affinity glucose transporter

Question 25

Describe GLUT4:

Answer 25

Km for glucose: 2-10
Expressed predominantly in insulin responsive cell types: Skeletal muscle, cardiac muscle, adipose tissue
Insulin responsive glucose transporter.

Question 26

What is GLUT4 responsible for?

Answer 26

Mediating insulin-sensitive glucose transport especially important for facilitating peripheral glucose disposal after a meal when blood glucose is high.

Question 27

Describe the experiment carried by Suzuki and Kono, Cushman and Wardzala to demonstrate how insulin stimulate uptake of sugar in fat and muscle:

Answer 27

They used cytochalasin B (CB) that binds to glucose transporter in a non-competitive manner. They quantiatively measured how many GLUT could be found in the PM or in the IM fractions from -Insuling and +Insuling treated adipocytes by measuring equilibrium binding of radioactive CB.

Question 28

What did both Cushman and Wardzala, Suzuki and Kono found?

Answer 28

PM of adipocytes treated with insulin had much higher CB binding than non-insuling treated cells and insulin treated adipocytes had much lower CB binding values in their IM compared to the untreated adipocytes.

Question 29

How did Cushman and Wardzala, Suzuki and Kono interpreted their results?

Answer 29

Insulin stimulates the movement or translocation of glucose transporters from the IM to the PM to facilitate glucose uptake.

Question 30

Which transporters has been found to translocate from IM to PM in response to insulin?

Answer 30

GLUT4, in both adipocytes and muscle and heart cells.

Question 31

Why are the finding that GLUT4 translocate from IM to PM in muscle and heart in response to insulin important? (3 reasons)

Answer 31

1) Muscle is the principal tissue for glucose use in the fed state
2) Muscle is a primary insulin target tissue
3) Muscle must be fully responsive to insulin in order to lower blood glucose level and therefore must have and efficient blood to muscle glucose transport system.

Question 32

What might be one of the causes for high blood glucose during diabetes?

Answer 32

Defects in either muscle sensitivity to insluing or in GLUT4 translocation.