Glucose Homeostasis- Facilitative and Secondary active transport

Question 1

What are the basic mechanisms for nutrient absorption by the cell?:

Answer 1

• Passive Diffusion ( Bilayer or Channel) - slow process- relies on solute to be very soluble in solution
• Facilitative transport - does not require energy , protein helps transport- as long as there is a concentration gradient
• Primary Active transport ( against concentration gradient )
• Secondary active transport ( symport, Antiport ) - , it is reliant on primary active transporters. Symport- both solutes are going in the same direction. Antiport- opposite direction of both solutes.

Question 2

what is Glucose oxidation important for ?

Answer 2

• represents a major source of energy in mammalian cells.

in order for glucose to be utilitsed as energy - it needs to be transported into the cells!.

Question 3

how is glucose transported across the plasma membrane ?

Answer 3

• by two classes of glucose carriers
  1. Sodium - Coupled glucose transporters ( requires sodium)
  2. Facilitative Glucose transporters ( only transport if there is a concentration gradient )

these two classes of glucose transporters are functionally and structurally distinct.

Question 4

Where are these two transporters located?

Answer 4

• Na - dependent glucose transporter - absorptive and reabsorptive epithelia such as intestine and Kidney
• Glucose transport from the lumen of the intestine or the nephron occurs ACTIVELY against its concentration gradient by coupling glucose uptake with that of Na.

Question 5

give an example of a secondary active transporter?

Answer 5

• usually Na+ coupled transporter in mammalian cells
• these use energy of Na+ gradient to move co-substrates against their electro – chemical gradient

e.g 2Na and 1 glucose transported into the cell.

the sodium coupled glucose transport works in symport

secondary active transport - antiport - one sodium for 1 proton ,

tertiary active transport system - proton coupled peptide transport

Question 6

What is the stoichiometry ?

Answer 6

• Na+ coupling ratios
• e.g. glucose and sodium ratio in the intestines.
  e. g. 2 (NA): 1 ( glucose )

Transport Stoichiometry is important physiologically for a Na+ coupled symporter , equilibrium ratio for cotransported solute (e.g. sugar , amino acid )

Question 7

What is the stoichiometry equation?

Answer 7

( refer to lecture page 4 for equation !!! )

the equation relates the concentrating power of the transporter to teh N+ gradient ( chemical and electrivcval ) each contributes roughly (x10) and coupling ratio (n)

equilibrium is not achieved in vivo because of energy losses, transporter ‘slippage’ and dissipation of (s) gradient by other mechanisms.

Question 8

What is the SGLT family?(1)

Answer 8

• sodium - glucose transporters- symporter!
• SGLT1 - high affonity
  - glucose km =0.15mM
  - stoichiometry = 2 Na+ =1 sugar
  -high concentrative capacity (x10,000)
  but needs energy from 2 NA+
  - found in intestines !!
• SGLT2 - low affinity
  - glucose Km -2mM
  - Stoichometry -1Na+:1 sugar
  - lower concentrative capacity - ( x100)
  but needs energy from 1Na+
  - found in kidneys !!!

Question 9

What is reverse Hydration therapy?

Answer 9

• sugar drinks to reverse intestine and kidney job to reabsorb water!!!

Question 10

what is the Ion- coupled transporters of amino acids? (2)

Answer 10

• symporters
• secondary active transport
• 20 common amino acids (AAs)
  A A transporters selective for particular type of AA substrate. Selectivity related to :-
  —-Charge ( anionic , cationic , zwitterionic/neutral)
  —- size ( small or large neutral)
  —- structure ( e.g. N in glutamine , asparagine , histidine )

several families of Na+ dependent transporter system with variety of transport stoiciometry

(e.g. GAT family :- family of Na+ and Cl- coupled transporter

• GAT1-3:- for GABA (neurotransmitter)
• GLYT1-2:- for glycine - roles in inhibitory neurotransmission

Question 11

What is the (Na+ + K+ K Cl-) cotransporters (NKCC) ?(3)

Answer 11

symporters - secondary active transport

properties:
1Na:1K:2Cl

inhibition by bumentanide . piretanide ..furosemide ( inhibits triple cotransporter in nephron- which promotes water loss in the body , results in edema or swollen ankles - if you have water retention!)

function:
- cotransport in epithelial Na Cl absortpion
- TALH: counteercurrent multiplier (‘loop diuretics)
- cell volume regulation
- modulation of neurotransmission

Question 12

What are three examples of secondary active transport symporters?

Answer 12

• the SGLT family
• the Ion- coupled transporters of amino acids
• (Na+ + K+ K Cl-) cotransporters (NKCC)

Question 13

What are two examples of secondary active transport - antiporters ?

Answer 13

1. Na+/H+ exchangers (NHE1-5)

2. Na+/Ca2+ exchanger

Question 14

What is the Na+/H+ exchanger?(1)

Answer 14

• UNIVERSAl
  three important transport related functions:

1. epithelial absorption and secretion ( NHE 2,3,4)
   ( e.g. NHE3 for intestinal Na+ absorption , acid
   microcclimate and renal HCO3- reabsorption )
2. cell volume regulation (NHE1)
3. pHi regulation (NHE 1,5)

Question 15

Seperation of NHE1 functions?

Answer 15

• ion translocation by ubiquitous NHE1 changes pHi and cell volume , in turn regulating cell proliferation and migration. NHE 1 activity modulated by phosphorylation!

NHE1 also binds to ERM ( actin binding) proteins.

Question 16

What is the Na+ / Ca+ exchanger antiporter? (2)

Answer 16

• moves sodium into the cell , when calcium moves out of the cell.- it is important that calcium is kept in check !!!
• Na inwards/ Ca outwards - notably in cardiac muslce- the skeletal and cardiac muscle need Ca it is very important for contractile force !!!

contributes to keeping cell (ca+) low
stoichometry ( 3Na :1 Ca2+)

ouabain used in heart failure - inhibits the NA/K pump
therefore it would reduce the Na gradient and cell Ca increases due to reduced NCX1 activity - increased contractile force then !!!!

Question 17

What is the second class of glucose carriers called?

Answer 17

• Facilitative glucose transporters - these are expressed in every cell in the body!
• these are integral membrane proteins
• present on the surface of all cell membranes
• they transport glucose down a concentration gradient by facilitative diffusion ( accelerate a form of passive transport)
• energy independent process
• can operate bi-directionally and mediate trans-cellular glucose transport, but always in the direction of the chemical solute gradient

( e.g. apical , basal and between cells glucose movement )

Question 18

What is the facilitative GLUT family?

Answer 18

• 13 functional facilitated hexose carriers , saturable , stereoselective
• 12 transmembrane domain containing protein
• every transporter in this family has a unique last 20 amino acids

Question 19

There are 3 classes of the GLUT transporters what are they?

Answer 19

• Cclass 1 - high affinity binding proteins : GLUT1,3,4
  - low affinity binding protein: GLUT 2 ( glucose transporter)
• Class 2 - very low affinity for glucose - transport fructose- GLUT 5,7,9,11 ; ( myoinositol transporter (HMIT1) - fructose transporter

Class 3- GLUT 6,8,10,12 - transporters based on sequence similarities

most members of class 2 and 3 have been identified recently in homology searches of sequence information provided by the various genome projects

Question 20

What are the Characteristics of the 4 functionally classified class 1 GLUT transporters ?

Answer 20

GLUT1: ( house keeping sugar cell!)

• KM of glucose = 2-3
• tissue distribution: - widely expressed ; high concentrations in the RBCs, the brain and endothelial cells.- provides every cell with basal needs !
• charachteristics:- constitutive glucose transporter

GLUT2:- ( expressed in kidney, intestines and liver )
- Km of Glucose ( loop value) - 25
tissue distribution:- kidney , small intestine , liver ,pancreatic beta cells.
- charachteristics :- low affinity glucose ( doe not become saturated easily ) transporter ,has a role in sensing gluclose concentrations in islets. ( transporter found on basolateral membrane )
N.B ON GRAPH ON P12 OF LECTURE NOTES , IT DOES NOT LOOK LIKE IT WPULD GET SATURATED - AND SEEMS WOULD NEED HIGH GLUCOSE TO BE SATURATED.

GLUT3:-

• Km of glucose- lowest
• tissue distribution:- neurons , brain, placenta (makes sure foetus always gets required glucose-even at the expense of mother !!!)
• Charachteristics- high affinity glucose transporter

GLUT4:- ( insulin regulated transporter) :-

• Km of glucose- 2-10
• tissue distribution:- skeletal muscle , cardiac muscle , adipose tissue. ( these are all insulin targetted tissues,)

Question 21

type two diabetes?

Answer 21

• glucose is a signal for release of insulin , glucose is phosphorylated allowing oxidatiion to occur , which generates ATP
• membrane is depolarised , and membrane potential shifts to -55
  opening Ca channels- which stimulates insulin secretion , B-cell releases insulin! - which lowers blood sugars!

this is the regulation of glucose levels
- type two diabetes cells become resisitant to this process!

N.B. type 1 diabetes dont have cells with insulin and so cannot reduce blood sugar levels.

Question 22

What is the insulin reponssive glucose transporter ?

Answer 22

GLUT4

this is expressed in predominantly insulin- responsive cell types e.g. Adipose tissue, skeletal muscle and heart !

this is responsible for mediating insulin- sensitive glucose transport - especially important for facilitating peripheral hlucose disposal after a meal when blood glucose is high ! ( implications for diabetes)

Question 23

What did Cushman and wardzala , and suzuki and kono do ?

Answer 23

• they proposed the mechanism by which insulin stimulated glucose transport in rat fat cells.

they used a fungal metabolite (CB) which speciifically binds to facilitative glucose transporters in a non competitive manner !

however CB binding does not discrimintae between transporter isoforms as it binds to them all !
binding site is thought to be part of the cytoplas,ic domain of the transporter
( refer to p14 of notes for diagram of study )

results:
- plasma membrane of fat cells treated with insulin had much higher CB binding than non- insulin treated cells !
BUT
these insulin treated fat cells have much lower CB binding values

in other words - insulin stimulated the movedment / translocation of glucose transporters from inner membranes to the plasma membrane and this could facilitate insulin stimulatd glucose transport

insulin caused an increase in the PM and a decrease in the IM of the GLUT4 transporter

importance:
- muscle is a primary insulin target tissue ! , muscles must be fully responsive to insulin in order to lower blood glucose and must therefore have an efficient blood to muscle glucose transport system.

overall it has been found that when Insulin and Exercise have been involved - there is more GLUT 4 in plasma membrane !!!