M&R Session 2: Permeability and transporters

Question 1

Name molecules that can passively diffuse across biological membranes

Answer 1

Water (osmotic gradient: up concentration gradient of a solute)
Carbon dioxide
Oxygen
Urea
Ethanol
Small hydrophobic molecules e.g. steroids, benzene

Question 2

What are aquaporins?

Answer 2

Specific water tubules found in the proximal kidney tubule

Question 3

What is the role in membrane proteins in mediating movement of hydrophilic molecules?

Answer 3

Maintain intracellular pH and ionic composition
Regulate cell volume
Extrusion of waste from metabolism and toxins
Generate ionic gradients for electrical excitability

Question 4

What is facilitated diffusion? (form of passive transport)

Answer 4

Ion channels that increase permeability for a polar substance, e.g. in erythrocyte membranes Cl- can move through due to the presence of band 3 protein which is an anion exchanger (low permeability in phosphatidylserine membrane)
Saturable-each carrier molecule can only interact with small number of molecules at a time, and there’s a finite number of carriers in each membrane
Some of these pores are gated

Question 5

Ligand-gated ion channels?

Answer 5

Facilitated diffusion. Open or close by conformational change when ligand binds
E.g. nAChR (NA+ flows in on ACh binding), ATP-sensitive K+ channel (K+ kept inside when ATP binds).

Question 6

Voltage-gated ion channels?

Answer 6

Facilitated diffusion. Open or close in response to potential difference across membrane.
E.g. Na+ channel

Question 7

What are connexins?

Answer 7

Gap junction proteins involved in facilitated diffusion pore gating. Closed when cellular Ca2+ goes above 10um.

Question 8

What is the purpose of active transport?

Answer 8

To overcome unfavourable chemical or electrical gradients. The movement of ions is coupled to a thermodynamically-favourable reaction

Question 9

Primary active transport?

Answer 9

Free energy directly from activity of ATP-dependent pumps (ATPases)

Question 10

Secondary active transport?

Answer 10

Free energy indirectly from other sources, e.g. the gradient of other substances (often Na+), light and high potential electrons. Often at expense of ATP hydrolysis, primary energy source is used indirectly

Question 11

Uniporter?

Answer 11

One species transporter to other side of membrane

Question 12

Symporter?

Answer 12

A cotransporter in which one the transfer of one molecule depends on the simultaneous transfer of another in the same direction

Question 13

Antiporter?

Answer 13

A cotransporter in which one the transfer of one molecule depends on the simultaneous transfer of another in the opposite direction

Question 14

Gradient of Na+ across membrane?

Answer 14

Inwards
Inside conc: 12mM
Outside conc: 145mM

Question 15

Gradient of K+ across membrane?

Answer 15

Outwards
Inside conc: 155mM
Outside conc: 4mM

Question 16

Gradient of Cl- across membrane?

Answer 16

Inwards
Inside conc: 4mM
Outside conc: 120mM

Question 17

Gradient of Ca2+ across membrane?

Answer 17

Inwards
Inside conc: 10-7 mM
Outside conc: 1.5mM

Question 18

What is the Na+-glucose?

Answer 18

Symporter

Moves Na+ and glucose in

Question 19

Na+/K+-ATPase

Answer 19

Primary active transport. Antiport
3 Na+ out for 2 K+ in per ATP hydrolysed
Found in all cells to maintain Na+ and K+ gradients (NOT RMP)
Drives secondary active transport for pH control, cell volume regulation, control of Ca2+, absorption of Na+ in epithelia and nutrient uptake
P-type ATPase as ATP phosphorylates aspartate

Question 20

Plasma membrane Ca2+ ATPase (PMCA)?

Answer 20

Primary active transport
Expels Ca2+ in order to maintain higher Ca2+ outside than in
High affinity, low capacity (removes residual Ca2+)

Question 21

Sarcoplasmic reticulum Ca2+-ATPase (SERCA)?

Answer 21

Primary active transport
Found on ER
High affinity, low capacity (removes residual Ca2+)
Ca2+ moved into SR for storage. Important in signalling

Question 22

Na+-Ca2+ exchanger (NCX)?

Answer 22

Secondary AT, antiport
3 Na+ out, 1 Ca2+ in … Can antiport either way depending on gradient
electrogenic as current flows in direction of Na+ gradient
Low affinity, high capacity (removes majority of Ca2+)
Membrane-potential dependent: inhibited in depolarisation and activity reverses [reperfusion ischaemia injury in MI]

Question 23

Mitochondrial Ca2+ uniporters?

Answer 23

Facilitated transport

Operate at high [Ca2+]in to buffer potentially damaging levels

Question 24

Passive transporter of potassium ions?

Answer 24

Voltage-operated K+ channel

Question 25

Na+/H+ exchanger (NHE)?

Answer 25

Secondary active transport
Uses Na+ moved out by Na+ pump to drive Na+ in and H+ out
Electroneutral: 1 Na+ in for 1 H+ out
Found in most cells, regulates pH, cell volume and cell growth
Activated by growth factors
Inhibited by amiloride

Question 26

Na+-bicarbonate-chloride cotransporter (NBC)?

Answer 26

H+ moves out, Na+ moves in, Cl- moves out; allowing HCO3- in (or reverse)
Alkalinises cell and regulates volume in some cells
Electroneutral, coupled to AE

Question 27

Anion exchanger (AE)?

Answer 27

HCO3- out, Cl- in through band 3 protein

Acidifies cell and regulates volume

Question 28

Na+-HCO3- cotransport?

Answer 28

Some cells. Symporter
1 Na+ and 3HCO3- in
Alkalinises cell

Question 29

How is cellular [Ca2+] controlled by membrane proteins?

Answer 29

PMCA
SERCA
Facilitated diffusion channels
NCX
Mitochondrial uniports

Question 30

How is cellular pH controlled by membrane proteins?

Answer 30

First buffering capacity but when exceeded needs transporters. Na+-K+-ATPase used a lot as maintains Na+ gradient
Acidification: opposed by NHE (expels H+) or inward movement of bicarbonate ions (e.g. NBC)
Alkalisation: opposed by expelling bicarbonate via AE

Question 31

How might a cell prevent cell shrinking?

Answer 31

Influx of osmotically-active ions such as Na+, K+ and Cl-, therefore water will also enter

Question 32

How might a cell prevent cell swelling?

Answer 32

Efflux of osmotically-active ions such as Na+, K+, Cl-, so water will also leave. Able to do this passively because the Na+/K+ ATPase maintains a low [Na+]in and provides a driving force for the movement of these ions to resist swelling

Question 33

What effect does ion transport to change cytplasmic pH have on the membrane potential?

Answer 33

No effect: transport is electroneutral so osmotic strength of cytoplasm can be varied without affecting the MP

Question 34

State the approximate fluid volumes for a typical 70kg person

Answer 34

Approx. 42L total aqueous volume
28L intracellular
14L extracellular: 3L plasma, 10.5L interstitial fluid, 0.5L transcellular fluid
Man ~60% water, women ~45-50% water (as more fat)

Question 35

How do the kidneys reabsorb bicarbonate?

Answer 35

Normally: proximal tubule reabsorbs all bicarbonate and retains for pH buffers
Uses NHE, Na+ pump and carbonic anhydrase

Question 36

What is the clinical relevance of renal Na+ handling?

Answer 36

Transport mechanisms can be blocked to treat hypertension, by diuretics (as decreased water will decrease blood pressure)

Question 37

How do loop diuretics reduce hypertension?

Answer 37

Block the Na+/K+/2Cl- symporter in the thick ascending limb

Partial inhibition of sodium uptake to reduce circulating blood pressure

Question 38

How do thiazides treat hypertension?

Answer 38

Block the NCCT (Na+/Cl- symporter) in the distal convoluted tubule

Question 39

How does spironolactone reduce hypertension?

Answer 39

Blocks ROMK and Na+/K+ ATPase in the cortical collecting duct
Stops aldosterone binding

Question 40

How does amiloride treat hypertension?

Answer 40

Blocks ENaC (epithelial Na+ channel) in the cortical collecting duct

Question 41

What is the action of aldosterone in the kidney?

Answer 41

Upregulates transporters causing increased sodium and water retention

Question 42

What is the function of ADH in the kidney?

Answer 42

Stimulates the formation of aquaporins so water is taken up more efficiently