Session 4

Question 1

Outline what a membrane potential is, how the resting potential of a cell
may be measured, and the range of values found

1. What is a mp?
2. How is the RMP measured i.e units?
3. Give examples of the RMP is different cells

Answer 1

1. magnitude of an electrical charge that exists across a plasma membrane and is always expressed as the potential inside the cell relative to the extracellular solution
2. Millivolts (mV) [1mV = 10-3 V]
3. – Cardiac and skeletal muscle: -80 to -95 mV
   – Nerve cells: -50 to -75 mV
   – Erthyrocytes have the smallest

Question 2

What is the mp in skeletal muscle, erythrocytes and neurones

Answer 2

Question 3

What instrument can be used to measure the mp?

Answer 3

The MICROELECTRODE is a fine glass pipette

Tip diameter is <1 µm

Can penetrate cell membrane

Filled with a conducting solution (KCl)

Question 4

Explain the concept of selective permeability LO

Establishment of the membrane potential

Two factors are important for the generation of the membrane potential. They are:

Answer 4

• Asymmetric distribution of ions across the plasma membrane
– (i.e., ion concentration gradients)

• Selective ion channels in the plasma membrane
– K+, Na+, and Cl − channels are the most important channel types for most cells;
however, there are many cells in which other channels are important as well.
• (Can you name an extra one? Can you name two?)

Question 5

Ion Channel properties:(3)

Answer 5

1. Selectivity: for one (or a few) ion species. Na+, K+, Ca2+, Cl-, cation channels.
2. Gating: the pore can open or close by a conformational change in the protein
3. Rapid ion flow: always down the electrochemical gradient

Question 6

Describe how the resting potential is set up given the distribution of ions across
cell membranes LO

What is the conc of Na+, K+ , Cl- and A- intra and extracellularly ?

Answer 6

Question 7

Describe how the resting potential is set up given the distribution of ions across
cell membranes LO

What ion channel and ion sets up the resting membrane potential and how

Answer 7

• For most cells, open K+ channels dominate the membrane ionic permeability at rest.
• When these are equal and opposite, there will be no net movement of K+, but there will be a negative CHARGE across the membrane – i.e. the resting membrane potential
• Thus, the resting membrane potential arises because the membrane is selectively permeable to K+
  How big would such a membrane potential be ?

Question 8

If a membrane is selectively permeable to K+ alone, its membrane potential will be at ?

Answer 8

E_k

Question 9

Answer 9

Question 10

1. That for most cells ? dominate the resting membrane permeability
2. E_k =
3. Extracellular:
4. Intracellular:

Answer 10

1. open K+ channels
2. -95 mV (from Nernst equation)
3. [K+] o = 4.5 mM
4. [K+] i = 160 mM

​

Question 11

Cardiac muscle (-80 mV), nerve cells (-70 mV):

Resting membrane potential is quite close to EK Not exactly at EK (less negative): ?

Answer 11

membrane not perfectly selective for K+

Question 12

Cells with lower resting membrane potentials: Somewhat lower selectivity for K+ : increased contribution from other channels, e.g.

Answer 12

smooth muscle cells (-50 mV); erythrocytes virtually no selectivity for K+ (-9 mV)

Question 13

Skeletal muscle:
Many ? open in resting membrane
Resting potential ≈ -90 mV
Close to both ?

Answer 13

Cl- and K+ channels

ECl and E_k

Question 14

• To be able to outline the major physiological roles of:

– Sodium-potassium ATPase (Na+-K+-ATPase, ‘The Na pump’)

– Plasma membrane Ca2+ ATPase (PMCA)

– Sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA)

– Sodium calcium exchanger (NCX)

– Sodium hydrogen exchanger (NHE)

– Anion exchanger (AE)

• To be able to discuss how ion transporters work together in cell physiology
• To be able to discuss how ion transport contributes to:

– The control of resting intracellular Ca2+ concentration

– Cellular pH regulation

– Cell volume regulation

– Renal bicarbonate reabsorption

– Renal Na+ ion handling

Question 15

Na+-K+-ATPase (Na pump) - Functions

Answer 15

1. Forms Na+ and K+ gradients
   – Necessary for electrical excitability
   – (only contributes about - 5 mV to the resting membrane potential)
2. Drives Secondary Active transport
   – Control of pH_i
   – Regulation of cell volume and [Ca2+]_i
   – Absorption of Na+ in epithelia
   – Nutrient uptake, e.g. glucose or amino acids from the small intestine

Question 16

Draw/name all the ca channels found in the cell

Answer 16

Question 17

Control of resting [Ca2+]i - summary

1. Primary active transport
2. Secondary active transport
3. Facilitated transport

Answer 17

1. – PMCA expels Ca2+ out of the cell
   • High affinity, low capacity (removes residual Ca2+)

– SERCA accumulates Ca2+ into the SR/ER
• High affinity, low capacity (removes residual Ca2+)

2. – Na+-Ca+-exchange (NCX)
   • Low affinity, high capacity (removes most Ca2+)
3. – Mitochondrial Ca2+ uniports
   • Operate at high [Ca2+]i to buffer potentially damaging [Ca2+]

Question 18

Sodium Calcium Exchanger (NCX) activity is membrane potential-dependent. Thus explain how it acts during depolarisation and polarisation

Answer 18

Question 19

Explain what happens to the Na+ Ca2+ exchanger during Ischaemia

Answer 19

Question 20

To be able to outline the major physiological roles – Sodium hydrogen exchanger (NHE) LO

• Exchanges extracellular Na+ for intracellular H+
• Electroneutral 1:1 exchange
• Regulates pHin
• Regulates cell volume
• Activated by growth factors
• Inhibited by amiloride (a potassium sparing diuretic)

Cancer produces lots of protons thus more active

Question 21

Function of AE

Answer 21

• cell volume regulation
• Acidifies cell

Question 22

To be able to discuss how ion transport contributes to:
– The control of resting intracellular Ca2+ concentration

Answer 22

Question 23

To be able to discuss how ion transport contributes to:
– Cellular pH regulation

Answer 23

Question 24

​Coordination of intracellular pH regulation

Answer 24

pH is held at the set point. Any drift away from this pH is corrected by the increased activity of either the Na+-H+- or Cl–HCO3- exchangers

Question 25

To be able to discuss how ion transport contributes to: – Cell volume regulation LO 1. Transport of osmotically ‘active’ ions, e.g. ? or organic osmolytes aa out of cell. Water follows.

Answer 25

Na+, K+, Cl-

Question 26

Mechanisms that resist cell swelling

Answer 26

Question 27

Mechanisms to resist cell shrinking

Answer 27

Question 28

How is bicarbonate reabsorped by the proximal tubule?

Answer 28

Question 29

Describe Na+ reuptake by the kidney in the TAL? And which drug effects sodium uptake in the TAL?

Answer 29

furosemide

Question 30

Describe Na+ reuptake by the distal convoluted tubule? And the drug which effects its uptake?

Answer 30

Question 31

Describe Na+ reuptake in the cortical collecting duct and the drug which prevents the uptake

Answer 31

Spironolactone: mineralocorticoid receptor antagonist

Question 32

Describe Na+ reuptake in the cortical collecting duct and the drug which prevents the uptake

Answer 32

Question 33

• ‘Pumps’ are only found on the plasma membrane and use ATP as a primary energy source • ATP-dependent channels found in other parts of cells are called ‘Primary Active Transporters’ • Channels and ion transporters tend to not work alone but in a concerted manner • Channels and transporters tend to work in an electroneutral manner to move not only ions and small molecules, but also molecules • Cell volume, calcium, and pH homeostasis is controlled through the actions of ATP-dependent pumps and cell- and tissue-specific ion transporters