Lab 3 (Properties of Epithelia)

Question 1

Define electrochemical equilibrium

Answer 1

This is a state of balance due to the equal action of opposing forces. The electrochemical equilibrium is a point at which the electrical and chemical driving forces for an ion to move across a membrane balance each other out. At this point, there will be a charge difference (voltage) across the membrane known as an equilibrium potential. This voltage/potential can be calculated using the Nernst equation

Question 2

What is the equation for Ohm’s Law?

Answer 2

V = IR

Question 3

Why could the measured and calculated values be different?

Answer 3

• experimental error and variability
• temperature
• the calculated values assume only movement of cations but it might be slightly permeable to anions
• we assume K+ is the only cation that goes through

Question 4

What are two reasons why the calculated membrane potential between nerve cell and artificial membrane is different, although the ion concentrations are identical?

Answer 4

• Different permeabilities to cations (nerve cells have a different permeability for K+ and Na+, artificial membrane have the same permeability for Na+ and K+)
• different permeabilities to anions (a nerve cell has some permeability to Cl- but the artificial membrane is not permeable to Cl-)

Question 5

What are we doing in the third experiment?

Answer 5

We add 3mol/L KCl solution to one side of the membrane causing a passive movement of K+ ions which can be measured as an electrical current

Question 6

How do we measure the Isc per unit?

Answer 6

measured Isc / surface area

14μA / 3.14 cm^2

Question 7

Which transport protein is primarily responsible for establishing the active driving force in cells?

Answer 7

Na+/K+ ATPase

Question 8

To measure active transport in a living epithelium what would the transepithelial voltage have to be initially?

Answer 8

If we want to measure active ion transport in a
living epithelia we want :
• the only driving force to be the electrochemical gradient generated by ATP.
• the only voltage to be generated by active ion transport
• Therefore, we would want the same solutions
on both sides and the transepithelial voltage
to be 0.
• The active transport would then generate the
voltage we would short circuit (V=0 mV).

Question 9

What does VIP do?

Answer 9

It activates Cl- secretion by:

• binding to its receptor in the basolateral membrane (GPCR)
• Gs:-
• activating adenylyl cyclase
• ATP -> cAMP
• cAMP activates PKA
• PKA phophorylates K+ channels and the CFTR
• increase Cl- secretion measured as an increase in Isc

Question 10

What happens to the Isc due to the effect of VIP?

Answer 10

The measured Isc increased after the VIP which means that there was more current flowing due to the movement of ions

Question 11

What happened to the Isc due to CF even as a result of VIP?

Answer 11

The measured Isc decreased after the CF which means that there was less current flowing