Introduction

Question 1

1. in what direction does absorption occur?
2. in what direction does secretion occur?
3. what is transcellular transport?
4. what is paracellular transport?

Answer 1

1. A > BL
2. BL > A
3. transport through cells
4. transport between cells

Question 2

1. give 3 examples of a) tight epithelia and b) leaky epithelia
2. what is Rte?
3. what is Vte?
4. compare tight and leaky epithelia in terms of:
   a) Rte
   b) Vte
   c) Flux
   d) H2O permeability

Answer 2

1a. distal tubule, frog skin, stomach
1b. proximal tubule, small intestine, gall bladder
2. transepithelial resistance. essentially how easily paracellular transport occurs across an epithelia
3. transepithelial potential. net potential across the epithelia; essentially the sum of A and BL Vm
4a. >2000Ω.cm² in tight epithelia, because paracellular transport is more difficult. <200Ω.cm² in leaky epithelia
b. approx 50mV in tight epithelia, net transport is large as ions cant move back; approx 0mV in leaky epithelia, as ions can move back so overall net movement is small
c. flux is small in tight epithelia, and large and isosmotic in leaky epithelia, due to the ease of paracellular movement
d. H2O perm is low in tight epithelia and high in leaky epithelia.

Question 3

1. what assumption is made when using western blotting?
2. what can immonostaining be used for?
3. what can radioactive compounds be used to measure?
4. what is a problem when using fresh tissues/cells, cultured cells or whole animals as epithelial preparations?
5. what is the problem when using overexpression systems as epithelial preparations?
6. what are positive/outward currents?
7. what are negative/inward currents?

Answer 3

1. that the antibody undergoes specific binding to the protein of interest
2. to reveal protein location, which might provide an indication of function
3. flux
4. the protein of interest may have a low expression level making single channel recordings impossible
5. may not be physiologically relevant, i.e. loss of regulatory proteins.
6. cations out, anions in
7. cations in, anions out.

Question 4

1. describe the set up of the ussing chamber
2. what is the short circuit current
3. what law can be used to work out Rte from ussing chamber recordings
4. what is the polarity of the potential dependent upon?
5. what Vte is produced by the loss of anions and why?
6. what Vte is produced by the loss of cations and why?

Answer 4

1. epithelial sheet is clamped between 2 chambers containing 2 solutions. 2 electrodes measure Vte, while another 2 electrides inject a current of a known amount which shifts the Vte
2. the amount of current needed to drive Vte to zero
3. ohms law (R=V/I)
4. upon which side of the epithelia the reference electride is placed
5. positive Vte, as it leaves behind a positive charge
6. negative Vte, as it leaves behind a negative charge