Epithelial Transport

Question 1

What is the normal plasma volume?

Answer 1

3L

Question 2

What is the normal volume of lymph?

Answer 2

100ml

Question 3

Give the free ion distribution across cell membranes

Answer 3

Outside = Na (133-146), Cl (95-108), Ca (2.1-2.6), K (3.5-5.3)

Inside = Na (10-12), Cl (4.2), Ca (10^-7), K (140-155)

Question 4

What is the role of primary active transport in the nephron?

Answer 4

Sets up Na/K gradients

Question 5

Describe Na+ reuptake by the thick ascending limb

Answer 5

Na/K ATPase creates gradient via capillary

NKCC2 (Na K Cl co-transporter) able to take Na from the tubular fluid

ROMK (renal outer medullary potassium channel) K into the tubular fluid

Question 6

Where do loop diuretics work?

Answer 6

On NKCC2 in the thick ascending limb

Block Na reuptake therefore excreting more fluid

Question 7

Describe Na+ reuptake by the distal convoluted tubules?

Answer 7

Na/K ATPase creates gradient via capillary

NCCT (Na Cl co-transporter)

ENaC (epithelial channel) electrochemical gradient taking Na from tubule fluid

Question 8

Which drugs work on the DCT and which pumps are involved?

Answer 8

Thiazides = block NCCT

Amiloride = block ENaC

Block Na reuptake, therefore, excreting more fluid

Question 9

Describe Na+ reuptake by the cortical collecting duct

Answer 9

Na/K ATPase creates gradient via capillary

Aquaporin = water uptake

ENaC = Na uptake

CIC = chlorine uptake

ROMK = K removal

Question 10

What hormones act on the cortical collecting duct?

Answer 10

ADH = enhances aquaporins

Aldosterone = enhances ENaC and ROMK

Question 11

Which drugs work on the cortical collecting duct and which pumps are involved?

Answer 11

Spironolactone = acts as aldosterone antagonist therefore inhibiting ENaC and ROMK

Amiloride = block ENaC

Block Na reuptake therefore excreting more fluid

Question 12

Describe Na+ and bicarb reuptake in the early proximal tubule

Answer 12

Na pump = sets up the Na+ gradient

NHE (Na/H exchanger) = H combines and forms carbonic acid, that then dissociates, CO2 is then absorbed, carbonic acid is reformed which splits to form the original H and bicarb

Anion exchanger = bicarb in for Cl out

Question 13

Which drugs work on the early proximal tubule and which pumps are involved?

Answer 13

Amiloride = blocks NHE

Block Na reuptake therefore excreting more fluid

Question 14

Outline Cl reabsorption in the proximal tubule

Answer 14

Na pump sets up Na gradient

NHE = removes H, forms formic acid which goes back into the cell, then forms formate

Anion exchanger = removes formate from the cell but puts Cl into the cell

Question 15

How can Cl move through tight junctions?

Answer 15

Going down the tubule the conc of Cl increases = high enough conc for it to move through tight junctions into the cell

Question 16

Describe phosphate reuptake in the proximal tubule

Answer 16

Na/K ATPase creates gradient via capillary

Na dependent phosphate transporter = co-transports 2Na in with 1Pi in

Phosphate conc rises and passively diffuses into the capillary

Question 17

Phosphate reuptake in the proximal tubules is hormonally regulated by what?

Answer 17

PTH

Vit D

Question 18

How does glucose reuptake into the proximal tubules take place?

Answer 18

Na/K ATPase creates gradient via capillary

SGLT2 (Na-Glucose transporter 2) into cell

GLUT2 from cell to capillary

As glucose conc rises all the transporters become saturated and more is seen in the urine

Question 19

How does AA reuptake into the proximal tubule occur?

Answer 19

Na/K ATPase creates gradient via capillary

Na-dependent AA transporter

Passive AA transporter into the capillary

Question 20

How does urea reuptake into the proximal tubule occur?

Answer 20

Na/K ATPase creates gradient via capillary

Na-dependent urea transporter

Passive urea transport into the capillary

Question 21

Outline receptor-mediated endocytosis

Answer 21

Ligand binds ligand

Coated pit created

Pinches off

Uncoated vesicle fused with lysosome

Dissociation of receptor

E.g. Albumin, vit B12 via transcobalamin, vit D3,

Question 22

Describe Ca2+ reuptake in the proximal tubule

Answer 22

TRPV6 Ca channels = into the cell

Na/K ATPase creates gradient via capillary

NCX = Na/Ca exchanger into the capillary

Question 23

How is K secreted in the thick ascending limb?

Answer 23

ROMK (renal outer medullary K channels)

Blocked by spironolactone = less water secreted

Question 24

How are organic acids secreted in the proximal tubule?

Answer 24

Acid-base antiporter