lecture 5/6- Molecular Mechanisms of Epithelial Ion and Fluid Transport

Question 1

movement of sodium outside to inside

Answer 1

entry through sodium channels down an electrochemical gradient (facilitated passive diffusion)
sodium removed by Na/K ATPase which requires ATP
sodium is moved from luminal side into the blood

Question 2

movement of chloride from outside to inside

Answer 2

chloride move between the cells (paracellular diffusion) through tight junctions
movement of cl maintains electroneutrality
now we have sodium chloride moved across

Question 3

water follow the NaCl by

Answer 3

osmosis through aquaporins or tight junctions

Question 4

NKCC1 transports on basolateral membrane

Answer 4

1 sodium 1 potassium and 2 chloride into the cell

Question 5

secondary active transporter as it uses the gradient set up by

Answer 5

Na/K ATPase

Question 6

on apical membrane the Cl channels we have are

Answer 6

CFTR and calcium activated Cl- channel (CaCC)
In CF, CFTR is defective, so salt and fluid secretion is inhibited

Question 7

R domain is the part of CFTR that is phosphorylated by

Answer 7

protein kinase A and this phosphorylation causes the channel to open

Question 8

CFTR activity is regulated by

Answer 8

PKA and ATP

Question 9

phosphorylation induces ATP binding of the

Answer 9

NBDs (nucleotide binding domains)

Question 10

conformational change in NBDs transmitted to

Answer 10

membrane spanning domains causing the pore to open

Question 11

ATP is hydrolysed and the pore closes

Answer 11

dephosphorylation of the R domain by protein phosphatases closes the channel even in the presence of ATP

Question 12

patch clamp technique allows

Answer 12

you to make electrical measurements of ions moving through ion channels
shows that ATP alone has no response

Question 13

calcium activated chloride channels (CaCC) are activated by

Answer 13

a rise in calcium. calcium binds to CaCC and you get chloride release

Question 14

examples of CaCCs are

Answer 14

TMEM16A and B

Question 15

TMEM16A

Answer 15

10 transmembrane domains
calcium ions bind to glutamate residues in one of the alpha helices of intracellular loop 3
alpha helices move apart opening the pore enabling chloride transport

Question 16

chloride can be brought back into the cells in exchange for

Answer 16

bicarbonate by SLC26A anion exchanger(AE)

Question 17

sodium move with bicarbonate by

Answer 17

NBC (Na+ dependent HCO3- cotransporters) on the basolateral membrane that supplies cytosolic HCO3- for the exchanger

Question 18

SLC26A and CFTR join together by scaffold protein allows

Answer 18

the activation of CTR to witch on AE activity

Question 19

in CF, AE activity is inhibited reducing net HCO3- and

Answer 19

fluid secretion

Question 20

pancreas is made up of 2 cells

Answer 20

duct and acinar cells

Question 21

acinar cells

Answer 21

secrete a variety of digestive enzymes and a low volumes of NaCl rich fluid into ducts using TMEM16A (CaCC) channels

Question 22

duct cells

Answer 22

transport digestive enzymes to SI. Produce a high volume of NaHCO3- rich solution using both CFTR and SLC26A6

Question 23

in CF theres still secretion of enzymes and NaCl fluid but theres no NaHCO3- fluid secretion as the CFTR and AE dont function

Answer 23

therefore you cant solubilize the enzymes and they cause blockages in the ducts. the enzymes get activated and start to self digesting the pancreas

Question 24

ENaC mediated fluid absorption occurs in the

Answer 24

colon

Question 25

sodium linked absorptive ion transporters

Answer 25

utilises SLC26A6 and A3 and they couple to a sodium hydrogen exchanger (NHE3)
sodium and chloride goes in and HCO3 and H+ goes out and forms co2 and water
if A3 is dysfunctional it causes Cl- losing diarrhoea

Question 26

nutrient absorptive transporters

Answer 26

eg glucose and amino acids but brings water with it

Question 27

cholera toxin (CT) inhibits NaCl and fluid absorption and stimulates CFTR mediated Cl/HCO3 and fluid secretion

Answer 27

leads to secretary diarrhoea

Question 28

CT and binds ADP ribose to the alpha s subunit

Answer 28

blocks GTP hydrolysis by a covalent modification
therefore alpha s subunit is switched on continuously and therefore always making cAMP
leads to an uncontrolled overstimulation of CFTR resulting in excessive salt and water loss into the intestinal lumen causing diarrhoea

Question 29

oral rehydration therapy

Answer 29

uses solute carriers which brings in solutes and water will follow