Water Balance in the GI tract

Question 1

What is a passive process driven by the transport of solutes (particularly Na+) from the lumen of the intestines to the bloodstream?

Answer 1

Absorption of water

Question 2

How much water enters the tract per day?

Answer 2

9.3 litres

Question 3

How much water is absorbed by the small intestine?

Answer 3

8.3 litres

Question 4

How much water enters the large intestine, and of that what percentage is absorbed?

Answer 4

1 litre, 90% absorbed

Question 5

What has 100ml in faeces and what has 50ml, along with, bilirubin and bacteria?

Answer 5

100ml water

50ml cellulose

Question 6

What is defined as a loss of fluid and solutes from the GI tract in excess of 500ml per day?

Answer 6

Diarrhoea

Question 7

What is intestinal fluid movement always coupled to?

Answer 7

Solute movement

Question 8

What two ways may water move?

Answer 8

Transcellular

Paracellular

Question 9

What two prinicple mechanisms of Na absorption occurs throughout the small intestine and is most omportant in the post-prandial period (also occurs in the colon in the new born)?

Answer 9

Na+/glucose co-transport

Na+/amino-acid co-transport

Question 10

What prinicple mechanism of Na movement occurs in the duodenum and jejunum and is stimulated by HCO3?

Answer 10

Na+/H+ exchange

Question 11

Where does Na+/H+ exchange occur?

Answer 11

In duodenum and jejunum

Question 12

What stimulates Na+/H+ exchange?

Answer 12

HCO3

Question 13

Where and at what time does the parallel Na+/H+ and Cl-/HCO3 exchange occur?

Answer 13

In the ileum and colon, most important in the interdigestive period

Question 14

What principle mechanism for Na exchange occurs in the colon (distal) and is regulated by aldosterone?

Answer 14

Epithelial Na+ channels (ENaC)

Question 15

What to mechanisms of postprandial Na+ absorption are examples of secondary active transport and are electrogenic?

Answer 15

Na+/glucose and Na+/amino acid co-transport

Question 16

Collectively, what does the overall transport of Na+ generate?

Answer 16

A transpepithelial potential (Vte) in which the lumen is negative

Question 17

Due to the transepithelial potential in which the lumen is negative, what does this drive?

Answer 17

Parallel absorption of Cl-

Question 18

Where does Na+/H+ exchange in the jejunum occur at?

Answer 18

Both the apical and baseolateral membranes (via different isoforms of the exchanger)

Question 19

During Na+/H+ exchange in the jejunum, what two things contribute to transepithelial movement of Na+ and the regulation of intracellular pH?

Answer 19

NHE2 and NHE3

Question 20

What is exchange at the apical membrane, in the jejunum stimulated by?

Answer 20

The alkaline environment of the lumen due to presence of bicarbonate from the pancreas

Question 21

In the jejunum, what is absent?

Answer 21

A parallel Cl–HCO3 exchanger

Question 22

What is the primary mechanism of Na+ absorption in the interdigestive period?

Answer 22

Na+/H+ and Cl-/HCO3- exchange in parallel

Question 23

In Na+/H+ and Cl-/HCO3-, what is absorption like?

Answer 23

Electroneutral

Question 24

In Na+/H+ and Cl-/HCO3-, what is it regulated by?

Answer 24

Intracellular cAMP, cGMP and Ca2+, all of which reduce NaCl absorption

Question 25

What is reduction in NaCl absorption a cause of?

Answer 25

Diarrhoea

Question 26

Explain how secretory diarrhoea is caused due to infection with E. coli?

Answer 26

Enterotoxin from which activates adenylate cyclase and increases intracellular cAMP, reducing NaCl absorption

Question 27

What mediates electrogenic Na+ absorption in the distal colon?

Answer 27

Epithelial Na+ channels (ENaC)

Question 28

What is ENaC increased by?

Answer 28

Aldosterone

Question 29

What are the three actions of aldosterone on ENaC?

Answer 29

1. opens ENaC 2. Inserts more ENaC into membrane from intracellular vesicle pool 3. Increases synthesis of ENaC and Na+/K+-ATPase

Question 30

What two routes can Cl absorption occur passively on?

Answer 30

Transcellular and paracellular

Question 31

In the small intestine, what is the driving force for Cl- absorption provided by?

Answer 31

Lumen negative potential due to electrogenic transport of Na+ in Na+/glucose and Na+/amino-acid co-transporty]

Question 32

In the large intestine, what is the driving force provided by for the absorption of Cl-?

Answer 32

Lumen negative potential due to electrogenic movement of Na+ through ENaC.

Question 33

What is another mechanism for Cl- absorption that occurs in the ileum, proximal and distal colon?

Answer 33

Cl-HCO3- exchange

Question 34

What is another mechanism for Cl- absorption that occurs in the ileum and proximal colon?

Answer 34

parallel Na+/H+ and Cl-/HCO3 exchange

Question 35

What occurs at a basal rate but is usually overshadowed by a higher rate of absorption?

Answer 35

Cl- secretion

Question 36

What cells does Cl- secretion occur from?

Answer 36

Crypt cells

Question 37

What membrane does Cl- absorption into cell before secretion occur at?

Answer 37

Basolateral

Question 38

What are the three processes involed on the basolateral membrane in Cl- secretion?

Answer 38

1. Na+/K+ATPase 2. Na+/K+/2Cl-co-transporter (NKCC1) 3. K+ channels (IK1 and BK)

Question 39

In Cl- secretion, what does low intracellular Na+ drive?

Answer 39

Inward movement of Na+, K+ and Cl- via NKCC1

Question 40

In Cl- secretion, once low intracellular Na+ drives inward movement of Na+, K+ and Cl- via NKCC1, what occurs next?

Answer 40

K+ recycles via K+ channels, but intracellular concentration of Cl- increases providing an electrical chemical gradient for Cl- to exit cell via CFTR on teh apical membrane.

Question 41

In Cl- secretion, once Cl- has exited the cell via CFTR on the apical membrane, what happens?

Answer 41

Lumen negative potential develops providing voltage-dependent secretion of Na+ through paracellular pathway

Question 42

What has to be active for Cl- secretion to occur?

Answer 42

CFTR

Question 43

What 4 substances activate CFTR?

Answer 43

1. bacterial enterotoxins 2. Hormones and neurotransmitters 3. Immune cells products 4. Some laxatives

Question 44

Activation of CFTR also occurs indirectly as a result of the generation of what three second messengers?

Answer 44

1. cAMP 2. cGMP 3. Ca2+

Question 45

What two things does the Cl- conductance mediated by CFTR result from?

Answer 45

1. Opening of channels at the apical membrane | 2. Insertion of channels from intracellular vesicles into the membrane

Question 46

What causes metabolic acidosis, due to loss of HCO3 and hypokalaemia?

Answer 46

Diarrhoea

Question 47

What 4 factors can lead to impaired absorption of NaCl and so lead to dairrhoea?

Answer 47

1. Congenital defects - congenital chloridorrhoea absence of Cl--HCO3 exchanger 2. Inflammation 3. Infection 4. Excess bile acid in colon

Question 48

What is a cause of non-absorbable solutes in intestinal lumen - leading to diarrhoea?

Answer 48

Lactase deficiency

Question 49

What provides a classic example of excessive secretion to lead to diarrhoea?

Answer 49

Cholera

Question 50

Once cholera toxin has entered the enterocyte, what does it enzymatically inhibit?

Answer 50

GTPase activity of the Gsalpha subunit.

Question 51

What does enzymatically inhibited GTPase increase the activity of?

Answer 51

Adenylate cyclase leading to increased cAMP

Question 52

What does cAMP stimulate and what does it lead to?

Answer 52

CFTR - hypersecretion of Cl- with Na+ and water following

Question 53

What does this describe: 2Na+ binds to SGLT1, affinity for glucose increases so glucose binds, Na+ and glucose translocate from extracellular to intracellular, 2Na+ dissociates, affinity for glucose falls so glucose dissociates and the cycle repeats?

Answer 53

Rehydration therapy exploting SGLT1

Question 54

What types of drugs have anti-diarrhoeal activity/

Answer 54

Morphine and opiate drugs

Question 55

What do opiates do to enteric neurones?

Answer 55

Inhibit them causing hyperpolarisation via activation of u-opoid receptors

Question 56

What do opiates do to peristalsis and segmentation?

Answer 56

Decreased peristalsis and increased segmentation (i.e. constipating)

Question 57

What do opiates do to fluid absorptin?

Answer 57

Increase it

Question 58

What drugs cause constriction of pyloric, ileocaecal and anal sphincters?

Answer 58

Opiates

Question 59

Name three major opiates used in diarrhoea?

Answer 59

Codeine Diphenoxylate Loperamide