Secretion and Digestion Physiology

Question 1

resting vs active parietal cells

Answer 1

rest: proton pumps (H/K ATPases) are sequestered in tubulovesicles
active: tubulovesicles fuse w/ intracellular canaliculi to become continuous w/ lumen

Question 2

how is acid produced in parietal cells

Answer 2

carbonic anhydrase converts water and CO2 to bicarb and acid

secretion of protons accompanied by equivalent release of bicarb from basolateral Cl/HCO3 antiporter, called alkaline tide

Question 3

stimulants for acid production

Answer 3

ACh from nervous system on M3 receptors

histamine from ECL on H2 receptors

gastrin from G cells to gastrin receptors

*gastrin and ACh also stimulate histamine release

Question 4

stimuli for gastrin

Answer 4

GRP downstream of vagus

peptides and AAs, gastric distension, raised pH

Question 5

3 phases of acid secretion

Answer 5

1. cephalic- interdigestive, vagal stimulation from pharynx signals and thought of eating (pH less than 2)
2. gastric phase- increase in pH in stomach stimulates gastrin (inhibits somatostatin), two other pathways: short pyloro pyloric reflexes and longer vagovagal reflexes (stomach to vagal nuclei and back)
3. intestinal- protein digestion products stim G cells

Question 6

challenges for carb/protein absorption vs lipids

Answer 6

carb/proteins require transporters to cross membrane, but lipids need to be hydrophilic and soluble w/i intestinal lumen

Question 7

4 stages of lipid digestion

Answer 7

1. emulsification
2. hydrolysis
3. micellization
4. packaging and transport of chylomicrons

Question 8

emulsification location and fn

Answer 8

starts in stomach

mechanical digestion and churning creates smaller fat particles- i micrometer

Question 9

bile role in emulsification

Answer 9

bile salts and lecithin have both hydrophilic and hydrophobic ends, helps make fat particles dissolve

Question 10

lipid hydrolysis location and fn

Answer 10

done by lipases

gastric lipase from chief cells work better in acidic, but works best on short/mediium TGs because only cleaves one side of glycerol

pancreatic lipase from pancreatic acinar duct cells better in alkaline and can cleave both sides of glycerol, so performs majority of work

Question 11

relation of pancreatic lipase and bile

Answer 11

bile acids inhibit fn of pancreatic lipase

made up for w/ colipase from pancreatic acinar cells, helps bind bile acids and lipase to restore lipolysis

Question 12

micellization role

Answer 12

prevent reverse rxn of lipolysis and accumulation of free fatty acids in intestine

Question 13

process of micellization

Answer 13

dependent on bile salts and lecithin- these help trap free fatty acids in a nucelus surrounded by a polar shell to make them soluble

Question 14

how are micelles absorbed by enterocytes

Answer 14

move easily into unstirred water layer over top of brush border

acidity of water layer releases fatty acids from micelles, they can then diffuse across membranes by simple diffusion (except things like cholesterol)

micelles not necessary, but make things more effecient

Question 15

describe metabolic trapping

Answer 15

FFAs are packaged in to TGs, cholesterol esters, fat soluble vitamins to prevent diffusion back across intestinal epithelium

Question 16

production of chylomicrons

Answer 16

reassembled lipids are attached to apolipoproteins , chylomicrons have core of TGs surrounded by these proteins, phospholipids, and CE

then transported basolaterally

Question 17

apoprotein disturbance

Answer 17

w/o apoproteins, TGs accumulate in cytosol

Question 18

chylomicron transport

Answer 18

enter lacteal in the vilus, drain into lymphatic system then thoracic lymphatic duct, then subclavian vein

Question 19

4 types of carbs ingested

Answer 19

starch, sucrose, lactose, maltose

Question 20

starch components

Answer 20

amylose and amylopectin, two glucose polymers

Question 21

molecules for carb digestion

Answer 21

salivary amylase (destroyed in stomach) and pancreatic alpha amylase (most, breaks starch into alpha limit dextrans)

Question 22

rate limiting step in sugar intake

Answer 22

not digestion, but absorption of monosaccharides across brush border

Question 23

source of fiber

Answer 23

cellulose and hemicellulose, we dont have enzymes for digesting the beta glucose bonds

Question 24

3 monosaccharide transporters

Answer 24

SGLT-1: Na and glucose across apical membrane, depends on secondary acitve transport to move galactose and glucose (lactose derivatives)

GLUT-5: fructose across apical membrane (Fru and Glu from sucrose)

GLUT-2: glucose across basolateral and into portal circulation

*GLUT 2,5 rely on diffusion

Question 25

how is protein digestion diff from carbs

Answer 25

many different AA and some better absorbed as oligomers

peptide transporters mean protein digestion often happens in cytosol rather than lumen

Question 26

result of protein digestion from pepsin in stomach

Answer 26

pepsin is specific and limited, only a few free AAs enter intestine, mostly large peptides

Question 27

most proteolysis occurs in…

Answer 27

SI lumen- via pancreatic proteases

Question 28

contrast two types of pancreatic proteases

Answer 28

endo: trypsin, chymotrypsin, elastase- cleave and internal amide bonds
exo: carboxypeptidase A and B- cleave at terminal AA

Question 29

how are pancreatic proteases activated

Answer 29

via trypsin, which is converted from trypsinogen by enterokinase on duodenal epithelium

premature activation causes autodigestion and acute pancreatitis

Question 30

how are peptides absorbed

Answer 30

large ones are poorly absorbed, further hydrolyzed by brush border peptidases

small ones absorbed independent of Na gradient, depend on inward H+ gradient maintained via apical Na/H exchange

peptides hydrolyzed in cytosol to AA via cytoplasmic peptidases