Secretion and Digestion Physiology Flashcards
resting vs active parietal cells
rest: proton pumps (H/K ATPases) are sequestered in tubulovesicles
active: tubulovesicles fuse w/ intracellular canaliculi to become continuous w/ lumen
how is acid produced in parietal cells
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
stimulants for acid production
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
stimuli for gastrin
GRP downstream of vagus
peptides and AAs, gastric distension, raised pH
3 phases of acid secretion
- cephalic- interdigestive, vagal stimulation from pharynx signals and thought of eating (pH less than 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)
- intestinal- protein digestion products stim G cells
challenges for carb/protein absorption vs lipids
carb/proteins require transporters to cross membrane, but lipids need to be hydrophilic and soluble w/i intestinal lumen
4 stages of lipid digestion
- emulsification
- hydrolysis
- micellization
- packaging and transport of chylomicrons
emulsification location and fn
starts in stomach
mechanical digestion and churning creates smaller fat particles- i micrometer
bile role in emulsification
bile salts and lecithin have both hydrophilic and hydrophobic ends, helps make fat particles dissolve
lipid hydrolysis location and fn
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
relation of pancreatic lipase and bile
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
micellization role
prevent reverse rxn of lipolysis and accumulation of free fatty acids in intestine
process of micellization
dependent on bile salts and lecithin- these help trap free fatty acids in a nucelus surrounded by a polar shell to make them soluble
how are micelles absorbed by enterocytes
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
describe metabolic trapping
FFAs are packaged in to TGs, cholesterol esters, fat soluble vitamins to prevent diffusion back across intestinal epithelium
production of chylomicrons
reassembled lipids are attached to apolipoproteins , chylomicrons have core of TGs surrounded by these proteins, phospholipids, and CE
then transported basolaterally
apoprotein disturbance
w/o apoproteins, TGs accumulate in cytosol
chylomicron transport
enter lacteal in the vilus, drain into lymphatic system then thoracic lymphatic duct, then subclavian vein
4 types of carbs ingested
starch, sucrose, lactose, maltose
starch components
amylose and amylopectin, two glucose polymers
molecules for carb digestion
salivary amylase (destroyed in stomach) and pancreatic alpha amylase (most, breaks starch into alpha limit dextrans)
rate limiting step in sugar intake
not digestion, but absorption of monosaccharides across brush border
source of fiber
cellulose and hemicellulose, we dont have enzymes for digesting the beta glucose bonds
3 monosaccharide transporters
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
how is protein digestion diff from carbs
many different AA and some better absorbed as oligomers
peptide transporters mean protein digestion often happens in cytosol rather than lumen
result of protein digestion from pepsin in stomach
pepsin is specific and limited, only a few free AAs enter intestine, mostly large peptides
most proteolysis occurs in…
SI lumen- via pancreatic proteases
contrast two types of pancreatic proteases
endo: trypsin, chymotrypsin, elastase- cleave and internal amide bonds
exo: carboxypeptidase A and B- cleave at terminal AA
how are pancreatic proteases activated
via trypsin, which is converted from trypsinogen by enterokinase on duodenal epithelium
premature activation causes autodigestion and acute pancreatitis
how are peptides absorbed
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