Pancreas physiology

Question 1

Pancreatic hydrolases

Answer 1

• Pancreatic hydrolase nzs are stored as inactive precursors (zymogens) to prevent auto-catalysis of the pancreas
• Enterokinase (on the luminal membrane of SI) activates trypsinogen to trypsin, and trypsin activates the remaining zymogens
• Pancreatic zymogens are released upon increases in intracellular [Ca], due to CCK or ACh binding to their GPCRs

Question 2

Action of CCK and ACh

Answer 2

• Both activate their respective GPRC
• The Ga subunits from these receptors activate PLC-B (phospholipase C beta), which hydrolyzes PIP2 (phosphatidylinositol bisphosphate) to IP3 (inositol triphosphate) and DAG (diacylglycerol)
• IP3 receptor is on ER, and is a ligand-activated Ca channel
• Thus IP3 will increase [Ca] via efflux of Ca from ER
• Ca will activate calmodulin-dependent kinase and PKC
• DAG will also activate PKC

Question 3

Secretory vesicle exocytosis

Answer 3

• Both docking and fusion of vesicles are mediated by interactions btwn T and V SNARES on the vesicular and plasma membranes
• Both docking and fusion via SNARES also requires high levels of Ca
• Normally there is a sub apical microfilament network barrier preventing exocytosis
• When Ca, calmodulin-kinase, and PKC are active, there is rearrangement of the microfilament network
• The network moves from blocking vesicular fusion to surrounding the vesicle, then myosin-like motors push the vesicle to membrane so the contents are expelled
• Extra membrane from the vesicle is recycled

Question 4

Ca oscillations

Answer 4

• Giving ACh leads to oscillations of intracellular Ca (Ca returned to ER via SERCA)
• This is because cholinesterases breakdown ACh rapidly, but then another ACh binds to the receptor
• Thus there are cycles of activation and inactivation of the GPCR and thus oscillations of Ca and other second messengers w/in the cell
• Upon giving a high enough ACh concentration, the secretion rate of amylase plateaus out at a maximal secretion rate

Question 5

Supramaximal stimulation 1

Answer 5

• Upon giving high enough doses of CCK, amylase secretion will rise but then fall sharply in “supra maximal stimulation”
• In supramaximal stimulation the Ca oscillations stop and [Ca] just remains elevated
• Supramaximal stimulation occurs b/c CCK is not rapidly broken down like ACh, thus there can be constitutive activation of the CCK receptor and thus constitutively elevated [Ca]

Question 6

Supramaximal stimulation 2

Answer 6

• Constitutively elevated Ca leads to a loss of exocytosis because Ca levels must fall for the actin microfilaments to reform after an exocytosis
• Since the network is incapable of returning to baseline when Ca is elevated, exocytosis cannot continue
• This can be seen in alcoholics on binges who do not eat
• When they eat their first meal after the binge there is supra maximal stimulation of CCK (can result in acute hemorrhagic pancreatitis)

Question 7

Acute hemorrhagic pancreatitis

Answer 7

• Seen in pregnancy (occlusion of bile ducts by stones) and alcoholics (first meal after binge)
• Zymogen vesicles are transformed into aqueous vacuoles, then activated due to interaction w/ lysosomal hydrolyses
• The cell ruptures and active hydrolyses are released into stroma causing damage
• This leads to inflammatory responses that cause further damage (PMNs)
• Overall this is due to lysosomal proteases (cathepsins) aberrantly accumulating in the same vacuoles as the zymogens and activating the zymogens (trypsin)

Question 8

Lysosomal hydrolases

Answer 8

• Procathepsins
• Prophospholipases
• Glycohydrolases (lysozyme)

Question 9

How lysosomal nzs and zymogens cross paths 1

Answer 9

• Normal flow of zymogen vesicle formation: TGN-> vesicles carrying zymogens fuse into dense aggregates and excess membrane is recycled to TGN
• Normal flow of lysosomal nzs: TGN to late endosome (either directly or first to early endosome), then to mature lysosome
• Both pathways originate from TGN

Question 10

How lysosomal nzs and zymogens cross paths 2

Answer 10

• When there is constitutively high Ca there are changes in cytoskeleton that prevent lysosomal nzs from being transported to late endosome (from early endosome and TGN)
• Since the lysosomal nzs cannot get to late endosome they reflux into TGN/early endosome
• Active cathepsins will activate procathepsins in the TGN
• This together w/ the inability to fuse zymogen vesicles w/ the apical membrane (due to supra maximal stimulation-> high Ca) leads to mixing of zymogens and lysosomal nzs in the TGN and thus activation of the zymogens