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GI > Paracrine Exocrine Secretions + Control > Flashcards

Paracrine Exocrine Secretions + Control Flashcards

1
Q

Secretions form acini?

A
  • Acini form sacs → connect to ductal tree → enters duodenum at main common bile duct
  • Intercalated ducts →intralobular ducts → extralobular ducts →interlobular ducts → main duct enters duodenum at common bile duct

found throughout the pancreas
Secrete:

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2
Q

Secretions of Islets of Langerhans?

A

insulin (β cells 65%)
glucagon (α cells 20%)
somatostatin (δ cells 10%)

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3
Q

Role of exocrine (acinar) cells?

A

secrete pancreatic enzymes into pancreatic duct secrete digestive enzymes stimulated by CCK

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4
Q

Whta’s exocrine portion of pancreas composed of?

A

acinar + duct cells

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5
Q

Role of duct cells?

A

release the aq NaHCO3 solution stimulated by secretin

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6
Q

Features of acinar cells?

A

-Pyramidal shape
-Arranged as acini sacs(clusters of cells resembling a many-
lobed berry)
-Microvilli extend into ducts
-Intercalated ducts lined by squamous epithelial cells
-Intralobular ducts lined by cuboidal or low columnar epithelium
-Interlobular ducts lined with columnar epithelium + goblet cells
-Pancreatic duct enters dudenum at common bile duct

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7
Q

What does acinar consist of?

A

pyramidal epithelial cells → digestive enzymes

  • microvilli extend from apical surface of epithelial cells into lumen of acini (tubes in tubular glands)
  • intercalated ducts begin within acini + duct cells (centroacinar cells)
  • intercalated ducts form smallest pancreatic duct system
  • drain into intralobular ducts lined with cuboidal epithelium
  • intralobular ducts lead into larger interlobular ducts in fibro-collagenous interlobular connective tissue surrounded by columnar epithelium
  • duct size increases
  • surrounding connective tissue denser
  • wall of pancreatic duct has smooth muscle
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8
Q

Where do microvilli extend from?

A

apical surface of epithelial cells into lumen of acini (tubes in tubular glands)

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9
Q

Describe endocrine pancreas

A
  • Islets of Langerhans: clusters of endocrine cells located throughout pancreas
  • Abundant in tail since blood vessels + each with its own capillaries
  • Insulin: β cells 70% of pancreatic endocrine cells - increase glucose uptake into cells
  • Glucagon: α cells 20% of pancreatic endocrine cells -increases gluconeogenesis, glycogenolysis which increases blood glucose levels
  • Somatostatin: 5-10% of pancreatic endocrine cells, acts locally as paracrine agent, inhibits insulin, glucagon production + inhibits gut peptides secretin, CCK, gastrin, motilin
  • Other cell types secrete VIP
  • Enterochromaffin cells secrete serotonin, motilin, substance P
  • PP or F cells secrete pancreatic polypeptide (1-2% of pancreatic cells)
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10
Q

Features of PP cells?

A
  • Cells in pancreas head that secrete pancreatic polypeptide (PP)
  • Helps to self-regulate pancreatic secretion activities (endocrine + exocrine)
  • Effects on hepatic glycogen levels + secretions from GI
  • Secretion increased after protein meal, fasting, acute hypoglycaemia
  • Somatostatin + IV infusion of glucose decrease its secretion
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11
Q

What does acini contain?

A

numerous mitochondria, nucleus, zymogen granules (filled with enzyme precursors) located towards apex

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12
Q

Role of centroacinar cells?

A

lines intra-acinar portion of duct

initiates alkalinisation of pancreatic juice

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13
Q

Where are acini?

A

salivary glands + pancreas.

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14
Q

Where does each acini drain into?

A

a duct

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15
Q

Role of exocrine pancreatic enzymes?

A
  • Required for digestion
  • Produced in acinar cells
  • Stored in cytoplasmic zymogen granules
  • Zymogen granules accumulate cell apex + released by exocytosis
  • Acinar cells arranged in secretory acini which drain into intercalated ducts -> interlobular ducts -> main pancreatic duct
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16
Q

Features of centroacinar cells?

A

=intercalated ducts since represent beginning of intercalated ducts

  • Spindle-shaped cells in exocrine pancreas
  • Extension of intercalated duct cells into each pancreatic acinus
  • Nuclei in centre
  • Initiates alkalinisation of pancreatic juice.
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17
Q

Features of intercalated + intralobular ducts?

A
  • Secrete Na+ and HCO3- rich fluid
  • Intercalated ducts take HCO3- to intralobular ducts which become extralobular ducts
  • HCO3- helps keep zymogens inactive – maintains pH >7.5
  • Premature activation of enzymes within pancreatic acinar cells –> onset of acute pancreatitis
  • Secretion mainly via HCO3-/Cl- exchanger regulated by CFTR
  • HCO3- released when secretin meets centroacinar + duct cells to buffer acidic chyme
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18
Q

Features of basal lamina?

A
  • Layer of extracellular matrix secreted by epithelial cells
  • Epithelium sits on it
  • Constitute a portion of basement membrane.
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19
Q

Describe how acinar cells connect to duodenum

A
  • adjacent pyramidal acinar cells oriented
  • to form acini
  • groups of acini form lobules
  • each lobule, ductules join to form intralobular ducts
  • lobules drain into extralobular ducts
  • converge into main collecting duct
  • connecting with duodenum at common bile duct
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20
Q

Functional units of exocrine pancreas?

A

pancreatic acinar cells

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21
Q

Describe pancreatic ductal system

A

Intercalated ducts- squamous epithelium
Intralobular ducts-cuboidal or low columnar epithelium
Extralobular ducts
Interlobular ducts – columnar epithelium/goblet cells
Pancreatic duct

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22
Q

What are intralobular cells composed of?

A

centroacinar cells

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23
Q

Pancreatic exocrine functions?

A
  • Secretes 1.5L of fluid/day
  • Na+ and HCO3- rich juice, albumin, globulin, digestive enzymes
  • Acinar cells secrete digestive enzymes to breakdown carbohydrates, fats, proteins, nucleic acids
  • Enzymes secreted in inactive form to prevent autodigestion
  • Activation of enzymes occurs in duodenum
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24
Q

Anions + cations secreted in pancreatic juice?

A

Anions : HCO3-, Cl-, SO42-, HPO42-
Cations : Na+, K+, Ca2+, Mg2+
Name the

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25
Q

Enzymes secreted in pancreatic juice?

A 
trypsinogen
chymotrypsinogen, 
elastase,
carboxypeptidase
pancreatic
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26
Q

What neutralises gastric acid?

A

HCO3- rich juice + secretions from gallbladder

↑pH to 6or7

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27
Q

Diff between pancreatic vs salivary gland secretion?

A

HCO3- rich hypertonic juice

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28
Q

Describe how composition of juice modified as it travels through duct

A
  • Cl- actively exchanged for HCO3- by epithelial cells
  • [HCO3-]pancreatic + rate of production ∝ [HCO3-]blood
  • H+ actively eliminated
  • more CO2 + HCO3- produced in blood
  • H+ exchanged for K+, Na+
  • H+ neutralises HCO3-
  • H2CO3 formed
  • CO2 diffuses in
  • forms H2CO3 + H2O
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29
Q

Effect of low secretion rate?

A

then most of fluid secreted by intralobular ducts

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30
Q

Role of CFTR?

A
  • Cystic fibrosis transmembrane conductance regulator protein + chloride channel in vertebrates encoded by CFTR gene to maintain balance of salt + water on surfaces in body eg lungs, gallbladder, pancreas
  • Functions as channel across membrane of cells that produce mucus, sweat, saliva, tears, digestive enzymes
  • In apical membrane of epithelial cells throughout body for fluid + electrolyte transport
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31
Q

Role of HCO3-/Cl- exchanger?

A

For secretion

Regulated by CFTR

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32
Q

Effects of secretin on composition + volume of pancreatic juice?

A

-Reciprocal change in conc of Cl- and HCO3-
-Fall in amylase conc reflects dilution effect as volume of
pancreatic juice increases
-Level of HCO3- > Cl-

33
Q

Effects of secretin on composition + volume of pancreatic juice?

A

-Reciprocal change in conc of Cl- and HCO3-
-Fall in amylase conc reflects dilution effect as volume of
pancreatic juice increases
-Less time for HCO3-/Cl- exchanger so level of HCO3- > Cl-

34
Q

Enzyme secretion of pancreas?

A

3 major types of enzymes

  • Proteolytic enzymes (trypsin, chymotrypsin and carboxypeptidase)
  • Amylase
  • Lipase
  • Ribonuclease
  • Deoxyribonuclease
35
Q

Diagnostic test for acute pancreatitis?

A

level of pancreatic α-amylase increased

36
Q

What activates trypsin?

A
  • secreted as trypsinogen

- activated by enteropeptidase + by trypsin

37
Q

Substrate of trypsin?

A

Proteins + polypeptides

38
Q

Substrate of pancreatic lipase?

A

TG

39
Q

What activates pancreatic α-amylase?

A

Cl-

40
Q

Substrate of pancreatic α-amylase?

A

starch

41
Q

Substrate of ribonuclease?

A

RNA

42
Q

Substrate of deoxyribonuclease?

A

DNA

43
Q

What activates elastase?

A

trypsin

44
Q

Substrate of elastase?

A

elastin

45
Q

What activates phospholipase A2?

A

trypsin

46
Q

Substrate of phospholipase A2?

A

phospholipids

47
Q

What activates chymotrypsins?

A
  • secreted as chymotrypsinogen

- activated by trypsin

48
Q

Substrate of chymotrypsins?

A

proteins + polypeptides

49
Q

What activates carboxypeptidase A + B?

A

trypsin

50
Q

Substrate of carboxypeptidase A + B?

A

proteins + polypeptides

51
Q

What activates colipases?

A

trypsin

52
Q

Substrate of colipases?

A

fat droplets

53
Q

Role of proteolytic enzymes + eg?

A

Involved in digestion of proteins so pancreas at risk of
autodigestion so enzymes released as proenzymes

eg enterokinases (enteropeptidase), amylase, lipase

54
Q

Role of enterokinases (enteropeptidase)?

A

-Converts trypsinogen -> trypsin within brush border

55
Q

Inhibitors of autodigestion?

A
  • kazal inhibitor, enzyme Y (chymotrypsin C, caldecrin)

* intracellular pH zymogen granule = acidic

56
Q

Role of trypsin?

A
  • Trypsin autocatalytic (autoactivation of other proenzymes)
  • Acute pancreatitis: trypsin activates phospholipase A2 (PLA 2) in pancreatic duct
57
Q

Role of phospholipase A2 (PLA 2)?

A

converts lecithin -> isolecithin

58
Q

Effect of isolecithin?

A

disrupts pancreatic tissue, membrane damage, necrosis

59
Q

Role of amylase + eg?

A

carbohydrate digestion
converts polysaccharides -> disaccharides
eg starch -> maltose

60
Q

Role of lipases?

A

fat digestion

converts TG -> FA + glycerol

61
Q

Effect of pancreatic insufficiency of lipase?

A

malabsorption of fats = steathorrea

62
Q

Role of enzyme Y (chymotrypsin C (caldecrin))

A
  • At high [Ca2+] of duodenum it allows activation of trypsinogen
  • At low [Ca2+] of duodenum it facilitates degradation of trypsin
63
Q

Percentage of secreted enzymes that reach small intestine?

A

Amylase: 75%
Trypsin: 20%
Lipase: 1%

64
Q

What suggests bidirectional permeability of basolateral membrane to digestive enzymes?

A

Chymotrypsinogen + amylase cross basolateral membrane of pancreas

65
Q

How much carb reduced to polysaccharide?

A
  • 1/3 carb reduced to polysaccharides by the time chyme leaves the stomach
  • 2/3 digested by pancreatic amylase.
66
Q

Role of Bowman-Birk inhibitor?

A

Digestive enzyme in some food

eg soybeans stops trypsin + chymotrypsin activities

67
Q

Control of pancreatic secretions?

A
  • Neuroendocrine signals
  • Vagal (para) stimulation: enhances secretion rate of enzymes + aq components of pancreatic juice
  • Symp stimulation: inhibits secretion (decreased blood flow?)
  • Secretin + CCK: stimulate secretion of pancreatic fluid
68
Q

How cephalic phase controls pancreatic secretions?

A

Vagal (ACh + VIP) stimulation of gastrin release from antrum → some protein-rich pancreatic juice

69
Q

How gastric phase controls pancreatic secretions?

A

Distension (vagal reflex on fundus or antrum), AA + peptide (in antrum) stimulated gastrin secretion → release of enzyme-rich pancreatic juice

70
Q

How intestinal phase controls pancreatic secretions?

A

Secretin + CCK

Chyme in duodenum + jejunum induces secretion of pancreatic juice

71
Q

Effect of vagotomy on chyme?

A

Vagotomy → 50%↓ in response to chyme

72
Q

How secretin controls pancreatic secretions?

A
  • From duodenal and jejunum
  • Induces pancreatic duct cells to secrete HCO3-rich pancreatic juice, but ↓ enzyme content
  • Stimulates bile production
73
Q

How CCK controls pancreatic secretions?

A
  • From duodenal + jejunum in response to FA (AA, peptides)
  • Stimulates pancreatic acinar cells to synthesise + release enzyme-rich pancreatic juice
  • Stimulates secretion of conc bile for fat absorption
  • Contracts gallbladder + relaxes the sphincter of Oddi → release of bile + HCO3- into duodenum
  • Potentiates effects of secretin (weak agonist of acinar cells)
74
Q

Effect of abnormal mucus blocks ducts in pancreas?

A
  • Affects pancreatic secretion –> affects digestion

- Carb, protein, lipid digestion affected

75
Q

Effect of obstruction of pancreatic duct by mucus?

A
  • Prevents release of digestive enzymes needed to breakdown food + absorb nutrients from food
  • Contributes to gastrointestinal symptoms in patients with cystic fibrosis (CF)
76
Q

Major symptoms of CF?

A
  • Due to dysfunction of exocrine glands, sweat glands secrete excessive salt
  • Pancreatic ducts blocked with thickened mucus –> pancreatic insufficiency
  • Lungs produce thickened mucus
77
Q

Consequences of CF?

A

-Severe malabsorption
-Steatorrhoea
-Recurrent chest infections
-98% sterile males due to congenital bilateral aplasia of the vas deferens (CBAVD) as vas deferens fails to develop
(vas deferens carries newly made sperm to back of prostate gland where it joins outlets of seminal vesicles, sperm released into semen during intercourse)

78
Q

How do CF patients digest food?

A

Take enzyme-replacement drugs but still malnourished since enzymes taken not good substitutes for natural

GI (15 decks)

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