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MONOGASTRIC GI > 5.2 – Secretory Functions of SI and Diarrhea > Flashcards

5.2 – Secretory Functions of SI and Diarrhea Flashcards

1
Q

What are the secretory cells within SI crypts?

A

-crypt stem cells
-crypt enterocytes
-goblet cells
-enteroendocrine cells
-Paneth cells
-M-cells

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2
Q

Crypt stem cells:

A

-pluripotent stem cell population
-at base of crypts
*do not migrate from the base of the crypts

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3
Q

What cells arise form crypt stem cells?

A

-crypt secretory cells
-mucus secreting goblet cells
-enteroendocrine cells
-Paneth cells

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4
Q

Crypt enterocytes:

A

-microvilli at apical surface
-primary function is to secret Cl, Na and water into lumen
-migrate up the crypt and then up the villus

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5
Q

What happens when crypt enterocytes migrate up the crypt and then up the villus?

A

-phenotype switches to become an ABSORPTIVE enterocyte
*lamellipodia allows them to walk up the crypt
>1-2 days to migrate
>3-4 days to reach tip of villus=slough off

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6
Q

Goblet cells:

A

-migrate out to the villi
-become more numerous from duodenum to ileum
-secret mucus

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7
Q

Enteroendocrine cells:

A

-remain near base of crypt
-contain secretory granules contain the hormone they will secrete

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8
Q

What are some examples of hormones that enteroendocrine cells will secrete?

A

-CCK
-secretin
-gastrin
-VIP
-GLP-1
-peptide YY
-somatostatin
-ghrelin
-serotonin

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9
Q

Paneth cells:

A

-do NOT migrate
-secrete antibacterial substances (lysozyme, phospholipase, defensins)
*not found in dog, cat and pig

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10
Q

M-cells (or dome cells):

A

-not derived form crypt stem cells
-associated with Peyer’s patches=part of the immune system

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11
Q

What are some secretory cells with the SI villus?

A

-villous absorptive enterocytes
-Goblet cells
-M-cells (not as numerous as in crypts)

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12
Q

Villous absorptive enterocytes:

A

derived from crypt secretory enterocytes
-begin to secret enzymes within apical membrane microvilli and transport proteins required for absorption

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13
Q

Structure/shape of all cells lining mucosal surface of GIT:

A

-apical and basolateral membranes
-adjacent cells linked by ‘tight junctions’
>impermeable to viruses, bacteria, large molecules
>impermeable to water and small ions (can be overcome if electrochemical gradient is sufficient=’leaky’)

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14
Q

Brunner’s glands:

A

-in upper duodenum submucosa
-produce mucous-rich alkaline secretion (mucous containing bicarbonate)
*stimulated by secretin in response to low pH

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15
Q

Acinar cells of the Brunner’s glands secret:

A

-mucus

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16
Q

Ductal cells of Brunner’s glands role:

A

-add K+ and Na+
-remove Cl-
*forms an alkaline secretion

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17
Q

What does the lamina propria of the SI contain?

A

-numerous lymphocytes
-muscularis mucosa

18
Q

What do you need to understand about the crypt enterocyte secretion of Cl, Na and water to understand the etiology of diarrhea?

A

Na+ provides electrochemical force required to transport of AA, sugars, phosphate
-water reduces osmolarity of digesta

19
Q

Duodenum and jejunum: what senses changes within the lumen?

A

-vagal and ENS sensory afferent neurons
>increased osmolarity
>increased stretch
>increased AA
>reduced pH
*medulla initiates PS efferent stimulation of crypt cells

20
Q

What will the vagus nerve release in response to stretch or osmotic changes?

A

-ACh
>activation of muscarinic receptor stimulates G-protein activation of phospholipase-A =catalyzing production of IP3

21
Q

What happens when IP3 is produced?

A

-acts on membranes of internal cell organelles that store Ca2+=Ca channels open
-Ca binds to calmodulin=activates it
-Ca-calmodulin complex interacts with a Cl pump=opens
>activates an ATPase to release energy to transport the Cl- against the concentration gradient

22
Q

Cl channel pump activated to pump Cl- against concentration gradient:

A

-cystic fibrosis transmembrane conductance regulator protein
-Cl- pumped into lumen is replaced by a Cl from ECF
>Na follows to maintain osmotic neutrality

23
Q

What happens due to negative charge of Cl and high Na in ECF?

A

-causes Na ions to move to lumen across tight junction opening
>water follows through specialized water channels

24
Q

What are the 2 mechanisms of diarrhea?

A

-transit disorder
-osmotic diarrhea

25
Q

Transit disorder:

A

-‘hypermotility’=rapid intestinal transit due to rapid peristalsis

26
Q

What may accelerate transit time?

A

-increased volume of fluid in lumen by increase secretion
-incomplete absorption

27
Q

Osmotic diarrhea:

A

-accumulation of non-absorbable solute in the gut lumen (ex. high protein diet to pigs)
-poor absorption pathway for solute
-overwhelmed transport systems
-loss of absorptive surfaces (villus shedding)

28
Q

Diarrhea is an alteration in:

A

-unidirectional secretion into intestinal lumen
-unidirectional absorption into intestinal mucosa

29
Q

When does diarrhea occur?

A

-net fluid absorption exceeds secretion

30
Q

What can cause net fluid absorption to exceed secretion?

A

-increase secretion exceeding normal absorption (secretory)
-inhibit absorption, exceeding normal secretion (malabsorptive)
-combination of both (secretory and malabsorptive, often called secretory)

31
Q

How can you localize diarrhea based on mucous?

A

-SI: none
-LI: yes

32
Q

How can you localize diarrhea based on blood?

A

-SI: digested (darker)
-LI: fresh (bright red)

33
Q

How can you localize diarrhea based on weight loss?

A

-SI: no
-LI: yes

34
Q

How can you localize diarrhea based on volume:

A

-SI: larger
-LI: smaller

35
Q

How can you localize diarrhea based on increased frequency?

A

-SI: no
-LI: yes

36
Q

What is the general mechanisms of diarrhea pathogens?

A

-increase secretion through direct activation of secretory cells
>usually via increase activation of phosphorylation pathway
-activation of NES (indirect activation of secretory cell)
-inhibition of absorptive transporter
-disruption of tight junctions
-reducing absorptive cells

37
Q

Some pathogens and poisons cause damage which causes release of

A

-prostaglandins

38
Q

Prostaglandins will diffuse through the lamina propria to reach a:

A

-receptor on the basolateral membrane of the crypt cell
>GPCR linked to phospholipase-A=increase IP3=Ca channels open=secretion of Cl, followed by Na and water

39
Q

What happens with bacterial toxin and serotonin secreting cells next to a crypt cell?

A

-activates receptors at the apical membrane of the EEC=serotonin cells fuse with basolateral membrane=secretes serotonin into lamina propria
>diffuses towards receptor in basolateral membrane of crypt cell=conformation change in receptor=open Ca channels
>Ca enters cell=Ca-calmodulin complex and activation of Cl transporter, Na and water follow Cl across tight junction

40
Q

Infectious secretory diarrhea (ex. due to cholera or E. coli):

A

-increase in cAMP
-induces serotonin release from enterochromaffin cells
>stimulate secretion
>release of NTs from myenteric submucosal plexus
*blood flow in gut wall is enhanced due to vasodilation by VIP and NO

41
Q

What is the outcome of diarrhea?

A

-fluid loss
-HCO3- loss
-dehydration
-acidosis
-death

MONOGASTRIC GI (24 decks)

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