GI - laxatives Flashcards by Emily Liu

what is constipation

slowed passage of food through GI tract

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what % of people are affected by constipation

50+%

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what are 6 possible causes of constipation

drugs (opiates, antidepressants, iron suppl.)
diet
hydration
exercise
hormones
IBS

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what are purgatives / what do they do

accelerate passage of food through intestine and evacuation of bowels

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what are 4 examples of purgatives

bulk laxatives, osmotic laxatives, stimulant purgatives, stool softeners

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what are 2 uses for purgatives

relieve constipation or to clear bowel prior to surgery or examination

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what are agents that increase motility of GI tract without causing purgation/ evacuation

D2 antagonist

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what do D2 antagonists do to GI system

increase motility of GI tract without causing purgation/ evacuation

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what are bulk laxatives

polysaccharide polymers not broken down by digestion in the upper part of the GI tract

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what do bulk laxatives do in the lumen

form a bulky hydrated mass in gut lumen

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what are 2 examples of bulk laxatives

methyl cellulose, ispaghula husk

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how do bulk laxatives work

mechanical distention promotes peristaltic reflex

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how long does it take for bulk laxatives to work

slow action (several days) but not absorbed

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are there any unwanted effects with bulk laxatives and why

no because it is not absorbed

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what are 2 examples of osmotic laxatives

lactulose and macrogols

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what is lactulose

osmotic laxative, disaccharide of fructose and galactose

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what are macrogols

inert ethylene glycol polymers (its okay if you dont know i think)

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what kind of laxative is methyl cellulose

bulk

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what kind of laxative is ispaghula husk

bulk

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what kind of laxative is lactulose

osmotic

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what kind of laxative is macrogols

osmotic

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how do osmotic laxatives work

pooly absorbed, so produce osmotic load in lumen that draws water into gut lumen, softens stool and stretches gut to protmote peristalsis and defacation

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how long do osmotic laxatives take to work

2-3 days

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are there any bad side effects of osmotic laxatives and why

no because it is not absorbed

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how do stimulant laxatives work

increase electrolyte and water secretion and/or increases peristalsis

what are 2 examples of stimulant laxatives

bisacodyl and senna

what is bisacodyl

stimulant laxative, suppository that can stimulate defacation in 15 mins

what is senna

stimulant laxative

how does senna work

active component (a glycoside) directly stimulates nerves in myenteric plexus to increase peristalsis and defection

how is senna often prescribed/used

in combination with stool softeners for chronic oopioid induced constipation

can you get dependent on purgatives

yes, with prolonged use

what do muscarinic agonists do to GI?

enhance contractions uncoordinatedly- little or no net propulsive movement

what do cholinomimetics] do to GI?

enhance contractions uncoordinatedly- little or no net propulsive movement

what do cholinesterase inhibitors do to GI?

enhance contractions uncoordinatedly- little or no net propulsive movement

how much of the bodys dopamine does the GI produce

50%

does GI functions change in parkinsons and why

yes because GI tract makes 50% of dopamine

what does dopamine do to gut motility (general)

mediates negative feedback mechanism to modulate gut motility

when is DA release in the GI

released at the same time as ACh during peristalsis

what does DA do to ACh release

inhibits

what does DA do to muscles

causes relaxation

how do D2 antagonists work

block the effects of endogenous dopamine

what do D2 antagonists do/what do they cause + how

by inhibiting smooth muscle relaxation and enhance ACh release --> increased contractility and peristalsis

what do D2 antagonists do to smooth muscle

inhibit relaxation

what do D2 antagonists do to ACh release

increase because it inhibits endogenous dopamine release that usually inhibits it

what do D2 antagonists do to contractility and peristalsis

increase

what is a way that you can negate the effects of D2 antagonists on the GI

with anticholinergic drugs like antropine

why would atropine make D2 antagonists have little effect

because blocking muscarinic receptors at the end will make D2 antagonists have little effect

what does dopamine do when it activates post synaptic D2 receptors

causes relaxation

what is more important: potentiating effects of D2 blockers on ACh release (presyn. D2) OR block of DA-mediated relaxation (postsyn. D2)

potentiating effects of D2 blockers on ACh release (presyn. D2) is more important maybe cause ACh is really good at making contractions, its better to do that than try to stop when DA does relaxation

are cholinergics good alone at constipation? why?

no, they increase contractility but not propulsion

what is metoclopramide

D2 antagonist

what is the general thing that metoclopramide does

increases gastric emptying

how does metoclopramide increase gastric emptying

relaxes pyloric sphincter as well as smooth muscle

what is a secondary use of metoclopramide and how

antiemetic due to action in CTZ

what are 2 bad things about metoclopramide

- decreases absorption of drugs normally absorbed in stomach (esp. weak acids) - crosses BBB< long term can cause dyskinesias

what is domperidone

a D2 antagonist

what is domperidone also used for besides constipation + how

stimulating lactation in new mothers by stimulating prolactin release

does metoclopramide cross the BBB

yes

does domperidone cross the BBB

how does domperidone stimulate prolactin release

D2 agonism decreases prolactin release, so antagonist at D2 will increase

which constipation drug may also increase prolactin release

domperidone

which cell type releases 5-HT

enterochromaffin cells

what 3 main things trigger 5-HT release

food constituents, stretching of gut wall, endogenous mediators like gastrin

what is 5-HT important for

the peristaltic reflex

what nerves does 5-HT stimulate

sensory nerves in the myenteric plexus

how does 5-HT cause peristalsis

by stimulating sensory nerves in the myenteric plexus

where are 5-HT4 receptors (2)

presynaptic in the myenteric plexus

what does 5-HT4 receptor activation cause

enhanced ACh release

what does 5-HT4 receptor activation do to ACh release

enhance

what does 5-HT4 receptor activation do to contraction

increase

how well do 5-HT4 agonists work for GI

really good for GI motility

what is pruclopride

5-HT4 agonist

what is a downside to using pruclopride

increased cardiac event risk, withdrawn from market

what is an example 5-HT4 agonist

pruclopride