24. Drugs Affecting Gut Motility Flashcards
How is gastric motility controlled?
Myogenically, neuronally, and hormonally.
What is meant by myogenic control of gastric motility?
Rhythmic contraction (slow waves of depolarisation through smooth muscle), passive currents through gap junctions, driven by interstitial cells of Cajal that act as pacemakers of electrical activity.
What is meant by neural control of gastric motility?
Intrinsic and extrinsic control, stimulation of post-ganglionic cholinergic enteric nerves -> increase force of gut contraction vs stimulation of non-adrenergic inhibitory nerves -> inhibits contraction.
What are the networks making up the neuronal supply for gastric motility?
Local nerves - enteric nervous system, Auerbach’s plexus - between circular and longitudinal layers of muscles, Meissner’s plexus - in submucosa, Henle’s plexus - in circular muscle adjacent to submucosa, Cajal’s plexus - in circular muscle adjacent to longitudinal muscle.
What is the intestine-intestinal inhibitory reflex?
Distension of one intestinal segment causes complete intestinal inhibition.
What is the anointestinal inhibitory reflex?
Distension of the anus causes intestinal inhibition.
What are the gastrocolic and duodenocolic reflexes?
Stimulation of motility after material has entered the stomach or duodenum.
What happens in emesis to the anatomy of the GI tract?
The pyloric sphincter closes while cardia and oesophagus relax, gastric contents are propelled by contraction of abdominal wall and diaphragm, glottis closes with elevation of the soft palate to prevent entry of vomit into trachea and nasopharynx.
What are the control centres and neurotransmitters involved in emesis?
Vestibular apparatus - ACh, H1; medullary centre - ACh, H1, 5-hydroxytyptamine (5-HT3); postrema on the floor of the 4th ventricle - dopamine.
Name three dopamine D2 receptor antagonists.
Domperidone, metoclopramide, phenothiazines.
Name two 5-HT3-receptor antagonists.
Ondansetron and granisteron.
Name an anti-muscarinic.
Hyoscine.
Name two histamine H1 receptor antagonists.
Cyclizine, promethazine.
Name a cannabinoid used in anti-emesis.
Nabilone.
Name a benzodiazapine used in anti-emesis.
Lorazepam.
What is the action of domperidone, the D2 antagonist?
Acts on postrema on the floor of the 4th ventricle and on stomach to increase rate of gastric emptying.
What are the indications for domperidone use?
Acute nausea/vomiting.
What are the ADRs of domperidone?
Stimulates prolacting release, rarely dystonia.
What is the route of administration of domperidone?
Oral or PR.
What is the action of ondansteron, the 5-HT3 antagonist?
5-HT into the gut causes vagal stimulation, 5HT3 antagonists work on the postrema on the floor of the 4th ventricle and against the vagal afferent nerves in GI.
What are the indications for ondansteron use?
High doses in radiation sickness and chemotherapy, post-operative.
What is the route of administration for ondansteron?
IV, IM, or orally.
How can the anti-emetic effect of ondansteron be enhanced?
A single dose of corticosteroid.
What are the ADRs of ondansteron?
Headaches, constipation, flushing.
What are the actions of metoclopramide?
D2 antagonism acting on 4th ventricle and gastric emptying, anti-cholinergic effects, and blocks vagal afferent 5-HT3R.
What are the indications for use of metoclopramide?
GI cause for N&V, migraine, post-op.
What are the routes of administration of metoclopramide?
Oral, IM, IV.
What is the half life of metoclopramide?
4hrs, short.