Canonical Question

GIT Pharm – Gastric fluid

V5 J4.ii Historical V4 O2.ii 1 appearance

Master answer

CLASS	EXAMPLES	MECHANISM OF ACTION
Anticholinergics	Hyoscine Atropine	– M1-Ach-R antagonism (NTS, CTZ, VC) – Small antihistamine and D2 antagonist effects
Antihistamines (H1)	Cyclizine Promethazine	– H1 antagonism- VC, vestibular nucleus and CTZ – Anti-muscarinic effects- NTS, CTZ, vomit center – D2 antagonism (GIT, CTZ)
5-HT3 Antagonists	Ondansetron	– Peripheral in GIT – Central at VC and CTZ
Dopamine Antagonists	1. Phenothiazines – Prochlorperazine (stemetil) 2. Butyrophenones – Droperidol, domperidone 3. Benzamides – Metoclopramide	– Decreased sensitivity of visceral afferents to vomit center – Central D2 blockade increased threshold at CTZ Other effects: – Inhibition of 5-HT3 – Anti-H1 effects
Steroids	Dexamethasone	– Proposed to act centrally to inhibit prostaglandin synthesis and inhibit endorphin receptors
Miscellaneous	1. Propofol 2. Benzos 3. Cannabinoids 4. NK1- Receptor antagonists (aprepitant)	– Propofol and BZD- GABAergic inhibition of VC – Cannabinoids- Direct CTZ and VC inhibition – NK1 antagonists inhibit VC

Selective stimulation of gastric muscarinic receptors
- acts primarily by augmenting release of ACh and perhaps by inhibition of 5-HT release
- Coordinated gastric, pyloric and duodenal activity contraction (accelerates gastric emptying)
- Increased amplitude and frequency of Longitudinal muscle contraction
- Lowers threshold for peristaltic reflex to occur
Reduces intestinal muscle fatigue
Direct action on smooth muscle to increase tone (including increased LOS)
relaxes the pyloric sphincter

Exam	Exact wording	Relationship	Success
2012A Q23	Classify antiemetic drugs and give an example from each group (60% of marks). Outline the gastrointestinal effects of metoclopramide (40% of marks).	historical_member	—