Pharmacology of Gastrointestinal Motility

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Questions and Answers

Which of the following correctly describes the mechanism of action of bulk laxatives?

They absorb water, forming a viscous gel that stimulates bowel movements. (correct)

They exert detergent-like effects to promote stool softening.

They create an osmotic load that distends the gut and induces peristalsis.

They directly increase smooth muscle tone, leading to quicker transit.

What adverse effect is commonly associated with osmotic laxatives?

Severe dehydration

Flatulence

Electrolyte imbalances

Abdominal cramps (correct)

For which patient group should magnesium salts in osmotic laxatives be avoided?

Small children and patients with poor renal function (correct)

Patients with chronic constipation

Patients on a high-fiber diet

Patients suffering from IBS

Which of the following describes the action of docusate sodium?

It acts like a detergent promoting softer stools. Signup and view all the answers

In which condition are bulk laxatives not recommended?

Intestinal pathologies causing obstruction Signup and view all the answers

What is the primary mechanism of action for bisacodyl?

Increase electrolyte secretion and stimulation of enteric nerves Signup and view all the answers

In which situation is senna recommended despite its potential carcinogenic effects?

For terminally ill patients requiring bowel stimulation Signup and view all the answers

What is a common side effect experienced with stimulant laxatives like bisacodyl and senna?

Abdominal cramping Signup and view all the answers

What is the main indication for using prokinetic drugs such as prucalopride?

Irritable bowel syndrome (IBS) related constipation Signup and view all the answers

Which of the following laxatives acts as an irritant purgative?

Senna Signup and view all the answers

Which patient population should avoid stimulant laxatives like bisacodyl and senna?

Patients with bowel obstruction Signup and view all the answers

How do serotoninergic agonists function in improving gastrointestinal motility?

By increasing cholinergic activation Signup and view all the answers

Which of the following statements about domperidone is accurate?

It is primarily a prokinetic agent for enhancing gastric motility Signup and view all the answers

What is the expected response time for bisacodyl when administered as a suppository?

Within 15–30 minutes Signup and view all the answers

Which of the following laxatives is specifically indicated for preparation prior to surgery?

Sodium picosulfate Signup and view all the answers

Study Notes

Lesson 21: Gastrointestinal Motility

Gastrointestinal motility refers to the movement of food through the GI tract.
Drugs can alter GI tract motility in different ways.

Drugs Altering GI Tract Motility

Purgatives: Accelerate food passage through the intestine.
Prokinetic drugs: Increase GI smooth muscle motility without causing purgation.
Antidiarrheal drugs: Decrease GI motility.
Antispasmodic/spasmolytic drugs: Decrease smooth muscle tone.

Laxative Agents

Purgatives/laxatives:
- Bulk laxatives: Ispaghula husk (first choice for slow action).
- Osmotic laxatives: Lactulose.
- Faecal softeners: Docusate.
- Stimulant purgatives: Senna
Mechanism of action (Bulk laxatives):
- Water-attracting polymers form a bulky hydrated mass, improving fecal volume and consistency, producing peristalsis.
- Takes 1-3 days to work.
- First choice for chronic constipation and long-term treatment.
- ADRs: Flatulence and abdominal distension; may cause obstruction in patients with intestinal pathologies.
Mechanism of action (Osmotic laxatives):
- Poorly absorbed in the intestinal wall, producing an osmotic load.
- Osmotic action retains a large volume of fluid in the intestine, speeding up transit.
- This increased volume distends the colon, leading to a purgative effect.
- Effect within an hour
- ADRs: Abdominal cramps may occur; avoid magnesium salt in children and patients with poor renal function.
Mechanism of action (Faecal softeners):
- Similar to detergents, they act in the GI tract to soften stools and have a weak stimulant effect.
- ADRs: Can cause impaired absorption of fat-soluble vitamins (A, D, E)
Mechanism of action (Stimulant laxatives):
- Biscodyl, Sodium picosulfate: Increase electrolyte and water secretion by the mucosa and increase peristalsis via enteric nerve stimulation.
- Bisacodyl can be administered orally or through suppository, leading to defecation within 15-30 minutes.
- Indications: Temporary constipation.
- Senna and Dantron (anthraquinones): Directly stimulate the myenteric plexus, increasing peristalsis and defecation.
  - Indications: Temporary constipation; use cautiously or avoid in vulnerable populations.
  - Contraindications: Patients with bowel obstructions, mental disorders, or preparing for digestive tests.
  - May lead to dependence (atonic colon) if used excessively.

Prokinetic Drugs

Mechanism of action: Stimulate and improve intestinal transit by increasing GI cholinergic activation, leading to increased stomach emptying and peristalsis.
Serotonin agonists (e.g., prucalopride): Used when other treatments fail (IBS constipation). Tegaserod works similarly,
Dopamine antagonists (e.g., metoclopramide): A marked acceleration of gastric emptying. Used for gastro-oesophageal reflux and disorders of gastric emptying, doesn't work well for paralytic ileus.
Other agents: Lubiprostone, naloxegol, and methylnaltrexone.
- Lubiprostone promotes chloride and fluid secretion into the small intestine, resulting in softer stools. Indications include constipation due to opioids and IBS.
- Naloxegol acts as a μ-opioid receptor antagonist to reduce gut motility and hypertonicity associated with opioid-induced constipation, without impacting central analgesic effects.
- Methylnaltrexone, a peripheral opioid receptor antagonist, is used to treat opioid-induced constipation.

Antidiarrheal Agents

Mechanism of action: Designed to reduce gut motility.
- Loperamide: Selectively reduces the frequency of abdominal cramps. Reduces the passage of faeces and shortens illness duration. Primarily used for travellers' diarrhea.
- Muscarinic antagonists(e.g., Atropine, Hyoscine): Rarely used for diarrhea due to their broader effects on other systems (anticholinergic effects)
Adsorbents: Kaolin, pectin, chalk, charcoal, and methylcellulose. These preparations may adsorb microorganisms or toxins or coat and protect the intestinal mucosa (but no strong evidence supporting latter)

Treatment of Chronic Bowel Disease

Irritable bowel syndrome (IBS): bouts of diarrhea (IBS-D), constipation (IBS-C), or abdominal pain (IBS-M)
Inflammatory bowel disease (IBD): Ulcerative colitis and Crohn's disease.
- Ulcerative colitis: Inflammation limited to the colon, predominantly in the lower left abdomen. Ulcers penetrate only the inner lining of the abdomen.
- Crohn's disease: Inflammation that can affect any part of the GI tract, with discontinuous patches. Ulcers penetrate the entire thickness of the abdomen.
Treatment strategies target specific symptoms and consider the underlying mechanisms (gut-brain axis alterations, hypersensitivity) and may include diet adaptation, loperamide for diarrhea and laxatives for constipation.
5-HT treatments: Examples include alosetron, ramosetron (5-HT3 receptor antagonists). Tegaserod (5-HT4 receptor agonist)
Glucocorticoids: Prednisolone or budesonide, for short-term management of acute inflammation.
Aminosalicylates: Sulfasalazine is broken down to sulfapyridine and 5-aminosalicylic acid (5-ASA). The main action is inhibition of leukotriene and prostaglandin synthesis.
Immunosuppressants: Methotrexate, cyclosporin, tacrolimus, azathioprine, 6-mercaptopurine, used in severe cases. Sodium cromoglycate also sometimes used to manage GI symptoms as an antihistamine.
Biological therapy: (e.g., infliximab): -Monoclonal antibodies to target factors in inflammation. -Indicated where other treatments are insufficient. -Examples: infliximab, vedolizumab, and ustekinumab.
Other treatments
- Eluxadoline, a mixed μ/κ opioid-receptor agonist and δ-receptor antagonist, is primarily used for the treatment of IBS with diarrhea.
- Linaclotide, a guanylate cyclase-C agonist and used for treating symptoms of moderate to severe IBS with constipation.

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Description

This quiz explores the mechanisms and drug classes that alter gastrointestinal motility. You'll learn about purgatives, prokinetic agents, and antidiarrheal medications, as well as their functions in managing GI conditions. Test your knowledge on laxative agents and their modes of action.