GI Flashcards
(13 cards)
What is the molecular mechanism of acid secretion in the stomach?
Parietal cells secrete HCl via the H⁺/K⁺ ATPase on the apical membrane. Stimulated by histamine (H₂ receptor → ↑cAMP), acetylcholine (M3 receptor → ↑Ca²⁺), and gastrin (CCK-B receptor → ↑Ca²⁺). All enhance H⁺ secretion.
How does acetylcholine affect GI smooth muscle?
Acetylcholine binds to M3 muscarinic receptors → activates IP3/DAG pathway → ↑ intracellular Ca²⁺ → activates MLCK → smooth muscle contraction.
What is the role of nitric oxide in GI motility?
NO activates guanylate cyclase → ↑ cGMP → activates PKG → stimulates MLCP → smooth muscle relaxation.
What cells regulate GI motility through neurotransmitter release?
Enteric neurons in the myenteric plexus coordinate contraction and relaxation by releasing ACh (contracts circular muscle) and NO/VIP (relaxes longitudinal muscle).
What is peristalsis and how is it regulated?
Peristalsis is the coordinated, rhythmic contraction and relaxation of circular and longitudinal muscles. Initiated by distension of the gut wall → sensory neurons → interneurons → motor neurons. Proximal contraction (ACh), distal relaxation (NO/VIP).
How is mucus secretion regulated in the GI tract?
Goblet cells secrete mucus. Stimulated by acetylcholine binding to M3 receptors → Ca²⁺-mediated exocytosis. Mucus lubricates and protects against mechanical and microbial damage.
What is the function of the intestinal barrier and what structures support it?
Enterocytes form a physical barrier sealed by tight junction proteins (occludins, claudins). Barrier prevents pathogen entry while allowing selective ion and water movement.
How does the GI tract regulate fluid absorption and secretion?
Na⁺ absorbed via SGLT1 and ENaC; Cl⁻ via CFTR and DRA. Water follows osmotically. Secretagogues like VIP or prostaglandins stimulate Cl⁻ secretion → increased water movement into lumen.
How do oral rehydration solutions work?
ORS contains glucose and Na⁺ to drive Na⁺/glucose cotransport via SGLT1, enhancing water absorption. Also promotes Cl⁻ and HCO₃⁻ absorption for osmotic balance.
What is the molecular target of hyoscine butylbromide (Buscopan) in IBS treatment?
Hyoscine butylbromide is a muscarinic M3 receptor antagonist → blocks ACh-induced smooth muscle contraction → reduces GI spasms and cramping.
How does the pH vary along the GI tract and why is it important?
Stomach is acidic (pH ~2) from HCl; duodenum slightly acidic due to chyme; small intestine becomes neutral (HCO₃⁻ from pancreas); caecum slightly acidic (SCFA production). pH affects enzyme activity and drug solubility (pKa considerations).
How do drugs for constipation typically work at the molecular level?
Stimulate Cl⁻ secretion via guanylate cyclase C activation (e.g., linaclotide) or EP4 receptor (prostaglandins) → activate CFTR → water follows osmotically → softens stool.
How do antidiarrheal drugs like loperamide work?
Loperamide is a μ-opioid receptor agonist in the enteric nervous system → ↓ ACh release → reduced peristalsis and secretion → increased transit time and fluid absorption.
