Seasons Wk 5 LG pt.2

Pharmacology and Therapeutics of Acid-Reducing Drugs

Where are gastric parietal cells primarily located?

Answer: In the fundus and body of the stomach.

Extra Information:

• Parietal cells secrete hydrochloric acid.

• Their secretion is controlled by neural, endocrine, and paracrine pathways.

What are the three major stimulatory control pathways for gastric acid secretion?

Answer: Neural acetylcholine, endocrine gastrin, and paracrine histamine.

Extra Information:

• These pathways act synergistically on parietal cells.
• They ultimately stimulate the H+/K+ ATPase proton pump.

What are the major inhibitory regulators of gastric acid secretion emphasized in the material?

Answer: Somatostatin and prostaglandins.

Extra Information:

• Both act as local brakes on acid secretion.
• They help protect the stomach from excessive acid exposure.

Which nerve provides the major neural stimulation of gastric acid secretion?

Answer: The vagus nerve.

Extra Information:

• Vagal activation occurs during stimuli such as stomach distension and the cephalic phase.

What neurotransmitter does the vagus nerve release to directly stimulate parietal cells?

Answer: Acetylcholine.

Extra Information:

• Acetylcholine binds M3 receptors on parietal cells.

Which receptor does acetylcholine activate on gastric parietal cells?

Answer: The M3 muscarinic receptor.

Extra Information:

• The M3 receptor is Gq-coupled.
• Its activation increases intracellular calcium.

What G-protein is coupled to the parietal-cell M3 receptor?

Answer: Gq.

Extra Information:

• Gq signaling increases cytosolic calcium.
• Increased calcium promotes proton-pump activation.

What intracellular signal increases when acetylcholine activates M3 receptors?

Answer: Intracellular calcium.

Extra Information:

• Calcium-dependent signaling activates protein kinases that stimulate acid secretion.

Trace the direct vagal pathway stimulating gastric acid secretion.

Answer: Vagus nerve → acetylcholine → M3 receptor on parietal cell → Gq activation → increased intracellular calcium → protein kinase activation → H+/K+ ATPase stimulation → acid secretion.

Extra Information:

• This is the direct neural pathway to the parietal cell.

What peptide does the vagus nerve release to stimulate antral G cells?

Answer: Gastrin-releasing peptide.

Extra Information:

• Gastrin-releasing peptide stimulates gastrin secretion.

What does gastrin-releasing peptide do in gastric acid regulation?

Answer: It stimulates antral G cells to release gastrin.

Extra Information:

• This is an indirect vagal mechanism for increasing gastric acid secretion.

How does vagal stimulation affect somatostatin-secreting D cells?

Answer: It inhibits them.

Extra Information:

• Inhibiting D cells reduces somatostatin release.
• This removes an inhibitory brake on acid secretion.

What is the combined indirect effect of vagal stimulation on gastric acid secretion?

Answer: It stimulates gastrin release through GRP and suppresses somatostatin release from D cells.

Extra Information:

• Both actions favor increased acid production.

Which cells secrete gastrin?

Answer: Antral G cells.

Extra Information:

• Gastrin functions as an endocrine regulator of gastric acid secretion.

What stimuli trigger gastrin release from G cells?

Answer: Gastric distension, gastrin-releasing peptide, and amino acids or peptides.

Extra Information:

• Gastrin is then released into the systemic circulation.

Why is gastrin considered an endocrine regulator?

Answer: It is released into the bloodstream and travels through the systemic circulation to its targets.

Extra Information:

• It can act directly on parietal cells and indirectly through ECL cells.

Which receptor does gastrin activate on parietal cells?

Answer: The CCKB, or CCK2, receptor.

Extra Information:

• This receptor is Gq-coupled.

What G-protein is coupled to the CCKB receptor?

Answer: Gq.

Extra Information:

• Like the M3 receptor, it increases intracellular calcium.

What second-messenger signal does gastrin increase in parietal cells?

Answer: Intracellular calcium.

Extra Information:

• Gastrin and acetylcholine therefore share a calcium-based stimulatory pathway.

Trace the direct gastrin pathway stimulating acid secretion.

Answer: Gastrin → CCKB receptor on parietal cell → Gq activation → increased intracellular calcium → proton-pump stimulation → gastric acid secretion.

Extra Information:

• This is the direct endocrine effect of gastrin.

What is the major indirect mechanism by which gastrin stimulates gastric acid secretion?

Answer: It stimulates enterochromaffin-like cells to release histamine.

Extra Information:

• The material describes this as gastrin's most potent acid-stimulating pathway in humans.

Which cells release histamine in the stomach?

Answer: Enterochromaffin-like cells.

Extra Information:

• They are commonly abbreviated ECL cells.
• Histamine acts locally on nearby parietal cells.

Why is histamine classified as a paracrine regulator of acid secretion?

Answer: It is released locally from ECL cells and diffuses to nearby parietal cells.

Extra Information:

• It does not need to travel through the systemic circulation to exert its effect.

Which receptor does histamine activate on parietal cells?

Answer: The H2 receptor → histamine type 2 receptor

Extra Information:

• H2 receptor activation strongly stimulates acid secretion.

What G-protein is coupled to the parietal-cell H2 receptor?

Answer: Gs.

Extra Information:

• Gs activation increases cyclic AMP.

What second messenger increases after H2 receptor activation?

Answer: Cyclic AMP.

Extra Information:

• Cyclic AMP activates protein kinase A.

Which protein kinase is activated by histamine-induced cyclic AMP?

Answer: Protein kinase A.

Extra Information:

• Protein kinase A helps stimulate the H+/K+ ATPase proton pump.

Trace the histamine pathway stimulating gastric acid secretion.

Answer: ECL cell histamine release → H2 receptor activation → Gs activation → increased cyclic AMP → protein kinase A activation → H+/K+ ATPase stimulation → gastric acid secretion.

Extra Information:

• This is the major stimulatory paracrine pathway.

Compare acetylcholine, gastrin, and histamine signaling in parietal cells.

Answer: Acetylcholine and gastrin use Gq and increase calcium, while histamine uses Gs and increases cyclic AMP.

Extra Information:

• All three ultimately stimulate the H+/K+ ATPase.

What stimulates somatostatin release from antral D cells?

Answer: Low gastric pH, especially below about pH 3.

Extra Information:

• Somatostatin provides negative feedback when the stomach becomes highly acidic.

Which cells release somatostatin in the stomach?

Answer: Antral D cells.

Extra Information:

• Somatostatin acts as an inhibitory paracrine signal.

How does somatostatin directly affect parietal cells?

Answer: It inhibits them through Gi-coupled receptors that decrease cyclic AMP.

Extra Information:

• Reduced cyclic AMP decreases stimulation of the proton pump.

What G-protein mediates somatostatin's direct inhibitory effect on parietal cells?

Answer: Gi.

Extra Information:

• Gi decreases intracellular cyclic AMP.

How does somatostatin affect gastrin release?

Answer: It inhibits gastrin release from G cells.

Extra Information:

• This decreases endocrine stimulation of acid secretion.

How does somatostatin affect histamine release?

Answer: It inhibits histamine release from ECL cells.

Extra Information:

• This decreases paracrine stimulation of parietal cells.

What are the three major inhibitory actions of somatostatin in gastric acid regulation?

Answer: It directly inhibits parietal cells, inhibits gastrin release from G cells, and inhibits histamine release from ECL cells.

Extra Information:

• These actions provide strong negative feedback against excessive acid secretion.

Which prostaglandin is emphasized as an inhibitory paracrine regulator of gastric acid secretion?

Answer: Prostaglandin E2.

Extra Information:

• Prostaglandins help protect gastric mucosa.

How do prostaglandins reduce gastric acid secretion?

Answer: They directly inhibit parietal cells and suppress histamine release from ECL cells.

Extra Information:

• Their inhibitory effect contributes to mucosal protection.

What is the final common pathway of gastric acid secretion?

Answer: The apical H+/K+ ATPase proton pump.

Extra Information:

• All major stimulatory pathways ultimately increase its activity.

What enzyme forms carbonic acid inside the parietal cell?

Answer: Carbonic anhydrase.

Extra Information:

• It catalyzes the reaction of carbon dioxide and water to form carbonic acid.

What reaction does carbonic anhydrase catalyze in gastric parietal cells?

Answer: Carbon dioxide plus water are converted into carbonic acid.

Extra Information:

• Carbonic acid then dissociates into hydrogen and bicarbonate ions.

What ions are produced when carbonic acid dissociates inside a parietal cell?

Answer: Hydrogen ions and bicarbonate ions.

Extra Information:

• Hydrogen is secreted into the stomach lumen.
• Bicarbonate is transported into the blood.

What does the apical H+/K+ ATPase do?

Answer: It secretes hydrogen ions into the gastric lumen in exchange for potassium.

Extra Information:

• This pump is the terminal step in acid secretion.

How does chloride reach the gastric lumen?

Answer: It diffuses through separate chloride channels.

Extra Information:

• Chloride combines with secreted hydrogen ions to form hydrochloric acid.

How is hydrochloric acid formed in the gastric lumen?

Answer: Secreted hydrogen ions combine with chloride ions.

Extra Information:

• The hydrogen is secreted by the H+/K+ ATPase.

What happens to bicarbonate generated inside the parietal cell?

Answer: It is transported across the basolateral membrane into the bloodstream through a chloride-bicarbonate exchanger.

Extra Information:

• This exchange contributes to the postprandial alkaline tide.

What is the alkaline tide?

Answer: A temporary rise in blood pH after a meal caused by bicarbonate leaving parietal cells and entering the bloodstream.

Extra Information:

• It accompanies gastric acid secretion.

Trace the cellular production and secretion of gastric hydrochloric acid.

Answer: Carbon dioxide + water → carbonic acid via carbonic anhydrase → hydrogen + bicarbonate → hydrogen secreted apically through H+/K+ ATPase → chloride exits through channels → hydrogen and chloride form HCl, while bicarbonate exits basolaterally into blood.

Extra Information:

• Basolateral bicarbonate extrusion causes the alkaline tide.

What is Zollinger-Ellison syndrome?

Answer: A syndrome caused by a gastrin-secreting neuroendocrine tumor called a gastrinoma.

Extra Information:

• Excess gastrin causes extreme gastric acid secretion.

How does a gastrinoma affect gastric acid secretion?

Answer: It causes unregulated endocrine stimulation of parietal cells.

Extra Information:

• This can overwhelm normal somatostatin-mediated feedback.

What clinical gastrointestinal findings are associated with Zollinger-Ellison syndrome?

Answer: Severe hyperacidity and recurrent peptic ulcers that are difficult to treat.

Extra Information:

• Hypertrophy of the gastric mucosa can also occur.

What was the historical rationale for using milk to treat heartburn?

Answer: Milk temporarily coats and soothes the gastric and esophageal lining.

Extra Information:

• The relief is short-lived.

Why can whole milk worsen heartburn after temporary relief?

Answer: Its fat and calcium-containing components stimulate hormones that increase gastric acid secretion.

Extra Information:

• This can produce acid rebound.

Which hormones can be stimulated by the fat content of whole milk according to the material?

Answer: Gastrin and cholecystokinin.

Extra Information:

• Their stimulation can worsen gastric acid production.

Why may low-fat milk be less likely to worsen reflux than whole milk?

Answer: It provides some coating effect with less fat-mediated stimulation of acid secretion.

Extra Information:

• It still does not function as a true acid-neutralizing therapy.

What is a vagotomy?

Answer: Surgical severing of vagal nerve branches supplying the stomach.

Extra Information:

• It was historically used to reduce gastric acid secretion.

How does vagotomy directly reduce gastric acid secretion?

Answer: It eliminates vagal acetylcholine stimulation of parietal-cell M3 receptors.

Extra Information:

• This removes a major neural stimulatory pathway.

How does vagotomy indirectly reduce gastric acid secretion?

Answer: It prevents vagal GRP stimulation of G cells and reduces downstream gastrin and histamine signaling.

Extra Information:

• It therefore affects both direct and indirect acid-stimulating pathways.

Why is vagotomy rarely used today for acid-related disease?

Answer: Modern pharmacologic therapies are highly effective and vagotomy can cause significant gastric dysmotility.

Extra Information:

• Delayed gastric emptying is an important complication.

What are antacids?

Answer: Weak bases that chemically neutralize existing gastric hydrochloric acid.

Extra Information:

• They do not prevent new acid from being produced.

What chemical products are formed when an antacid neutralizes gastric acid?

Answer: Salt and water.

Extra Information:

• Some antacids can also generate carbon dioxide depending on their chemical composition.

Why do antacids provide rapid symptomatic relief?

Answer: They neutralize acid already present in the gastric lumen.

Extra Information:

• They act faster than drugs that reduce future acid secretion.

Do antacids decrease acid production by parietal cells?

Answer: No. They neutralize existing acid but do not suppress its production.

Extra Information:

• This distinguishes them from H2 blockers and proton pump inhibitors.

How do antacids affect pepsin activity?

Answer: Raising gastric pH above about 4 decreases pepsin's proteolytic activity.

Extra Information:

• This reduces pepsin-mediated injury to gastric and esophageal mucosa.

What does the mnemonic SCAM represent for antacids?

Answer: Sodium bicarbonate, Calcium carbonate, Aluminum hydroxide, and Magnesium hydroxide.

Extra Information:

• Each has a characteristic adverse-effect profile.

What is a major adverse effect of sodium bicarbonate antacids related to carbon dioxide production?

Answer: Gastric distension and belching.

Extra Information:

• Carbon dioxide gas is generated during acid neutralization.

What systemic acid-base abnormality can excess sodium bicarbonate cause?

Answer: Metabolic alkalosis.

Extra Information:

• Sodium bicarbonate is highly soluble and systemically absorbed.

What antacid is calcium carbonate?

Answer: Calcium carbonate is the active ingredient in products such as Tums.

Extra Information:

• It is highly effective but can be absorbed systemically.

What adverse gastrointestinal effect is associated with calcium carbonate?

Answer: Constipation.

Extra Information:

• Excessive intake can also cause milk-alkali syndrome.

What is milk-alkali syndrome?

Answer: Hypercalcemia, metabolic alkalosis, and renal insufficiency caused by excessive calcium carbonate intake.

Extra Information:

• Confusion and dehydration can also occur clinically.

What adverse gastrointestinal effect is associated with aluminum hydroxide?

Answer: Constipation.

Extra Information:

• Aluminum inhibits gastrointestinal smooth muscle contraction.

What electrolyte abnormality can aluminum hydroxide cause?

Answer: Hypophosphatemia.

Extra Information:

• Severe depletion can contribute to muscle weakness and osteomalacia.

What long-term skeletal complication can severe aluminum-induced hypophosphatemia cause?

Answer: Osteomalacia.

Extra Information:

• This reflects impaired mineralization from phosphate depletion.

• a bone disease in adults that causes bones to become soft and weak because they do not harden properly

What adverse gastrointestinal effect is associated with magnesium hydroxide?

Answer: Diarrhea.

Extra Information:

• Magnesium acts as an osmotic laxative.

Why are aluminum hydroxide and magnesium hydroxide often combined?

Answer: Their opposing gastrointestinal effects can partially offset each other.

Extra Information:

• Aluminum causes constipation.
• Magnesium causes diarrhea.

Which antacids are associated with constipation?

Answer: Aluminum hydroxide and calcium carbonate.

Extra Information:

• Magnesium hydroxide instead causes diarrhea.

Which antacid is most associated with diarrhea?

Answer: Magnesium hydroxide.

Extra Information:

• Remember: Magnesium makes you go more.

Why can chronic heavy calcium carbonate use be dangerous?

Answer: It can cause milk-alkali syndrome with hypercalcemia, metabolic alkalosis, and renal dysfunction.

Extra Information:

• Constipation can also occur.

Compare the main limitation of milk, vagotomy, and antacids in heartburn management.

Answer: Milk can stimulate acid rebound, vagotomy is invasive and causes dysmotility, and antacids only neutralize existing acid without reducing production.

Extra Information:

• Modern antisecretory drugs provide more targeted therapy.

What are the major H2 receptor antagonists listed in the material?

Answer: Cimetidine, famotidine, nizatidine, and ranitidine.

Extra Information:

• Ranitidine is noted as withdrawn in the material.

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What are the major proton pump inhibitors listed in the material?

Answer: Omeprazole, esomeprazole, lansoprazole, and pantoprazole.

Extra Information:

• Members of this class commonly end in -prazole.

What is the mechanism of action of H2 receptor antagonists?

Answer: Competitive, reversible blockade of H2 receptors on gastric parietal cells.

Extra Information:

• This blocks histamine-mediated stimulation of acid secretion.

Where are H2 receptors located on gastric parietal cells?

Answer: On the basolateral membrane.

Extra Information:

• Histamine normally binds these receptors to activate a Gs signaling pathway.

What signaling pathway do H2 receptor antagonists block?

Answer: The histamine H2 → Gs → adenylyl cyclase → cyclic AMP pathway.

Extra Information:

• Blocking this pathway reduces proton-pump activation.

How potent is acid suppression with H2 receptor antagonists compared with PPIs?

Answer: H2 receptor antagonists provide moderate acid suppression.

Extra Information:

• The source describes suppression of roughly 60–70% of 24-hour acid secretion.

What type of gastric acid secretion is particularly well suppressed by H2 receptor antagonists?

Answer: Nocturnal acid secretion.

Extra Information:

• Histamine plays an important role in nighttime acid production.

🌙 Nocturnal acid = acid PRODUCTION at night

What is the mechanism of action of proton pump inhibitors?

Answer: Irreversible covalent inhibition of the active H+/K+ ATPase proton pump.

Extra Information:

• This blocks the terminal step of gastric acid secretion.

Why are PPIs more potent than H2 receptor antagonists?

Answer: PPIs block the final common proton pump used by all major stimulatory acid-secretion pathways.

Extra Information:

• H2 blockers only block the histamine pathway.

Approximately how much acid suppression can PPIs achieve according to the material?

Answer: Up to about 98–99%.

Extra Information:

• This is substantially more profound than H2 receptor blockade.

Are PPIs active when administered?

Answer: No. They are administered as inactive lipophilic prodrugs.

Extra Information:

• They require activation in an acidic environment.

Why are oral PPIs enteric coated?

Answer: To prevent premature activation and destruction in the acidic gastric lumen.

Extra Information:

• They must reach the small intestine before absorption.

Where are oral PPIs absorbed?

Answer: In the small intestine.

Extra Information:

• They then enter the systemic circulation and reach parietal cells.

Where are PPIs activated?

Answer: In the highly acidic secretory canaliculi of active parietal cells.

Extra Information:

• Acid protonates the prodrug.

What happens to a PPI in the acidic parietal-cell canaliculus?

Answer: It becomes protonated and is converted into an active thiophilic sulfenamide.

Extra Information:

• The positively charged active drug becomes trapped in the canaliculus.

What is acid trapping of PPIs?

Answer: Protonation in the acidic canaliculus creates a charged active form that cannot readily diffuse back out.

Extra Information:

• This concentrates the drug at its target site.

How does the activated PPI bind the H+/K+ ATPase?

Answer: It forms a covalent disulfide bond with sulfhydryl groups on the proton pump.

Extra Information:

• This irreversibly inactivates the pump.

Why does PPI action last much longer than its plasma half-life?

Answer: Acid secretion resumes only after new proton pumps are synthesized.

Extra Information:

• The covalent inhibition is irreversible.

Approximately how long does synthesis of new proton pumps take according to the material?

Answer: About 36 hours.

Extra Information:

• This contributes to the prolonged pharmacologic effect of PPIs.

Trace PPI activation and action.

Answer: Enteric-coated prodrug → small-intestinal absorption → systemic circulation → parietal cell → acidic canaliculus protonation → active sulfenamide formation → acid trapping → covalent H+/K+ ATPase inhibition → profound acid suppression.

Extra Information:

• The pump remains inactive until new pumps are synthesized.

When should PPIs generally be taken?

Answer: About 1 hour before a meal, ideally before breakfast.

Extra Information:

• This allows peak drug levels to coincide with active proton pumps.