Gastric Acid Secretion Made Easy

1. H+ and HCO3- are produced from CO2 and H2O.

2. H+ is secreted into the lumen by H+/K+ ATPase pump.

3. HCO3- moves out of the cell, across the basolateral membrane via antiport with Cl-.

4. Cl- diffuses passively into the lumen via Cl- channel.

Phases	Acid production	Trigger	Mechanism	Inhibition
1. Cephalic	30%	Psychic stimuli	Vagal stimulation	Loss of appetite/Depression (lack of parasympathetic center stimulation)
2. Gastric	60%	Presence of food in stomach (mechanical and chemical)	Distension: Vagovagal and local reflex Chemical (high pH): G cells release Gastrin	Low PH: Gastrin secretion decreases Emotional distress: Sympathetic stimulation
3. Intestinal	10%	Presence of food in duodenum (chemical)	Chemical (low pH): Intestinal gastrin released	Distension, Fatty acids, Hypertonic chyme: a. Enterogastric reflex b. Intestinal hormones (secretin, GIP, CCK, VIP)

Cholinergic stimulation and Gastrin both also stimulates Histamine secretion from ECL cells which in turn stimulates acid secretion.

1. Reduction of gastric acid secretion: H2 antihistaminics, Proton pump inhibitors (PPI), M1 anticholinergics, PG analogs (EP3 receptor)

2. Neutralization of gastric acid: Antacids

3. Ulcer protectives: Sucralfate, Bismuth, PG analogs

4. Ulcer healing: Carbenoxolone

5. Anti-H. pylori medications: Amoxicillin, metronidazole, tinidazole, tetracycline

Name	Source	Stimulus	Mechanism	Action
Gastrin	G cells (gastric antrum)	Stomach distension	Hormone	HCl, pepsinogen, IF secretion Gastric motility Trophic to gastric mucosa
CCK	I cells (duodenum)	Partially digested peptides and triglycerides	Hormone	Enzyme rich pancreatic fluid Gallbladder contraction and sphincter of Oddi relaxation Gastric emptying reduction Trophic to pancreatic acinar cells Satiety
Secretin	S cells (duodenum/proximal jejunum)	Acid chyme, fatty acids	Hormone	Bicarbonate rich pancreatic fluid Gastric acid reduction Trophic to pancreatic acinar cells
VIP	Neuron (GI tract, pancreas)	Neural	Neurocrine	Pancreatic and intestinal secretion Inhibits gastric acid and pepsinogen secretion
Somatostatin	D cells (pancreas, stomach)	Fat, bile salts, glucose in intestinal lumen	Paracrine/Neurocrine	Decreases acid, pepsin, gastrin, pancreatic enzyme secretion Decreases insulin and glucagon Stimulates gastric mucous production