GI Physiology

Question 1

Gastrin

Answer 1

G cells-antrum of the stomach

↑ Gastric H+ secretion

↑ Growth of gastric mucosa

↑ Gastric motility

Regulation:

↑ by stomach distention/alkalinization, aa’s, peptides, vagal stimulation.

↓ by stomach pH < 1.5, or acidic conditions

Notes:↑↑ in Zollinger-Ellison Syndrome

↑ by chronic PPI use.

Phenylalanine and tryptophan are potent stimulators.

Question 2

Cholecystokinin (CCK)

Answer 2

I cells of duodenum and jejunum.

↑ Pancreatic secretions

↑ Gallbladder contraction

↑Sphincter of Oddi relaxation

↓Gastric emptying

Regulation: ↑ by FA’s and aa’s

Note: CCK acts on neural muscarinic pathways to stimulate pancreatic secretions.

Question 3

Secretin

Answer 3

S cell of the duodenum.

↑ Pancreatic HCO^-₃

↑ Bile secretion

↓ Decreases gastric secretion

Regulation:↑ by acid and FA’s in lumen of duodenum

Notes: ↑ HCO3- neutralizes gastric acid of duodenum, allowing pancreatic enzymes to function.

Question 4

Somatostatin

Answer 4

δ/D cells of Pancreatic islets, GI mucosa

↓ Gastric acid and Pepsinogen secretion

↓ Pancreatic and small intestine fluid secretion

↓ Gall bladder contraction

↓ Decreases insulin and glucagon release

Regulation:

↑ by acid

↓ by Vagal stimulation

Note: Inhibitory hormone. GHIH effects inhibits digestion and absorption of substances needed for growth.

Inhibits: secretin, gastrin, Insulin, glucagon, Vasoactive Intestinal Peptide (VIP), CCK

Question 5

Glucose-dependent Inulinotropic Peptide (GIP)

Answer 5

K cells of the duodenum, jejunum

Exocrine: ↓ Gastric H+ secretion

Endocrine: ↑ Insulin release

Regulation: ↑ by FA’s, aa’s, and oral glucose.

Note: AKA Gastric inhibitory peptide (GIP).

An oral glucose load is used more rapidly vs the equivalent given by IV due to GIP secretion.

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Question 6

Vasoactive Instestinal Peptide (VIP)

Answer 6

Parasympathetic ganglia in sphincters, GB, and Small intestine (SI).

↑ intestinal water and electrolyte secretion

↑ relaxation of intestinal smooth muscle and sphincters.

Regulation: ↑ by distention and vagal stimulation.

                   ↓ by adrenergic input

Note: VIPoma- non-α, non-β islet cell pancreatic tumor that secretes VIP. Copious Watery Diarrhea, Hypokalemia, and Achlorhydia (WDHA syndrome).

Question 7

Nitric Oxide

Answer 7

Vascular endothelium?

↑ smooth muscle relaxation, including Lower Esophageal Sphincter (LES).

Note: Loss of NO secretion is implicated in ↑ lower esophageal tone of achalasia.

Question 8

Motilin

Answer 8

Secreted by Small intestine

Produces migrating motor complexes (MMC’s)

Regulation: ↑ in fasting state

Note: Motilin receptor agonists (such as erythromycin) are used to stimulate peristalsis.

Question 9

Intrinsic Factor

Question 10

Intrinisic Factor

Answer 10

Parietal cells

B12 binding protein.

Required for vitamin B12 uptake in the terminal ileum

Note: Autoimmune destruction of the parietal cells leads to chronic gastritis and pernicious anemia. (B12 is cofactor for RBC production)

Question 11

Gastric Acid (HCl)

Answer 11

Parietal cells secrete HCl

↓ stomach pH

Regulation:

↑ by histamine, ACh, Gastrin

↓ by somatostatin, GIP, prostaglandin, secretin

Note: Zollinger-Ellison syndrome/Gastrinoma- gastin secreting tumor, causes continuous secretion of acid which can lead to ulcers or steatorrhea because low duodenal pH inactivates the pancreatic lipases necessary for fat digestion.

Question 12

Pepsin

Answer 12

Cheif cells of stomach

Protein digestion

Regulation: ↑ by vagal stimulation, local acid

Note: Inactive pepsinogen→ pepsin by H+

Question 13

HCO3-

Answer 13

Mucosal cells of stomach, duodenum (Brunner’s glands), salivary glands, pancreas.

Neutralizes acid

Regulation: ↑ by pancreatic and biliary secretion with secretin.

Note: HCO3- is trapped in the mucus that covers the gastric epithelium