GI Regulatory Substances

Question 1

Gastrin

Answer 1

G cells (antrum of stomach, duodenum)

INC gastric H+ secretion
INC growth of gastric mucosa
INC gastric motility

REGULATION
INC by stomach distention/ alkalinization, amino acids, peptides, vagal stimulation via gastrin-releasing peptide (GRP)
DEC by pH

Question 2

Somatostatin

Answer 2

D cells (pancreatic islets, GI mucosa)

DEC gastric acid and
DEC pepsinogen secretion
DEC pancreatic and small
intestine fluid secretion
DEC gallbladder contraction
DEC insulin and glucagon
release

REGULATION
INC by acid
DEC by vagal
stimulation

*Inhibits secretion of various
hormones (encourages
*somato-stasis).

*Octreotide is an analog used to treat acromegaly, carcinoid syndrome, and variceal bleeding

Question 3

Cholecystokinin

Answer 3

I cells (duodenum, jejunum)

INC pancreatic secretion
INC gallbladder contraction
INC sphincter of Oddi
relaxation
DEC gastric emptying

REGULATION
INC by fatty acids, amino acids

Acts on neural *muscarinic
pathways to cause pancreatic secretion

Question 4

Secretin

Answer 4

S cells (duodenum)

INC pancreatic HCO3–
secretion
INC bile secretion
DEC gastric acid secretion

REGULATION
INC by acid, fatty acids in lumen of duodenum

INC HCO3– neutralizes gastric acid in duodenum, allowing pancreatic enzymes to function.

Question 5

Glucose-dependent

insulinotropic peptide

Answer 5

K cells (duodenum, jejunum)

Exocrine:
DEC gastric H+ secretion
Endocrine:
INC insulin release

REGULATION
INC by fatty acids, amino acids, oral glucose

Also known as gastric inhibitory peptide (GIP).

Oral glucose load leads to
INC insulin compared to IV
equivalent due to GIP
secretion.

Question 6

Motilin

Answer 6

Small intestine

Produces migrating motor
complexes (MMCs)

REGULATION
INC in fasting state

Motilin receptor agonists (eg,
erythromycin) are used to
stimulate intestinal peristalsis.

Question 7

Vasoactive intestinal polypeptide (VIP)

Answer 7

Parasympathetic ganglia in
sphincters, gallbladder, small intestine

INC intestinal water and
electrolyte secretion
INC relaxation of intestinal
smooth muscle and sphincters

REGULATION
INC by distention and vagal
stimulation
DEC by adrenergic input

VIPoma—non-α, non-β
islet cell pancreatic tumor
that secretes VIP. Watery
Diarrhea, Hypokalemia,
and Achlorhydria (*WDHA
syndrome).

Question 8

Nitric oxide

Answer 8

INC smooth muscle relaxation, including lower esophageal sphincter (LES)

Loss of NO secretion is
implicated in INC LES tone of
achalasia

Question 9

Ghrelin

Answer 9

Stomach

INC appetite 

REGULATION
INC in fasting state
DEC by food

INC in Prader-Willi syndrome.
DEC after gastric bypass surgery

Question 10

Intrinsic factor

Answer 10

Parietal cells (stomach)

Vitamin B12–binding protein (required for B12 uptake in terminal ileum)

Autoimmune destruction
of parietal cells–>*chronic
gastritis and *pernicious
anemia.

Question 11

Gastric acid

Answer 11

Parietal cells (stomach)

DEC stomach pH

REGULATION
INC by histamine, ACh, gastrin
DEC by somatostatin, GIP, prostaglandin, secretin

Question 12

Pepsin

Answer 12

Chief cells (stomach)

Protein digestion

REGULATION
INC by vagal stimulation, local acid

Pepsinogen (inactive) is converted to pepsin (active) in the presence of H+.

Question 13

Bicarbonate

Answer 13

Mucosal cells
(stomach, duodenum, salivary glands, pancreas) and Brunner glands (duodenum)

Neutralizes acid

REGULATION
INC by pancreatic and biliary
secretion with secretin

Trapped in mucus that covers the gastric epithelium.