GIT Regulators

Question 1

Gastrin

Answer 1

Type: Endocrine regulator. Structurally homologous to CCK. Gq receptors.

Released by: G-cells in gastric antrum, duodenum, jejunum.

Function: Increase H+ secretion by gastric parietal cells. Stimulate growth of gastric mucosa.

Stimulated by: Small peptides & aas in stomach, stomach distension, vagal stimulation (via gastrin-releasing peptide).

Inhibited by: Increasing H+ in stomach. Somatostatin.

Question 2

Cholecystokinin (CCK)

Answer 2

Type: Endocrine regulator. Structurally homologous to CCK.

Released by: I Cells of duodenal & jejunal mucosa.

Function: Stimulates gallbladder contraction & sphincter of Oddi relaxation. Stimulates pancreatic enzyme secretion. Potentiates secretin-induced stimulation of pancreatic HCO3- secretion. Inhibits gastric emptying.

Stimulated by: Small peptides & amino acids. Fatty acids & monoglycerides (fat containing meals).

Question 3

Secretin

Answer 3

Type: Endocrine regulator. Structurally homologous to glucagon.

Released by: S cells of duodenum.

Function: Stimulates pancreatic HCO3- secretion. Stimulates HCO3- and H2O secretion by liver, and increases bile production. Inhibits H+ secretion by gastric parietal cells.

Stimulated by: H+ in duodenal lumen. Fatty acids in duodenal lumen.

Question 4

Gastric Inhibitory Peptide (GIP)

Answer 4

Type: Endocrine regulator. Incretin.

Released from: K cells of duodenum & jejunum. (not on slides)

Function: Enhances insulin secretion in response to oral carbohydrates. Inhibits H+ secretion by gastric parietal cells.

Question 5

Glucagon like peptide (GLP-1)

Answer 5

Type: Endocrine regulator. Incretin.

Released from: Intestinal enteroendocrine L-cells & brainstem. (not on slides)

Function: Enhances insulin secretion in response to oral carbohydrates.

Question 6

Somatostatin

Answer 6

Type: Paracrine regulator.

Released from: Endocrine cells throughout GIT mucosa. Gi receptors.

Function: Inhibits release of all GI hormone. Inhibits gastric H+ secretion.

Stimulated by: Luminal H+

Inhibited by: Vagal stimulation

Question 7

Histamine

Answer 7

Type: Paracrine regulator. Gs receptors.

Released from: Mast cells of gastric mucosa.

Function: Increases gastric H+ secretion directly. Potentiates effects of gastrin & vagal stimulation (both increase H+ secretion).

Question 8

Vasoactive Intestinal Peptide (VIP)

Answer 8

Type: Neurocrine regulator. Homologous to secretin in both structure & function.

Released from: Neurons in GIT mucosa & smooth muscle.

Function: Relaxation of GI smooth muscle, including lower esophageal sphincter. Stimulates pancreatic HCO3- secretion. Inhibits gastric H+ secretion.

Question 9

Gastrin-releasing peptide (GRP, also bombesin)

Answer 9

Type: Neurocrine regulator.

Released from: Vagus nerve that innervates G cells.

Function: Stimulates gastrin release from G-cells.

Question 10

Enkephalins (met-enkephalin & leu-enkephalin)

Answer 10

Type: Neurocrine regulator.

Released from: Neurons in GIT mucosa & smooth muscle.

Function: Contraction of GI smooth muscle, particularly sphincters: lower esophageal, pyloric, ileocecal. Inhibits intestinal secretion of fluid and electrolyes.

Note- forms basis of usefulness of opiates in treatment of diarrhea.