Describe
the physiological actions of gastrin in the human gut.
Outline:
·
Effects on:
- acid secretion
- GI motility
- pancreatic secretion
- growth of mucosa
·
Regulation of
secretion
Essay:
Gastrin is a polypeptide hormone secreted by G cells in the pyloric
glands and in the duodenum. G cells are flask-shaped, with a broad base
containing many gastrin granules and a narrow apex that reaches the mucosal
surface. Microvilli project from the apical end into the lumen. Receptors
mediating gastric responses to changes in gastric contents are present on the
microvilli.
The principal action of gastrin is stimulation of gastric acid and pepsin
secretion. Gastrin binds to its membrane receptor, and elevates intracellular Ca2+
levels through phospholipase C. This promotes the fusion of the tubulovesicles
in the parietal cells with the apical membrane, increasing the number of H+-K+-ATPase
molecules there. Gastrin increases gastric and intestinal motility, facilitating
the mixing of ingested food particles with gastric juice and its breakdown
(mechanical digestion). Gastrin causes contraction of the musculature that
closes the gastroesophageal junction though this effect is of questionable
physiological significance. It is also necessary for the proper growth of the
gastrointestinal mucosa (tropic action).
Gastrin secretion is affected by the contents of the stomach, the rate of
discharge of the vagus nerves, and blood-borne factors. Postganglionic vagal
fibers innervating the G cells secrete gastric-releasing polypeptide (rather
than acetylcholine) that stimulates gastrin release. Gastric secretion is
increased by the presence of products of protein digestion in the stomach,
particularly amino acids which act directly on the G cells. Phenylalanine and
tryptophan are particularly effective.
Acid in the antrum inhibits gastrin secretion. The effect of acid is the
basis of a negative feedback loop regulating gastrin secretion. Gastrin
secretion is also inhibited by the hormones secretin, GIP, VIP and glucagon.