In my review, I would be discussing about 3 hormones of the gastrointestinal tract, namely, Cholecystokinin (CCK), Gastrin and somatostatin. Gastrin is a gastrointestinal hormone produced and secreted by the G-cells of the stomach, and is one of the main GI hormones playing a role in digestion. The G-cells are located in the gastric pits, which are especially found in the antrum region. On the other hand, the receptors of the G-cells are present in the Enterochromaffin-like cells and also the parietal region of the stomach.
Some amount of G-cells is also found in the duodenal bulb. Recent research has demonstrated that gastrin has also been detected in the central nervous system (CNS), peripheral nervous system, pituitary gland, genitourinary system, adrenal glands, lungs, respiratory tract, cancers, benign tumors, pancreas of the fetus, etc (Bowen, 2003, Ganong, 2005 & Feldman, 2006). The gastrin hormone appears as a linear peptide (chain of amino acids attached one to another in a length-wise fashion). Initially, the pre-progastrin precursor produced contains about 101 amino acids.
Through the process of translation, gastrin is produced into active forms such as G34, G17 and G14. G34 is also known ‘big gastrin’, whereas G-14 is known as ‘mini gastrin’. In the pregastrin precursor, there are about 101 amino acids from 4 different groups, namely the 21 amino acids signal peptide group, the 37 amino acids spacer sequence group, the 34 amino acid gastrin component and a 9 amino acid at the carboxyl terminal. Using several enzymes produced in the GIT, pregastrin is converted into various forms of the gastrin hormone.
Pentgastrin is the five C-terminal amino acids of gastrin that is responsible for its biological activity. These pentagastrin components are similar in both, cholecystokinin and gastrin. This is suggestive that both of them may have some mode of actions that are similar (Feldman, 2006). The G-cells that produce gastrin appear flask-shaped. The base of these cells is broad and contains gastrin granules. On the other hand, the apex is narrow with projections into the mucosal surface.
At the apex of the G-cells, microvilli are located that guide the secretions into the lumen. Any changes that occur with the contents of the stomach are relayed back to the G-cells through receptors located in the microvilli. During the fetal period, the pancreas also contains G-cells. Some of these G-cells may persist and may result in the formation of Gastrinomas (gastrin-secreting tumors). The G-cells are also secreted by the pituitary gland (both anterior and posterior lobe), hypothalamus and medulla oblongata (Ganong, 2005).