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Lumone
Lumone is the generic name applied to Enteric peptides secreted by the intestinal
tract, especially the stomach and duodenal region to stimulate the secretion of
gastrointestinal hormones (GI hormones) and Insulin. These are potential luminal
factors (traveling the luminal route of intestine) that have clinical significance in
connection with diabetes. Two important luminones are Incretin and Apelin.
Incretin
Incretins are a group of gastrointestinal hormones that cause an increase in the
amount of insulin released from the beta cells of the islets of Langerhans after
eating, even before blood glucose levels become elevated. They also slow the rate
of absorption of nutrients into the blood stream by reducing gastric emptying and
may directly reduce food intake. As expected, they also inhibit glucagon release
from the alpha cells of the Islets of Langerhans.
The Incretins are hormones that work to increase insulin secretion. The Incretin
concept was developed when it was observed that there is substantially more
insulin secreted in response to oral glucose versus intravenous glucose. It was
hypothesized that glucose in the digestive tract activated a feed forward
mechanism that increased insulin secretion, anticipating the rise in blood glucose
that would occur following absorption of ingested carbohydrates. There are two
main Incretin hormones in humans, GIP (glucose-dependent insulinotropic
peptide; also known as gastric inhibitory peptide) and GLP-1(glucagon-like
peptide-1). Both hormones are secreted by endocrine cells that are located in the
epithelium of the small intestine. The endocrine cell senses an increase in the
concentration of a substance in the lumen of the digestive tract (like glucose), and
this acts as the trigger for hormone secretion. Incretins are carried through the
circulation to their target tissue: the pancreatic beta cells. Incretin stimulation of
beta cells causes them to secrete more insulin in response
Apelin
Apelin is a potential luminal factor, i.e. a lumone that stimulates CCK secretion.
Apelin is not produced by the intestine; however, Apelin may travel by a
luminal route from the stomach to the intestine to stimulate CCK release. Apelin is
a recently characterized peptide that was isolated from bovine stomach extracts
based upon its ability to increase the extracellular acidification rate in Chinese
hamster ovary cells transfected with a G protein-linked orphan receptor called the
APJ receptor. Apelin is named after APJ endogenous ligand.
Like many other regulatory peptides, pharmacological studies indicate that Apelin
has multiple biological activities. Reported actions for Apelin include inhibition of
pro-inflammatory cytokine production by mouse spleen cells, chemotactic activity
on CHO-A10 cells, and lowering of blood pressure and stimulation of drinking
behavior in rats, and Apelin is thought to function as a co-receptor with CD4 in the
process of HIV infection. Apelin is produced in several tissues of the body,
including the heart, brain, lung, pregnant and lactating breast, and GI tract.
Proxenopsin and Leptin
Other gastric peptides, including, Proxenopsin and Leptin have been shown to be
secreted into the gastric lumen where they are either processed by pepsin or
travel to the duodenal lumen to stimulate CCK secretion.
D.Mohankumar