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.
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 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.