“Incretins are hormones released from the gastrointestinal tract into the circulation in response to nutrient ingestion. Incretins enhance glucose-stimulated insulin secretion. Preproglucagon is expressed in the αcells of the endocrine pancreas, L cells of the intestine (distal ileum and colon), and neurons located in the caudal brainstem and hypothalamus. It is cleaved into multiple different products, including glucagon and two of the incretins, oxyntomodulin and glucagon-like peptide-1 (GLP-1). Oxyntomodulin and GLP-1 are released from L cells in the distal ileum and colon in response to ingestion of nutrients. Oxyntomodulin binds the GLP-1 receptor that is expressed in the nucleus of the solitary tract in the brainstem and in the arcuate nucleus. Oxyntomodulin inhibits gastric acid secretion, decreases gastric emptying, and decreases pancreatic enzyme secretion which is likely related to decreased gastric output [76]. Administration of oxyntomodulin in humans has been found to suppress ghrelin levels [77], decrease body weight and appetite, decrease leptin, and increase adiponectin levels presumably secondary to loss of adipose tissue [78].
GLP-1 leads to delay in gastric emptying, stimulation of glucose-dependent insulin secretion, inhibition of glucagon secretion, and stimulation of somatostatin secretion. GLP-1 binds to its receptor, a G-protein coupled receptor that belongs to the class B family, including receptors for glucagon and GIP [79]. The GLP-1 receptor is expressed in a wide range of tissues, including the pancreatic islet cells, lung, heart, kidney, stomach, intestine, pituitary, skin, vagus nerve, and several regions of the CNS including the hypothalamus and brainstem. Peripheral and central GLP-1 administration activates neurons in the arcuate and paraventricular nuclei, nucleus of the solitary tract, and area postrema [80] leading to decreased appetite. GLP-1 administration promotes satiety and has beneficial effects on glucose homeostasis.
The properties of GLP-1 have made it a useful drug target. GLP-1 is released rapidly into the circulation after oral nutrient ingestion, and its secretion occurs in a biphasic pattern starting with an early (within 10–15 minutes) phase that is followed by a longer (30–60 minutes) phase [81]. The half-life of GLP-1 is less than 2 minutes owing to rapid inactivation by the enzyme DPP-IV, which also cleaves PYY. This is the basis for the development of exenatide (Byetta), a subcutaneously administered DPP-IV-resistant GLP-1 receptor agonist.” Int J Pediatr Endocrinol. 2009; 2009: 141753.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2777281/
http://www.yourhormones.info/hormones/glucagon-like-peptide-1/
References
Int J Pediatr Endocrinol. 2009; 2009: 141753. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2777281/