Glucagon-like peptide 1 (GLP-1) in the gastrointestinal tract of the pheasant (Phasianus colchicus)

Chicken Intestinal L Cells and Glucagon-like Peptide-1 Secretion

Many types of endocrine cells have been identified in the gastroenteropancreatic system of vertebrates, which have subsequently been named with alphabet (s). L cells which secrete the glucagon-like peptide (GLP)-1 are scattered in the intestinal epithelium. This review discusses the morphological features of chicken L cells and GLP-1 secretion from intestinal L cells. L cells, identified using GLP-1 immunohistochemistry, are open-type endocrine cells that are distributed in the jejunum and ileum of chickens. GLP-1 co-localizes with GLP-2 and neurotensin in the same cells of the chicken ileum. Intestinal L cells secrete GLP-1 in response to food ingestion. Proteins and amino acids, such as lysine and methionine, in the diet trigger GLP-1 secretion from the chicken intestinal L cells. The receptor that specifically binds chicken GLP-1 is expressed in pancreatic D cells, implying that the physiological functions of chicken GLP-1 differ from its functions as an incretin in mammals.

Immunohistochemical distribution of glucagon-like peptide 1 in the digestive system of different aquatic vertebrates

The distribution of glucagon-like peptide 1 (GLP-1)-positive cells in digestive tracts and pancreases of aquatic vertebrates was investigated by immunohistochemical staining method. The results suggested that GLP-1-positive cells were distributed in the columnar mucous epithelium and tubular glands of lamina propria in the digestive system. However, GLP-1-positive cells were also found in subepithelial lamina propria of the mucosae and muscularis in each segment of the digestive tract of Rana nigromaculata. The distribution densities of these cells reached peaks in the stomachs, and the middle or end segments of small intestines of Chinese softshell turtle, Bufo gargarizans, R. nigromaculata and catfish, and there was the third distribution density peak in the rectum of catfish. The total amount or overall density of GLP-1-positive cells varied a lot in the digestive tracts of different animal species. The distribution density was relatively low in the digestive tract of chub and reached the maximum in the digestive tracts of snakehead and catfish, but no GLP-1-positive cells were found in the digestive tract of bighead carp. GLP-1-positive cells were densely distributed in the pancreases of Chinese softshell turtle, B. gargarizans and R. nigromaculata. These cells spread over the superficial layers of islets or scattered in exocrine pancreas in the pancreas of B. gargarizans, spread in the endocrine cells or scattered in the pancreas of Chinese softshell turtle, scattered in the pancreas of R. nigromaculata and distributed in the superficial layers of islets in the pancreas of catfish.

Ultrastructural study on colocalization of glucagon-like peptide (GLP)-1 with GLP-2 in chicken intestinal L-cells

Colocalization of glucagon-like peptide (GLP)-1 with GLP-2 in L-cells was investigated in the chicken ileum by using double immunofluorescent and immunocytochemical techniques. Ultrastructural features of L-cells were also clarified in this study. L-cells showing immunoreactivity for both GLP-1 and GLP-2 were distributed in the whole ileum. They showed comma-like or flask-like shape and were located in epithelium of crypts and lower part of intestinal villi. L-cells showing GLP-1-immunoreactivity only were found in epithelium of lower and middle parts of intestinal villi. Transmission electron microscopy indicated that L-cells identified by colloidal gold-labeled immunocytochemistry were covered apically with microvilli, open-type and contained many secretory granules in their perikarya. These secretory granules without halo were round to oval in shape and showed moderate electron density. The longest and shortest diameters of secretory granules were 355 ± 62 nm (mean ± SD) and 287 ± 48 nm, respectively. Double labeling immunocytochemistry using two different sizes of particles (6 and 12 nm in diameter) of colloidal gold revealed that GLP-1 colocalized with GLP-2 in the same secretory granules. This study advances new morphological data about the endocrine system of the chicken small intestine.

The influence of restricted feeding on glucagon-like peptide-1 (GLP-1)-containing cells in the chicken small intestine

The influence of restricted feeding on the distribution of glucagon-like peptide-1 (GLP-1)-containing endocrine cells in the chicken small intestine was investigated using immunohistochemical and morphometrical techniques. This study demonstrated that the restricted feeding had an influence on the activity of GLP-1-immunoreactive cells in the chicken small intestine. There were differences in the localization and the frequency of occurrence of GLP-1-immunoreactive cells in the small intestine between control and restricted groups, especially 25% feed supply group provided with 25% of the intake during the adapting period. GLP-1-immunoreactive cells in the control chickens were mainly located in epithelium from crypts to the lower part of intestinal villi. Those in restricted groups, however, tended to be located from crypts to the middle part of intestinal villi. The frequency of occurrence of GLP-1-immunoreactive cells was lowest in the control group, medium in 50% feed supply group and highest in 25% feed supply group at each intestinal region examined in this study, that is, increased with the advancement of restricting the amount of feed supply. These data show that the quantity of food intake is one of signals that have an influence on the secretion of GLP-1 from L cells in the chicken small intestine.