Proceedings of The Physiological Society
AstraZeneca (2010) Proc Physiol Soc 18, C13 and PC13
Inhibition of GIP and GLP-1 secretion through Gi-coupled receptor activation
C. E. Moss1, A. M. Habib1, F. M. Gribble1, F. Reimann1
1. Clinical Biochemistry, University of Cambridge, Cambridge, United Kingdom.
Background and aim:Glucose-dependant insulinotropic polypeptide (GIP) is released in response to nutrient ingestion from specialised enteroendocrine cells (K-cells) scattered throughout the proximal gut epithelium. As an incretin GIP stimulates insulin secretion from the pancreatic β-cell, and together with the other incretin (glucagon-like peptide 1 (GLP-1) secreted from L-cells) is responsible for ~50% of insulin release following oral glucose administration. In addition GIP has been postulated to link over nutrition to the development of obesity, as pharmacological or genetic interference with GIP signalling proved protective in several rodent obesity models. The aim of this study was to investigate if GIP secretion can be inhibited through activation of Gi-coupled receptors. Results: Using recently developed transgenic mice with fluorescently labelled K- or L-cells, expression of Gi-coupled receptors was investigated by hybridising mRNA isolated from FACS sorted K-cells, L-cells or non-fluorescent cells from these sorts to Affymetrix 430 2.0 chips. The cannabinoid receptor 1 (Cnr1) mRNA was found to be highly enriched in both small intestinal K- and L-cells relative to the non-fluorescent cells, while it seemed to be absent from colonic L-cells. By contrast, somatostatin receptor 5 (SstR5) seemed to be highly expressed in both small intestinal and colonic L-cells and to a lesser extent in K-cells. To investigate if stimulation of these Gi-coupled receptors affects incretin release, secretion experiments were performed on primary murine cultures of epithelial cells from the duodenum (top ten cm of intestine) and the colon/rectum. GIP and GLP-1 secretion, assayed by ELISA, was stimulated by 10 mM glucose + 100 µM 3-isobutyl-1-methylxanthine (IBMX). Addition of 100 nM somatostatin inhibited both GIP (61±0.2% n=10) and GLP-1 (≥89% n=9) secretion from small intestinal cultures and GLP-1 secretion from colonic cultures (52±0.6% n=9), an effect partially suppressed by coincubation with a SSTR5 selective antagonist. Addition of the CB1R agonist methanandamide (10 µM) attenuated GIP secretion (58±0.3% n=7) and this inhibition was completely suppressed by the concomitant presence of the antagonist AM251 (1 µM; n=7). AM251 alone was without effect on either basal or stimulated GIP secretion. Neither methanandamide nor AM251 did affect GLP-1 secretion from either duodenal or colonic cultures. Conclusions: Somatostatin inhibits the secretion of both incretins partially through the activation of SSTR5. GIP secretion can be inhibited by CB1R-activation, which somewhat surprisingly did not affect GLP-1 secretion. The absence of AM251 effects on basal and stimulated secretion argues against a tonic inhibition of K-cells by endocannaboids.
Where applicable, experiments conform with Society ethical requirements