- • In intestine, nutrients including glucose and amino acids and non-nutrients including bile acids increase secretion of anti-diabetic gut peptides such as gluco-insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1) and peptide tyrosine tyrosine (PYY).
- • Facilitative glucose transporter pathways in addition to active electrogenic transporter pathways contribute to GIP, GLP-1 and PYY secretion; in particular, the facilitative glucose transporter 2 (GLUT2) is involved.
- • Sucralose, in the presence of glucose, can strongly and acutely upregulate GIP, GLP-1 and PYY secretion in a time scale of minutes.
- • Amino acid-stimulated GIP, GLP-1 and PYY secretion is acutely regulated by the calcium-sensing receptor (CasR).
- • The results establish new functions for GLUT2 and CasR as regulators of gut peptide secretion that sense nutrients and provide signalling pathways for the release of GIP, GLP-1 and PYY.
Abstract Intestinal enteroendocrine cells (IECs) secrete gut peptides in response to both nutrients and non-nutrients. Glucose and amino acids both stimulate gut peptide secretion. Our hypothesis was that the facilitative glucose transporter, GLUT2, could act as a glucose sensor and the calcium-sensing receptor, CasR, could detect amino acids in the intestine to modify gut peptide secretion. We used isolated loops of rat small intestine to study the secretion of gluco-insulinotropic peptide (GIP), glucagon-like peptide-1 (GLP-1) and peptide tyrosine tyrosine (PYY) secretion stimulated by luminal perfusion of nutrients or bile acid. Inhibition of the sodium-dependent glucose cotransporter 1 (SGLT1) with phloridzin partially inhibited GIP, GLP-1 and PYY secretion by 45%, suggesting another glucose sensor might be involved in modulating peptide secretion. The response was completely abolished in the presence of the GLUT2 inhibitors phloretin or cytochalasin B. Given that GLUT2 modified gut peptide secretion stimulated by glucose, we investigated whether it was involved in the secretion of gut peptide by other gut peptide secretagogues. Phloretin completely abolished gut peptide secretion stimulated by artificial sweetener (sucralose), dipeptide (glycylsarcosine), lipid (oleoylethanolamine), short chain fatty acid (propionate) and major rat bile acid (taurocholate) indicating a fundamental position for GLUT2 in the gut peptide secretory mechanism. We investigated how GLUT2 was able to influence gut peptide secretion mediated by a diverse range of stimulators and discovered that GLUT2 affected membrane depolarisation through the closure of K+ATP-sensitive channels. In the absence of SGLT1 activity (or presence of phloridzin), the secretion of GIP, GLP-1 and PYY was sensitive to K+ATP-sensitive channel modulators tolbutamide and diazoxide. l-Amino acids phenylalanine (Phe), tryptophan (Trp), asparagine (Asn), arginine (Arg) and glutamine (Gln) also stimulated GIP, GLP-1 and PYY secretion, which was completely abolished when extracellular Ca2+ was absent. The gut peptide response stimulated by the amino acids was also blocked by the CasR inhibitor Calhex 231 and augmented by the CasR agonist NPS-R568. GLUT2 and CasR regulate K- and L-cell activity in response to nutrient and non-nutrient stimuli.