Nutrient-dependent secretion of glucose-dependent insulinotropic polypeptide from primary murine K cells

Glucose-dependent insulinotropic polypeptide (GIP) is an incretin hormone with anti-apoptotic effects on the pancreatic beta cell. The aim of this study was to generate transgenic mice with fluorescently labelled GIP-secreting K cells and to use these to investigate pathways by which K cells detect nutrients.

Transgenic mice were generated in which the GIP promoter drives the expression of the yellow fluorescent protein Venus. Fluorescent cells were purified by flow cytometry and analysed by quantitative RT-PCR. GIP secretion was assayed in primary cultures of small intestine.

Expression of Venus in transgenic mice was restricted to K cells, as assessed by immunofluorescence and measurements of the Gip mRNA and GIP protein contents of purified cells. K cells expressed high levels of mRNA for Kir6.2 (also known as Kcnj11), Sur1 (also known as Abcc8), Sglt1 (also known as Slc5a1), and of the G-protein-coupled lipid receptors Gpr40 (also known as Ffar1), Gpr119 and Gpr120. In primary cultures, GIP release was stimulated by glucose, glutamine and linoleic acid, and potentiated by forskolin plus 3-isobutyl-1-methylxanthine (IBMX), but was unaffected by the artificial sweetener sucralose. Secretion was half-maximal at 0.6 mmol/l glucose and partially mimicked by alpha-methylglucopyranoside, suggesting the involvement of SGLT1. Tolbutamide triggered secretion under basal conditions, whereas diazoxide suppressed responses in forskolin/IBMX.

These transgenic mice and primary culture techniques provide novel opportunities to interrogate the mechanisms of GIP secretion. Glucose-triggered GIP secretion was SGLT1-dependent and modulated by K(ATP) channel activity but not determined by sweet taste receptors. Synergistic stimulation by elevated cAMP and glucose suggests that targeting appropriate G-protein-coupled receptors may provide opportunities to modulate GIP release in vivo.