1975-P: Increased Pancreatic a Cell Ca2+ Oscillations Explain Hyperglucagonemia in High-Fat Diet-Fed Mice

Abstract

Glucagon secretion from pancreatic α cells is elevated in obesity, exacerbating hyperglycaemia and contributing to the development of diabetes. α cells exhibit intracellular calcium (Ca2+i) oscillations, which drive glucagon secretion. To understand these oscillations and their contribution (if any) to the diabetic phenotype, we developed a mouse model for studying α cell Ca2+i, and used this to investigate α cell Ca2+i dynamics in mice exposed to a high fat diet (HFD). Mice expressing GCaMP3 specifically in α-cells (GcgCre x GCaMP3 reporter mouse) were fed a control (10% fat; CTL) or high fat diet (60%; HFD) for 12 weeks. Mice then underwent a glucose tolerance test (GTT). Ca2+i imaging and glucagon secretion experiments were carried out on isolated (ex vivo) islets. HFD mice had impaired glucose tolerance compared to CTL mice (p<0.0001, n=25 mice). Plasma glucagon was suppressed by high glucose in CTL (p=0.01), but not HFD mice (p=0.65; n=10-20 mice). In islets from both CTL and HFD mice, Ca2+i oscillation frequency was suppressed by high glucose (p<0.0001 n=200-250 cells, 6 islets, 3 mice). However, oscillation frequency was higher in HFD mice vs. CTL (p<0.0001; n= 203-252 cells, 6-9 islets, 3 mice). Oscillation amplitude was not altered by glucose in CTL or HFD mice (p=0.076 and 0.87 respectively, 31-67 cells, 6-7 islets, 3 mice). Pharmacological approaches suggested that the increase in oscillation frequency was not due to altered α cell fat metabolism (p=0.5 38-68 cells, 4 islets, 3 mice). Insulin secretion from ex vivo islets was elevated in HFD vs. CTL mice, although this did not reach significance (p=0.084; N=6 mice). These preliminary data suggest that hyperglucagonemia in HFD mice may have origins intrinsic to the islet. This was not due to reduced paracrine regulation by insulin, as insulin secretion was elevated in HFD mice. Future experiments will aim to fully power this study and understand whether α cell intrinsic mechanisms contribute to the hyperglucagonemic phenotype. Disclosure J. Kellard: None. L. Briant: None. J.G. Knudsen: None. P. Rorsman: None. Funding UK Wellcome Trust