Glucose-induced cAMP elevation in β-cells involves amplification of constitutive and glucagon-activated GLP-1 receptor signalling

Aim: cAMP typically signals downstream of G(s)-coupled receptors and regulates numerous cell functions. In beta-cells, cAMP amplifies Ca2+-triggered exocytosis of insulin granules. Glucose-induced insulin secretion is associated with Ca2+- and metabolism-dependent increases of the sub-plasma-membrane cAMP concentration ([cAMP](pm)) in beta-cells, but potential links to canonical receptor signalling are unclear. The aim of this study was to clarify the role of glucagon-like peptide-1 receptors (GLP1Rs) for glucose-induced cAMP signalling in beta-cells. Methods: Total internal reflection microscopy and fluorescent reporters were used to monitor changes in cAMP, Ca2+ and ATP concentrations as well as insulin secretion in MIN6 cells and mouse and human beta-cells. Insulin release from mouse and human islets was also measured with ELISA. Results: The GLP1R antagonist exendin-(9-39) (ex-9) prevented both GLP1- and glucagon-induced elevations of [cAMP](pm), consistent with GLP1Rs being involved in the action of glucagon. This conclusion was supported by lack of unspecific effects of the antagonist in a reporter cell-line. Ex-9 also suppressed IBMX- and glucose-induced [cAMP](pm) elevations. Depolarization with K+ triggered Ca2+-dependent [cAMP](pm) elevation, an effect that was amplified by high glucose. Ex-9 inhibited both the Ca2+ and glucose-metabolism-dependent actions on [cAMP](pm). The drug remained effective after minimizing paracrine signalling by dispersing the islets and it reduced basal [cAMP](pm) in a cell-line heterologously expressing GLP1Rs, indicating that there is constitutive GLP1R signalling. The ex-9-induced reduction of [cAMP](pm) in glucose-stimulated beta-cells was paralleled by suppression of insulin secretion. Conclusion: Agonist-independent and glucagon-stimulated GLP1R signalling in beta-cells contributes to basal and glucose-induced cAMP production and insulin secretion.