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 β‐cells, cAMP amplifies Ca(2+)‐triggered exocytosis of insulin granules. Glucose‐induced insulin secretion is associated with Ca(2+)‐ and metabolism‐dependent increases of the sub‐plasma‐membrane cAMP concentration ([cAMP](pm)) in β‐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 β‐cells. METHODS: Total internal reflection microscopy and fluorescent reporters were used to monitor changes in cAMP, Ca(2+) and ATP concentrations as well as insulin secretion in MIN6 cells and mouse and human β‐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 Ca(2+)‐dependent [cAMP](pm) elevation, an effect that was amplified by high glucose. Ex‐9 inhibited both the Ca(2+) 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 β‐cells was paralleled by suppression of insulin secretion. CONCLUSION: Agonist‐independent and glucagon‐stimulated GLP1R signalling in β‐cells contributes to basal and glucose‐induced cAMP production and insulin secretion.