Glucose- and forskolin-induced increases in cytoplasmic cAMP. Effects of glucose on δ-cell cAMP–glucose elevates δ-cell intracellular cAMP. (A) Representative recordings of changes in CFP (blue traces, F/F₀) and YFP (yellow traces, F/F₀) in non–δ- (upper traces) and δ-cells (middle traces) expressing the genetically encoded cAMP sensor Epac2-camps evoked by adding 2.5 µM forskolin (indicated by the horizontal bar). The ratio of CFP and YFP (CFP/YFP, bottom traces) indicates cytoplasmic cAMP ([cAMP]_i) in non–δ- (black trace) or δ-cells (red trace). (B) Glucose- (20 mM) and forskolin-induced (2.5 µM) increases in [cAMP]_i measured in non–δ- (black) and δ-cells (red) reported by Epac2-camps. (C) Bar graph comparing glucose- and forskolin-induced increases in [cAMP]_i in δ- (n = 10 cells; expressing tdRFP) and non–δ-cells (n = 20 cells; cells not expressing tdRFP). Responses have been normalized to the level observed at 1 mM glucose. *, P < 0.05 vs. 1 mM glucose. (D) Relationship between islet cAMP content and somatostatin secretion measured in the presence of 1 mM (red triangle) and 10 mM glucose (red rectangle) or 0.2 mM tolbutamide (black rectangle; added at 1 mM glucose). *, P < 0.05 vs. somatostatin secretion in presence of tolbutamide; †, P < 0.05 vs. somatostatin secretion measured at 1 mM glucose in the absence of tolbutamide. cAMP content is higher (P < 0.05) in the presence of 10 mM glucose than at 1 mM glucose alone or 1 mM glucose in the presence of tolbutamide (n = 8 independent experiments from more than three mice). a.u., arbitrary unit.