A cAMP and Ca²⁺ coincidence detector in support of Ca²⁺-induced Ca²⁺ release in mouse pancreatic β cells

The blood glucose-lowering hormone glucagon-like peptide-1 (GLP-1) stimulates cAMP production, promotes Ca²⁺ influx, and mobilizes an intracellular source of Ca²⁺ in pancreatic β cells. Here we provide evidence that these actions of GLP-1 are functionally related: they reflect a process of Ca²⁺-induced Ca²⁺ release (CICR) that requires activation of protein kinase A (PKA) and the Epac family of cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs). In rat insulin-secreting INS-1 cells or mouse β cells loaded with caged Ca²⁺ (NP-EGTA), a GLP-1 receptor agonist (exendin-4) is demonstrated to sensitize intracellular Ca²⁺ release channels to stimulatory effects of cytosolic Ca²⁺, thereby allowing CICR to be generated by the uncaging of Ca²⁺ (UV flash photolysis). This sensitizing action of exendin-4 is diminished by an inhibitor of PKA (H-89) or by overexpression of dominant negative Epac. It is reproduced by cell-permeant cAMP analogues that activate PKA (6-Bnz-cAMP) or Epac (8-pCPT-2′-O-Me-cAMP) selectively. Depletion of Ca²⁺ stores with thapsigargin abolishes CICR, while inhibitors of Ca²⁺ release channels (ryanodine and heparin) attenuate CICR in an additive manner. Because the uncaging of Ca²⁺ fails to stimulate CICR in the absence of cAMP-elevating agents, it is concluded that there exists in β cells a process of second messenger coincidence detection, whereby intracellular Ca²⁺ release channels (ryanodine receptors, inositol 1,4,5-trisphosphate (IP₃) receptors) monitor a simultaneous increase of cAMP and Ca²⁺ concentrations. We propose that second messenger coincidence detection of this type may explain how GLP-1 interacts with β cell glucose metabolism to stimulate insulin secretion.