ABSTRACT
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 beta 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 beta 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 beta 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 beta cell glucose metabolism to stimulate insulin secretion.
