Generation of nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose by glucagon-like peptide-1 evokes Ca2+ signal that is essential for insulin secretion in mouse pancreatic islets

OBJECTIVE - Glucagon-like peptide-1 (GLP-1) increases intracellular Ca2+ concentrations ([Ca2+), resulting in insulin secretion from pancreatic beta-cells. The molecular mechanism(s) of the GLP-1-mediated regulation of [Ca2+](i) was investigated. RESEARCH DESIGN AND METHODS - GLP-1-induced changes in [Ca2+](i) were measured in P-cells isolated from Cd38(+/+) and Cd38(-/-) mice. Calcium-mobilizing second messengers were identified by measuring levels of nicotinic acid adenine dinucleotide phosphate (NAADP) and cyclic ADP-ribose (ADPR), using a cyclic enzymatic assay. To locate NAADP- and cyclic ADPR-producing enzyme(s), cellular organelles were separated using the sucrose gradient method. RESULTS - A GLP-1-induced [Ca2+](i) increase showed a cooperative Ca2+ signal, i.e., an initial [Ca2+](i) rise mediated by the action of NAADP that was produced in acidic organelles and a subsequent long-lasting increase of [Ca2+](i) by the action of cyclic ADPR that was produced in plasma membranes and secretory granules. GLP-1 sequentially stimulated production of NAADP and cyclic ADPR in the organelles through protein kinase A and cAMP-regulated guanine nucleotide exchange factor II. Furthermore, the results showed that NAADP production from acidic organelles governed overall Ca2+ signals, including insulin secretion by GLP-1, and that in addition to CD38, enzymes capable of synthesizing NAADP and/or cyclic ADPR were present in P-cells. These observations were supported by the study with Cd38(-/-) beta-cells, demonstrating production of NAADP, cyclic ADPR, and Ca2+ signal with normal insulin secretion stimulated by GLP-1. CONCLUSIONS - Our findings demonstrate that the GLP-1-mediated Ga2+ signal for insulin secretion in pancreatic P-cells is a cooperative action of NAADP and cyclic ADPR spatiotemporally formed by multiple enzymes.