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 Ca ²⁺ concentrations ([Ca²⁺]_i), resulting in insulin secretion from pancreatic p-cells. The molecular mechanism(s) of the GLP-1-mediated regulation of [Ca²⁺]_i was investigated. RESEARCHDESIGN AND METHODS-GLP-1-induced (changes in [Ca²⁺] _i were measured in β-cells isolated from Cd38^+/+ and Cd38^-/- mice. Calcium-mobilizing second messengers were identified by measuring levels of nicotinic acid adenine dinucle-otide 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 [Ca ²⁺]_i rise mediated by the action of NAADP that was produced in acidic organelles and a subsequent long-lasting increase of [Ca ²⁺]_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 β-cells. These observations were supported by the study with Cd38^-/- β-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 Ca²⁺ signal for insulin secretion in pancreatic β-cells is a cooperative action of NAADP and cyclic ADPR spatiotempo-rally formed by multiple enzymes.