Abstract

Objective: Glucagon-like peptide-1 (GLP-1) increases intracellular Ca²⁺ concentration ([Ca²⁺]_i) that results in insulin secretion from pancreatic β 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 β 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 (cADPR) using cyclic enzymatic assay. To locate NAADP and cADPR producing enzyme(s), cellular organelles were separated using sucrose gradient method.

Results: GLP-1-induced [Ca²⁺]_i increase showed a cooperative Ca²⁺ signal, that is, an initial [Ca²⁺]_i rise mediated by the action of NAADP which was produced in acidic-organelles, and a subsequent long-lasting increase of [Ca²⁺]_i by the action of cADPR which was produced in plasma membranes and secretory granules. GLP-1 sequentially stimulated production of NAADP and cADPR in the organelles through PKA and Epac. 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 cADPR were present in β cells. These observations were supported by the study with Cd38^-/- β cells, demonstrating production of NAADP, cADPR and Ca²⁺ signal with normal insulin secretion stimulated by GLP-1.

Conclusion: Our findings demonstrate that GLP-1-mediated Ca²⁺ signal for insulin secretion in pancreatic β cells is a cooperative action of NAADP and cADPR spatiotemporally formed by multiple enzymes.