Nicotinic acid-adenine dinucleotide phosphate-sensitive calcium stores initiate insulin signaling in human beta cells

Recent studies suggest a role for autocrine insulin signaling in beta cells, but the mechanism and function of insulin-stimulated Ca2+ signals is uncharacterized. We examined Ca2+-dependent insulin signaling in human beta cells. Two hundred nanomolar insulin elevated [Ca2+](c) to 284 +/- 27 nM above baseline in approximate to30% of Fura-4F-loaded cells. Insulin evoked multiple Ca2+ signal waveforms, 60% of which included oscillations. Although the amplitude of Ca2+ signals was dose-dependent between 0.002 and 2,000 nM, the percentage of cells responding was highest at 0.2 nM insulin, suggesting the interaction of stimulatory and inhibitory pathways. Ca2+-free solutions did not affect the initiation of insulin-stimulated Ca2+ signals, but abolished the second phase of plateaus/ oscillations. Likewise, inositol 1,4,5-trisphosphate (IP3) receptor antagonists xestospongin C and caffeine selectively blocked the second phase, but not the initiation of insulin signaling. Thapsigargin and 2,5-di-tert-butylhydroquinone (BHQ) blocked insulin signaling, implicating sarcoplasmic/endoplasmic Ca2+-ATPase (SERCA)-containing Ca2+ stores. Insulin-stimulated Ca2+ signals were insensitive to ryanodine. Injection of the CD38-derived Ca2+ mobilizing metabolite, nicotinic acid-adenine dinucleotide phosphate (NAADP), at nanomolar concentrations, evoked oscillatory Ca2+ signals that could be initiated in the presence of ryanodine, xestospongin C, and Ca2+-free solutions. Desensitizing concentrations of NAADP abolished insulin-stimulated Ca2+ signals. Insulin-stimulated Ca2+ signals led to a Ca2+-dependent increase in cellular insulin contents, but not secretion. These data reveal the complexity of insulin signal transduction and function in human beta cells and demonstrate functional NAADP-sensitive Ca2+ stores in a human primary cultured cell type.