Generation of specific Ca2+ signals from Ca2+ stores and endocytosis by differential coupling to messengers

It remains unclear how different intracellular stores could interact and be recruited by Ca2+-releasing messengers to generate agonist-specific Ca2+ signatures. In addition, refilling of acidic stores such as lysosomes and secretory granules occurs through endocytosis, but this has never been investigated with regard to specific Ca2+ signatures. In pancreatic acinar cells, acetylcholine (ACh), cholecystokinin (CCK), and the messengers cyclic ADP-ribose (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), and inositol 1, 4,5-trisphosphate (IP3) evoke repetitive local Ca2+ spikes in the apical pole. Our work reveals that local Ca2+ spikes evoked by different agonists all require interaction of acid Ca2+ stores and the endoplasmic reticulum (ER), but in different proportions. CCK and ACh recruit Ca2+ from lysosomes and from zymogen granules through different mechanisms; CCK uses NAADP and cADPR, respectively, and ACh uses Ca2+ and IP3, respectively. Here, we provide pharmacological evidence demonstrating that endocytosis is crucial for the generation of repetitive local Ca2+ spikes evoked by the agonists and by NAADP and IP3. We find that cADPR-evoked repetitive local Ca2+ spikes are particularly dependent on the ER. We propose that multiple Ca2+-releasing messengers determine specific agonist-elicited Ca2+ signatures by controlling the balance among different acidic Ca2+ stores, endocytosis, and the ER.