Identication of cyclic ADP-ribose-dependent mechanisms in pancreatic muscarinic Ca²⁺ signaling using CD38 knockout mice

We showed that muscarinic acetylcholine (ACh)-stimulation increased the cellular content of cADPR in the pancreatic acinar cells from normal mice but not in those from CD38 knockout mice. By monitoring ACh-evoked increases in the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) using fura-2 microfluorimetry, we distinguished and characterized the Ca²⁺ release mechanisms responsive to cADPR. The Ca²⁺ response from the cells of the knockout mice (KO cells) lacked two components of the muscarinic Ca²⁺ release present in wild mice. The first component inducible by the low concentration of ACh contributed to regenerative Ca²⁺ spikes. This component was abolished by ryanodine treatment in the normal cells and was severely impaired in KO cells, indicating that the low ACh-induced regenerative spike responses were caused by cADPR-dependent Ca²⁺ release from a pool regulated by a class of ryanodine receptors. The second component inducible by the high concentration of ACh was involved in the phasic Ca²⁺ response, and it was not abolished by ryanodine treatment. Overall, we conclude that muscarinic Ca²⁺ signaling in pancreatic acinar cells involves a CD38-dependent pathway responsible for two cADPR-dependent Ca²⁺ release mechanisms in which the one sensitive to ryanodine plays a crucial role for the generation of repetitive Ca²⁺ spikes.