Gastrin releasing peptide and cholecystokinin employ different intracellular pathways to elicit similar safe Ca2+ signals
Repetitive cytosolic Ca2+ spikes in pancreatic acinar cells, elicited by low (physiological) concentrations of acetylcholine (ACh), cholecystokinin (CCK) and gastrin releasing peptide (GRP), control secretion of digestive enzymes, whereas high-intensity stimulation induces sustained Ca2+ elevation initiating acute pancreatitis. Since the discovery of the Ca2+-releasing function of inositol trisphosphate (IP3), it has been generally assumed that Ca2+ signaling relies on the phospholipase C - IP3 pathway. We have now compared the mechanism of action of the three physiological stimulants, all acting on different receptors, but each coupled to the IP3 pathway. Unlike ACh, low physiological concentrations of CCK and GRP cannot elicit Ca2+ signals without co-operation of an additional intracellular mechanism. CCK-elicited Ca2+ signaling requires activation of intracellular receptors for nicotinic acid adenine dinucleotide phosphate (NAADP), whereas this is not the case for the action of GRP that nevertheless relies on the operation of CD38, the enzyme involved in the synthesis of both cyclic ADP ribose and NAADP. Even Ca2+ signals elicited by ACh are partially dependent on CD38. It is engagement of these additional non-IP3 pathways that allows low concentrations of secretagogues to elicit safe Ca2+ spiking and therefore secretion, obviating the need for potentially toxic high levels of agonists.