Gastrin releasing peptide and cholecystokinin employ different intracellular pathways to elicit similar safe Ca ²⁺ signals

Repetitive cytosolic Ca ²⁺ 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 Ca ²⁺ elevation initiating acute pancreatitis. Since inositol trisphosphate (IP ₃ ) was discovered as an intracellular Ca ²⁺ releasing messenger, it has been assumed that a major class of G-protein coupled receptors relies on this pathway. We have now compared the mechanisms of action of the three physiological stimulants, all acting on different receptors, but each coupled to the IP ₃ pathway. Low concentrations of CCK and GRP cannot elicit Ca ²⁺ signals without co-operation of an additional intracellular mechanism. CCK-elicited Ca ²⁺ signalling 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 Ca ²⁺ signals elicited by ACh are partially dependent on CD38. It is engagement of these additional non-IP ₃ pathways that allows low concentrations of secretagogues to elicit safe Ca ²⁺ spiking and therefore secretion, obviating the need for potentially toxic high levels of secretagogues.