Local calcium spiking in pancreatic acinar cells.

A few years ago, my laboratory obtained evidence for local agonist-evoked repetitive Ca2+ spikes in single pancreatic acinar cells. We have now confirmed this and shown that regular cytosolic Ca2+ spikes evoked by low concentrations of acetylcholine or inositol 1,4,5-trisphosphate (InsP3) are confined to the secretory granule area at the luminal pole of the cells. The local subcellular signals probably arise because the first internal messenger (InsP3), generated from the basolateral membrane, can diffuse rapidly, whereas the Ca2+ released from the more responsive secretory granule region has a very restricted mobility. Local Ca2+ spikes are useful from an energetic point of view and also help to avoid undesirable activation of Ca(2+)-dependent processes. Another messenger, cyclic ADP-ribose, may also regulate intracellular Ca2+ release. In pancreatic acinar cells cyclic ADP-ribose induces repetitive Ca2+ spikes localized in the secretory granule area; these spikes are blocked by ryanodine, but also by the InsP3 receptor antagonist heparin. Ryanodine abolishes or markedly inhibits agonist-evoked Ca2+ spiking, but enhances the frequency of spikes evoked by internal InsP3 application. These results indicate that both ryanodine and InsP3 receptors are involved in Ca2+ spike generation in pancreatic acinar cells, and that both InsP3 and cyclic ADP-ribose may act as internal messengers.