Ca2+ waves require sequential activation of inositol trisphosphate receptors and ryanodine receptors in pancreatic acini.

The inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) and the ryanodine receptor (RyR) are the principal Ca2+-release channels in cells and are believed to serve distinct roles in cytosolic Ca2+ (Ca(i)2+) signaling. This study investigated whether these receptors instead can release Ca2+ in a coordinated fashion. Apical and basolateral Ca(i)2+ signals were monitored in rat pancreatic acinar cells by time-lapse confocal microscopy. Caged forms of second messengers were microinjected into individual cells and then photoreleased in a controlled fashion by either UV or 2-photon flash photolysis. InsP3 increased Ca(i)2+ primarily in the apical region of pancreatic acinar cells, whereas the RyR agonist cyclic adenosine diphosphate ribose (cADPR) increased Ca(i)2+ primarily in the basolateral region. Apical-to-basal Ca(i)2+ waves were induced by acetylcholine and initiation of these waves was blocked by the InsP3R inhibitor heparin, whereas propagation into the basolateral region was inhibited by the cADPR inhibitor 8-amino-cADPR. To examine integration of apical and basolateral Ca(i)2+ signals, Ca2+ was selectively released either apically or basolaterally using 2-photon flash photolysis. Ca(i)2+ increases were transient and localized in unstimulated cells. More complex Ca(i)2+ signaling patterns, including polarized Ca(i)2+ waves, were observed when Ca2+ was photoreleased in cells stimulated with subthreshold concentrations of acetylcholine. Polarized Ca(i)2+ waves are induced in acinar cells by serial activation of apical InsP3Rs and then basolateral RyRs, and subcellular release of Ca2+ coordinates the actions of these 2 types of Ca2+ channels. This subcellular integration of Ca2+-release channels shows a new level of complexity in the formation of Ca(i)2+ waves.