Although the role of secretory granules as the inositol 1,4,5-trisphosphate (IP 3 )-sensitive intracellular Ca 2؉ store and the presence of the IP 3 receptor (IP 3 R)/Ca 2؉ channel on the secretory granule membrane have been established, the identity of the IP 3 R types present in the secretory granules is not known. We have therefore investigated the presence of different types of IP 3 R in the secretory granules of bovine adrenal medullary chromaffin cells using immunogold electron microscopy and found the existence of all three types of IP 3 R in the secretory granules. To determine whether these IP 3 Rs interact with CGA and CGB, each IP 3 R isoform was cotransfected with CGA or CGB into NIH3T3 or COS-7 cells, and the expressed IP 3 R isoform and CGA or CGB were co-immunoprecipitated. From these studies it was shown that all three types of IP 3 R form complexes with CGA and CGB in the cells. To further confirm whether the IP 3 R isoforms and CGA and CGB form a complex in the secretory granules the potential interaction between all three isoforms of IP 3 R and CGA and CGB was tested by co-immunoprecipitation experiements of the mixture of secretory granule lysates and the granule membrane proteins. The three isoforms of IP 3 R were shown to form complexes with CGA and CGB, indicating the complex formation between the three isoforms of IP 3 R and CGA and CGB in the secretory granules. Moreover, the pH-dependent Ca 2؉ binding property of CGB was also studied using purified recombinant CGB, and it was shown that CGB bound 93 mol of Ca 2؉ /mol with a dissociation constant (K d ) of 1.5 mM at pH 5.5 but virtually no Ca 2؉ at pH 7.5. The high capacity, low affinity Ca 2؉ -binding property of CGB at pH 5.5 is comparable with that of CGA and is in line with its role as a Ca 2؉ storage protein in the secretory granules.The secretory granules of endocrine cells, neurons, and neuroendocrine cells contain many hormones, ions, peptides and proteins, including 40 mM Ca 2ϩ and 1-2 mM chromogranins A and B in addition to high concentrations of hormones (1-6). The secretory granule contents are secreted to the extracellular space and then into the bloodstream during exocytosis, which is initiated by a sudden increase of intracellular Ca 2ϩ concentration (7). In bovine adrenal medullary chromaffin cells the secretory granules occupy ϳ10% of the total cell volume (8), thereby storing a majority of the intracellular Ca 2ϩ of the cell in the secretory granules. Hence it appears inevitable for the secretory granules to participate in the control of intracellular Ca 2ϩ concentrations. Consistent with this notion, the secretory granules from adrenal medullary chromaffin cells (9), pancreatic acinar cells (10), and the goblet cells (11) concentrations but also in exocytotic processes. Despite the importance of the IP 3 -sensitive intracellular Ca 2ϩ store role of secretory granules, the study of IP 3 R/Ca 2ϩ channels in the secretory granules did not begin until the secretory granule Ca 2ϩ storage protein chromogranin A was shown to...