Chromogranins A and B (CGA and CGB) are high capacity, low affinity calcium (Ca2+) storage proteins found in many cell types most often associated with secretory granules of secretory cells but also with the endoplasmic reticulum (ER) lumen of these cells. Both CGA and CGB associate with inositol 1,4,5-trisphosphate receptor (InsP3R) in a pH-dependent manner. At an intraluminal pH of 5.5, as found in secretory vesicles, both CGA and CGB bind to the InsP3R. When the intraluminal pH is 7.5, as found in the ER, CGA totally dissociates from InsP3R, whereas CGB only partially dissociates. To investigate the functional consequences of the interaction between the InsP3R and CGB monomers or CGA/CGB heteromers, purified mouse InsP3R type I were fused to planar lipid bilayers and activated by 2 microM InsP3. In the presence of luminal CGB monomers or CGA/CGB heteromers the InsP3R/Ca2+ channel open probability and mean open time increased significantly. The channel activity remained elevated when the pH was changed to 7.5, a reflection of CGB binding to the InsP3R even at pH 7.5. These results suggest that CGB may play an important modulatory role in the control of Ca2+ release from the ER. Furthermore, the difference in the ability of CGA and CGB to regulate the InsP3R/Ca2+ channel and the variability of CGA/CGB ratios could influence the pattern of InsP3-mediated Ca2+ release.