Distribution profile of inositol 1,4,5-trisphosphate receptor isoforms in adrenal chromaffin cells

Given the importance of inositol 1,4,5-trisphosphate receptor (IP3R)/Ca2+ channels in the control of intracellular Ca2+ concentrations, we determined the relative concentrations of the IP3R isoforms in subcellular organelles, based on serially sectioned electron micrographs. The endoplasmic reticulum (ER) was estimated to contain 15–20% of each of the three IP3R isoforms while secretory granules contained 58–69%. The nucleus contained ∼15% each of IP3R-1 and -2, but 25% of IP3R-3, whereas the plasma membrane contained ∼1% or less of each. These suggested that secretory granules, the nucleus and ER are at the center of IP3-dependent intracellular Ca2+ control mechanisms in chromaffin cells.