Abstract
Given the importance of inositol 1,4,5-trisphosphate receptor (IP 3R)/Ca 2+ channels in the control of intracellular Ca 2+ concentrations, we determined the relative concentrations of the IP 3R 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 IP 3R isoforms while secretory granules contained 58–69%. The nucleus contained ∼15% each of IP 3R-1 and -2, but 25% of IP 3R-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 IP 3-dependent intracellular Ca 2+ control mechanisms in chromaffin cells.
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