Contributions of intracellular compartments to calcium dynamics: Implicating an acidic store

Many cells show a plateau of elevated cytosolic Ca2+ after a long depolarization, suggesting delayed Ca2+ release from intracellular compartments such as mitochondria and endoplasmic reticulum (ER). Mouse pancreatic beta-cells show a thapsigargin-sensitive plateau ('hump') of Ca2+ after a 30 s depolarization but not after a 10 s depolarization. Surprisingly, this hump depends primarily on compartments other than the mitochondria or ER. It is reduced by only 22% upon blocking mitochondrial Na+-Ca2+ exchange and by only 18% upon blocking ryanodine or IP3 receptors together. Further, the time course of ER Ca2+ measured by a targeted cameleon does not depend on the duration of depolarizations. Instead, the hump is reduced 35% by treatments with the dipeptide glycylphenylalanine beta-napthylamide, a tool often used to lyse lysosomes. We show that this dipeptide does not disturb ER functions, but it lyses acidic compartments and releases Ca2+ into the cytosol. Moreover, it induces leaks in and possibly lyses insulin granules and stops mobilization of secretory granules to the readily releasable pool in beta-cells. We conclude that the dipeptide compromises dense-core secretory granules and that these granules comprise an acidic calcium store in beta-cells whose loading and/or release is sensitive to thapsigargin and which releases Ca2+ after cytosolic Ca2+ elevation.