Contributions of intracellular compartments to calcium dynamics: implicating an acidic store.
Many cells show a plateau of elevated cytosolic Ca(2+) after a long depolarization, suggesting delayed Ca(2+) release from intracellular compartments such as mitochondria and endoplasmic reticulum (ER). Mouse pancreatic beta-cells show a thapsigargin-sensitive plateau ('hump') of Ca(2+) 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(+)-Ca(2+) exchange and by only 18% upon blocking ryanodine or IP(3) receptors together. Further, the time course of ER Ca(2+) 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 Ca(2+) 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 Ca(2+) after cytosolic Ca(2+) elevation.
Duman
Joseph G
JG
Department of Physiology and Biophysics, University of Washington, Seattle WA 98195, USA.
Chen
Liangyi
L
Palmer
Amy E
AE
Hille
Bertil
B
D000143:Acids / Q000737:chemistry; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D004594:Electrophysiology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D050417:Insulin-Secreting Cells / Q000378:metabolism; D042541:Intracellular Space / Q000737:chemistry* / Q000378:metabolism*; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D018408:Patch-Clamp Techniques
D000143:Acids; D002118:Calcium