Degranulation of rough endoplasmic reticulum in M phase and adenosine-triphosphate-treated interphase cells is reversible.
In the preferential harvesting of rounded mitotic (M phase) cells of human Chang liver monolayer cultures by mechanical agitation in Ca(2+)-free phosphate-buffered saline, degranulation of endoplasmic reticulum (ER) was observed. Mitotic cells are known to have a series of Ca2+ transients and, without being subjected to Ca(2+)-free washings, did not have degranulated ER. Quiescent cells incubated with 0.7 mM adenosine 5'-triphosphate (ATP) in Ca(2+)-free HEPES-buffered saline produced very similar ER degranulations. Confocal argon laser imaging of fluo-3-loaded cells showed a Ca2+ transient peaking at 2 min after ATP treatment. In the absence of extracellular Ca2+, transients of Ca2+ elevation in the cytosol would exit the cell in a down-gradient, draining the ER Ca2+ stores. Substituting ATP with 1 microM brominated A23187 calcium ionophore in the incubation that contained 1-100 mM CaCl2, respectively, did not produce ER degranulation, thereby excluding raised cytosolic Ca2+ per se as the cause of ER degranulation. In fact, incubation with 0.7 mM ATP in the presence of 1-5 mM CaCl2 failed to produce ER degranulation. ER degranulated cells, from treatment with ATP without extracellular Ca2+ as well as from Ca(2+)-free washings at M phase, could be rescued by subsequent incubation in growth medium that contains Ca2+ whereupon the rounded cells re-flatten (a round-to-flat change) and have well-defined rough ER. It therefore seems possible for Ca2+ depletion, or at least a reduction, to be causally related to ER degranulation. If that were the case, ER granularity would appear to be a facultative rather than a constitutive state.
Sit
K H
KH
Department of Anatomy, National University of Singapore.
Bay
B H
BH
Wong
K P
KP
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology*; D015550:Cell Degranulation / Q000187:drug effects*; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D006801:Humans; D007399:Interphase; D008099:Liver / Q000166:cytology / Q000502:physiology; D008854:Microscopy, Electron; D008938:Mitosis
D000255:Adenosine Triphosphate; D002118:Calcium