AJP - Cell Physiology, Vol 263, Issue 3 C584-C589, Copyright © 1992 by American Physiological Society
Volume-sensitive Ca influx and release from intracellular pools in gastric parietal cells
P. A. Negulescu, B. Munck and T. E. Machen
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
The effects of osmotically induced changes in cell volume on cytoplasmic
free Ca (Cai) were studied in parietal cells from intact rabbit gastric
glands using digital image processing of fura-2 fluorescence. In resting
unstimulated cells, Cai was unaffected by either cell swelling or shrinking
when osmolarity was varied between 200 and 400 mosM (isotonicity 290 mosM).
However, when cells were swelled in a 165 mosM solution (55% tonicity), a
biphasic Ca increased was observed. On average, Cai increased transiently
from 80 to 218 nM before stabilizing at approximately 140 nM. The peak was
due to release from intracellular pools because it was present in Ca-free
solutions while the sustained elevation was dependent on external Ca. In
carbachol-stimulated cells, Ca influx was most sensitive to cell shrinkage.
For example, addition of 25 mM sucrose (108% tonicity) caused a 30%
decrease in the sustained carbachol-stimulated Cai increase (plateau). In
contrast, carbachol-stimulated cells were relatively insensitive to cell
swelling, with a 30% decrease in tonicity causing only a 15% increase in
the plateau. However, as in the unstimulated cells, extreme (55% tonicity)
swelling caused additional increases in Cai levels. The carbachol-dependent
effects of changes in cell volume on Cai could be mimicked by treating
cells with thapsigargin, an inhibitor of Ca pumps of intracellular
membranes that also has been shown to stimulate Ca entry. Thus, although
extreme swelling conditions (55% tonicity) could elicit Cai increases in
either the presence or absence of agonist, agonist was required to observe
Cai decreases due to cell shrinkage.(ABSTRACT TRUNCATED AT 250 WORDS)
