AJP - Cell Physiology, Vol 263, Issue 5 C978-C985, Copyright © 1992 by American Physiological Society
Kinetics of nucleocytoplasmic Ca2+ transients in DDT1 MF-2 smooth muscle cells
B. Himpens, H. De Smedt and R. Casteels
Physiological Laboratory, K. U. Leuven, Belgium.
The free calcium concentrations in the nucleus ([Ca2+]n) and in the
cytoplasm ([Ca2+]c) of cultured DDT1 MF-2 smooth muscle cells were
estimated using the fluorescent dye indo-1. With the use of confocal
microscopy, line scans were made during the onset and the differential rise
of the Ca2+ signal elicited by the agonists histamine and ATP. The results
confirm our earlier findings that in these cells [Ca2+]n at rest was lower
than [Ca2+]c. The present experiments show that this gradient over the
nuclear envelope was also preserved in Ca(2+)-free solution containing 2 mM
EGTA, underlining the selective barrier function of the nuclear envelope.
During stimulation with histamine, an early Ca2+ rise in the vicinity of
the nuclear envelope was found in contrast to the delayed Ca2+ rise 2
microns away on both sides of the envelope. This suggests the release of
Ca2+ stored in the envelope and the perinuclear sarcoplasmic reticulum. The
time course for reaching a uniform Ca2+ concentration ([Ca2+]u = [Ca2+]n =
[Ca2+]c) in the nuclear and cytosolic compartment varied with the agonist
used for stimulation and was dependent on the external Ca2+ concentration.
The value of this uniform Ca2+ concentration itself was, however,
independent of the type of stimulation. After reaching [Ca2+]u, a further
rise occurred with [Ca2+]n becoming larger than [Ca2+]c. It is postulated
that a critical Ca2+ concentration must be reached to induce this
differential Ca2+ rise by releasing Ca2+ from an intranuclear Ca2+ store.
