AJP - Cell Physiology, Vol 263, Issue 1 C95-105, Copyright © 1992 by American Physiological Society
Differences in regulation between nuclear and cytoplasmic Ca2+ in cultured smooth muscle cells
B. Himpens, H. De Smedt, G. Droogmans and R. Casteels
Department of Physiology, K. U. Leuven, Gasthuisberg, Louvain, Belgium.
The free Ca2+ concentrations in the nucleus ([Ca2+]n) and cytoplasm
([Ca2+]c) of cultured smooth muscle cells were estimated using the
fluorescent dye indo-1 and the ACAS 570 confocal laser microscope. In
resting DDT1MF2 smooth muscle cells [Ca2+]n was found to be lower than
[Ca2+]c. Both values increased transiently in response to histamine (100
microM), but during this stimulation [Ca2+]n exceeded [Ca2+]c. Maximal
increase of [Ca2+]n was observed in the center of the nucleus, and a
maximal increase of [Ca2+]c was observed in the immediate vicinity of the
plasma membrane. A similar response was obtained with other agonists, such
as carbachol or ATP. Comparable results with ATP were obtained in cultured
aorta cells. The differential rise of [Ca2+]n over [Ca2+]c in DDT1MF2 cells
did not occur during either spontaneous release of Ca2+ or Ca2+ release
induced by caffeine (7.5 mM). The differential rise during histamine
stimulation was abolished by the presence of the intercalating substance
ethidium bromide. Thapsigargin, a presumed specific inhibitor of the
endoplasmic reticulum Ca(2+)-Mg(2+)-adenosine-triphosphatase, abolished the
Ca2+ gradient between nucleus and cytosol at rest. During subsequent
histamine stimulation the Ca2+ increase was largely blocked in both
compartments and attained similar levels. We propose that the lower value
of [Ca2+]n at rest is dependent on an active Ca2+ extrusion system. The
differential rise of [Ca2+]n over [Ca2+]c during agonist stimulation can be
explained by an influx of Ca2+ from perinuclear stores and/or by a release
of intranuclear Ca2+ possibly mediated by a process dependent on the
inositol lipid metabolism.
