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Journal of Neuroscience, Vol 14, 5834-5843, Copyright © 1994 by Society for Neuroscience
Sodium/calcium exchange in rat cortical astrocytes
WF Goldman, PJ Yarowsky, M Juhaszova, BK Krueger and MP Blaustein
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.
Regulation of the cytosolic free Ca2+ concentration ([Ca2+]cyt) by an Na/Ca
exchanger was studied in primary cultured rat cortical astrocytes.
[Ca2+]cyt was measured by digital imaging in cells loaded with fura-2. The
resting [Ca2+]cyt, approximately 150 nM, was only slightly increased by
reducing the extracellular Na+ concentration ([Na+]o) to 6.2 mM, or by
treating the cells with ouabain for 15 min (to raise cytosolic Na+).
Following treatment with ouabain, however, lowering [Na+]o caused [Ca2+]cyt
to rise rapidly to approximately 1300 nM. When Ca2+ sequestration in
intracellular stores was blocked by thapsigargin, lowering [Na+]o increased
[Ca2+]cyt to approximately 1500 nM in the absence of ouabain. The
low-[Na+]o-stimulated rise in [Ca2+]cyt was abolished by removal of
external Ca2+, but was not blocked by the Ca2+ channel blocker verapamil,
or by caffeine or ryanodine, which deplete an intracellular Ca2+ store
responsible for Ca(2+)-induced Ca2+ release. These data suggest that Na+
gradient reduction promotes net Ca2+ gain via Na/Ca exchange. Normally,
however, a large rise in [Ca2+]cyt is prevented by sequestration of the
entering Ca2+; this buffering of cytosolic Ca2+ can be circumvented by
blocking sequestration with thapsigargin, or overwhelmed by enhancing net
Ca2+ gain by pretreating the cells with ouabain. The presence of Na/Ca
exchanger protein and mRNA in the astrocytes was confirmed by Western and
Northern blot analyses, respectively. Immunohistochemistry revealed that
exchanger molecules are distributed in a reticular pattern over the
astrocyte surface. We suggest that the Na/Ca exchanger plays a role in
regulating both [Ca2+]cyt and the intracellular stores of Ca2+ in
astrocytes, and may thus contribute to the control of astrocyte
responsiveness to neurotransmitters and neurotoxins.
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