Sodium/calcium exchange in rat cortical astrocytes

Abstract

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.

On the Role of Voltage-Dependent Calcium Channels in Calcium Signaling of AstrocytesIn Situ

“…The source of Ca 2ϩ that permits these changes can be either intracellular, i.e., release from stores, and/or extracellular, i.e., entry through plasma membrane channels and ion exchangers. Although a contribution of the Na ϩ and Ca 2ϩ exchanger cannot be excluded (Goldman et al, 1994), our results suggest that the release of Ca 2ϩ from intracellular Ca 2ϩ stores and the Ca 2ϩ release-activated Ca 2ϩ influx are major mechanisms used by astrocytes to achieve rises in their [Ca 2ϩ ] i that could have functional significance for the triggering of Ca 2ϩ -dependent events.…”

Na + pump low and high ouabain affinity α subunit isoforms are differently distributed in cells

“…The location of the high affinity isoforms appears to provide a clue because the SR and ER sequester Ca 2ϩ , and the Ca 2ϩ released from these organelles controls numerous cell activities. A critical link between the Na ϩ pump and cell Ca 2ϩ is the PM Na ϩ ͞Ca 2ϩ exchanger, which is also localized to PM regions that overlie junctional SR and ER (11,14,15,31). The energy required to control cytosolic Ca 2ϩ via the Na ϩ ͞Ca 2ϩ exchanger is derived from the Na ϩ electrochemical gradient generated by the Na ϩ pump (12).…”

GABA uptake-dependent Ca 2+ signaling in developing olfactory bulb astrocytes

“…It has been shown that inhibition of NCX can result in increased Ca 2ϩ concentrations in other cells, including glial cells (39). In olfactory bulb astrocytes, this increase in the cytosolic Ca 2ϩ concentration depended on extracellular Ca 2ϩ .…”