Sodium/calcium exchange in rat cortical astrocytes.
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.
Goldman
W F
WF
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.
Yarowsky
P J
PJ
Juhaszova
M
M
Krueger
B K
BK
Blaustein
M P
MP
D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000737:chemistry / Q000187:drug effects / Q000378:metabolism*; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002352:Carrier Proteins / Q000032:analysis* / Q000378:metabolism; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D018076:DNA, Complementary / Q000032:analysis; D017136:Ion Transport; D008856:Microscopy, Fluorescence; D010042:Ouabain / Q000494:pharmacology; D012313:RNA / Q000032:analysis; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D012433:Ryanodine / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D014700:Verapamil / Q000494:pharmacology
D002352:Carrier Proteins; D018076:DNA, Complementary; D012333:RNA, Messenger; D013729:Terpenes; D012433:Ryanodine; D002110:Caffeine; D010042:Ouabain; D012313:RNA; D019284:Thapsigargin; D012964:Sodium; D014700:Verapamil; D000252:Calcium-Transporting ATPases; D002118:Calcium