Modulation of two functionally distinct Ca2+ stores in astrocytes: role of the plasmalemmal Na/Ca exchanger.
Mechanisms that regulate the amount of releasable Ca2+ in intracellular stores of cultured mouse astrocytes were investigated using digital imaging of fura-2 loaded cells. At rest, the cytoplasmic Ca2+ concentration, [Ca2+]cyt, was about 110 nM. In the absence of extracellular Ca2+, cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca(2+)-ATPase, induced a transient, four-fold increase in [Ca2+]cyt due to the release of Ca2+ from inositol triphosphate (IP3) sensitive stores. Caffeine (CAF), which releases Ca2+ from Ca(2+)-sensitive stores, induced a two-fold increase in [Ca2+]cyt. The CPA- and CAF-sensitive stores could be released independently. Changes in the amplitudes of the Ca2+ transients were taken as a measure of changes in store content. Removal of extracellular Na+ or addition of ouabain, which inhibit Ca2+ extrusion and promote Ca2+ entry across the plasmalemma via the Na/Ca exchanger, caused minimal increases in resting [Ca2+]cyt but greatly potentiated both CPA- and CAF-induced Ca2+ transients. The amount of Ca2+ releasable from the IP3(CPA) sensitive store was directly proportional to cytosolic Na+ concentration (i.e., inversely proportional to the transmembrane Na+ electrochemical gradient). Under these reduced Na+ gradient conditions, little, if any, Ca2+ destined for the ER stores enters the cells through voltage-dependent Ca2+ channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2+ stores, the larger, IP3- (CPA-) sensitive, and the smaller, Ca(2+)- (CAF-) sensitive. The Ca2+ content of both ER stores can be regulated by the Na/Ca exchanger. Thus, the magnitude of cellular responses to signals that are mediated by Ca2+ release induced by the two second messengers, IP3 and Ca2+, can be modulated by factors that affect the net transport of Ca2+ across the plasmalemma.
Golovina
V A
VA
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201, USA.
Bambrick
L L
LL
Yarowsky
P J
PJ
Krueger
B K
BK
Blaustein
M P
MP
D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors / Q000378:metabolism; D002138:Calibration; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure; D004791:Enzyme Inhibitors / Q000494:pharmacology; D016257:Fura-2; D007091:Image Processing, Computer-Assisted; D007211:Indoles / Q000494:pharmacology; D051379:Mice; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects / Q000378:metabolism
D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D007211:Indoles; D010726:Phosphodiesterase Inhibitors; D019831:Sodium-Calcium Exchanger; D002110:Caffeine; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase; D002118:Calcium; D016257:Fura-2; C000543:cyclopiazonic acid