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Modulation of two functionally distinct Ca2+ stores in astrocytes: Role of the plasmalemmal Na/Ca exchanger

✍ Scribed by Vera A. Golovina; Linda L. Bambrick; Paul J. Yarowsky; Bruce K. Krueger; Mordecai P. Blaustein

Publisher
John Wiley and Sons
Year
1996
Tongue
English
Weight
983 KB
Volume
16
Category
Article
ISSN
0894-1491

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✦ Synopsis

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 +Icrt, was about 110 nM. In the absence of extracellular Ca", cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca2+-ATPase, induced a transient, four-fold increase in [Caz+lm due to the release of Ca2+ from inositol triphosphate (IP,) sensitive stores. Caffeine (CAE'), which releases Ca2-from Ca2 ' -sensitive stores, induced a two-fold increase in ICa2+l,.

The CPA-and CAE'-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 N d C a exchanger, caused minimal increases in resting [Ca2'l,,, but greatly potentiated both CPA-and CAF-induced Ca2' transients. The amount of Ca2+ releasable from the IPS (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 Ca" channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2-stores, the larger, IPS-(CPA-) sensitive, and the smaller, Ca2 -(CAE'-) sensitive.

The Ca2+ content of both ER stores can be regulated by the NdCa exchanger. Thus, the magnitude of ccllular 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. Q 1996 Wiley-Liss, Inc

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