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) Ca²⁺-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 Ca²⁺-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, Ca²⁺- (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. © 1996 Wiley-Liss, Inc.