Store-operated Ca2+ entry in astrocytes: Different spatial arrangement of endoplasmic reticulum explains functional diversity in vitro and in situ

Ca2+ signaling is the astrocyte form of excitability and the endoplasmic reticulum (ER) plays an important role as an intracellular Ca2+ store. Since the subcellular distribution of the ER influences Ca2+ signaling, we compared the arrangement of ER in astrocytes of hippocampus tissue and astrocytes in cell culture by electron microscopy. While the ER was usually located in close apposition to the plasma membrane in astrocytes in situ, the ER in cultured astrocytes was close to the nuclear membrane. Activation of metabotropic receptors linked to release of Ca2+ from ER stores triggered distinct responses in cultured and in situ astrocytes. In culture, Ca2+ signals were commonly first recorded close to the nucleus and with a delay at peripheral regions of the cells. Store-operated Ca2+ entry (SOC) as a route to refill the Ca2+ stores could be easily identified in cultured astrocytes as the Zn2+-sensitive component of the Ca2+ signal. In contrast, such a Zn2+-sensitive component was not recorded in astrocytes from hippocampal slices despite of evidence for SOC. Our data indicate that both, astrocytes in situ and in vitro express SOC necessary to refill stores, but that a SOC-related signal is not recorded in the cytoplasm of astrocytes in situ since the stores are close to the plasma membrane and the refill does not affect cytoplasmic Ca2+ levels.