Cellular mechanism for spontaneous calcium oscillations in astrocytes

To determine the Ca²⁺ source and cellular mechanisms of spontaneous Ca²⁺ oscillations in hippocampal astrocytes.
Methods: The cultured cells were loaded with Fluo-4 AM, the indicator of intracellular Ca²⁺, and the dynamic Ca²⁺ transients were visualized with confocal laser-scanning microscopy.
Results: The spontaneous Ca²⁺ oscillations in astrocytes were observed first in co-cultured hippocampal neurons and astrocytes. These oscillations were not affected by tetrodotoxin (TTX) treatment and kept up in purity cultured astrocytes. The spontaneous Ca²⁺ oscillations were not impacted after blocking the voltage-gated Ca²⁺ channels or ethylenediamine tetraacetic acid (EDTA) bathing, indicating that intracellular Ca²⁺ elevation was not the result of extracellular Ca²⁺ influx. Furthermore, the correlation between the spontaneous Ca²⁺ oscillations and the Ca²⁺ store in endoplasmic reticulum (ER) were investigated with pharmacological experiments. The oscillations were: 1) enhanced when cells were exposed to both low Na⁺ (70 mmol/L) and high Ca²⁺ (5 mmol/L) solution, and eliminated completely by 2 μmol/L thapsigargin, a blocker of sarcoplasmic reticulum Ca²⁺-ATPase; and 2) still robust after the application with either 50 mol/L ryanodine or 400 μmol/L tetracaine, two specific antagonists of ryanodine receptors, but depressed in a dose-dependent manner by 2-APB, an InsP3 receptors (InsP₃R) blocker.
Conclusion: InsP₃R-induced ER Ca²⁺ release is an important cellular mechanism for the initiation of spontaneous Ca²⁺ oscillation in hippocampal astrocytes.