Effect of metformin on spontaneous calcium signaling in cultured astrocytes during normoxic and hypoxic conditions.

Astrocyte function is controlled by intracellular Ca2+ signaling. On the other hand, hypoxia influences calcium dynamics and its homeostatic range because of reduction of ATP synthesis, which inhibits ATP dependent processes. By using Ca2+ sensitive fluorescence dye, we studied how metformin changed spontaneous oscillating Ca2+ signals in soma of astrocytes in monocultures, prepared from rat brains. Mild hypoxic conditions (2 % O2) applied for 24 h had no effect on astrocyte viability; however, it reduced the relative amplitude of Ca2+ signals, slowed the decay of the signals, and increased the period of spontaneous oscillations. Lower concentrations of metformin, 250 μM or 500 μM, applied before hypoxia reduced this influence by partially restoring the amplitude, fastening the decay, and reducing the period of Ca2+ signaling. In contrast, higher concentration, 1 mM of metformin exaggerated the effects of hypoxia by reducing signals, slowing their decay and prolonged the period between signals. Unexpectedly, in astrocytes grown under normoxic conditions, all concentrations of metformin after 1 h of application had effects similar to hypoxia for Ca2+ signaling. In conclusion, our data show that mild hypoxia reduces Ca2+signaling in astrocyte cell monocultures, and low concentrations of metformin under mild hypoxic conditions help to rescue the functioning of astrocytes by conditioning the cells to prolonged hypoxic influence.