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Keywords:

  • energy metabolism;
  • glycolysis;
  • oxidative phosphorylation;
  • glia;
  • mitochondria

Abstract

The influence of physiological Ca2+-inducing stimuli and agents mimicking ischemic conditions on mitochondrial potential was studied in postnatal (P1) hippocampal astrocytes. Cytosolic Ca2+ loads with characteristic kinetics of rise and duration, detected by Fura-2, were provoked by extracellular Ca2+ influx, release from InsP3-sensitive intracellular stores, or inhibition of the reloading of endoplasmic reticulum Ca2+ stores. Inhibitors of mitochondrial respiration caused only moderate release of Ca2+ from intracellular stores, inducing a rise of less than 60 nM. The maximal Ca2+ rise was found with InsP3-mediated responses (500 nM; via ATP) or with ionophore (4-Br-A23187)-mediated Ca2+ influx from extracellular medium (770 nM). Remarkably, all these agents causing significant rise of cytosolic Ca2+, only minimally depolarized the mitochondria. Membrane potential of mitochondria was monitored by Rh123 or TMRE. Depolarization was only found with very high cytosolic Ca2+ levels (above 60 μM; measured by fura FF). These were achieved with external Ca2+ influx by ionophore in combination with inhibition of glycolysis. Thus, mitochondria in the astrocytes are obviously not sensitive to moderate cytosolic Ca2+ loads, irrespective of the source of Ca2+. Furthermore, isolated rat brain mitochondria display a low sensitivity of respiratory activity to Ca2+, which is consistent with the data obtained with the astrocytes in vitro. The capacity of isolated mitochondria to build up a potential was gradually reduced at low micromolar Ca2+ and totally compromised only at Ca2+ concentrations in the 100 μM range. © 2001 Wiley-Liss, Inc.