Mitochondrial polarization in rat hippocampal astrocytes is resistant to cytosolic Ca(2+) loads.
The influence of physiological Ca(2+)-inducing stimuli and agents mimicking ischemic conditions on mitochondrial potential was studied in postnatal (P1) hippocampal astrocytes. Cytosolic Ca(2+) loads with characteristic kinetics of rise and duration, detected by Fura-2, were provoked by extracellular Ca(2+) influx, release from InsP(3)-sensitive intracellular stores, or inhibition of the reloading of endoplasmic reticulum Ca(2+) stores. Inhibitors of mitochondrial respiration caused only moderate release of Ca(2+) from intracellular stores, inducing a rise of less than 60 nM. The maximal Ca(2+) rise was found with InsP(3)-mediated responses (500 nM; via ATP) or with ionophore (4-Br-A23187)-mediated Ca(2+) influx from extracellular medium (770 nM). Remarkably, all these agents causing significant rise of cytosolic Ca(2+), only minimally depolarized the mitochondria. Membrane potential of mitochondria was monitored by Rh123 or TMRE. Depolarization was only found with very high cytosolic Ca(2+) levels (above 60 microM; measured by fura FF). These were achieved with external Ca(2+) influx by ionophore in combination with inhibition of glycolysis. Thus, mitochondria in the astrocytes are obviously not sensitive to moderate cytosolic Ca(2+) loads, irrespective of the source of Ca(2+). Furthermore, isolated rat brain mitochondria display a low sensitivity of respiratory activity to Ca(2+), 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 Ca(2+) and totally compromised only at Ca(2+) concentrations in the 100 microM range.
Kahlert
S
S
Institute of Neurobiochemistry, Medical Faculty, Otto-von-Guericke-University Magdeburg, Leipziger Str. 44, 39120 Magdeburg, Germany.
Schild
L
L
Reiser
G
G
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D002545:Brain Ischemia / Q000378:metabolism* / Q000503:physiopathology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D003574:Cytochrome c Group / Q000378:metabolism / Q000494:pharmacology; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism*; D004734:Energy Metabolism / Q000187:drug effects / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D006019:Glycolysis / Q000187:drug effects / Q000502:physiology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism* / Q000503:physiopathology; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism; D007476:Ionophores / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D026901:Membrane Transport Proteins / Q000187:drug effects / Q000502:physiology; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*; D010085:Oxidative Phosphorylation / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar
D003574:Cytochrome c Group; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007476:Ionophores; D026901:Membrane Transport Proteins; D000255:Adenosine Triphosphate; D002118:Calcium