Mitochondrial polarization in rat hippocampal astrocytes is resistant to cytosolic Ca²⁺ loads

Abstract

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.

High Cyclophilin D Content of Synaptic Mitochondria Results in Increased Vulnerability to Permeability Transition

“…Primary rat cortical astrocytes have a greater Ca 2ϩ uptake capacity than do cerebellar granule neurons, and this uptake capacity is enhanced by CsA in astrocytes but not in neurons (Bambrick et al, 2006). Also consistent with the low cyclophilin D content of astrocytic mitochondria is their relative resistance to large cytosolic Ca 2ϩ loads (Kahlert et al, 2001). Previously, a role for CypD in the impaired Ca 2ϩ handling of synaptic mitochondria had been questioned based on the inability of 1 M CsA to increase Ca 2ϩ uptake capacity (Brown et al, 2006).…”

Dual inhibition of sodium-mediated proton and calcium efflux triggers non-apoptotic cell death in malignant gliomas

“…In normal astrocytes and other normal cell types, collapse of the transmembrane mitochondrial potential does not demonstrably increase [Ca 2+ ] i (Kahlert et al, 2001). In U87 glioma cells, FCCP caused a large increase in [Ca 2+ ] i and a negligible [Ca 2+ ] i increase in normal astrocytes (Fig 3C).…”

Swelling of mitochondria in cultured rat hippocampal astrocytes is induced by high cytosolic Ca2+ load, but not by mitochondrial depolarization

“…Subsequent additional application of oligomycin (10 μM) evoked a small dequenching peak. B: Cytosolic Ca 2+ (open squares) was transiently increased by ATP up to 800 nM [16] by release from the endoplasmic reticulum by P2Y receptor stimulation, but there was no influence on the mitochondrial potential (filled triangles). Dequenching peak (mitochondrial depolarization) was observed with FCCP/oligomycin.…”