The mitochondrial membrane potential and Ca²⁺ oscillations in smooth muscle

Ca²⁺ uptake by mitochondria might both modulate the cytosolic Ca²⁺ concentration ([Ca²⁺]_c) and depolarize the mitochondrial membrane potential (_m) to limit ATP production. To investigate how physiological Ca²⁺ signaling might affect energy production, _m was examined during Ca²⁺ oscillations in smooth muscle cells. In single, voltage-clamped smooth muscle cells, inhibition of mitochondrial Ca²⁺ accumulation inhibited inositol (1,4,5)-trisphosphate [Ins(1,4,5)P₃]-evoked Ca²⁺ release and prolonged the time required for restoration of [Ca²⁺]_c following activation of plasmalemmal Ca²⁺ currents (I_Ca). Ca²⁺ could be released from mitochondria immediately (within 15 seconds) after a [Ca²⁺]_c rise evoked by Ins(1,4,5)P₃ or I_Ca. Despite this evidence of mitochondrial Ca²⁺ accumulation, no change in _m was observed during single or repetitive [Ca²⁺]_c oscillations evoked by these conditions. Occasionally, spontaneous, repetitive, persistent Ca²⁺ oscillations were observed. In these cases, mitochondria displayed stochastic _m depolarizations, which were independent both of events in neighboring mitochondria and of the timing of the [Ca²⁺]_c oscillations themselves. Such _m depolarizations could be mimicked by increased exposure to either fluorescence excitation light or the _m-sensitive dye tetramethylrhodamine ethyl ester (TMRE) and were inhibited by antioxidants (ascorbic acid, catalase, Trolox and TEMPO) or the mitochondrial permeability transition pore (mPTP)-inhibitor cyclosporin A (CsA). Individual mitochondria within smooth muscle cells might depolarize during repetitive Ca²⁺ oscillations or during oxidative stress but not during the course of single [Ca²⁺]_c transients evoked by Ca²⁺ influx or store release.