Stimulation‐Induced Mitochondrial [Ca 2+

Changes in mitochondrial matrix [Ca(2+)] evoked by trains of action potentials were studied in levator auris longus motor terminals using Ca(2+)-sensitive fluorescent indicator dyes (rhod-2, rhod-5F). During a 2500 impulse 50 Hz train, mitochondrial [Ca(2+)] in most wild-type terminals increased within 5–10 s to a plateau level that was sustained until stimulation ended. This plateau was not due to dye saturation, but rather reflects a powerful buffering system within the mitochondrial matrix. The amplitude of this plateau was similar for stimulation frequencies in the range 15–100 Hz. Plateau amplitude was sensitive to temperature, with no detectable stimulation-induced increase in fluorescence at temperatures below 17 °C, and increasing magnitudes as temperature was increased to near-physiological levels (38 °C). When stimulation ended, mitochondrial [Ca(2+)] decayed slowly back to prestimulation levels over a time course of hundreds of seconds. Similar measurements were also made in motor terminals of mice expressing the G93A mutation of human superoxide dismutase 1 (SOD1-G93A). In mice > 100 days old, all of whom exhibited hindlimb paralysis, some terminals continued to show wild-type mitochondrial [Ca(2+)] responses, but in other terminals mitochondrial [Ca(2+)] did not plateau, but rather continued to increase throughout most of the stimulus train. Thus mechanism(s) that limit stimulation-induced increases in mitochondrial [Ca(2+)] may be compromised in some SOD1-G93A terminals