Stimulation-induced mitochondrial [Ca²⁺] elevations in mouse motor terminals: Comparison of wild-type with SOD1-G93A

Changes in mitochondrial matrix [Ca²⁺] evoked by trains of action potentials were studied in levator auris longus motor terminals using Ca²⁺-sensitive fluorescent indicator dyes (rhod-2, rhod-5F). During a 2500 impulse 50 Hz train, mitochondrial [Ca²⁺] 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 [Ca²⁺] 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 [Ca²⁺] responses, but in other terminals mitochondrial [Ca²⁺] did not plateau, but rather continued to increase throughout most of the stimulus train. Thus mechanism(s) that limit stimulation-induced increases in mitochondrial [Ca²⁺] may be compromised in some SOD1-G93A terminals.