Evidence that mitochondria buffer physiological Ca2+ loads in lizard motor nerve terminals

It was already known that calcium ions regulate synaptic activity, but the role of mitochondria in buffering calcium in motor nerve terminals remained unclear. Prior studies showed that mitochondria can take up calcium, but the timing and extent of this process during physiological stimulation had not been directly observed. Earlier work suggested that cytosolic calcium levels rise rapidly during nerve stimulation, but the contribution of mitochondria to this process was uncertain. This gap motivated researchers to investigate how mitochondria respond to calcium influx in real time. No prior work had resolved whether mitochondrial uptake occurs immediately or after a delay. This uncertainty drove the need for high-resolution imaging techniques to track calcium dynamics in motor nerve terminals. The lack of direct evidence for mitochondrial buffering during normal neuromuscular transmission prompted this study. The researchers aimed to clarify the temporal relationship between cytosolic and mitochondrial calcium changes during stimulation.

The aim of this study was to determine whether mitochondria in motor nerve terminals can buffer physiological calcium loads during normal neuromuscular transmission. The researchers focused on the timing and magnitude of calcium uptake in mitochondria compared to cytosolic calcium levels. They wanted to test if mitochondrial calcium uptake occurs immediately or after a delay following stimulation. The study sought to clarify how mitochondrial buffering affects cytosolic calcium dynamics during nerve activity. The researchers also aimed to assess the impact of mitochondrial function on calcium regulation when the proton gradient was disrupted. They hypothesized that mitochondria may act as a secondary buffer after an initial rapid calcium rise in the cytosol. This study addressed the uncertainty about the role of mitochondria in maintaining calcium homeostasis in nerve terminals. The findings could help explain how presynaptic calcium levels are regulated during normal and pathological conditions.

The researchers used a rapidly scanning confocal microscope to measure calcium concentrations in motor nerve terminals. Cytosolic calcium was tracked using Oregon Green BAPTA 5N, injected ionophoretically into the nerve terminals. Mitochondrial calcium was measured using rhod-2, which was bath-loaded as dihydrorhod-2. The nerve terminals were stimulated with brief trains of action potentials at frequencies between 25 and 100 Hz. The researchers recorded calcium dynamics in response to 100 to 250 stimuli per train. They observed the time course of cytosolic and mitochondrial calcium increases during stimulation. To test mitochondrial function, they added carbonyl cyanide m-chlorophenyl hydrazone (CCCP) to disrupt the mitochondrial membrane potential. The study compared calcium levels before and after CCCP application to assess mitochondrial buffering capacity.

Cytosolic calcium levels increased rapidly at the start of stimulation and then rose more slowly over time. Mitochondrial calcium remained unchanged for the first fifteen to twenty stimuli before rising rapidly and then more slowly. The onset of mitochondrial calcium increase coincided with the slowing of cytosolic calcium rise. Both cytosolic and mitochondrial calcium levels increased with higher stimulation frequencies. After stimulation ended, cytosolic calcium decayed much faster than mitochondrial calcium. When CCCP was added, cytosolic calcium rose more rapidly and reached higher levels than normal. CCCP also prevented the increase in mitochondrial calcium levels during stimulation. These findings suggest that mitochondria contribute to buffering cytosolic calcium during normal neuromuscular transmission.

The authors propose that mitochondria in motor nerve terminals buffer cytosolic calcium during normal stimulation. Their findings suggest that mitochondrial uptake of calcium occurs after an initial rapid rise in cytosolic calcium. The delay in mitochondrial calcium increase coincides with the slowing of cytosolic calcium accumulation. This buffering effect may help regulate presynaptic calcium levels during neuromuscular transmission. The study shows that mitochondrial function is necessary for this buffering process. When the mitochondrial proton gradient was disrupted, cytosolic calcium levels rose more rapidly and reached higher levels. The researchers suggest that mitochondria may act as a secondary buffer after an initial calcium influx. These results support the hypothesis that mitochondria play an important role in maintaining calcium homeostasis in nerve terminals.