To investigate mitochondrial responses to repetitive stimulation, we measured changes in NADH fluorescence and mitochondrial membrane potential (Ψm) produced by trains of action potentials (50 Hz for 10-50 s) delivered to motor nerve terminals innervating external intercostal muscles. Stimulation produced a rapid decrease in NADH fluorescence and partial depolarization of Ψm. These changes were blocked when Ca2+ was removed from the bath or when N-type Ca2+ channels were inhibited with ω-conotoxin GVIA, but were not blocked when bath Ca2+ was replaced by Sr2+, or when vesicular release was inhibited with botulinum toxin A. When stimulation stopped, NADH fluorescence and Ψm returned to baseline values much faster than mitochondrial [Ca2+]. In contrast to findings in other tissues, there was usually little or no poststimulation overshoot of NADH fluorescence. These findings suggest that the major change in motor terminal mitochondrial function brought about by repetitive stimulation is a rapid acceleration of electron transport chain (ETC) activity due to the Ψm depolarization produced by mitochondrial Ca2+ (or Sr2+) influx. After partial inhibition of complex I of the ETC with amytal, stimulation produced greater Ψm depolarization and a greater elevation of cytosolic [Ca2+]. These results suggest that the ability to accelerate ETC activity is important for normal mitochondrial sequestration of stimulation-induced Ca2+ loads.
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