Changes in mitochondrial Ca2+ detected with Rhod-2 in single frog and mouse skeletal muscle fibres during and after repeated tetanic contractions

The present study investigated mitochondrial Ca²⁺ uptake and release in intact living skeletal muscle fibres subjected to bouts of repetitive activity. Confocal microscopy was used in conjunction with the Ca²⁺-sensitive dye Rhod-2 to monitor changes in mitochondrial Ca²⁺ in single Xenopus or mouse muscle fibres. A marked increase in the mitochondrial Ca²⁺ occurred in Xenopus fibres after 10 tetani applied at 4 s intervals. The mitochondrial Ca²⁺ continued to increase with increasing number of tetani. After the end of tetanic stimulation, mitochondrial Ca²⁺ declined to 50% of the maximal increase within 10 min and thereafter took up to 60 min to return to its original value. Depolarization of the mitochondria with FCCP greatly attenuated the rise in the mitochondrial Ca²⁺ evoked by repetitive tetanic stimulation. In addition, FCCP slowed the rate of decay of the tetanic Ca²⁺ transient which in turn led to an elevation of resting cytosolic Ca²⁺. Accumulation of Ca²⁺ in the mitochondria was accompanied by a modest mitochondrial depolarization. In contrast to the situation in Xenopus fibres, mitochondria in mouse toe muscle fibres did not show any change in the mitochondrial Ca²⁺ during repetitive stimulation and FCCP had no effect on the rate of decay of the tetanic Ca²⁺ transient. It is concluded that in Xenopus fibres, mitochondria play a role in the regulation of cytosolic Ca²⁺ and contribute to the relaxation of tetanic Ca²⁺ transients. In contrast to their important role in Xenopus fibres, mitochondria in mouse fast-twitch skeletal fibres play little role in Ca²⁺ homeostasis.