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 int act living skeletal muscle fibres subjected to bouts of repetitive activity . Confocal microscopy was used in conjunction with the Ca2+-sensitive dye R hod-2 to monitor changes in mitochondrial Ca2+ in single Xenopus or mouse m uscle fibres. A marked increase in the mitochondrial Ca2+ occurred in Xenop us fibres after 10 tetani applied at 4 s intervals. The mitochondrial Ca2continued to increase with increasing number of tetani. After the end of te tanic stimulation, mitochondrial Ca2+ declined to 50% of the maximal increa se 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. I n addition, FCCP slowed the rate of decay of the tetanic Ca2+ transient whi ch in turn led to an elevation of resting cytosolic Ca2+. Accumulation of C a2+ in the mitochondria was accompanied by a modest mitochondrial depolariz ation. In contrast to the situation in Xenopus fibres, mitochondria in mous e toe muscle fibres did not show any change in the mitochondrial Ca2+ durin g repetitive stimulation and FCCP had no effect on the rate of decay of the tetanic Ca2+ transient. It is concluded that in Xenopus fibres, mitochondr ia play a role in the regulation of cytosolic Ca2+ and contribute to the re laxation of tetanic Ca2+ transients. In contrast to their important role in Xenopus fibres, mitochondria in mouse fast-twitch skeletal fibres play lit tle role in Ca2+ homeostasis.