Mitochondrial Calcium Oscillations in C2C12 Myotubes

Abstract

Mitochondrial Ca2+ concentration ([Ca2+] m) was monitored in C2C12 skeletal muscle cells stably expressing the Ca2+-sensitive photoprotein aequorin targeted to mitochondria. In myotubes, KCl-induced depolarization caused a peak of 3.03 ± 0.14 μM [Ca2+]m followed by an oscillatory second phase (5. 1 ∓ 0.1 per min). Chelation of extracellular Ca2+ or blockade of the voltage-operated Ca2+ channel attenuated both phases of the KCl response. The inhibitor of the sarcoplasmic reticulum Ca 2+-ATPase, cyclopiazonic acid, reduced the amplitude of the KCl-induced [Ca2+]m peak and prevented the oscillations, suggesting that these were generated intracellularly. No such [Ca 2+]m. oscillations occurred with the nicotinic agonist carbachol, cyclopiazonic acid alone, or the purinergic agonist ATP. In contrast, caffeine produced an oscillatory behavior, indicating a role of ryanodine receptors as mediators of the oscillations. The [Ca2+]m response was desensitized when cells were exposed to two consecutive challenges with KCl separated by a 5-min wash, whereas a second pulse of carbachol potentiated [Ca2+]m, indicating differences in intracellular Ca2+ redistribution. Cross-desensitization between KCl and carbachol and cross-potentiation between carbachol and KCl were observed. These results suggest that close contacts between mitochondria and sarcoplasmic reticulum exist permitting Ca2+ exchanges during KCl depolarization. These newly demonstrated dynamic changes in [Ca2+]m. in stimulated skeletal muscle cells might contribute to the understanding of physiological and pathological processes in muscular disorders.