Alteration in calcium handling at the subcellular level in mdx myotubes

In this study, we have tested the hypothesis that augmented [Ca2+] in subce llular regions or organelles, which are known to play a key role in cell su rvival, is the missing link between Ca2+ homeostasis alterations and muscul ar degeneration associated with muscular dystrophy. To this end, different targeted chimeras of the Ca2+-sensitive photoprotein aequorin have been tra nsiently expressed in subcellular compartments of skeletal. myotubes of mdx : mice, the animal model of Duchenne muscular dystrophy. Direct measurement s of the [Ca2+] in the sarcoplasmic reticulum, [Ca2+](sr), show a higher st eady state level at rest and a larger drop after KCl-induced depolarization in mdx compared with control myotubes. The peaks in [Ca2+] occurring in th e mitochondrial matrix of mdx myotubes are significantly larger than in con trols upon KCl-induced depolarization or caffeine application. The augmente d response of mitochondria precedes the alterations in the Ca2+ responses o f the cytosol and of the cytoplasmic region beneath the membrane, which bec ome significant only at a later stage of myotube differentiation. Taking in to account the key role played by mitochondria Ca2+ handling in the control of cell death, our data suggest that mitochondria are potential targets of impaired Ca2+ homeostasis in muscular dystrophy.