Na(+)/Ca(2+) exchange in human myotubes: intracellular calcium rises in response to external sodium depletion are enhanced in DMD.

This study aims to investigate the sodium/calcium exchanger expression in human co-cultured skeletal muscle cells and to compare the effects of Na(+)/Ca(2+) exchange activity in normal and dystrophic (Duchenne's muscular dystrophy) human co-cultured myotubes. For this purpose, variations of intracellular calcium concentration ([Ca(2+)](int)) were monitored, as the variations of the fluorescence ratio of indo-1 probe, in response to external sodium depletion. External sodium withdrawal induced [Ca(2+)](int) rises within several seconds in both normal and Duchenne's muscular dystrophy myotubes. These Na(+)-free-induced [Ca(2+)](int) elevations were attributed to the reverse mode of the Na(+)/Ca(2+) exchange mechanism since the phenomenon was dependent on extracellular calcium concentration ([Ca(2+)](ext)), and since it was sensitive to external Ni(2+) ions. Amplitudes of Na(+)-free-induced [Ca(2+)](int) rises were significantly greater in Duchenne's muscular dystrophy cells than in normal ones. Such a difference disappeared when the sarcoplasmic reticulum was pharmacologically blocked, suggesting that the reverse mode of the Na(+)/Ca(2+) exchange mechanism was able to generate enhanced calcium-induced calcium-release in Duchenne's muscular dystrophy myotubes. Immunostaining images of Na(+)/Ca(2+) exchanger (NCX) isoforms, obtained by confocal microscopy, revealed the presence of NCX1 and NCX3 at the sarcolemmal level of both normal and Duchenne's muscular dystrophy myotubes. No differences were observed in the location of NCX isoforms expression between normal and Duchenne's muscular dystrophy co-cultured myotubes.